by
Leif E. Andrén
Intergovernmental Oceanographic Commission
UNESCO
Place de Fontenoy
75700 Paris - France
Abstract
An overview is given of the most important categories of pollutants and their effects on aquatic life with special reference to aquaculture. Examples are given of cases of pollution of African waters which are reported to have damaged aquaculture and fisheries, or which are threatening aquaculture development. Proposals are made for measures to protect aquaculture from losses due to pollution.
Résumé
Une vue d'ensemble est donnée des catégories les plus importantes de polluants et leurs effets sur la vie aquatique avec référence spéciale à l'aquaculture. Des exemples sont données de cas de pollution dans les eaux africaines qui ont endommagé l'aquaculture et les pêches ou menacé le développement de l'aquaculture. Des propositions sont faites pour prendre des mesures de protection pour l'aquaculture par suite de pertes dues à la pollution.
In 1972 the ACMRR Working Party on Aquaculture (FAO-ACMRR, 1972) warned that unless adequate measures were taken urgently, existing aquaculture might be seriously affected in many areas and further expansion made extremely difficult, and that adverse effects of pollution on aquaculture was likely to become more acute with increasing industrialization and urbanization. Some 8–10 percent of the oyster culture grounds in the United States had been closed as a result of sewage and industrial pollution. Eutrophication of the Inland Sea of Japan had rendered 20–30 percent of its aquaculture areas unfit for fish or shellfish culture. It was reiterated that the IPFC Symposium on Coastal Aquaculture in Bangkok, Thailand, (November, 1970) had shown that a large majority of the developing countries of the region faced the same problem.
During the relatively short time that aquaculture has been practised in Africa a range of different geographical areas have seen various types of aquaculture develop, and many examples can be given of different environmental situations and socio-economical contexts, where some form of aquaculture is being experimented with or, at best, commercially operated. Therefore, the environmental problems facing the aquaculturists can be quite diverse.
The collective experience in Africa of conflicts between water quality requirements for aquaculture and other human activities is very limited compared to that, for example, in Europe or South East Asia.
Some of the more important types of environmental problems that can occur regardless of the state of industrialization or urbanization, are for instance alteration of natural environmental parameters, and spreading of organisms damaging to aquaculture or to human health (biological pollution) such as water-borne pathogenic micro-organisms or excessive growth of weeds. However, with the growth of industry and intensified crop production new problems are inevitably created. Practically all kinds of pollution can negatively affect aquaculture in one way or another; for instance, it can hinder operations, disturb life-processes of the cultured species and their food organisms, degrade their environment or, finally, deteriorate their quality as human food products. The cultured species have a limited capacity to avoid or escape hazardous pollutants, therefore aquaculture is more sensitive to pollution than open water fisheries.
Evidently there are many ways of using pollutants in a way beneficial to aquaculture, for instance, use of various types of toxicants (insecticides, molluscicides, piscicides and even petroleum) for protection of the cultured stocks, and use of wastes for nutrient supply. In order to limit the presentation these aspects have been largely left aside.
2.1 Water exchange and currents
In aquaculture projects dependent on free-flowing water for supply of nutrients and removal of waste products, and possibly for recruitment of stocks, production can be harmfully affected by the effects of human activities in the vicinity that alter the dynamic properties of the water-body. Such changes can be brought about, for instance, by construction of piers and dams, deposition of waste material or sediments and by increased exposure to erosion and subsequent shore degradation. The effect can be noticed as a change in current velocity, turbidity, sedimentation or ventilation in the cultured system.
In connection with construction of the port of Cotonou, Dahomey, the hydrodynamic properties of the lagoon were altered, so that a sand bar on the shore was prevented from rebuilding. The resulting increase of saltwater intrusion into the lagoon had ecological consequences. (FAO, 1969).
Constructions made by aquaculturists in esturaries and lagoons can affect current patterns so that sedimentation results in undesired locations. Odum (1972) points out that with very intensive raft culture of molluscs dissolved oxygen can be significantly lowered downstream of the rafts. The efficient filtering by the molluscs can endanger recruitment of species with planktonic larvae in nearby, confined water-bodies.
In South Africa mariculture is on the whole less threatened by industrial and domestic pollution than by a physical degradation of estuaries and lagoons, and fish farming practices may even further deteriorate the situation unless performed with particular regard to the sensitivity of the environment. Therefore, a development of estuarine aquaculture in Natal is expected to be slow and perhaps directed toward types of operations, the environmental effects of which are easily controlled (A.E.F. Heydorn, Oc.Res.Inst., pers. comm., 1975).
2.2 Salinity
With altered exchange of water with the open sea the salinity regime of semi-enclosed coastal waters will change. The diversion of the Umfolozi River (Natal) which used to drain into the St. Lucia Lake System, resulted in a considerable elevation of salinity in parts of the system. The result was that a previous problem of siltation caused by a combination of increased erosion and elimination of buffering swampy areas, was replaced by a problem of hypersalinity (Heydorn, 1973). Increase of salinity can also be used to advantage. In connection with mullet culture in the Tunis lagoon an experiment was made to control excessive growth of the algae Ulva lactuca and Enteromorpha intestinalis. (Pillai, unpubl.) An area of approximately 20 ha was closed off from the lagoon. As a result of evaporation during the hot season the salinity gradually rose to 80 p.p.t. and the algae were destroyed. During the following winter a rich phytoplankton production was observed, but no Ulva or Enteromorpha. The mullet fry that were then stocked grew from 2.5–3 cm to 8–10 cm in six months. During the following summer no macro-algae appeared, and dissolved oxygen at dawn was about 6 p.p.m. By comparison, in nearby mullet ponds that had not been exposed to hypersalinity, oxygen dropped to zero in the mornings. This suggests that the decomposition of macro-algae played an important role in the oxygen balance in these ponds and that the algae cannot establish themselves on the bottom when a heavy plankton growth reduces light penetration sufficiently.
2.3 Suspended solids and siltation
Deforestation, overgrazing and altered techniques in agriculture have increased erosion significantly; during the rainy season runoff into rivers, lakes and estuarine waters cause extreme turbidity and silt deposition. This phenomenon is causing increasing concern to fisheries, as production in estuarine and lagoon waters is threatened, and reservoirs and irrigation channels otherwise suitable for fish production are adversely affected. Many studies have been made on this subject (EIFAC, 1964; Alabaster, 1972). It should be remembered also that destruction of vegetation fringing the water, or even at some distance from it, can also affect the nutrient supply and the degree of exposure to toxicants.
Problems related to silting were reported from several countries in East Africa and Chad and from South Africa. In Kenya it is a serious problem. For instance, the Attú (Galana) and Tana Rivers carry heavy loads of silt that pollute the inshore waters near Malindi (Kongere, 1973) and silting threatens coral biota of the Watamu Marine National Parks (R.E. Morris, EAMFRO, pers.comm., 1975). Turbidity is also serious in the Nairobi River, where it adds to the problems of industrial and domestic waste, and where fish stocks are being depleted as a result.
Turbidity from storm runoff is difficult to avoid or prevent. It was specifically mentioned as a problem to aquaculture in Kenya (P. Kamande, Dept. of Fisheries, 1975, pers. comm.), but is certainly experienced in many other places. Okorie (unpubl.) informs that in the Lake Baringo basin the aquatic environment has deteriorated through siltation to the extent that maintenance of the existing fishery is endangered. This problem is causing concern to fishery authorities in Madagascar who fear damage to shrimp resources, although in this case the damage would be less due to direct effects of the silt than to pesticide residues adsorbed to the particles. Intensive agriculture with concomitant erosion is causing gradual filling-up of natural pools in places in Rhodesia (G.A. Reeskamp, Makoholi Exp.Stat., pers.comm., 1975). The fish-culture centres at Kpéwa and Ná in Togo have ponds with direct connections to nearby streams. When turbid waters entered the ponds by accident a lowering of oxygen levels was observed and, occasionally, mortalities occurred. The dead fish showed deposition of silt particles on their gills. Cases of infection by fungi (saprolegniosis) were also observed in connection with increased turbidity (J.Y. Kusiaku, Serv.Pèche, pers.comm., 1975).
In Natal there is a strong runoff from land and in several of the estuaries and lagoons only organisms that can cope with the turbidity during the rainy season can survive.
In Lagos area sand excavation in the lagoon and associated streams is thought to contribute to the diminishing of fish populations and in some cases aquaculture production (O. Bakare, Fisheries Division, Min.Agr., pers.comm., 1975).
Turbidity affects aquatic organisms and can cause considerable changes in community structure. Physically it impairs light penetration and, therefore, reduces photosynthesis and sometimes food-uptake. It changes bottom characteristics and smothers some attached species. Fish and shellfish respond physiologically in several ways. Oysters are sensitive to high particle content which interferes with oxygen and food intake, as it causes a decrease of pumping rate and clogging of branchiae. This fact has caused concern among oyster culturists in Kenya and Tanzania, where oyster beds are also exposed to additional environmental stresses. Fish mortality is sometimes caused by particles clogging the gills. Cultured fish species vary in their response to content of suspended particles, but frequently a reduction of growth rate, and sometimes also in survival rate, can be observed.
Inert or non-toxic substances such as china clay, gypsum or red mud from bauxite industry and fly-ash from power stations have similar effects on the ecosystem. Dredging and dumping (e.g., of sludge) can also have damaging effects depending on the nature of the bottom and characteristics of the material dumped. It should be noted that during dredging new surfaces become exposed in the water-sediment interface. Nutrients as well as toxic metal ions may leak into the water and become available to organisms.
The beneficial effects of nutrient supply and its use for aquaculture will be discussed elsewhere. Related problems have been considered in a large number of studies (Allen, 1970; Chan, 1974; McGarry 1971, 1972; and Slack, 1974). A total re-circulation system involving production of algae, fish, pigs and vegetables, as well as heat energy was described by Richard (1973). Some implications of supply of nutrient-rich pollutants, other than their beneficial role as fertilizers in aquaculture, should also be considered. For instance, municipal waste water often consists of a mixture of pollutants including heavy metals, chlorinated hydrocarbons, detergents and pathogens, it also has a high content of organic matter and, therefore, imposes a high BOD.
3.1 Domestic waste as a source of undesirable nutrients
Uncontrolled discharge of domestic waste and of effluents with high organic content from sewage plants with only partial treatment are causing undesirable enrichment of waters in many places in Africa, and aquaculture as well as fisheries are often affected adversely. In most countries sewage is not considered a problem for aquaculture, rather the opposite. However, a number of African water bodies have attracted attention because of their state of pollution from sewage, or mixed domestic and industrial pollution; examples include Lake Tunis, Lake Maryut and Lagos Lagoon. In the vicinity of a number of cities such as Nakuru, Nairobi, Addis Ababa, Tananarive and Mwanza, Lagos, and many others, recipient water bodies are known to be polluted. Around cities with rapid growth this effluent can be expected to impose an increasing (organic) load on water bodies nearby.
Domestic waste may be accompanied by other pollutants that cause acute or sublethal effects. Such pollutants are not always easily identified. Exceptional cases were, for instance, phenols applied to sewage for sanitary reasons which resulted in tainting of fish in the Lagos Lagoon, and accumulation of heavy metals in fish of the North Lake Tunis, which receives sewage from parts of Tunis city.
3.2 A case of study: Lake Tunis
Lake Tunis can be taken as an example of a confined coastal water body receiving an organic load exceeding its purification capacity. Originally a lagoon, it now has connection with the sea only through channels - one direct and three indirect ones via a navigation channel leading to the harbour of Tunis. It has an area of nearly 30 km2 and an average depth. of 90 cm. The annual fish catch in the lake is about 300 t. Raw sewage from Tunis is discharged into the lake. Partially treated sewage is also discharged from a treatment plant. As a whole the lake has a high self-purification capacity. However, the high nutrient level of the water and leakage of nutrients from the bottom mud produce an environment that is extremely favourable to production of algae (Ulva and Enteromorpha) and of plankton. During the summer (July-September) plankton blooms (also red tides) occur every year, sometimes in connection with massive fish kills (between 50–100 t). The mortality is caused by oxygen depletion, which in turn is probably a result of several factors, such as plant respiration during the night, degradation of organic substance from sewage and from degenerating algae, and the lowered oxygen content of the water due to high water temperature.
It has been shown that large diurnal oxygen variations can cause reduction in fish growth or even reduction in weight; Payne (1971) points out that several life functions of three species of Tilapia are affected by such variations.
The bottom mud in the western part of the lake is releasing nutrients into the water. According to an ambitious plan for restoration of water quality sewage sludge accumulated on the bottom in the western part of the lake will be dredged and deposited on land. The run-off water will be drained into the lake after reduction of phosphorous and nitrogen by chemical precipitation (by lime) and air-stripping. The present treatment plant will be re-constructed for increased treatment efficiency. The water circulation in the lake and the exchange with the sea will be regulated by automatic gates during critical periods of the year. These gates promote a one-way, east-west circulation: sea - lake - navigation channel - sea. The circulation will be completely driven by water-level fluctuations and will be facilitated by dredging and broadening of the eastern-most and most sea-ward channel.
The lake fishery depends on natural recruitment of young fish. Therefore, the gates will allow free passage of water into the lake between December and April to ensure such recruitment. Special pontoon-equipped amphibious skimmers are used for removing floating algae. Even if these vehicles do not cope with the large masses of algae produced they will contribute to the removal of organic matter and toward a reduction of the amounts of organic matter depositing on the bottom of the lake along these shores where the algae are driven by the winds.
It is too early to predict the degree of success of this project. Several factors still pose problems. For instance, there are large areas with reefs of a tube-building polychaete Mercierella enigmatica which are obstructing circulation. Ideally there should be a complete stop to discharge of organic matter and nutrient salts into the lake and much will depend on the attained reduction of nutrient supply. Sediments will continue to leak nutrients into the water. Before the dredging in the western part, the sediment there contributed 500–900 mg NH4-N and 25–30 mg PO4-P/m2/day (Björk, 1972). Further, as is normally the case outside points of sewage discharge, the sediments have elevated levels of heavy metals. It is not yet known to what extent these will be mobilized through the sediment water interface.
3.3 Land runoff
Fertilization due to runoff from farm land is a common problem to fisheries in Europe. In Africa application of fertilizers on agricultural land seems rarely to be intensive, and its contribution can be considered negligible in comparison with other potential sources of eutrophication. Reduction of a beneficial natural supply of organic material can occur where a productive shore vegetation such as mangroves is cleared for urban or industrial development. In such cases the aquatic biota (e.g., coral reefs, lagoon biota) may receive a diminished input of nutrients and also become more exposed to deposition of inorganic material (Canestri, Ruiz 1973).
3.4 Organic industrial wastes
In many places organic wastes from industry is contributing visibly to pollution of waters by causing turbidity and oxygen depletion. Fortunately, aquaculturists have been able to take advantage of waste material from a number of such industries for feeding of fish or fertilizing of ponds. This applies especially to industries processing agricultural products such as cereals (like rice bran), oil cakes, coconut waste and brewery waste. Wastes from dairy industry and slaughter houses can also be used for pond fertilization and as fish food.
Cases of damage to aquaculture or shallow water fisheries is reported from some localities in Africa. At Banjul (Gambia) a groundnut mill is affecting oyster production in a nearby river. Lagos Lagoon receives, among other wastes, large amounts of wood and bark fragments in connection with log handling (O. Bakare, pers.comm., 1975).
Sugar processing plants in Rhodesia are polluting the Sabi-Lundi river system (G.A. Reeskamp, pers.comm., 1975). Several industries (soap, vegetable oil, a slaughter house) will probably be joined by a fish meal factory at Mwanza at the southern end of Lake Victoria, but damage to fisheries has not been observed so far (K.H. Ibrahim, Freshw.Fish.Inst., pers.comm., 1975) In many places in East Africa coffee-pulping plants, sisal processing and sugar refining cause high BOD in receiving waters (e.g., in Nyando River). Paper mills are adding a serious threat to fisheries in some rivers (e.g., R. Nzoia, Kenya).
3.5 Oxygen and other gases
The common denominator for such wastes is the high biochemical oxygen demand (BOD), that is of greater significance generally than associated toxic pollutants. The impact on the receiving water can be checked by ensuring adequate initial dilution and mixing. When such wastes are used in fishculture great attention must be given not to overload the system. The symptoms of over-load are well known to aquaculturists. In the early morning, when oxygen levels are at a minimum, due to the combined effect of microbial decomposition and plant respiration, fish will show signs of distress, or at worse, will be found dead or dying.
Too-high oxygen levels in the water can also be caused by pollution. In an eutrophied water body with very intensive photo-synthesis, oxygen levels can rise to supersaturation and gas-bubble-disease may result in fish.
Decomposition under anaerobic conditions gives rise to dangerous concentrations of H2S and NH3. In mild cases of oxygen depletion in small ponds it may be sufficient to whip the water with sticks to improve the situation. In commercial fishculture a number of different types of aerators, aero-hydraulic guns and air-and-water injection apparatus (for aeration of putrid bottom) come into use (Anon., 1973 a).
Dissolved oxygen requirements of freshwater fishes of Europe have been reported (FAO/EIFAC, 1973); the relationship between oxygen and pollution in Egyptian lakes has been studied by Saad (1970).
4.1 Pathogenic microbes, parasites and other health hazards
It has been shown that water-borne diseases can be transferred to humans through consumption of fish or shellfish that were exposed to sewage. Where raw sewage is fed into fish ponds or discharged in the vicinity of shellfish beds, there is a serious risk of infection, because sewage, even if partially treated, can constitute a “soup” of pathogenic and saprophytic bacteria, viruses and parasites. Filter feeders like oysters accumulate these organisms and cause human infections unless sterilization or purification takes place before consumption.
It should be noted that even if standard coliform counts are well within safety limits, this does not guarantee, for example, that infectious hepatitis does not present a problem, because in saline water viral inactivation is slower than that of coliforms (Wood, 1970). There is now also evidence (Vickery, 1973) that Coxackie virus A in oysters and mussels causes enteroviral infection in man.
Bacteria that can be spread to man through aquaculture include Clostridium, Shigella and Salmonella spp. If fishery products are handled or stored under unsanitary conditions there is a risk of botulism. It is under such conditions that Vibrio parahaematolyticus can thrive; this is a marine bacterium that commonly causes gastroenteritis. It is, for instance, responsible for about 20 percent of all cases of food poisoning in Japan.
The risk for infection by protozoans and metazoan parasites through sewage has also been shown to exist.
There are other aquaculture practices that may favour the spread of diseases by providing suitable environments for biological pollutants (weeds and animals) which in turn act as vectors, or habitats for pathogenic organisms of significance both to human health or to the cultured species. The increased practice of pond culture and rice-cum-fish culture may favour the spread of the vector snails of bilharzia, unless suitable counter-measures are taken. Two species of the parasitic trematode Schistosoma are causing two types of bilharzia in humans. The trematodes depend on specific aquatic snails (Biomphalaria sp., Bulinus spp., or Physopsis sp.) for completion of part of their life cycles. The remedy normally sought is to kill the snails by molluscicides or eradicate weeds that provide food and shelter for the snails through use of herbicides or biological control methods. Coche (1967) and Meschkat (1967) describe several experiments aimed at biological control of snails in Africa. Problems of this nature pose a serious threat to aquaculture development in large parts of Africa. Epidemics have been reported from Lake Volta, Ghana, and Yaoundé, Cameroon. In Lake Volta two species of the snail Bulinus and the Ewe fisher, an aquatic bird, act as hosts for two species of Schistosoma. Lavoipierre (1973) reports that experiments are now being made to kill the larvae of the schistosomes (but no other aquatic animals) by a biocide incorporated in an india-rubber matrix which dissolves slowly in water.
Another serious problem is that of mosquitoes (Anopheles spp., Stegomya sp.) that are vectors for human diseases like malaria, yellow fever, filariasis, dengue fever and encephalitis that breed in ponds and rice fields. Again, beside using pesticides, biological control is being tried to eradicate the mosquitoes and to remove or kill plants that give shelter.
The black fly (Simulium) is a vector for onchocerciasis - river blindness. As the name implies, the disease is associated with flowing waters (rivers and channels) where the fly breeds. In tropical West Africa, where the disease is common, attention has to be given not to favour reproduction or survival of the fly in connection with aquaculture development.
In this context it may be mentioned that pollution may favour development of diseases in cultured fish. It has been suggested that turbidity can favour some diseases; domestic wastes can produce skin ulcerations and teratogenic formations. Elvers of European eel (Anguilla anguilla) suffered mass mortalities from myxo-bacteria attacking the gills, to which, it is believed, they become susceptible due to pollution effects (Anon, 1972). It is likely that stress increases the susceptibility to infection, and different pollutants or changes in environmental conditions may well create enough stress for diseases to become manifest.
To this list of dangers to animals and humans can be added intoxication by biotoxins. Especially common is paralytic shellfish poisoning (PSP) which is caused by accumulation in filter-feeding bivalves of saxotoxin derived from dinoflagellates. When these organisms (e.g., species of Gonyaulax and Gymnodinium) are exposed to favourable ecological conditions, possibly related to temperature, nutrients and other biotic factors, a population explosion occurs which can be observed as a “red tide” phenomenon. It has been observed in several continents including Africa, and both animal (fish and birds) and human mortalities have been reported as a result of the curare-like toxin for which no antidote is known (FAO, 1974). For instance, at Mogadiscio in 1972 a large fish kill was attributed to red tide.
4.2 Plants as biological pollutants
Undesirable production of aquatic plants is another problem of “biological pollution” that is highly significant to aquaculture and is closely linked to human wastes.
There are many species of plants, floating, submersed, etc. that have caused great losses in several fields of human activity, including food production, and immense expenditure in terms of human labour and eradication costs. In Africa the most serious problems from floating weeds are caused by Eichhornia crassipes (Mart.) (water hyacinth), Salvinia auriculata Aublet (water fern) and Pistia stratiotes L. (water lettuce) (Holm et al., 1969). These plants interfere seriously with fishing operations and aquaculture. They reduce water flow in drainage and irrigation channels and increase evaporation. They provide shelter for low value fish species and for vectors of diseases such as mosquitoes (Mansonia sp.) that carry encephalomyelitis and rural filariasis. The Mansonia larvae take their oxygen directly from the roots of Pistia and not from the water surface, and are therefore not easily eradicated as long as Pistia is present.
The production of large masses of weeds consumes large amounts of nutrients. It is estimated (Gupta, 1973) that aquatic weeds can remove up to 5 900 kg/ha of N and up to 590 kg/ha of P from the water. Therefore, it is not surprising that nutrient-rich waters give the most luxuriant growths of weeds.
In other waters, however, weeds compete for nutrients with phytoplankton and, hence, with fish production, which is also hampered by lowered oxygen tension under the mat of weeds. Weeds also significantly reduce photosynthesis below the water surface due to reduced light. Further, when large amounts of plant material decay there will be a further oxygen demand which may lead to fish kills.
George (unpubl., a) reports that the masses of weeds behind the Djebel Aulia Dam (Sudan) have caused an increase in overall plankton density, with a concomitant lowering of dissolved oxygen; details on the quantitative relation between phyto- and zoo-plankton were not available.
The water hyacinth is now generally distributed in Africa. It is impeding fisheries and transport in the Congo/Zaire River. The White Nile is badly infested. Elevated levels of nitrates and phosphates in Lake Mellwaine, Rhodesia, due to sewage pollution and, possibly, land runoff are thought to have contributed to the rich growth of water hyacinth on the lake (G.A. Reeskamp, pers.comm., 1975). Salvinia (“Kariba weed”) together with Pistia have been common, since the early sixties, on Lake Kariba, Rhodesia, and are now floating in vast mats on the lake. Salvinia is common in South Africa, in the Congo/Zaire River and in Cameroon. Vossia and Polygonum have spread in Volta Lake (FAO, 1969) and are contributing to public health problems because of their association with bilharzia vectors.
The control of weeds by mechanical means has many advantages, but is labour-demanding and needs to be repeated more often than chemical treatment. It removes nutrients from the water; this may or may not be an advantage depending on water quality. This approach does not cause water pollution, but may produce good fodder for animals and an excellent fertilizer (Aboaba, 1973). Chemical treatment may cause some problems to aquaculture; these will be discussed below. Biocontrol experiments have been carried out with a number of plant-eating organisms - fish, birds, manatees, snails, grasshoppers and beetles. For small fish ponds fish, like the common carp, the white amur and tilapia as well as ducks and geese may turn out to provide sufficient control in combination with mechanical removal.
5.1 General aspects
Pollution by petroleum and its derivatives has significance to fisheries in coastal areas, in harbours and in limited areas near certain industries. After the closure of the Suez Canal a number of countries along the coasts of Africa noted a considerable increase of shore-pollution by oil and tar from tankers. The opening of the canal recently should ease this problem, although certain West African countries will probably notice no difference, and on the Red Sea and North African coasts things may grow worse.
The most noticeable nuisance of petroleum to fishermen is probably smearing of equipment and clogging of nets. However, oil residues have been shown to affect aquatic life in several ways, such as altering bottom structure, smothering of fixed epifauna and plants, and reduction of light penetration. When taken up by fish and shellfish (at ppm levels) unpleasant flavour is acquired that renders the products unmarketable.
The use of dispersants in areas near the shore is still a much debated question. In connection with establishment of national plans for oil combatting, several governments have recommended against the use of dispersants in the near-shore and littoral zone, except under special circumstances, since they are considered more damaging than removal with mechanical means. Less and less toxic dispersants have been developed and it may be preferable to use a suitable dispersant against an oil slick outside the coast rather than to ignore it and let it drift ashore.
5.2 Biological effects
Crude oil is a very complex mixture of compounds and there is still some controversy about its modes of action and effects on aquatic life. There is some evidence that oil floating on the surface is damaging to animals living at the water surface, and toxicity of components incorporated into the water mass has been proved. Many complaints from fisheries interests have been precipitated as a result of the smothering effect of oil reaching the littoral.
The components of petroleum that become incorporated in the water body have a biological significance and damage can be important in waters with limited capacity for dispersion, such as lagoons and ponds. Opinions are quite divided as to the extent of toxicity of petroleum to aquatic life. Due to the numerous difficulties with analyses, with bioassay design, and with inter-comparability of results of investigations, scientists are still feeling their way toward an understanding of biological responses to different types of mixtures of hydrocarbons and petroleum.
The reactions vary considerably between species, between life-stages of the same species and also with varying environmental conditions. Anderson et al. (1974) made a careful test with the fish, sheepshead minnow, Cyprinodon variegatus, and the silverside, Menidia beryllina, and Fundulus similus and with the crustaceans Mysidopsis almyra (mysid), Penaeus aztecus, brown shrimp (postlarvae) and Palaemonetes pugio (grass shrimp). The authors obtained TLm-values (48h) for the crustacean species exposed to Bunker C and No. 2 fuel oil ranging between about 1–10 ppm. Highest values were obtained for Penaeus postlarvae, which is interesting since young stages often have been found more susceptible than adults. However, the test did not provide for comparison between different stages within one and the same species. TLm for fish was one to two orders of magnitude higher, than for the shrimps.
It should be noted that responses such as depression of respiration can be observed at sub-lethal concentrations and that ecological effects may occur considerably below that level. In cases of chronic hydrocarbon pollution, significant ecological effects can be expected at concentrations below 1–10 ppm.
It is normally found that refined oils are one or several orders of magnitude more toxic than crudes. There are also differences in toxicity between crude oils from different areas of extraction.
When crude oil is released part of it evaporates and part enters the water body or dispersed form. The water-soluble fractions tend to become enriched in aromatic compounds which are much more water-soluble than alkanes. They will contain a high amount of light aliphatic and light aromatic hydrocarbons. By contrast, the water-soluble fractions of refined oils tend to contain higher concentrations of di- and tri-aromatic components (Anderson et al., 1974) and, as could be expected, they tend to have higher acute toxicity to aquatic organisms. Toxicity of water-soluble fractions of crude oil is caused to a great extent by mono-aromatic hydrocarbons. Much of the low-boiling, most volatile (and toxic) components of the oil evaporate within hours in hot climates, but remaining, heavier polynuclear fractions may have long-term biological effects.
As shown in the example above, fish are less susceptible to oil than filter-feeding molluscs. However, fish are nevertheless affected and lowering of tolerance to other environmental stresses have been shown as well as reduction of growth, as a result of exposure to sublethal concentrations.
5.3 Some cases of petroleum-pollution in Africa
It is quite evident from a number of reports that many countries have a disturbing extent of oil pollution caused by tankers frequenting major harbours, or just passing at some distance from the coast. In the Mediterranean oil and tar balls are found on most open shores. Fish kills due to releases in confined areas have occurred, e.g., in Tunisia.
George (unpubl.) gives several examples of coastal oil pollution in Sudan with damage to coastal water communities. In Ethiopia oil pollution is of importance only near the refinery at Assab but is expected to become serious after opening of the Suez canal. This also applies to the northern Somalia coasts.
In Kenya and Tanzania oil and tar is drifting in from the sea. In addition, tankers and other vessels calling at Mombasa and Dar-es-Salaam pollute nearby waters. Oily wastes from industries at Kisumu, Nakuru, Mwanza, Mombasa and Dar-es-Salaam pollute inland and coastal waters. In the Mozambique channel oil pollution is becoming more evident. South African coasts appear to be particularly subjected to tanker accidents, and some West African nations have continuing problems with petroleum due to oil extraction. Even in Gambia there is fear that oil and tar pollution will eventually affect aquaculture (Jallow, Fisheries Division, pers. comm., 1975).
In Nigeria oil pollution is causing serious concern to fisheries, since oil wells are located off the Niger delta where there is an important fishery of the prawn, Penaeus duorarum, and other species, which depend on brackish-water nurseries for part of their development (Tobor, 1972). Mass mortalities of fish have been caused by accidental discharges; according to Bakare (pers. comm., 1975) oil pollution contributed to a major set-back of large-scale brackish water aquaculture, the effects of which extended to the Lagos, Mid-Western and River States and included some inland waters. Binet and Marchal (1970) describe similar problems in the Ivory Coast.
It is difficult to distinguish between the impacts of single pollutants when a number of these interact in the same site, as in the Lagos area. Ezenwa (unpubl.) holds petroleum at least partly responsible for an annual loss of 5 million fry of mullet, 2 million of snapper and of 1.5 million eel larvae, plus damage to populations of several other species, including shrimps. High mortality in connexion with stocking of mullet in inland waters was also attributed to oil (T. Orekoya, Fed. Dept. of Fisheries, pers. comm., 1975).
There is evidently a real basis for the concern about oil pollution expressed by fishermen and aquaculturists in several African countries, as far as coastal areas are concerned. In inland waters the problems are, at worst, local.
5.4 Petroleum refineries and petro-chemical industry
Refineries have been mentioned several times as sources of water pollution. Apart from more or less accidental release of oil during handling of petroleum these industries can give rise to a number of problems. Discharge of phenolic compounds, acid waters, ammonia cyanide, chromate, detergents, solids and heat can also be expected from refineries. Although methods for prevention of such pollution exist, the required facilities are often lacking or, if available, often function badly.
The petro-chemical industry produces probably between 3 000 and 4 000 different compounds and can give rise to toxicity, turbidity, high BOD and pH-alterations and, rather often, tainting of fish.
This large group of chemical compounds comprises pesticides, herbicides and numerous other types of biocides, and a large number of other chemicals. Some common chemicals of environmental importance are: surfactants, phenolic compounds, organic acids, phthalates, styrenes and related compounds, and various chlorinated hydrocarbons including PCBs.
6.1 Pesticides
The halogenated hydrocarbons have a wide use in pest control because of their high toxicity and persistence in the environment. Add a moderate cost for production and their usefulness for crop protection is evident - as is their ecological hazard. The organochlorine pesticides - DDT, Dieldrin, Endrin, Aldrin, BHC, HCH, etc. - are well known and widely used. Evidently some of the pesticides distributed will not remain in the target area. In fact, about 50 percent of DDT sprayed by aeroplane reaches the ground hundreds or thousands of kilometres away. The pesticides easily reach streams, lakes and coastal lagoons. Most of it becomes adsorbed to particles, and in turbid waters, common in tropical Africa, pesticides rapidly become buried in the sediment (Ulfstrand, Södergren, 1972; Sserunjoji, 1974).
The effects of organochlorines on fish are by now rather well documented. These aspects were reviewed by Johnson (1968) and since then by several others. They are readily accumulated into fish and can attain levels up to 106 those in the environment.
In a laboratory food-chain experiment, Metcalf (1974) found that Gambusia, that was at the top of the food chain, obtained body residues of DDE that were 85 000 times higher than in the water through trophic magnification; corresponding value for DDT was 110 000 times. It is also established that crustaceans are perhaps two orders of magnitude more vulnerable to organochlorines than fish. Although molluscs are generally much more tolerant to such chemicals than shrimps, it has been found that shell formation is inhibited at a level of a few ppb in the water and Roberts (1975) obtained sublethal effects in bivalves at concentrations of 0.05 mg 1- of DDT, and at about ten times higher concentration in the case of endrin and dieldrin.
Dieldrin, gamma-HCH, Abate and fenthion are used in public health programmes. DDT and malathion are more commonly used, especially for malaria control. Malathion use will probably increase in the future, whereas DDT and HCH may remain at the same level. Goldberg (in press) quotes estimates of DDT requirements in Africa during 1975 to be 113 tons of technical grade, 816 tons 75 percent (wdp) and 159 tons 25 percent (wdp) for malaria eradication purposes only. The same report foresees an annual future use of DDT for agriculture in Africa in the order of 14 500 tons. The Indicative World Plan for Africa South of Sahara calculates a need for pesticides in the region which is six times higher in 1985 than 1962 for realization of its development plans. It cannot be expected that countries (for instance Chad that depends on cotton export for about 80 percent of her income of foreign exchange) will give up using pesticides such as DDT because of environmental concerns, or to protect their fisheries. But one can hope for a gradual increase in the use of more degradable chemicals and, albeit slowly, an increase of integrated control methods. The aquaculturists, therefore, must remain vigilant. They are obliged to acquire a capability to monitor toxic chemicals or at least to judge where and when danger exists.
A large number of countries from Algeria to Mozambique grow cotton - Egypt and Sudan are the largest producers. Cotton alone consumed in 1972 about 2 200 tons of DDT. In the near future the need may become about six times as high. The amount of pesticides utilized today in cotton production is very large: about 600 000–800 000 imperial gallons of DDT and Endosulfan in Sudan alone. In addition, HCH, Diel, Sevin, Azodrin and Dimethoate and many other formulations, are used. Several other crops, such as cocoa, coffee, groundnuts and bananas require use of large amounts of pesticides. Gamma-BHC is used against the capsid bug in cocoa production (Ghana, Nigeria, Ivory Coast and Cameroon). Fungicides and DDT, Lindane, etc., are used in coffee production in Ivory Coast, Uganda, Kenya and Zaire. HCH is used for rice protection, etc. The list is long, and it is of interest to the aquaculturist to know about formulations and application rates, because he may experience the negative effects of windblown aerosols, and of accidental discharges for various reasons.
In Egypt fish kills were reported to be common in small canals and in Lake Quarun as a result of air spraying, discarded cans and careless washing of contaminated clothes. It cannot be excluded that there is a large build-up of organochlorines in the sediments of Lake Nasser. Before the construction of the High Dam such residues were transported with the suspended solids to the Mediterranean. George (unpubl.) reports on use of pesticides in Sudan and on related damage to fisheries.
In the Malagasy Republic rice is a very important crop and much could be gained by expanding fish culture in rice paddies. Experience from Taiwan is, however, not encouraging, since rice-cum-fish culture there has ceased due to incompatibility with pesticide application (Pierce, 1968).
Other problems have been reported. In the Rift Valley and Lake Victoria basins there is important cotton production, and drainage of pesticides to fishing waters is causing great concern to fisheries managers in the region.
In connexion with fish-cum-rice culture, which is employed increasingly in the Malagasy Republic, and tried successfully in Dakahlia, Egypt and Cameroon, there seems to be an unresolved problem in that the rice needs insecticides to yield acceptable harvests, but fish and their food organisms suffer from it. Kenya has had a more than ten-fold increase in pesticide use over a decade.
6.2 PCBs
Polychlorinated Biphenyls (PCBs) in the environment originate mainly from industrial use, for instance, as electrical insulators and as plasticizers in paints and resins as well as algicides. Analytically, they are cumbersome to differentiate from some other organochlorines, but their presence should always be suspected and because of their great persistence and high toxicity, they can constitute a serious hazard to aquatic life.
6.3 Herbicides
Herbicides are used widely, e.g., for control of aquatic weeds. Of the many formulations used, fenoxyacetic acids are among the most common. Of these, 2,4,5-T is the most toxic to fish, probably due to its content of impurities - the very toxic dioxins (Jensen, Renberg, 1973; Schwetz et al., 1973). The commonly used 2,4-D does not contain this impurity. There is evidence (Lichtenstein et al., 1974) that herbicides can considerably increase toxicity of insecticides through synergism. This effect is, however, significantly reduced by suspended silt.
Of the many inorganic elements there are several metals that have attracted attention as pollutants with serious ecological effects. The most important metals in this context are probably mercury, lead, cadmium and, perhaps, chromium, zinc and selenium. The one metal which has caused greatest alarm recently is mercury. Mercury compounds have a long life in the aquatic environment before they are eventually buried in deep sediments. Mercury is readily accumulated in aquatic organisms, including fish and shellfish. It has a high toxicity to mammals.
Methyl mercury is the most toxic mercurial compound. Although normally available in sea water at concentrations about 0.1 percent of that of the inorganic form, any increase of almost any kind of mercury compound may lead to increased methyl-mercury contents in organisms due to transformations in the environment. The normal levels of mercury in fish range between 0.01 and 0.2 ppm. Therefore, any increase of mercury by pollution may lead to hazards to human health.
Through bioaccumulation and biological magnification during transfer through food chains, the levels fish products in some instance have exceeded adopted standards for food, and species of fish caught within certain areas in Europe have been black-listed for sale, with a resulting economic loss to fishermen.
In some instances insufficient information and a distorted view of prohibitive measures, have caused buyers to refuse even those fishery products which were not contaminated, or which are well within safety limits. The economic effects assumed great proportions recently when tuna, one of the most popular fish products in the U.S.A., was temporarily declared unsafe for human consumption due to high levels of mercury found in some samples of canned fish. The swordfish fishery has been hard hit by economic effects of the rather high concentration of mercury found in this fish, most of which may be of natural origin.
Although heavy metals can be expected to appear in waste waters from many different kinds of industries, and in municipal sewage waters of African cities, no reports on elevated levels in aquatic organisms from Africa are available, probably due to the scarcity of laboratories equipped for this kind of analysis in this area.
There is much room for argument about the seriousness of pollution to aquaculture, but one thing is clear: pollution problems are here to stay and, if anything, they can be expected to aggravate.
A means to ensure successful aquaculture (implying also freedom from disturbing pollution) is good management. Prerequisites for good management are manpower, knowledge and facilities. Manpower may not be a problem, but know-how, sometimes quite inadequate even in Europe, can be improved only by education and training. Politicians need to be educated about essential environmental protection aspects in national economic planning, farmers how to use pesticides safely, fishermen how to use fish poisons without harming aquafarming. Aquaculturists need also to be aware of methods for the safe utilization of sewage and sublethal effects of toxicants, etc.
Facilities for surveyance and monitoring of pollutants need be instituted, perhaps first in connexion with regional aquaculture institutes that would also provide farming and specialized assistance, then in national and provincial centres.
Research and studies are required, also preferably coordinated by regional centres, on water quality criteria for aquaculture in Africa and on suitable, less costly, methods for detection of pollutants. Special problems such as persistence of pesticides under natural conditions need urgent study.
My warm thanks are due to colleagues from many African countries and elsewhere who have provided valuable information by correspondence, both those quoted in the text and others who have contributed to the coverage of this wide subject.
Aboaba, F.O., 1973 Management of aquatic vegetation. World crops, Jan./Feb. 1973, 21–4 p.
Alabaster, J.S., 1972 Suspended solids and fisheries. Proc.Roy.Soc., London, B. 180:395–406
Allen, G.H., 1972 The constructive use of sewage with particular reference to fish culture. In Marine Pollution and Sea Life (Ed. M. Ruivo), Fishing News (Books) Ltd., 624 pp. London 1972
Anderson, J.W. et al., 1974 Characteristics of dispersion of water-soluble extracts of crude and refined oils and their toxicity to estuarine crustaceans and fish. Marine Biology, 27:75–88
Binet, D. and E. Marshal, 1970 Sur la présence de résidus d'hydrocarbures dans les eaux ivoriennes, Rapport scientifique, No. RS-5/70 du Project de développement de la pêche pélagique côtière. (IVC 6/288)
Björk, S., 1972 Swedish lake restoration system gets results, AMBIO, 1(5):153–65
Canestri, V. and O. Ruiz, 1973 The destruction of mangroves, Mar.Poll.Bull. 4(12):183–5
Chan, G.L., 1974 The use of pollutants for aquaculture - conditioning of wastes for aquaculture. In Proceedings of the 15th Session of IPFC, Wellington, New Zealand, 18–27 Oct. 1972, IPFC and FAO Reg. Office, Bangkok, 1974. pp. 84–91
Coche, A.G., 1969 Fish Culture in Rice Fields. A world-wide Synthesis. Hydrobiologia, 30:44 p.
EIFAC, 1964 Working Party on Water Quality Criteria for European Freshwater Fish, Water Quality Criteria for European Freshwater Fish: Report on finely divided solids and inland fisheries, EIFAC Tech.Paper (1):21 p. FAO, Rome
EIFAC, 1973 Water Quality Criteria for European Freshwater Fish: Report on dissolved oxygen and inland fisheries, European Inland Fisheries Advisory Commission (EIFAC) Technical Paper (EIFAC/T/19), pp. 1–10, FAO, Rome
Ezenwa, B., Unpubl. The problems of pollution as affecting aquaculture in Nigeria, Unpubl. manuscript
FAO, 1969 Report of the Ad Hoc Consultations by West African Fishery Officers on Problems of Inland Fisheries, Accra, Ghana, 29 March 1969, FAO Fisheries Reports, No. 79 (FRi/R79-En), FAO, Rome, October 1969
FAO, 1974 Fish and shellfish hygiene. Report of a WHO Expert Committee convened in cooperation with FAO, Geneva 18–24 September 1973. FAO and WHO, 1974
FAO-ACMRR, 1972 Report of the First Session of the ACMRR/IABO Working Party on Aquaculture, Rome 17–23 May 1972 (ACMRR:7/73/WP6) FAO, Rome, 16 p.
FAO-IPFC, 1974a Proceedings, 15th Session, Wellington, New Zealand, 18–27 October 1972. Sect. II: Coastal Aquaculture and Environment, IPFC Secretariat, FAO Reg. Office for Asia and the Far East, Bangkok, 91 p.
FAO-IPFC, 1974b Report of the Second Session of the IPFC Working Party on Aquaculture and Environment. Presented at the 16th Session of IPFC Jakarta 30 Oct. -6 Nov. 1974. Jakarta, 26–29 Oct. 1974 (IPFC/74/5) 9 p.
George, T.T., Unpubl. Water pollution in relation to aquaculture in Sudan. Manuscript at the CIFA Second Session and Symposium on Aquaculture in Africa, Accra, Sept. 1975
Goldberg, E.D., in press Health of the Oceans, submitted for publication by UNESCO-IOC
Gupta, O.P., 1973 Aquatic weed control. World Crops, July/August: 182–90
Heydorn, A.E.F., 1973a Aquaculture: a substitute for management of living resources, J.sth.Afr. Wildl.Mgmt.Ass., 3(2): 109–14
Heydorn, A.E.F., 1973b The interdependence of Marine and Estuarine Ecosystems in South Africa. S.Afr.J. of Science, (69):18–24
Holm, L.G., L.W. Weldon and R.D. Blackburn, 1969 Aquatic weeds, Science, 166(3906):699–709
Jensen, S. and L. Renberg, 1973 Chlorinated dimers present in several technical chlorophenols used as fungicides, Environmental Health Perspectives, Exper. Issue No. 5, pp. 37–9, U.S. Dept. Health Education and Welfare
Johnson, D.W., 1968 Pesticides and fishes - a review of selected literature, J.Am.Fish.Soc., 97 (4):398–424
Kongere, P.C., 1973 Pollution in Kenya, Informal report presented at the FAO/SIDA Training Course on Marine Pollution in Relation to the Protection of Living Resources, Gothenburg and Stockholm, 31 July-9 September 1973 (FIR/TPLR/73/R8). Limited Distrib.
Lavoipierre, G.J., 1973 L'épidémie de schistosomiase du lac Volta. J.Méd. Lyon, 54 (1250):883–7
Lichtenstein, E.P., T.T. Liang and B.N. Anderegg, 1974 Synergism of Insecticides by Herbicides under various Environmental Conditions. Envir.Conserv., 2(2):148
McGarry, M.G., 1971 Water and Protein Reclamation from Sewage. Process.Biochemistry, Jan. 1971
McGarry, M.G., 1972 Reuse of human wastes in agriculture. Thai J.Agr.Sci., (5):183–94
Meschkat, A., 1967 The status of warm-water fish culture in Africa. Proceedings of the World Symposium on warm-water pond fish culture, FAO Fish.Rep.No. 44, Vol. 2:88–108 (FRi/R44.2/Tri) FAO, Rome
Metcalf, R.L., 1974 A laboratory model ecosystem for evaluating the chemical and biological behaviour of radio-labelled micropollutants. Comparative studies on Food and Environmental Contamination. Proc.Symp.Nuclear Techniques in Comp.Studies on Food and Envir.Contam., organized by FAO/IAEA/WHO, Otaniemi, Finland, 27–31 August 1973, IAEA (STI/PUB/348), pp. 43–8
Odum, W.E., 1972a Effects of Aquaculture on the Shallow Water Environment. Paper presented at the ACMRR/IABO Working Party on Aquaculture, FAO Rome, 17–23 March 1972 (ACMRR/IABO:1/72/8), Limited Distrib.
Odum, W.E., 1972b The Potential of Pollutants and other Environmental Alterations to Adversely Affect Aquaculture. Paper presented at the ACMRR/IABO Working Party on Aquaculture, Rome, 17–23 May 1972 (ACMRR/IABO:1/72/7), Limited Distrib.
Okorie, O.O., Unpubl. Environmental constraints to aquaculture development in Africa. Manuscript presented at CIFA, Second Session and Symposium on Aquaculture in Africa, Accra, Ghana, Sept. 1975
Orekoya, T., Unpubl. The problems of pollution as affecting aquaculture in Nigeria.
Payne, A.I., 1971 An experiment on the culture of Tilapia esculenta (Graham) and T. zillii (Gervais) (Cichlidae) in fish ponds. J.Fish.Biol., (3):325-40
Peres, J.-M., 1974 Marine communities in the Mediterranean with examples of degradation resulting from pollution. Paper presented at FAO Consultation on the Protection of Living Resources and Fisheries from Pollution in the Mediterranean, Rome, 19–23 Feb. 1974. FAO, Rome, FID:PPM/74/Inf.6
Pierce, P.C., 1968 Fish production possibilities for Ghana's proposed irrigated paddy rice scheme. USAID/GHANA 9th Annual Agricult.Conf., pp. 114–32
Pillai, T.G., Unpubl. On the possibilities for aquaculture development in Tunisia. Unpublished manuscript submitted to Dept. of Fisheries, FAO
Richard, C., 1973 One useful way to dispose of the wastes in a South Pacific piggery. Paper presented at the Meeting of the South Pacific Commission, Noumea, New Caledonia, Sept. 1973
Roberts, D., 1975 Sub-lethal effects of chlorinated hydrocarbons on bivalves. Mar.poll.bull., 6(2):20–4
Saad, M.S.H., 1970 Dissolved oxygen as an indicator of water pollution in Egyptian brackishwater lakes. Marine Pollution and Sea Life (Ed. M. Ruivo), Fishing News (Books) Ltd., 624 p. London 1972
Schwetz, B.A. et al., 1973 Toxicity of chlorinated dibenzo-p-dioxins. Environmental Health Perspectives, Exper. Issue No. 5:87–99, Sept. 1973, U.S. Dept. Health Education and Welfare
Slack, E.B., 1974 Sewage and aquaculture production. Proceedings of the 15th Session of IPFC, Wellington, New Zealand, 18–27 Oct. 1972. IPFC and FAO Reg. Office, Bangkok, 1974. pp. 70–5
Sserunjoji, J.M.S., 1974 A study of organochlorine insecticide residues in Uganda with special reference to Dieldrin and DDT. Comparative studies on Food and Environmental Contamination. Proc.Symp.Nuclear Techniques in Comp.Studies in Food and Envir. Contam., organized by FAO/IAEA/WHO, Otaniemi, Finland, 27–31 August 1973, IAEA (STI/PUB/348), pp. 43–8
Swift, D.R., 1972 Some physical and chemical changes in water beneficial to aquaculture. Paper presented at the ACMRR/IABO Working Party on Aquaculture, Rome, 17–23 May 1972 (ACMRR/IABO:1/72/6)
Tobor, J.G., 1972 Marine pollution problems in Nigeria. Informal report presented at the FAO/SIDA Training Course on Marine Pollution in Relation to the Protection of Living Resources, Göteborg, Sweden, 2 May-3 June 1972, FIR/TPLR/72/R1. 2 p.
Ulfstrand, S. and A. Södergren, 1972 Organochlorine residues in East African birds, AMBIO 1(4): 150-1
Wood, P.C., 1972 The principles and methods employed for the sanitary control of molluscan shellfish. Marine Pollution and Sea Life (Ed. M. Ruivo), Fishing News (Books) Ltd., 624 pp. London, 1972
Anon, 1972 Bacterial gill disease of European elvers. FAO Aquacult.Bull., 4(4):13
Anon, 1973a Control of mud gas production in ponds. FAO Aquacult.Bull., 5(2):9
Anon, 1973b Removal of excess nutrients from fish plants. FAO Aquacult.Bull., 5(3-4):8
by
O.O. Okorie
East African Freshwater
Fisheries Research Organization,
P.O. Box 343, Jinja, Uganda
Abstract
The current status of aquaculture in Africa is discussed, and the various environmental factors limiting its development are analysed. The role of temperature, unsuitability of the physical environment and turbidity as regulating factors are discussed in relation to the propagation of exotic and indigenous species in ponds and natural lakes. The incidence of predation, the problem of the non-breeding exotic species or the breeding prolificity of the indigenous species under African environmental conditions are mentioned as some of the restraining influences on the development of aquaculture on the continent. Health hazards associated with African aquaculture, the deleterious effects of land and water use malpractices on aquatic resources, the inadequate application of artificial feeds in fish ponds, the difficulty in rearing or obtaining fry for use in rural production ponds and the sheer expense and logistical problems encountered in the translocation of fish between African water bodies are identified as some of the problems facing African aquaculture. It is recommended that regional aquaculture research Centres for north, west, east and central Africa be set up to tackle these problems.
Résumé
L'état actuel de l'aquiculture en Afrique est discuté et les différents facteurs limitant de son développement sont analysés. Le rôle de la température, l'inadaptation de l'environnement physique et de la turbidité comme facteurs de régulation sont discutés en relation avec la propagation en étangs et en lacs naturels d'espèces exotiques et indigènes.
L'influence de la prédation, le problème des espèces exotiques stériles ou de la fécondité incontrolée des espèces indigènes, dans les conditions de l'environnement africain, sont présentés comme quelques uns des facteurs limitant le développement de l'aquiculture sur le continent.
Les problèmes sanitaires associés à l'aquiculture africaine, les effets néfastes de pratiques non adaptées, sur les terres sur les eaux et sur les ressources aquatiques, le mauvaise usage des aliments composés dans les bassins d'élévage, la difficulté d'obtenir ou d'élever des alevins pour l'utilisation dans les étangs de production en zone rurale, le coût réel et les problèmes logistiques rencontrés dans la distribution des poissons entre les différents zones aquicoles sont identifiés comme des problèmes auxquels les pays africains ont à faire face.
Il est recommandé que des centres régionaux de recherches d'aquiculture situés au nord, à l'ouest, à l'est, et au centre de l'Afrique scient établis afin qu'ils abordent ces problèmes.
Artificial propagation of fish in lacustrine, brackish, marine and manmade impoundments have been recognized as an invaluable source of supplementary protein material in the diets of the peoples of the developing countries. Within the last hundred years fish culture techniques have attained a significant measure of success in Asia and the Far East to the extent that artificial propagation of fishes is now a permanent feature of the rural agricultural economy in that part of the world.
In Africa however, though the need to augment the protein content in the diet of African peasantry has long been recognized, fish culture as a means of achieving this end has not yet been sufficiently developed in Africa to the degree where this source of much-needed protein plays a significant role not only in nutrition per se but also as an integral part of the socio-economic fabric of the rural African society.
In the context of this discussion the term “aquaculture” in all its ramifications is being used to denote the specific art of the introduction of fish into lacustrine and riverine environments or impoundments (ponds and manmade lakes) for the purpose of propagating these species in their new environments. Exotic fish species were introduced into parts of Africa from Europe and North America. These include members of the Salmonidae (e.g., the American brook trout, Salvelinus fontinalis), Centrarchidae (Micropterus salmoides the large-mouth black bass) and Cyprinidae (the European carp, Cyprinus carpio). Other fish species indigenous to Africa were introduced into waters in which they were non-endemic. To this category belong some members of the Cichlidae (the tilapias), the Clupeidae (Stolothrissa spp. and Limnothrissa spp.) and Centropomidae (e.g., Nile perch, Lates niloticus).
Various reasons have been given to support the introduction of exotic species to African aquatic environments and/or African indigenous species to waters in which they are foreign; or even more significant the use of “exotic” species for cultivation in African ponds. In some large African water bodies the endemic species are thought not to have adequately “colonized” the environment to a point where an economically viable fish production is sustained. To correct this imbalance a non-endemic species is introduced which it is hoped would be more successful in “invading” the environment and could therefore be exploited on a larger scale. Such is the reasoning behind the attempted introductions of the prolific breeding and commercially important Lake Tanganyika clupeids, Limnothrissa spp. and Stolothrissa spp. into Lake Nyasa where the endemic pelagic cyprinid Engraulicypris sardalla occurs irregularly and also into Lake Bangweulu where the endemic pelagic species Engraulicypris moeruensis remain scarce and commercially insignificant. Introductions of Stolothrissa spp. were also made, albeit unsuccessfully, into Lake Kivu.
Other introductions have been made in situations where a voracious predatory species (e.g., Lates niloticus or M. salmoides) are used to aid in the control of a prolific breeding endemic species of little commercial significance. The predator given the abundant food supply flourishes and becomes the commercial mainstay. In cases where the endemic species is economically desirable the predator is introduced to control excessive fry and fingerlings which are most vulnerable to predation.
Certain species have been introduced into African waters primarily as instruments of biological control, for instance T. zillii are used because this species feeds mainly on aquatic vegetation and could be used successfully in situations where the presence of both rooted and floating vegetation are detrimental to fish life. Apart from attempts at introducing fish species into African natural and manmade lakes, persistent efforts have been made since the turn of this century to culture and domesticate certain indigenous and exotic fish species in African rural fishponds.
Fish culture development with varying degrees of success have been reported in South and Central Africa, the Congo Basin, Cameroon, Nigeria, Egypt and the Sudan, but nowhere in Africa has aquaculture been as successful or played a significant role in augmenting the protein diet of the population as in the Far East and other tropical Asian countries. This paper seeks to ascertain to what extent the degree and scope of success of aquaculture development in Africa has been restricted by the operation of those environmental factors (physical, ecological and human) that are essentially of African identification both in character and content.
3.1 Ecological constraints
Temperature is one of the most important environmental factors in the selection of a particular species for pond culture. It is known that most species of fish do not spawn unless the water temperature is within certain limits. T. mossambica, one of the most desirable species for pond culture in Africa, is known to be affected badly by low temperatures. Chimits (1955) notes that its optimum temperature lies between 20 and 35°C, the northeast limit for its survival being latitude 30°N. Koura and Bolock (1958) describe an attempt to culture this fish in Egyptian ponds, which failed because of very low water temperatures (less than 14°C for periods exceeding 10 days) during four successive Egyptian winters when the experiments were carried out. T. mossambica on account of its relatively higher rate of growth than the three native species of Tilapia in Egypt (T. nilotica, T. galilea and T. zillii) would have been a preferred species for culture but for its mortality in low winter temperature.
Though the European common carp - C. carpio has been successfully cultured in Asia and the Far East and reportedly flourishes in South Africa, the success achieved in rearing this fish in tropical Africa has been very limited. It is possible that there exists an upper temperature limit for this species in spite of its much publicized wide tolerance of diverse ecological conditions. Within certain limits it is conceivable that temperature could limit reproduction by its effect on fertility of the adults or even the survival of the young after hatching.
Attempts were made in the late forties to introduce American brook trout (S. fontinalis) into a number of farm dams in the Kenya highlands (2 000–4 000m above sea level), since the water in these high altitude dams is far too cold to support tilapia. The trout grew but could not spawn since they had no access to gravel spawning grounds even though in Sweden they do spawn in lakes without any gravel facilities. It was suggested that the reason for the failure was due to environmental limiting factors, possibly that the water temperature, 51°F (10.4°C), might have been too high, and also the relative hours of light and darkness may have been unsuitable, the fish getting no winter periods with short hours of daylight.
Someren and Whitehead (1960) showed that the growth of male T. nigra (especially in weight) at Sagana fishponds, Kenya, varied from month to month according to fluctuations in external environmental conditions, the most important external factor being ambient water temperature; rises and falls in mean temperatures of only a few degrees causing marked increases or decreases in weight achieved.
In certain parts of Africa, water supply to fishponds is derived from perennial streams which, during periods of prolonged heavy rainfall, are subject to considerable increase in turbidity. High turbidity due to suspended clay particles as a result of heavy rainfall and surface run-off has become a permanent feature in most African fishponds. In experimental pond studies Buck (1956) demonstrated some important effects of turbidity on growth rates of certain fishes. At the end of two growing seasons the average total weight of fish in “clear” ponds (average turbidities less than 25 ppm) was about 1.7 times that of fish in ponds of “intermediate” turbidities (ranging from 25 to 100 ppm) and approximately 5.5 times greater than that of fish in “muddy” ponds (turbidities exceeding 100 ppm). Of the three species used the large-mouth bass, M. salmoides was the most adversely affected by high turbidity.
3.2 Biological interactions in the ecosystem
The incidence of predation within natural or cultured fish populations is more common in African freshwater systems than is perhaps experienced elsewhere, which implies that the fish to be cultivated should not only be able to obtain sufficient food supply within the aquatic environment but should also be protected from too intensive a predation by other fishes, frogs, tadpoles, and aquatic birds.
The large-mouth bass, M. salmoides has been used to get rid of excessive tilapia fry in African ponds with very mixed results. Bard (1960), in Cameroon, used the cichlid, Hemichromis fasciatus, that could destroy a large percentage of undesirable tilapia fry in ponds, but has the disadvantage of being a prolific breeder itself. M. salmoides introduced into Lake Naivasha (Kenya) and L. niloticus introduced into Lake Victoria, have been “accused” of being responsible for the declining tilapia fishery in both water bodies. Attempts to introduce the pelagic freshwater clupeids, Limnothrissa miodon and Stolothrissa tanganicae into any new African manmade or natural lakes would have to be evaluated against the predatory menace of such species as the Hydrocyon vittatus which are endemic in such African water bodies.
Carp culture in the African environment demands a tougher measure of predator control than otherwise would have been the case in Europe or the Far East, for such virulent predators as water beetles, tilapia fry and fingerlings, and frogs (Rana spp. and Xenopus spp.) which do manage to penetrate filter screens (where these exist) into the pond water, cause untold damage to carp eggs and fry. At the carp farm in Jos, Nigeria, it was estimated that a 50 percent mortality rate results from the predatory activities of these pests during carp breeding operations (Fed.Fish.Service, 1963).
One of the outstanding limiting factors to aquaculture in the African environment has been the problem of regulating the breeding capacity of the cultivated species, whether “exotic” or those indigenous to African freshwaters. There are two facets to this problem. The first is the need to induce spawning in those species that do not breed naturally, perhaps due to the unsuitability of the environment. An example in this category is the introduced European common carp, C. carpio. The second is the need to repress breeding in those species with prolific breeding habits. This applies to the indigenous cichlids.
Carps have flourished in lower and middle latitude rivers in South Africa where ecological conditions are congenial. Carp culture has been successful in some parts of South Africa, Southern Rhodesia and Madagascar where climatic conditions are very similar to those prevailing in the Jordan Valley in Israel where production in ponds of up to 5 000kg/ha/year is recorded (Maar, 1960). It is thought that given favourable environmental conditions, production of carp in ponds can be better than that of tilapia in some parts of Africa. Both in Asia and the Far East carp production in ponds has been raised to such a level of proficiency that it now occupies a position of eminence in rural agricultural economy. This has been made possible due not only to the availability of natural breeding stock, but also to the perfection of induced spawning techniques (Chaudhuri, 1966).
Bard (1960) notes that M. salmoides reproduces and grows to an acceptable size in natural lakes, but would not breed in ponds in which it was introduced in Cameroon. But when introduced into a crater lake at roughly the same altitude as the ponds, they prospered. The lake is 70 m deep (pH 7.0) while the ponds were never deeper than 2.5 m (pH 5.9). Black bass might prefer the cooler depths of the lake water to the warm, shallow ponds, and might have found adequate nest-making facilities in the volcanic crevices which abound in the lake. Bard (op.cit.) also notes that Heterotus niloticus in Cameroon would not breed in small ponds of less than 2 ha. At the Panyam Fish Farm in Jos, Nigeria, it was reported (Fed.Fish.Service, 1963) that ponds (about 2 000 m2) used for carp breeding are larger than those used in Europe because under African conditions, carp fry cannot safely be moved into storage or production ponds until they are 1–2 months old.
L. niloticus which would have been another desirable species for pond culture unfortunately has a high oxygen demand and is adversely affected by silty floodwater entering its environment. Oxygen deficiency and high turbidity are common characteristics of most African fishponds.
On the second front, there is no doubt that early and prolific breeding under tropical pond conditions is perhaps the greatest disadvantage in the use of the indigenous cichlid species for aquaculture in Africa. It is argued that in certain African situations total production from tilapia culture in terms of absolute weight in a unit area are vastly in excess of those obtained from carp culture in temperate countries. But these high-weight yields from tilapia ponds result from a mixed population of inumerable but uneconomic undersize adults, fry and fingerlings, a consequence of unrestricted breeding in production ponds.
In situations where mixed culture of carp and tilapia is tried in African ponds, tilapia invariably becomes a nuisance, posing a threat not only as a predator of carp eggs and larvae but also in competing with the fingerlings for food. Since cichlids occur naturally in many African river systems, problems arise where these cichlids inadvertently enter stocked ponds through sluices, and through their prolific breeding interfere to the detriment of the stocked and more desirable species.
3.3 Human factors
It is now expedient to examine the status of the African rural fish farmer within the environment in which he operates, and to ascertain to what extent his particular idiosyncrasies and the demands of the environment have placed limitations not only on his capacity to maximize the yield of the cultured species in the aquatic environment, but also on the economic returns from such ventures. It is common knowledge that in Africa, fishponds, manmade lakes or other forms of aquatic facilities used for aquaculture have become special health hazards to the human population within their vicinity. It is possible that by providing excellent breeding places for malaria vectors - especially the Anopheles (A. gambiae and A. funestus), the role of fishponds and basins in the spread of malaria cannot be ignored, especially in those parts of the continent where fishponds provide the only free surface water available to mosquitoes under dry-season conditions.
Bard (1960) reports that individuals living around fishponds in the Yaoundé area in Cameroon suffered from intestinal bilharziasis from the Schistoma mansoni transmitted by Biomphalaria camerunensis (molluscs: planorbis) abundant in the ponds. Bilharziasis, he surmises, could constitute a serious threat to the future of fish breeding in that part of Africa. In Ghana, Obeng (1966) reports that the Volta Lake ecosystem encourages the establishment of populations of the molluscs Bulinus forskalii and B. rholfsi, both of which act as intermediate hosts of vesical schistosomiasis in the Volta Lake basin.
Schistosomiasis is such a debilitating human disease that successful aquaculture development in areas in which it is endemic can easily be frustrated. The use of a cichlid fish, Serranochromis macrocephala, a snail-eater from the Congo, as a biological control has proved unsuccessful. An additional health problem is posed by the presence of the Mansonia sp. of mosquitoes which transmit rural filariasis. The question arises whether the economic returns in terms of profitability and other subtle contributions to the rural agricultural economy accruing from aquaculture development in Africa, can be equated positively against the debilitating effect of diseases and infestations resulting from these practices.
Awareness of the deleterious effects of land and water use malpractices on aquatic resources has increased in the past few years. The increasing activities of man through technological innovations especially in pursuit of the utilization of natural resources in both terrestrial and aquatic environments in Africa, have led to both the denudation of terrestrial landscapes, and the lowering of the quality of aquatic environments to the detriment of economically important aquatic resources. Some ill-conceived agricultural projects, indiscriminate deforestation, and non-regulation of the course of mountain torrents and flash-floods, have increased the discharge and sedimentation of silt into ecosystems where aquaculture can be instituted. One good example can be found in the Lake Baringo basin, Kenya, East Africa, where endemic commercial fish stocks are being gradually depleted due, by and large, to the deterioration of the water quality through silting, and where introduction of exotic species to replenish the diminishing stock would be confronted with a hostile environmental ecosystem.
In Asia and the Far East where aquaculture has attained a significant position in the rural economy, it has been emphasized that supplemental feeding in fishponds is a sine qua non for the maintenance of high productivity, yet Vandor Lingen (1960) and Huet (1970) have stressed the fact that particularly amongst peasant fish farmers in Africa, there is an inclination not to maintain adequate artificial feeding in rural fishponds, resulting in comparative poor yields and subsequent loss of interest by the fishpond proprietor. This is especially true in those private fishponds where natural production in the ecosystem is also minimal.
Artificial feeds recommended for use in rural fishponds include cereals, especially rice-bran, all forms of brewer's waste, dried or fresh slaughterhouse wastes, oilcakes, or a system where animal husbandry is integrated with fish culture. The argument is advanced that though an African rural fish breeder may grow rice, he might not have the bran. It is also to be noted that breweries, rice and oil mills and abattoirs are normally located at an industrial urban area far removed from his operational base. The procurement of artificial feeds to service fishponds in rural areas could therefore be prohibitively expensive for the peasant fish farmer unless his ponds are cited in relation to both the local agricultural and industrial economy. In the majority of cases in Africa this convenient situation does not arise. The result being that the feeding of fish in ponds deteriorates to the detriment of the growth and breeding capacity of the cultured species.
In order to obviate the perennial problem of runting and uncontrolled breeding of tilapia populations in rural fishponds, monosex culture has been strongly recommended, especially as it is found that males grow faster and attain larger maximum lengths than females in most cichlids. But then the fry for stocking have to be fairly big for the sex to be distinguishable. Although Brown and Van Someren (1953) and a few other workers have described various methods for separating the sexes, the African rural fish farmer is ill-equipped to carry out this exercise efficiently in his rearing ponds, and worse still he would have some difficulty obtaining monosex fry from any other source for his production ponds.
There is no doubt that the introduction of the pelagic freshwater clupeids, Limnothrissa miodon and Stolothrissa tanganicae from Lake Tanganyika into other large African water bodies would be a sound economic proposition if these clupeids succeed in colonizing their new environment.
However, experience has shown from attempts made to introduce these clupeids into Lakes Kivu, Nyasa and Bangweulu that the sheer logistical requirements necessary to transport the numbers of these species adequate to colonize these water bodies are yet beyond the capability of African fisheries administrators.
It is now necessary to consider in what ways the African rural fish farmer can use the technical expertise developed in fish culture in Asia and the Far East to weaken the resistance of his environment to aquaculture in Africa. Is it feasible for the peasant fish farmer or African fisheries administrators to modify the physical, ecological and socio-economic environment in order to favour aquaculture development in Africa?
The solution lies in the establishment of regional aquaculture research centres (for the north, west, east and central Africa) funded by participating individual governments and relevant international organizations and supported by a cadre of technical expertise in the field of pisciculture. These regional centres would tackle not only the problems of the irregular or non-breeding traits of exotic species, but also the breeding prolificity of the indigenous species used for pond culture. Hybrid stocks need to be developed that would survive and propagate in environments that hitherto were deemed unsuitable for the parent stock.
Other areas of concern that could be tackled on a regional basis would include the development of cheap and efficient artificial feeds that could be compounded easily from raw materials that abound in African rural communities, improved techniques for the raising and distribution of fry and fingerlings to fish farmers, and continuing effort at the eradication of the debilitating diseases that result from fish culture practices in Africa.
5. REFERENCES
Bard, J., 1960 The breeding of Heterotis niloticus. In Symposium on Problems of Major Lakes. CCTA/CSA
Bard, J., Propagation of parasitic diseases from fish-breeding ponds in the Cameroons. In Symposium on Problems of Major Lakes. CCTA/CSA
Brown, J.M. and V.D. Van Someren, 1953 New Fish Culture methods for Tilapia in East Africa. Nature, Vol. 172, p.330
Buck, D.H., 1956 Effects of turbidity on fish and fishing. Trans.North Amer.Wildl.Conf., 21:249–61
Chaudhuri, H., 1966 Breeding and Selection of Cultivated Warm-water fishes in Asia and the Far East - A review. FAO World Symposium on Warm-water fish culture. FR:IV/R-3, 1-37
Chimits, P., 1955 Tilapia and its Culture. FAO Fisheries Bulletin, Vol.VIII, No.1
Federal Fisheries Service, Lagos, 1963 Carp farming on the Jos Plateau. Bulletin de L'lfan, t. XXV, Série A. 285–98
Huet, M., 1970 Textbook of fish culture. Breeding and cultivation of fish. Brussels, Editions Ch. De Wyngaert, 436 p.
Koura, R. and A.R. El Bolock, 1958 Age Growth and Survival of Tilapia mossambica (Peters) in Egyptian ponds. Hydrobiol.Dept., Inst. of Freshwater Biol.Cairo. Notes and Memoirs No.41
Maar, A., 1960 Carp culture in Africa South of the Sahara. In Symposium on Problems of Major Lakes. CCTA/CSA
Obeng, L.E., 1966 The invertebrate fauna of aquatic plants of the Volta Lake in relation to the spread of helminthic parasites. In International Symposium on Man-made Lakes, Accra
Van der Lingen, M.I., 1960 The use of animal manures and the integration of animal keeping and fish culture in Tilapia culture. In Symposium on Problems of Major Lakes. CCTA/CSA
Van Someren, V.D. and P.J. Whitehead, 1960 The Culture of T. nigra (Gunther) in ponds. IV - The seasonal growth of male T. nigra. E.A.Agric.Forestry, Jour., Vol.XXVI, No.2:79–86
by
T.T. George
Fisheries and Hydrobiological Research Section,
Khartoum, Sudan
Abstract
In Sudan the problem of pollution in the marine and inland resources exists to some degree but has not become a serious problem as in other industrial countries. The main source of marine pollution is oil and chemicals discharged from ships at the harbour. Though the problem is not yet severe, it has become an urgent necessity to formulate pollution control laws to promote aquacultural development of cultivable indigenous fish and shellfish species such as Chanos chanos, Mugil cephalus and Penaeus monodon as well as exotic species and also safeguard the existing culture industry of Pinctada margaritifera in Dongonab Bay.
The inland waters of Sudan, with the exception of irrigation canals in agricultural schemes, particularly the Gezira Scheme, are not very much polluted with domestic, industrial and agricultural pesticides and hence the biological principles relating to the living organisms in the water are not affected. Besides, industrial development will not pose a serious threat in the future due to existing legislative measures. That is why there is a great scope for the development of aquaculture especially in private poultry farms situated on the Nile banks; there is also a promising field in the rain-water fed reservoirs or ‘haffirs’ of Western Sudan and other similar places. However, the extensive and intensive use and harmful effects of insecticides, larvicides and molluscicides pose serious hazards to aquacultural development in the Gezira irrigation canals and hence present a crucial problem to the production of more protein-rich food. That is why there is a need for well-coordinated administrative and technical measures to cope with pollution in these canals before aquaculture could be beneficially developed.
Résumé
Au Soudan, le problème de pollution des ressources marines et continentales existe à un certain degré mais n'a pas encore atteint le stade dangereux affectant d'autres pays industriels. La principale source de pollution marine est le mazout et les effluents chimiques déversés par les bateaux dans les ports. Bien que le problème ne soit pas encore grave, il est urgent de formuler des lois sur le contrôle de la pollution afin de promouvoir le développement piscicole des espèces d'élevage de poisson et coquillages locaux tels que Chanos chanos, Mugil cephalus et Penaeus monodon, autant que des espèces exotiques et de sauvegarder l'industrie piscicole de Pinctada margaritifera existant dans la Baie de Dongonab.
Les eaux continentales du Soudan, à l'exception des canaux d'irrigation des plans agricoles, en particulier le plan Gezira, ne sont pas trop pollués par les résidus domestiques, les pesticides industriels et agricoles, et de ce fait les principes biologiques inhérentsaux organismes vivant dans l'eau ne sont pas affectés. En outre, compte tenu des mesures législatives en vigueur le développement industriel ne sera pas une menace sérieuse à l'avenir. C'est pourquoi l'on peut envisager de développer l'aquiculture, en particulier en conjonction avec l'élevage de volaille pratiqué dans les fermes situées sur les rives du Nil; un domaine également prometteur est celui des réservoirs d'eau de pluie ou ‘haffirs’ du Soudan occidental et autres lieux similaires. Néanmoins, l'emploi intensif et extensif des insecticides, larvicides et molluscicides et leurs effets nuisibles sont des risques graves pour le développement aquicole dans des canaux d'irrigation de Gezira et, en conséquence, pose un problème crucial en vue de la production d'aliments riches en protéines. C'est ce domaine qui nécessite l'établissement de mesures administratives et techniques visant à affronter la pollution dans ces canaux avant que le développement de l'aquiculture puisse être envisagé avec profit.
The Sudan (lat 23° and 3° North, long 22° and 39° East) has a coastline of 480 km (300 mi) on the Red Sea and 6 500 km (over 4 000 mi) of river Nile waters (Fig. 1). Besides, there are the waters of ponds, rain-fed reservoirs (‘haffirs’) and irrigation canals in agricultural schemes. These extensive freshwater resources cover an estimated two million ha.
In Sudan, the problem of pollution has not become a posse ad esse as in other countries where there is rapid industrialization; marine and inland pollution exist to some degree. However, no detailed study is available on the problem of pollution particularly in the Sudanese coast of the Red Sea. Therefore, an attempt is made in this paper to elucidate the problem of pollution in Sudan and its relation to the development of aquaculture.
At Port Sudan there is a Shell Petroleum refinery but no effluents whatsoever are discharged into the sea exceeding the following standards: tasteless, colourless, pH 6–9, 5 ppm oil content, 0.2 ppm settleable solids, 30 ppm suspended solids, 1 ppm phenols and no sulphides. However, the main source of marine pollution is oil and chemicals which are pumped into the harbour by ships cleaning tanks or leaked from pipes or spilt accidentally. That is why the Cambridge Coral Starfish Research Group (CCSRG) (1974) wrote “what we find difficult to understand is why shipping is allowed to pollute the waters of the Democratic Republic of Sudan”.
The culture of marine animals and plants in Sudanese coastal waters has not yet been fully utilized, though the black-lip pearl shell, Pinctada margaritifera (L.) variety erythraensis Jameson (Arabic: Sadaf), has been cultured since 1905 at Dongonab Bay, about 110 mi north of Port Sudan. In 1969 mass mortality of P. margaritifera in this Bay was reported by Japanese pearl culture experts working there at that time but unfortunately it was never determined whether this was due to disease, pollution or other factors. At Port Sudan the marine garden was also partially damaged by oil pollution and completely destroyed when the bedding of the harbour in its vicinity was deepened and the turbid waters deposited silt on the living corals.
It is true that “Port Sudan's harbour is unique in the sense that its wide entrance, open to the prevailing winds, allows constant renewal of water from outside but if the harbour waters become so polluted, as more and more oil, chemicals and organic rubbish are poured into the harbour, that they no longer sustain reef animals, they will destroy coral communities outside the harbour. This is because the inflow of wind-borne water from outside necessitates a counter-current of polluted harbour water flowing out of the harbour. Therefore, it has now become an urgent necessity to take measures, in the form of pollution-control laws to promote and control the development of the natural resources of the coast. However, research is required before control measures can be formulated to the long-term benefit of the Sudan.” (CCSRG, 1974). Besides, to avoid damage of similar natural gifts of the Red Sea as the marine garden, oil boom for oil spill containment should also be used.
The inland waters of Sudan, with the exception of irrigation canals in agricultural schemes, particularly the Gezira Scheme, are not polluted to any significant degree with domestic, industrial or agricultural wastes or pesticides.
3.1 Legislation
The Ministry of Health has now finalized a National Water Pollution Control Act suggested in 1972 which provides effluent standards for both sewage and industrial wastes. The law prohibits the discharge of sewage, treated, partially treated or raw into the river or its branches since it has been estimated that the sewage produced by one adult and discharged into a water body results in a daily oxygen demand of 115 g, equivalent to the dissolved oxygen content of 10 000 litres of saturated freshwater (Mellanby, 1967). The law permits, in exceptional cases, discharge of treated industrial wastes that are in compliance with the effluent standards set by the law: no toxic chemicals, suspended solids not more than 30 ppm and BOD not more than 20 ppm (official records, Sanitary Engineering Division, Ministry of Health). It is worth mentioning that these regulations were being observed by the authorities in the Ministry of Health long before the formulation of this law.
3.2 Indirect Pesticide Pollution in the Nile and Irrigation Canals
3.2.1 The problem of aquatic midges
Some aquatic midges, particularly simuliids and chironomids are known as serious pests in Sudan (El Bashir, unpublished; Quttubuddin, unpublished; Satti et al., unpublished) the main species is Cladotanytarsus lewise Freeman, known locally as “Nimitti”. They occur along the Nile in a belt extending from south of Sennar to Wadi Halfa and they have been increasingly felt since the establishment of the Sennar Dam of the White Nile in 1925. It is also anticipated that this chironomid problem will become acute and alarming when Lakes Nasser and Nubia are resettled (Satti et al., unpublished). In 1957, when DDT was sprayed on the Blue Nile river in an attempt to control aquatic midges, a great number of fish died and analysis revealed visceral DDT at concentrations of 2.5 ppm in Labeo spp. and 70 ppm in Synodontis spp. Toxicity to fish in subsequent trials was avoided by spraying of DDD (Brown et al., 1961). However, Satti et al. (unpublished) reported that “special interest lies in finding out methods of control of adults without greatly affecting their aquatic stages which are known to be useful fish food”, while El Bashir (unpublished) pointed out that “the chemical control of these insects is rather a difficult task, because it endangers the life of other aquatic animals such as fish, and also because of possible pollution of drinking water as organochlorine compounds are relatively stable substances”. The organophosphate Abate is now used particularly in the southern region of Lake Nubia and the canals of the Gezira Scheme as this compound has a relatively low fish toxicity and a short residual action, hence less danger of chronic toxicity and prolonged environmental contamination (El Bashir, unpublished). Patterson et al. (1964) tested both Baytex and Abate against fish amphipods and shrimps and found them ineffective at 0.2–0.25 lb/ac (0.22–0.28 kg/ha).
3.2.2 The problem of Eichhornia crassipes
In the upper reaches of the White Nile all the way to Jebel Aulia Dam (45 km south of Khartoum) the problem of the water hyacinth, Eichhornia crassipes (Mart) Solms, exists. The occurrence of this weed, which belongs to the monocot family Portederiaceae, was first reported by Wickrama-Sekara in April 1958 when it was occupying about 700 mi (1 130 km) river water from Shambe on the Bahr El Jebel to north of Kosti. By 1960 it infested the whole stretch of the White Nile from Juba (1 754 km) to Jebel Aulia as well as its tributaries and swamps; its spread northward has been checked by the Jebel Aulia Dam, a natural barrier (Abu-Gideiri et al., 1974; Gay, 1958; Dissogi, 1974). The presence of this plant is considered a sort of pollution because of its harmful effects on primary production and fish. Studies have shown that the dissolved oxygen at a depth of 30 cm decreases from 99 to 20 percent saturation, or from 7.6 mg/l to 1.8 mg/l under average conditions (WHO, 1972). Bishai (1961) pointed out that “the deficiency of O2 content renders the breeding and nursery areas unsuitable for fish life and that the presence of the hyacinth causes decrease in light penetration in the water, thus hindering the process of photosynthesis, restricting the growth of phytoplankton, zooplankton and hence the food available for fish. Also the plant provides a breeding and nursing habitat for mosquitoes which cause an increase in the incidence of malaria (Burton, 1960; Correa et al., 1941; Seabrook, 1962) and harbours the vector snails, Bulinus sp., and Biomphalaria sp. of Schistosomes (Dissogi, 1974). However according to Abu-Gideiri et al. (1974) the accumulation of the weeds behind the Jebel Aulia Dam has resulted in an appreciable increase in overall plankton densities and control of the species representation. This is correlated with changes in water chemistry as is evident from comparison of present data with those of Talling (1957): while an average of 6.8 mg/l of dissolved oxygen was obtained in the past, the average obtained now ranges between zero and 3.4 mg/l during August-October; 2 mg/l of carbon dioxide changed to 38–41 mg/l; 6 mg/l of Silicon are now 20–28 mg/l; pH dropped from an average of 8.1–8.6 to an average of 6.2–7.6 at present; phosphate increased from an average of 0.15 mg/l to 0.3–0.9 mg/l, etc. The increase in plankton provided favourable conditions for plankton-feeding fish to dominate the population in the area.
Chemical approach has been the only method used for the control of E. crassipes along the Nile. Yearly, a few thousand gallons or about 500 tons of the herbicide 2, 4-D are applied to the White Nile (El Bashir, unpublished; Saad, unpublished). This herbicide is sprayed at a dose of 4 lb of active ingredient/ac (4.4 kg/ha) from motor-powered knapsack sprayers, specially designed 30 in (75 cm) steel spray launches, Uni-Mog high pressure spraying vehicles and aircraft sprayers (Cessna 180 and Pipper, Fig. 2). This chemical is applied over an enormous volume of moving water, and as a result no significant residues are likely to occur in the waters of the White Nile (Saad, unpublished). On the other hand, El Bashir (unpublished) pointed out that the acute toxicity of the substance to fish and other animals is rather low, but long-term and side effects are not yet known.
3.2.3. The problem of agricultural pests on public health
The major sites of freshwater pollution lie in the irrigation canals of the Gezira, Managil Extension, Khashm el Girba Project, Suki Project and Guneid Sugar Estate where the intensive and extensive uses of pesticides and molluscicides pose a serious threat to aquaculture development.
In the Gezira Scheme, the canalization network consists of 5 649 km (3 530 mi) with a depth ranging from 0.50 to 0.75 m (Anon., 1955). In this scheme, more than 85 percent of the pesticides imported into Sudan is used for the control of cotton pests (El Bashir, unpublished). In the season 1946/7 an area of only 8 346 feddans (1 feddan = 1.038 ac) was sprayed while in 1972/3 an area of nearly 750 000 feddans was covered (Saad, unpublished). Moreover, prior to the 1962/3 season, cotton used to be sprayed once with a single insecticide (DDT) but today the same plot may have to be sprayed up to seven times during the growing season (El Amin et al., unpublished). Besides, other crops than cotton are also treated. That is why about one million tons of formulated insecticides costing nearly £Sd. 2.5 million were imported into the country in 1971 (El Bashir, unpublished). Table I shows the insecticides most commonly used today and their application rate.
Due to accidental aerial spray or drift of these pesticides across the irrigation canals, many fish have been found floating in the canals of the Gezira (Fig. 3), Managil Extension, Suki and Khashm el Girba Projects, particularly when DDT or Thiodan were sprayed on cotton (Saad, unpublished). According to Trazwell (1963) fish are very sensitive to the chlorinated hydrocarbons. Endrin, used during 1960–65 for protection of cotton, had been reported to have killed a great number of fish in the irrigation canals (Saad, unpublished). This organo-chlorine compound is regarded as highly toxic to fish and its use was subsequently discontinued. “In addition to the drifts of the sprayed chemicals, contamination of water occurs through dumping of surplus insecticides, washing of empty containers or that of contaminated clothes. Such contamination has been experienced in the Blue Nile Province during the anti-malaria activities, as many monkeys died by drinking from contaminated canals” (Saad, unpublished).
According to Abdel Nour (1972) and Haridi et al. (1973), Anopheles gambiae, the principal malaria vector in Sudan, has developed double resistance to the chlorinated hydrocarbon insecticides, DDT and Dieldrin, in the Gezira irrigated area due to the continuous application of the said insecticides on cotton. As an alternative, the organo-phosphorous compound Abate (formulation 500 E (50% W/U) emulsifiable concentrate), is used as a larvicide in the canals where it is applied at a concentration of 0.05 percent at the rate of 10 ml/m2 so as to give ultimately 50 g of the active ingredient/ha (Haridi et al., 1973).
Schistosomiasis is another major health problem in the Gezira canals as well as in other irrigated areas. Copper sulphate was used as a molluscicide for the Gezira Scheme at an initial concentration of 30 ppm followed by continuous application of 0.125 ppm to create a chemical barrier (Amin, 1972a; El Nagar, 1958; Sharaf El Din and El Nagar, 1955). Trials for replacing CuSO4 by N-Trithymorpholine (Frescon), applied at a dose of 0.075 ppm, were undertaken by Amin (1972b).
TABLE I
The insecticides most commonly used today at the Gezira Scheme and their application rate (Agricultural Research Corporation)
| Insecticide | Dosage Rate | |||
| Ekatin 25% E.C. | 0.25 l/feddana | |||
| Malathion 57% E.C. | 1.0 | lb | a.i./feddan | |
| Sevin 85% W.P. | 1.5 | " | " | " |
| Anthio 25% E.C. | 0.5 | " | " | " |
| Azodrin 55.2% W.S.C. | 0.25 | " | ||
| Dimecron 50% E.C. | 0.5 | " | " | " |
| Ekatin 25% E.C. | 0.25 | l/ | feddan | |
| Metasystox R 50 E.C. | 0.15 | " | " | |
| Nexion 25% E.C. | 0.5 | " | " | |
| Rognor 32% E.C. | 0.4 | lb | a.i./feddan | |
| Thimul 35% E.C. | 0.9 | " | " | " |
| DDT 25% E.C. | 1.0 | " | " | " |
| Dimethoate 32% E.C. | 0.4 | " | " | " |
| Torbidan 10: E.C. | 2.0 | L. | bulk | |
All the facets of aquatic pollution problems in Sudan, as indicated above, are more or less related indirectly to the development of the aquaculture industry. That there is a need to formulate pollution control laws in the Red Sea is never disputed but the problem of marine pollution is not severe at present. Therefore, there is a good scope for developing aquaculture of such indigenous species (Reed, 1964) such as the milkfish, Chanos chanos (family Chanidae) and Mugil cephalus (family Mugilidae) and also a number of shellfish, either indigenous such as the grass shrimps Penaeus monodon or exotic and yet to be introduced.
On the other hand, there is no significant pollution in the Nile System and even the development of industrial programmes will not pose a treat in the future; this is mainly due to legislative measures already elucidated above. Thus, the scope for using the waters of the Nile System seems to be very promising in developing aquaculture particularly at private poultry farms which are generally situated along the Nile banks. Also, in the ‘haffirs’ of Western Sudan, where there is no pollution due to pesticides or herbicides, aquaculture can be developed to a great extent.
The heavy use of pesticides in the Gezira Scheme poses serious hazards to aquaculture development in the canals. Little information is available on the toxicity of the various pesticides and molluscicides to endemic fish species and additional research is required before control measures can be formulated. Studies are now in progress on residues of organochlorine insecticides in the flesh of fish surviving in the Gezira canals. Preliminary results indicated the prevalence of DDT type compounds at levels of 0.1–4.0 ppm varying with species such as T. nilotica, Lates niloticus, Clarias angularis, Hydrocyon forskalii, being highest in the last (El Zorgani, personal communication). There is, therefore, a strong need for well coordinated technical measures to cope with pollution in these canals before aquaculture can be developed. In fact, the production of additional protein-rich food through aquaculture in the irrigation canals of the Gezira Scheme and other similar places is urgently needed; the problem is how best this can be achieved.
I am indebted to Dr. Y.B. Abu-Gideiri, Head, Zoology Department, University of Khartoum, for useful suggestions and to Dr. G.A. El Zorgani, Entomologist, Agricultural Research Corporation, for providing research data on residues of organochlorine insecticides in fish flesh.
Abdel Nour, O.M., 1972 The insect problem in the Sudan. Symposium on the Environment and Development in Arab countries. Arab League Educational and Cultural Organization/Sudan National Council for Research, Khartoum, Sudan
Abu-Gideiri, Y.B. and A.M. Yousif, 1974 The influence of Eichhornia crassipes Solm on planktonic development in the White Nile. Arch.Hydrobiol., 4:463–7
Amin, M.A., 1972a The control of Schistosomiasis in the Gezira, Sudan. The use of copper sulphate and mechanical barriers. S.M.J., Vol. 10, No. 2
Amin, M.A., 1972b Large-scale assessment of the molluscicides copper sulphate and N-Trithy-morpholine (Frescon) in the north group of the Gezira irrigated Area, Sudan. Trop.Med.Hyg., Vol.75, 169–75
Bishai, H.M., 1961 The effect of the water hyacinth, Eichhornia crassipes, on the fisheries of the Sudan. University of Khartoum, Hydrobiological Research Unit, 7th Annual Report
Brown, A.W. 1961 et al., Insecticidal operations against Chironomid Midges along the Blue Nile. Bull.Ent.Res., 51:789–801
Burton, G.J., 1960 Studies on the bionomics of mosquito vectors which transmit filariasis in India. Ind.J.of Malariology, 14:81–108
Cambridge Coral Starfish Research Group, 1974 Conservation manifesto for protection of the marine environment of the Sudanese Red Sea and its coral reefs. Publication of Coral Conservation Trust, 18p.
Corea, R.R. and A. de S. Ramos, 1941 The discovery of Anopheles darlingi and A. oswaldori naturally infected with malaria parasites in the south of Sao Paulo State. Folio Clin. et Biol., 13:183–91
Dissogi, L.A.-G., 1974 Some aspects of the biology and control of the water hyacinth (Eichhornia crassipes Solm). M.Sc. Thesis
El Amin, E.T. et al., Cotton pests and their control in the Sudan. (Unpublished report)
El Bashir, E.-S.M., (Unpublished) A note on the chemical control of the Nile Midges (Nimitti).
El Bashir, Pesticides and pollution of human environment in the Sudan. (Unpublished)
El Nagar, H., 1958 J.Trop.Med.Hyg., 61:231–5
Gay, P.A., 1958 Eichhornia crassipes in the Nile of the Sudan. London, Nature, 182:538–9
George, T.T., The history and status of fish culture in Sudan and the urgency of an experimental project for its development into an industry: a review. FAO/CIFA Symposium on Aquaculture in Africa, Accra, Ghana, 1975.
Haridi, A.M., M.A. Akood and A.D. Nugud, 1973 Laboratory and field evaluation of the larvicide Abate against Anopheles gambiae in the Sudan. (In press)
Mellanby, E.K., 1967 Pesticides and pollution. London, Collins, 221p.
Petterson, R.S. and D.L. Von Windeguth, 1964 The effects of Baytex on some aquatic organisms. Mosq.News, 24(1):46–9
Qutubuddin, M., The Nimitti problem at Khartoum. (Unpublished)
Reed, W., 1964 Red Sea fisheries of Sudan. Game and Fisheries Section, Ministry of Animal Resources, Sudan Government
Saad, A.-A., Pesticide Chemicals in the Sudan. Port Sudan, 88p. (Unpublished)
Satti, M.H. and O.M. Abdel Nour, Chironomidae as a cause of nuisance and allergic manifestations. (Unpublished)
Seabrook, H.L., 1958 Aquatic weed control. Proc.Soil Crop Soc.Pla., 18:210–5
Sharaf, E.-D. and H. El Nagar, 1955 J.Trop.Med.Hyg., 58:260
Talling, J.F., 1957 The longitudinal succession of the water characteristics in the White Nile. Hydrobiologia, 2:73–89
Trazwell, C.M., 1963 Hazards of pesticides to fish and the aquatic environment. Pesticides, their use and effect. New York, Albany
WHO, 1972 Seminar on water pollution control. Khartoum, EM/SEM.WAT.POLL.CTRL/DR.7
Anon., 1955 The Sudan Today, Social and Economic Progress. Ministry of Social Affairs, Sudan Government
Fig. 1 Map of Sudan showing marine and inland water resources
Fig. 2. Spraying of E. crassipies with the herbicide 2, 4-D.
Fig. 3. Dead Tilapia nilotica floating in a Gezira minor canal after areal spraying with insecticides.
