An important characteristic of small water bodies is that, because of their small size, they can be controlled relatively easily. Relatively few are privately-owned, except in South Africa and Zimbabwe (where 61% of them, or 41% of their area, is privately-owned), and most are either state-owned or communal property, in which case they can be controlled by local governments or traditional leaders. The ability to regulate the fisheries in small water bodies increases the possibilities of enhancing their productivity, and various methods have been used to do this.
Introducing fish species into water bodies is a management measure that has been used to improve fisheries in lakes, reservoirs and rivers throughout the world. If successful, it can be an extraordinarily cost-effective technique, and one that can transform fisheries. Well-known examples in Africa include the introduction of Limnothrissa miodon into Lakes Kivu and Kariba, which brought about a ten-fold increase in their fisheries productivity. Introduced fish can also have detrimental environmental effects, as in the case of Lates niloticus in Lake Victoria (Craig, 1992), and proposals to introduce fish species should give due regard to their potential environmental impacts.
Most fish introductions are likely to be carried out once, or a few times, until the new species is fully established and capable of maintaining its population through natural reproduction. The periodic stocking of fish is perhaps more common and is a useful means of managing small or seasonal water bodies or supplementing natural recruitment (Welcomme and Henderson, 1976). It is usually uneconomical in larger water bodies when there is a wide variety of species that reproduce naturally. Restocking of large lakes or rivers involves only species of exceptional value or ones that do not require food with high levels of protein as fry or fingerlings. The circumstances in which fish might be stocked include:
when there is an unutilized ecological niche, as there was in Lake Kariba prior to the introduction of Limnothrissa miodon. In another case, the bottlenose Mormyrus longirostris was introduced into Lake Mutirikwe in southeast Zimbabwe, and greatly increased the fish yield without any effect on other species (J.L. Minshull, personal communication);
to restore species diversity, especially in man-made lakes where species that should be in the system were absent when the dam was closed. This commonly occurs because dams tend to be closed in the dry season and many migratory species are absent then. An example of such a situation is the Chimutsi Dam in Zimbabwe, which apparently lacks the catfish, Clarias gariepinus, although they occur in the river below. Evidently they are unable to ascend the dam's spillway (B.E. Marshall, unpublished observations);
when fish stocks have been depleted through overfishing, pollution or drought. The stocking of hatchery-raised fingerlings is a widely practised means of improving or restoring fisheries yields, although its use in southern Africa has been limited because of a lack of hatchery facilities. A noteworthy restocking exercise was carried out recently in Zimbabwe. Hundreds of small dams dried out completely during the 1991/92 drought and they were restocked during the 1993/94 rainy season, not with hatchery-bred fish but with fish taken from those dams that did not dry out (C. Nugent, personal communication; van der Mheen, 1994); and
in response to specific problems, such as the use of Grass Carp Ctenopharyngodon idella to control troublesome weeds. Other problems that might warrant the introduction of a fish species include possible control of vector-borne diseases like schistosomiasis, malaria or filariasis.
Fish communities that develop in reservoirs, following the construction of a dam, may differ considerably from those of the original river. Those species which are better-adapted to riverine conditions tend to decrease whilst those that can adapt to lacustrine conditions increase. This is clearly illustrated in Lake Kariba, where Labeo and Distichodus species have declined whilst tilapias have increased, except in the western basins, which are influenced by the Zambezi River (Kenmuir, 1984). Another species that tends to disappear from reservoirs is Barbus marequensis, which, in Zimbabwe, is common in rivers but is absent from most reservoirs (Marshall, 1982). The decline of these species is usually due to an inability to breed in reservoirs, and, in many countries, especially in Asia, they are stocked into them into them in order to improve their productivity (Lu Xiangke, 1992). There are few examples of this practice in any of southern African countries, perhaps because tilapias are the most important species and they have little difficulty in breeding in reservoirs.
Since the supply of fingerlings is often a major constraint in stocking programmes, there are several criteria that should determine the species being considered. They should reproduce easily and frequently, whilst the technology required to propagate them should be simple and inexpensive, so that it can be done by the local population. They should feed low on the food chain, and be capable of rapid growth in order to maximize yield (which is why tilapia and carp are popular species). Finally, they should be preferred by the local population so that there are no difficulties in disposing of the catch.
Transporting fingerlings over long distances increases their mortality and should be avoided whenever possible (Lu Xiangke, 1992). Survival rates of 90% have been recorded if fry are stocked directly from a hatchery into reservoirs, and methods of doing this should be examined. This can be accomplished by rearing fingerlings (i) in cages set in the water body itself, (ii) in terraced fish ponds set close to or even below the reservoir's maximum water level, or (iii) in coves blocked-off by means of nets or earth and stone barriers across their mouths.
The advantages of blocked-off coves is that they can be constructed after draw-down, thus saving on labour, material and investment. They are also more stable and the depth of the water can be regulated according to the needs of the fish. By adjusting the water level in relation to the size of the fish, the use of fertilizers can be reduced whilst maintaining the growth of plankton. Harvesting is relatively easy, of course, because the fish can simply be discharged by draining the water directly into the reservoir. The disadvantages are that suitable sites may be rare, that poorly-constructed coves will leak, or that predators will find their way into them. Good management is also necessary in order to maximize the growth and survival of the fingerlings (Lu Xiangke, 1992).
The number of fingerlings that should be stocked depends on the fertility of the water and the species that is being used. In China, carp are stocked at a density of 75 to 300 kg/ha in order to realize an annual production of 375–1 500 kg/ha, depending on the level of fertilization. For tilapias, if stocked in early spring and harvested in winter, it might be possible to stock fertile reservoirs with 750–2 250 fingerlings per hectare, depending on their size when initially stocked. Another approach depends on the size of the reservoir, i.e., those smaller than 4 ha should have at least 9 kg of fingerlings, those from 4 to 8 ha should have at least 18 kg, whilst those over 8 ha need no less than 27 kg.
Most countries of southern Africa are susceptible to drought, and it is probably the single most important constraint on inland fisheries in the region. The extraordinarily severe drought of 1991/92 caused most of the smaller dams in the country to dry out, and severely depleted fish resources in most areas. Because so few fish were left in the river systems, it was decided to restock as many as possible during the following rainy season. This operation is described more fully elsewhere (van der Mheen, 1994), but the main features can be summarized here since this programme provides valuable lessons in the large-scale management of small water bodies.
It was obviously impossible to stock all the reservoirs that had dried up in the drought, but fish were finally put into about 600–700 reservoirs. Special equipment was developed to transport the fish, which had to be captured from dams which still held water, as the available hatchery facilities could not supply enough (many lacked water). No attempt was made to restore the natural population and the introductions were largely restricted to tilapias. Oreochromis mossambicus was put into 94% of the dams, O. macrochir into 17% of them, Tilapia rendalli into 50% of them and T. sparrmanii into 3% of them. It was assumed that other species would eventually be able to recolonize the dams.
A total of 4 500 kg of fish was caught, with an average fish weight of 16 g. Although some mortality occurred, most of them were successfully stocked, and it was estimated that about 70% of the dams were effectively restocked, i.e., the fish established themselves. Stocking was ineffective in a number of dams, mostly less than 2 ha in area, which dried up again after restocking. The programme proved to be relatively cost-effective, since the cost of captured fish was about half that of hatchery-reared stock. The major limitation was the speed with which the operation had to be carried out and the lack of opportunities for detailed monitoring. Nevertheless, this programme proved to be extremely effective and is a good example of what can be done to mitigate the effects of drought.
Increasingly complex methods of fish stocking lead ultimately to some kind of aquaculture. Aquaculture is, of course, a specialized topic that is not necessarily relevant to the management of small water bodies. The fundamental difference between small water body fisheries and aquaculture is that in the latter the environment in which the fish are grown can be controlled to a large extent. Nevertheless, some aquaculture practices can be used to enhance fisheries in small water bodies, and can be considered here.
Extensive and semi-intensive aquaculture systems involve a low level of capital investment, technology and management. These methods are inefficient and expensive in larger waters but can enhance fish production in small ones. They generally require an input in the form of stocking, fertilization or regular supplementary feeding. This can be cost-effective if locally available foodstuffs and farm by-products (e.g., rice bran, ground maize, cotton seed, brewery wastes, sugar-cane wastes, slaughterhouse wastes, coffee pulp and hulls, cassava wastes, etc.) are readily available. Fertilizing the water with manure can also promote fish production.
The use of farm residues or manure is not widely practised in the countries of southern Africa. Some areas, especially those that are semi-arid, produce very little of this material and it is in any case used for other purposes, such as manuring croplands. Commercial fertilizers or fish foods are far too expensive and are not likely to be used to any extent.
The food supplied to the fish is often of a low grade (10–30% protein) and supplements natural production, which can also be stimulated by the application of fertilizers (Redding and Midlen, 1991). These methods can lead to considerable improvements in the productivity of small water bodies (Figure 22). It should be noted that the gains were much smaller in the relatively large barrage ponds (i.e., on the main stream of the river) than they were on the much smaller diversion ponds (i.e., into which water was diverted). This suggests that not all small water bodies are suitable for this type of enhancement and care is needed in selecting those in which to apply these practices.
|Notes: Oreochromis niloticus was used except in D, where O. macrochir was used.|
Treatments: A. no management; 1 fish/m2. B. chicken manure; 1 fish/m2. C. chicken + pig manure; 1 fish/m2. D. & E. manure + rice bran (once daily); 1 fish/m2. F. manure + rice bran (twice daily); 1 fish/m2; harvested twice annually. G. manure + rice bran (thrice daily); 2 fish/m2; harvested twice annually. H. manure + rice bran + termites + bloodmeal (thrice daily); 3 fish/m2; harvested thrice annually.
Figure 22 Effects of fertilizing, feeding, stocking and harvesting on aquaculture yields in barrage and diversion ponds
(From Palm, 1989)
All aspects of the production cycle are controlled in intensive culture. Capital costs are high, especially since high quality foods are supplied, and a considerable investment in technology and management skills is needed. In addition, it requires fish that have a high conversion rate, grow rapidly and command a higher price in the market. The fish need to be able to resist stress, parasites, diseases and poor water quality (Palm, 1989). Intensive aquaculture systems are generally of little concern to the management of small water bodies, except perhaps for those which confine fish in cages or pens. These methods are more likely to be used in small water bodies and merit further discussion.
Cage and pen systems can be set up in existing water bodies and use relatively simple technology. Although their initial costs may be considerable, their operating costs are relatively low. Floating cages made of netting material can be used in large, deep and moderately rich reservoirs. In this situation, with supplementary feeding, cage culture should be successful because the epilimnion is relatively warm, with adequate dissolved oxygen and relatively rich in plankton (Baluyut, 1983).
Although cage culture is a means of producing high quality protein relatively cheaply, it may have some detrimental environmental effects. In Malaysia and Singapore, for example, the culture and harvesting of planktivorous species is said to clean up eutrophic waters, but elsewhere cage culture causes eutrophication, which leads to reduced production (Beveridge, 1984). Water quality is certainly an important consideration, especially if there are a large number of cages placed in a relatively small body of water. This would be a serious problem in southern Africa during the hot, dry season, when water levels are at their lowest. Low levels of dissolved oxygen can be expected under these circumstances and the risk of fish deaths is likely to increase.
Cage culture is not practised widely in southern Africa, although there is some in Zimbabwe. A system that operated in the Mazvikadei Dam (2 300 ha) used 25 cages, each with a capacity of 8.5 m3. Because crocodiles are present in the lake, the cages were made out of wire mesh and securely locked. Oreochromis niloticus was stocked with 50 g fingerlings at a density of 235/m3. They were fed with locally made pellets containing 25% protein, and harvested after 180 days, when they weighed around 250 g, giving a growth rate of about 1.25 g/day. Cage culture was also carried out in Lake Kariba, where 250 fingerlings were stocked in each cage at a density of 25 g/m3 and the growth rate was about 1 g/day. Feeding amounted to 60–70% of the operating costs of these enterprises1. Mutsekwa (in preparation) reported that artisanal cage culture in small water bodies is practicable using 2 cages of 36 m3 or 8 cages of 9 m3 which are stocked with 25 g fingerlings and reared to 120 g. Nevertheless, the returns are unpredictable and depend on the cost of fingerlings, feed and the fish themselves. A careful analysis is therefore required before any small-scale cage culture system is embarked upon.
The main differences between pens and cages are that fish grown in pens have access to the bottom and can feed on benthic organisms, and some species seem to grow better in cages than in pens (Beveridge, 1984). Pens are much larger than cages and are usually owned by companies or individuals, which limits the number of people who can benefit at any site.
According to Balarin and Haller (1982), the advantages of cage or pen culture are:
the initial investment is relatively small.
The disadvantages are:
labour costs are relatively high.
It should also be noted that cage culture is most suited to planktivorous fish or those that feed on benthos or detritus. Microphagous species, like Oreochromis niloticus, O. mossambicus and O. aureus, are better suited than macrophagous ones like Tilapia rendalli or T. zilli (Coche, 1982). These methods can also affect the enhancement of fisheries in small water bodies, since the cages and pens are located in shallow areas which may be important for the spawning of wild fish.
1. Both of these enterprises have run into difficulties since this review was written, and the operations at Mazvikadei Dam have been closed down. This was because of (i) low lake levels, which made the cost of pumping water prohibitive, and (ii) an inability to compete with marine fish imported from Namibia and South Africa. Such fish became widely available in Zimbabwe following economic reforms, and may have a significant effect on inland fisheries in the region.
Many of the most important fish species in small water bodies are vulnerable to disturbance in their breeding areas. Anadromous species like Labeo or Hydrocynus and, to some extent, Clarias, have to move up rivers to breed and may be concentrated in large numbers in their breeding areas. They can then be caught in such large numbers that their population is significantly reduced. In Zambia, for example, the commercially valuable stocks of Labeo altivelis in Lake Mweru were destroyed on their breeding run up the Luapula River and have never recovered (Jackson, 1976). The tilapias are also vulnerable to beach seines during their breeding period, when the males congregate in arenas. This was found to be the case in the Makungwa dam (Zambia) in 1992, where outsiders were using small-meshed gillnets as beach seines and had prevented the recruitment of tilapias. Consequently, there were few tilapias in the catches, which mostly consisted of the small Barbus paludinosus, which is a less valuable species (Maes, Ersdal and Mutale, in preparation).
Protecting these spawning areas is therefore of great importance and, once they have been identified, active steps should be taken to protect them. These could include the construction of spawning beds along the reservoir shore, as in Indonesia (Baluyut, 1983), or the designation of protected areas, as in Lake Cohoha, Burundi. Fishing can also be prevented in some areas by putting in obstructions like cut trees or car skeletons (Horemans and Maes, 1989).
Most of the artisanal fisheries in small water bodies are characterized by a variety of fishing methods of varying efficiency. Not all of the methods are employed regularly and the choice may be influenced by water levels, wind, the behaviour and availability of fish, and so on. Gillnets, when permitted by legislation (which may not always be the case, as in Lesotho), are the only fishing gear that is likely to be used regularly in most small water bodies.
Since fishing effort has a major impact on fish stocks, it is necessary to monitor the efficiency of each type of gear and their catch composition. Data from Africa are scarce, but Amarasinghe and De Silva (1989) showed that beach seines diminished fish stocks in Sri Lankan reservoirs, partly because of their effect on the breeding of tilapias. Most African reservoirs contain submerged trees, which give the fish a great deal of protection. For example, in Makungwa Dam, Zambia, only two-thirds of the area is accessible to beach seines. Consequently, these nets probably do not have a significant effect on fish stocks.
Regulations governing fisheries will obviously vary from one country to another, but should be based upon biological knowledge, while taking economic factors into account. In most southern African countries, except for Lesotho, Botswana and Tanzania, fishing regulations are well defined and specify the minimum mesh size of the gear, closed seasons, licensing, and the prohibition of illegal fishing. Some countries in the region have realized the importance of fishing regulations. Thus the Government of Lesotho has asked the FAO Legal Office for assistance in drafting new ones, which will include the control of fishing in small water bodies.
These regulations will not be effective if they are neither respected nor enforced, and, in practice, they are widely ignored. This has created many problems, and one solution might be to limit the number of prohibited types of gear, in order to control them better. Another approach, adopted in Zimbabwe and Botswana, is to license a group of fishers, who will then regulate access themselves in order to protect their resource.