PART II
MANAGEMENT OF FISHERIES IN RIVER AND LAKE BASINS: BASIC AND APPLIED STUDIES FOR FISHERY MANAGEMENT INFORMATION ACQUISITION/THEORIE ET PRATIQUE DE LA COLLECTE DE DONNEES SUR L'AMENAGEMENT HALIEUTIQUE DANS LES BASSINS FLUVIAUX ET LACUSTRES (continued)

SESSION IV   BASIC AND APPLIED STUDIES FOR FISHERY MANAGEMENT INFORMATION ACQUISITION/LA COLLECTE DE DONNEES SUR L'AMENAGEMENT HALIEUTIQUE: THEORIE ET PRATIQUE (continued)

THE IMPORTANCE OF CLARIAS SPP. IN THE FISHERIES DEVELOPMENT OF THE ANAMBRA RIVER BASIN, NIGERIA

IMPORTANCE DE L'ESPECE CLARIAS DANS L'AMENAGEMENT HALIEUTIQUE DU BASSIN HYDROGRAPHIQUE DE L'ANAMBRA, AU NIGERIA

by

J.E.B. Awachie
and
G.M.H. Ezenwaji
Hydrobiology/Fisheries Research Unit
Department of Zoology
University of Nigeria
Nsukka, Nigeria

1. INTRODUCTION

The Anambra basin lying between 6°–7.8°N latitude and 6°40'–7°30'E longitude, is one of the richest, if not the richest, area for agricultural and fisheries production in the Nigerian Lower Niger (Mutter, 1973; Awachie, 1976; Awachie and Walson, 1978). Principle crop products include a wide variety of large yams (Dioscorea spp.) sweet potatoes, cassava, and rice, while fish production is dominated by clariids, Gymnarchus and mormyrids which are available throughout the year.

The continuing siting of many agricultural and fisheries projects, including the World Bank's Rice Projects in the Anambra basin, is a clear indication of the great potentials of the area inspite of the adverse effects of Kainji on the extent of its rich alluvial farm lands and natural floodplain production systems. It is not surprising therefore that the Anambra-Imo basin was recently created to ensure the continued development and management of the rich natural resources of the basin within the current national integrated rural development and ‘green revolution’ policy.

2. ASPECTS OF THE BIOLOGY OF CLARIAS SPP. IN THE ANAMBRA BASIN

Various biological, biochemical and production studies on Clarias spp. are currently being undertaken at the Hydrobiology Fisheries Laboratories at Nsukka in an effort to provide comprehensive basic and applied data for the development of the yield potential of the species.

2.1 Species Composition

There has been considerable doubt about the number of species of Clarias in the Anambra basin. Sydenham (1978) redescribed the type specimens of six species of the genus Clarias from West Africa and indicated the case for a revision. Walson (1978) took the fingerlings of Clarias lazera (Cuvier and Val, 1840) only, during his 24-month Atalla fishery sampling programme of the Anambra and Lower Niger systems. It is noteworthy that all the fingerlings were from the Anambra basin. Azugo (1978) while undertaking a survey of Helminth parasites of fish of the basin examined 16 C. lazera and 21 C. anguillaris L. 1762; both occurring in stretches of the river investigated.

In this study, three species of Clarias were found to be commonly landed by the fishermen, viz.: C. lazera, C. anguillaris and C. submarginatus. It is clear therefore that there are at least three species of Clarias in the basin.

2.2 Food and Feeding Habits

The dominant food items of Clarias spp. in the basin are benthic fauna especially Chironomidae, Odonata and Ephemeroptera nymphs, as well as associated detritus. This contrasts sharply with Thomas's (1966) finding for C. senegalensis, C. mossambia and C. gariepinus in a savanna man-made lake in Ghana - a feature which may be related to essentially different hydrologic regimes and therefore fauna of the two environments.

Large C. lazera and C. anguillaris (above 50 cm standard length) also feed significantly on zooplankton and smaller fish. Fish does not appear to form an important element in the diet of C. submarginatus, but other smaller mid-water organisms, particularly Daphnia and Cyclops, were taken in large numbers. It is interesting to note that the known diet of C. senegalensis and C. ngamensis more closely resembles that of their smaller anguilliform shallow-water relatives which are more stenophagous and feed mainly on arthropods as recorded by Corbet (1961) for C. carsoni, Jubb (1967) for C. submarginatus, and Willoughby and Tweedle (1978) for C. ngamensis.

From available data the three Clariid species of the Anambra basin appear to be opportunistic feeders, feeding at any time of the day with the intensity of feeding increasing appreciably during the night and in the early hours of the morning. Laboratory studies with C. submarginatus support the above field observation. Experiments in which either light floating or heavy-sinking feeds were administered indicate that C. submarginatus take in more food at night but that the intensity of feeding was about the same if the aquarium was kept for long periods in the darkroom. Vision may therefore be important in day feeding but it would appear from preference of C. submarginatus for night feeding, that the sense organs situated at the barbels are more important.

Adaptations which have relevance to both bottom feeding and low visibility situations are exhibited by the three Clarias spp. found in the basin. These include retention of the palatine - maxillary articulation as a hinge for probing movements of maxillary barbel independent of the mouth (Alexander, 1965), reduction of the importance of the eyes, development of a rich network of sensory organs on the body head, lips and barbels (Angelopoulo, 1947, cited by Bruton, 1979; Lowe-McConnel, 1975) and the further development of the suck and bite of food intake (Bruton, 1979).

2.3 Breeding Biology

Because of the importance of species of Clarias in tropical countries, data on its breeding biology is gradually accumulating, but is not yet sufficiently detailed for the exploitation of the full potential of the genus, especially in West Africa.

Spawning migration

There is concensus that the spawning migration is related to hydrological parameters. Greenwood (1955), Hall (1968), Awachie and Walson (1978), Welcomme (1975, 1979) and Clay (1979) have recorded that the spawning migration in Clarias spp. commences with the initial flood resulting from summer rains usually associated with high temperature. In Florida (USA), where rainfall is distributed throughout the year, Idyll (1969) observed that C. batrachus breed all the year round. In Israel, where the seasonal rainfall occurs in the winter months, the Nile catfish, C. lazera, breeds at the beginning of the hot season before the winter floodwaters have totally receded (Clay, 1979).

In the Anambra basin spawning is seasonal and migration occurs at the beginning of the rainy season, April to June, when water temperature is 26.5°–32°C. The behaviour of the three species is similar. Fish move up to the main river channels and then fan out into the tributary streams and rivers on the floodplain. Groups of fish may be, at this time, found congregating at the inlet channels to tributary streams, swamps, lakes and ponds probably in anticipation of further rise in water level to facilitate their movement into their destination/breeding grounds. This behaviour renders them highly susceptible to overexploitation.

The final migration to the spawning site, typically a swampy area or shallow pond with liberal growths of Nymphaea, Pistia, Ceratophylum and some Graminae and Cypraceae, is triggered by heavy downpours. Shoals of Clarias (mainly C. lazera and C. anguillara and fewer C. submarginatus) have been observed pairing, wriggling, walking and grunting as they move over wet grass.

As described by Clay (1979) and Richter (1976) for C. lazera, and Van der Waal (1974) and Bruton (1979) for C. gariepinus, spawning generally occurs at night and early in the morning (18.00–05.00 hours). Observed couples appear to mate for less than 30 seconds. The process of distributing the eggs has not been observed in the Anambra basin, but Greenwood (1955) and Van der Waal (op. cit.) reported that the ova are spread over a wide area by the female flicking its tail vigorously, since the eggs are adhesive.

Based on detailed observations made at Iyi-Efi pond on the floodplain, six gonadal maturation stages are readily recognizable for C. submarginatus, and these are also valid for C. lazera and C. anguillaris, viz.:

1. Immature

   ♀: Ovary light red, highly vascularized and elongated. No eggs visible through the ovarian membrane.

   ♂: Testes grey but ramified with blood vessels and elongated. Edges smooth.

2. Developing

   ♀: Ovary light red, vascularized, small developing eggs visible to the naked eye through the thick ovarian membrane. Increase in ovary weight.

   ♂: Testes pale and less vascularized. Edges slightly serrated with slight milky patches - perhaps due to development of milt.

3. Mature

   ♀: Eggs clearly visible through the ovarian membrane; eggs are yellowish in colour, rounded and growing larger. Ovary wall thin and transparent.

   ♂: Edges of testes highly convoluted and milky white.

4. Ripe

   ♀: Ovary fully distended. Eggs not released when the abdomen is gently pressed but the eggs are now big, yellowish-brown and appear as discrete bodies through the very thin ovarian membrane.

5. Running

   ♀: The ovary and eggs are similar to the “ripe” condition, but the big eggs can now be released by stripping or rough handling.

6. Spent

   ♀: Ovaries slightly flabby and light red with few large brown eggs visible, especially toward the distal end of the ovary. Sometimes, however, no eggs are seen. This stage comes immediately after spawning and before the ovaries revert to the Developing Stage (II).

   ♂: Testes slightly flabby with very small milky-white patches still at distal end of the edges. Later, testes revert to Stage II condition.

The fecundity (the number of ripening eggs in the female prior to the next spawning period) of Clarias submarginatus ranged from 1 974 for a 10-cm to 9 310 for a 29.5-cm fish. The fecundity of either C. lazera or C. anguillaris is by far greater. For C. lazera, it ranged from 4 576 for a 24-cm to 95 168 for a 72-cm female fish; for C. anguillaris 5 125–105 000 for 25-cm and 85-cm fish, respectively. The above data are compared with fecundities of other studied Clarias spp. in Table 1.

Table 1

The fecundity of different Clarias spp.

Differences in the size of eggs of the three species occurring in the basin were noted, viz.: 0.35–1.75 mm for C. submarginatus, 0.42–1.40 mm for C. anguillaris and 0.535–1.75 mm for C. lazera. Further analysis on the eggs of C. submarginatus showed a bimodal distribution with peaks around 0.35 mm and 1.21 mm. Available data on the egg diameter of studied species of Clarias are summarized in Table 2.

Table 2

Egg diameter of various Clarias spp.

Ovaries from non-ripe or spent Clarias submarginatus contain ova less than 1.050 mm in diameter. Because of this finding it is suggested that eggs over 1.050 mm are shed in a current spawning season and those below during the next. As the bimodal distribution in the size of the ova is established by ranges between 0.35–1.05 mm and 1.05–1.75 mm with peaks at 0.35 and 1.21 mm, respectively, it is thought that a possible implication of this finding is that with good environmental conditions and simulated triggers, Clarias spp. might be induced to spawn twice or more in culture practices.

2.4 Distribution and Abundance

The three Clarias spp. occur in practically all parts of the basin, in both the main river channels and the floodplain. However, C. submarginatus seems to be most hardy and widely distributed, having been caught in areas, e.g., very shallow pools along dry small river channels, where none of the other two species were ever taken.

All three species however display seasonal changes in abundance with peak catches from October to January and April to June. Low harvest periods are February–March and July–September, the latter period being the high water period in the basin. On the whole, C. submarginatus is more abundant than either C. lazera or C. anguillaris which has the lowest catch.

2.5 Productivity and Economic Importance

Accurate estimates of the total production/yield of Clarias spp. in the basin are unavailable and difficult. This has been largely due to the absence of relevant basic studies, the difficult terrain and inaccessibility of many of the rich fishing areas/villages, and the large extent of the productive areas which according to Awachie (1976), is over 40 km wide in some stretches. However data from the four major landing sites/fish markets - Otuocha, Opotoo, Enugu-otu and Ogrugu - indicate a fairly substantial yield. In June 1980, a total of 80 t of Clarias was landed during the day at the above four towns. Thus during the lower peak period of April–June, about 240 t are landed during the day. i.e., excluding landings made late in the evenings and early mornings not recorded by us. During October–January when Clarias are caught in their greatest numbers, an average of 110 t/month is landed at the four landing posts, giving a total of 440 t/month.

Allowing for the unaccounted evening and early morning landings indicated above and taking into consideration the estimated landings at Otuocha and Onitsha main fish markets only, during the remaining off season months of the year, Clarias yield from the basin may amount to a minimum of 1 000 t/y, i.e., about 17 percent of the over 6 000 t annual fish production from all fisheries sections in the Anambra basin (Awachie and Hare, 1977). Despite the fact that only available data from only the four major fish markets were considered, and that consumption by fishermen and the floodplain population is not taken into account, it is clear that the Clarias fishery makes an important contribution to the fisheries production of the basin. Indeed, apart from Gymnarchus, it is the most demanded and expensive genus in the basin.

Clarias spp. not only keep well in both fresh and dried conditions, but are highly valued items in Nigeria. Holden and Reed (1972) and Ezenwa (1976) noted that “they are extremely popular with the consumer and the flesh is very good to eat”. Similar situations obtain in Thailand with C. batrachus and C. macrocephalus (Bardach, Ryther and McLarney, 1972).

The economic importance of Clarias spp. in the Anambra basin is enhanced by its hardiness and adaptability to adverse environmental conditions especially low oxygen levels in warm to hot and dirty/turbid waters. Thus live Clarias are transported to distant markets, many days/weeks after capture, in crowded containers ranging from calabashes to empty kerosine tins, with little or no apparent adverse effects. They are held in the above containers for several days in the retail markets. The above qualities of Clarias and the fact that they are able to utilize a wide variety of food items - specimens examined from the basin had in their stomach almost every item found in the river, viz.: fish, mud, frogs, insects, rotten vegetation, both phyto- and zooplankton, and bird feathers.

Both smoke-dried and fresh fish are equally relished. Depending on size and species, 1 kg of fresh fish costs US$ 4.00–7.00, while smoke-dried material may fetch up to US$ 8.00 for large specimens. For most rural communities in the basin as elsewhere, Clarias provides one of the cheapest local dietary animal protein, as well as the main source of income, especially during the dry season when floodplain lentic waters are harvested.

3. CLARIAS AND CAPTURE FISHERIES

Aspects of the fish and fisheries of the basin have been studied by Awachie (1973, 1976), Walson (1978) and Azugo (1978) inter alia. Data made available from the above studies indicate that the basin, especially the lower half, is rich in fish of commercial importance.

3.1 Main Fish Species and Fisheries

As elsewhere in the main inland water systems of Nigeria, fish of commercial importance in the Anambra basin are dominated by the Cichlidae and siluroid complex. The main elements which support main fisheries are listed in Table 3.

Table 3

Fish of commercial importance in the Anambra basin

3.2 Hydrologic Regime and Fisheries

Although three seasons exist on a Nigerian-wide basis, viz.: wet/rainy season, dry season, and the harmattan; the last season is more clearly marked in northern Nigeria where it could play a significant role in the life of fish. In the Anambra basin, the harmattan lasts for a very short period and for this reason only the dry and rainy seasons have significant effects on the lives of aquatic organisms as they determine, to a large extent, the level of water in the basin bodies of water.

The Anambra basin is a typical tropical flood river system with an alternation of the flood and dry phases which follow the seasonality of the rains, which in turn, determine the seasonality of fisheries activities. The rainy season lasts from April to September in the basin and causes the “white flood” experienced in the area (Awachie, 1976). However, the flood phase does not correspond precisely with the rainy season but lasts from July to December. Attention is drawn to the fact that in the main River Niger, into which the Anambra flows, the flood phase may last from September to April as noted by Welcomme (1975). In the Anambra basin, flood recession period is about mid-October to December, by the end of which the dry phase fishery is in full swing.

3.3 Fishing Methods

The gear, craft and fishery methods of the basin in relation to the efficiency and productivity of the various fisheries has been investigated by Awachie and Uzoechina (in press). In addition to standard modern gear introduced into the basin by business enterpreneurs and the Anambra State Fisheries Division, the rich floodplain fisheries thrive on a wide array of traditional gear, and craft which have been evolved over the centuries in response to the widely different habits and habitats of the equally vast variety of freshwater fishes, as well as as the marked seasonal variation in their environment.

Thus in the 40-km wide floodplain, Clarias spp. are fished with a variety of gear whose detailed operations depend on the dynamics of the flood water level and behaviour of the fish.

A very interesting local method of fishing Clarias and, indeed, other siluroids, is locally called ‘akalla’. A remarkably efficient method, it consists of a network of tree branches stuck into the mud in swamps and shallow portions of large ponds, and aims at getting fish to congregate in the area. Later the area is partially encircled by bamboo fence about 1.8 m high. The unit is completely encircled early in the morning of the day of harvest. Fish are taken with clap and scoop nets. A similar procedure, the ‘acadja’ is practised in the lagoons of Benin Republic (FAO, 1978; Welcomme, 1979).

The above method, as well as many others, are used during early flood phase when many siluroids, especially the Monchokidae and Clariidae, are on their spawning migration. Extensive damage is thus done to the above fisheries at this time, since large and gravid C. lazera and C. anguillaris are caught in large numbers.

The foul-hook system locally called ‘mari-mari’ is another effective gear used to great advantage in this period. Unbaited and set across partially cleared spawning routes, this novel gear, after heavy downpours, may take up to 40 Clarias, mostly C. submarginatus, in a 200-hook (mostly No. 17) long-line.

Set nets, various traps, baskets, as well as matchets and spears are used to good effect as Clarias wriggle over shallow water and wet grass. Most of the local fishing methods have little, if any, regard to the survival of exploited fish stock. A fishery might be expected to be damaged or destroyed by the annual massive fishing of gravid specimens. However in the Anambra basin the feared detrimental effect on the Clarias fishery is not yet apparent to most people even though catch data analysis would point to some decrease in the size of fish taken in the last ten years.

Difficulties in getting access to the floodplains during the early and late flood phase impair fish production by migrant artisanal and commercial fishermen. Recently, attempts are being made by the Hydrobiology/Fisheries Research Unit of the Department of Zoology, University of Nigeria at Nsukka to improve communication into the inaccessible parts of the floodplain by the introduction of flat-bottom boats, made by Lake Chad Research Institute. Although the results of preliminary trials are good, the comparatively high cost of these boats will militate against their wide adoption by the average fisherman in the near future.

3.4 Fish Handling, Processing and Distribution

For Clarias catching, handling, processing and distribution present little or no problems as indicated earlier. Fresh Clarias has remarkable keeping quality and gets to most distant markets of the basin in good condition. During glut periods, the fish is smoke-dried, a procedure that improves the demand and fetches higher prices. Smoke-dried Gymnarchus, Mormyrids and Lates also fetch very high prices in the distant markets than fresh fish.

It is to be noted that most of the other commercially important species are readily sold fresh at the many landing posts, towns and villages distributed all over the basin. Sundried fish, comprising mainly the small-sized species are distributed during the dry season. Like the Clariids, Protopterus is distributed alive for up to three months at 25°–30°C.

Available distribution channels vary in different sectors of the basin. The most prevalent distribution system which involves beach-head middlemen (mainly women) who then sell to retailers, deprives fishermen of equitable profit from their labour for the middlehands sell at an average of 200 percent gain to the retailers who in turn may make gains of up to 50 percent of their outlay. In spite of the realization of the above fact, fishermen have been unable, hitherto, to form viable cooperatives to protect their interests.

For future increased fish output from the basin, it would be necessary to improve on the earnings of both professional and part-time local fishermen by ensuring higher prices for their products. To achieve this, it would seem obvious that a detailed study aimed at identifying the impediments to viable fishermen's cooperatives should be undertaken soonest.

4. CLARIAS AND CULTURE FISHERIES

4.1 Present Status in the Basin

Although culture fisheries have lagged behind capture fisheries among traditional riverine fishing communities in Nigeria (Awachie, 1976), considerable strides have been made in the Anambra basin especially around Otuocha. The excellent handling qualities and ecophysiological adaptations of both young and adult Clariids to hot and low oxygen levels in the murkey waters of floodplain ponds and pools have, from all available records, contributed to the early development of indigenous cultural procedures in the basin.

Consequent on their experience with Clarias fingerlings introduced into ponds and pools located near their villages, the realization that floodplain ponds are a richer and more certain source fish than the river channels during the dry season, and the recent, largely successful, attempts at semi-intensive culture of Clarias (as advocated by Awachie, 1976), by the Fisheries Department of the Anambra State Ministry of Agriculture and Natural Resources, many fishermen now undertake semi-intensive polyculture of Clarias and other hardy floodplain species. Water level during the dry season is enhanced by the daming of inlet channels to natural ponds. In distant ponds, pools and swamps, extensive culture practices as described by Awachie (op. cit.) remain the rule. At present, there is no intensive culture in the basin.

The contribution of Clarias in extensive polyculture varies with the size of the culture media. Thus Clarias spp., especially S. submarginatus, predominate in small to medium-sized ponds (up to 700–3 500 m²), where H. niloticus, G. niloticus, C. obscura and Tilapia spp. are present in reasonable numbers. In large pools, swamps, ponds and lakes, Clarias is found in smaller numbers. As recorded by Awachie (op. cit.) and Awachie and Hare (1977), most semi-intensively cultured ponds are privately-owned and located along farm routes and in villages. Because of its wide ecological distribution, C. submarginatus fingerlings are most commonly stocked in these ponds.

On yields from these ponds, Awachie (op. cit.) has observed that over 70 percent of fish in the local markets in the Anambra basin area consist of genera ‘cultured’ in these ponds.

4.2 Clarias Culture - Problems and Prospects

From the foregoing it would seem clear that the basin has great potential for successful development and expansion of fish culture. Cultivable species with favourable biological characteristics, e.g., Clarias, Heterotis, Tilapia, etc., are available. There are many small- and medium-sized natural ponds which can be modified/converted for culture (Awachie, 1976; Awachie and Hare, 1977). Swamp rice fields at Opotoo and Uzo-Uwani areas, which provide good spawning grounds for Clarias spp., particularly C. sumbarginatus, whose fingerlings (5–7 cm) are collected with baskets from October to January, are also at hand for ricefish culture. Added to the above, are the facts that wastes from crop agriculture, e.g., ground nut wastes, rice husk/bran, damaged corn, etc., which provide cheap sources of food and fodder abound, and that so far no fish diseases of epizootiologic importance have been reported from the basin. For instance, although bacterial, crustacean nematode, platyhelminth, and acanthocephalan parasites attack Clarias spp. (Khalil, 1971; El Bolock and El Sarnagawi, 1976; Awachie, Ilozumba and Azugo, 1978), none is known to pose a serious problem to Clarias fishery in the basin.

Polyculture of Clarias spp. with Tilapia, the planktivore Heterotis, benthivore Citharinus, herbivore Distichodus, and Synodontis in suitable natural ponds which simulate the natural production association in the basin, has very good prospects and should be used to provide the technical bench mark data for artificial pond culture for which there is considerable basin-wide demand and enthusiasm.

4.3 Role of Current Development Projects

The future rate of development of fish culture in the basin will depend, to a great extent, on the combined and/or coordinated efforts of the State Fisheries Division and the Anambra-Imo Basin Development Authority, especially on the latter which is specifically charged with the integrated development of the basin.

Studies on the breeding biology of Clarias in bamboo pens are currently underway by the State Fisheries Division personnel while a planned Fish Hatchery and Fish Farm Demonstration Centre at Umuekete, near Otuocha, by the same department is yet to take off because of lack of funds. An early implementation of the latter project will provide the necessary infrastructure and impetus for fish culture activities in the basin.

5. OPTIMIZATION OF THE CLARIAS FISHERY

With the establishment of a basin authority for the Anambra and Imo rivers, serious attempts to develop all the resources of the Anambra basin in an integrated manner, may be expected to be intensified during the current national quinquennial development plan period (1980–85). The rich renewable resources of the basin lie in its extensive floodplains. These floodplains should be managed for the full realization of their potential for both dry and wet season crops of which fisheries are but one component as observed by Welcomme (1975).

In order to optimize the exploitation of the basin's rich land and water resources, as well as develop relevant agro-based and allied industries, there is urgent need to formulate, ab initio, overall and integrated management strategies based on sound pre-development studies, part of which has been undertaken by Skoup and Co., for the basin authority.

Fish is a major resource of the basin and unless suggested, detailed studies on fisheries are undertaken early, then the great potential of the basin for fisheries production may be jeopardized.

Strategies for the optomization of the fisheries, especially Clarias fishery, should include as follows:

1. Development of management procedures including applicable regulations to safeguard spawners and fingerlings from massive and indiscriminate exploitation. The season or period for the collection of fingerlings from the floodplain for culture purposes has to be fixed.

2. The potential for induced second breeding in the year indicated by biological data on Clarias should be fully developed in order to enhance culture fisheries and the restocking of ponds, swamps and lakes which may be cut off by water control developments elsewhere within or around the basin (see below).

3. The proposed hatchery near Otuocha should concentrate on the development of local species for culture. Large-scale local fish seed production, and trials of mono and polyculture of Clarias and other species outlined above, should be embarked on side by side, in both natural and excavated ponds in order to compare the economics of production in both systems and thus optimize production. The natural production systems are to include those obtained by closing inlet channels to large ponds with earth dams in order to retain a large volume of water for a long time for fish production (Welcomme and Hagborg, 1977).

4. To check on excessive pollution by biocides and eutrophication from fertilizers of field crops, suitable drainage systems are to be provided for farms and associated industrial plants.

5. As post-Kainji studies in the basin have shown that Clarias is apparently the only fish which did not exhibit appreciable depressed productivity with the lowering of the flood level in the basin, probably because of its adaptive capabilities, priority attention is to be given to the fullest development of both capture and culture fisheries for the genus. This is particularly important in order to offset the expected loss in fisheries production which will result in further loss of wetlands in the basin if the proposed dams around Jebba (below Kanji on the Niger) and at the Lokoja confluence of the Niger and Benue Rivers are built.

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Khalil, L.F., 1971 Checklist of the Helminth parasites of African freshwater fishes. Tech.Commun.Commonw.Inst.Helminthol.Farnham, (42):80 p.

Khan, H.A., 1972 Breeding of catfish under controlled conditions. Barrackpore, Central Inland Fisheries Research Institute, Silver Jubilee Volume:73–8

Lowe-McConnell, R.H., 1975 Fish communities in tropical freshwaters: their distribution, ecology and evolution. London, Longman, 337 p.

Mookerjee, H.K. and S.R. Mazumdar, 1950 Some aspects of the life history of Clarias batrachus. Proc.Zool.Soc.Bengal, 3:71–9

Mutter, N.E.S. (ed.), 1973 Report of the Lower Niger Survey Appraisal mission. London, Overseas Development Administration, PROP/14/73

Nawar, G. and E.G. Yoakim, 1962 A study on the fecundity of the Nile catfish, Clarias lazera. Ann.Mag.Nat.Hist., 13(5):385–9

Reed, W., 1969 Report to the Government of Nigeria on fishing technology relating to river and swamp fisheries of Northern Nigeria. Based on the work of W. Reed. Rep.FAO/UNDP(TA), (2711):90 p.

Richter, C.J.J., 1976 The African catfish, Clarias lazera, a new possibility for fish culture in tropical regions. In Aspects of fish culture and fish breeding, edited by E.A. Huisman, pp. 51–72

Sydenham, D.H.J., 1978 Redescriptions of the type specimens of six clarid species (Pisces) from western Africa. Zool.J.Linn.Soc., 64:347–71

Thomas, J.D., 1966 On the biology of the catfish, Clarias senegalensis, in a man-made lake in the Ghanaian savannah with particular reference to its feeding habits. J.Zool., 148:476–514

Van der Waal, B.C.W., 1974 Observations on the breeding habits of Clarias gariepinus. J.Fish Biol., 6:23–7

Van der Waal, B.C.W., and H.J. Schoonbee, 1975 Age and growth studies of Clarias gariepinus in the Transvaal, South Africa. J.Fish Biol., 7:227–33

Walson, E.C., 1978 The ecology of the Atalla lift-net fishery of the Anambra and Lower Niger River systems. M. Philos. Thesis, University of Nigeria, Nsukka, Nigeria.

Welcomme, R.L., 1975 The fisheries ecology of African floodplains. CIFA Occas.Pap., (4):40–81

Welcomme, R.L., 1979 Fisheries ecology of floodplain rivers. London, Longman, 317 p.

Welcomme, R.L. and D. Hagborg, 1977 Towards a model of a floodplain fish population and its fishery. Environ.Biol.Fish, 2(1):7–24

Willoughby, N.G. and D. Tweedle, 1978 The ecology of commercially important species in the Shire Valley fishery, Southern Malawi. CIFA Doc.Tech./Doc.Tech.CPCA, (5):137–52

BIOLOGICAL CONSEQUENCES OF FLUCTUATIONS IN LAKE LEVEL

CONSEQUENCES BIOLOGIQUES DES FLUCTUATIONS DU NIVEAU DES LACS

by

A.J. McLachlan
Department of Zoology
The University
Newcastle upon Tyne, England

1. INTRODUCTION

Fluctuations in water level are a characteristic of the aquatic habitat. Predictability, frequency and magnitude, however, all vary greatly. Relative to the size of the habitat those in the marine environment are minute. In contrast, freshwaters experience major fluctuations in habitat size through variations in rainfall, evaporative losses or artificial manipulation.

Whatever the nature of the freshwater habitat, such fluctuations share biological consequences. One example illustrates the point: it concerns the role of terrestrial plants growing on lake bottom exposed by a recession in water level. No attempt is made to review the literature. Instead data are drawn from two lakes, both in Central Africa, with which I have had personal experience. Lake Kariba, the first of the giant man-made lakes to be constructed in Africa; and Lake Chilwa. The latter is a natural lake subject to marked changes in water level which sometimes results in its drying out, as it did most recently in 1968.

2. CONSEQUENCES OF A DROP IN WATER LEVEL

Grasses and fast-growing floodplain plants such as Aeschynomene pfundii Taub, flourish in the draw-down zone when the lake is low. In man-made lakes such as Kariba, dead trees may also be exposed by receding water. This organic matter provides food for terrestrial animals, including elephants and wood-boring beetles, principally Xyleborus torquatus Eichh. Both change the plants in some way, converting grass to dung or creating burrows in the wood of trees.

3. CONSEQUENCES OF REFLOODING

3.1 Terrestrial Vegetation as a Habitat

Following a rise in water level, modified trees and remaining grass provide a new habitat for aquatic animals, largely immature stages of Chironomidae and Ephemeroptera.

That exposure to influences from the terrestrial ecosystem can be important in developing the plants as a habitat is illustrated by the case of submerged Colophospermum mopane Kirk ex Benth. woodland (Figure 1). When first flooded this woodland provides a substratum for aquatic fauna. At this stage animals, largely larval chironomids of the genus Dicrotendipes, are confined to tree surfaces. On reflooding, however, aquatic animals are able to make use of the burrows created by the terrestrial wood-borers at low lake level. The mayfly Povilla adusta Agnew is the dominant species making use of the new habitat, its appearance being accompanied by a substantial increase in the total weight of fauna supported per square metre of submerged woodland.

Flooded terrestrial vegetation, especially grasses, are a transient habitat decomposing and disintegrating shortly after flooding. It is very noticeable that in the brief time when tissues have been softened by decomposition, but before break-up of the plants, the habitat supports the largest biomass of fauna. The legume Aeschynomene and the grass Diplachne fusca (L.) Beaur, provide examples. In both cases (Figure 2) there is nearly a thousand-fold increase in dry weight of fauna per unit area of vegetation. Biomass changes are again accompanied by a change in dominant species, Povilla and Chironomus being found mining in the softened plant tissues. The end result is similar to that following the attack of trees by wood-borers. Micro-organisms, however, take over the role in the case of these ‘softer’ plants.

Figure 1 Changes in submerged trees as a habitat for aquatic animals. Top, Colophospermum mopane woodland, on first flooding, inhabited by Dicrotendipes sp. Middle, woodland exposed to terrestrial wood-borers by a recession in water level. Bottom, reflooded woodland inhabited by Povilla adusta nymphs. Weight of aquatic animals per square metre of woodland is given together with the dominant species in each case. Values are the arithmetic mean and 95 percent confidence limits of replicate observations. (After McLachlan, 1970 and 1974.)

CHANGES RESULTING FROM THE DEATH OF TWO PLANT SPECIES

Figure 2 Changes resulting from the death of two plant species. Aeschynomene pfundii and Diplachne fusca. Dominant animal taxa are indicated in each case. Top, Dicrotendipes sp. Bottom left, Povilla adusta. Bottom right, Chironomus sp. Numbers are arithmetic mean and 95 percent confidence limits for the dry weight of fauna m^-2 stand of each plant. (After McLachlan, 1974 and 1975.)

The story, especially concerning Aeschynomene is more complex than this, though. An early result of decomposition is that the plant breaks free from the mud and assumes a horizontal floating position. Fauna are normally confined to a ring, about 4 cm wide, where the stem emerges from the water and where epiphytic algae are abundant. The change in orientation means that this belt now occupies the entire length of the stem. In addition, since the uprooted plant now floats this zone is no longer continually moving up and down accompanying small water level changes. A much more stable habitat is the outcome, the figure of C.4 000 mg shown in Figure 2 being accounted for partly by a ten-fold increase in bark surface dwelling Dicrotendipes larvae. Relative stability due to floating, of course, is a property enjoyed all along by inhabitants of Diplachne.

3.2 Terrestrial Vegetation as a Source of Food and Nutrients

When terrestrial vegetation, either due to micro-organisms or wood-borer attack, approaches its peak as a habitat, a second partly overlapping effect becomes evident. This entails the release of detritus and plant nutrients. Blooms of algae are conspicuous at this stage and detritevores, such as chironomids living in the lake mud, respond strongly (Figure 3), presumably to improved feeding.

Figure 3 Consequences of the decomposition of flooded grass and dung. Top, grasses growing on exposed mud flats grazed by terrestrial mammals. The dominant mud dwelling invertebrate at this stage is Dicrotendipes sp. Bottom, flooded grass and dung accompanied by the appearance of Chironomus sp. in the lake mud. Biomass values are given (arithmetic mean and 95 percent confidence limits m^-2 lake mud) for the lake dwellers. (After McLachlan, 1970a and 1974)

Again there is an interaction with the terrestrial ecosystem. In this case there is evidence that the activity of grazing mammals is involved in pushing up the concentration of decomposition products in shallow water to a critical point. Phosphate ions, for example, are released more rapidly from dung than from grass as follows:

Grass 900 ± 100 μe 1^-1 week ^-1

Dung 1 850 ± 210 μe 1^-1 week ^-1

(Values are the arithmetic mean and 95 percent confidence limits).

It is conceivable that detritus and nutrients would otherwise be released too slowly to reach concentrations producing the dramatic changes illustrated in Figure 3.

4. CONCLUSIONS

Water level fluctuations can be seen, therefore, much like tides in the oceans, to promote an interaction between terrestrial and aquatic ecosystems. The result is an enhancement in production in both systems. The affect is illustrated here with reference to organisms near the bottom of the food chain. Indirectly, of course, consumers including fish are also involved.

Predictability of fluctuations and time of year at which they occur are important factors in determining whether the situation can be exploited by organisms. At best fluctuations can be viewed as a system of following such as that used in agriculture for thousands of years and in the management of freshwater systems, notably in rice paddy management.

REFERENCES

McLachlan, A.J., 1970 Submerged trees as a substrate for benthic fauna, in the recently created Lake Kariba (Central Africa). J.Appl.Ecol., 7:253–66

McLachlan, A.J., 1970a Some effects of annual fluctuations in water level on the larval chironomid communities of Lake Kariba. J.Anim.Ecol., 39:79–90

McLachlan, A.J., 1974 Development of some lake ecosystems in tropical Africa, with special reference to the invertebrates. Biol.Rev.Camb.Philos.Soc., 49:365–97

McLachlan, A.J., 1975 The role of aquatic macrophytes in the recovery of the benthic fauna of a tropical lake after a dry phase. Limnol.Oceanogr., 20:54–63

HYDROGEOLOGIC FACTORS AND AQUATIC POLLUTION IN INTEGRATED RIVER BASIN DEVELOPMENT AND MANAGEMENT WITH PARTICULAR REFERENCE TO FISHERIES

FACTEURS HYDROGEOLOGIQUES ET POLLUTION AQUATIQUE DANS L'AMENAGEMENT ET LA MISE EN VALEUR INTEGRES DES BASSINS HYDROGRAPHIQUES, NOTAMMENT DU POINT DE VUE DE LA PECHE

by

B.C.E. Egboka
Department of Geology
University of Nigeria
Nsukka, Nigeria

1. INTRODUCTION

Capture fisheries, fish farming and sport fishing have played major roles for a long time in the cultural life and the economy of the various ethnic groups in Africa. The Argungu fishing festival in Northern Nigeria is a highly renowned annual traditional event. Unlike in many developed countries where fishing in inland waters is mainly for sport and leisure, the majority of people who fish in Africa depend on it for their livelihood. Thus, any large-scale pollution and contamination of surface waters is damaging to the fisheries industry, and would destablize the means of subsistence of many people.

Heavy pollution loads that endanger aquatic life and the environment are the direct consequences of industrialization and technological advancement. Today in the industralized world, efforts are being made to check or control the transport of pollutants through the environment, particularly in the surface and ground water flow systems. Unfortunately, grave and irreversible harm has already been done. Municipal and industrial sewage were dumped into rivers or lakes, sanitary landfills were improperly sited and poorly managed, radioactive waste is still being piled up in temporary disposal sites, oil spills and gas flaring are common, and acid rain, which presents a threat, is ignored. The pollutants generated from these wastes eventually reach the streams, rivers and lakes through the hydrologic cycle, and often at concentration levels that are hazardous to aquatic life.

The pollution problems in the industrialized world were created because basic preventive measures were either not taken or were ignored. In the present fever to industrialize, African countries must learn from the mistakes of the developed world, particularly in the area of fisheries planning and management. Today, many rivers and lakes in Africa are relatively safe from pollution, and offer high potentials for the fisheries industry. However, the fast rate of industrial growth by many African countries is already threatening the environment with large-scale pollution.

The purpose of this paper is to review briefly hydrogeologic concepts and problems in order to enable the investigator or the planner to understand more fully the relationship between environmental pollutants and fisheries industries; and to describe the basic hydrogeologic techniques that can enable adequate monitoring of the environment for an advance warning of any pollution threats. Finally, brief case histories will be outlined to highlight these objectives.

2. HYDROGEOLOGIC CONCEPTS

2.1 Hydrologic Cycle

The hydrologic cycle involves the continuous circulation of water between the ocean, atmosphere and land. The inflow into surface and ground water systems comes in form of precipitation while outflow occurs as streamflow, evapotranspiration and ground water flow. Thus, according to Freeze and Cherry (1979), “a watershed must be envisaged as a combination of both the surface drainage area and the parcel of subsurface soils and geologic formations that underlie it.” The aquatic life of any stream, river and lake may be affected by changes within the environment and must be considered in the planning or management of the drainage system.

2.2 Hydrogeologic Parameters

Various hydrogeologic parameters are very useful in understanding the hydrologic processes that occur in a watershed. The relevant parameters include hydraulic conductivity, ground water velocity and residence time. Other related parameters are transmissivity, specific yield, storage coefficient, specific discharge and total discharge.

Hydraulic conductivity (K) is a basic flow parameter in hydrogeology and related disciplines. Its importance derives from its significance in the Darcy equation that forms the basis of mathematical solutions to various ground water flow problems. The Darcy equation may be represented thus:

	Q	=	-K i A	(1)
where	Q	=	the flow rate or flux [L3/T]
	K	=	hydraulic conductivity [L/T]
	i	=	hydraulic gradient i.e., change in hydraulic head [L] over the
			distance or depth of measurement [L]
	A	=	cross-sectional area [L2]

The specific discharge (V_d) is also expressed as:

(2)

and has the dimension of velocity. The specific discharge is also called the Darcy velocity or Darcy flux and is a macroscopic concept that permits the measurement of average hydraulic values in the porous media. While V_d is fictitious, the actual velocity (V_a) is a real parameter that considers the microscopic intergranular flows, and V_a is obtained by dividing V_d with the porosity (n) of the medium.

The residence time (t) at a point or front within the ground water flow system indicates the time when the water at the point or front entered or recharged the flow system. It may be calculated from the relationship:

(3)

where ℓ is the distance or depth of penetration and V_a may be obtained as described above. Residence times may also be established with environmental isotopes, such as bomb tritium and carbon-14. The transmissivity, storage coefficient and specific yield are useful in the fields of water resources evaluation and management studies.

2.3 Sources of Pollutants

Point sources

The pollutants from the point sources affect the hydrologic system locally. The point sources include waste lagoons and barnyards, septic systems, landfills or refuse dumps, disposal sites for radioactive wastes and waste heat, industrial and oil spills. Pollution from point sources have been very common and can be easily monitored.

Distributed sources

The pollutants from distributed sources are more widely and regionally spread and may be much more difficult and costly to monitor and control. The acid or alkaline rain from precipitation, fallouts from thermonuclear tests or from nuclear power plants, agricultural fertilizers, pesticides and detergents are the chief pollutants that come from distributed sources.

3. PROBLEMS ASSOCIATED WITH RIVER BASIN DEVELOPMENT AND MANAGEMENT

3.1 Abiotic Environmental Factors

The following abiotic environmental factors must be considered in any integrated river basin development and management. The factors include ground water availability, soil suitability for waste disposal, soil susceptibility to flooding and water table fluctuations, soil erodability, water quality and contamination.

The fluctuations of the water table, the ease of recharge for the aquifer, and the amount of water available in the aquifer that underlie the streams, rivers and lakes aid in determining the water levels of such surface waters and their seasonal fluctuations. The suitability of the soil for waste disposal must be determined before any refuse dump or landfill is established. In badly located landfills, the leachates may be transported along the ground water flow system into the surface waters and pollute them. Sometimes, the engineered lining of the landfills with ‘impermeable’ clays hardly prevents the contaminant migration.

The permeability of the soil materials and the depth to the water table determine the degree of flooding within the watershed. The soil erodability and stream velocity assist in the determination of the sediment load and competence. Weathered soil materials are, thus, eroded and transported by streams and rivers into lakes. Information from the pattern of sediment transport, surface runoff contribution, stream capacity, velocity and competence is pertinent for the purpose of assessing the self-adjusting and purifying capabilities of the lakes.

The water quality and contamination are factors that must be recognized in order to preserve life within any watershed. The waters that flow into and out of the watershed should be monitored as described below (Section 4), to get advance warning on possible pollution of the aquatic environment that may endanger the fisheries development and management.

3.2 Biotic Environmental Factors

The biotic environmental factors include the sensitivity of vegetation, fish and wildlife populations to development. In Africa, several large and small dams have been constructed, and extensive areas of land have been flooded. Few investigations have been done to assess the extensive damages caused to the plant and animal life by the man-made lakes. In excavation and mining sites, loads of sediment and mine wastes are dumped on the ground surface. Weathering and erosion generate wastes that are transported into the surface waters and pollute them. Thus, within a given watershed, the environmentally sensitive areas should be established for better management and planning processes.

3.3 Human Factors

Sociological, psychological and anthropological factors have been down-played or totally ignored in integrated river basin development and management. The impact of any water resources or fisheries development on the people must be assessed. Man-made lakes may adversely affect the life of the people as a result of the loss of their lands, homes and possibly the traditional or local fish farming. Studies that focus on adequate compensation and/or proper enlightenment of the affected population, should be a part of the investigations. Some of the lakes are dedicated to deities such that fisheries development in these waters are regarded as desecration, and is strongly resisted by the people. In some of these lakes, fishing is completely prohibited. In these situations, the role of the government and institutions of learning to educate the people becomes obvious before any development programme is embarked upon. Finally, location of fisheries development projects should be made with a sound political judgement based on an integrated scientific investigation.

3.4 Lack of Personnel and Finance

Lack of personnel and finance have bedevilled many river basin development programmes in Africa. Several river basin development authorities have been established. Many of these lack the technical expertise and research scientists required to manage them effectively. For a meaningful running of the various river basin development authorities, research in the fields of hydrogeology, limnology, geotechnical engineering, geomorphology, soil classification and genesis, surface hydrology, hydrogeochemistry, isotope hydrology, climatology, rural sociology, etc., are highly essential. The integrated evaluation and analysis of the detailed results provide reliable data to facilitate solutions to problems of fisheries development.

3.5 International Relations

Various lakes and river basins are jointly owned by different countries. Joint international programmes for the development and management of the surface waters exist between some countries. In Europe and America strong treaties covering the use of the waters are enforced by the committed countries. Such treaties check or reduce the pollutant loads to rivers and lakes, reduce overfishing, and prevent rapid declines in water levels in the watershed. In Africa today, such joint projects between nations exist, but more cooperation and understanding between the committed nations is still needed in the execution of their programmes.

4. HYDROGEOLOGIC TECHNIQUES

4.1 Sampling and Monitoring Network

Multilevel samplers and bundle piezometers described in detail by Pickens et al. (1978) and Egboka et al. (1980), respectively, are sampling and monitoring devices that may be easily installed in a watershed at relatively no great costs. These devices provide ground water samples at depth intervals ranging between 0.15 and 0.61 m.

The multilevel samplers are designed to provide vertical profiles of geochemical parameters. Each bundle of samplers may consist of any number of 0.0032 m OD polypropylene tubes with small No. 100 mesh nylon screens attached to their extremities. The number of tubes may range between 10 and 80 which are connected to the sampler pots in the sides of a 0.038 m ID polyvinyl chloride pipe. These are lowered to the desired depth in the aquifer after drilling.

In a piezometer bundle that contains nine individual piezometers, eight of them are made of 0.0095 m ID polyethylene tubes, perforated over a 0.15 m interval and wrapped with No. 100 mesh nylon screen. The eight piezometers are strapped around a ninth piezometer in the centre. This ninth one consists of a length of 0.0127 m ID polyvinyl chloride pipe perforated over the bottom 0.30 m and wrapped with nylon screen. The 0.0127 m ID PVC pipe performs a dual function: serving as structural support for the 0.0095 m ID polyethylene tubes during installation, and as a deep piezometer for ground water sampling and monitoring. The bundle piezometers and multilevel samplers have been successfully used in North America for watershed studies.

For ground water flow and seepage studies in shallow lake and stream beds, Lee and Cherry (1978) have described the use of seepage meters and mini-piezometers. The two devices provide data for constructing the flow net, for the ground water flux and velocity. They also provide effective means of sampling ground water inflow to or outflow from the lakes and streams for the study of the ground water-surface water interactions.

Water samples are analysed for various geochemical parameters that may include Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, SO^2-₄, NO^-₃, PO^3-₄, HCO^-₃, and CO^2-₃. Aquaculture and water quality standards require that these geochemical parameters fall within acceptable ranges of concentrations. Other measurements include turbidity, Eh, pH and dissolved oxygen.

4.2 Use of Environmental Isotopes

The radioactive isotopes tritium and carbon-14, and the stable isotopes oxygen-18 and deuterium, are very useful in understanding the various processes involved in the hydrologic cycle. The radioisotopes provide information for the evaluation of the residence times, pore water velocities and for the study of hydrodynamic dispersion. The stable isotopes and radioisotopes also provide a means of identifying sources of recharge into the ground water flow systems, and separating the catchment areas within a given watershed.

The bomb tritium of the thermonuclear era that has been released into the atmosphere since 1953 enables one to identify the pre- and post-1953 waters. Thus, ground water as old as, or younger than 27 years can be traced along the flow system. Carbon-14 enables one to determine the age of water older than 27 years and ‘dead’ water that is thousands of years old. Oxygen-18 and deuterium are enriched as a result of evaporation effects, and may become enriched in landfills, and can be used as effective tracers for different sources of flow and recharge.

4.3 Pollutant Discharge and Surface Waters

The following approaches discussed in details in Gillham et al. (1978) provide means of estimating the loads of pollutant that may be discharging into streams, rivers or lakes. A knowledge of the amount of pollution that enters a surface water body helps in an effective management process.

(a) Area of influence around piezometer nests

Piezometers are monitoring equipment that are also used for ground water sampling. A collection of piezometers at a place and installed at various depths forms a piezometer nest. Collected water samples are analysed for the particular contaminant that is required. Several nests within a watershed yield data for the estimation of the pollutant loads, which is carried out with the following formula:

where P_t = total pollutant in ground water storage (kg)

A_i = area of influence of piezometer nest (m²)

P_c = pollutant concentration of aquifer at a piezometer nest (mg/l)

C_f = correction factor to convert mg to kg, litres to m³, and to account for the assumed aquifer porosity

n  = the number of areas of influence

(b) Flow net analysis

The potential distribution within the watershed is mapped and a representative cross-section is chosen and the cross-section is subdivided into smaller sections for the flow net analysis.

Q = Kb∆ ω

(5)

where Q = discharge through each section (cm³/s)

K = hydraulic conductivity (cm/s)

b = saturated thickness of the polluted aquifer (cm)

∆ω = width of each section through which Q is calculated (cm)

∆h = change in hydraulic head (cm),

∆ℓ = length over which ∆h was taken (cm)

(c) Discharge based on ground water residence time

The average ground water residence time is established. The load of pollutant in ground water storage is estimated. The annual discharge of pollutant to the surface water is estimated by dividing the load of pollutant in storage by the residence time.

(d) Discharge at the weir

The concentrations of the pollutants are measured and monitored with respect to time. Stream discharge values are calculated from the stream hydrograph and stream velocity measurements. The combination of the measured concentrations and the discharge values gives the pollutant discharge estimates.

4.4 Hydrodynamic Dispersion

The physical processes that control solute (or pollutant) migration through the porous media are advection and hydrodynamic dispersion. Advection is the process by which the dissolved constituents are transported along the flow system by the bulk motion of the flowing ground water. These constituents are thereby transported at the rate equal to the average linear ground water velocity, and they maintain a downflow sharp vertical tracer front. This is referred to as piston or plug flow.

For the case of hydrodynamic dispersion, the solute spreads out from the path of flow, both in the longitudinal and transverse directions to give a sigmoid-shaped front. Hydrodynamic dispersion comprises two main components: mechanical or hydraulic dispersion due to pore water velocity variation, and molecular diffusion due to the thermal kinetic energy of the solute particles. Mechanical dispersion is predominant at high velocities, and molecular diffusion effect becomes significant only at very low velocities.

The steady-state, uniform flow, one-dimensional transport equation for a constituent through a homogeneous, isotropic granular aquifer may be represented thus:

(6)

where c = solute concentration in solution [M/L³] at time t [T]

D_L = coefficient of hydrodynamic dispersion in the longitudinal direction along the flow path [L²/T]

V_L = average linear ground water velocity [L/T]

= change in solute concentration with change in time

= hydrodynamic dispersion term

= advection term

The differential equation, under the specified boundary and initial conditions, may be solved analytically or numerically to know the dissolved constituent that remains after dispersion. A radioactive decay term and retardation term may be added for the radionuclides and retarded solutes, respectively. Two- and three-dimensional analysis for equation 6 are also available.

The hydrodynamic dispersion coefficient is expressed further as:

where α_L with dimension [L] is the characteristic property of the porous medium called dispersivity. D* is the molecular diffusion coefficient and is expressed thus (Bear, 1972):

where   D_ω = diffusion coefficient in the uncontaminated water [L²/T]

τ_o   = tortuosity of the medium

Tortuosity is a number with a high degree of uncertainty. Bear (1972) gave 0.67 as a usable number for τ_o although numbers ranging between 0.1 and 1.0 have been used.

The calculation of the dispersion coefficient in the porous media in a watershed is very essential for estimating the amount of pollutant entering streams, rivers and lakes. Dispersion causes mixing of the polluted and unpolluted waters, and the subsequent dilution of the pollutant. Thus a high dispersion coefficient may result in low concentrations of the pollutant reaching the surface waters, and vice versa. Also, a highly retarded constituent is transported at a very slow rate along the flow system while a radionuclide might have decayed to harmless levels before reaching the surface waters depending on the distance and time of travel. Thus, the effect of hydrodynamic dispersion must be considered in any integrated river basin development.

5. CASE HISTORIES

The Agulu, Ulasi, Otiba and Uchu Lakes on both sides of the Agulu-Nanka gully complex, Anambra State, Nigeria, are presently being studied. Proposals have been presented to the Ministry of Science and Technology, Nigeria, for an integrated multi-objective investigation by a multi-disciplinary research group at the University of Nigeria, Nsukka. These lakes lie on the flanks of a major ground water divide, and have been receiving sediments eroded from the extensive gully system (Egboka and Nwankwor, submitted for publication). The lakes have also served for traditional fisheries and sports. However, the gullying in the area and the development of urban centres have attracted the attention of governments and tourists.

The Kainji Dam in Northern Nigeria has caused a great lowering of the water level of River Niger in the downstream areas of the country. Existing islands within the river have been widened while new ones have emerged. Presently, more dams are being planned or are being constructed, and these will definitely reduce the water level further and throw more traditional fishermen out of their job.

One of the results of Nigeria's colonial history is the development of urban centres near many rivers. Up till now, domestic sewage and industrial wastes are dumped into these rivers and lakes. The result is unsightly and ill-semelling river banks. It is yet to be estimated how far these waste disposal practices affect the fisheries industry in these rivers and lakes. Lake Nwangene that flows into River Niger at Ouitsha, Anambra State, Nigeria, is gradually disappearing as a result of the heavy solid waste being recklessly dumped into it. Similar practices are reported in Jebba, Lokoja and Idah along the River Niger; Yola and Makurdi along the Benue River; Abeokuta along Ogun River, and Kaduna along Kaduna River (Fanira, 1972).

In 1972, under the Great Lakes Water Quality Agreement, a joint United States and Canada effort established the PLUARG, an acronym for “Pollution form Land Use Activities Reference Group”. The joint effort has been aimed at dealing with pollution from non-point sources and its effect on water quality in the Great Lakes. The problem areas PLUARG has tackled included: urban areas, forested areas, liquid, solid and deepwell disposal areas, shoreline landfilling activities and lakeshore and riverbank erosion. Results from the investigations are being used to plan and manage the already highly polluted Great Lakes of North America and their river basins.

To check the extensive environmental pollution of many surface waters in the United States, the Government established the Environmental Protection Agency (EPA) and empowers it to monitor and check the disposal of any liquid and solid wastes, and any emissions from the industries. Tough environmental laws against any type of pollution have been made, and stiff penalties exist to punish any individuals or companies that are caught polluting the environment. Many companies are reported to be exporting their industrial wastes to some countries, and some African countries are reported to have agreed to receive these hazardous wastes. Some of the wastes have very long life that enables them to be transported to the streams, rivers and lakes from the disposal sites.

Many companies operate their industries in Africa with little or no supervision. Their industrial activities highly pollute the environment. In the oil-producing zones of Nigeria, extensive oil spills are common, and flaring of gasses has been a continuing process. According to FAO (1971), the oil pollution “problem is more pronounced (and is becoming chronic) in oil-producing countries like Nigeria, where heavy pollution occurs near offshore drilling operations, refineries and harbours; but other coastal States are also having their beaches fouled by oil from tankers discharging at sea”. In the tin-mining town of Jos, Plateau State, Nigeria, scattered large mounds of excavated earth with their adjacent artificial lakes are found all over the area, thereby destroying the aesthetic beauty of the environment.

6. CONCLUSIONS

In an integrated multi-objective river development and management for fisheries in Africa, the role of the hydrologic cycle must be well understood and considered in the various stages of the planning process. It should be realized that leachates from waste disposal sites or industrial emissions might eventually reach the surface waters and pollute them. The industrialized world has tough laws to protect their environments, and enforce these laws with stiff penalties. This is seriously lacking in less industrialized nations, particularly in African countries. Several sampling and monitoring equipments that may be installed cheaply are available. Mathematical formulae that are useful for calculating the load of pollutants getting into surface waters are also available. Any effective planning process must also consider the sociological impact of development on the people.

REFERENCES

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