4. FISHERY RESOURCES OF NIGERIAN INLAND WATERS Contd...

4.11. RESERVOIRS

These include reservoirs and ponds created by damming seasonal and perennial rivers. In both cases the reservoirs so formed have areas far in excess of the original surface areas of the rivers that were dammed. In most cases the surrounding farm lands become either permanently or seasonally inundated during the flood season. Most reservoirs are annually filled to capacity during the flood season and, because of water loss through evaporation, seepage and discharge either from spill-ways or outlet channels for either irrigation or power generation, the water level drops drastically during the dry season resulting in a marked reduction in reservoir volume and exposure of a vast area of reservoir floor.

In Kainji Lake for instance, the maximum surface area of the reservoir is 1280 km² during high and 660 km² during the low water seasons with mean depths of 11 and 8.8 metres respectively. The change in volume results in about one third of the area of the lake floor being exposed (Halstead, 1973). Kainji Lake has the greatest draw-down compared with other major lakes in Africa.

The reduction in the volume of reservoirs results in higher fish catches by commercial fishermen, partly because the fish are subject to heavy predation as the water recedes from the flooded bush, and terrestrial and littoral aquatic vegetation, and partly because the fish become concentrated in a smaller volume of water (Ita, 1978a). The littoral margins of the reservoir serve as breeding and nursery grounds for most species which, in the river, utilized seasonally flooded plains and swamps as their breeding and nursery grounds. However, as the water recedes gradually from the bush, the small fishes become exposed to heavy predation by carnivorous fishes, reptiles and birds.

Periodic exposure of the bottom of a reservoir or pond helps to promote the fertility of the soil and hence that of the water during the subsequent flooding season. As the bottom dries out and cracks open, the access of abundant oxygen speeds up the process of decomposition and the pH of the bottom soils is increased. Under these conditions there may be a release of certain fertilizing substances from organic colloidal systems making available greater quantities of potassium and phosphate (Bennett, 1971).

There are twelve major reservoirs in Nigeria with an estimated surface area of about 303,600 ha and numerous smaller reservoirs with estimated surface area of about 98,900 ha. Fish ponds are estimated at 5,500 ha (Table 2).

4.11.1. RESERVOIR PRODUCTIVITY

It has been established that the mineral content of the water expressed as total dissolved solids (or alkalinity or conductivity) can be used as a rough indicator of the edaphic conditions which play a fundamental role in determining the biological productivity of reservoirs or lakes. Ryder's (1965) Morpho-Edaphic Index (MEI) has been found very useful in predicting fish yield both in temperate and tropical lakes. The index is expressed as:

The index determined for any lake can be used to approximate its potential fish yield if matched against the regression curve of MEI versus fish yield in 31 African lakes estimated in Henderson and Welcomme (1974).

Table 11. Variation in the conductivity of some Nigerian and other African reservoirs and their major inflowing rivers.

* = Data extrapolated from mean percentage of the first four

A comparison of the conductivity of some Nigerian and other African Rivers and the lakes formed by their impoundment (Table 11) reveals that in all cases the conductivity was higher in the lake than in the original river. The percentage increase was estimated for four major lakes (Kainji, Volta, Nasser and Kariba) and the mean percentage increase used to estimate the expected conductivity in Lakes Kossou and Bakolori, for which only figures for the rivers were recorded in the literature. The percentage increase in conductivity was also matched against the flushing rate (i.e. rate of water replacement in the lakes) of the various lakes and there appears to be an increase in conductivity with decrease in the flushing rate.

Shallow lakes are more productive than deep lakes on account of the presence of shallow littoral margins which serve as breeding and nursery grounds for fish. Besides, in shallow lakes there is complete mixing of the surface and bottom waters giving rise to uniform distributions of dissolved oxygen allowing free movement of fish to exploit the available food sources in all the habitats. In contrast, deep lakes often possess steeply sloping shores, not ideal for fish breeding and nursery areas and are often stratified with little or no oxygen in the deeper parts, thus rendering the rich organic matter at the bottom unavailable to bottom feeding fish.

4.11.2 ESTIMATES OF FISH LANDINGS FROM KAINJI AND OTHER RESERVOIRS IN NIGERIA

Bazigos (1974) estimated the total fish landings in Kainji Lake between 1969 and 1974 and gave projections for other years (Table 12). Ekwemalor (1977) also computed the catch assessment records for Kainji Lake for 1977 and repeated the frame survey for 1978. The author of this report has utilized the same methods to conduct frame surveys and for the computation of fish landings in other, smaller reservoirs including: Jebba Reservoir (Ita et.al., 1984; 1985); Shiroro Reservoir, Niger State (Ita and Mohammed et.al., 1986); Kiri Reservoir, Gongola State (Ita and Ismaila et.al., 1986); some reservoirs in Kano and Kaduna States (Ita et.al., 1986a; 1986b; also Ita and Mohammed, 1988); and Eleiyele and Asejire Reservoirs, Oyo State (Akinyemi et.al., 1989).

In view of the extremely high cost of sending staff to the field for these surveys, a way had to be found for deriving relationships between estimates of commercial landings from a fishing village (expressed as catch per boat in kg) and catch per 1,000 m² of a graded fleet of gillnet set simultaneously along with the catch statistics survey team. The relationship was strongly correlated (Fig. 4). From this regression, it is possible to convert landings of local fishermen per boat into catch per unit effort in gillnet fleets for each station sampled and for the whole lake and vice versa.

As discussed in Ita (1986), the comparison is limited only to artisanal fisheries with relatively low efficiency gears such as gillnet, hooks, traps and castnets. It is useful in extrapolating yield indices of commercial landings in different zones of a reservoir where survey cost is a major constraint on organized statistical surveys of artisanal fish landings. It is also possible to estimate total fish landings in small reservoirs if the total boat count is known after a Frame Survey. Table 12 shows the results of frame and catch assessment survey data for some reservoirs sampled in Nigeria and case studies of some of these reservoirs are discussed below.

An attempt is made in the following section to present the case studies (undertaken by the author) of some reservoirs in Nigeria in which the results of quantitative fish population surveys have been used in advancing management recommendations for increasing fish production. In Nigeria however, fisheries management science is still in its infancy and the management scientist is rendered helpless in the sense that recommendations aimed at improving or protecting the fishery of most reservoirs cannot be implemented on account of the lack of inland fishery policies in both the States, and at the Federal level. However, it is hoped that with the continuous dissemination of information about the need for conservation and management of inland fishery resources, awareness will gradually increase, both at the State and Federal levels, and the need to implement some of the policies advocated by the Institute will become obvious.

4.11.3. CASE STUDIES OF FISHERIES OF SOME RESERVOIRS IN NIGERIA

KAINJI LAKE:

Kainji Lake was impounded in August 1968 creating a water surface area of 1,280 km². Active fishery investigation on the lake started in August 1969 with the assistance of the Food and Agricultural Organization of the United Nations (FAO). There has been an uninterrupted progression from a Project in 1968 to an Institute from 1975 and Lake Kainji has become one of the most intensively studied lakes both in Nigeria and in Africa as a whole as evidenced by the volume of information that has been accumulated, particularly in the fisheries section of the Institute (Ita, 1984b).

Table 13 shows the data on commercial fish landings from the lake in relation to the number of fishing boats in comparison to the catch in experimental gillnet expressed in kg per 1,000 sq. metres of gillnet between 1969 and 1978. Ita (1982) compiled all available evidence to show that Kainji Lake was over-fished. Among the evidence advanced was:

1. The observed decline in the experimental catch per unit effort and the simultaneous decline in commercial landings in spite of the relatively constant number of fishing boats.

Table 12. Artisanal fisheries statistics of some reservoirs in Nigeria.

Fig. 4. Fish landings in relation to catch per 1000m² of graded gillnet fleet.

Table 13. Estimates of commercial landings per boat and yield in kg/1000m² of experimental gillnet in Kainji Lake (1969–1978)

Source: Ita (1982)

2. The decline of catches in larger mesh nets and the consistently higher catches in small mesh nets is shown in Figure 5.

3. The effect of low productivity of the littoral zones:
   The standing crop of fish in the lake estimated between 1975 and 1976 varied from 24 kg/ha to 787 kg/ha within the 21 sampled stations. The mean standing crop for all stations was about 240 kg/ha. It was also concluded that the extensive seasonal drawdown area during the low water season exposed the majority of juvenile fishes, hitherto hidden amidst vegetation along the littoral zone, to intensive exploitation in small mesh nets. The few mature adults that escaped capture during this period succeeded in migrating back to the littoral coves and swamps to breed during the flood season. This natural control helps to sustain the fishery of the lake at a low level of production, thus preventing total depletion of the stock.

4. The effect of high concentration of fishermen on catch per unit effort:
   As a result of unregulated fishing in Kainji Lake, the annual mean catch per boat dropped from 9.4 mt in 1969 to 1.4 mt in 1978. On the other hand, the number of fishing boats increased from 1,800 in 1969 to 3,500 in 1971 and has since fluctuated slightly between 3,500 and 3,300 from 1971 to 1978. In a regulated fishery a lake with a maximum surface area of 1,280 km² would be expected to cater for 1,280 boats or double that number of fishermen. Between 1985 and 1990, the States bordering the lake (Niger, Sokoto and Kwara) have promulgated their fisheries edicts to this effect. These edicts have yet to be enforced, hence the fishery is still largely unregulated.

Fig. 5. The percentage occurrence of fish in nets of different mesh sizes.

Table 14. Observed maximum weight, density and standing crop per hectare (1976) for the major commercial species in Kainji lake, with special reference to gillnet selectivity
(Source: Ita (1984b); SL = standard length)

5. Management strategy:

   It is a known fact that fishery management strategies have to be lake specific. A strategy such as stocking, which could be economically feasible in a small reservoir with low fish production, would be extremely uneconomical in a large reservoir such as Kainji. Moreover, the decline in catch per unit effort has not drastically affected the species diversity in the lake which, under adequate management, could recover its production. Since the decline in catch per unit effort resulted from over-exploitation, the management strategy prescribed for the lake was not hinged around the fish population but rather directed at the control of the fishing population through registration and licensing of fishermen, in order to ultimately control the number of people registered to fish in the lake.

   Mesh size regulation was also advanced as a possible strategy in controlling the catch of juvenile fishes by licenced fishermen. Detailed analysis of mesh size selection of some commercially important species revealed that only 7 out of the 30 most abundant and economically important species could be captured with a 5 cm (2 inch) mesh net after they have attained sexual maturity (Table 14). The remaining 23 species were still immature when captured in 5 cm nets. However, by increasing the mesh size from 5 cm to 7.5 cm (3 inch), about 14 out of the 30 commercially important species could reproduce at least once before being captured. In particular, the forage species dominated by the family Cichlidae (the tilapias) which made up about 60% of the standing crop would reproduce before they were captured.

   All other management techniques (to be discussed later), based on control of fishing, can be applied to Kainji Lake provided they are effectively enforced. The strategies for such enforcement will also be discussed later.

TIGA LAKE (Kano State):

Tiga Lake impounded in 1974, with a surface area of 178 km², was until 1983 the second largest man-made lake in Nigeria. The lake was first surveyed in 1976 (Ita, 1979a) less than two years after impoundment and resurveyed in 1982 (Ita, 1983).

The relative composition by number of fish families in Tiga Lake two years after impoundment was compared with that of Kainji Lake (Table 15). The comparison revealed that the tilapias were less well established in Kainji Lake two years after impoundment than in Tiga Lake after the same period. Standing crop estimates based on a cove rotenone survey revealed a mean standing crop of 118 kg/ha for Tiga Lake in 1976 and 17.6 kg/ha in 1982 (Table 16). There was a significant decrease in littoral standing crop and catch per unit effort in shore and bottom gillnet samples. The surface layers of the open water was consistently low in fish production and yield.

As at 1976, there was no visible threat to the fishery of this reservoir. Only a few fishermen using gourds as floats were observed fishing for subsistence. It was estimated that with a surface area of 178 km² the lake could conveniently cater for about 200 to 300 fishermen. The low fish catch at the surface of the open water was attributed to the absence of forage species, such as the pelagic clupeids, which are observed to school along with carnivores in the open water of Kainji Lake. It was therefore recommended that freshwater clupeids be transplanted from Kainji to Tiga Lake to boost the pelagic production and reduce the concentration of Hydrocynus sp. preying on tilapia along the shore (see below).

During the second survey in 1982 a total of 27 fishing villages were identified along the lake shore. About 286 fishing boats were counted out of which 42 had outboard engines. In addition, about 251 gourds were recorded in 16 villages. Using an average of 2 fishermen per fishing boat an estimated total of over 800 fishermen were recorded on the lake, including those with gourds. This revealed that the lake had become overpopulated with fishermen on account of the absence of controls for licensing and registration.

In an earlier recommendation in 1976 it was stated that with only gillnets, traps and hooks as possible fishing gear, there could be no fear of overfishing provided the minimum mesh size of 76.2 mm (3 inches) was enforced. As of 1990, Kano State still had no fisheries regulations, notwithstanding the fact that the State has the largest surface area of reservoirs in the country estimated at 42,773 ha in 1983 (Ita and Sado, 1984 and Ita and Sado et. al., 1985). The commonest mesh sizes of gillnet and castnets used as at 1982 ranged from half inch to two and a half inch stretched mesh. These were used in exploiting A lestes, Barbus and juveniles of tilapia which were common along the littoral margins of the lake. The unregulated number of fishermen, combined with unregulated and uncontrolled fishing methods, had resulted in the sharp decline in fish production and yield observed in 1982.

Table 15. Percentage number of fish families in gillnet catches at Kainji and Tiga lakes two years after impoundment.

The introduction of freshwater sardines to occupy the pelagic zone of the lake has now been initiated by the National Institute for Freshwater Fisheries Research but it may take some years before the impact of such an introduction will be felt by the local fishermen. However, the need for the traditional method of fishery regulation, through the promulgation and enforcement of fishery edicts, cannot be overlooked if the maximum fish production and yield is to be obtained from this reservoir.

Table 16. The mean standing crop per hectare along the littoral zone of
Tiga Lake and catch/1000 m² of gillnet in 1976 and 1982 (Ita, 1983).

JEBBA LAKE:

Jebba Lake was created in July 1983 by damming the River Niger about 100 km below Kainji Lake at Jebba in the north-west of Nigeria. The lake is about 100 km long and 12 km at its widest point. The maximum depth is 105 m, the mean depth 11 metres and surface area 303 km². A survey embracing both experimental gillnetting and a commercial fishery survey was conducted between October and November, 1983 and later, in September/October 1984, to evaluate the current status of the lake fishery (Ita and Omorinkoba et.al., 1984 and 1985).

In the 1983 experimental survey, a total of 18 fish families and 50 species were recorded in the catch from 15 stations. The family Cichlidae was the most prominent with 27.9% of the total catch (Table 17). Again, unlike Kainji Lake, Jebba Lake showed signs of becoming a tilapia lake barely 5 months after impoundment. A comparison of the relative composition of the fish families in Kainji Lake one and two years after impoundment (1969/70) with Jebba Lake 5 months and one year 5 months after impoundment shows that the families Schilbeidae, Citharinidae and Centropomidae dominated the catch in Kainji Lake soon after impoundment, while the Cichlidae maintained their leading position in Jebba Lake in 1984, followed by the Schilbeidae. The Characidae succeeded in dominating the catch in Kainji Lake two years after impoundment.

The habitat distribution of the fish in Jebba showed a shift in abundance from shore (47.9%) in 1983 to surface (42.2%) in 1984. This was possibly a reflection of the increase in the abundance of the family Schilbeidae, a major pelagic group in the surface of open water.

The relative catch by mesh size showed that 1 inch mesh nets had the highest catch by number both in 1983 and 1984 suggesting that the majority of fish caught were juveniles. There was a decreasing trend in abundance with increasing mesh size although three inch mesh caught the greatest weight of fish in 1983 showing that bigger fish were caught in that mesh. The average mean catch per 1,000 m² of gillnet fleet was 3.0 kg in 1983 and 3.2 kg in 1984. The difference, although slightly positive, is not significant.

A comparison of the relative composition of the catch in commercial landings showed that carnivores topped the list in 1983 with Clarias and Lates comprising about 26.5% by weight of the total fish landed. In 1984 there was a shift in dominance from carnivores to herbivores with Labeo and tilapias dominating both by number and weight. The mean weights of fish caught by fishermen during the two surveys were large which indicates that they were using mostly larger mesh gillnet.

The frame survey statistics for the two surveys revealed that the number of fishing villages and camps had increased from 17 in 1983 to 32 in 1984. Similarly, the number of boats on the lake had also increased from 266 (68 with engines) in 1983 to 369 (91 with engines) in 1984. A total of 551 professional fishermen was counted in 1983 but in 1984 the number had increased to about 1,025 fishermen. This lake, with its surface area of 303 km², could cater for about 300 boats and 600 fishermen. By 1984 the lake had already become overpopulated. The average catch per boat dropped slightly from 8.5 kg in 1983 to 7.6 kg in 1984. Like other newly impounded reservoirs, such as Tiga and Shiroro, Jebba Reservoir had the potential of being a highly productive lake in time if appropriate control measures were taken. Management recommendations prescribed for Kainji, Tiga and Shiroro (see below), all of which are classified as larger reservoirs, also apply to Jebba Reservoir.

The statistics available from the 1983 survey revealed that the lake was not yet overpopulated with fishermen. It was timely then to license and register the appropriate number of fishermen expected to fish in the lake, prior to the influx from other lakes of excess fishermen in 1984. The lake is bordered by Niger State on the eastern shore and by Kwara State on the west but with the transfer of Borgu Local Government to Niger State, both Kainji and Jebba Lakes (formerly shared by Kwara and Niger States) are now controlled by Niger State. At the time of the surveys, neither of these two States had any fishery edict and therefore the lake was open to exploitation by fishermen from different parts of the country, including fishermen previously operating on Kainji Lake.

Table 17. Percentage composition of fish in Jebba Lake by number, in gillnet catches (estimated from three gillnet fleets) and the relative distribution in the shore, surface and bottom samples.

Many States are currently becoming aware of the need for promulgating and enforcing their fisheries edicts in order to attract international assistance in fisheries development. Kwara State has recently promulgated an edict which has yet to be implemented and enforced. Given adequate time, resources and technical knowhow, all the States in the country could eventually have adequately implemented and enforced fishery edicts. The National Institute for Freshwater Fisheries Research, New Bussa, will for its part, attempt to bring about harmonization among the States sharing the resources of most inland lakes and rivers in the country.

SHIRORO RESERVOIR (Niger State):

Shiroro Reservoir was created in May, 1984 by damming the Kaduna River at Shiroro village. The reservoir has an estimated surface area of 312 km² and a mean depth of 22.4 metres. It is now the second largest man-made lake in Nigeria followed by Jebba and Tiga. The Shiroro Hydropower Reservoir, like most other newly created large man-made lakes in Nigeria and throughout the tropics, was expected to provide favourable conditions for large scale fish production and fishery development. In order to substantiate this assumption a fishery survey was carried out in December 1984 and April 1985, about 7 and 11 months respectively after the impoundment of the reservoir and after only the first breeding season. The objective of the survey was to determine the status of the fishery and the level of exploitation by the local fishermen.

Both an experimental survey with a graded fleet of gillnet, together with frame and catch assessment surveys of the commercial fishery were conducted. The results revealed that the family Cichlidae (tilapia) dominated the catch (Table 18), as earlier observed for Jebba Lake one year after impoundment (Ita and Omorinkoba et.al., 1984) showing that in spite of its relatively deep mean depth (22.4 m) compared with Jebba (11.0 m) the lake was likely to become a tilapia lake. However, with Lates niloticus (family Centropomidae) ranking third in terms of abundance (16.1%) in experimental samples and fourth (5.8%) in commercial landings (Tables 18 & 19) it was obvious that the reservoir, with its rocky landscape, was likely to provide ideal habitats for the breeding of Lates.

Tables 18. Relative composition of fish species in gillnet samples in Shiroro Reservoir (April 1985)

Species recorded only in 1 inch (25.4 mm) include:
Alestes nurse, Eutropius niloticus, Labeo parvus and Petrocephalus bene

Table 19. Relative composition of fish landed by commercial (artisanal) fishermen at Shiroro Lake in April 1985.

The mean sizes of fish recorded in both experimental and commercial catches revealed that the majority of fish caught were small, primarily due to the fact that the reservoir was only about one year old. Even within this period the lake was flooded with a total of 1,173 boats out of which 456 were gourds and 104 were abandoned boats while 151 boats were mechanized, showing signs of a very active fishery on the lake. The overall mean catch per boat recorded during the survey was 7.1 kg. This compared favourably with the 8.5 kg/boat recorded for Jebba Lake (Ita and Omorinkoba, et.al. 1984) one year after impoundment.

A preliminary report of the survey was sent to the Niger State Government advising that fishing on this lake should be brought under control by limiting the number of registered boats (canoes) on the lake to 300, together with 600 fishermen inclusive of those using gourds as floating objects in fishing. It was also recommended that a minimum mesh size of 76.2 mm (3 inches) be enforced in order to protect the tilapia population and allow for effective growth of all the fish species before capture. The State Government had earlier promulgated the State Fisheries Edict which has yet to be enforced.

KIRI RESERVOIR (Gongola State):

Kiri Reservoir is situated on the Gongola River about 28 km upstream of the confluence of the Gongola River with the Benue. The reservoir, with a surface area of 115 km², was constructed primarily for irrigation by the Savanna Sugar Company and was being managed by the Upper Benue River Basin Development Authority (UBRBDA). The author became interested in this reservoir when it was discovered, during a visit to Tallum Irrigation Project of the UBRBDA, that the daily fish landings by the staff at the station ranged from 40 – 150 kg per day, mostly tilapia. Such a high catch was comparable only to the early commercial landings from Kainji Lake between 1969 and 1970. It was therefore decided to conduct a preliminary survey of the commercial fishery of the lake with the objective of proposing some management and development strategies to help sustain this high fish catch (Ita and Ismaila et al., 1986).

Frame and catch assessment surveys of the artisanal fisheries were conducted late in December, 1984 and between March and April 1985. A total of 1,101 full-time fishermen were recorded in 14 villages along the lake. Also 407 fishing boats were counted of which 81 had outboard engines.

An estimated total fish landing of 61.7 mt was recorded in December 1984 and about 350.4 mt in March, 1985. March was regarded as the peak of the dry season with the lowest water level which is usually accompanied by the highest fish landings, as observed in other big reservoirs. The mean catch per boat in December 1984 was 4.9 kg while in March 1985 it was 27.7 kg. The total annual fish landing was estimated at 2,473 mt (215 kg/ha), taking into consideration both the good and bad fishing seasons.

Although no experimental survey was conducted, the relative composition of the fish in the commercial landings showed that tilapia was again the dominant fish in the lake in terms of abundance, while Clarias dominated in terms of weight, followed by tilapia. The dominance by weight of carnivores in the commercial landings was attributed to the capture method and was therefore not a true indication of the forage to carnivore ratio in the lake.

With a surface area of 115 km² the reservoir might be expected to cater for only 115 boats and 230 fishermen. At the time of the survey both the numbers of boats and of fishermen were more than four times higher than this. This was also reflected in the yield estimated at 215 kg/ha which was extremely high for an artificial water body. With the high number of boats and fishermen recorded it was predicted that the reservoir would be overfished if adequate control measures were not taken. The State Government was advised to speed up action on the promulgation of the inland fisheries edict for the State in order to empower the Fisheries Department to control the number of boats and fishermen through a licensing system, and to regulate the mesh sizes of nets used for fishing.

IITA RESERVOIR (Oyo State):

The International Institute for Tropical Agriculture (IITA) at Ibadan, in Oyo State has a small irrigation and water supply reservoir with a surface area of about 78 hectares. It was first surveyed in May 1978 at the invitation of the IITA management, to determine its productivity (Ita, 1978b).

Old catch records held by the Ministry of Agriculture and Natural Resources (MANR), Ibadan, revealed that the yield from the reservoir had been sustained at about 7.6 mt (97.5 kg/ha)) with minor fluctuations for the previous 4 years. About 98% by number of the catch during those 4 years comprised mostly Tilapia. Predominantly 89 mm (3.5 inch) and 102 mm (4 inch) mesh gillnet had been used. Other species recorded in the catch between 1976 and 1977 (Moriarty, 1977) included Hepsetus odoe (3 specimens), Clarias lazera (13), Pelmatochromis (Chromidotilapia) guentheri (35) and Hemichromis fasciatus (13). Within the same period about 3,978 specimens of tilapia were captured.

From this record, it was obvious that the mesh sizes of the gillnet used exploited mostly the tilapias, to the exclusion of other species. It was therefore decided to conduct an intensive survey of short duration using diverse gear and multimesh gillnet fleets to update the available information. Other gears used included long line with hooks, wire traps, and a beach seinenet with the intention of establishing their relative effectiveness for the different fish species.

The traditional multimesh gillnet fleet used in sampling at Kainji Lake was used for the survey. This consisted of seven nets each 30 m long and 3 m deep with meshes ranging from 5 cm to 18 cm (2, 2.5, 3, 3.5, 4, 5 and 7 inches stretched mesh). The long-line consisted of 150 hooks (No. 10) at intervals of 1.5 m and baited with assorted baits (e.g. live fish, pieces of dead fish, earthworms and soap). Also 10 bean shaped 1.5 inch mesh wire traps, baited with palm nuts, were set in shallow areas. Beach seine trials were also carried out at a temporarily cleared beach since the lake was not completely cleared of palm tree stumps and the shore line was covered with thick mats of Nymphaea (Water lily). These covered an estimated one third of the surface area of thereservoir thus providing cover for young fish. The reservoir was described as eutrophic, being supplied constantly with nutrients from fertilized farmlands and from treated sewage effluent.

Table 20. Relative composition of fish species in multimesh gillnet fleets at IITA reservoir, Ibadan (Ita, 1978b)

A total of seven species were identified during the survey with tilapias dominating the catch both by relative number and weight (51.9 and 59.3% respectively), followed by Chromidotilapia and Hepsetus in terms of abundance (Table 20). About 83.2% by number and 79.5% by weight of all fish caught at night came from the shore nets set close to marginal vegetation, while 16.8% and 20.5% by number and weight respectively, were recorded in the open water surface set. The mean catch per fleet along the shore was 13.7 kg compared with 10.6 kg at the surface. The overall mean catch per 1,000 km² of gillnet fleet was 20.6 kg.

A comparative evaluation of the efficiency of the assorted fishing gears showed that the multimesh fleet of gillnet was the most effective for capturing all the representative species in the lake in relation to their relative abundance but that Clarias were not well represented in gillnet catches, possibly because in most cases the nets did not touch the bottom of the vegetation fringe which was much deeper than the depth of the net (3m). The maximum depth sampled along the vegetation was 6 metres. Active long-line fishing was recommended for harvesting the Clarias along the vegetated zone. Traps were also effective in capturing most of the representative species with the exception of Clarias. It was concluded that with an increase in the number of traps, the quantity and variety of fish caught could be sustantially increased. It was estimated that a total catch of 20 kg of assorted fish per day could be recorded if about 50 traps were set at strategic positions.

It was observed that beach seining had great potential if more beaches of approximately 100 metres long by 70 metres wide were cleared for seining. In Kainji Lake a mean catch of 10 – 40 kg per haul has been recorded. About double this quantity was estimated for the IITA reservoir.

Comparative production and yield estimates using data for Kainji Lake and catch per unit effort data for IITA Reservoir revealed that the reservoir, with a catch per unit effort of about 21 kg per 1,000 m² of gillnet, was about four times more productive than Kainji Lake. It was estimated that an annual yield of about 22 mt (282 kg/ha/yr) could be sustained if a multimesh fleet, capable of cropping all the representative species in the reservoir, was used. A multimesh fleet comprising 63.5 mm (2.5 inch) mesh for the capture of Hepsetus and Chromidotilapia. 76.2 mm (3 inch) for Chromidotilapia and Clarias and 101.6 mm and 127.0 mm (4 and 5 inches) for the capture of Tilapia and Clarias was recommended, each measuring about 1,000 m² in surface area. An estimated total fish landings of 122 kg/day was predicted for the four major fish (Sarotherodon, Oreochromis, Hepsetus and Chromidotilapia) using a fleet consisting of the four mesh sizes (2.5", 3", 3.5" and 4") each measuring 1,000 m² in surface area with a total area of 4,000 m². The above recommendation was not popular since the staff were not interested in buying Hepsetus and Chromidotilapia.

Stocking: The introduction of new species into the reservoir was recommended provided the species were carefully chosen in order to maintain a balanced population. Swingle (1950) defines a balanced population as one in which the ratio of forage to carnivorous (F/C) species ranges from 1.4 to 10.0. The F/C ratio in the gillnet sample for IITA was 2.8. Swingle (op.cit.) maintains that F/C ratios from 3 – 6 appeared to be the most desirable since he observed that 77% of the best producing populations fell within that range. The low F/C ratio for this reservoir could be explained by the fact that harvesting within the last five years was concentrated mostly on forage species, particularly tilapias. It was suggested that the ratio could be increased by systematic cropping of the carnivorous species. With an increased F/C ratio, Lates niloticus (the sport fish most desired among the staff) could then be introduced to promote sport fishing. Other species recommended for introduction were Heterotis niloticus, a plankton feeder, and common carp, a bottom feeder, or grass carp to control the aquatic vegetation.

Reappraisal Survey (1986):

After the 1978 survey, systematic harvesting continued throughout 1978 and 1979 with regular documentation of species and mesh size statistics. A repeat survey was conducted in August 1986 to evaluate the status of the commercial fisheries in the reservoir (Ita, 1987a) and to update the catch statistics. Information obtained from the IITA Management revealed that Lates niloticus had been introduced into the reservoir in 1980 to promote sport fishing.

The results of the 1986 survey revealed that tilapia, mostly Sarotherodon galilaeus, which dominated the catch in 1978, was still the dominant species in the reservoir. Five new species were added to the checklist of species in the lake while three of the species recorded in 1978, were not captured in 1986 namely Clarias spp. and Parachanna sp. possibly because no long-line was set and, as discussed earlier, suspended gillnet were not very effective at catching Clarias spp.

A drastic decline in the populations of Chromidotilapia (1.0%) and Hepsetus (0.3%) was observed compared with their relative composition in 1978 (25% and 19% by number respectively). The catch per unit effort had also decreased from 20.6 kg/1,000 m² of net fleet in 1978 to 12.6 kg/1,000 m² (in 1986) of the standard gillnet fleet used in the two surveys. The mean weight of Sarotherodon captured had however, increased from 407 g/fish in 1978 to 521 g/fish in 1986. Estimates of the efficiency of the gillnet mesh sizes in the fleet used for sampling the reservoir in 1986 revealed that the highest catches were recorded in 3.5, 4.0 and 5.0 inches.

Catch statistics were documented for 3 months in 1978, 12 months in 1979, 10 months in 1980 and 8 months in 1986. No record was obtained for 1981 – 1985. Estimates based on available records for a 25 days sampling schedule in the month showed that about 19.5 mt of fish could have been landed in 1978, 16.5 mt in 1979, 8.2 mt in 1980 and 13.8 mt in 1986. The estimated landings in 1978 and 1979 (19.5 mt and 16.5 mt respectively) appeared much closer to the predicted estimate of 22 mt than those of 1980 and 1986. This was attributed to the fact that fishing with a multimesh fleet combination as recommended earlier was effected in 1978 and 1979. Subsequently a change in mesh sizes was effected in favour of bigger fishes. Also, the introduction of Lates in 1980 possibly contributed to the elimination of the large populations of Chromidotilapia and Hepsetus.

The factors responsible for the high fish production in the IITA Reservoir were identified as the washing down of inorganic fertilizers from the surrounding farmlands into the lake, coupled with the recycling of sterilized sewage back to the reservoir. This model is worthy of emulation by other agricultural enterprises in Nigeria. The IITA reservoir appears to be the only protected reservoir in the country with regulated fishing and accurate catch statistics records.

BAKOLORI RESERVOIR:

A survey of Bakolori Reservoir was undertaken between April and May 1982 in order to provide background information for prediction of the productivity of Goronyo Reservoir then under construction (Ita and Balogun, 1982 and 1983).

Bakolori Reservoir with a surface area of 80 km² (8,000 ha) was constructed by Impresit Bakolori (Nig.) Limited. Construction started in 1975 and the dam across the River Sokoto was commissioned in 1978. The maximum length of the lake at full storage capacity is 19 km. The 1982 survey period corresponded with the lowest drawdown of the reservoir. The shoreline was completely devoid of vegetation, but covered with decaying trees and shrubs of uncleared bush which rendered gillnetting extremely difficult.

Three stations were blocked and sampled with rotenone. The family Cichlidae, as observed in other reservoirs, (e.g. Tiga, Shiroro, Jebba) dominated the catch (Table 21) with an estimated ichtyomass of 23.2 kg/ha out of a total of 31.5 kg/ha thus making up about 73.6% of the total biomass of fish captured. The overall density of fish per hectare from the three stations sampled was about 3,708 fish/ha.