Kainji Lake is a reservoir formed by the closure of the Niger River at the Kainji Dam in 1968, Figures 1 and 2. Descriptions of the river and the lake are given by Motwani, (FAO, 1970a), Turner (FAO, 1970b), Lelek (FAO, 1972) and Henderson (in press). Some details of the lake morphometric, hydrologic and physical-chemical characters are noted below:
| Latitude: 9°50 to 11°00 N | ||
| At full supply level: | ||
| Elevation: 465 feet above sea level | ||
| Area: approx. 1 270 km2 | ||
| Volume: | approx. 14 km3 | |
| Length: | 136 km | |
| Depth: | maximum 60 m mean 11 m | |
| Annual drawdown: 10 m | ||
| Ratio annual outflow: volume is 4:1 | ||
| Water fluctuation regime: | ||
| Full: November–March | ||
| Drawdown: | April–August | |
| Fillings: | September–October | |
| Water temperature: | ||
| December–January isothermal at 22–25°C | ||
| February–May stratification with deepening thermocline 25–29°C | ||
| June stratification destroyed by drawdown | ||
| July–November cooling (rain), 27–28°C | ||
| Notes Top metre of lake sometimes 32–37° C | ||
| Secchi disc transparency: | ||
| Maximum: | 3 m May–June | |
| Minimum: | 0.1 m September | |
| Total dissolved solids: | ||
| estimated 35–40 mg/1 | ||
Fish populations in the lake have been studied by catches of gillnet fleets; shoreline sampling by electrofishing, castnetting, seining, and rotenone poisoning; pelagic fish sampling by light attraction; and commercial catch assessment samples. Since these studies began in 1969 the results (FAO, 1970b, FAO, 1972, Ita, 1972) show that the fish populations have been in a state of flux and may not stabilize for some time in the future. One of the recent developments is an increase in the Tilapia population (mainly T. galilaea) which now contribute substantially to the commercial catch.
The fish which have provided material for this report were captured during 1969 to early 1973, by means of experimental gillnets, castnets, electrofishing. and dipnets (for locations see Figure 2). Gillnets included mesh sizes of 1½, 2, 2½, 3, 3½, 4, 5, 6 and 7-inch stretched mesh, set overnight at both surface and bottom. Gillnet stations were located throughout the lake but most Tilapia were caught only in the shallow sets. Castnet catches in nets of 1½,2y2,3 and 5-inch stretched mesh by both experimental fishing crews (of professional fishermen) and commercial fishermen were examined from September 1972 to February 1973. Dipnet samples of Tilapia fry were made with a mosquito-mesh hand net in shallow water in November and February 1973 at five locations around the southern half of the lake. Electrofishing samples along short stretches of shoreline at various stations were made between October 1970 and January 1973, with a Smith-Root Electrofisher Type VI.
The gillnet and castnet samples of fish were measured in fresh condition, and all measurements for the relationship of length: weight and standard length: fork length are from these samples. Measurements of electrofishing and dipnet samples were made on material preserved in 10 percent formalin.
