The four species of Tilapia recorded in the lake are all widely distributed around the entire shore length, although there are major differences in relative abundance among species. The Warra region with its widespread area of emergent grass is notably rich in T. galilaea. and the intensive castnet fishery here harvests large numbers. The Yelwa and Papiri areas also support major castnet fisheries which harvest Tilapia among other species. Host of the shallow grassy bays harbour good populations of T. galilaea with lesser amounts of T. nilotica and T. zillii. Lelek (FAO, 1972) notes that Cichlids (including Tilapia) were abundant on shorelines with aquatic vegetation, but were scarce or absent where little vegetation occurred.
Abundant samples of fry (T. galilaea with fewer T. nilotica) were obtained by dipnet near Shagunu; at a village on the north shore of the Swashi Bay mouth; at Warra beach, at Monai beach; and near the Kainji Dam looks. The favoured habitat has a gently-sloping (less than 5 percent) mud and organic debris bottom of 1 to 6 inches deep, protected on the lakeward side by emergent weeds. Here the fry receive maximum protection from aquatic predators, the benefit of the warmest water, and nearby weeds in which to hide from non-aquatic predators.
When the survey fishermen were asked to collect fry, they generally went to an area such as described, near a village where women were washing clothes at the beach. Presumably nutrients in the detergent used for washing encouraged the growth of food upon which the fry feed, or perhaps the agitation of the water by humans made food easy to collect.
Only T. galilaea and T. nilotica were found in this habitat, never accompanied by T. zillii or t. dageti in The periods of dipnet sampling (November and February).
Both T .galilaea and T. nilotica are known to be mouthbrooders. Direct evidence of the reproductive season by observations of fish with eggs or fry in their mouths has not been obtained in the lake, although many hundreds of fish (particularly T. galilaea) haw ben examined by the Shagunu research. workers and others. Banks, Holden, and McConnell (1966) report the capture of two T. galilaea carrying eggs in the river near Shagunu before the lake formation, but give no date. In a nearby drainage (the lower Yobe River flowing into Lake Chad) Tobor (1970) reports that large female T. galilaea captured in late February were carrying eggs in their mouths.
Hatching of T. galilaea takes place in the mouth and the young are retained to a length of 12 mm T.L. (9-T0 mm S.L.)at the age of 10–15 days, depending on the temperature (Fryer and Iles, 1972). T. nilotica releases fry at about the same size. Indirect evidence of spawning is available from the length-frequency distribution of fry collected by electro fishing and dipnets, Tables 4 and 5.
Fry of T. galilaea of a size indicating release from the parental mouth within a week or two (up to 14 mm S.L.), were found in catches each month from August to March, Table 4. The only month samples when only larger fry were found was June. The actual spawning would have taken place a month previous to the catches. More intensive sampling might well have shown some reproduction through the year; the present data suggest greatest spawning activity in October, November and February, the period of filling and full supply level of the lake. Spawning of T. galilaea here seems related to rising water levels and recently-flooded shorelines. This agrees with Lelek's (FAO, 1972) observation that female ovaries of this species were ripening from August to January and February coinciding with a rise in water level until the months of high water level.
Indirect evidence or reproduction for T. nilotica (Table 5) suggests recent spawning in September and November. The difficulty of separating the smaller fry of these from T. galilaea may have masked better evidence of reproduction of T. nilotica. This problem merits further study utilizing the criterion of spine counts to differentiate between very small T. nilotica and T. galilaea.
Turner (FAO, 1970b) records that of 138 Hydrocynus spp. stomachs which contained food, in only four were Tilapia identified (species not given). Tilapia were not identified in stomach examinations of Chrysichthys spp., Schiibe mystus, Eutropius niloticus. Synodontis membranaceous or Lates niloticus.
Mr. N. Willoughby (personal communication) while studying Syndontis spp. in Kainji Lake, reports that these are basically detritus feeders and not serious predators on Tilapia. On rare occasions S. schall and S. ocellifer were found to have eaten Tilapia fry about 13 mm in length. Mr. R.Sellick (personal communication) while working on Alestes spp. here found no predation on Tilapia fry. Dr. D.S. Lewis (personal communication) notes that most predaceous fishes in the lake occasionally eat Tilapia up to 150 ran, but that Tilapia are not major prey species for piscivores, at least after the fingerling stage. The expert has seen two T. galilaea in apparently healthy condition but with the caudal peduncle missing, and received reports of others. This is attributed by the fishermen to attacks by Hydrocynus brevis.
An observation of piscivorous feeding by T. galilaea was made at the village of Faku (1/4 mile downstream from Kainji Dam) on 8 December 1972. Two boys were angling with small hooks baited with Sierrathrissa leonensis 30–35 mm long, successfully catching (at the time of visit) 29 T. galilaea ranging in length from 120–200 mm S.L. Faku is the centre of a thriving ‘atalla’ net fishery which captures small clupeids, and many of these are washed out of the fishermen canoes each day. The small T. galilaea were conditioned to eating these, and were seen doing so as one canoe was washed out. While T. galilaea is essentially a planktonophage, these fish (in common with many other species) are opportunistic feeders in specialized situations. This observation suggests the possibility of developing a sport fishery for T. galilaea in Kainji Lake.
