On the biology of the catfish Clarias senegalensis, in a man‐made lake in the Ghanaian savanna with particular reference to its feeding habits

Abstract

The present study of Clarias senegalensis C. & V., from a small man‐made lake in the coastal savanna of Ghana, was initiated because this species has not been investigated previously and there is a need to have precise information regarding its general biology and food niche both from the theoretical and practical viewpoints. This catfish becomes sexually mature when about 32 cm in length and once a year, after the onset of the major rains in April, or early May, they ascend the feeder stream to spawn in its flood zone. Spent fish soon return to the lake after spawning but smaller immature fish do not do so until September. As with other piscivorous fish the feeding intensity is low and with one exception the stomachs of the monthly samples were on the average less than half full. The suggestion of a slight seasonal increase in the amount of food consumed from August onwards appears to be correlated with a seasonal increase in standing crop and is followed later by an increase in condition factor. C. senegalensis is extremely euryphagous but it subsists mainly on organisms swimming in midwater including fish such as Tilapia, zooplankton and insects, those living on the surface of the sediment and small organisms swimming near the surface of the sediment. Animals from these habitats have a higher forage ratio than those living under stones, in the sediment or in shallow water or in vegetation close to the shore. Vegetation, adventitious food and detritus are of little consequence to C. senegalensis in Nungua lake and it is suggested that the latter is not exploited because the sediments are poor in organic matter. The euryphagous habits of C. senegalensis are shared by other Clarias species and this is reflected in several anatomical adaptations concerned with feeding. Seasonal changes in the dietaries are very slight because the catfish do not exploit phytophilous species to any great extent even when they become abundant during the wet season. For this reason it is doubtful whether they are of value in controlling important vectors of diseases many of which are phytophilous. Although there is a suggestion that larger fish eat more vegetation, phytophilous species and adventitious food the evidence for ontogenetic changes in the deitaries are very slight and fish in all the size groups eat minute food organisms such as copepods and also large ones such as fish including Tilapia. The possibility of combining Clarias with Tilapia in fish ponds is, therefore, worthy of consideration. The factors which cause expansion and contraction of food niches are discussed.