Abstract:
The African catfish Clarias gariepinus is a non-native fish species to Lake Naivasha. The timing and method of its introduction into the Lake is not clearly known, but it is thought that it probably came through the inflowing rivers; Malewa, Gilgil and Karati. The biology and ecology of catfish in Lake Naivasha is poorly known. This lack of knowledge makes it difficult to estimate the trophic level, reproductive capacity and ecological role of the species in the Lake. Therefore, this study aimed to assess the feeding habit, fecundity and biology of C. gariepinus. The study was conducted between November 2015 and January 2016 in four stations. The feeding habit was assessed by analysing the stomach contents of 400 fish using the point method. The major food items consumed were zooplankton 24.6%, fish 23.1% and insects 21.4%. Ontogenic shift in the diet was evident where juvenile fish fed on high proportion of phytoplankton, insects and detritus while adults fed more on cray fish and zooplankton. Food selection using Strauss linear index revealed positive prey selectivity for Bacillariophyceae, Chlorophyceae and Cyanophyceae and negative selection for Desmidiaceae, Dinophyceae and Euglenophyceae. The sex ratio was significantly different from the hypothetical ratio of 1:1 (χ2=77.28, df = 5, P < 0.05). Length-Weight relationship of males and females of C. gariepinus among all sampling stations was not significantly different (t=0.373, df = 6, P > 0.05) from the isometric exponent value of 3. Relative condition factor showed that C. gariepinus in Lake Naivasha are in a good condition with a value of 1 except at Crescent Island station with a value of 0.95. Length at first maturity (Lm50) of females and males was 18.9 cm and 42 cm respectively. Fecundity ranged from 1,260 to 354,361 eggs. There was significant variation in fecundity between different size classes (one way ANOVA, F = 9.13, df = 2, P < 0.05). The relationship between fecundity with total length and total weight were curvilinear while the relationship of fecundity with ovary weight was linear. A significant correlation was observed between fish condition factor and pH (r = 0.83, P < 0.05) and dissolved oxygen with fish length-weight relationship (r = 0.87, P < 0.05). The relationships between temperature and conductivity with condition factor and length-weight relationship were not significant (P > 0.05). This study found out that C. gariepinus is a generalist and has an opportunistic feeding behaviour ingesting a variety of food organisms ranging from phytoplankton to fish. In addition, the biology of C. gariepinus is affected by key water quality parameters such as pH and dissolved oxygen in Lake Naivasha.
