Abstract:
Lake Naivasha fishery is economically important to the communities living around the lake and nationally. The communities derive economic benefits directly and indirectly through fish sales and utilization of fish for food as a source of protein. The fishery is primarily based on exotic species, supported by six main species, namely; Blue spotted tilapia (Oreochromis leucostictus), Nile tilapia (Oreochromis niloticus), Red-bellied tilapia (Tilapia zilli), Largemouth bass (Micropterus salmoides), Common carp (Cyprinus carpio) and African sharp tooth catfish (Clarias gariepinus). Among the commercial fisheries, O. leucostictus is the most desired but less abundant compared to C. carpio, the most abundant fish in the lake. O. leucostictus was introduced into the lake in 1956 from Lake Victoria and quickly established itself while C. carpio was accidentally introduced in 1997. However, since the introduction of C. carpio, the populations of O. leucostictus have been on the decline due to reasons that are not yet well understood. The aim of this study was to assess the feeding and reproductive ecology of O. leucostictus and C. carpio in Lake Naivasha. Fish samples were collected monthly using gill nets (35 mm to 70 mm mesh size) between July and December 2013. Stomach contents of all the sampled fish were analysed using point method to determine the type of food items consumed. Detritus contributed 50% of the diet of O. leucostictus and 63% of the diet of C. carpio. The second most dominant food item for both O. leucostictus and C. carpio was algae contributing
35% and 30% respectively. This study demonstrates that both O. leucostictus and C. carpio in Lake Naivasha are predominantly detritus feeders. The ratio of males to females of O. leucostictus and C. carpio was tested using the Chi-square test. The ratio was 1.51:1 and 1.28:1 respectively. There was no significant difference between the sex ratios (P > 0.05) in the two species. There was suppressed fecundity in O. leucostictus probably caused by destruction of the spawning grounds through rooting and digging action of C. carpio. Length-weight relationships for males and females of both fish species showed that all the samples had negative allometric growth with the allometric coefficient value (b) being <3. A comparison of the two fish species showed C. carpio had a health condition factor of 1.51 while O. leucostictus had 1.32, indicating good healthy conditions. Therefore, it can be concluded that the feeding and reproductive habits of C. carpio has disrupted the natural environmental conditions of Lake Naivasha causing a decline in the numbers of O. leucostictus through alteration of its feeding and reproductive strategies.
