Changes in population characteristics and diet of Nile tilapia Oreochromis niloticus (L.) from Nyanza Gulf of Lake Victoria, Kenya: what are the management options?

Abstract

Length frequency data collected from 1998 to 2001 from commercial landings was used to estimate asymptotic length (L∞), growth coefficient (K), mortality (Z, F, M), growth performance index (φ ) and exploitation rate (E) of Oreochromis niloticus from the Nyanza Gulf of Lake Victoria, Kenya. Studies on the diet of O. niloticus collected by demersal trawl and seining between 1998 and 2000 were also conducted. Length frequency data were analyzed using the FISAT software (an FAO-ICLARM Stock Assessment Tool package). The L∞ had a mean value (±S.D) of 58.78 ± 2.42 cm TL, K of 0.59 ± 0.05 yr−1, Z of 2.16 ± 0.40 yr−1, M of 1.00 ± 0.06 yr−1, F of 1.12 ± 0.34 yr−1, E of 0.48 ± 0.11 and φ of 3.31 ± 0.04. Length at first entry into the fishery (L50) was observed at 26.18 ± 12.50 cm TL. Recruitment occurred throughout the year, with two peaks corresponding with the rainy seasons. A comparison with previous studies in the gulf indicates that O. niloticus is now caught at a smaller mean size, whereas K, Z, and M have increased. Fish appeared to become sexually mature at a smaller size. Nile tilapia originally known to be herbivorous, feeding mostly on algae, has diversified its diet to include insects, fish, algae and plant materials, all being important food items. The changes in population characteristics and diet of O. niloticus are discussed in context of changes occurring in the lake and its surroundings.