SPATIAL – TEMPORAL DISTRIBUTION AND ABUNDANCE OF LARVAL STAGES OF NILE PERCH (Lates niloticus LINNAEUS 1758) IN LAKE VICTORIA, KENYA

Abstract

Lake Victoria is a relatively dynamic ecosystem with constant changes in both biotic and abiotic variables primarily influenced by human activities. Studies in the ecology and fisheries in Lake Victoria has mostly concentrated in the adult stages of Lates niloticus, the most important species in the lake. This studies have mostly attributed population decline to overfishing and pollution effects. There is little information in the contribution of larval dynamics and ecology in the fishery of the lake. This study therefore aimed to bridge this gap by providing data in the spatial – temporal distribution and abundance of larval stages of Nile perch (Lates niloticus Linnaeus 1758) in Lake Victoria, Kenya. Nile perch Larvae were sampled from 2011-2013 at 29 stations in the Winam Gulf of the Lake by Tows using a combination of plankton net mesh sizes. Juveniles(less than 10cm) were sampled in 2011-2013 by trawling with cod end covered with 3 mm mosquito net and gillnetting using 25 mm meshed nets. Larval abundance ranges from a low of 1 to a high of 600 with distribution in sheltered bays and river mouth stations. Peak larval abundance occurred in the month of February, August, September, November and December indicating spawning period for L. niloticus that coincided with full moon lunar phases. GLM analysis indicated the distribution of larvae was influenced significantly by water turbidity. Other water quality parameters had no significant effect on larval abundance. Spatially Nile perch distribution showed small sized fishes (< 5 cm) were abundant in inshore stations than open lake stations. The study generated VBGF for Nile perch in the Winam Gulf as L∞ = 169 cm, with an exploitation rate (E) of 0.57 and mortality coefficient(Z) of 0.96 yr-1). Results of food analysis indicated the fish to feed proportionately higher on Caridina nilotica. The ontogenetic shift in diet occurred at three different levels, larval stages fed purely on zooplankton, juveniles shifted to Caridina and thereafter fish dominated diets. Management measures based on the findings of this study include protection of the 200 m from the shore through legal prohibition of fishing and any other activity in the lake, further studies of near shore hydro-dynamics are required to provide insights on factors influencing the abundance and distribution of larvae and juveniles of Nile perch in the lake and the extent and nature of diel vertical migration to determine whether it is related to predator avoidance, feeding or energetics as has been observed for zooplankton.