DIET AND SOME BIOLOGICAL ASPECTS OF SILVER CYPRINID, Rastrineobola argentea (Pellegrin, 1904) IN LAKE VICTORIA, KENYA.

Abstract

Studies on the diet and some biological aspects of Rastrineobola argentea were conducted between August 2014 and March 2015 in the Kenyan waters of Lake Victoria. Stomach contents of 1154 specimens collected from commercial fishers and experimental seining were quantitatively analysed. Juveniles of R. argentea under 30 mm SL fed almost exclusively on zooplanktons, while adult fish larger than 40 mm SL preferred insects. Changes in diet require morphological and physiological changes which are not yet well developed in young fish, thus, they can easily digest zooplankton that also satisfy their demand for protein. Diel feeding regime suggested that R. argentea is a visual feeder. There was spatial variation in the diet with rotifers, copepods and insects dominating the diet of R. argentea at Dunga, Ngegu and Wichlum stations. The change in diet among stations could be attributed to environmental changes shaping the zooplankton community in Lake Victoria. Results for length-weight relationship revealed that R. argentea experienced positive allometric growth pattern (b>3). The regression slope b was significantly different between the sexes (ANCOVA: F=18.13, p=0.001) and within the stations (ANCOVA: F=10.23, p=0.032). The highest value of b was recorded in male fish from Dunga (3.40) and the lowest in female fish from Ngegu (2.92). The fish population was dominated by females with an overall male to female ratio of 1.00:1.38. The highest length at 50% maturity (LM50) was recorded in male fish from Honge (39.8 mm SL) and the lowest in female fish from Ngegu (35.7 mm SL). The mean (±SE) condition values recorded were 1.096±0.01 in males and 1.097±0.01 in females with range values of 0.78-1.59 and 0.77-1.57 respectively. The condition factor did not differ significantly between males and females (Kruskal Wallis test; H = 0.03, p=0.86). The mean (±SE) Gonadal Somatic Index (GSI) recorded were 1.08±0.04 (males) and 6.02±0.14 (females) with range values of 0.02-3.38 and 0.34-17.39 in males and females respectively. Rastrineobola argentea showed peak condition and GSI in September/February and October/March for open waters and Nyanza gulf respectively. The following recommendations were outlined from this study: sampling for diet study during the day when the fish feeds, further study on change in diet with season, not assume isometric growth for this fish and to revise the mesh-size for the fishery due to the small size at maturity.