Analysis of Growth and Mortality from daily Growth Increments in the Otoliths of Dagaa (Rastrineobola Argentea) in Nyanza Gulf, Lake Victoria, Kenya

Abstract

Pnor to 1960, Rastrineobola argentea was of little economic importance in terms of catches in Lake Victoria. Catches have increased in the last 15-20 years and it is now become the second most important commercially targeted fish species. Growth and mortality parameters of R. argentea, a tropical cypnnid, were estimated using growth increments in otoliths and length-frequency analysis to gather more biological data on the species. The Gompertz growth curve yielded the best fit for the juvenile population. Growth and population parameters for the commercial catch show a growth rate coefficient (K) of 1.8 yr'1 with L*. of 5.0 cm standard length (SL) in Nyanza Gulf and K of 1.5 yr'1 with L„ of 6.5 cm SL in the open waters site. Instantaneous growth rates decreased with age, with fish from open waters showing a more gradual decline. The weight (g)-length (mm) relationship is W = 5.562L,J. Fish immersed in 600 mg/L of tetracycline hydrochloride showed its incorporation within 12 hours and increments were likely formed daily. Juvenile fish mortality ranged from 11.3 to 29.9 yr'1. Total mortality (Z) for adults estimated from length-converted catch curves was 4.0 and 4.8 yr'1 for Nyanza Gulf and the open waters respectively. Fishing mortality (F) estimated from catch and biomass was 0.98 yr'1 for Nyanza Gulf, while that of open waters from length-converted catch curve was 1.4 yr'V The exploitation rate is 0.25 and 0.29 for Nyanza Gulf and open waters respectively. Two annual breeding peaks were observed in both Nyanza Gulf (May/ October) and open waters (May/November). Length and age at recruitment for L50% for Nyanza Gulf was 11.6 mm and 34.5 mm in open waters, corresponding to age of 44 and 175 days respectively In comparison with published data on the growth and mortality of some small pelagics of African waters, R. argentea had low values of L,,, K, M and Z.