The Reproductive biology of the thumbprint emperor, lethrinus harak (forsskal 1775) from the marine waters of Kenya with special emphasis on its fecundity estimates

Abstract

The reproductive biology of the Thumbprint Emperor, Lethrinus harak (Forsskal 1775) was studied between April 95 - March 96 off the Kenya coast. The emphasis of the research was to compare its fecundity estimates Using volumetric and stereologica1 methods. Sampling was done twice a month during spring tides at Gazi and Msambweni areas south of Mombasa. Beach seine nets of variable mesh sizes (bag 28mm Wings 60mm) were used to catch fish. A total of 827 fishes was collected of which 386 were males, 426 were females and sex of the remaining 15 was not identified. The fish caught were measured for total length, standard length and weighed for their total body weight. .They were then dissected and each gonad assigned to a maturity stage by visual inspection. For females, these maturity stages were later validated by histological examination of the internal organization of the ovary and by taking oocyte diameter measurements. The relationship between total length and total body weight for males, females and the unidentified fishes was determined separately. As the student's t-tests showed no significant

differences in the regression coefficients b of these groups, the groups were thus pooled and a common regression line, Log10 W = 2.97 Log10 L - 1.79 determined. The overall sex ratio did not depart from the expected ratio of one male to one female. But when data were stratified by size, chisquare tests showed that males were predominant in the smaller size classes while females predominated in the larger size classes. Length at which 50% of males and females attain sexual maturity was found to be 24.5 cm and 26.1 cm total length respectively. L. harak populations at the Kenya coast has a prolonged spawning season extending from October to April with possibly two peaks during October and again in February based on the cyclical changes in the gonadosornatic index and the relative condition factor; and the occurrence of high percentages of running and spent fishes in the catches during these months. Intra ovarian oocyte diameter frequency showed that L. harak has a group synchronous oocyte development where there IS a unimodal distribution in stage 1, 2, 3 and 6 ovaries and bimodal

one in stage 4 and 5 ovaries. In the later case, there is no distinct size separation between the resting and the developing oocytes suggesting that these oocytes are released In batches over a long period. Comparative fecundity estimates made using volumetric and stereo logical methods showed that volumetric estimates were 23% lower than stereolgical ones. A mean fecundity of 476,000 oocytes per female was determined. High positive correlation was found to exist between fecundity and ovary weight.