Aspects of the reproductive biology and fishery of the blue marbled parrot fish leptoscarus vaigiensis (quoy and gaimardi, 1824) in Kenya shallow inshore waters

Abstract

The reproductive biology, growth parameters, mortalities and exploitation rate of Leptoscarus vaigiensis were investigated in a shallow inshore lagoon in the south coast of Kenya. The overall male to female sex ratio was 1:1.4. Statistical analysis by Chi-square showed that there is no significant different from the 1:1 ratio (P> 0.05). The monthly male to female sex ratio did not show any consistent trend that could have been related to the reproductive cycle. Oocyte- size-frequency distribution coupled with histological analysis of the ovary indicated unimodal distribution of oocytes in stages 1,2,3 and 6. A bimodal distribution of oocytes is evident in ovaries at maturity stages 4 and 5. The bimodal distribution included one mode of mature oocytes and the other mode made of immature oocytes. These modes were not completely separated from the immature stock, and some intermediate oocytes were evident between. the two modes. However, these oocytes contained cytoplasmic vacuoles meaning that such oocyte will mature and will be spawned together with the large sized ones. There is no progression of the mature oocytes in ovaries in stage 6 of sexual maturity. At this stage, the ovaries contained only immature stock of oocytes, with few degenerating oocytes. It was be inferred that an individual fish probably spawns only once during the year ofstudy. However, the GSI and Kn calculated for the fish population, showed that th~r.e were two peak spawning times during the year of study, one between March and June"with a peak in April, and the other between November and December. These months fall within the two intermonsoon periods that are experienced in the East African region. There is therefore a possibility that there are two breeding populations of L. vaigiensis at the area of study with each having peak spawning at a specific intermonsoon period. In the present study, the potential fecundity of L. vaigiensis ranged between 186,000 to 1,806,000 eggs. A mean potential fecundity of 674,000 eggs per female was determined. Fecundity was significantly related to body weight and body length. The fish became sexually mature at 16.8 cm, total length. The length-weight relationships calculated for males and females were not significantly different (ANeOV A: F = 14.9; P = 0.68) and a single regression was calculated which adequately described the length-weight relationship of this species. The relationship obtained was Logn, W = 2.86 LoglO L - 1.595. Growth and mortalities were determined from length frequency data. The growth constants were estimated as K = 1.5 per year, Loc = 28.9 cm and <1>1 = 3.10. Natural mortality was estimated from Pauly's empirical formula (Pauly 1980) which was found to. be 2.30. Total (Z) and fishing mortality (F) were estimated as 3.52 and 1.15 respectively. The estimated exploitation rate (F/Z) estimated to be 0.33 is below the optimum exploitation rate of 0.5. This could be an indication that the stock is not optimally exploited, though growth overfishing was deduced to be taking place since the recruitment into the fishery occurs when the fish is only 11.7 ern (total length) and the fish seems to mature at 16.] cm (total length).