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Some management aspects of pre-recruitment ecology of the freshwater sardine Limnothrissa miodon in Lake Kariba Mtsambiwa, Morris Zororai


The early life history of the freshwater sardine Limnothrissa miodon (Boulenger, 1906) was investigated with the view of establishing the factors behind recruitment variability in the Lake Kariba sardine fishery. Environmental factors behind recruitment variability were investigated through the examination of otolith microstructure from field captured sardine larvae samples while historical length-frequency data were analysed to establish size-at-recruitment. The larval ecology study established that food was the only possible limiting factor as far as growth and survival are concerned. Growth was described by the von Bertalanffy Growth Function while instantaneous mortality rates were obtained from catch curve analysis and both estimates were observed to fluctuate from month to month. The presence of larvae in the littoral zone through the sampling period implied continuous recruitment while fluctuation in larval catches was assumed to indicate that recruitment variability was probably established at an earlier life history stage. Response of larvae to light during capture has serious management implications to the fishery in that fishing in shallow areas would result in increased larval mortality which could be detrimental to recruitment. The occurrence of larvae in most of the shoreline sampled implies that Limnothrissa miodon in Lake Kariba utilizes most of the shoreline as nursery grounds. From analysis of combined historical commercial data collected from both Zambia and Zimbabwe from 1982 to 1992, it was observed that the sardine recruited to the fishery at 41 mm in length and that based on regression analysis, the mean size of fish in the fishery has decreased by 7% and 4% in Zambia and Zimbabwe respectively. This reduction in size however, does not pose a threat to the fishery since at the current mean size in the catch is greater than the recorded size-at-maturity. The fish therefore have a chances of spawning at least once before being harvested. It was further established that due to migration to the pelagic zone as the fish grew larger, the current minimum mesh size of 8 mm was appropriate provided fishing was restricted to areas deeper than 15 m where the adult population occurs.

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