Aspects of the upwelling regime between Cape Point and Cape Agulhas,South Africa |
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| Abstract: | A progressive increase in mean ovary mass (standardized by the cube offish length) over time was observed in specified length classes of pilchard Sardinops ocellatus sampled from purse-seine catches off the west coast of South Africa. This, in earlier investigations, was considered to have been a density-dependent response following the collapse of the stock in the mid 1960s, and it appeared to result mainly from the increasing frequency of occurrence of pilchard entering the batch-spawning cycle, particularly among fish of 16–20 cm Lc . The possibility that the observed trend was an artifact of changing distribution of fishing or shifting seasonality of spawning relative to the three-month period of sample collection was rejected following analysis of variance on spatially disaggregated data and inspection of seasonal patterns in ovary mass. Other causes of the increase in mean ovary mass could have been density-dependence, declining age structure, changes in the environment and selection for early maturity under prolonged high rates of mortality. The data are critically examined to evaluate the likelihood of each of these explanations. |
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| Keywords: | behaviour breeding grounds group composition South Atlantic |
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