Estimation of the annual consumption of food by Cape hake Merluccius capensis and M. paradoxus off the South African west coast |
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| Abstract: | Information on hake stomach contents collected during research cruises off the west coast of South Africa between 1988 and 1990 is analysed. Estimates of the annual consumption and daily ration of the Cape hakes Merluccius capensis and M. paradoxus are obtained. Coefficients of variation (CVs) for these estimates are estimated by a bootstrap variance-estimation procedure. Cape hakes are opportunistic feeders. Juveniles feed mainly on crustaceans and the diet becomes increasingly piscivorous with age. Anchovy were the dominant dietary item of juvenile M. capensis, although this may be a reflection of the fact that anchovy were readily available at the time of the surveys. The large number of anchovy found in the diet during this study highlights the dangers of extrapolating consumption estimates for opportunistic feeders to other time periods. Pooling of data across geographic and seasonal strata introduces substantial bias in estimates of consumption and daily ration for only the pelagic prey species. Estimates of annual consumption are highly sensitive to the cruise selected to provide the estimates of numbers-at-length, because these latter estimates vary substantially between cruises. Furthermore, estimates of annual consumption and daily ration by prey species are very imprecise due to the effects of small sample sizes, the opportunistic nature of and natural variability associated with feeding. This implies that it is unlikely that model-estimation procedures which utilize these data will be able to provide particularly precise predictions. Therefore, before any larger-scale stomach collection exercise is undertaken, it is advisable to perform simulation studies to assess the sampling intensity required to achieve the desired levels of accuracy and precision for predictions from multispecies model-estimation procedures which make use of such data. |
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