INFLUENCE OF SAMPLE SIZE AND SAMPLING FREQUENCY ON THE QUANTITATIVE DIETARY DESCRIPTIONS OF A PREDATORY FISH IN THE BENGUELA ECOSYSTEM

Abstract

Spatial and temporal variation in the diet and feeding intensity of snoek, Thyrsites atun, a top predator of the southern Benguela, was investigated to provide information on data requirements for accuracy of annual diets as inputs to ecosystem models. Appropriate sample sizes to produce accurate daily or event-scale diet descriptions were investigated by means of a posteori tests. Cumulative prey diversity curves showed that 55 (± 25) stomachs containing food are required to accurately quantify presence and absence data of prey species. Differences in the percentage contribution of dominant prey in consecutive and cumulative 10-stomach classes, indicates that a minimum of 75–80 (± 25) stomachs containing food are necessary to describe the proportion by weight of primary prey. Diets of snoek (proportions by weight) sampled inshore (shallower than 50 m and within 25 km of the coast) off the Cape Peninsula during six consecutive weeks (i.e. one day per week) in autumn 2001 were highly variable. Comparing diet for the six-week period with that of diets from consecutive 10-sample weekly units revealed 95% similarity at 70 samples per week. Seasonal diets between Cape Columbine and Cape Hangklip also varied, but there was no evidence of a predictable seasonal pattern in diet that could be related to prey life history; two-way nested ANOSIM revealed that seasonal prey proportions across years were statistically less similar than those within years. Snoek spawn offshore in winter/spring. Statistical differences between inshore (<50m) and offshore (>150m) prey composition were largely influenced by the absence of anchovy Engraulis encrasicolus and much larger proportions of Cape hake Merluccius spp. and lanternfish Lampanyctodes hectoris in the offshore diet. Feeding intensity (in terms of proportions of fish with prey and mean stomach fullness) was strongly seasonal and highest during the spawning season. Sampling programmes for the southern Benguela should account for spatial and temporal variation in diet and feeding intensity of predators if accurate annual dietary descriptions are to be achieved. The results of this study indicate that a minimum of 70 snoek stomachs (containing food) should be collected at each sampling event.

Afr. J. mar. Sci. 26: 205–217