RELATIONSHIPS BETWEEN PRODUCTION AND CONSUMPTION OF PREY SPECIES BY RESIDENT FISH IN THE BOT, A COOL TEMPERATE SOUTH-AFRICAN ESTUARY

Data on the biomass, diets, feeding cycles and consumption rates of the resident fish of the Bot estuary are presented, together with figures for the biomass and production of invertebrates. These data are used to quantify the impact of the fish on their prey populations and to examine how the fish apportion their food resource. Calculations of total daily consumption by the six resident predatory fish species ranged from 3·48 to 5·66% body mass day-1 and their combined annual consumption was 5·99 g m-2. The total standing stock of potential prey was 7·1 g m-2 and total annual production was calculated was 35 g m-2. Consumption by the fish therefore accounted for 17% of secondary production, or 30% of the production by prey species, suggesting that predation by fish was not likely to be having a major impact on the prey resource and that food was not in short supply. A few species do, however, have a substantial proportion (>80%) of their production consumed by fish. Most of the fish consumed a wide variety of prey but there was considerable subdivision of diet with no two species sharing their most important prey category. Dietary niche width for the different species was significantly correlated with their abundance but not with their average overlap with the other species, implying that the most successful species are those with the broadest niches and that specialization is a dubious means of reducing competition. © 1990.