Anchovy foraging: simulating spatial and temporal match/mismatches with zooplankton

Abstract

Prey abundance is only a useful correlate of fish growth rate if it is considered within a match/mismatch hypothesis framework. We developed a model to explore the extent to which the growth rates of juvenile anchovies (Engraulis capensis) are influenced by temporal and spatial match/mismatches with the copepod Calanoides carinatus, a major component of their diet. The distributions of copepods and anchovies are dynamic, so that predation by anchovy may dramatically impact on its prey's spatial distribution. Predation rates by resident fish were commensurate with the average and maximum specific copepod production rates at fish densities of approximately 1 and 3.5 t·km-2, respectively. The spatial arrangement of fish biomass and the temporal overlap between fish and copepod abundances substantially modified both anchovy and copepod growth estimates. Predicted optimal shoal sizes suggest that additional factors such as predator avoidance or the energetics of maintaining school structure may be important. The model emulated observed variability in anchovy growth rates and confirmed the prediction that the availability of high-sustained abundances of food along the West Coast may be a critical "bottleneck" contributing to the strength of recruitment to the South African pelagic purse-seine fishery.