Body size and vigilance mediate asymmetric interference competition for food in fish larvae

Interference competition for food is difficult to measure because feeding rates altered by behavioral interactions of competitors are confounded by exploitative competition, the depletion of food over time. We quantified the magnitude of interference competition between individuals using a combination of experiments and computer simulations, which allowed us to remove the effect of exploitative competition. We used this approach to test the hypothesis that interference competition reduces feeding rates of young red drum (Sciaenops ocellatus) and that the magnitude of interference competition is related to phenotypic traits, such as the sizes of competitors and behavioral interactions between them. In laboratory experiments, we measured prey consumption and behavioral interactions between 2 fish of different sizes (7- to 25-mm standard length). We used computer simulations to remove the effects of exploitative competition and found that prey consumed by focal individuals decreased in the presence of competitors due to interference competition and that the decrease was stronger when focal individuals were smaller than competitors and when the competing individuals were generally larger. We also found greater impairment of prey consumption by focal individuals when they were more vigilant (more reactive) to the presence of competitors. A better understanding of the magnitude of asymmetry in interference competition has important implications for understanding population dynamics, spatial distribution, and reproductive strategies of many organisms. This is the first study to quantify the asymmetry in interference competition when prey density changes over time.