Influence of Body Size and Prey Type on the Willingness of Age-0 Fish to Forage under Predation Risk

Abstract

Ontogenetic diet shifts have been shown to be the outcome of changes either in vulnerability to predators or foraging efficiency for different prey types. More information is needed on how changes in vulnerability and prey value with forager size interact to influence the responses to predation risk. Piscivorous fishes are a significant source of predation risk for many other fish species, but as young of the year (age 0) they themselves are vulnerable to predators. We experimentally compared the foraging behavior of three vulnerable size-classes of largemouth bass Micropterus salmoides with respect to zooplankton, chironomid larvae, and bluegills Lepomis macrochirus when a larger predator was visible and when none were present. The size-classes of age-0 largemouth bass represented a documented ontogenetic diet progression from zooplankton to benthic invertebrates to fish. The size of the largemouth bass influenced their consumption rate and amount of foraging activity in the presence of a predator, but prey type did not. All sizes of largemouth bass reduced their foraging time and total consumption when a larger predator was visible except for age-0 largemouth bass that are typically piscivorous in the wild. Predation risk may affect the ontogeny of piscivory by decreasing the feeding rate on all prey types, changes in vulnerability with size having more influence on risk-taking than foraging efficiency for different prey types.