Big and brave: size selection affects foraging under risk of predation in juvenile rainbow trout, Oncorhynchus mykiss

Abstract

Abstract. In this study, the hypothesis that selection for large body size under relaxed predation pressure will increase the tendency of juvenile rainbow trout to forage under threat of predation was tested. In eight replicate groups of juvenile hatchery trout, the behaviour of the six largest (by weight) and the six smallest juveniles was compared. Based on a series of feeding competition trials, large trout had significantly higher mean dominance ranks than small trout. In a second experiment the willingness to forage under exposure to a live predator (an adult rainbow trout) was compared between the two size classes of trout. Larger trout were significantly more willing to risk exposure to a live predatory rainbow trout in order to get access to food than were small trout. However, similar-sized dominant and subordinate trout were equally willing to forage under threat of predation. Finally, to determine whether large and small juveniles were taking equivalent risk, their susceptibility to the predator was compared. Standardized encounters between the predator and individual juveniles revealed no significant difference in predation susceptibility between large and small trout. It is suggested that selection for large body size under reduced predation pressure will shift the optimal trade-off between energetic gain and mortality risk in the favour of 'high gain-high risk' phenotypes. This may cause directional selection on the genetic basis of feeding behaviour.