Visual Prey Detection Responses of Piscivorous Trout and Salmon: Effects of Light,Turbidity,and Prey Size

Abstract

AbstractVisual foraging models provide a useful framework for predicting distribution,foraging success,and predation risk in pelagic communities; however,the visual prey detection capabilities of different predator species within and among taxonomic groups have not been sufficiently evaluated. Our primary objective was to more adequately characterize variation in the reaction distances of piscivorous salmonids by evaluating important anadromous taxa. We measured reaction distances of yearling Chinook Salmon Oncorhynchus tshawytscha and adult Coastal Cutthroat Trout O. clarkii clarkii to fish prey over a range of prey sizes and ecologically relevant light and turbidity levels. Reaction distances of Coastal Cutthroat Trout increased rapidly with increasing light intensity (lx) and attained an average maximum of 187.1 cm above a light threshold of 18.0 lx. Reaction distances of Chinook Salmon increased at a slower rate to a maximum of 122.1 cm above a light threshold of 24.9 lx,declined exponentially with turbidity beyond a threshold of 1.65 NTU,and declined for prey sizes less than 50 mm FL. Reaction distances of Coastal Cutthroat Trout were consistently higher than those of Chinook Salmon across all light levels; this difference could not be attributed to the greater FLs of the Coastal Cutthroat Trout. Results from this and previous studies show that the functional form of reaction distance is similar across piscivorous salmonid species and life stages,but the magnitude of the response can vary considerably. Therefore,to adequately predict the strength of predation effects in pelagic communities,species‐ and life‐stage‐specific responses must be considered.