Season- and size-dependent risk taking in juvenile coho salmon: experimental evaluation of asset protection

Using juvenile coho salmon, Oncorhynchus kisutch, we tested predictions ari sing from dynamic optimization models of foraging under predation risk. Coh o juveniles from two size groups raised in the laboratory were individually fed varying food rations. Their willingness to risk Predation was measured as the time to resume foraging after presentation of a predator model. Sma ll fish (mean weight 1.5 g) resumed feeding earlier than larger fish (3.5 g ) as predicted by dynamic models under summer photoperiod but not under aut umn photoperiod. Contrary to predictions, larger fish did not increase risk taking and small fish decreased risk taking between summer and autumn trea tments. Food ration significantly influenced time to resume feeding only in small coho. A simple mechanistic model we proposed to explain feeding moti vation under risk as a function of body size and prior growth rate was not sufficient to explain observed variation in risk taking. This study suggest s that coho salmon use photoperiod and their own body size as cues for long -term, state-dependent adjustments of feeding behaviour. The lower risk tak ing of larger fish is probably an example of asset protection, whereby larg er animals accept less predation risk to protect their greater accumulated fitness value. The decrease of risk taking in small fish in the autumn was possibly caused by a switch of life history trajectory towards delayed smel ting. (C) 1999 The Association for the Study of Animal Behaviour.