Asset protection in juvenile salmon: how adding biological realism changes a dynamic foraging model

The "asset-protection principle" created by Clark is based on a dynamic programming model and states that individuals should (1) become more averse to predation risk as they accumulate fitness assets but (2) generally be more willing to accept predation risk later in the foraging season. To test whether these predictions hold under biologically meaningful foraging parameters, I constructed a dynamic model of the optimal trade-off between foraging and predator avoidance in juvenile salmon. The model incorporates temperature and body-size dependent bio-energetic constraints typical for juvenile fish, which grow by orders of magnitude over a season. In its simplest form using seasonally constant growth potential and a linear over-winter survival function, my results equal those of Clark's model. Adding a fitness function and environmental data from field studies accentuates the asset-protection effect and fundamentally changes the seasonal pattern of optimal effort. Simulation of typical poor feeding conditions in mid-summer yields the prediction of increased foraging in the spring in anticipation of worsening conditions. Increasing overall predation risk results in smaller fish at the end of the season with a trade-off between summer and winter survival. The model generates testable predictions for juvenile salmon and provides insights for other organisms (particularly poikilotherms) that are subject to size-dependent or seasonally changing foraging dynamics. Copyright 2002.