Migration strategies of young fishes under temporal constraints: the effect of size-dependent overwinter mortality

Habitat selection by fishes involves balancing the benefits of foraging opp ortunities and the risk of predation. Size-dependent overwinter mortality i mposes a time constraint on behavioral programs of juvenile fishes: individ uals must achieve a sufficient size by the end of the growing season to sur vive. I developed a dynamic optimization model to evaluate how severity of size-dependent overwinter mortality alters habitat use by minnows. The litt oral zone was the safest habitat with respect to predation. In lakes where zooplankton exhibit diel vertical migration, fish migrate to the pelagic at night and return to the littoral during the day to maximize foraging rates . Model results demonstrated that size-dependent overwinter mortality shoul d lead to development of more pronounced predation risk-taking by fishes in lakes with severe winters. Individuals are predicted to initiate diel litt oral-pelagic migrations at smaller sizes as winter severity increases or as hatch date within a season increases. This analysis suggests that the time constraint imposed by size-selective overwinter mortality has important co nsequences for life history strategies of juvenile fishes. The effects of t ime constraints on behavior are expected to be especially important near th e high latitudinal limits of a species' geographic distribution.