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 opportunities and the risk of predation. Size-dependent overwinter mortality imposes a time constraint on behavioral programs of juvenile fishes: individuals must achieve a sufficient size by the end of the growing season to survive. I developed a dynamic optimization model to evaluate how severity of size-dependent overwinter mortality alters habitat use by minnows. The littoral 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 should lead to development of more pronounced predation risk-taking by fishes in lakes with severe winters. Individuals are predicted to initiate diel littoral-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 consequences for life history strategies of juvenile fishes. The effects of time constraints on behavior are expected to be especially important near the high latitudinal limits of a species' geographic distribution.