How Body Size and Food Availability Influence First-Winter Growth and Survival of a Stocked Piscivore

Abstract

The first winter of life can play an important role in the success of age-0 fishes. First-winter survival is often size dependent, with larger fish exhibiting higher survival than small fish. Cohorts of age-0 saugeye female Walleye Sander vitreus × male Sauger S. canadensis stocked into Ohio reservoirs exhibit overwinter shifts toward larger body sizes; however, it is unclear whether growth, size-dependent mortality, or size-dependent emigration underlie this phenomenon. Saugeye may experience low prey availability during the overwinter period, making them especially vulnerable to starvation. Furthermore, survivors emerging from winter in poor energetic condition may experience reduced spring foraging success and growth. We used a combination of overwinter PIT tag studies in the field and overwinter outdoor pool experiments to understand these direct and indirect effects of winter on survival and size distributions of cohorts of saugeye. Using PIT tags to allow us to track growth of individuals in reservoirs, we found that saugeye of all sizes increased in length over winter, there was no evidence of size-dependent overwinter mortality, and rates of emigration out of the reservoir were greater for large saugeye than small saugeye. Thus, only growth rate, and not mortality or emigration biased toward small fish, can explain the observed overwinter shift in size distributions. In pool experiments, we found no direct effects of winter on survival, even in the complete absence of food, and no negative consequences of starvation over the winter on the ability of saugeye to resume feeding in the spring. Our results suggest that shifts in size distributions of first-year cohorts over winter are driven by growth rather than mortality. Neither direct effects of first winter on survival nor indirect effects mediated through effects of starvation on future foraging ability are important in recruitment success of saugeye in Ohio reservoirs.

Received March 18, 2014; accepted June 30, 2014

ACKNOWLEDGMENTS

We thank E. Williams, B. Gunther, and M. Bahler for their tireless efforts in the field and laboratory. Thanks are also extended ODNR-DOW Districts 1 and 3 and the Inland Fish Research Unit for their support in the field. Funding for this research was provided by ODNR-DOW Federal Aid in Sport Fish Restoration Project F-69-P, Fish Management in Ohio.