Selective Harvest Effects on Body Size and Productivity of Exploited Walleye Stocks in North Temperate Lakes

Long-term overharvest and environmental change can depress fish productivity and lead to a collapse of stocks. In particular, selective harvest of large individuals in recreational fisheries may induce changes in individual-level traits (e.g., size-at-maturity) and in turn reduce population sustainability. We assessed region-wide trends in body size and productivity of >350 walleye (Sander vitreus) stocks in Wisconsin lakes over more than two decades using a multilevel mixed effects model with harvest rates and other lake-level covariates. Our analyses showed that body size of younger walleye has gradually increased in the past decade, whereas body size of older fish has declined. These changes in body size were negatively correlated with harvest rates and population density, but positively correlated with growing degree days. Declining stock size in many of these lakes over the past decade may have relieved density-dependent effects on growth of younger fish, whereas continued size-selective harvest may have prevented older fish from reaching their growth potential. Furthermore, region-wide trends showed that biomass and production rates have declined at faster rate than population size. These findings suggest that the current harvest rate set for these stocks may be unsustainable at some lakes.