Empirical Evidence for Depensation in Freshwater Fisheries

Inland fisheries face increasing threats to their sustainability. Despite speculation that depensation may exacerbate the effects of stressors on population resiliency, depensation has not been empirically explored in freshwater fisheries. Declining productivity of Walleye Sander vitreus populations in northern Wisconsin foreshadows an underlying change in naturally reproduced juvenile Walleye survival. We used long-term stock and recruitment data from lakes in the Ceded Territory of Wisconsin to quantify density-dependent trends in juvenile Walleye survival and tested for the prevalence of depensation using the q parameter of Liermann and Hilborn (1997). Of 82 Walleye populations evaluated, about half exhibited depensatory recruitment. An analysis of the global q for all populations examined suggested that the posterior probability of depensatory dynamics was about 0.89. In addition, there were few clear cases of compensation-most populations exhibited weak density dependence. The general lack of strong compensatory recruitment across Walleye populations could leave these stocks vulnerable to stressors and unresponsive to rehabilitation. We present multiple lines of evidence to suggest that depensation is a plausible phenomenon explaining declines in Walleye populations in the Ceded Territory of Wisconsin and may be implicated in other invisible collapses of freshwater fisheries.