Empirical Evidence for Depensation in Freshwater Fisheries

Abstract

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.

Plasticity in Abundance and Demographic Responses of Walleye to Elevated Exploitation in a North Temperate Lake

“…For Sherman Lake, compensatory age-0 to age-1 survival and a significant decline in length at maturity appears to have resulted in a relatively stable adult stock size with correspondingly low and variable age-0 recruitment. Sherman Lake exhibits strong resilience to exploitation in its stock-recruitment relationship and does not show strong support for depensatory recruitment dynamics (Tsehaye et al 2016;Sass et al 2021). Clearly, the weak adult abundance response to elevated exploitation did not negatively influence recruitment by reducing stock size below a critical depensatory threshold, which has also been observed in other Walleye exploitation studies .…”

“…For example, 10 years of 35% annual exploitation on Big Crooked Lake, Wisconsin, significantly reduced the total adult Walleye density from about WALLEYE RESPONSES TO ELEVATED EXPLOITATION 541 16 to 10 fish/ha, with total adult density being reduced to about 7 fish/ha during the later stages of the study . According to Tsehaye et al (2016) and Sass et al (2021), Sherman Lake and Big Crooked Lake were highly resilient to elevated-exploitation rates given their strong, compensatory stock-recruitment relationships and lack of depensatory recruitment dynamics. Nevertheless, other Walleye populations in the CTWI may not be as resilient to elevated exploitation and many show depensatory recruitment dynamics (Rypel et al 2015(Rypel et al , 2018Embke et al 2019;Sass et al 2021).…”

“…Except for juveniles, growth responses were less evident than in previous exploitation studies, suggesting that the magnitude of adult density reduction, lake productivity, diversity of the fish community, and whether fish allocate surplus energy to reproduction or growth may be important and likely differs among lakes and Walleye populations (Walters et al 2000;Sass and Kitchell 2005;Schmalz et al 2011;. If elevated exploitation is considered as a management strategy to improve individual growth rates in Walleye populations, we recommend that managers consider the magnitude of adult abundance reduction needed, within-lake productivity and fish community characteristics, length at maturity implications and associated surplus energy allocation, and whether the population exhibits depensatory recruitment dynamics (Walters et al 2000;Sass and Kitchell 2005;Sass et al 2021). It is also noteworthy to consider that elevated exploitation did not improve population size structure or the number of large fish in the Sherman Lake population.…”

Unveiling the recovery dynamics of walleye after the invisible collapse

“…While it is possible that Walleye were headed toward extinction due to recruitment failures that occurred at low SSB in at least some lakes (i.e., depensatory stock-recruitment; Walters and Kitchell 2001;Sass et al 2021), assessments showed that many of these populations were "rescued" by strong recruitment events that occurred in the late 1990s to early 2000s and before SSB could have recovered following fishing mortality reductions in 1996. This finding raises the important possibility that at least some Walleye populations have always been sustained by occasional episodic recruitment events, and persisted under high fishing pressure because favorable recruitment events occurred even though population age structures and egg production were severely impacted by high fishing rates (see Sullivan 2003;Sullivan 2004;this study).…”

Slow social change: Implications for open access recreational fisheries

“…Second, depensatory decreases in per capita recruitment at low abundance can lock a population into a cycle of decline once abundance has been reduced below a critical threshold. Density‐dependent Allee effects or cultivation–depensation food web dynamics can cause these depensatory responses (Kramer et al., 2009; Sass et al., 2021; Walters & Kitchell, 2001).…”