Gill net catchability of walleye (Sander vitreus): Are provincial standards suitable for estimating adult density outside the region?

Abstract

Standard gill netting protocols are increasingly used to assess freshwater fish populations. Understanding the catchability (q) of fish following these protocols enables direct estimation of density, which has advantages over relative abundance from analytical and applied research perspectives. However, catchability is complex. The application of q estimated in one region may be inappropriate for another if physical and biological processes driving catchability differ. Cross-region assessments of q are needed to assess the applicability of estimates over a broader geographic and environmental range. In this study, we evaluated whether the global estimate of q (1.044 ha/gang; variance = 0.2268) derived by Giacomini et al. (2020) for walleye Sander vitreus ≥ 350 mm total length (TL) was suitable for application in two Colorado, USA reservoirs supporting recreational fisheries and wild spawn operations. We followed provincial standards in Ontario and Quebec, Canada (i.e., Fall Walleye Index Netting) combined with other corrections for size-dependent catchability to estimate the density of mature, adult walleye ≥ 470 mm TL for comparison to independent estimates from mark-recapture analyses. Following similar methodologies, we found that the global estimate of q combined with size-dependent gill net retention coefficients underestimated the density of adults by nearly 80%, but uncertainty in point estimates can be high. Results highlighted the need to better understand sources of uncertainty, including factors influencing size-dependence in gill net encounter when extending estimates of q to other size classes of fish, systems, and regions. Estimates of catchability for large-bodied adult walleye in our systems were consistent, but relatively low, ranging from 0.203 ha/gang (95% confidence limits = 0.133–0.298) to 0.227 ha/gang (0.091–0.556). We expand the geographic and environmental range of paired mark-recapture and gill net Catch Per Unit Effort information and discuss potential factors contributing variability to estimates of catchability for adult walleye.