Water clarity and temperature effects on walleye safe harvest: an empirical test of the safe operating space concept

Abstract

Successful management of natural resources requires local action that adapts to larger‐scale environmental changes in order to maintain populations within the safe operating space (SOS) of acceptable conditions. Here, we identify the boundaries of the SOS for a managed freshwater fishery in the first empirical test of the SOS concept applied to management of harvested resources. Walleye (Sander vitreus) are popular sport fish with declining populations in many North American lakes, and understanding the causes of and responding to these changes is a high priority for fisheries management. We evaluated the role of changing water clarity and temperature in the decline of a high‐profile walleye population in Mille Lacs, Minnesota, USA, and estimated safe harvest under changing conditions from 1987 to 2017. Thermal–optical habitat area (TOHA)—the proportion of lake area in which the optimal thermal and optical conditions for walleye overlap—was estimated using a thermodynamic simulation model of daily water temperatures and light conditions. We then used a SOS model to analyze how walleye carrying capacity and safe harvest relate to walleye thermal–optical habitat. Thermal–optical habitat area varied annually and declined over time due to increased water clarity, and maximum safe harvest estimated by the SOS model varied by nearly an order of magnitude. Maximum safe harvest levels of walleye declined with declining TOHA. Walleye harvest exceeded safe harvest estimated by the SOS model in 16 out of the 30 yr of our dataset, and walleye abundance declined following 14 of those years, suggesting that walleye harvest should be managed to accommodate changing habitat conditions. By quantifying harvest trade‐offs associated with loss of walleye habitat, this study provides a framework for managing walleye in the context of ecosystem change.

Species-specific preferences drive the differential effects of lake factors on fish production

“…This result is not surprising because walleye have evolved subretinal tapetum lucidum that allow them to feed, grow, and reproduce at low light levels (Ryder 1977;Lester et al 2004). Other studies have found that walleye population biomass is greater in dark lakes (Lester et al 2004;Hansen et al 2019).…”

Using Radiotelemetry to Evaluate Poststocking Survival and Behavior of Large Fingerling Walleye in Three Iowa, USA, Lakes

“…), and habitat loss (Hansen et al. ). Regardless of the mechanism, the lack of Walleye recruitment is concerning, as it creates problems for sustainable Walleye fisheries.…”

“…Fish populations are largely regulated through recruitment patterns (Ricker 1975); however, Walleye recruitment is declining in some locations (Hansen et al 2017;Rypel et al 2018), whereas Walleye cannot naturally reproduce or their recruitment is limited in other locations (Mitzner 2002;Reed and Staples 2017;Koch et al 2018). Although the reasons for poor recruitment are not clear, potential mechanisms that have been hypothesized include competition or predation (Fayram et al 2005;Fielder et al 2007), climate variability (Beard et al 2003;Hansen et al 2017), and habitat loss (Hansen et al 2019). Regardless of the mechanism, the lack of Walleye recruitment is concerning, as it creates problems for sustainable Walleye fisheries.…”

Frontiers in modelling social–ecological dynamics of recreational fisheries: A review and synthesis

“…A. Hansen et al, 2019). Yet these sorts of dynamics received very little attention in the studies that we reviewed.…”