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.
There is a fundamental conflict between harvesting fish and conserving their biomass. Managers mediate this conflict with regulations that control fishery methods and amounts of harvest. In most recreational fisheries, aside from closed seasons, the precise control of fishing effort is difficult to achieve because fisher entry into a managed area is often unlimited and because effort can be influenced by both direct and indirect factors. Choosing the best fishing regulations is also complicated by a need to jointly regulate and accommodate the desires of different user groups who share the fishery. Regulations may need to account for (1) low‐consumptive uses of fish populations that occur from catch‐and‐release fishing by recreational anglers and/or (2) both tribal subsistence and commercial fishers. We applied a suite of graphical techniques to data on a shared fishery, that for Walleye Sander vitreus in Mille Lacs Lake, Minnesota, to examine the trade‐offs between fishery yield and spawner biomass on a per‐recruit basis across a range of harvest tactics, so that fisheries managers could simultaneously evaluate a variety of regulations. We also used Bluegill Lepomis macrochirus data from several Minnesota lakes to further evaluate the utility of our approach. Some regulations were uniformly better than others because they provided higher yield for a given spawning biomass, and higher spawning biomass for a given yield at equilibrium, under a range of plausible levels of fishing effort. For a given level of fishing effort, we were able to identify a frontier in the yield–biomass space whereby an increase in either the yield or the biomass could only be achieved by a reduction in the other. In the absence of density‐dependent changes to growth, maturity, or mortality, regulations that included a minimum length produced more optimal yield and spawner biomass than those that did not include a minimum length. We reduced the daunting task of choosing from dozens of regulations by considering just a few graphs that best demonstrated the trade‐offs offered by a suite of regulations.Received December 24, 2014; accepted August 21, 2015
Walleye ( Sander vitreus) are an ecologically important species managed for recreational, tribal, and commercial harvest. Walleye prefer cool water and low light conditions, and therefore changing water temperature and clarity potentially impacts walleye habitat and populations across the landscape. Using survey data collected from 1993 to 2018 from 312 lakes in Minnesota, we evaluated the relationship between thermal-optical habitat and the relative abundance of small (0–300 mm), medium (300–450 mm), and large (450 + mm) walleye. Thermal-optical habitat was positively correlated with the relative abundance of small and medium walleye but not large walleye. Walleye were more abundant in larger, naturally reproducing lakes opposed to smaller, stocked lakes. Thermal-optical habitat changed in 59% of lakes since 1980 (26% increasing and 33% decreasing) and appears to be driven primarily by changes in water clarity and thus optical habitat area. Our study provides important insights into local and regional drivers that influence walleye populations that can be used to assist fisheries managers in setting population goals and managing harvest.
The prevalence of catch‐and‐release angling for many species has increased over the past several decades. A potential benefit of catch‐and‐release fisheries is higher catches for anglers due to multiple captures of individual fish within a season. We term the measure of this benefit “recycling rate,” defined as the total catch in a fishing season divided by the number of individuals caught at least once. Multiple‐capture studies are common in the literature, but our recycling rate is a new metric that could be helpful in evaluating recreational fisheries. Estimates of recycling rate will be dependent on several factors, especially the distribution of angling selectivities among individual fish, which is generally unknown. We compared several models that estimated recycling rate based on different assumptions about angling selectivity. Application of the models to Smallmouth Bass Micropterus dolomieu data from Lake Mille Lacs, Minnesota, demonstrated that estimates of recycling rate were robust to assumptions about the distribution of angling selectivity.
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