Visualizing Trade-Offs between Yield and Spawners per Recruit as an Aid to Decision Making

Abstract

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

ACKNOWLEDGMENTS

We thank Michael H. Prager, the late Ransom A. Myers, and the members of the MNDNR and Great Lakes Indian Fish and Wildlife Commission 1837 Fisheries Committee for their helpful suggestions. We also thank Charles Anderson, John Hoxmeier, Neil Kmiecik, Don Pereira, Dan Daugherty (AFS editor), the anonymous AFS associate editor, and three anonymous reviewers for providing insightful comments on previous drafts. We thank Mike McInerny and Cindy Tomcko for their assistance with the Bluegill data. The study was funded in part by the Federal Aid in Sport Fish Restoration (Dingell–Johnson) Act. This is Virginia Institute of Marine Science Contribution 3519.