Influences of Stock Intermixing and Productivity on Harvest Policy Performance: a Simulation-Based Evaluation of Lake Whitefish Management in the Upper Great Lakes

The influence of stock intermixing on harvest policy performance is a major concern to fisheries managers because of the potential to overharvest low productivity stocks. If productivity of mixing stocks is not equal, harvest of fish originating in more productive stocks can overshadow declining harvest attributable to less productive stocks, leading to unnoticed depletion of less productive stocks. These “hidden” collapses could lead to sub-optimal fishery outcomes and loss of genetic diversity. Recent research in the Great Lakes suggests that intermixing of distinct lake whitefish (Coregonus clupeaformis) spawning stocks during commercial fishing seasons has increased in recent decades, raising concerns regarding sustainability of some of these stocks. Management agencies in the Great Lakes have already developed plans to move toward managing some intermixing stocks based on pooled assessments, but insufficient research has been conducted to support this action. Our analysis used a management strategy evaluation approach to explore how the current 65% constant-mortality harvest control rule might affect stock sustainability and fishery yield under differing levels of mixing and stock productivity. We also investigated several alternative harvest control rules to compare the existing control rule to more conservative alternatives. For this research, we developed a stochastic simulation model based loosely on lake whitefish stocks in northern lakes Huron and Michigan that incorporated all aspects of the management system (population dynamics, data collection, stock assessment, control rule implementation). Harvest policy performance was evaluated based on mean commercial fishery yield, inter-annual variation in commercial fishery yield, and proportion of years spawning stock biomass declined to less than 20% that of unfished spawning stock biomass (a proxy for risk). Under these performance metrics, desirable attributes of a harvest policy are high and stable yields and low risk of depleting stock biomass. Our analysis suggests that low productivity stocks generally exhibit low average yields and greater risk under the existing control rule when compared to intermediate and high productivity stocks. We also found that patterns of intermixing influenced the performance of control rules for stocks with differed productivity characteristics. Management that accounts for both productivity and rate of intermixing has a better chance of providing desirable fishery outcomes.