Lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush) are important species to commercial fisheries in Lake Huron. Lake trout dominated harvest until their near extirpation in the 1950s, and lake whitefish are currently the primary commercially harvested species in the lake. Lake trout are currently stocked in the lake with the goal of re-establishing self-sustaining populations. Fishery management objectives include continued harvest of lake whitefish and restoration of lake trout. Bycatch of lake trout in the commercial lake whitefish fishery creates potential tradeoffs for managers in reaching both objectives. Changes in the food web of Lake Huron may influence interactions between lake trout, lake whitefish, and the lake whitefish fishery in unforeseen ways. The research described herein assesses tradeoffs in objectives for lake whitefish harvest and lake trout restoration in Lake Huron by simulating harvest policies for the commercial lake whitefish fishery using the food-web modeling software Ecopath with Ecosim. Chapter 1 provides background information on biological changes within Lake Huron's food web as well as on Ecopath with Ecosim, Chapters 2 and 3 describe questions encountered while constructing the model, Chapter 4 describes the results of policy simulations, and Chapter 5 provides overall conclusions. Two critical questions arose during construction of the food-web model: 1) what is the effect of adjusting for imbalances in data inputs on simulation results, and 2) how to include invasive species in dynamic model simulations. I examined the effect on simulation results in Ecosim resulting from different approaches of adjusting data inputs to meet the requirements of mass balance in Ecopath (Chapter 2). I found that reasonable changes in data inputs had less effect on simulation results than did changes in vulnerability parameters in Ecosim, and that the effect was smaller when the food web had changed substantially. I compared four methods for including species invasion in Ecosim models (Chapter 3). I found that all four methods allowed the model to reasonably reproduce observed biomass patterns; however they differed in terms of their complexity to implement. Harvest policies were simulated after the questions in balancing and species invasion were addressed (Chapter 4). As expected, I found that, among the policies I considered, policies where bycatch rates of lake trout were reduced in the lake whitefish fishery performed best at simultaneously meeting management objectives for both species. Indirect interactions between lake trout and lake whitefish were minimal; the two species interacted primarily via harvest in the lake whitefish fishery. Future levels of environmental productivity substantially influenced both the magnitude of expected biomass and harvest of lake trout and lake whitefish, as well as the relative performance of management options. Furthermore, the simulation results were sensitive to alternative assumptions about the strength of trophic interactions between predators and their prey (vulnerabilities). In summary, although environmental productivity and vulnerability parameters influenced the forecasted performance of management options, policies where lake trout bycatch rates were reduced invariably performed best. Under original best-fit values for vulnerabilities, indirect interactions between lake trout and lake whitefish were minimal and the two species remained linked primarily by interactions within the fishery, however under alternative and plausible values of vulnerabilities the magnitude of indirect interactions increased.
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