**The title, authors, and abstract for this completion report are provided below.† For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**
A Decision Analysis for Multispecies Harvest Management of Lake Huron Commercial Fisheries
Quantitative Fisheries Center
Department of Fisheries & Wildlife
Michigan State University
East Lansing, MI, 48824-1222
Tradeoffs between achieving desired yield objectives for lake whitefish (Coregonus clupeaformis) and restoration objectives for lake trout (Salvelinus namaycush) were assessed for harvest policies affecting coldwater commercial fisheries in Lake Huron. Lake whitefish are targeted in the majority of commercial fisheries operating in Lake Huron, but these fisheries also capture lake trout as bycatch. Lake trout were nearly extirpated from Lake Huron by the 1950s, and substantial stocking efforts have been underway for decades to aid in recovery. Ongoing or expanded harvest of lake whitefish may negatively affect rehabilitation efforts for lake trout. Additionally, Lake Huron has undergone substantial changes to its food web in the last two decades. Dreissenid mussels and round gobies (Neogobius melanostomus) have invaded and thrived in Lake Huron. Abundance of several prey fish species has declined as has abundance of Diporeia, a primary food source for lake whitefish. These ecosystem changes may also affect tradeoffs for coldwater commercial fishing policy in Lake Huron. To assess these tradeoffs, we developed a food-web model (Ecopath with Ecosim: EwE) for the coldwater community in the main basin of Lake Huron and used this model to compare harvest policies and evaluate the importance of key system uncertainties to policy rankings. We engaged Lake Huron fishery stakeholders in two workshops to help guide model development. Obtaining a balanced EwE model, and appropriately including invasive species in the dynamic simulations both proved difficult, and prompted additional simulation studies. We found that in general, dynamic simulations in Ecosim are not highly sensitive to ad hoc balancing adjustments, but that sensitivity increases as the strength of trophic interaction among groups increases. We compared four methods for incorporating invasive species into the EwE model and concluded that initializing invasive species biomasses before actual invasion at very low biomasses, and maintaining them at low levels by imposing an ad hoc mortality until the time of invasion was reasonably good at reproducing observed time series of all groups. The completed EwE model was used to simulate changes to fishing mortality targets, to the season in which fishing occurred, and to the type of gear used. Conversions of gill nets to trap nets resulted in the maintenance of lake whitefish harvest and 15% increases in lake trout biomass over the status quo policy. Changes in fishing seasons varied among policies, but resulted in at most a 14% increase in lake trout biomass, and a 39% increase in lake whitefish harvest. Changing fishing mortality targets revealed the expected tradeoffs between lake whitefish harvest and lake trout biomass. In general, changes in harvest were greater than changes in biomass as fishing mortality targets changed, suggesting increases in harvest could be achieved without large decreases in biomass, but raising questions about the modelís representation of compensatory processes for both species. Our assessment of the significance of uncertainties about future environmental productivity, diet, and strength of trophic interactions revealed that the first of these had the greatest effect on model outcomes, but did not alter the relative performance of the policies. The other two uncertainties had lesser effects than changes in productivity, and influenced lake whitefish harvest and biomass much more than that of lake trout. We found little evidence for substantial indirect interactions between lake trout and lake whitefish, leading us to conclude that the commercial fishery is the primary factor that links these two groups. Future work on balancing tradeoffs in the commercial fisheries should therefore focus on direct interactions with the fishery (i.e. bycatch reduction), rather than on indirect interactions through the food web.