Efficacy of the Gull Island Shoal Lake Trout Refuge in Lake Superior
Akins, Andrea L.
University of Wisconsin-Stevens Point, College of Natural Resources
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Historically, Lake Superior supported one of the largest and most diverse lake trout fisheries in the Laurentian Great Lakes. The Apostle Islands region is unique in Lake Superior, with a diversity of shoals that each supported unique spawning stocks of lake trout. Lake trout stocks collapsed throughout Lake Superior because of excessive fishery exploitation and sea lamprey predation, so stocking, sea lamprey control, and fishery regulations were enacted to support stock restoration. A refuge was established around Gull Island Shoal to enable recovery of the lake trout stock that spawned on this historically important spawning shoal in the Apostle Islands region. In the early 2000s, evidence suggested that stocks in the Apostle Islands Region were rehabilitated and near carrying capacity. My objective was to determine if future sustainability of lake trout stocks will depend on the Gull Island Shoal refuge. I constructed an age-structured simulation model to assess the effect of excluding and including the refuge, as a harvest management tool, on sustainability. The model was used to estimate median abundance, probability of collapse, and time to extinction. Median abundance was estimated for age 4, age 4-and-older, and age 8-and-older lake trout, and probability of collapse was estimated for age 4-and-older and age 8-and-older lake trout. Harvest was simulated as a range of commercial fishing mortality rates, while holding recreational mortality constant, a range of recreational mortality rates, while holding commercial mortality constant, and a range of equal commercial and recreational mortality rates. Natural mortality was modeled as a fixed-base rate and a randomly varying sea lamprey mortality rate. Recruit abundance was modeled as a density-dependent function of adult lake trout abundance. Random process error was incorporated into sea lamprey mortality and recruitment sub-models to simulate model uncertainty. Parameter uncertainty was incorporated into the recruitment sub-model. Combinations of fishing mortality rates were varied under different movement rates to assess effectiveness of the refuge on lake trout sustainability. In general, median abundance of age 4, age 4-and-older, and age 8-and-older collapsed at lower instantaneous fishing mortality rates when the refuge was abandoned. With the refuge in place, the fishing mortality rate that resulted in collapse depended on the rate of movement into and out of the refuge. When movement was low, too many fish stayed in the refuge, and conversely, when movement was high, too many fish became vulnerable to fishing, so the refuge was ineffective at both low and high rates of movement. With the refuge in place and movement into and out of the refuge, extinction did not occur at any level of simulated fishing mortality. In contrast, when the refuge was removed, extinction occurred for all combinations of commercial and recreational fishing mortality. In conclusion, the lake trout population in Eastern Wisconsin waters of Lake Superior was sustained by the refuge under even impossibly high fishing mortality rates, whereas removal of the refuge increased the risk of collapse at much lower fishing mortality rates. Without the refuge, exploitation rates cannot remain at current levels because no fish outside or inside the refuge would be protected from exploitation to sustain a fishable population. Therefore, retaining the refuge is the optimal solution to long-term sustainability of the lake trout population in eastern Wisconsin waters of Lake Superior.