Dunlop, Erin S

Lake Huron's food web has experienced drastic changes in response to multiple stressors including declines in offshore productivity, decreased trophic transfer efficiency and a transformation of the benthic food web. However, how these changes have affected the diets and isotopic niches of predatory fish is largely unexplored. My diet study analysed stomach contents from five predatory fish species (lake trout, lake whitefish, chinook salmon, rainbow trout, and walleye) from the Ontario waters of Lake Huron. My isotopic study focused on lake trout and lake whitefish, based on community concerns that recovering lake trout are competing with or consuming lake whitefish. By contrast, I found that lake whitefish were a minimal component of lake trout diets, and the diet and isotopic overlap between these two species was low overall, but varied regionally. Both the dietary and isotope analyses reflect the high regional diversity of energy sources used by predatory fish.

Author Keywords: Food web, Isotopic Niche, Lake Huron, Predator-prey, Regime shift, Regional Energy Sources

Lake whitefish (Coregonus clupeaformis) are ecologically, economically, and culturally significant across their range, yet little is documented about the species' spawning behaviour. Recruitment of lake whitefish has dramatically declined over the past two decades across many regions of the Laurentian Great Lakes, most notably in lakes Huron and Michigan, prompting research efforts to fill critical knowledge gaps in the life history of this species. Understanding the reproductive ecology of lake whitefish, including documenting aspects of spawning behaviour, may reveal clues about factors contributing to the declines. This study used fine-scale acoustic telemetry to characterize sex-specific movement patterns and habitat associations of lake whitefish during the spawning season. A VEMCO Positioning System (VPS) was deployed at an active spawning shoal in Georgian Bay (Lake Huron), with guidance provided by Saugeen Ojibway Nation members with local ecological knowledge about the shoal and the lake whitefish population. In the fall of 2020, 50 lake whitefish were captured and tagged at the spawning shoal; 28 of those fish were observed the following spawning season in 2021. A clear increase and decrease was observed in the presence of individuals in the study area over the spawning period. Both sexes moved into shallower waters at night and deeper waters during the day at rates of 0.162 m/h and 0.08 m/h, respectively. Decreased rates of movement and horizontal distance travelled were detected during the day leading up to a peak activity phase where 22 fish were present. At night during this peak activity phase, males moved 30.0% faster on average than females (0.204 ± 0.13 and 0.143 ± 0.10 m/s, respectively). This study provided a unique opportunity to work with local First Nations to understand lake whitefish spawning behaviour, providing insights into the reproductive ecology of a population undergoing concerning declines in recruitment and abundance.

Author Keywords: Coregonine restoration, Coregonus clupeaformis, Fine-scale acoustic telemetry, GLATOS, Spawning habitat, Two-Eyed Seeing

Fisheries assessment and management approaches have historically focused on individual species over relatively short timeframes. These approaches are being improved upon by considering the potential effects of both broader ecological and evolutionary processes. However, only recently has the question been raised of how ecological and evolutionary processes might interact to further influence fisheries yield and sustainability. My dissertation addresses this gap in our knowledge by investigating the role of eco-evolutionary dynamics in a commercially important lake whitefish fishery in the Laurentian Great Lakes, a system that has undergone substantial ecosystem change. First, I link the timing of large-scale ecological change associated with a species invasion with shifts in key density-dependent relationships that likely reflect declines in the population carrying capacity using a model selection approach. Then, using an individual-based model developed for lake whitefish in the southern main basin of Lake Huron, I demonstrate how ecosystem changes that lower growth and recruitment potential are predicted to reduce population productivity and sustainable harvest rates through demographic and plastic mechanisms. By further incorporating an evolutionary component within an eco-genetic model, I show that ecological conditions also affect evolutionary responses in maturation to harvest by altering selective pressures. Finally, using the same eco-genetic model, I provide a much-needed validation of the robustness of the probabilistic maturation reaction norm (PMRN) approach, an approach that is widely used to assess maturation and infer its evolution, to ecological and evolutionary processes experienced by exploited stocks in the wild. These findings together highlight the important role that ecological conditions play, not only in determining fishery yield and sustainability, but also in shaping evolutionary responses to harvest. Future studies evaluating the relative effects of ecological and evolutionary change and how these processes interact in harvested populations, especially with respect to freshwater versus marine ecosystems, could be especially valuable.

Author Keywords: Coregonus clupeaformis, density-dependent growth, fisheries-induced evolution, individual-based eco-genetic model, Lake Huron, stock-recruitment