Spatial and Temporal Dynamics of Top Predators in a Large Lake

Environmental degradation is the legacy of the Anthropocene era and maintaining ecosystem health is a major challenge for managers globally. Restoration ecology aims to improve ecosystem health through reintroduction and rehabilitation of important native species to maintain or increase biodiversity and achieve stable and resilient communities. Non-native species may affect reintroduction efforts through interactions in time and space. This dissertation proposes that understanding the spatio-temporal ecology of two species along with their energy demands and growth (i.e., bioenergetics), would provide key clues for untangling the complexities of species interactions and further the knowledge on influences of non-native species on native species rehabilitation. Lake Ontario has two native salmonids undergoing rehabilitation and four introduced salmonids and provided the ideal model system for this research. Here, I focused on lake trout (Salvelinus namaycush), a native top predator extirpated in the 1950s and currently under rehabilitation and the introduced Chinook salmon (Oncorhynchus tshawytscha). Quantifying the spatio-temporal ecology of each species in Lake Ontario in the first two research chapters provided baseline information used in the last two research chapters for assessment of their interactions and their bioenergetics. Specifically, lake trout seasonal distributions showed inter-individual variation in home range size, winter and summer habitat preference, and long-distance movement behaviour. Chinook salmon showed moderate segregation occurring between immature and sub-adult individuals in both the horizontal and vertical planes, suggesting interactions with lake trout may vary by size-class. Spatial utilization overlap and fine-scale co-occurrence of the two species were assessed next, and the results revealed that the species segregated horizontally, except during the summer when segregation was vertical. The final research chapter showed that lake trout is occupying temperatures below the optimum and growing below its potential, while Chinook salmon occupied temperatures close to their optimum and their growth was nearing the species’ potential under present conditions. Further, results from warmer temperatures and diet composition change scenarios revealed that prey quality was more important in determining growth than temperature (i.e., habitat). Overall, this dissertation enhanced the understanding of lake trout and Chinook salmon spatial ecology, interactions and the relative influences of temperature and diet on their bioenergetics. Taken together, these results suggest that limited interactions occur between the two species and that Chinook salmon do not affect lake trout negatively, and thus are not an impediment to their restoration. Chinook salmon’s presence in the lake appears to induce lake trout re-distribution and thus, limit their overlap and foraging on Alewife (Alosa pseudoharengus), a prey fish that impedes lake trout reproduction, suggesting that Chinook salmon indirectly facilitates lake trout restoration. Understanding the spatio-temporal ecology of these species together with their growth under varying scenarios, furthered our knowledge of the complexity of their interactions when the two species co-occur. The use of a multi-disciplinary approach (i.e., spatial ecology and bioenergetics) improved result interpretations, showcasing that this approach would provide a useful framework in any ecosystem where species interactions are poorly understood. These results provide relevant information on biodiversity targets aiming to improve ecosystem health that include seemingly conflicting objectives where restoration of native species is important, but the maintenance of non-native species is also highly desirable.

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