Therrien, Christian2026-04-302026-04-302026-04-302026-04-28https://hdl.handle.net/10012/23130Reintroduction programs are important tools for reversing extirpations of fishes in freshwater environments, but only 25-50% of reintroduction programs are successful. Exotic species are present in many habitats that are targeted for restoration, and exotic species can negatively affect reintroduced species through predation, competition, and changes in the quantity and/or quality of available prey. Thiamine (vitamin B1) is an important nutrient in the diet of fishes. There is growing concern that consumption of thiaminase, an enzyme that degrades thiamine and is found in the exotic prey fish rainbow smelt (Osmerus mordax), may be impacting reintroductions of lake trout (Salvelinus namaycush) in lakes where they are present. Ongoing programs to reintroduce lake trout in Ontario, specifically in the Sudbury Basin and Lake Ontario, provide an opportunity to study how the outcome of reintroduction efforts is influenced by (1) the presence of exotic rainbow smelt; and (2) identity of source population (with specific focus on previous exposure to exotic prey species). I used a multi-lake study to determine if presence of exotic rainbow smelt resulted in decreased tissue thiamine concentrations in lake trout and compared fish body condition and growth between lakes that do and do not contain rainbow smelt. I found that while muscle thiamine concentrations of lake trout did not differ between lakes with and without rainbow smelt, an instance of thiamine deficiency was observed in a lake that contains rainbow smelt. I also found that body condition was significantly lower in lake trout from lakes with rainbow smelt and the asymptotic length of lake trout populations declined with increasing proportion of smelt in the diet. My results suggest that consumption of exotic rainbow smelt can cause thiamine deficiency, decrease body condition and lower asymptotic length in lake trout, and ultimately may be a factor contributing to the failure of programs to reintroduce lake trout throughout Ontario. Next, I compared the effects of diets designed to mimic the thiaminase activity of exotic prey fishes, including rainbow smelt, on survival and performance-related traits between two hatchery strains (Seneca Lake and Slate Island) of lake trout that differ in historical exposure to diet-derived thiaminase. Results indicated that, regardless of strain, the diet containing thiaminase negatively affected most performance-related traits and while most of the negative effects of the thiaminase diet did not differ between the two strains, an increase in red pigmentation and decrease in survival was observed in Seneca Lake fish that had received the thiaminase diet. By comparing the response of two hatchery strains of lake trout to a diet containing thiaminase, I found evidence that pre-existing adaptations to thiaminase differ among hatchery strains of lake trout. Of the hatchery strains I examined, the Slate Island strain is the best environment match and is likely the most suitable for reintroduction into lakes where exotic prey fishes occur. Lastly, I captured a fish of presumed wild origin from Tyson Lake, which has been subjected to reintroductions of lake trout since the 1970s but previously had no records of natural reproduction. Using a combination of methods, I confirmed the fish was, indeed, of wild origin and represents the first documentation of natural reproduction in Tyson Lake. The fish was assigned to both the Lake Manitou and Iroquois Bay strains, which suggests that these two strains of lake trout may be best suited for reintroduction into lakes in the Sudbury Basin, due to their ancestry match to extirpated populations of lake trout in the area. By incorporating multiple methods, this thesis identifies some of the major factors influencing the success of reintroductions of lake trout and advances our understanding of best practices for species reintroductions.enfishinvasive speciesspecies reintroductionslake troutthiamine deficiencyDoctoral Thesis