Spatial and trophic niches through ontogeny and the influence on native species restoration: Using Lake Ontario Atlantic salmon (Salmo salar) as a model species

Species reintroductions are an important aspect of conservation biology to prevent biodiversity loss, and post-release monitoring can help in understanding and improving restoration success. Generally, population sizes are monitored to determine if reintroductions are successful, however, it does not reveal why it is a success or failure. This thesis proposes that by understanding a species ecological niche within the introduced abiotic and biotic habitat throughout its life history, the mechanisms behind restoration success or failure can be better determined, as well as assist management with improving the potential for species restoration. Once extirpated, Atlantic salmon (Salmo salar) have been reintroduced into Lake Ontario, however, numbers of returning adults remain low. Thus, focusing on Lake Ontario Atlantic salmon provided an opportunity to assess restoration potential and improve restoration success. The spatial and trophic niches of Lake Ontario Atlantic salmon were assessed at various life stages. This thesis begins with understanding the seasonal trophic niche of juvenile Atlantic salmon stocked into streams with different fish communities, and specifically to determine if trophic interactions and other species abundances limit restoration success. Stream resident fish communities appeared to partition resources across seasons such that abundant species had low trophic niche overlap with young-of-year Atlantic salmon minimizing overall competition given available resources. The next chapter followed the migration success of Atlantic salmon smolts coming from different rearing environments. Acoustic telemetry revealed that there was similar migratory performance and no impacts from weirs, yet survival was greater in naturally-reared smolts than hatchery-reared smolts. Survival was lowest at the release site indicating pre-migration mortality, and specifically high stocking-related mortality of hatchery-reared smolts. Overall, when either stocked as parrs or smolts there was low mortality during the actual migration. Lastly, this thesis investigated the trophic and spatial niches of adult Atlantic salmon within the lake environment and compared them to the salmonid community. All salmonids primarily consumed alewife and exhibited some overlap in trophic niche due to this prey commonality and similarity in stable isotope values. Spatially, Atlantic salmon are using slightly different habitats than the other salmonids, regardless of consuming similar prey, thus limiting the spatial niche overlap within Lake Ontario. This thesis has increased our understanding of Lake Ontario Atlantic salmon. There were no major ecological niche overlaps with other salmonids, limiting resources, or reduced survival of Atlantic salmon, from juveniles and smolts in the river to adults in the lake. However, this research revealed a few indirect aspects that could affect success and potentially limit restoration (e.g., stocking related survival, thiamine deficiency, spawning success) which could be further researched. Understanding ecological niches can be useful beyond Atlantic salmon restoration in Lake Ontario such as with other reintroduction projects, stocking programs and fisheries management. Researching different aspects of a species ecological niche, like its trophic and spatial niches at various life stages, provides management with information to increase the potential for reintroduction success, such that ultimately, reintroductions may be a more effective tool towards species conservation and increasing biodiversity