Temporal and spatial differences in smolting among sockeye salmon (Oncorhynchus nerka) populations throughout fresh- and seawater migration and the effect of water temperature on the smolt window

Salmon smolts undergo physiological changes in the spring that are important for successful migration to seawater. Species that are widely distributed may differ in timing of physiological changes associated with smelting. In my first study, I compared indices of smolt characteristics among populations that differ in migration distance to the ocean. Fraser River sockeye salmon from four regions in the watershed were intercepted at different times during migration to characterize the parr-smolt transformation. Gill Na⁺/K⁺-ATPase (NKA) activity was highly variable at the start of migration, and was not explained by the distance from the ocean. Gill NKA activity changes with migration were also highly variable, but consistently smolts in the ocean had the highest gill NKA activities. The nature of smelting appears to be dynamic and variation was not based on the region of origin, timing during migration, or on the year of migration. The duration of time when anadromous salmon are able to survive in seawater – the smolt window – is influenced by temperature. In my second study, I found that warm water temperature abbreviated the smolt window. Additionally, isoforms of the gill NKA enzyme and endocrine signals suggest that the stimulus for smelting occurred prior emigration from the natal lake. Modeling the thermal experience that smolts encountered as they migrated downstream to the ocean in 2012 suggested Chilko fish did not experience temperatures as warm as the temperatures that abbreviated the smolt window in my study. Furthermore, climate change projections for temperature may not limit successful emigration of Chilko sockeye salmon smolts from central British Columbia to the ocean – but changes in other abiotic and biotic factors may confound this prediction.