Hot or Not: Early Development Thermal Reaction Norms among Sockeye Salmon Populations

Climate warming will have serious repercussions for salmonids, requiring an understanding of local variation and adaptive potential of wild populations. Populations may vary in their ability to cope both behaviourally and physiologically with warming temperatures.  Early life history stages are thought to drive local thermal adaptation, especially in salmonids, which have thermally sensitive incubation and early development periods. The Fraser River is collectively home to one of the largest salmon migrations in the world, with over 100 reproductively distinct populations of sockeye salmon within the watershed. These populations differ in temporal spawning period, adult spawning migration distance, migratory challenge, parental morphology, and historical incubation temperatures. These factors, driving adaptation and selection during early development, are critical in determining the dynamics of such populations; however, among-population responses to thermal stress during incubation are not well known.  To assess the differences in population-level responses to temperature, sockeye salmon families were created using gametes from 8 populations in the Fraser River watershed and 1 from the Columbia River watershed.  These populations spanned a range of spawning locales from coastal to far in the interior, with a range of local historical thermal regime. Families were fertilized and incubated at three supra-optimal temperatures (10°, 14°, 16° C) in a common garden experiment until emergence. The 14° and 16° treatments resulted in reduced fertilization and survival during embryonic, post-hatch and post-emergence stages. Delayed mortality effects were seen with the 14° and 16° treatments for some populations. Inter-and intra-population variation in survival increased with temperature. Within populations, alevin and fry were smaller under higher temperature treatments. Egg, alevin and fry size significantly differed between stocks with extremely different life histories. Several populations exhibited a significantly higher thermal tolerance, while others showed a severe inability to develop, survive, or even successfully fertilize at high temperatures. The question becomes if populations have a sufficient ability to thermally adapt, and if adaptations can respond quickly enough to maintain population viability.   Collectively, these findings highlight the need for concern at the population level for sockeye salmon, especially for regions experiencing rapid climate warming.