Phenological Diversity of Salmon Smolt Migration Timing within a Large Watershed

Although there is growing concern that climate change might drive phenological mismatches between predators and prey, it is possible that within- and among-species phenological variation provides resilience against such a mismatch. One key life history event that may be vulnerable to climate-induced mismatch is the seaward migration of juvenile salmon relative to the spring bloom of their marine zooplankton prey. Here, we quantified phenological diversity of out-migration timing among salmon populations within a large watershed and its implications for climate mismatches with marine zooplankton. Specifically, we sampled juvenile Sockeye Salmon Oncorhynchus nerka throughout the spring and early summer in the estuary of the Skeena River, a vast watershed with numerous locally adapted salmon populations that support commercial, recreational, and First Nations fisheries, and we used genetic stock identification to link the fish to their population of origin. We found that Sockeye Salmon were migrating through the estuary for more than 50d, with peak emigration for different populations varying by over 5weeks. The out-migration timing of specific populations was related to geographic factors, including elevation of the rearing lake and theriver distance between individual rearing lakes and the estuary, with different populations arriving 1.5d later for every 100m of elevation or 3d later for every 100km of river distance. Concurrent with sampling, we quantified the estuarine prey of juvenile Sockeye Salmon; zooplankton species composition and abundance varied throughout the smolt migration period, and the different salmon populations encountered different prey abundances upon ocean entry. Together, these results indicate underappreciated phenological diversity in this harvested metapopulation, which may contribute to response diversity and metapopulation-level resilience to climate change.