Abstract The Northeast Pacific harbors the majority of North American and a substantial subset of Asian Pacific salmon stocks (Oncorhynchus spp.), yet little is known about the factors shaping ecosystem composition and salmon distribution during the oceanic winter. Accordingly, fluctuating salmon marine survival, specifically of imperiled southern populations, remains poorly understood. The International Year of the Salmon initiative has conducted three winter expeditions to the Northeast Pacific to uncover these factors using a multidisciplinary approach, including eDNA surveys. Here we report on the results of the 2022 large-scale four vessel ecosystem-wide eDNA survey at the end of the oceanic winter, spanning 2.2 million km2 of open ocean salmon habitat. Through multi-amplicon metabarcoding targeting diverse metazoan lineages, we identify distinct ocean biomes shaped by macroscale oceanographic and bathymetric features like currents and the continental shelf. Salmon exhibited species-specific distribution across the Northeast Pacific and were associated with mesoscale anticyclonic eddies in the open ocean. Salmon showed species-specific positive association with potential prey species and competitors, but some were negatively associated with specific cnidarians and harmful algae. The localized onset of spring blooms lead to a rapid transition of ecosystem composition within weeks from copepod-dominated winter communities to complex communities of forage fish and higher trophic levels, including all salmon species as well as apex predators. This bloom community was composed of members of open ocean and coastal biomes, forming a hybrid community, suggesting targeted migration of mobile species to areas of high biological activity. Together, we document the distinct biomes salmon navigate in a species-specific manner over the oceanic winter at unprecedented taxonomic and spatial scale, identify key species they interact with, and observe rapid temporal transitions in ecosystem composition associated with the onset of spring blooms, improving our understanding of salmon ocean ecology.
Deeg et al. (Sat,) studied this question.