Capelin Size, Condition, and Abundance Through Multiple Heatwaves in Alaska

Capelin (Mallotus spp.) are pelagic forage fishes that can be especially abundant in sub-arctic marine ecosystems and are important prey for upper trophic-level consumers. Abundance and distribution of capelin have been linked to ocean temperature, but the magnitude and directionality of thermal sensitivity can vary regionally. We used data spanning two decades (2002–2022) from autumn surface trawl surveys in three Alaska large marine ecosystems to evaluate how size, body condition, abundance, and distribution of age 1+ (60–130 mm) capelin responded to two marine heatwaves occurring within a 6-year period from 2014 to 2019. Body size did vary with ocean temperature and declines in body condition during heatwave years were observed in the Chukchi Sea and Gulf of Alaska where water temperatures were approaching the lower and upper bounds, respectively, of the species' known temperature range. There was no clear directional change in body condition during heatwave years in the Bering Sea where temperatures were intermediate among the three regions. Changes in capelin abundance, however, were dramatic and primarily driven by reduced encounter probability, which declined to zero in the upper water column of the Chukchi Sea and Gulf of Alaska during heatwave years. When capelin were encountered, however, catch-per-unit-effort was similar among regions and years. In the Bering Sea, encounter probability fluctuated but remained above zero during all years, with core areas of biomass distribution near St. Lawrence Island. Changes in abundance (primarily) and body condition reduced energy available to capelin predators by up to 600-fold during marine heatwave years.