Unexpected Positive Effects of the Pacific Marine Heatwave on Zooplankton and Juvenile Salmon in the Salish Sea

The 2014-2016 Pacific marine heatwave (a.k.a. the Blob) resulted in unprecedented temperature anomalies and highly visible negative consequences for Northeast Pacific ecosystems, including mass seabird mortalities and extensive harmful algal blooms. However, impacts on the Salish Sea were not as apparent, nor as readily understood due to a relative paucity of historical data from the region. Here we use data from several monitoring programs in the Strait of Juan de Fuca, San Juan Islands, and Puget Sound, Washington (together comprising the southern Salish Sea) to explore spatial and interannual variability in ecosystem response to conditions over 2014-2019—before, during, and after the warm anomalies. Strong bottom-up effects of the physical changes occurred, including shifts in plankton phenology and biomass; responses differed across sub-regions of the Salish Sea, with differences driven largely by local oceanographic conditions and the degree of connection to the Pacific ocean. In sharp contrast to expectations based on coastal changes, zooplankton biomass and juvenile salmon growth were strongly elevated during the warm years in Puget Sound. Furthermore, coho salmon that out-migrated from Puget Sound rivers in 2015 during the peak of the warm anomalies survived and returned in higher numbers than expected. This decoupling of previously-established relationships between temperature, zooplankton, and salmon growth and survival—in which high temperature = low zooplankton, low growth, and poor survival—suggests a challenge for fishery managers who seek ecosystem indicators of population success across broad spatial and temporal scales. These and other contrasts observed in zooplankton response among systems and regions are explored to understand the physio-chemical underpinnings of the observed changes and their implications for fish populations.