Pacific salmon are an ecologically important species and provide subsistence, commercial, and recreational fishery opportunities. New research found that the cold, low salinity inshore waters of Icy Strait in Southeast Alaska may have provided a temporary buffer from marine heatwave conditions in the Gulf of Alaska for four species of migrating juvenile salmon. The marine heatwaves occurred in 2014–2016 and 2019. Prior studies have found connections between juvenile Pacific salmon survival and growth rates to oceanographic conditions and food availability in the marine environment.
Recent marine heatwaves with persistent warm seawater temperatures above normal conditions in the Gulf of Alaska resulted in multi-year warming throughout the water column. This persistent warming was correlated to reduced condition and abundance of some key prey species, which led to widespread effects throughout the food web. Following the warm anomalies in 2014–2016 and 2019, multiple adult salmon stocks in the Gulf of Alaska declined. This prompted U.S. state and federal agencies to issue statewide fishery disaster declarations for multiple fisheries.
Warming trends were likely linked to the declines of adult salmon in the Gulf of Alaska. “We expected that this might be due to changes to their prey, potentially at the juvenile stage, because we have evidence that changes in diets are related to marine temperature. However, we found that the body condition of juvenile salmon did not show a significant relationship with temperature in the inside waters of Southeast Alaska,” said Mariela Brooks, research chemist, Auke Bay Laboratories, Alaska Fisheries Science Center.
During heatwave years, temperature of marine waters increased in Southeast Alaska, but were still cooler than waters offshore in the Gulf of Alaska. Brooks and her fellow scientists suspect that the unique geography and oceanography of Southeast Alaska kept waters relatively cool. This supported a diversity of prey for juvenile salmon, which allowed them to switch between food sources and buffer the potential negative effects of heatwaves. Within this unique marine habitat, the body condition of observed juvenile fish in Southeast Alaska inside waters did not decline during marine heatwaves prior to migrating into the northeast Pacific Ocean.
Our long-term surveys support salmon harvest forecasts in Southeast Alaska by tracking juvenile abundance. The findings of this study confirm the value of these inside waters as early marine habitat. They also suggest that declines in adult salmon returns during marine heatwaves were likely driven by conditions beyond Southeast Alaska. These results indicate that survival bottlenecks may emerge after juveniles migrate offshore, especially during marine heatwave events. Understanding the full migratory pathway, and when and where mortality occurs, is essential for improving predictions of salmon survival and forecasting.
