Federal fisheries scientists who have been studying Western Alaska chum salmon for nearly two decades say recent marine heatwaves in the eastern Bering Sea and Gulf of Alaska may have had a critical role in the survival of juvenile chum salmon.
The study, published Nov. 30 by NOAA Fisheries, states that researchers also suspect the marine heatwaves subsequently impacted adult chum salmon returning to western Alaska rivers.
The study was led by Ed Farley, a program manager with the Alaska Fisheries Science Center and co-author Kathrine Howard, a statewide fishery scientist with the Alaska Department of Fish and Game.
“Recent declines in chum salmon and subsequent closures of commercial and subsistence fisheries in western Alaska, coinciding with years of record warm water temperatures, has heightened the urgency for this research,” Farley said.
“Many people are dependent on salmon in Alaska for food security, cultural traditions and local economies,” he added. “Through this and continuing work, we hope to provide information to help subsistence and small-scale commercial fisheries and state and federal resource managers plan and adapt to climate change.”
According to the study, juvenile chum salmon were more abundant during the more recent and exceptionally warm marine period, from 2014 through 2019, compared to the previous heatwave from 2003-2005 and cold period that ran from 2006-2013. This increase in juvenile abundance, however, did not lead to an increase in adult chum salmon returns, the study noted.
While juveniles were larger in size during the 2014-2019 warm period, their body condition was poorer, and they consumed lower quality prey, the result of which was they had fewer energy reserves and a lower probability of surviving their first winter. This may have led to lower adult returns in recent years, the research suggests.
“Our data suggest a shift in how juvenile chum salmon are allocating energy during their first year at sea,” Farley said. “This is a critical period for them and our results illustrate how anomalous events in marine ecosystems can impact their survival and future returns.”
The study also notes that the northern Bering, Chukchi and Beaufort seas have been subject to accelerated warming and extremes in seasonal sea ice extent. In the Northern Bering Sea, there were unprecedented reductions in seasonal sea ice during the winter of 2017-18, followed by an increase in warm southerly winds in February of 2019 and early ice retreat.
The ecosystem responded to these unusual events with unusually warm spring and summer sea temperatures and a reduced cold pool, that natural thermal barrier created by melted sea ice between the northern and southern Bering Sea ecosystems.