NOAA Study Focuses on Marine Heatwave Impacts on Chum Salmon

Chum salmon. Photo: NOAA Fisheries

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.

There was an expansion of subarctic fish species into the northern Bering and southern Chukchi seas, an increase in seabird die-offs, a reduction in high-fat chum salmon prey, and run sizes of western Alaska chum salmon run returns were at record low levels.

Chum salmon sped most of their lives in the marine environment. In western Alaska, juvenile chums enter the marine waters of the northern Bering Sea from mid-June to mid-July and spend their first summer there feeding and growing along the northern Bering Sea shelf.

In late fall and early winter, these juvenile chums migrate out of the Bering Sea and into the Gulf of Alaska, to spend their first winter at sea.

For the next one to four years, they migrate between the Gulf of Alaska in winter and the Gulf of Alaska and Bering Sea in summer months, before heading back to their natal streams to spawn.

Researchers observed that when these juvenile salmon first enter the marine environment, they tend to allocate energy to rapid growth, then switch their focus later in the season to fat storage. By focusing first on faster growth rates, they protect themselves against being eaten by predators.

Researchers speculate that chums overwinter survival was affected by exposure to two separate warming events which occurred within their early marine and winter habitats. As juveniles when they first entered the marine environment, they were subjected to warmer than average temperatures in the northern Bering Sea. Those that survived and migrated to the Gulf of Alaska to overwinter were also exposed to warmer than average temperatures.  

When exposed to these higher-than-normal ocean temperatures, their metabolic rates increased, requiring them to seek more food for growth, but prey available within their early marine habitat was of lower quality. Another issue is that typically prey availability during winter decreases.

Researchers found that juvenile chum salmon fed on a variety of prey during the warm and cold periods, but that during warm periods there was an increased percentage of lower quality prey available, especially during the recent warm period. 

These juvenile chums also faced potential increased competition from other chum salmon stocks in the Gulf of Alaska in winter months.

“It is really these potential interactions among sea temperature, prey quality and prey quantity that can affect energy accumulation or fat storage in juvenile chum salmon during their first year at sea,” Howard said. “These interactions may play a significant role in survival during that first winter.”

The abundance of juvenile chum salmon in the northern Bering Sea increased during the more recent warm period. Scientists suspect that warmer water in natal rivers and streams could have improved the freshwater survival of young chum salmon and enabled more juveniles to make it downstream to the ocean.