A report drawing on 17 years of data found that recent “marine heatwave” events in the eastern Bering Sea and Gulf of Maine may have helped juvenile chum salmon flourish in warmer water.

But the increase in juvenile chum abundance did not yield an increase in adult returns to Alaska rivers as biologists expected, the National Marine Fisheries Service reported Nov. 30.

The new study from the Alaska Fisheries Science Center and Alaska Department of Fish and Game found that juvenile (first ocean year) chum salmon were more abundant during the more recent an exceptionally warm marine period (2014–2019) compared to previous warm (2003–2005) and cold (2006–2013) periods. However, this increase in juvenile abundance did not lead to an increase in adult returns as expected.

Researchers also observed that during the 2014-2019 warm period, juveniles were larger in size but in poorer body condition. These salmon consumed lower quality prey. As a result, they had fewer energy reserves and a lower probability of surviving their first winter. Scientists speculate this may have led to lower adult returns in recent years.

“Our data suggest a shift in how juvenile chum salmon are allocating energy during their first year at sea. This is a critical period for them and our results illustrate how anomalous events in marine ecosystems can impact their survival and future returns,” said Ed Farley, lead author and manager at the Alaska NOAA science center’s Ecosystem Monitoring and Assessment Program.

Arctic regions including the northern Bering, Chukchi, and Beaufort seas have been affected by faster warming and extremes in seasonal sea ice extent. In the northern Bering Sea, unprecedented reductions in seasonal sea ice occurred during winter of 2017–2018. This was followed by an increase in warm southerly winds during February 2019 and early ice retreat. 

The ecosystem response to these extreme events was rapid. 

·       Unusually warm spring and summer sea temperatures 

·       Reduced cold pool (natural thermal barrier created by melting sea ice between the northern and southern Bering Sea ecosystems)

·       Expansion of subarctic fish species into the northern Bering and southern Chukchi seas, 

·       An increase in seabird die-offs

·       Reduction in high-fat chum salmon prey

·       Declines in run sizes of western Alaska chum salmon to record low levels  

·       Chum salmon spend most of their life in the marine environment. In western Alaska, juvenile chum salmon enter the marine waters of the northern Bering Sea from mid-June to mid-July. They spend their first summer at sea feeding and growing along the northern Bering Sea shelf. 

·       During late fall and early winter, western Alaska juvenile chum salmon migrate out of the Bering Sea and into the Gulf of Alaska. This is where they spend their first winter at sea. Over the next one to four or more years, they migrate between the Gulf of Alaska during winter and the Gulf of Alaska and the Bering Sea during summer. After chum salmon mature, they return to their natal rivers to spawn.  

·       Juvenile chum salmon tend to allocate energy to rapid growth when they first enter the marine environment. Later in the season, energy is allocated to fat storage. Faster growth rates early on reduce the chances that a juvenile salmon will become a meal for predators. Larger juvenile salmon that attain sufficient energy reserves by the end of summer/early fall also have a greater probability of surviving the winter. 

·       Scientists speculate that chum salmon overwinter survival was affected by exposure to two separate warming events that 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 in their habitats, their metabolic rates increased, requiring them to seek more food for growth. The prey that was 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. However during warm periods, researchers observed that there was an increased percentage of lower quality prey available, especially during the recent warm period. 

·       Scientists saw a shift in prey to cnidaria jellyfish during warm years. The caloric content of this jellyfish is roughly half that of other juvenile chum salmon prey. 

·       Juvenile chum salmon stomach contents contained jellyfish during the first warm period. In the second warm period, jellyfish were proportionally more dominant in the stomach contents. This was also the period with the lowest fat content values for juvenile chum salmon, suggesting a direct connection between late summer fat storage and prey quality.

·       Juvenile salmon potentially faced increased competition among other chum salmon stocks that are also distributed in the Gulf of Alaska during winter. 

“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. These interactions may play a significant role in survival during that first winter,” said Kathrine Howard, a co-author and fishery scientist with the Alaska Department of Fish and Game.   

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. This may have enabled more juveniles to make it downstream to the ocean. 

While a strong relationship between juvenile chum salmon abundance and adult returns to rivers in the northern Bering Sea has not been established, the expectation was the greater abundance of juvenile chum salmon seen in the recent warm period would herald higher adult returns to the region three to four years later. 

But it did not happen as those juvenile salmon entered the marine environment  – they didn’t find the food they needed to attain sufficient fat reserves prior to winter. Scientists think that shortfall in food supply hurt the juvenile salmons’ survival, and may have contributed to recent annual variation in adult survival.  

“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,” said Farley.

“Many people are dependent on salmon in Alaska for food security, cultural traditions and local economies,” said Farley. “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.”