After looking at nearly two decades of survey data, scientists found evidence to suggest that recent marine heatwave events in the eastern Bering Sea and the Gulf of Alaska may have played a key role in juvenile chum salmon survival. Scientists also suspect this impacted subsequent adult returns to western Alaska rivers.
In a new study published, Alaska Fisheries Science Center and Alaska Department of Fish and Game scientists 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 of the Ecosystem Monitoring and Assessment Program.
Changing Arctic Conditions
Arctic regions including the northern Bering, Chukchi, and Beaufort seas have been experiencing accelerated 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
Critical Stages in Western Chum Salmon’s Life
Chum salmon spend the majority 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 1-4 or more years, they migrate between the Gulf of Alaska during winter and the Gulf of Alaska and the Bering Sea during summer. After they 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.
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