New research compiled by NOAA Fisheries shows that adult Pacific cod are moving into the northern Bering Sea in summer months due to warming ocean waters, prompting questions about how changes in spawning habitat might impact productivity of these fish in the future.
Researchers with the Alaska Fisheries Science Center (AFSC) note that bottom trawl surveys in 2017 and 2019 estimated that 30% to 40% of the biomass of Eastern Bering Sea Pacific cod was in the Northern Bering Sea. The proportion of the biomass in the Northern Bering Sea has gone down in recent years corresponding to cooler conditions, they said.
Researchers are currently working on quantifying the impact of Pacific cod as predators in the northern Bering Sea by looking at their diets and their energy needs, said Lauren Rogers, a fisheries research biologist.
“We know adults are moving northward, but what does that mean for other life stages?” asks Jennifer Bigman, study lead and a postdoctoral fellow at NOAA Fisheries.
“Evidence from tagging studies and our work indicates that Pacific cod are not spending the winter in the northern Bering Sea and appear to migrate back to their typical spawning grounds in winter to spawn,” she said. “This means we don’t expect them to produce offspring in the northern Bering Sea. Our work suggests that it is too cold for eggs to survive to hatch in the northern Bering Sea.”
The collaborative research, conducted with biologists from the Cooperative Institute for Climate, Ocean and Ecosystem Studies at the University of Washington, predicts how thermally suitable habitat for spawning Pacific cod in the Bering Sea may shift over the coming century. Research shows that current spawning areas will likely get too warm for egg development and hatching, but that the northern Bering Sea will still be too cold for successful spawning even by the end of the century.
“Understanding how warming will affect spawning habitat is a critical step in predicting and mitigating the effects of climate change on Pacific cod populations and fisheries,” said Bigman.
Researchers are integrating physiological data with climate modelling to provide insight into how climate may drive change. From laboratory experiments they conducted they learned that Pacific cod eggs have a very narrow thermal window for successful development and hatching. Juveniles and adults, by comparison, tolerate a much wider temperature range. The egg stages may act as a bottleneck for the species’ ability to adapt to warming, they said.
Recent studies linked reduced thermal spawning habitat with a stark decline in Pacific cod production in the Gulf of Alaska, after an unprecedented marine heat wave from 2014 to 2016, but the Bering Sea is colder than the Gulf of Alaska.
Bigman said that much of the Bering Sea is too cold for eggs to successfully hatch and that this research was undertaken to find out whether or when warming might affect Pacific cod populations through shits in spawning habitat.
“Our results suggest that thermally suitable spawning habitat has not limited the reproductive potential of Pacific cod in the eastern Bering Sea in the past—nor is it likely to in the future,” she said.
