NOAA trawl survey: Cold pool returns in the Bering Sea
By Megan Gannon
The Bering Sea’s cold pool, a critical part of the seafloor ecosystem, had shrunk to a worrying degree in recent years, but it is continuing to slowly return, according to the latest results of NOAA’s bottom trawl survey.
Duane Stevenson, the senior lead for the Bering Sea bottom trawl survey for NOAA Fisheries, who works with the agency’s division of Resource Assessment and Conservation Engineering, or RACE, presented the results of the survey in a Strait Science presentation last week.
“This year, we had a relatively cold pool of water in the northern Bering Sea and actually extending south of St. Matthew Island onto the central part of the Bering Sea shelf,” Stevenson said. “This is interesting, because over the past several years, we haven’t really seen much of a cold pool, certainly not extending down into the eastern Bering Sea survey area. This year, we definitely saw more of a cold pool than we have in recent years.”
When seasonal sea ice melts, very cold water sinks, creating a persistently frigid layer at the seafloor that tends to stay under 2°C most of the summer. This cold pool is thought to act as a barrier to keep some species from crossing into the eastern Bering Sea shelf and northward toward the Bering Strait. But declining sea ice is disrupting this phenomenon. After record-low sea ice extent in the winter of 2017/2018, the cold pool was severely diminished in the summer of 2018. That unprecedented ice loss continued in the next winter, which again had dire consequences for the 2019 cold pool.
“2019 was the year that we saw the smallest cold pool of any year that we’ve done these surveys,” Stevenson said. “However, we saw a little bit more cold water in 2021, and even more so in 2022. So, in general, the temperature pattern that we saw on the further bottom temperature of the Northern Eastern Bering Sea survey area in 2022 was pretty similar to what we saw in 2017. I certainly wouldn’t call it a cold year. But it looks like temperatures are moving back to the long-term mean.”
NOAA established its annual trawl surveys of the Eastern Bering Sea in 1982, and initiated more occasional surveys of the Northern Bering Sea, which includes the Norton Sound, in 2010. The temperature readings come from a trawl net that the researchers deploy to sample species in the bottom 6 to 10 feet of the water column. The survey, which was completed this year by researchers aboard the F/V Vesteraalen and F/V Alaska Knight, allows them to track overall changes in food web and species distribution and to monitor environmental conditions and collect data on genetic information.
Stevenson reported that some species showed encouraging signs that they might be increasing in numbers in the northern Bering Sea, at least compared to last year. For example, Saffron cod, also called tomcod, seems to be bouncing back after a few bad years.
“In the earliest survey years, not only the cold year of 2010, we saw really good numbers of Saffron cod in the Northern Bering Sea, particularly in Norton Sound, in 2017 and 2019,” Stevenson said. “Then last year, we saw almost none. We saw a huge decline in the amount of Saffron cod in the Northern Bering Sea. The good news is this year, they seem to have come back to some extent.”
Their biomass in the Northern Bering Sea was estimated to be 27,738 metric tons this year—a 178 percent increase from the 9,974 metric tons estimated last summer.
The researchers saw a similar pattern with Arctic cod, which used to be seen throughout the Bering Sea. The fish seem to have made a strong retreat after the warm years, and while they still aren’t back in the Eastern Bering Sea, “it looks like the colder water is allowing them to come back into the Northern Bering Sea—a little bit, at least,” Stevenson said. The biomass of Arctic cod was estimated to be 387 metric tons, up 367 percent from the 83 metric tons estimated in 2021.
While the researchers only recorded about 50 individual blue king crabs last year, they collected about 150 this year. In the Northern Bering Sea, they are estimated to have a biomass of 3,014 metric tons this year.
“The numbers are small but we seem to be moving in the right direction,” Stevenson said.
Other species did not seem to be bouncing back from recent declines. Shorthorn sculpin, for example, is another Arctic species that seemed to especially decline in the warm years. But even as colder temperatures returned, they still experienced a 52 percent decline in biomass in 2022 compared with last year’s numbers. Stevenson said the surveyors saw too few specimens to create a length frequency distribution for the population.
Walleye Pollock caught the attention of fisheries researchers when they started to be seen in large numbers in the Bering Sea, particularly north of St. Lawrence Island, during the recent warm years. The concentrations of those fish declined in 2021 and then again in 2022. The researchers observed a similar pattern with Pacific cod.
The halibut population, meanwhile, seemed to be stable in the region.
“We’re right around 20,000 to 30,000 metric tons total, throughout the entire time series for the Northern Bering Sea, so no big changes in halibut biomass,” he said. He did note, however, that there seemed to be fewer big catches in the Northern Bering Sea; most of the fish they were seeing were around 40 to 60 centimeters in length.
Though Pacific herring seemed to experience the biggest loss in biomass in the Northern Bering Sea—losing 82 percent—the population has greatly increased in the Eastern Bering Sea.
“It seems like the herring population is doing well, but they’re distributed a little bit more southerly than they have been in the previous couple of years,” Stevenson said.
UAF Alaska Sea Grant agent Gay Sheffield, who was moderating the presentation, encouraged participants to reach out to her if they wanted a PDF of the survey results. She noted that the RACE Division also made an effort to distribute hard copies to residents of the region. While the surveys are helping scientists establish baselines for tracking various effects of ecosystem changes, Stevenson also said he hoped the results would at least help put in context anecdotal information he often hears from local communities about what they’re able to catch, and what they’re not able to catch anymore.
“It sounds like there are a lot of things that used to be in abundance that are just not anymore,” Stevenson said. “People are concerned about that, and I’m hoping that our data can at least [tell] people how widespread those issues are. I think that’s the real value of our data from these surveys.”