Warm water fish migrating to Northern Bering Sea are thriving

Lyle Britt, NOAA fisheries research biologist visited Nome last week with colleague Duane Stevenson to report on impacts of sea ice loss on the Bering Sea ecosystem and the creatures within it.
Britt supervises teams of fisheries research biologists conducting surveys of Bering Sea and arctic ground fish surveys. Britt’s team has been on the water surveying the Southeastern Bering Sea and the Northern Bering Sea for six weeks. The assessment surveys monitor the same GPS points, or stations—144 of them in the Northern Bering Sea— in the same places each season to achieve base lines for a variety of investigations.
The program uses a bottom trawl device to sample fish and invertebrate organisms — crabs, snails, urchins and species that are foundation prey in the food web that lead up to larger fish, marine mammals and human consumption on which regional communities depend for food and commercial value. The bottom trawl has a mouth 50 feet wide and about 10 feet high. The chartered fishing vessels dragging them have labs aboard where scientists count, weigh, measure and study the composition and health of the species collected.
Other Alaska Fisheries Science Center surveys of the surface and middle water column of the Northern Bering Sea in August and September revealed a significant happening: the presence of larvae and age zero and age one fish apparently spawned by mature warm-water fish that have traveled north from the South Bering Sea or possibly from Russia. Scientists cannot say for sure.
While the bottom trawl 2019 survey shows the biomass of walleye pollock in the Northern Bering Sea had fallen by 11 percent compared to 2017 estimates, the abundance (estimated number of fish in the survey area) of pollock had grown by 59 percent, increased to 2.9 billion fish from 1.8 billion fish in 2017. Much of this increase in number was due to more young fish in the area.
The increased catch of the very young pollock and cod strengthens the biologists’ hypothesis that these species are overwintering in the region during periods of low ice covering and spawning.
Findings from the 2019 Northern Bering Sea survey overall again show that large numbers of sub-arctic species of fish—walleye pollock, Pacific cod, flathead sole, etc.—distributed throughout the region.
The Southeastern Bering Sea has been studied as a separate ecosystem since 1982. Now the dramatic loss of sea ice and very warm water temperatures in the Northern Bering Sea and Chukchi Sea regions have launched boat-loads of ocean and fisheries scientists wanting to find out about impacts on the Bering Sea ecosystem. Changes in warming of the waters, distribution and abundance and sizes of some fish are happening fast.
“The climate change and warming and collapse of the cold pool are very familiar to people up here because it is happening in your back yard, Britt said at a session of the Strait Science program Nov. 7 at Northwest Campus UAF.
Research scientists surveyed the Northern Bering Sea in summer 2010. That has been the base year for additional surveys in 2017 and 2019.
However, based on findings in 2017, Britt and NOAA scientists performed a Rapid Response Survey in a part of the Northern Bering Sea in 2018. Findings in that limited survey are not being included a planned biennial time series: 2017, 2019, 2021, although 2018 findings generated more questions to which researchers are working to find answers and to form hypotheses. Now another complete Bering Sea survey is in the planning for 2020 and already partially funded, according to Britt.
A tongue of cold briny water going down through the Bering Sea, a cold pool, that has been a thermal barrier to discourage warm water fish in the Southeastern Bering Sea from distribution to the Northern Bering Sea had disappeared in 2018 according to sensors used in the bottom trawl survey.
The cold pool is approximately a 30-meter layer of cold bottom water that is 2°C or less and occurs near the sea floor.
In this season of 2019, the bottom trawl survey found a small part of the cold pool returned in the northwest Bering Sea.
Pacific cod, large fish that cannot viably operate in very cold water, which used to appear in trace numbers in the Northern Bering Sea, are now appearing there in increasing numbers. A few have appeared even farther north into the Chukchi Sea, according to other surveys.
The dramatically increased biomass of cod and pollock in the Northern Bering Sea have attracted operators of long line fishing vessels to the vicinity of St. Lawrence Island, where they have not fished in the recent past.
The NOAA scientists did not come off a research vessel tied up at Port of Nome and head to town. That ship has sailed weeks ago. Britt, Stevens and Maggie Mooney-Seus, NOAA Communications program manager for Alaska Fisheries Science, flew to Nome to deliver new information from the survey assessment to communities that depend on marine resources for food, spiritual and economic well-being.
However, the information goes both ways, according to Britt. They shared information in conferences with groups who must adapt to pressures on the ecosystem. Local people with long histories of keeping records of catches and observing fish and where they hang out have contributed invaluable assistance, according to Britt.
What findings in the survey were the most dramatic to Lyle Britt?
Certainly the appearance of larval and very young sub-arctic fish in the Northern Bering Sea, he said. “One of the bigger things that came out of our 2019 survey for us is that we’re getting better confirmation,” Britt said. “So now we’ve got repeated warm years to where we’re seeing similar patterns to where we have a better understanding of how some of these species and parts of the ecosystem are responding to the lack of a cold pool and the warmer conditions.
“I think we’re feeling a little more certain about understanding what we’re seeing. And one of the things that allows scientists to do is start asking better questions, to move forward and better understand the changes that are happening in the ecosystem.” He said that everything was just completely new after the 2017 survey. “We had no idea of knowing whether or not the next year would be warm or cold, whether species would continue to react the same way or whether the entire year was novel,” Britt continued.
“In 2018 with the rapid response survey, the environment changed even more in a quick way to the point where we felt actually during season that we needed to try to expand our sampling to at least have an understanding of how that even greater environmental change was having an effect in this region.
“Now 2019 is really the first chance we’ve had to repeatedly sample up here using the same methods, the standardized methods, the same way as we did in 2017. So, this is our first chance to have comparable data for two warm years to start making a hypothesis, testing and comparisons. So this year has much less to do with the drama or wow factor and much more to do with our having a more cohesive understanding of what warming may have for Northern Bering Sea ecosystem,” he concluded.
Strait Science is a lecture series sponsored by Alaska Sea Grant and Northwest Campus UAF.

Sunday, Nov. 17, 2019

This article was corrected to reflect the right spelling of Duane Stevenson, rather than Dwayne Stevens, and the correct title for Maggie Mooney-Seus, NOAA Communications program manager for Alaska Fisheries Science.