GIVING TO SCIENCE—A large Pacific Cod specimen caught just south of Little Diomede Island is being weighed and measured prior to collection of otoliths (ear bones) for determining the age of the fish and its overall condition.

NOAA survey shows shocking lack of thermal barrier between northern and southern Bering Sea

NOAA Fisheries scientists conducting their annual trawl survey of the southern Bering Sea ecosystem survey found unprecedented conditions of warm ocean temperatures and significant changes in the cod and pollock numbers and conditions. 
As a result, they received emergency funding to unexpectedly bring their trawl survey north from St. Matthew Island to Diomede Island to investigate.
With their survey still underway, what they have found is a shocking lack of the colder ocean waters that separate the northern and southern Bering Sea marine ecosystems. The “thermal curtain” is another expression for “cold pool” that acts as a barrier to keep some species—pollock and Pacific cod, for example — from migrating across the eastern Bering Sea shelf and northward toward the Bering Strait.
 The scientists are expecting some potentially big changes in fish species distribution due to the disappearance of a “cold pool” within the Bering Sea south of St. Lawrence Island. The cold pool is created by super-cooled briny water made from salt leaking from winter sea ice and dropping to the seafloor.  This annual thermal barrier typically keeps the fishes of the southern Bering Sea in their preferred warmer water.
After many years of diminishing sea ice, the past winter of 2017 was unprecedented with near open water conditions throughout the winter months.   It seems that the extensive open water season last winter was long enough to make a big difference.
Britt tells more about how they found the familiar cold pool missing.
“We started this year’s southern survey going east to west right across the shelf from the Aleutians to St. Matthew Island. We had watched this winter as to the unprecedented lack of sea ice in the Bering Sea. We noticed that it was extraordinarily low. It was very ephemeral and gone. It had us very much worried about water temperatures for summer 2018,” he recounted.
As part of the survey, the scientists monitor the ocean temperatures near the seafloor because of the importance of the thermal barrier formed by cold salty waters associated with sea ice coverage that is formed annually.
“Its extent from one year to the next —due to the extent and timing of the annual sea ice coverage in the Bering Sea— can have a huge impact on the distribution and types of fish populations between the northern and southern Bering Sea shelf,” Britt continued.
“At the same time, it becomes a refuge for species from the Arctic that are adapted to those temperatures. It gives them an ability to kind of sneak down in here and use resources in an area that wouldn’t otherwise be available,” Britt explains. “The extent or length of that cold pool can be really valuable in structuring fish distributions and populations throughout the region.
“As we started moving across [the Bering Sea shelf], to the north we were not recording any 2°C water. For the first time in 37 years of surveying the Bering Sea, we could not find the cold water barrier.”
More detailed information will be available this November when the survey has been completed and the results have been analyzed.
The NOAA Fisheries surveys the southern Bering Sea annually but has been surveying the northern Bering Sea only two years, in 2010 and 2017, and intends to perform another in 2019, adopting a biennial survey schedule after that.
This NOAA fish survey is for science and all data are publically accessible to commercial and subsistence stakeholders alike. The group is using a small test trawl net to assess the types, ages, diet and numbers of fish and other marine life throughout the Bering Sea, to assess the health and status of the ecosystem.
“What do you think we found in 2017? About a 6,000 percent increase in the biomass [volume] of pollock and a 900 percent increase in the number of cod in this northern area, with a huge population at the south east corner of St. Lawrence Island, lots along the Russian border, all the way up to the Bering Strait and not nearly as much distribution down throughout the Southern Bering Sea,” Britt told a public gathering at a Strait Science Series presentation at Northwest College UAF last Wednesday.
He concedes the 2010 to 2017 increase in cod numbers was dramatic because previously there were so few Pacific Cod in the northern section of the Bering Sea.  
 Britt came off the F/V Alaska Knight in Nome last week. A group of scientists were conducting an unexpected trawl survey in the northern Bering Sea, based on unexpected results in the 2018 southern Bering Sea survey.
“This really gets things on our radar screen. What’s going on? Why are the fish heading north?” Britt and other research scientists wonder.
Mimi Farley of Nome has taken part in commercial crab and fish harvests with her family business for decades.
“For the past two years or so I have seen more than the usual number of cod and pollock in our crab pots when we pull them up. Last year they were small. This year they are really big,” she reported.
Britt confirmed that all the cod they found were big.  “Where did all the young fish go?” Britt wondered. “Also, we really did not get a lot of pollock. The pollock were not in the area of the shelf where they usually feed a large fishery. At the same time, we were getting reports of temperature anomalies—jelly fish sightings, things in this region that we wouldn’t expect to hear about.”
In the 2010 survey, the researchers found an average cold pool along the area of the ice extend. The cold pool was not strong but it was enough to structure the populations of fish in these regions, according to Britt. 
In 2017, researchers found a cold pool running between St. Lawrence Island and St. Matthew Island.
“What do you think we found this year?” Britt asked. “No cold pool. None. In 37 years of surveys, this is the first time we have not found in a single water station with a bottom temperature of less than 1° Celsius,” Britt said. “When we got to the point in the survey and realized there was not going to be a cold pool, we’re not seeing pollock the way we should see them, we are not seeing invertebrate population where we should be seeing them. And, we’re hearing about seabird die-offs.”
Charles Lean, fish biologist observing systems in local waters for 40-odd years noted Monday that several years back the cod and pollock were small, but now he sees larger ones, and by his estimates, two times as many Pacific cod and about 10 times more pollock hereabouts in the Northern Bering Sea.
At the presentation at Northwest Campus UAF, Britt selected photos from one station to explain the degree of change the team sees in this region. Britt pointed on a map to a locale in the northwest corner of the Northern Bering Sea survey area where he could see the Diomede Islands from the vessel. Britt has a photo from the 2010 survey at the station that shows a laundry basket sized container of fish.
“What you see her is a lot of small fish. There are saffron cod, a lot of Arctic cod, capelin, sand lance, four or five kilos of catch. Our net opening is about 50 feet wide.
“What do you think we saw in 2017? On our sorting table is just short of one ton, many yellow fin sole and pollock. Essentially the invertebrates, shrimp, crab, are the same players in 2017.
“But, holy cow! What do you think we saw two days ago? Almost entirely fat, adult pollock. This catch is just shy of five tons!”
Someone in the audience says, “Wow.”
“Of that five tons, 4.1 tons are adult pollock,” Britt said. “There were also 800 pounds of every size of cod.”
But they did almost no invertebrates like crab. “The invertebrate population should have been the dominant catch in the trawl in this region,” he said.
“This is just south of the Bering Strait, about 18 miles southwest of Diomede Island,” Britt said.
“Was that just a 30 minute tow with your net?,” someone asked.
“Exact same tow length, exact same procedure, exact same station, in all three,” Britt responded.
What does this mean for the Bering Sea?
Without the annual cold water barrier this year, there is no separation between the southern and northern Bering Sea. “We are all in this together now,” Britt said.
Scientists were asking questions. Did the fish migrate from the southern Bering Sea?
“Those [in the southern Bering Sea] might have died,” Britt suggested. “They might have come from the Russian side, we don’t have any idea. They might be a mixture of both.
Are they in the Chukchi Sea?
We just don’t know. There is great pressure on the fitness of other organisms, Britt pointed out. “They have to eat something.”
What about the marine mammals, the birds, the bird die-offs?  Is this all related?
Researchers don’t really have the data for answers.
 “The Bering Sea is broad and deep in the south but shallow and narrow in the north, so another thing we are worried about is there is less habitat, less food source for this huge biomass of pollock and cod,” Charles Lean said in an interview.
“Are they going to be eating us out of house and home?”
“So they’ve displaced saffron cod, Arctic cod, sand lance, capelin, all the forage food have moved on,” Lean said. “Those are what feed the birds. They’ve driven those away.
That’s why we are having the bird die-off.”
The big picture comes out to ecological transformation, changes in marine fisheries management, and more than an inkling of the warm future of global oceans.

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