TRAWL SURVEY—Lyle Britt, fish biologist, sorts crab taken from the sea floor in summer 2018 NOAA survey of northern Bering Sea. Britt and team of scientists have been researching effects of climate warming and receding sea ice with startling results.

Scientists research link between warming waters and fish redistribution

Based on scientists’ summer surveys, drama is happening in the northern Bering Sea—warming waters, changes in fish distribution, receding sea ice and a missing thermal barrier to fish migration north and south or east and west, are just examples of findings by NOAA trawl surveys in recent years.
Whatever the causes, climate change is happening based on documented evidence in the Arctic. However, Lyle Britt, a fish biologist who has been leading NOAA survey projects, will say “it is a real case of a lot of unknowns.”
National Oceanic and Atmospheric Administration has been conducting annual trawl surveys in the southern Bering Sea since 1982. Now drastic changes have drawn interest to the northern Bering Sea.
“I am presenting the information, but we have a large group of researchers working very diligently to work up data from this summer’s surveys,” Britt said.
“The overarching thing is that we seem to be in a warming period right now. That’s a little bit spooky for people,” Britt said.  
However, the warm period may not be forever. “Overall models people have put forth say that for this region, there will still be some year-to-year variability. So nothing says that we are absolutely going to stay warm. Over a long period of time we should see a general warming and less and less sea ice. That doesn’t mean that next year or even this winter, there won’t be a cold period for things to reset themselves a little bit, or, continue to be warm. And that is why exactly we can’t really model what’s going to happen with the fish population,” he said.
By a small difference, maybe a couple thousand, there are more cod in the northern Bering Sea than in the southern Bering Sea, according to Britt. However, no one knows where they came from, whether they will stay, or whether they will survive.
“We have to keep in mind we’ve really been here surveying in a comprehensive sense only two times. Once in 2010, which was a cold year, and we had a good cold pool at that time, and in 2017, last year, when there wasn’t a very strong cold pool, but there was some remnants of a cold pool.”
 A cold pool is defined as bottom waters that are 2°C or colder.
 Notably, in the 2018 trawl survey, the team found a shocking lack of the colder ocean waters that separate the northern and southern Bering Sea marine ecosystems. This cold pool, when present, acts as a thermal curtain. 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.
They are crediting some potentially big changes in fish species distribution 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 the extensive open water season last winter was long enough to make a big difference.
“This year, 2018, was an informative survey rather than a comprehensive survey. It doesn’t have the same power as the stuff we did those previous two years. We didn’t do the same number of stations; we didn’t do the same extent; we didn’t get into Norton Sound. All three years aren’t really directly comparable—and we had limited time,” Britt explained. “So we had a colder year, we had a warmer year, and we had some information for this year. Where the real power in our type of data comes is the more years we get in, the better off we are. The positive side is that we have secured at least some funding to come back again next year.”
Britt and a team of scientists surveyed stations set in a grid pattern in the northern Bering Sea earlier this summer and gave a preliminary report in a presentation sponsored by UAF Northwest Campus and Alaska Sea Grant in Nome in August.
He returned to Nome again earlier this month to share data after sorting his findings and measuring the biomass, ages and sizes of the species of fish taken in the trawl survey.
Fish are on the move also, another challenge to understanding the Bering Sea ecosystem. Scientists think the absence or presence of the cold pool influences the fish distribution. The cold pool varies from one year to the next, with a lot having to do with the strength and coverage of the previous winter’s ice.
“If you have a really cold winter with lots of ice with long residency time, you get a year like 2012, where you get a large amount of cold water along the bottom that will reach down to the Alaska Peninsula and even reach up toward Bristol Bay,” Britt said. “In years where you have less residency of ice, and maybe not as thick and strong, you get something like 2011. You still have a cold pool, but it doesn’t extend all the way down.”
The ice melts and leaves cold water. As the ice melts, the heavier brine drops down and flows along the bottom.
 “I’m really interested in how this affects the fish and other organisms,”  Britt told the audience at the Grand Hall at UAF Northwest Campus.
“We have what we call a series of domains. The area along the coast of Alaska we call the inner domain, with water less than 50 meters deep and a little warmer in most years. Then we have the cold pool region, between about 50 meters and 100 meters deep, where most of the cold pool forms, which we call the middle domain. The outer domain is 100 meters to 200 meters depth of our survey grid.”

Thermal curtain
 “There are a lot of species, Pacific cod and walleye cod included, that really metabolically cannot deal well with this cold pool water. It makes it harder for them to do things, like digest their food, so they don’t like to enter it if they don’t have to, so they try to stay out of it,” Britt explained. “So this water is like a de facto barrier to fish movement east and west across the Bering Sea.
“In some years there is less a barrier to north and south, but still a barrier. Sometimes we have warm years, like 2003, 2004, 2005, 2014, 2015, 2016, 2017 and this year, 2018. In earlier years there was a reduction in the cold pool, but always some, until we get to 2018. No cold pool all the way up through the Bering Sea region to your back yard,” Britt said. “There is no barrier for fish passage from a thermal perspective.”
Based on summer 2018 surveys, there were only a couple of stations in the southeast Bering Sea hinting at being in the cold pool area and effectively not a cold pool at all in the northeast Bering Sea—just a small residency of cold water west of St. Lawrence Island, according to Britt.
It is possible the biomass missing in the southeast Bering Sea went north, but that is a little too easy an explanation, according to Britt. “We really don’t know,” he said. “Between 2010 and 2017, that’s eight years. It could be possible theoretically that the fish have just done well and grown up here. Another thing that is possible is where are the fish located most? Right along the Russian border. It is very possible with the warming as well that we are seeing fish that have come east out of Russian waters into United States’ waters.”
This raises a lot of questions and issues that Britt wants to start addressing in 2019.
Two ways of going about it is by tagging fish and by studying genetics. Tagging fish to track movements opens up another set of unknowns. “Do those fish just go north and stay? Or when the ice starts to form and the weather gets cold, do they go south? Is this summer vacation for them, or is this a long term thing,” Britt and the groups of scientists he works with want to know. “If they came from Russia, do they go back west?
“If they stay north, are they able to reside under the ice and do well assuming we have ice, or will they not do well and we have a large mortality event. To all these questions we don’t have answers.”
Another scientist, Ingrid Spies, with NOAA, has been working on the underpinnings of a genetics program on Pacific cod to find out more about the origin of these fish and their movements. Cod usually stay in the deeper outer domain along the Alaska Peninsula.
 “In 2017 we saw that break down with fish spread across the shelf, a lot in Bristol Bay,  a lot in the northern Bering Sea region and in 2018,” Britt said. Even more shocking, there are a lot of Pacific cod in the northeastern Bering Sea region right now, according to Britt.
“When we look at biomass numbers, in 2010 there were 26,000 metric tons of cod; in 2017, 289,000 metric tons; in 2018, a whopping 564,000 metric tons,” according to Britt’s figures.
“In 2018, the biomass of Pacific cod was larger than we calculated for southeast Bering Sea,” Britt said, “so there are a lot more cod up here.”
When the surveyors look at length, they see similar to what they saw in walleye pollock, where there was not much of anything in 2010, but when they got into 2017, they got a lot of sizes—young fish and old fish.
“In 2018, you see a lot of young fish from 10 cm to 14 cm, that should be a one-year-old fish, even younger,” Britt said. “We are wondering how these fish got up here.
“Can a one-year-old or younger fish actually do a migration all the way up, or are these fish actually starting to reproduce in the northeast Bering Sea? “ Britt mused.
“We don’t know the answer because they could have come from Russia—which is a much shorter distance, but if the fish came from this region, how did the small young fish get here without coming from localized recruitment?
“We don’t have an answer, but it gives us a hint that maybe Pacific cod are hanging out here a bit.”
The scientists are getting basically no information from the Russian side, according to Britt, but scientists are working to change that. “I am very hopeful,” Britt said.

What is next?
The highest densities of pollock and cod in the Bering Sea are right up against the Bering Strait, according to surveys.
“We all know there isn’t some big magic gate sitting there that keeps the fish from continuing north,” Britt said. “There is a likelihood some of these fish have spilled into the Chukchi Sea.”
Britt and his crew will be back. “What happens in terms of this winter’s weather and ice and all will tell lot about where things are going to go, so 2019 will be very important,” he said. Meanwhile, local residents can help, telling local wildlife agencies their observations like unusual piles of frozen fish, strange fish in their catches, dead sea mammals, dead birds or other environmental changes.

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