Bottom trawl survey shows cold pool intact again

As colder conditions returned to the northern Bering Sea, pollock and cod that had invaded the region during warmer years seemed to be absent this summer. That’s what researchers discovered when they conducted a bottom trawl survey of the eastern and northern Bering Sea this year.

The 2023 results were presented in a Strait Science lecture by the survey’s lead research biologist Duane Stevenson, who works with NOAA Fisheries’ Resource Assessment and Conservation Engineering, or RACE, division.

The primary goal of these surveys is ecosystem monitoring, Stevenson said. The scientists track changes in the food web by measuring the volume (or biomass) and distribution of fishes, crabs and other creatures that live on or near the seafloor. This information contributes to stock assessments for commercially important species. And researchers like Stevenson are also increasingly interested in measuring environmental conditions like water temperature and salinity.

“We can’t really learn a whole lot about what’s causing the changes in the populations that we’re seeing without measuring some of the variables in the environment in which they’re living,” Stevenson said.

One of those environmental features is of special concern: the “cold pool.” This is the name for the persistently frigid layer of water at the seafloor which tends to stay under 2° Celsius (35.6° Fahrenheit) most of the summer.

The cold pool is thought to act as a barrier between ecosystems that keeps certain fish and sea creatures from migrating across the different habitats. The existence of the cold pool depends on sea-ice melt sinking from the surface. After a winter with extremely low levels of sea ice, the cold pool shriveled in the summer of 2018. After another low-ice year the next winter, the cold pool was basically non-existent in 2019.

Last year’s bottom trawl survey showed that the cold pool had largely returned. And now in this summer’s survey, it was about the same size. The difference was that this year, it had larger areas of really cold water, below 0° Celsius (32° Fahrenheit).

“The overall extent of that cold pool of water was the same, but the water itself was actually colder this year than last year,” Stevenson said.

Commercially valuable fish like pollock and cod seemed to follow the warm water into the northern Bering Sea during those summers when the cold pool was shrunken or non-existent. But now those populations seem to have disappeared, and the overall fish biomass in the northern Bering Sea was down.

“Those are the two species that were really responsible for the influx of fish biomass into the northern Bering Sea during those really warm years,” Stevenson said. “We were not seeing the big densities, the big populations of pollock and cod anymore in the northern Bering Sea that we once did.”

Meanwhile, populations of invertebrates like sea urchins seemed to be bouncing back. These had a 21 percent increase from last year.

“As the northern Bering Sea becomes colder, it becomes more like the Arctic system that it used to be,” Stevenson said. “We know that in general, Arctic marine systems are dominated by benthic invertebrates. So invertebrate populations that live on the bottom tend to be most of the biomass in the system. That system kind of got turned upside down when the water got really warm in the northern Bering Sea.”

But not every species has come back with the return of cold water in the Bering Sea. For example, Stevenson said one of the biggest “headscratchers” of the last few years is that Arctic cod have not returned, and this year the researchers found even less than they have before. They estimated that the entire biomass for Arctic cod in the eastern Bering Sea is just one metric ton.

Compared to last year’s survey results, some creatures saw significant increases in the northern Bering Sea, including sea peaches, tomcods and jellyfishes.

While numbers for red king crabs and blue king crabs are low in the eastern Bering Sea, Stevenson had good, or at least cautiously optimistic, news to report about crabs in the northern Bering Sea.

“It looks like we saw an increase in red king crab biomass this year relative to last year, but almost exactly the same as what we saw in 2021,” Stevenson said, adding that blue king crabs had slightly higher numbers this year.

Other species were holding steady compared to recent years, including the Alaska plaice and Alaska skate. Purple-orange sea stars remained ubiquitous.

Meanwhile, other species besides pollock and cod also saw declines, including Pacific herring, sea onions, northern rock sole and Bering flounder. The researchers saw fewer halibut in the in eastern and northern Bering Sea as well.

Worryingly low numbers recently forced Alaska’s first-ever closure of the winter snow crab season in the Bering Sea, and researchers are still trying to understand the population’s decline. The northern Bering Sea population of snow crabs is much smaller and tends to be relatively stable. That trend held true for this year’s bottom trawl survey.

“We don’t see the huge declines in the in the northern Bering Sea that we’ve seen in the eastern Bering Sea, and the distribution is pretty stable as well,” Stevenson said. “But what this tells us is that the reason that we saw this huge decline in the eastern Bering Sea is not because they were all moving to the northern Bering Sea.”

NOAA started its annual trawl surveys of the eastern Bering Sea in 1982. But this year only marks the sixth time the agency has extended the survey to the northern Bering Sea.

In addition to expanding the survey’s geographic range, the team has also been trying to expand the list of environmental factors it can monitor. Stevenson said the team started experimenting this year with measuring pH and dissolved oxygen levels in the water.

“We’ve all heard by now that the oceans of the world are becoming more acidic as they absorb more carbon dioxide from the atmosphere, and we know that that lowers the pH in ocean water,” he said.

The trawl net is not designed to collect the species that tend to accumulate toxins from harmful algal blooms, such as clams, mussels and cockles. But since harmful algal blooms have been a major concern for the region this past summer, Stevenson said that when these species show up in the net, the researchers send them in for testing.