Researcher reports declining numbers of fish after trawl survey
By Julia Lerner
Key marine species in the Northern Bering Sea, including several types of crab and fish, have seen significant population declines over the last several years, according to a new bottom trawl survey organized by the National Oceanic and Atmospheric Administration. The survey, conducted throughout the summer months in both the Northern and Eastern Bering Sea, explored the current state of the sea floor.
“Our results are not just about the fish,” explained Lyle Britt, director of the resource assessment and conservation engineering division of the Alaska Fisheries Science Center. “We also collect a lot of environmental data, in particular water temperature, and we actually track the cold pool.”
Britt presented the survey’s findings during last week’s Strait Science lecture to an audience composed of local subsistence hunters, scientists and researchers, and commercial fishery executives.
“We’re looking at the movement patterns of fish and crab and other invertebrates that live near or on the bottom [of the sea],” he explained. “We want to look at population size and structure, age, genetics for very important species. We also collect food web data, so we look at the stomachs and what everything is eating and how it relates. And we collect environmental data: water temperature, salinity, light. Most of this has been driven by concerns about the loss of sea ice.”
The cold pool Britt tracked “is essentially the remnant of seasonal ice from the previous winter,” he explained. “It establishes a kind of a barrier that historically made the Northern Bering Sea very arctic in its profile, whereas south of that barrier was a much more temperate, or sub-arctic community.”
Over the last decade, researchers noticed the cold pool shrinking significantly. In 2017, they realized the cold pool was no longer serving as a barrier between the marine populations of the southern and northern Bering Sea.
“We had a little more ice in the 2020-2021 winter than what we’ve had in the last couple years, but effectively, it still did not create much of a cold pool,” Britt said.
The location and size of the cold pool matters, and helps researchers track movement of arctic and sub-arctic fishes.
“It effectively is a physiological barrier to what we would call sub-arctic fishes,” Britt explained. “Things like pollock and cod, they prefer to not be in water that cold.”
In 2021, the sea temperatures were lower than they have been in recent years, potentially impacting the fish populations in the region.
The bottom trawl survey helps researchers to study the entire marine ecosystem of Alaska, Britt said. NOAA has been conducting these surveys for years. The Eastern Bering Sea survey has been conducted annually since 1982, and the Northern Bering Sea survey has been conducted several times over the last decade, including in 2010, 2017, 2018, 2019, and this year. Researchers were not able to conduct the survey in 2020 following widespread COVID-19 concerns but were able to make the trip this summer.
“Why are we doing these surveys?” Britt asked the audience. “It’s really about trying to monitor and assess the entire marine ecosystem of Alaska. … [There was] a growing concern regarding the loss of seasonal sea ice, and with that loss of seasonal sea ice, the real concern about how the ecosystem will respond.”
Britt and his team collected data points on dozens of fish and wildlife from 144 stations across the Northern Bering Sea. They collected samples and studied more than 48,000 fish from 21 species, as well as 19,518 carapace measurements from three species of crab. Researchers also studied the stomach contents from four species of fish.
Though Britt studied thousands of fish, he presented key findings on select populations, including red king crab and snow crabs, several species of cod, halibut and pollock. Almost every population showed declining numbers.
Researchers are concerned about the declining crab populations.
“Our estimates of snow crab populations have declined precipitously from the 2019 survey to the 2021 survey,” Britt said.
In 2010, researchers estimated about 319,000 metric tons of snow crab biomass in the Northern Bering Sea region. In 2017, the estimated population declined 29 percent to 225,000 metric tons. In 2019, estimates fell another 29 percent to 159,000 metric tons, and since then, researchers estimate the population has more than halved. Following the 2021 survey, they estimate just 73,994 metric tons of snow crab biomass in the region.
“The crabs that we saw when compared to the previous years are slightly smaller on average, but not in a grand way,” Britt said. “But the number of crabs we’re seeing is very low. We didn’t really see large pockets of population in 2021.”
Red king crab populations are also on the decline.
“The part of this that’s a little bit concerning is, at least in our survey data, we’re not seeing the same young crab that we saw before,” Britt said. “The red king crab that we’re seeing are pretty localized in the Norton Sound area, but they do extend out to almost to the eastern side of St. Lawrence Island.”
Researchers are able to track cohorts, effectively the same groups of a species over time as the animals age and grow. With red king crabs, Britt says they’re able to track one cohort growing, but they’re not seeing new cohorts cropping up.
Blue king crab populations, too, have halved. The number of crabs researchers have been able to catch and study fell from 122 blue king crabs in 2010 to just 66 this year.
“We’ve dropped below the biomass estimates that we had around 2010 [for the blue king crab],” Britt said. “We had about 2,000 metric tons in 2010, and now we’re down to around 1,000 metric tons, so about a 50 percent decline from 2010.”
Since 2019, the last time the survey was conducted, blue king crab population estimates declined about 12 percent.
Many fish populations in the region also showed significant levels of decline over the years, including the Arctic cod.
Arctic cod populations have almost completely collapsed since the 2010 survey.
“We’ve talked about these [fish] a lot in recent years when I’ve given these talks, because it’s been pretty sobering,” Britt said. “This is obviously a kind of arctic species and one of the ones that would use the cold pool as a refuge.”
Britt said Arctic cod populations were much larger in 2010, when the cold pool was much larger. That year, there were almost 38,000 metric tons of Arctic cod estimated in the Northern Bering Sea. That estimated number fell 90 percent in 2017, to just 3,906 metric tons. The numbers continued to decline, in 2019, where there was only 47 estimated metric tons. This year, estimates put the Arctic cod population around 83 metric tons.
“There is a little bit of an interesting and potentially bright spot here,” Britt said. “Temperatures are slightly cooler this year. Again, not cold, but a little bit cooler and we actually got 83 metric tons. That’s a 77 percent increase from 2019 levels, but still well below what we were seeing in the cold phase of 2010.”
The Saffron Cod, locally called tomcod, is an important subsistence fish, but reported populations have declined significantly since the 2010 bottom trawl survey.
In 2010, biomass estimates of the tomcod were around 90,299 metric tons. The population shifted slightly in 2017 to an estimated 76,238 metric tons, and then again to 81,269 metric tons in 2019. This year, researchers estimate only 9,972 metric tons of the species, an 88 percent drop from the year before.
Until more data is available, Britt says to take the tomcod data with a grain of salt.
“I would not try to read too much into the data for the species,” he said. “Their population moves around a lot and they can also be mid-water, so our offshore bottom trawl is not the best way to sample for Saffron Cod.”
The Pacific capelin also saw catastrophic declines since the 2010 survey.
In 2010, the capeline had an estimated biomass of 14,632 metric tons. In 2017, that number fell to just 179 metric tons. The population continued to shrink to just 50 metric tons in 2019, though grew by 52 percent to 76 metric tons this year.
“This kind of gives me a little glimmer of hope,” Britt said. “Where this is a more arctic species, [it’s] also a kind of a critical forage fish species for seabirds and marine mammals.”
Several other species showed population declines since the 2010 survey, including the Bering flounder, the Plain sculpin, the purple-orange sea star, the sea peach and jellyfish.
Shorthorn sculpin populations declined significantly. While biomass estimates in 2010 saw around 39,824 metric tons of fish, estimates in 2017 saw more than 111,000 metric tons of fish. Between 2017 and 2019, though, the population fell 87 percent to 14,159 metric tons. This year, biomass estimates just 7,626 metric tons of the fish.
Pacific halibut, a fish often subsistence harvested, is one of the few fish to show a population increase over the last decade.
In 2010, biomass estimates for Pacific halibut put the population around 23,333 metric tons. The population fell to only 18,507 metric tons in 2017 but rose to 25,722 metric tons in 2019. The estimates have remained steady since then, rising only 1 percent since 2019 to 25,995 metric tons in 2021.
Most bottom-dwelling arctic fish populations showed significant levels of decline, according to survey results, but southern fish, like the Pacific cod, have fared well since the 2010 survey.
Between 2010 and 2017, Pacific cod biomass estimates rose 887 percent, from 29,124 metric tons in 2010 to 287,535 metric tons in 2017. The estimates rose again in 2019 to just shy of 365,000 metric tons. but fell about 38 percent this year. Researchers estimate about 227,577 metric tons of Pacific cod this year, still significantly more than the population just a decade earlier.
“The numbers went from very few in 2010 during that cold [pool], to large numbers of very large Pacific cod in the region,” Britt said. “[This occurred] in the most recent years with the warming [sea temperatures].”
Britt says cod populations moved further south this year.
“One of the things that has actually changed, and this may be because conditions are a little bit cooler [this year], is that instead of seeing large, large, large catches of cod north of St. Lawrence island, we had some okay catches up there, but the predominance that we’re seeing for this region is actually what we consider to be more of the southern half of our Northern Bering Sea survey,” he said.
Walleye pollock, too, showed population growth during the warmer years.
“In the colder years, when that cold pool extended down, much of this sub-arctic population was not wanting to cross that barrier,” he explained. “When it broke down, fish moved across the shelf and started showing up in large numbers in the Northern Bering Sea, except for this year.”
In 2010, one of the colder years surveyed, researchers estimated around 21,141 metric tons of Walleye pollock in the Northern Bering Sea. In 2017, that number jumped over 6,000 percent, to 1,319,062 metric tons. Since then, the numbers have declined more than 50 percent, to 1,167,099 metric tons in 2019 and just 474,448 metric tons in 2021.
While 474,448 metric tons is significantly higher than the 2010 estimates, Britt says 2021 estimates are low across the board.
“Maybe that’s fish moving back down with the slightly cooler water more towards the southeastern Bering Sea region that we would expect them to be in, but when you look at our eastern Bering Sea estimates, [the Walleye pollock population] actually dropped 44 percent [this year],” Britt said.
This is the third-lowest estimate of Walleye pollock in the eastern Bering Sea shelf since the survey began in 1982.
Several species saw growth between 2010 and 2021 but began shrinking in recent years following changing climate and sea temperatures.
The Northern rock sole, the Alaska skate, the Rainbow smelts and the Pacific herring saw population growth between 2010 and 2019, but have since shrunk.
The Northern Neptune whelk, a type of sea snail, is one indicator of volatility in the region, Britt says.
The population, typically a very stable and even population, saw significant growth and population loss between 2010 and 2021. In 2010, researchers estimated the whelk’s biomass to be 110,916 metric tons. The population grew 61 percent in 2017 to 178,930 metric tons, and then fell significantly to 146,344 metric tons in 2019. This year, researchers estimate 114,179 metric tons of the snail.
“This may be indicative of a kind of an ecosystem in flux,” Britt said. “If you look prior to 2017, historically in our data set the species wasn’t quite this volatile. It tended to be pretty consistent in our catches, but in more recent years it’s become much more up-and-down patterns from one year to the next.”
Britt also shared information about ongoing special projects related to the Bering Sea, including cod tagging and harmful algal bloom sampling, a cooperative project between Kathi Lefebvre at the Northwest Fisheries Science Center and Gay Sheffield with the Alaska Sea Grant program. Sheffield is also the organizer of the Strait Science lecture series at UAF’s Northwest Campus, the program through which Britt delivered his presentation on Thursday.
Following Britt’s presentation, the several-dozen listeners had the opportunity to ask the researcher questions about his work, ongoing research collection and how his team worked with the indigenous communities in the region.
Throughout his presentation, Britt pointed to the support of indigenous communities in the region, and said their knowledge was helpful in data analysis, particularly during the months when the research teams were not actively present.
“I’m not convinced that all the 16 communities in this region are so keen to help with observations, giving you data, doing a lot of work for you,” said Austin Ahmasuk, the marine advocate at the Kawerak, Inc., Marine Program. “The North Pacific Fishery Management Council is destroying the Bering Sea ecosystem and that’s what we’re dealing with now, so I have a concern there and I’m wondering how you might be able to improve that trust.”
Ahmasuk also advised Britt that his characterizations of subsistence hunters and lifestyles was lacking.
“Not all subsistence resources populations or subsistence food local components related to subsistence are reviewed or evaluated by your work, so there needs to be some treatment of that,” he said. “As you noted, we have a better understanding of how changing fish stocks and species and climate change affect our subsistence lifestyle, and so the characterization that you made … needs to be revised in terms of subsistence, because there’s a much wider array of subsistence resources that we utilize.”
The Strait Science lecture series, co-produced by UAF Alaska Sea Grant and UAF Northwest Campus, connects scientists and researchers with residents in the region. The next presentation will take place on Thursday, November 11, where Jim Murphy, a research biologist with the Salmon Ocean Ecology and Bycatch Analysis group at the Alaska Fisheries Science Center, will discuss the 2021 Surface Trawl Survey.