Northern Bering Sea Trawl Survey Shows Dismal Outlook For Salmon

NOAA survey also showed lowest levels of herring, capelin ever seen; a third of pollock sampled had ichthyophonus parasite.

by | February 29, 2024

Off-kilter water temps and fish diets; pollock likely source of parasite infecting Chinook, says NOAA project

By Megan Gannon/Nome Nugget
February 16, 2024

The northern Bering Sea is still rebounding from a warm period four years ago, and juvenile Chinook and chum salmon stocks are still low in that ecosystem, according to the results of a recent surface trawl survey.

Jim Murphy, a research biologist with the Alaska Fisheries Science Center, gave a Strait Science lecture on Monday evening to describe the findings of 2023 federal survey.

NOAA initiated its northern Bering Sea surface trawl survey in 2002 to study juvenile salmon in the ocean. Over the last two decades, the project has expanded to look at forage fish, seabirds and marine mammals and benthic species, too.
The 2023 iteration of the survey confirmed what other scientists have reported: That water temperatures have dropped since the recent marine heatwave.

“We’re seeing a fairly abrupt cooling of the northern Bering Sea from the warm temperatures we saw in 2019,” Murphy said.
That cooling seems to have had effects on the makeup of species. The survey looks at zooplankton abundance, and Murphy reported that large copepod species seem to have bounced back after a record low abundance in 2019. Large copepod species are known to carry a lot of fat, Murphy said, so this particular prey group has been very important for understanding the transfer of fats to animals higher up the food chain. Meanwhile, the opposite pattern was true for small copepods, which increased in abundance in 2019 and have since dropped.

Copepods, or baby zooplankton, in Arctic waters Image credit: Oceanbites

Forage fish like herring and capelin also impacted by the temperature spike.

“We typically see higher biomass of herring and capelin in the northern Bering Sea, compared to the southern Bering Sea,” Murphy said. “But we saw both species drop to the lowest levels that we’ve seen in the entire history of this sampling program during 2019, that really warm year. We’re seeing some evidence of a recovery of the abundance of these species. But they haven’t returned to the levels that we saw in the previous cold years, and so obviously temperature will have a very direct and immediate effect on the abundance of these species and in their distribution.”

“This vulnerability (of herring and capelin) to temperature change is concerning because there are a lot of predator species that are dependent on these forage fish, such as Chinook salmon and seals.”

jim murphy, Noaa biologist, ak fisheries science center

Pollock is a likely source of deadly Chinook parasite

In 2021, the surveyors started looking for ichthyophonus in their samples. Ichthyophonus is a parasite that infects Chinook salmon and has been a major concern for Yukon River salmon. A third of the immature Chinook that the researchers collected last year had ichthyophonus, which was less than previous years. Murphy also said that a third of the one-year-old pollock samples had ichthyophonus and speculated that they are the source of the parasite that’s spreading to salmon.

“Whatever conditions that will cause Chinook salmon to feed more on pollock than they have historically is likely to be a source for why ichthyophonus would vary,” he said.

Image credit: Canadian Science Publishing

LW: Abstract of Journal of Fish Diseases, 2013

In 2003, the Alaska walleye pollock industry reported product quality issues attributed to an unspecified parasite in fish muscle. Using molecular and histological methods, we identified the parasite in Bering Sea pollock as Ichthyophonus. Infected pollock were identified throughout the study area, and prevalence was greater in adults than in juveniles. This study not only provides the first documented report of Ichthyophonus in any fish species captured in the Bering Sea, but also reveals that the parasite has been present in this region for nearly 20 years and is not a recent introduction. Sequence analysis of 18S rDNA from Ichthyophonus in pollock revealed that consensus sequences were identical to published parasite sequences from Pacific herring and Yukon River Chinook salmon. Results from this study suggest potential for Ichthyophonus exposures from infected pollock via two trophic pathways; feeding on whole fish as prey and scavenging on industry-discharged offal. Considering the notable Ichthyophonus levels in pollock, the low host specificity of the parasite and the role of this host as a central prey item in the Bering Sea, pollock likely serve as a key Ichthyophonus reservoir for other susceptible hosts in the North Pacific.

Journal of Fish Diseases, White, Morado, Freeman, 2013

LW: Here is a power point presentation titled “Investigating the impacts of Ichthyophonus on Yukon River Chinook salmon” by the AK Dept. of Fish and Game, 2023

Outlook for Chinook and Chum salmon “is probably not going to get much better.”

Murphy presented other evidence for how the warm period seemed to change the diets of Chinook salmon.

“In recent years, since this warm period, you’ve started to see a lot more diversity in the diets of juvenile Chinook salmon,” Murphy said. Juvenile chum diets are becoming more diverse, too.

The data is meant to help forecast salmon runs, and the outlook for Chinook and chum salmon continues to be grim.

“I think all the juvenile data for Chinook is indicating that things are probably not going to get much better,” Murphy said.  But he hoped that with temperatures in ocean water and freshwater starting to cool, the fish would have less issues during their spawning migration. “There could be some improvement there,” he said.

Although chum salmon seemed to be fattier and better in body condition last year, their abundance is still way below average.
While the researchers have been predicting the decline in salmon runs, they have been trying to figure out why some of their forecasts for certain stocks have not been matching up with reality.

“For the Canadian-origin stock group, the actual run sizes have been coming in well below the forecasted run sizes for about the last six years,” Murphy said. “We’re seeing the run sizes just not reaching the expected run sizes based on juvenile abundance…If you look at the total Yukon forecast, we’re not seeing that same pattern.”

Gay Sheffield, the Alaska Sea Grant agent who hosts the Strait Science series, asked Murphy if NOAA was looking at the overall carrying capacity of the ecosystem considering all these indicators of decline.

While Murphy said carrying capacity would be really difficult to get a handle on, he noted that some future research might be able to help put together a big picture look at what’s happening in the Bering Sea.

“In the next year or two is the North Pacific Research Board is going to launch their integrated ecosystem research plan, which is going to be basically building models studying the whole integrated ecosystem,” Murphy said. “So I think, once the proposals and the research come through to look at it from that perspective, I think we’ll have better answers as to what’s driving it.”

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About Laine

Laine Welch has covered the Alaska fish beat for print and radio since 1988. She also has worked “behind the counter” at retail and wholesale seafood companies in Kodiak and on Cape Cod.

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