Surface trawl survey reveals shifting fish populations

By Julia Lerner
Researchers are predicting low fish runs in the Norton Sound and Northern Bering Sea region again next year, according to research biologist Jim Murphy.
Murphy, who works with the Salmon Ocean Ecology and Bycatch Analysis Group at the Alaska Fisheries Science Center in Juneau, presented the findings of the recent 2021 surface trawl survey during a Strait Science event. The survey, which tracks marine life across the surface and midlevel of the northern Bering Strait, was conducted in September this year. Researchers studied salmon, seabirds, shrimp, zooplankton and several other marine species.
The surface trawl survey has been conducted every year for almost two decades, and Murphy says when the survey is conducted is crucial. “The timing of the survey was established at the beginning to match the timing of marine entry and dispersal of juvenile salmon from estuarine habitats, and we’ve attempted to keep the timing of the survey as consistent as possible.”
Though the primary purpose of the surface trawl is to track pelagic fish, or species found in the middle and upper water columns, and invertebrate populations, researchers also collect zooplankton and sediments, as well as bottom-dwelling fish, crab and invertebrates.
In addition to tracking marine life, the research team, comprised of researchers from U.S. Fish and Game, U.S. Fish and Wildlife Service, and the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory, also track water properties, including temperature and salinity, including other characteristics of the sea surface, at 46 primary stations across the Northern Bering Sea.
“We collected water for nutrients, environmental DNA, and also phytoplankton that includes some of the harmful algal bloom species,” Murphy said. “We also monitor the distribution abundance of marine birds during the survey.”
Marine bird populations have caused concern for Bering Strait residents, who have witnessed significant die-offs of several seabird species in recent years. Across the region, migratory seabird populations, including kittiwakes, auklets, murres and shearwaters, have been shrinking, and dead birds have appeared on regional beaches. At this point, though, researchers have not been able to provide a conclusive answer as to why these seabirds are experiencing such significant die-offs.
As part of their analysis, Murphy’s team tracked water temperatures at both the surface and the floor of the sea.
“Although the water temperatures cooled from the record warm temperatures we saw in 2019, the surface temperatures were slightly above average for 2021,” he told the audience on Thursday. “Temperatures on the bottom have a much greater range than the temperatures at the surface in the Northern Bering Sea. Surface temperatures range from approximately 7 to 11 degrees Celsius [44.6-51.8°F], whereas the bottom temperatures range from sub-zero temperatures to 11 degrees Celsius [lower than 32°F to 51.8°F].”
Sea temperatures helped the researchers understand shifting fish populations this year.
“Our catch of capelin tends to be very temperature-dependent, with higher catches of capelin in cold years,” Murphy explained. “The abundance of capelin is very low in recent low-ice years, so it was good to at least see a few capelin in 2021.”
Capelin are an important forage species in the northern Bering Sea, meaning larger marine species, including other fish and seabirds, rely on steady populations for food.
In a recent Strait Science lecture, Lyle Britt, the director of the resource assessment and conservation engineering division of the Alaska Fisheries Science Center, discussed the findings of the 2021 bottom trawl survey, where researchers studied bottom-dwelling marine populations, including the capelin. During the presentation, he also reported catastrophic low numbers of capelin, falling from 14,632 metric tons of biomass in 2010 to just 76 metric tons this year.
Several larger species tracked by the survey, including pollock and Pacific cod, are studied to understand their diets and role in the ecosystem.
“[We] study the growth, the diet and the condition of these two species in the Northern Bering Sea,” Murphy said.
Both species are relative newcomers in the ecosystem as sub-arctic fish, who typically stay in warmer waters. While there was a fairly large increase in young Pacific cod populations, cod returns are still relatively small when compared to the number of pollock the researchers trawled.
Other cod species, including the Saffron cod, locally known as tomcods, saw very low population estimates during the surface trawl survey.
“Our catch of saffron cod in 2021 was particularly low, with only a few saffron caught near Norton Sound and Bering Strait,” Murphy said. “We’ve seen some fairly large numbers of young saffron cod in our survey, particularly during 2014 through 2017, but we’ve seen very few of these young saffron cod in our survey since 2018. We’ve also seen a decline in our catch of older ages saffron cod.”
Britt’s bottom trawl survey saw similar results. In 2010, biomass estimates of the tomcod were around 90,299 metric tons, but this year, the bottom trawl researchers estimate only 9,972 metric tons of the species.
Both Murphy and Britt say the tomcod populations move around and can be difficult to study.
“Although [these findings] could reflect a northward shift in their distribution, similar to some of the distribution shifts that [Britt] showed of cod of Arctic cod and Pacific cod, however, saffron cod really are only present in these very shallow nearshore habitats, and so our catch of saffron cod in our survey may not be a very good index of their abundance,” Murphy said.
Pink salmon populations, too, were abysmal this year.
“The numbers that came back this year were lower than what we expected based on their juvenile abundance,” Murphy explained. “This may indicate that there can be a higher mortality of pink salmon and chum salmon that’s occurring offshore.”
Researchers rely on juvenile populations to predict future adult population sizes, as tracking the juveniles to adulthood provides significant information about species’ health.
Pinks are an important subsistence fish in the region, and after several record-breaking years, fishermen were troubled by the small catch this summer.
Murphy was not optimistic about the pinks returning next year.
“We’re expecting to see low numbers of pink salmon returning to the region in 2022,” he said.
Chinook salmon, too, saw a small return during the surface trawl survey.
“Juvenile [chinook abundance] was below average in 2021 and has been below average since 2017,” Murphy explained. “Our projection [for future years] … it’s probably going to be comparable, which was very low this year, but we’re not expecting things to improve from what we saw in terms of run size this year.”
Chinook and chum salmon were both found close to near-shore research stations, unusual for the fish species.
“This is even more atypical for chum salmon, as they tend to be much more broadly distributed than Chinook salmon,” Murphy explained. The index for chum has been “above average” since 2018, and the two highest abundance indexes were in 2019 and this year, he said. The relationship between juvenile and adult chum tends to be more variable than the relationships seen in Chinook.
“As we expected to see more adults retuning to the Yukon River based on their juvenile abundance in 2016, we believe that this is indicating that chum salmon are experiencing higher mortality during their later marine stages than we’ve seen in the past,” Murphy explained.
The model for studying chum mortality is still in development, but Murphy says it’s key to understanding the large decline of chum salmon in the Yukon River.
Several other species saw significant population declines, including herring, which saw the lowest biomass index in surface trawl history this year.
Murphy says the 2021 surface trawl survey saw one of the lowest forage fish indexes in survey history.
“By combining the catch of all these small fish species, such as herring, capelin and juvenile salmon, [researchers] constructed a forage fish index for the Northern Bering Sea,” he explained. “Although we’ve seen low values of this index in previous years, the index reached its lowest value in 2021, and this was mainly due to the low abundance of herring. However, the lack of other forage species like capelin is also contributing to the low index in 2021.”
Coho salmon, though, were seen in abundance this year.
Catches of the fish were “quite high,” and Murphy said their biomass index “was close to the highest that we’ve seen in the history of our sampling in the Northern Bering Sea. Hopefully, this means that coho runs will be much better next year.”
In addition to studying fish, the researchers tracked copepods, Chionoecetes crabs, shrimp, and seabirds.
Shrimp were the most abundant species of fish and invertebrates captured in the trawls this year, caught throughout the entire survey area, but had their highest density in Norton Sound.
Several kinds of copepods, very small species of zooplankton, were also abundant in the ecosystem. While the very small copepod populations have been stable over the years, large copepods, or copepods that are larger than two millimeters in length, have seen more variable populations.
“Large copepod species are less abundant, but they’re an important energy-rich food,” Murphy said. “Their abundance has been quite variable over time in the Northern Bering Sea. [The] abundance in 2021 was a bit higher than what we’ve seen in the more recent, low-ice years.”
Juvenile Chionoecetes crabs are snow crabs and tanner crabs smaller than 15 millimeters in length, meaning they’re smaller than the width of your index finger, Murphy said. At this stage in their life cycle, it’s almost impossible to distinguish between species, and very rarely captured in bottom-trawl surveys. Murphy says the chionoecetes were present in more intermediate depths of the sea and had abundance throughout the region.
Researchers from U.S. Fish and Wildlife, including Kathy Kuletz, have been monitoring seabirds in the region for almost a decade. “This has allowed us to establish a pretty good baseline for marine birds in the region,” Murphy explained.  During the survey, researchers identified three dead birds: one shearwater and two unidentified birds.
“This was generally lower than what we’ve seen in previous years, but it’s always a bit challenging to spot dead birds from a moving ship,” he said.
Live populations of shearwaters, auklets and murres the researchers tracked were typical for this time of year, Murphy said.
Though researchers finished the survey earlier this year, final, conclusive data won’t be available until they conduct a more thorough analysis in a lab. “We’ll have more information that will come available over this next year as some of the laboratory analyses are completed,” Murphy said. “These surveys have been conducted in the Northern Bering Sea for the last 20 years or so.”
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.