In a new study, NOAA Fisheries scientists have analyzed how oceanic conditions influence Chinook and chum salmon bycatch in Alaska’s pollock fishery in the eastern Bering Sea.
Alaska’s pollock fleet lands more than 2 billion pounds annually, but unintentional salmon catch remains a longstanding management concern, particularly regarding bycatch of Chinook and chum stocks, the migration routes of which overlap with pollock fishing grounds.
To find ways to mitigate the issue, the study, led by researchers at the Alaska Fisheries Science Center and partner institutions, analyzed more than a decade of observer data to identify environmental factors linked to salmon encounters.
“This is an issue that’s the subject of ongoing discussions at North Pacific Fishery Management Council meetings,” Lukas DeFilippo, a fisheries biologist and the study’s lead author, said. “There’s limited information available on how environmental factors affect bycatch, which could potentially be useful for informing ongoing scientific and policy discussions.”
Referencing observer data from 2011 to 2023 and using a modeling framework that accounted for fishing season, depth, and a range of oceanographic indicators, the study found that interactions between bottom depth and sea surface temperature anomalies consistently shaped bycatch rates, though effects varied by species and season.
During the A fishing season, when sea ice remains over parts of the eastern Bering Sea, years with more ice were linked to higher Chinook bycatch in most regions. Researchers said that one hypothesis for the phenomenon is that both fishing vessels and Chinook are pushed into ice-free areas, increasing overlap. Chinook bycatch typically declined later in A seasons as fish migrated. Chum salmon are usually absent from the shelf during this period.
In the B season, meanwhile, chum bycatch generally rose early, while Chinook bycatch increased later. Fishing deeper – about 475 feet or more – tended to reduce bycatch for both species, according to the study, but unusually warm years can drive chum into deeper waters, raising bycatch risk.
Additionally, warm water periods of Pacific Decadal Oscillation, which is a long-term climate pattern of sea surface temperature anomalies in the North Pacific Ocean, were broadly associated with higher B-season bycatch for both species. Shifts in the extent and position of the area’s cold pool, which is the mass of cold water at the bottom of the ocean left after sea ice melt, also influenced where bycatch hotspots formed.
“In a lot of marine ecosystems, oceanographic features have a significant impact on bycatch patterns, so it makes sense that the cold pool and Pacific Decadal Oscillation play an important role in salmon bycatch dynamics in the Bering Sea,” DeFilippo said.
DeFilippo described the work conducted for the study as an initial step toward identifying environmental indicators that may help reduce salmon bycatch. He noted that timing strategies have historically helped reduce Chinook interactions, but differing seasonal patterns for chum and Chinook make it harder to minimize bycatch of both species simultaneously.
“This work has been successful because of collaborations between Alaska Fisheries Science Center, North Pacific Fishery Management Council staff, the Alaska Department of Fish and Game, and the University of Alaska,” he said. “That kind of cooperation is critical for advancing future research focused on reducing salmon bycatch.”
