Wednesday November 19, 2025
NOAA —
A new study from NOAA Fisheries and its partners is exploring salmon bycatch in the eastern Bering Sea pollock fishery and how bycatch rates may relate to oceanographic conditions. This research deepens our understanding of oceanographic factors contributing to encounters between pollock fisheries and salmon. It also supports our efforts to simultaneously manage a sustainable Alaska pollock fishery and minimize bycatch.
Bycatch in the Pollock Fishery
Alaska’s pollock fishery in the eastern Bering Sea is one of the largest and most valuable in the world. On average, the fishery lands more than 2 billion pounds of pollock and contributes to nearly half of all global pollock production. However, bycatch of salmon presents challenges for this fishery.
Bycatch is the unintentional capture of animals that fishermen don’t want or can’t keep. In the Alaska pollock fishery, bycatch of Chinook and chum salmon—particularly those originating from western Alaskan populations—is a concern. NOAA Fisheries has implemented measures designed to minimize salmon bycatch, while providing the pollock fleet with the flexibility to harvest the total allowable catch for pollock. The commercial pollock fishery has also adopted its own measures to avoid salmon.
Western Alaska Chinook and chum salmon are essential to the wellbeing of communities in this region, but have experienced significant declines, particularly over the last decade. Many of these declines have been linked to unfavorable environmental conditions. These stocks are also vulnerable to bycatch due to their migration behavior and geographic overlap with pollock.
Minimizing bycatch of these species is a top priority for fisheries managers and the commercial fishing industry. In order to effectively minimize bycatch, we need to understand the dynamics that drive it. Lukas DeFilippo is a fisheries biologist at the Alaska Fisheries Science Center and lead author of the new publication, which began as an effort to address important knowledge gaps about salmon bycatch.
“This is an issue that’s the subject of ongoing discussions at North Pacific Fishery Management Council meetings,” DeFilippo says. “There’s limited information available on how environmental factors affect bycatch, which could potentially be useful for informing ongoing scientific and policy discussions.” This publication was a starting point for documenting and providing such information for fisheries managers and the fishing industry.
Environmental Conditions Impact Bycatch
To investigate bycatch dynamics, DeFilippo and his colleagues analyzed catch data collected by fishery observers from 2011 to 2023 in the eastern Bering Sea pollock fishery. Using a modeling framework that considers a wide range of factors, the authors examined bycatch patterns across salmon species and fishing seasons. They explored how bycatch rates responded to environmental conditions at multiple locations and different times of the year. They found that interactions between bottom depth and local sea surface temperature anomalies were consistently important in shaping bycatch rates. The effects differed by species and across fishing seasons.
The Alaska pollock commercial fishing year is divided into two seasons. During the A season (January into June), sea ice is still present in the eastern Bering Sea. In the B season, which spans from mid-June to November, much of this sea ice has melted. This leaves behind the cold pool, a mass of cold, sub-surface waters that has a strong effect on ecosystem conditions.
The study showed that temperature, depth, and season influence salmon bycatch in the Bering Sea pollock fishery. Oceanographic conditions such as the extent of sea ice and the size of the cold pool also influence bycatch. During the A-season, they found that years with more sea ice were associated with higher Chinook bycatch in most areas. More analysis is needed to tease out why, but one hypothesis is that both fishing boats and Chinook salmon were constrained to ice-free areas, thereby increasing their overlap. Later in the A season (May–June), Chinook bycatch typically declined due to seasonal migration and less overlap with the fishery. Chum are generally absent from the Bering Sea shelf during the A season.
In a typical B season (summer through fall), chum salmon bycatch generally increased earlier in the season, while Chinook bycatch increased later. Fishing deeper (roughly 145 meters or more) typically reduced bycatch for both Chinook and chum. However, in an abnormally warm year where surface temperatures are high, chum move to deeper water, which can increase their bycatch in the pollock fishery. For both species, warmer Pacific Decadal Oscillation phases were broadly tied to higher B-season bycatch risk. Furthermore, the extent and position of the cold pool shifted where bycatch hotspots occur, often along its edge.
“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 says.
Implications for Management
DeFilippo views this research as a first step in identifying factors that could help reduce salmon bycatch. While it is challenging to definitively pinpoint the factors that drive bycatch from fishery-dependent catch data alone, there are insights that could be useful for informing management and complementing ongoing industry-led bycatch avoidance efforts.
Historically, timing has been a useful consideration in reducing bycatch of Chinook salmon. While that can be effective, there are additional challenges when trying to reduce bycatch for multiple species. Given that chum salmon bycatch increases earlier in the B-season, while Chinook bycatch increases later in the B season, it’s more difficult to avoid bycatch of both species based solely on timing.
However, DeFilippo and his colleagues found a consistent response to depth between chum and Chinook salmon. This suggests that, when feasible, deeper pollock fishing could help reduce bycatch of both species simultaneously.
Other ongoing projects to reduce bycatch are focused on finer scale prediction. The goal is to forecast where and when we can expect to find chum and Chinook salmon so fishing fleets can avoid them. “This work has been successful because of collaborations between Alaska Fisheries Science Center, North Pacific Fishery Management Council staff, Alaska Department of Fish and Game, and the University of Alaska,” DeFilippo notes. “That kind of cooperation is critical for advancing future research focused on reducing salmon bycatch.”