Study Shows Pollock Stocks are Mixing More Due to Changing Ocean Conditions and Weather Patterns

October 26, 2020

With the loss of the cold water barrier in 2017-2019, historically present in the Bering Sea in summer months, young and adult pollock are spreading out and moving north.

Heat map showing variations in size of the Bering Sea cold pool in 2010, 2017, 2018 and 2019. The Bering Sea cold pool, a bottom water mass that is < 0-2o C resulting when winter sea ice recedes affects fish movement. Warmer water temperatures and reduced sea ice in recent years resulted in shrinkage of the cold pool. Figure: NOAA Fisheries.

New research confirms movement of adult and young pollock between Russian and U.S. waters. Scientists from the NOAA Alaska Fisheries Science Center, NOAA Research's Pacific Marine Environmental Laboratory and the Russian Research Institute of Fisheries and Oceanography, Pacific branch (TINRO) cite warmer Arctic temperatures, changing wind patterns, and shifting currents as contributing to stock shifts in the eastern and western Bering Sea.

“Our survey data have been indicating a northward shift in pollock distribution in the U.S. waters for the past several years,” said NOAA Fisheries Oceanographer and Lead Author for the study, Lisa Eisner. “But this is the first time we have been able to work together with Russian scientists to confirm both the northward shift and the long suspected movement of fish between the U.S. and Russia.”

To assess their effect on where pollock occurred, scientists from both countries looked at:

  • Biological data
  • Temperature data
  • Sea ice conditions (retreat timing and extent)
  • Model-based estimates of ocean circulation patterns 

They also considered other potential environmental factors driving the observed changes. 

Scientists attribute movements of adult fish to reductions in sea ice extent and a shrinking cold pool in 2017-2019 due to warmer water temperatures. The cold pool is a naturally formed band of bottom water over the continental shelf that is < 0-2o C.  It results from winter heat loss/mixing and melting sea ice creating a natural barrier to pollock movement northward. Adult pollock rarely venture into water colder than 0o C.  The smallest cold pool extent in the last two decades occurred in 2018 and 2019.

There were stronger northward/northwestward bottom currents over the eastern shelf in warmer years, particularly in 2018 and 2019. Scientists also found that these led to changes in distributions of both adult and age-1 (fish in their second year of life) pollock.

Pollock Distribution in Cold vs. Warm Years

Map showing 2010 approximate pollock distribution and representation of a cold year with a larger cold-pool.

(Image is representational for exact coordinates of fish distributions please refer to the research publication). Figure: NOAA Fisheries.

In 2010, a cold year, adult pollock had southerly and narrow distribution over the outer shelf in U.S. waters. Scientific surveys in Russian waters (the northwest Bering Sea) historically have documented pollock off Cape Navarin in southern Anadyr Bay in cold or average years. Fish in 2010 appear limited by the 0°C cold water located northeastward of this area.

Map showing 2017-2019 approximate pollock distribution and representation of warm years with a reduced cold-pool.

(Image is representational for exact coordinates of fish distributions please refer to the research publication). Figure: NOAA Fisheries.

In U.S. waters, adult pollock were concentrated north of St. Lawrence Island and had wider longitudinal distributions in the low ice years (2017-2019). They primarily occurred in waters with bottom temperatures between 0o C and 6°C. In Russian waters, adult fish were spread out, farther northeastward.

Map showing 2010 approximate age-1 pollock distribution and representation of a cold year with a larger cold-pool.

(Image is representational for exact coordinates of fish distributions please refer to the research publication). Figure: NOAA Fisheries.

Few age-1 pollock were observed on the south middle shelf to the east of St. Lawrence Island in 2010 when currents were weaker.

Map showing 2018 approximate age-1 pollock distribution and representation of a warm year with a reduced cold-pool.

(Image is representational for exact coordinates of fish distributions please refer to the research publication). Figure: NOAA Fisheries.

Age-1 pollock had higher densities over the inner eastern shelf in 2017-2019 compared to 2010 in U.S. waters. These young fish also occurred in waters with a wider bottom temperature range (as high as 10o C). Stronger northward bottom currents from the south middle shelf to east of St. Lawrence Island occurred during spring (March-May) in 2018 and 2019. This likely played a role in the distribution of fish northward. Scientists also saw pollock on the eastern inner shelf between Nunivak and St. Lawrence islands.  Overall, in 2018 and 2019, age-1 pollock had a larger cross shelf distribution with the high concentrations located near the eastern inner shelf. This was particularly the case in 2019 (large area with > 10,000 pollock km-2) in U.S. waters. 

In looking at historic data, scientists observed similarities in fish size and age groups in U.S. and Russian waters particularly in 2019. With declines in the cold pool, there appears to be more intensive mixing between the Russian stock as it moves north and eastward, and the U.S. stock as it moves north and westward.

“Specifically we show that pollock expanded their distribution to the area between St. Matthew and St. Lawrence islands in the U.S., and the area between St. Lawrence Island and Bering Strait in both U.S. and Russian waters,” said Eisner.

Implications of Changes in Pollock Distribution in the Bering Sea

For Eisner and fellow scientists the big question is how these environmental changes will affect pollock over the long term. 

For instance, typically during fall and winter as sea ice is forming in the northern Bering Sea, adult pollock move south to continue feeding in warmer areas. However, in years with reduced sea-ice extent, there may be less incentive to move to the most southern areas where their traditional spawning grounds are located. 

Early ice retreat and warmer temperatures during winter can also lead to earlier spawning and an earlier start of feeding migrations. Changes in hydrography can lead to changes in currents. These changes then have large impacts on planktonic prey resources for adults (who follow prey) and on juvenile pollock and other fish species, who are smaller and more susceptible to current flow.

In 2018 and 2019 juvenile pollock were observed in high densities in the southern Chukchi Sea on fisheries oceanography surveys, both in the U.S. and Russian sectors. Russian scientists observed high densities of adults in these areas too. This suggests that pollock have the potential to move northward from the north Bering Sea into the Chukchi as the climate warms and sea-ice diminishes. Whether it will be possible for pollock to colonize these northward regions remains an open question. A lot will depend on their temperature tolerance, prey resources, reproductive requirements, and predation pressure.

“This research is really critical because pollock are a key ecological component of the Bering Sea shelf food web supporting the largest commercial fishery in the U.S. by biomass,” said Robert Foy, Alaska Fisheries Science Center director. “To get an accurate assessment of pollock abundance so that resource managers can set sustainable catch limits, we have to be able to understand pollock distribution, which certainly looks different under a warm water regime.”

Scientists want to fully assess the potential for broad-scale changes in pollock distribution over the Bering Sea and environmental factors driving these changes. To do this, they must evaluate distributions across the entire shelf in both U.S. and Russian sectors. Continued collaboration and sharing of data between Russian and U.S. scientists is key to tracking these changes.