The Bering Sea’s Cold Pool: What the 2018 Bottom Trawl Survey Set Out to Find

Every summer NOAA Fisheries runs a bottom trawl survey across the southeastern Bering Sea shelf. In 2018, after an unusually warm winter and little sea ice in 2017/2018, the scientists going out wondered whether they would find a cold pool at all. If not, it would be the first time in the survey's history.

What the cold pool is

The cold pool is a large body of frigid water near the bottom of the eastern Bering Sea shelf, averaging below 2 °C (36 °F). It forms where sea ice covered the shelf, as the ice melts in the Bering Sea and in the Gulf of Anadyr, the Siberian gulf at the Bering Sea's northeastern corner.

It shapes where fish live, acting as both refuge and barrier:

• for arctic cod, it is a place to feed and spawn;
• for pollock and Pacific cod, it is a wall that limits their movement across the shelf and north toward the Bering Strait;
• the decline of snow crab over the southern shelf has been put down to the gradual warming of the water there, and other species have moved north as cold bottom water has retreated less far south.

The survey

The survey, then in its 37th year, runs from early June through August. At each station scientists record biological data on commercially important fish, crab and other species, which feed estimates of stock size and the decisions managers make. The key species are Alaska (walleye) pollock, Pacific cod, Greenland turbot, yellowfin sole, northern rock sole, and red king, snow and Tanner crab. The team also measures ocean conditions, including the bottom temperature at every station.

Surprises in earlier years

The summer of 2016 brought warmer than normal surface and bottom water, and sea ice along the shelf was limited the following winter. The survey coordinator at the Alaska Fisheries Science Center expected a small cold pool in 2017, like 2016's. Instead a late winter freeze and melt set one up, and in 2017 the cold pool reached unusually far south, from the U.S.–Russian border to the western edge of Bristol Bay.

Bottom temperatures during the 2016 survey: a small cold pool. NOAA Fisheries.

Bottom temperatures during the 2017 survey: the cold pool reached far south. NOAA Fisheries.

The last survey year with the cold pool that far south was 2012, after a winter (2011/2012) of colder than average bottom water and broad sea ice over the Bering Sea and the Gulf of Anadyr. Returning that summer, the scientists found an extensive cold pool, as expected.

After the cold winter of 2011/2012, the 2012 survey found an extensive cold pool. NOAA Fisheries.

Mean near-bottom temperatures in each survey year from 2001 to 2017. The red band marks the 95% decision limits around the overall mean (UD the upper limit, LD the lower). NOAA Fisheries.

What 2018 might bring

The coordinator said the team might not see a cold pool in the survey area at all, given how warm the winter had been and that the Gulf of Anadyr had not fully frozen. That would be a first, and would likely change where fish are found, with consequences for commercial fisheries and the communities that rely on them. Long-term data, he said, are what make it possible to understand the relationships in a marine ecosystem and manage it sustainably.

Through the season, the team posted maps of the bottom temperatures recorded at each station, covering the whole southeastern shelf, an area about the size of California.

The survey area. NOAA Fisheries.

Bottom temperatures as of 7 June 2018. NOAA Fisheries.

Bottom temperatures as of 29 June 2018. NOAA Fisheries.

Bottom temperatures as of 2 August 2018. NOAA Fisheries.

Further reading

• Scientific Teams Set Out to Track Unprecedented Changes in the Eastern Bering Sea
• 2019 Southeastern Bering Sea Shelf Bottom Trawl Survey Gets Underway
• The 2018 Eastern Bering Sea Continental Shelf and Northern Bering Sea Trawl Surveys: Results for Commercial Crab Species
• Bottom trawl surveys in the northern Bering Sea indicate recent shifts in the distribution of marine species (scientific paper)

Sources

• NOAA Fisheries, "2018 Southeastern Bering Sea Shelf Bottom Trawl Survey Gets Underway"; rewritten in hubnx's own words. Maps and charts: NOAA Fisheries, Alaska Fisheries Science Center; four of the season's eighteen heat maps are shown.