Juvenile Pollock Survival Better Than Expected

A new federal study suggests that young Pollock survived at a rate better than anticipated during the most recent warm phase in the Bering Sea, having found alternative resources not available during the last warming phase.

With 2017 showing signs of cooling, Pollock populations may have successfully weathered the warm years of 2014 though 2016, according to a report by scientists with the Alaska Fisheries Science Center published in the journal PLOS on June 28.

Juvenile Pollock need energy-rich prey to survive winter in the Bering Sea.

The study led by NOAA Fisheries scientist Janet Duffy-Anderson showed that Arctic algae are attached to the sea ice. When the ice melts, said Duffy-Anderson, “these algae are released into the water column to be eaten by larger, oil-rich zooplankton, which are in turn eaten by young Pollock fattening up to prepare for the Bering Sea’s harsh winter. This chain of events is critical to Pollock success.”

The 2001-2005 warm stanza triggered ecological changes that resulted in a decline in the number of walleye Pollock, ultimately leading to a 40 percent reduction in the fishing quota. Sea ice eventually returned to the southeast Bering Sea shelf, bringing cold sea temperatures from 2007 to 2013, with oil-rich prey. By 2013, recruitment to the Pollock fishery recovered completely.

When ocean conditions turned warm again in 2014, scientists were concerned this was the start of a new warm stanza. Pollock survival declined, as expected after 2014, and 2015 proved to be even warmer, but scientists said it was warm for a reason not seen before.

The year 2015 acted like a cold year, with strong winds from the north pushing Arctic sea ice southward to the southern Bering Sea, but the sea ice was stopped by “the Blob,” a mass of warm water from the Gulf of Alaska invading the Bering Sea.

By 2016, with sea ice absent, the water was very warm over the southern shelf, and while large copepod preys were scarce, krill remains in the area.

Young Pollock from the southern shelf may have taken refuge in the northern cold pool in 2015, feeding on fat-rich copepods or krill, and then in 2016 they consumed large numbers of krill, possibly remnant populations from 2015. This suggests that switching prey sources figured in their survival, said Duffy-Anderson. Successive warm years due to reduced Arctic winds, weak sea ice advance and warm ocean temperatures still, however, spell trouble for Pollock, she said.

The complete study appears online at http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0178955