The effects of a colder Bering Sea on the feeding habits of pollock
A three-year dip in Bering Sea temperature has caused a change in the distribution of the staple food of pollock.
The Bering Sea is that frigid strip of ocean between Alaska and Russia, separated from the North Pacific Ocean by the sweep of the 300 Aleutian Islands with their 57 volcanoes. Nearly all of these islands are part of Alaska, but at their extreme western end are the Komandorsky or Commander Islands, which are part of Russia. These are named after Commander Bering who was shipwrecked and died on the island now known as Ostrov Beringa, Bering Island.
The Bering Sea is considered to be one of the world's most productive fisheries and its northern portions are the home of sea ducks, grey whales, bearded seals and walruses, but a 30-year warming trend has been bad news for those animals that are adapted to a cold-water environment, causing them to migrate further north.
Yet in defiance of all trends, the last three years in the Bering Sea have been the coldest on record. A direct effect of this, according to research scientist Alexei Pinchuk of the Seward Marine Centre at the University of Alaska Fairbanks, is that the cold temperatures have helped to produce larger zooplankton, which may well be changing the feeding habits of Walleye pollock.
Pinchuk and his colleagues spent the last three years gathering zooplankton and studying the ways that changes in temperature have affected them and how these zooplankton shifts may affect the diet of the Walleye pollock.
The larger zooplankton like copepods and krill, have been seen to thrive in the chillier temperatures like those of the last three years and this is what the pollock have tended to eat.
Pinchuk also discovered that during these colder years arctic 'sand-fleas' have migrated south into the Bering Sea. These are a particularly popular food of young pollock, who prefer them to smaller zooplankton.
In warmer years, such as the record-breaking period between 2001 and 2005, smaller zooplankton tended to thrive and the research of Pinchuk and his colleagues suggests that this is what the younger pollock tend to eat. Larger pollock go after the larger plankton that are found in colder waters.
Thus, while younger pollock tend to do well in warmer temperatures, as they grow bigger they may not be able to find the larger zooplankton prey that they need in order to produce enough fat to see them through the winter. With no larger plankton to eat, the larger pollock turn to cannibalism and end up eating their smaller cousins.
So this colder period in the Bering Sea has been seen to be something of a bonus for the pollock. The smaller pollock have been able to add sand fleas to boost their diet and they no longer run such a great risk of being eaten by their larger cousins. With larger zooplankton now being more plentiful, large pollock have found a plentiful supply of their favoured food.
However, the cold period is not expected to last and scientists predict that the warming trend in the Bering Sea is set to resume.