Sonar Monitoring on Seafloor Shows Pollock Traveling Between U.S., Russia EEZs

Alaska pollock. Photo: NOAA Fisheries.

A new federal fisheries study released June 20 shows that substantial numbers of pollock travel seasonally between the two U.S. and Russian exclusive economic zones (EEZ).

The study, conducted from July 2019 to August 2020, used specially designed moorings, equipped with sonar on the seafloor, to acoustically monitor pollock abundance and movement between the two EEZs.

They documented pollock moving southeast over the maritime boundary in winter as the sea ice formed, but largely absent in late spring when ocean temperatures were near freezing and sea ice was still present, biologists with the National Oceanic and Atmospheric Administration said.

When moorings were deployed in summer 2019, the area was unusually warm but conditions were cooler in summer 2020, according to NOAA. Over the year of observations, 2.3 times more pollock moved into the U.S. EEZ in fall and winter then exited during the subsequent spring and summer.

Scientists believe the cooler conditions in 2020 led to fewer pollock moving into Russian waters than had the previous year.

According to Robert Levine, a scientist and lead author on a new paper published in the ICES Journal of Marine Science, research suggests that in years when water temperatures are warmer than average, the proportion of fish moving across the boundary is greater.

Scientists suspect that continued warming in the eastern Bering Sea will increase the proportion of the pollock stock found in Russian waters. Currently, pollock support the top U.S. commercial fishery, which harvests more than one million metric tons annually.

Survey ships involved in the study captured a snapshot in time with their modified version of sonar technology at the end of a multi-year period of record marine heatwaves in the Bering Sea.

The scientific team deployed echosounders on the seafloor at four sites along the U.S.-Russia Convention Line in the Bering Sea. The devices recorded the passage of fish above them for just over a year. The echosounders were also equipped with sensors to collect physical oceanographic data, which is linked to fish migratory behavior.

The echosounders emit an acoustic signal—a ping—that echo off the fish. Based on the strength of the echo, scientists estimate the number of fish in the water. They measure fish movements by tracking individuals as they move through the sonar beam in successive pings. The underwater devices can’t transmit data in real time, so they store the data until the devices are retrieved.