Underwater acoustic monitoring shows marine mammals’ movements
Last Thursday, NOAA’s Catherine Berchok gave a Strait Science presentation facilitated by UAF’s Northwest Campus on her research monitoring marine mammals in the Bering and Chukchi seas using underwater microphones. Her results show a distinct northward shift in many species.
Berchok is an acoustician, specializing in the use of sound to monitor marine life. She works with the Cetacean Assessment and Ecology Program at NOAA Fisheries, and has been running year-round acoustic monitoring in Western Alaska since 2007.
Her program uses 21 underwater microphones stretching from the Aleutians to the high Arctic. The microphones sit at the bottom of the ocean and record sound year-round. About once a year – with some unplanned breaks – she and her team travel to the microphones, haul them up with a ship’s crane and upload the data.
Since many marine mammals communicate in frequencies well out of range of the human ear, Berchok uses a computer to represent the data visually in a chart called a spectrogram.
“We analyze 100 percent of the recordings that are collected for 11 species of whales, seals and walrus, and also human-made sounds like vessels and air guns and environmental sound like ice,” she said.
Because of the sheer quantity of data, they don’t count every single time a sound pops up. Instead, they look at 10-minute intervals and record whether a sound was present or absent in each interval. Once the time-consuming process is finished, Berchok has a remarkably detailed record of what sounds were occurring where, and at what times of year.
The data she presented was far from complete – sometimes there are technical issues and the microphones stop recording, she said, and in other instances her team just hadn’t gotten around to analyzing the data yet.
Still, the data she could provide told a compelling story. Many species, both “ice-associated” species that follow the ice edge and “ice-avoidant” species that come in when the ice recedes, appeared to be moving north.
Bowhead whales, which are ice-associated, were less common in the North Bering Sea as ice levels decreased, and showed up more near the North Slope. Walruses were very closely tied to the ice south of St. Lawrence Island and receded as the ice receded, while they were present year-round in the Chukchi Sea.
Humpback whales, which usually migrate south during the winter, stayed year-round in parts of the Bering Sea that remained ice-free. Right whales were also detected farther north than usual, both by the microphones and by hunters on St. Lawrence Island.
Gray whales were present in most places with open water, although the frequency of their detections have decreased dramatically. Berchok said it’s tough to tell whether they’re actually gone or just not singing in the same way, but said the decrease was in line with the Unusual Mortality Event declared for gray whales and some other marine mammals in 2017.
In addition to the songs of marine mammals, Berchok’s microphones pick up man-made sounds. Ship engines constantly make noise, and fossil fuel companies use “air guns” to detect possible oil and gas deposits under the water, a method akin to sonar.
“When the Shell oil rig was up and all those vessels were there to support it, that’s when we had the most vessel presence in the open water season,” Berchok said, referring to Shell’s failed exploration effort in the Chukchi Sea.
Commercial shipping through the Bering Strait is a bit harder to detect, she said, because she can’t place microphones too close to the Russian border, but it’s something she’ll be watching for as Arctic shipping becomes more widespread.
Going forward, Berchok is hoping to expand her analyses to include animals that make higher-frequency sounds like belugas and bearded seals. She also emphasized her eagerness to cooperate with local communities and using her microphones to conduct research that they felt was important.
“If there’s a space or a location that would make more sense for us to be listening to that you’re very interested in, or a particular species that we haven’t presented here that you’re interested in, please let us know,” she said. Berchok can be reached via email at catherine.berchok@noaa.gov, or by phone at 206526-6331.