When eight young northern elephant seals suddenly began showing up at a deep-sea observatory, researchers were taken by surprise. Their repeated visits to the research site, otherwise a speck in the vast, dark ocean, wasn’t a chance occurrence, a new study reveals. The mammals were likely drawn to the area by the observatory’s sonar pulses.
A team of scientists made the discovery while studying the effects of light and bait on fish behavior at a research site located 645 meters (2,116 feet) deep, at Barkley Canyon, off the west coast of Canada. The researchers used a high-definition camera, hydrophone and acoustic imaging sonar, a system that transmits sounds to map and image features and animals.
While reviewing the footage, the team unexpectedly came upon at least eight male elephant seals (Mirounga angustirostris), ages 4-7, who visited the observatory multiple times between June 2022 and May 2023.
“These repeat visits suggest that the seals may be using the general site as a focal foraging area,” study lead author Héloïse Frouin-Mouy, a marine biologist and bioacoustician at the University of Miami, U.S., told Mongabay in an email.
The footage gave the researchers some unique insights into elephant seal behavior in the deep sea. For instance, the videos showed that the young male elephant seals preferred to eat sablefish (Anoplopoma fimbria) over snailfish (Careproctus spp.).
“This was evident when an elephant seal we named Dennis mistakenly caught a snailfish while attempting to capture a sablefish, only to quickly release it,” Frouin-Mouy said. “This is important because the prey of males, in contrast with females, is not well known.”
Moreover, footage of only male seals at the research site, but no females, suggests that the two have different foraging habitats and behaviors. “We know from previously published tagging studies that female northern elephant seals forage in the open ocean on pelagic species, while male northern elephant seals forage on benthic species at the ocean floor along the continental shelf,” Frouin-Mouy said.
Most intriguing, she added, was the elephant seals’ ability to locate the small research site in the open ocean, in complete darkness, multiple times. The researchers suspect that the observatory’s sonar had a role to play.
The sonar pulse frequencies fall within the hearing range of northern elephant seals, Frouin-Mouy said, meaning the mammals can detect the sounds. Furthermore, the same individual seals visited the site repeatedly, mostly to eat fish available there, sometimes 10 or 30 days in a row, but only when the sonar was active. After the sonar failed, their visits declined drastically.
“Our findings suggest that the northern elephant seals learned to associate the sonar noise with food availability at the site — a phenomenon known as the ‘dinner bell’ effect — and used this acoustic cue to visit the site more frequently,” Frouin-Mouy said.
Banner image of northern elephant seal foraging for food. Image courtesy of Ocean Networks Canada.
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