Harbor Seals Listening for Steelhead

The Nisqually Tribe is using harbor seals to collect data about the decline of steelhead in Puget Sound.

The tribe has partnered with NOAA Fisheries and the state Department of Fish and Wildlife to attach acoustic receivers to a handful of seals. The seal-mounted receivers will work with an array of stationary receivers to track the progress of Nisqually steelhead fitted with acoustic tags.

The tribe is concerned that ever-increasing populations of harbor seals are having an outsized impact on out-migrating steelhead.

“What we suspect is happening is that these seals are eating a lot of juvenile steelhead,” said Chris Ellings, the tribe’s salmon restoration manager. “But even if the seals aren’t eating the steelhead, they are both swimming in the same water and they’ll likely be able to hear the steelhead go by.”

The project partners captured and fitted juvenile Nisqually steelhead with acoustic transmitters. When a steelhead carrying a transmitter passes between a pair of receivers, its individual frequency is recorded and tracked for several hundred yards.

The study is part of the Salish Sea Marine Survival Project coordinated by Long Live the Kings.

Just because harbor seals are eating steelhead doesn’t make them responsible for the population’s decline. “What we’re looking at out there is an ecosystem out of whack,” said David Troutt, the tribe’s natural resources director. “Because seals are not finding the prey they historically have, such as forage fish like sand lance, they need to focus on already weak stocks of juvenile steelhead.”

The tribe funded the steelhead tagging project last year to observe the impact of a large number of transient orcas arrving in South Puget Sound.

“What we ended up seeing is that the places we knew there was orca activity, there was also a much greater survival of steelhead,” Ellings said. Transient orcas prey solely on marine mammals. Even if transient orcas don’t end up eating that many seals, their hunting activity could disrupt the seals’ ability to hunt steelhead, Ellings said.

Decades ago, the Nisqually River had one of the strongest runs of steelhead in Puget Sound, with more than 6,000 returning every year, but the population crashed almost 20 years ago. Tribal and state co-managers would like to see about 2,000 steelhead return to spawn every year to the Nisqually, but since 1993, fewer than 1,000 have come back.