Examining Seal Diet to Learn Impact on Salmon

Collecting seal scat may be the best way to see how many salmon are being eaten by marine mammals.

The Nisqually Indian Tribe is investigating seal diets in South Sound following the rapid increase of the harbor seal population the past few decades.

In late 2016, tribal researchers began observing marine mammal predation on winter chum salmon on the lower Nisqually River.

Tribal staff are working with the Salish Sea Marine Survival Study to collect scat at three sites, including the mouth of the Nisqually River. Samples are sent to a lab to measure genetic content.

The impacts of marine mammal predation on salmon are potentially serious.

The Nisqually Tribe has fished a complete chum fishery only once in the last seven years, while fishermen have observed marine mammals eating a large number of chum.

“We’re concerned that up to one-third of the incoming chum run is intercepted by seals and sea lions before making it to the spawning grounds,” said David Troutt,the tribe’s natural resources director.

The tribe also has partnered with Washington Department of Fish and Wildlife, and the National Oceanic and Atmospheric Administration to study Nisqually steelhead survival rates by tracking harbor seal predation. The partners inserted acoustic tags in steelhead and attached receivers onto harbor seals.

So far, researchers have found that the presence of other predators and prey might benefit salmon and steelhead. “In the places we knew there was orca activity, there was steelhead survival,” Troutt said. “We think the orcas either ate a lot of seals or chased them away and affected their behavior, resulting in more steelhead making it out to the ocean.”

Based on data from the acoustic tagging project, salmon and steelhead survival increased last year while seals apparently pursued anchovies.

“We want to see if that was a normal year, or if we were seeing something in isolation,” Troutt said. “By analyzing marine mammal scat and tracking Puget Sound survival of steelhead, we will get a much better idea of what makes up their diet and if they’re limiting salmon productivity.”