A recent study on the Nisqually estuary showed that about 16 percent of a steelhead population migrating from the Nisqually River to the Puget Sound ended up in the stomachs of a growing population of harbor seals.
The study, conducted by NOAA fisheries research biologist Megan Moore and her colleagues, is part of a larger examination of recovering populations of steelhead trout, which for the past three decades have largely been on the decline throughout the region.
In a research analysis posted in conjunction with Long Live the Kings, a sustainable fishing nonprofit based in Seattle, Moore says that the study began with releasing acoustic telemetry-tagged fish 12 miles up the Nisqually River.
This research has been going on for about a decade. Researchers with the Nisqually Indian Tribe have been conducting fish-tag studies along the Nisqually River since around 2008, Moore said.
Acoustic telemetry tags allowed the researchers to monitor steelhead smolt movement throughout the region as they made their way out of the sound and through the Straits of Juan de Fuca.
The first data point they found was pretty significant — less than 10 percent of juvenile steelhead from the Nisqually River were able to survive through to the Puget Sound.
“This work really highlights the fact that migration through Puget Sound is having a big impact on the number of adult steelhead that are returning to spawn,” Moore said in her analysis.
In an interview with the Nisqually Valley News, Moore said they also found that steelhead don’t stay for long in the area along the Nisqually estuary.
“These pieces of information led us to believe it was predation that was leading to these low rates of survival,” she said.
Researchers then looked to a growing population of harbor seals along the Nisqually estuary. These marine mammals generally have a fairly diverse diet to fulfill their high daily energetic requirements, Moore said.
But because the population was increasing, likely due to migration, they were impacting the fish population attempting to migrate through the lower Puget Sound.
It became clear the seals were having a significant impact on the migrating steelhead. They first noticed this when when data from the tags became unpredictable.
“We weren't expecting to find much evidence of mortality … (But) we started to understand this was a big factor in mortality with steelhead,” Moore said, adding that they’ve heard anecdotes from other researchers that harbor seal populations have increased significantly over the last four years.
Moore and her colleagues then began to use temperature-sensitive tags that could report when the steelhead were eaten by warm-blooded predators.
Similar studies on fish predation aren’t new, Moore says. Researchers have seen similar studies in the Duwamish River and along the Hood Canal estuaries.
Moore said their research found that the tags that recorded a significant increase in temperature also recorded a back-and-forth behavior along the estuary, indicative of a harbor seal’s behavior.
About 16 percent of the tags recorded this change in temperature and behavior.
Uneaten tags will normally show a linear path through the estuary, and generally will not stick around too long around the estuary.
A 16-percent decline in population is fairly significant when looking at the larger scale migration of the steelhead fish population. The Nisqually River basin makes up 3 miles of the 170-mile route migratory fish populations must make through the Strait of Juan de Fuca.
Moore said that from the Nisqually River to the Tacoma Narrows, the survival rate of steelhead is roughly 60 to 70 percent. That rate includes the 16-percent mortality rate seen at the Nisqually estuary.
The next thing they’ll attempt to understand is how significant this is compared with other hurdles that steelhead trout encounter.
Moore said the Washington State Department of Fish and Wildlife is working with the Nisqually Indian Tribe to collect seal scat. Analysis of those samples showed varying proportions of steelhead.
“We’re working with them to estimate the number of steelhead eaten using scat analysis methods, then plan to compare to the estimate calculated using telemetry methods,” she wrote.