Harbor seals have become prime suspects in the deaths of millions of young steelhead trout that die each year in Puget Sound, but the seals may not be working alone.
Disease and/or various environmental factors could play a part, perhaps weakening the young steelhead as they begin their migratory journey from the streams of Puget Sound out to the open ocean. Something similar is happening to steelhead on the Canadian side of the border in the Salish Sea.
More than 50 research projects are underway in Puget Sound and Georgia Strait to figure out why salmon runs are declining — and steelhead are a major focus of the effort. Unlike most migratory salmon, steelhead don’t hang around long in estuaries that can complicate the mortality investigation for some species.
The steelhead initiative was launched by the Washington Department of Fish and Wildlife and Puget Sound Partnership with funding from the Legislature. The steelhead work is part of the Salish Sea Marine Survival Project, which is halfway through its five-year term, according to Michael Schmidt of Long Live the Kings, which coordinates the effort in the U.S. The larger project involves at least 60 organizations, including state and federal agencies, Indian tribes and universities.
A new report on research findings for steelhead (PDF 9.8 mb) describes the most significant results to date for our official state fish, which was listed as “threatened” in 2007. While steelhead populations on the Washington Coast and Columbia River have rebounded somewhat since their lowest numbers in the 1980s, steelhead in the Salish Sea remain at historical lows — perhaps 10 percent of their previous average.
“Because steelhead are bigger and move fast through the system, they are easier to study (than other salmon species),” Michael told me. “It has been a lot easier to feel confident about what you are finding.”
Steelhead can be imbedded with tiny acoustic transmitters, which allow them to be tracked by acoustic receivers along their migration routes to the ocean. It appears that the tagged fish survive their freshwater journey fairly well, but many soon disappear once they reach Puget Sound. The longer they travel, the more likely they are to perish before they leave the sound.
While steelhead are susceptible to being eaten by a few species of birds, their primary predators appear to be harbor seals. These findings are supported by a new study that placed acoustic receivers on seals and observed that some of the transmitters embedded in steelhead ended up where the seals hang out, suggesting that the fish were probably eaten.
In a different kind of tagging study, Canadian researchers placed smaller passive integrated transponder (PIT) tags in a large number of coho salmon and attached devices to read the PIT tags on coho salmon.
“What is most interesting to date,” states a new report from the Pacific Salmon Foundation,“ (PDF 4 mb), “is that we only have confirmed feeding on tagged coho salmon by four of the 20 seals equipped with receivers. This suggests that feeding on juvenile salmon may be an opportunistic behavior acquired by a limited number of seals.”
New studies are underway to confirm steelhead predation by looking at fecal samples from seals in South Puget Sound. Researchers hope to figure out what the seals are eating and estimate steelhead consumption.
As I mentioned at the outset of this blog post, it may be more than a simple case of seals eating steelhead. For one thing, seal populations may have increased while their other food choices have decreased. Would the seals be eating as many steelhead if Puget Sound herring populations were close to their historical averages?
Other factors may be making young steelhead vulnerable to predation. A leading candidate is a parasite called Nanophyetus salmincola, which can infest steelhead and perhaps increase their risk of predation. The parasite’s life cycle requires a snail and a warm-blooded animal, as I described in a story I wrote for the Encyclopedia of Puget Sound — part of a larger piece about disease as a powerful ecological force. Anyway, the snail is found only in streams in South Puget Sound, which might help explain why steelhead deaths are higher among these South Sound populations.
Experiments are underway to compare the survival of two groups of identical steelhead, one group infested with Nanophyetus and one not.
Depending on funding and proper design, another experiment could test whether treating a stream to temporarily eliminate the snail — an intermediate host — could increase the survival of steelhead. If successful, treating streams to remove these snails could be one way of helping the steelhead. For these and other approved and proposed studies, check out the Marine Survival Project’s “2015-2017 Research Work Plan” (PDF 9.3 mb).
Other factors under review that could play a role in steelhead survival are warming temperatures and pollution in Puget Sound, which could help determine the amount and type of plankton available for steelhead and salmon. Could a shift in plankton result in less food for the small fish? It’s a major question to be answered.
I’ve mentioned in Water Ways (3/15/2010) that transient killer whales, which eat seals, sea lions and harbor porpoises, may be helping their distant cousins, the Southern Resident killer whales, which eat fish. Those smaller marine mammals compete for the adult salmon eaten by the Southern Residents. By clearing out some of those competitors, the transients could be leaving more salmon for the Southern Residents.
It may be too early to draw any firm conclusions, Michael Schmidt told me, but transient killer whales may be helping steelhead as well. Last year, when transients ventured into South Puget Sound and stayed longer than usual, the survival rate for steelhead from the nearby Nisqually River was the highest it has been in a long time.
Were the whales eating enough seals to make a difference for steelhead, or were the seals hiding out and not eating while the whales were around. Whether there were benefits for the steelhead, we could be seeing what happens when a major predator (orcas) encounters an abundance of prey (seals).