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
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.
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
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
“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
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
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
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
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
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).
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