Coho and chinook salmon have been successfully reintroduced
upstream of a diversion dam in Washington after a fish ladder was installed in
The reintroduction was accomplished without supplementation
by just allowing existing runs of salmon to recolonize up to 33 kilometers
(20.5 miles) of habitat in a pristine area previously occupied by the fish
prior to the dam’s construction.
The result of this passive reintroduction is that coho and
chinook salmon now spawn in habitat upstream of the Landsberg Diversion Dam on
the Cedar River in Washington.
That shows, according to a recent study, that the only
necessary action after removing a dam or circumventing a dam (in this case with
a fish ladder) is to allow naturally-spawning fish to move into the reclaimed
habitat on their own volition.
“Our study demonstrated that salmon are capable of rapidly
recolonizing habitat made accessible by removal or circumvention of migration
barriers,” said Joseph Anderson, research scientist with the Washington
Department of Fish and Wildlife. He added that the results are likely
transferrable to other projects, “particularly in cases where there is a
potential source population for recolonization spawning immediately below the
barrier and high quality habitat above the barrier, as on the Cedar River.”
Anderson and five other scientists published the article,
“Dispersal and productivity of chinook (Oncorhynchus tshawytscha) and coho (O.
kisutch) salmon colonizing newly accessible habitat,” online this month in the
Canadian Journal of Fisheries and Aquatic Sciences (http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2014-0180#.VKQYFCvF9qU).
Joining Anderson as study authors are Thomas Quinn,
professor at the University of Washington; William Atlas, graduate student at
Simon Fraser University; and Paul Faulds, Karl Burton and Michele Koehler,
fisheries biologists at Seattle Public Utilities.
The Cedar River flows west from the Cascade Mountains and
empties into Lake Washington in Seattle. The lake connects with Puget Sound
through a canal with locks to the west. Historically, the river flowed into the
Green River and directly into Puget Sound, but the Cedar River was diverted to
Lake Washington in 1916. Landsberg Diversion Dam (35 km – about 22 miles -
upstream from Lake Washington) blocked fish passage from 1901 until 2003, when
Seattle Public Utilities installed a fish ladder at the dam.
The study showed that salmon will immediately take advantage
of stream reaches made accessible by restoration or, in this case, providing
access to habitat upstream of a dam, and this will happen naturally without
human assistance, according to the study.
“These initial colonists, especially the coho salmon, were
remarkably successful, suggesting the removal or circumvention of barriers
throughout the Pacific Northwest offer promising opportunities for conservation
and recovery of anadromous salmonids,” the report concludes.
Coho salmon were more successful at recolonizing than were
chinook salmon. According to Anderson, a much higher proportion of coho salmon
in the second generation – fish that were the progeny of the first salmon to
spawn in the reintroduced habitat – than chinook salmon returned to the new
habitat above the dam to spawn. Initially, straying fish from nearby hatcheries
on Lake Washington also made up a part of the spawning salmon in the restored
habitat, but became a minor part of the fish spawning in following generations.
“This result helped us understand how patterns of homing
versus straying influence the rate of numerical population growth and the
transition from recolonization to self-sustaining reproduction,” he added.
The report pointed to similar recolonization of pink salmon
on the upper Fraser River in British Columbia, Canada, when fish passage
facilities were added to the Hell’s Gate dam.
“Thus, in situations where a migration barrier is removed
adjacent to a naturally reproducing, self-sustaining population of salmon,
transplanting or hatchery supplementation does not appear necessary for
population expansion,” the report concludes.
This may not always be the case: a recolonizing population
may fail if the population density does not exceed a critical abundance
threshold, according to the study. If that doesn’t happen, then active
recolonization strategies may be needed using transplanting or supplementation
to ensure that adequate numbers of fish reach the new habitat to spawn.
However, that “may compromise long-term conservation goals by altering
evolutionary and ecological processes.” Passive reintroduction will avoid that
In this case, passive reintroduction of both chinook and
coho salmon worked because both species already were spawning naturally
downstream from the dam, and a population of straying fish from Lake Washington
hatcheries initially helped to sustain the critical abundance needed.
“[S]almon are quite adept at taking advantage of
opportunities for recolonization, and so restoring access to high quality
habitat blocked by migration barriers is an effective salmon conservation
strategy,” Anderson said. He also suggested that the “removal or circumvention
of barriers that block high quality habitats adjacent to existing source
populations” should be prioritized in order to get maximum benefit from future
fish passage projects.
Each project will be different, he added, saying that
proximity to source populations, the quantity and quality of habitat above the
removed or circumvented barrier, survival during migration through passage
facilities, survival during the barrier removal process itself and the natural
history of salmon already in the watershed should be factors considered when
“identifying recolonization opportunities and managing salmon populations
following barrier removal.”