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Study: ‘Passive’ Salmon Recolonization Works Rapidly In Certain Situations When Barriers Removed
Posted on Friday, January 09, 2015 (PST)

Coho and chinook salmon have been successfully reintroduced upstream of a diversion dam in Washington after a fish ladder was installed in 2003.


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 (


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 risk.


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.”


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