by
Orlay W. Johnson
Thomas A. Flagg
Desmond J. Maynard
George B. Milner
F. William Waknitz
National Marine Fisheries Service
Northwest Fisheries Science Center
Coastal Zone and Estuarine Studies Division
2725 Montlake Boulevard East
Seattle, WA 98112
June 1991
This report summarizes biological information on coho salmon (Oncorhynchus kisutch) from the lower Columbia River* (LCR) gathered in conjunction with a U.S. Endangered Species Act (ESA) status review. Under the ESA, any "distinct population segment" of fishes qualifies as a "species" and is eligible for protection if it is threatened or endangered. National Marine Fisheries Service (NMFS) policy is that a population will be considered "distinct" for purposes of the ESA if it represents an evolutionarily significant unit (ESU) of the species as a whole (Waples 1991). The ESA evaluation of coho salmon is complicated because the LCR has sustained intense human perturbation in the last century. Commercial exploitation of coho salmon and degradation of salmon spawning habitat reduced coho salmon numbers to near extinction around the middle of this century. The advent of extensive artificial propagation of the species in hatcheries has "rebuilt" the runs to at or above historic levels. However, this process has changed the coho salmon runs in the LCR from predominantly naturally spawning fish to predominantly hatchery-maintained fish. Native runs, if they persist, would exist only as small remnant populations.
Coho salmon represent one of five species of anadromous Pacific salmon native to North America. Throughout their range, native coho salmon populations return to their natal streams to spawn from early fall to late spring. Fry emerge from egg nests (redds) between early March and July, rear in fresh water for a year, migrate to sea the next season, and return to spawn after 5 to 20 months of feeding in the open ocean.
The NMFS Northwest Region Biological Review Team (BRT) evaluated studies on natural history of coho salmon to determine if there are distinct differences between populations. The BRT also evaluated whether the characteristics of the life history of coho salmon might indicate that native populations could persist, considering the extensive hatchery releases, outplantings, overharvest, and habitat destruction in the LCR over the last 100 years.
Fidelity of homing in coho salmon under natural conditions is similar to that demonstrated for other species of Pacific salmon and appears sufficient to maintain populations with a similar level of distinctiveness. Different coho salmon populations show timing differences from fry emergence to time of adult spawner returns. Coho salmon show freshwater, estuarine, and ocean migratory patterns apparently determined by the geographic area of their natal streams. Homing and spawning behavior is complex and would suggest a selection mechanism that appears sufficient to reduce gene flow from nonnative populations. However, the BRT determined, from the available evidence, that the massive and extensive disruptions documented in coho salmon populations in the LCR have depleted native populations enough that population differences have been largely eliminated.
Prior to the 1900s, naturally produced coho salmon were widespread in the Columbia River Basin, with a historical center of abundance in the LCR. There were also large runs of coho salmon in the middle and upper reaches of the Columbia River and in the Snake River. All upper, middle, and Snake River runs were drastically reduced or destroyed by various factors prior to the 1950s, including overharvest and habitat destruction or blockage (Cramer et al. 1991).
It is impossible to accurately estimate the decline in LCR stocks of coho salmon, but the BRT estimated that the runs may have been reduced to less than 5% of historic levels by the late 1950s.
The drastic decline in coho salmon abundance initiated a widespread hatchery enhancement program after 1960 (ODFW 1990a,b; WDF 1991a). This program increased coho salmon populations in the Columbia River to or above historic levels. The total return to the river often exceeded 400,000 fish in recent years (Howell et al. 1985, CBFWA 1990, PFMC 1991).
The causes of the original decline in coho salmon were not eliminated by this extensive hatchery production. Overharvest, habitat blockage and destruction, and other activities detrimental to natural production continued. The result was a continued decline in naturally spawning runs while exploitation of hatchery fish continued at increased levels. The Columbia River is now managed almost entirely for commercial exploitation of hatchery fish.
It is believed that the majority of naturally produced coho salmon return to the LCR to spawn between early December and March (ODFW 1990b, 1991a,b; Cramer et al. 1991). The Oregon Department of Fish and Wildlife (ODFW) estimates there may be less than 195 of these fish in Oregon and that they may exist only as small, isolated populations in the Lewis and Clark and Sandy River systems (ODFW 1991c). No detailed information was available from the Washington Department of Fisheries (WDF) on possible locations where natural spawning may occur or on numbers of nonhatchery origin coho salmon that may spawn. The BRT estimated there may be only about 100 post-December spawning coho salmon in lower Columbia River tributaries in Washington.
There is a post-December run of naturally spawning fish that return to the North Fork of the Clackamas River in Oregon (Cramer 1991). Counts over the North Fork Dam fluctuated substantially between 1957 and 1989, but there is no apparent trend in abundance. This run was not part of the original petition for lower Columbia River coho salmon, but was reviewed by the BRT as a special case. Computer modeling indicates this population has less than a 0.1% chance of extinction during the next 100 years assuming no change to population parameters.
The BRT evaluated the outplanting history of hatchery populations in the LCR to determine if these populations could represent the historical ESU. The BRT determined that the history of hatchery populations shows an infusion of stocks from throughout the basin and coastal regions in Oregon and Washington (ODFW 1990a,b; 1991c; VTDF 1991a,d). Most hatcheries in the basin manage their hatchery populations for stocks which have either a predominately northern or southern ocean distribution from the mouth of the Columbia River (Cramer et al. 1991; VTDF 1991a). These stocks themselves are a mixture of populations, and the BRT was unable to determine if any LCR hatchery stocks represent historically distinct LCR populations.
The NMFS Northwest Fisheries Center collected coho salmon samples from the LCR for electrophoretic analysis. The BRT analysis and a review of the scientific literature of other genetic studies on LCR coho salmon were inconclusive in determining whether distinct coho salmon populations existed in the LCR.
Ceratomyxa shasta is a protozoan parasite of coho salmon which can cause large losses of adult fish. Stocks of coho salmon in the LCR, and some populations in coastal Oregon and Washington, are presumed to have an inherited resistance to the parasite. Studies suggest the presence of C. shasta may act to reduce introgression in LCR stocks from populations that lack resistance to the parasite.
The BRT concluded that the available data fail to identify an existing evolutionarily significant unit in the lower Columbia River. If one or more coho salmon ESUs are present in the Columbia River, fishery management actions and research studies have inadequately documented these populations.
The status review for the lower Columbia River coho salmon was conducted by the NMFS Northwest Region Biological Review Team (BRT). The extensive public record developed pursuant to this review and discussions of that record by the ESA Technical Committee formed the basis for this report. Members of the BRT for LCR coho salmon were David Damkaer, Thomas Flagg, Elizabeth Gaar, Lee Harrell, Orlay Johnson, Robert Jones, Conrad Mahnken, Gene Matthews, Desmond Maynard, George Milner, Gerald Monan, Ben Sandford, Michael Schiewe, Grant Thompson, Merritt Tuttle, William Waknitz, Robin Waples, John Williams, and Gary Winans.
* LCR is defined in this document as the Columbia River and its tributaries below Bonneville Dam, exclusive of the Willamette River. The mid Columbia River is defined as the area from Bonneville Dam to the confluence of the Snake and Columbia Rivers. The upper Columbia River is the area above this confluence, including the area above Grand Coulee Dam.