Status Review for Oregon's Umpqua River Sea-Run Cutthroat Trout
Orlay W. Johnson, Robin S. Waples,
National Marine Fisheries Service
The first issue addressed was the relationship between anadromous (sea-run) and nonanadromous resident and potamodromous (river-migrating) life-history forms in the Umpqua River. Based on studies of life-history traits in cutthroat trout and other salmonid species, we concluded that, at least until better information is developed, anadromous cutthroat trout should be considered in the same ESU as resident and potamodromous O. c.clarki.
We then addressed the issue of whether all life-history forms of Umpqua River O. clarki were reproductively isolated from O. clarki stocks in other coastal drainages. Because sea-run cutthroat trout in the Umpqua River have not been the focus of rigorous scientific inquiries, no genetic or tagging data were identified that are directly relevant to this issue. However, the preponderance of secondary information suggested that O. clarki as a group in the Umpqua River probably satisfy the ESU criterion of substantial reproductive isolation from other conspecific populations. Information evaluated in this context included genetic studies on anadromous cutthroat trout from Puget Sound and the Oregon coast, locations of possible anadromous O. clarki spawning sites in the Umpqua River Basin, and the possibility that the lower part of the mainstream Umpqua River is too warm for cutthroat trout during much of the summer and may be an isolating mechanism.
Factors considered for the second ESU criterion regarding ecological/genetic diversity include distinctive physical and environmental features of the Umpqua River drainage, lengthy freshwater migration for the anadromous form, distinctive run times of sea-run fish, and possible adaptations for dealing with high water-temperatures. The Umpqua River Basin has a variety of distinctive features, including one of the largest coastal basins in Oregon and headwaters that begin farther inland and at a higher elevation than most other Oregon coastal rivers. Further, anadromous cutthroat trout that spawn in the upper tributaries of the North and South Umpqua Rivers would migrate farther inland (240 to 280 km) than do cutthroat trout from most other rivers in North America.
However, other factors relating to ecological/genetic diversity were more difficult to evaluate because of the lack of information on O. clarki in the Umpqua River Basin. Historically, Umpqua River sea-run cutthroat trout apparently had a bimodal run-timing, with both peaks occurring slightly earlier than in other streams that have been studied. However, from 1961 to 1976, large numbers of sea-run cutthroat trout from the Alsea River hatchery were released below Winchester Dam on the North Umpqua River. During this time, there was a dramatic increase in numbers of adult cutthroat trout passing Winchester Dam. Alsea River fish have a slightly later run-timing than the Umpqua River fish, and a statistically significant shift toward later run-timing could be detected in fish that returned to Winchester Dam during this period of supplementation. After supplementation was stopped, there was a significant shift back toward the original run-timing, although the later (October) peak of the run virtually disappeared.
Although the pattern of abundance and tag/recovery data during the period of supplementation indicated that Alsea River hatchery fish returned as adults to Winchester Dam in some numbers, it is apparent that 15 years of hatchery releases did not result in a viable, self-sustaining population of naturally spawning fish. The unresolved issue is, What do the few remaining anadromous O. clarki represent: remnants of the original Umpqua River gene pool, descendants of the Alsea River hatchery fish, or a mixed lineage? The run-timing shift suggested that an indigenous component may remain.
The precarious status of the remaining sea-run fish in the Umpqua River is well documented in this review, but evidence is inconclusive as to the evolutionary heritage of these fish. Furthermore, we have concluded that resident and potamodromous fish should also be considered part of the ESU, but data on river-migrating fish within the Umpqua River are very limited, and the total information we were able to develop on abundance of resident O. clarki in the Umpqua River drainage amounted to a list of lakes and tributaries believed to contain cutthroat trout. However, even if the nonanadromous forms were determined to be healthy, risk of loss of the anadromous form still would be an ESA concern if the trait has a genetic basis and it contributes substantially to ecological/genetic diversity. Thus, after considering all available information, we concluded that there were two general approaches to the listing decision, given the available scientific information:
1) Since nonanadromous O. c. clarki are included in the ESU, the petition could be denied because the petitioned entities (North and South Umpqua River sea-run cutthroat trout) are not by themselves ESA "species." This, however, would not be a resolution of the issue.
2) A proposal to list could be based on the precarious status of sea-run cutthoat trout in the North Umpqua River. This would require taking a conservative approach, similar to the one used with Redfish Lake sockeye salmon, to each of the three major unresolved issues: the geographic boundaries of the ESU, the heritage of the remaining sea-run cutthroat trout, and the abundance and population trends of the resident and potamodromous fish. A framework for doing this would presumably involve the following assumptions:
1) All life-history forms of O. clarki in the Umpqua
River Basin are a single ESU distinct from other coastal populations.
2) This ESU represents the evolutionary legacy of the historical O. clarki population prior to the releases of Alsea River hatchery fish.
3) All life-history forms of O. clarki in the Umpqua River Basin have experienced extensive declines in abundance such that they are presently threatened or in danger of extinction; or alternatively, although there is little information regarding the abundance of nonadromous O. clarki in the basin, the depressed sea-run component of the population is a substantial and important component of the ESU and its loss would comprise the distinctness and viability of the inclusive ESU.