U.S. Dept Commerce/NOAA/NMFS/NWFSC/Publications
NOAA-NWFSC Tech Memo-19: Status Review for Klamath Mountains Province Steelhead
History of Hatchery Stocks and Outplantings

Steelhead Hatcheries

Annual hatchery production of steelhead on the west coast of North America increased from about 3 million juvenile steelhead in 1960 to almost 30 million in 1987 (Light 1989). The majority of hatchery produced steelhead (89%) are from the Pacific Northwest states of Idaho, Washington, and Oregon (Table 9), and this figure is dominated by steelhead from hatcheries concentrated in the Columbia River Basin (Light 1989).

Oregon Hatchery Stocks

The State of Oregon produces 22 hatchery stocks of coastal steelhead (ODFW 1986). Of these, we found that five stocks have a substantial history of production and/or stocking within the Klamath Mountains Province. As named by ODFW (1986), these five stocks are Alsea Winter-Run 43, Applegate Winter-Run 62, Chetco Winter-Run 96, Rogue Winter-Run 52, and Rogue Summer-Run 52.

Alsea Winter-Run 43--Steelhead originating from the Alsea River were planted in streams along Oregon's coast from the late 1930s through the 1960s; liberations of this stock since then have been limited to the coast between the Coquille and Salmon Rivers (ODFW 1986). Although specific information is not available regarding stocking locations of Alsea steelhead prior to 1980, it is likely that some introductions occurred in streams considered here.

Table 9. Steelhead smolt production by hatcheries, listed from north to south (Light 1989).
Location (number of
Average annual smolt
production, 1978-87
Percent of
coastwide total
Alaska (4) 62,000 0.2
British Columbia (22) 616,000 2.5
Washington (44) 6,782,000 27.6
Idaho (4) 10,320,000 41.9
Oregon (26) 4,537,000 18.4
California (9) 2,304,000 9.4
Total (109) 24,621,000 100.0

Applegate Winter-Run 62--This Cole Rivers Hatchery stock began in 1979; broodstock are collected from the Applegate River (ODFW 1986). The stock is propagated primarily to mitigate habitat loss due to Applegate Dam, and most of the plantings from this stock have been into the Applegate River (ODFW 1993a). From 1983 through 1993, an average of 126,000 steelhead from Applegate Winter-Run 62 were stocked in the Applegate Basin annually, at an average size of about 18 cm; occasionally, Applegate steelhead have been planted in a variety of lakes and ponds in southern Oregon (ODFW 1993a).

Chetco Winter-Run 96--Chetco Winter-Run 96 hatchery stock was established in 1970; broodstock are collected from steelhead returning to the Chetco River (ODFW 1986). As the Chetco River was stocked with Alsea Winter-Run 43 prior to 1970, the present stock may have some Alsea stock influence (ODFW 1986). Chetco Winter-Run 96 is reared at the Elk River Hatchery, then released into the Chetco River at an average length of 18 cm; from 1980 through 1993, an average of 62,000 Chetco Winter-Run 96 were released annually (ODFW 1993a).

Rogue Winter-Run 52--Culture of Rogue River winter steelhead at Cole Rivers Hatchery began in 1974 to mitigate loss of habitat due to Lost Creek Dam (ODFW 1986). Most of this stock is planted in the Rogue River; an annual average of 132,000 fish were planted from 1980 through 1993, at an average length of 20 cm (ODFW 1993a). Fish from this stock that are not used in the Rogue River are usually planted in local ponds and reservoirs to supplement the trout fishery (ODFW 1986). Most of these fish have gone to Lost Creek Reservoir, just upstream from the hatchery, and to Emigrant Reservoir on Bear Creek, in the Rogue River Basin near Ashland, Oregon (ODFW 1993a).

Rogue Summer-Run 52--This stock was established in 1962 with broodstock collected at Gold Ray Dam (ODFW 1986). Initially, Rogue Summer-Run 52 was reared at Butte Falls and Bandon Hatcheries, then released into the Rogue River. Since 1974, broodstock collection and smolt production have been conducted at Cole Rivers Hatchery, below Lost Creek Dam (ODFW 1986). From 1980 through 1993, an average of 181,000 Rogue Summer-Run 52 steelhead were released annually in the Rogue River, at an average length of 18 cm (ODFW 1993a). In addition, from 1990 through 1993 an average of 109,000 steelhead were released in Lost Creek Reservoir, at an average length of 10 cm; other plantings of this stock have occasionally occurred in other southern Oregon reservoirs and ponds (ODFW 1993a).

Applegate Summer-Run 62--Data from ODFW (1993a) indicate that in 1981 and 1982, a stock of Applegate summer-run steelhead was produced at Cole Rivers Hatchery. These fish were released primarily in the Applegate and Rogue River Basins (ODFW 1993a). These may actually have been winter steelhead (Evenson - footnote 6); ODFW (1986) does not mention this stock.

California Hatchery Stocks

California Department of Fish and Game produces steelhead at seven hatcheries, of which two (Iron Gate and Trinity River Hatcheries) are within the Klamath Mountains Province and another (Mad River Hatchery) is nearby. Iron Gate and Trinity River Hatcheries are mitigation hatcheries for habitat lost to power generating and water diversion dams; Mad River Hatchery is an enhancement hatchery (McEwan and Jackson in prep.). California Department of Fish and Game also administers several community-based steelhead rearing projects throughout northern California (McEwan and Jackson in prep.). Additionally, there is one USFWS hatchery in northern California (Coleman National Fish Hatchery, Sacramento River) and two locally operated hatcheries, Rowdy Creek (Smith River) and Prairie Creek (Redwood Creek), that also produce steelhead.

Iron Gate Hatchery--Iron Gate Hatchery is on the Klamath River near Hornbrook, California. Steelhead have been reared at this hatchery since 1966 (CDFG 1994a). Most broodstock are taken from steelhead returning to the hatchery; however, some eggs were imported from Trinity River Hatchery and from Cowlitz Trout Hatchery (Washington) in the late 1960s (CDFG 1994a). Steelhead releases from Iron Gate Hatchery have primarily been into the Klamath River. There have been several transfers of eggs from Iron Gate to Trinity River Hatchery, and occasionally Iron Gate Hatchery has supplied eggs or fingerlings to various other facilities within the Klamath Mountains Province (e.g., Humboldt State University, Six Rivers National Forest, and Tribal facilities; CDFG 1994a). Iron Gate Hatchery produces 200,000 steelhead smolts annually (McEwan and Jackson in prep.). Between 1972 and 1982, hatchery steelhead comprised an average of 7.8% of the steelhead runs on the Klamath River (McEwan and Jackson in prep.).

Trinity River Hatchery--The Trinity River Hatchery program began in 1958. To supplement steelhead returning to the hatchery, eggs and fingerlings have been imported from other facilities (CDFG 1994b). The most common source of eggs has been Iron Gate Hatchery, with annual transfers since 1974 (CDFG 1994b). Trinity River Hatchery has also received eggs and fingerlings from the Sacramento and Eel River Basins in California, Willamette River in Oregon (Roaring River Hatchery), and Washougal River in Washington (Skamania Hatchery). These latter transfers have been few and infrequent and apparently ceased after 1973 (CDFG 1994b). Most of the steelhead produced at Trinity River Hatchery are released into the Trinity River. Some transfer of eggs and fingerlings to Iron Gate and Mad River Hatcheries has occurred (CDFG 1994b). Trinity River Hatchery produces 800,000 steelhead smolts annually; hatchery contribution to the steelhead runs on the Trinity River was 20-34% for the run years 1980-83 (McEwan and Jackson in prep.).

Mad River Hatchery--Mad River Hatchery is outside of the Klamath Mountains Province; however, as a fishery enhancement facility, Mad River Hatchery has received steelhead eggs from a variety of locations, and steelhead from Mad River Hatchery have been stocked into numerous waters. Mad River Hatchery has received steelhead eggs from the following river basins: Trinity (CA), Eel (CA), San Lorenzo (CA), Smith (CA), Dry Creek (CA), and Washougal (WA). In 1978 and 1979, 284,000 steelhead eggs were transferred from Rowdy Creek Hatchery on the Smith River to Mad River Hatchery (CDFG 1994c).

Steelhead from Mad River Hatchery have been transferred to several facilities, including some within the Klamath Mountains Province. Most of these have involved the Smith River; between 1971 and 1981, 410,000 smolts and 41,000 fry were planted in the Smith River Basin from Mad River Hatchery (CDFG 1994c). Of these, 37% were of Smith River stock (CDFG 1994c).

Population Genetic Structure

Previous Studies

Numerous protein electrophoretic studies of population structure in coastal O. mykiss have been published since the mid-1970s. Allendorf (1975) first distinguished two major groups of O. mykiss in Washington, Oregon, and Idaho, separated geographically by the Cascade Crest; Allendorf termed these inland and coastal. These two groups have large and consistent differences in allele frequency that apply to both anadromous and resident forms. Subsequent studies have supported this finding (Utter and Allendorf 1977, Okazaki 1984, Schreck et al. 1986, Reisenbichler et al. 1992), and similar differences have been identified between O. mykiss from the interior and coastal regions of British Columbia (Huzyk and Tsuyuki 1974, Parkinson 1984).

Parkinson (1984) found substantial genetic differences among steelhead populations from adjacent drainages in British Columbia. Studies from Washington (Allendorf 1975, Reisenbichler and Phelps 1989) and Oregon (Hatch 1990, Reisenbichler et al. 1992) reported smaller differences between populations. Reisenbichler and Phelps (1989) and Reisenbichler et al. (1992) suggested that since both Washington and Oregon had far more extensive hatchery steelhead programs in the 1970s and early 1980s than did British Columbia, the relative homogeneity among populations in these states may be due to introgression of hatchery fish into naturally spawning populations. Furthermore, during that period, hatcheries in both Oregon and Washington predominately used steelhead that had originated from single within-state sources (the Green River in Washington and the Alsea River in Oregon).

Allozyme studies on Oregon steelhead, including some populations from the Rogue River Basin, have been reported by Hatch (1990) and Reisenbichler et al. (1992). Hatch (1990) surveyed 13 protein-coding loci in steelhead from 12 hatcheries and 26 coastal rivers or tributaries in Oregon. He found evidence for a north-south cline in allele frequencies in 5 of the 13 enzyme systems analyzed, but only in river systems larger than 350 km2. Hatch also reported that "the area south of the Coos River was marked by sharp transition in four different enzymes..." (p. 17) and that "the pattern of several alleles ending their detectable Oregon presence just north of Cape Blanco suggests that there is a less than average amount of straying between the populations north and south of this feature" (p. 33).

Reisenbichler et al. (1992) examined 10 polymorphic gene loci in steelhead from 37 natural and hatchery populations in the Pacific Northwest, including 24 from the Oregon coast and two in northern California (Trinity River summer-run and Mad River Hatchery winter-run). They did not discuss clines in allele frequencies; instead, they found evidence for genetic differentiation between some clusters of populations. For example, steelhead from north of the Umpqua River formed a separate cluster from steelhead in southern Oregon. The Trinity River sample was genetically similar to most of the Rogue River samples, but steelhead from the Mad River Hatchery were genetically distinct from other hatchery and natural populations in California and Oregon. Genetic differences between naturally spawning populations in separate drainages within clusters were not statistically significant and were similar in magnitude to those reported in coastal Washington (Allendorf 1975, Reisenbichler and Phelps 1989) and less than reported in British Columbia (Parkinson 1984). However, pair-wise comparisons revealed significant differences within drainages between hatchery fish and naturally spawning populations, including Cole Rivers Hatchery fish and Rogue River natural stocks.

In recent years, genetic methods that analyze DNA variation directly have seen increasing use with salmonids, and we are aware of two studies of mtDNA that address population structure in steelhead. In a study that remains unpublished, Buroker (footnote 3) examined restriction-fragment-length polymorphisms in mtDNA from 120 individuals from 23 major river systems from Alaska to California. He found no evidence for strong geographic structuring of populations, as most of the common clonal types were widely dispersed. However, Buroker found that steelhead from southern Oregon were highly diverse in mtDNA. In the 120 fish analyzed, 18 different mtDNA clonal types were observed. These clones were clustered into four lineages, all of which overlap in southern Oregon. The 12 fish examined from the Rogue River had 6 of the 18 mtDNA clonal types observed in the study.

In another study, Nielsen (1994) sequenced part of the D-loop section of mtDNA of steelhead and rainbow trout in California and found that a different allele was the most common in each of three geographic regions: north coast, central coast, and south coast. The boundary between the central and south coast regions corresponds to a natural biogeographic boundary near Point Conception. All of the samples, however, including those from the north coast area, were from south of Humboldt Bay.

Chromosome karyotypes in steelhead and rainbow trout have also been extensively studied (see review in Thorgaard 1983). In a survey of steelhead from Alaska to central California, Thorgaard (1983) found that although chromosome numbers ranging from 58 to 64 were observed, a 58-chromosome karyotype was the most common in most samples. In contrast to results for studies of morphological and allozyme characters, Thorgaard did not find chromosomal differences between interior and coastal O. mykiss populations. All interior/redband trout populations had predominately 58 chromosomes, as did most coastal rainbow trout and steelhead populations.

The exceptions to the 58-chromosome pattern, however, provide insight into population genetic structuring in O. mykiss. Two geographic regions were characterized by steelhead with 59 or 60 chromosomes: the Puget Sound/Strait of Georgia region and the Rogue River/northern California region. However, the karyotypes of fish from these two regions were different; northern fish with 59 or 60 chromosomes had a different number of subtelocentric and acrocentric chromosomes than did southern fish (Thorgaard 1977). Farther south, winter steelhead in the Mad and Gualala Rivers from northern California and resident trout from the San Luis Rey River in southern California had 61-64 chromosomes (Thorgaard 1983).

New Data

As part of the status review of Illinois River winter steelhead, NMFS biologists analyzed 15 new samples of coastal steelhead, focusing on the Illinois and Rogue River drainages but including samples from as far south as the Smith and Klamath Rivers in northern California (Busby et al. 1993). Genetic distance values (Nei 1978) were computed between each pair of populations based on 39 gene loci that were variable (polymorphic) in at least one sample. Busby et al. (1993) found that the three samples from north of Cape Blanco (Bandon Hatchery, Nehalem River, and Yaquina River), as a group, were genetically distinct from the more southerly populations. This is consistent with results reported by Hatch (1990) and Reisenbichler et al. (1992), who found evidence for some genetic differentiation between populations in northern and southern Oregon. In contrast, little geographic pattern was evident in samples from the area between Cape Blanco and the Klamath River. The four samples from the Illinois River did not form a coherent genetic group; in fact, three of the four samples were genetically more similar to samples from outside the Rogue River drainage than they were to other Illinois River samples.

For this expanded status review, we analyzed an additional five samples of steelhead collected from streams in northern California (Table 10). Collection and laboratory procedures were as described in Busby et al. (1993), and data were again gathered for 39 polymorphic gene loci. Results of unweighted pair-group method (UPGMA) clustering of pairwise genetic distance values are shown in the dendrogram in Figure 3. One of the new samples was from the Trinity River, and this sample showed a clear genetic affinity with the other steelhead from the Klamath River Basin as well as with those north to Cape Blanco. In contrast, the four samples from south of the Klamath River (Redwood Creek, Mad River wild and hatchery, and Eel River) were distinct genetically. In fact, genetic differences between steelhead populations from south of the Klamath River and areas to the north are considerably larger than the differences between steelhead from southern and northern Oregon.

Table 10. Steelhead populations examined in the genetic analysis. For run timing, W = winter, S = summer, and W/S = uncertain or a mixture of both forms.

Location Run Sample Population number
Population timing size (see Figure 4)
North of Cape Blanco
Nehalem River W 40 1
Yaquina River W 40 2
Bandon Hatchery
     (Coquille R. stock)
W 40 3
Cape Blanco to Klamath River Basin
Elk River W 40 4
Rogue River
     Lobster Creek W/S 40 5
     Little Butte Creek W/S 40 6
     Cole Rivers Hatchery W 40 7
Illinois River
     Grayback Creek W 40 8
     Briggs Creek W 40 9
     Lawson Creek W/S 40 10
     Indigo Creek W/S 30 11
Pistol River W 40 12
Winchuck River W 40 13
Smith River W 40 14
Klamath River W/S 40 15
Trinity River W/S 39 16
South of Klamath River Basin
Redwood Creek W/S 40 17
Mad River (hatchery) W 40 18
Mad River (wild) W/S 40 19
Van Duzen River W/S 40 20

Figure 3
Figure 3. Dendrogram depicting genetic relationships among 20 coastal steelhead populations analyzed for this study. The figure was constructed by clustering of Nei's (1978) unbiased, pairwise genetic distance values based on data for 39 polymorphic gene loci. Rogue River: 1 = Lobster Creek, 2 = Little Butte Creek; Illinois River: 1 = Indigo Creek, 2 = Briggs Creek, 3 = Lawson Creek, 4 = Grayback Creek. Letters identify three geographic regions: A = north of Cape Blanco; B = Cape Blanco through the Klamath River Basin (i.e., the Klamath Mountains Province); C = south of the Klamath River Basin.

As a group, the samples from south of the Klamath River Basin are characterized by divergent allele frequencies at the loci *FBALD-3, *GPIA, *LDHB-1, *NTP, *PGM-2, and *sSOD-1 (Fig. 4a-b). Based on the genetic data, Redwood Creek, the basin immediately south of the Klamath River, appears to be in a transitional zone; the sample from this stream falls out with the southern group but also has some genetic affinity with samples from the Klamath River and areas to the north. For the three loci shown in Figure 4b (*NTP, *PGM-2, and *sSOD-1), there is some evidence for north-south clines in allele frequency. The *sSOD-1 locus was one for which Hatch (1990) reported a cline in steelhead from Oregon rivers with basins larger than 350 km2. However, closer examination of Figure 4b indicates that the clines, if they exist, are not monotonic; in fact, there is little evidence of a cline within the Klamath Mountains Province for any of these three loci. Trends are apparent for these loci because allele frequencies for samples from within the Klamath Mountains Province are intermediate to frequencies for samples taken north or south of this area.

Figure 4a Figure 4b
Figure 4. Allele frequencies at selected gene loci in 20 coastal steelhead populations. Populations are arranged from north to south along the X-axis, and population numbers correspond to those shown in Table 10. Letters identify three geographic regions: A = north of Cape Blanco; B = Cape Blanco through the Klamath River Basin (i.e. the Klamath Mountains Province); C = south of the Klamath River Basin.

Because several out-of-basin steelhead stocks, including some from Washington State, have been used at the Mad River Hatchery, we considered the possibility that the genetic differences we saw in steelhead populations from south of the Klamath River were an artifact of stock transfers. Although we cannot discount genetic effects of these stock transfers on natural populations, they do not explain the observed genetic differences. We find substantial allele frequency differences between our sample of steelhead from the Mad River Hatchery and samples from the Skamania and Washougal Hatchery stocks (Phelps-footnote 7), which are among those that have been used in the Mad River Hatchery.

Discussion and Conclusions on the Species Question

In this section, we summarize evidence developed in the status review that is relevant to the two criteria, reproductive isolation and ecological/genetic diversity, that must be met for a population to be considered an ESU, and hence a species under the ESA.

Reproductive Isolation

Steelhead in general are believed to have strong tendencies to home to their natal stream, but there are few studies directly relevant to the area under consideration. There is evidence that some adult steelhead move between the Klamath, Rogue, and Smith Rivers, but it is not clear whether this "wandering" results in spawning in nonnatal streams.

Genetic information presented in the Illinois River winter steelhead status review (Busby et al. 1993) supported earlier findings that there is a genetic discontinuity (or at least a transition) between steelhead from coastal streams in southern and northern Oregon. The discontinuity/transition appears to occur in the vicinity of Cape Blanco, but sampling has not been sufficiently fine-scaled to precisely define the boundary.

For the present status review, we collected genetic data for five additional samples from northern California, including four from streams south of the Klamath River Basin. Whereas steelhead from the Klamath River and the Trinity River (a tributary to the Klamath River) do not differ substantially from steelhead populations to the north, there are large allele frequency differences between samples from the Klamath River Basin and those taken from rivers to the south. Genetic differences between steelhead from these two areas are larger than those found between southern and northern Oregon populations.

Within the area bounded by Cape Blanco and the Klamath River Basin (inclusive), there is evidence for genetic heterogeneity, suggesting a reasonable degree of reproductive isolation of individual populations. However, there is no clear geographic pattern to the genetic structuring that would allow us to identify major subgroups within this area.

Two seasonal run-types of steelhead are widely recognized in North America: summer-run and winter-run. These terms refer to the time of year at which adults enter fresh water to commence their spawning migration. In the Pacific Northwest, steelhead that enter fresh water between May and October are usually considered summer run, and steelhead that enter fresh water between November and April are usually considered winter run. In the Klamath River Basin, some biologists refer to fall-run steelhead; disagreement exists as to whether fall-run steelhead should be considered as summer-run, winter-run, or as a separate entity. In this status review, we consider fall-run steelhead from the Klamath River Basin to be part of the summer run.

Because the Illinois River winter steelhead petition focussed only on winter-run steelhead, and because the few summer-run steelhead populations in the area are depressed and difficult to sample, our genetic study also focussed on winter-run steelhead. However, genetic studies that considered both winter and summer steelhead from other areas have failed to find consistent genetic differences between run-types within a region. Although there are behavioral and ecological differences between summer and winter steelhead, sufficient evidence of reproductive isolation between these ecotypes within the geographical range of an ESU is lacking. Genetic evidence clearly supports a polyphyletic origin for coastal summer steelhead.

Patterns of ocean migration of salmon and steelhead may reflect reproductive isolation of spawning populations. Chinook salmon populations from south of Cape Blanco are generally considered south migrating (e.g., to ocean areas off southern Oregon and California), while most stocks from north of Cape Blanco are considered north migrating. Other studies suggest that coho salmon and steelhead from south of Cape Blanco may not be highly migratory, remaining instead in the highly productive oceanic waters off southern Oregon and California.

Ecological/Genetic Diversity

The Klamath Mountains Province extends from the vicinity of Cape Blanco in the north to the Klamath River Basin in the south. Geologically, the province is distinctive in that it includes northern extensions of formations typical of the California Coastal Ranges and the Sierra Nevada. Ecologically, the province includes areas that are warmer and drier than coastal regions to the north and south; interior valleys receive less precipitation than any other location in the Pacific Northwest west of the Cascade Range. The vegetation combines elements from California, the northern coast, and eastern Oregon, as well as a large number of endemic species (Whittaker 1960).

The nearshore ocean environment in this region is strongly affected by seasonal upwelling. The strength and consistency of upwelling south of Cape Blanco yields highly productive waters. The area of increased upwelling extends, with some local variations, as far south as 33°N latitude.

Studies of the zoogeography of freshwater fishes have consistently identified differences between the Rogue River Basin and streams to the north. A number of authors have also noted affinities between freshwater fish of the Klamath and Rogue River Basins. Ichthyofauna of coastal streams south of the Klamath River Basin are generally considered to be allied with the Sacramento River Basin. For marine fishes, Cape Mendocino has been identified as an important southern limit to the abundance of many northern species.

The half-pounder life history form of steelhead appears to be restricted to southern Oregon and northern California, having been described from the Rogue, Klamath, Eel, and Mad Rivers. The advantages of the half-pounder strategy are poorly understood; presumably, the fish are either seeking refuge from adverse conditions in the ocean or taking advantage of favorable conditions in fresh water. It is likely that expression of this life history strategy is due to a combination of genetic and environmental factors.


Several lines of evidence suggest Cape Blanco as the northern boundary for the ESU that contains Illinois River winter steelhead. Genetic and ocean distribution data suggest that there is substantial reproductive isolation between steelhead populations from north and south of Cape Blanco. Cape Blanco is also an approximate northern boundary for the Klamath Mountains Province, a local area of intense upwelling, the distribution of the half-pounder life history, and the Klamath-Rogue freshwater zoogeographic zone. To the south, Cape Mendocino is a natural landmark associated with changes in ocean currents and also represents the approximate southern limit of the half-pounder life history strategy. However, the Klamath River Basin forms the southern boundary of the Klamath Mountains Province as well as the Klamath-Rogue freshwater zoogeographic zone. Furthermore, genetic data show a sharp discontinuity between steelhead populations from the Klamath River Basin and those farther south. Therefore, we conclude that the geographic boundaries of the ESU that contains Illinois River winter steelhead extend from Cape Blanco in the north and include the Klamath River Basin in the south.

There is no question that diversity in run-timing is an important component of the overall diversity of steelhead within this ESU, and this diversity may be in part genetically based. However, we have little direct information about the degree of reproductive isolation of the different steelhead runs in any stream within the proposed ESU. Furthermore, previous genetic studies suggest that summer- and winter-run steelhead are not independent, monophyletic groups over broad geographic regions. Based on available evidence, therefore, we conclude that all runs of steelhead (those termed summer-, fall-, and winter-run) within these geographic boundaries should be considered part of the same ESU.

We have found no direct evidence regarding the relationship between anadromous and nonanadromous O. mykiss within the geographic area of this status review. Studies from other geographic areas indicate that the two forms within an area can be genetically more similar to each other than either is to the similar form from outside the area. On the other hand, studies of a number of species of salmonids (including O. mykiss) have repeatedly found evidence for reproductive isolation between anadromous and nonanadromous forms from the same geographic area (reviewed by Johnson et al. 1994). We therefore conclude that, until information specifically for O. mykiss populations within the Klamath Mountains Province becomes available, only anadromous fish should be considered part of the ESU. Nevertheless, we recognize the possibility that some resident populations within the geographic boundaries of the ESU may have a close affinity with anadromous populations, and such resident populations could be considered part of the ESU if information becomes available demonstrating that the two forms share a common gene pool.

Table of Contents