U.S. Dept Commerce/NOAA/NMFS/NWFSC/Publications

NOAA-NWFSC Tech Memo-27: Status Review of West Coast Steelhead
INTRODUCTION

The U.S. Endangered Species Act (ESA) is intended to conserve threatened and endangered species in their native habitats. Under the ESA, vertebrate populations are considered "species" if they are "distinct." According to National Marine Fisheries Service (NMFS) policy, a salmon population or group of populations is considered "distinct" and hence a "species" under the ESA if it represents an evolutionarily significant unit (ESU) of the biological species.

The NMFS has received three petitions to list populations of steelhead (anadromous Oncorhynchus mykiss) as threatened or endangered "species" under the ESA. The ESA stipulates that, if a petition is found to present substantial information that a listing may be warranted, NMFS must conduct a status review and issue a determination on its findings within 1 year. On 6 May 1992, NMFS was petitioned by the Oregon Natural Resources Council and 10 co-petitioners to list Oregon's Illinois River winter steelhead (ONRC et al. 1992). NMFS concluded that Illinois River winter steelhead by themselves did not constitute an ESA "species" (Busby et al. 1993, NMFS 1993a). At the same time, however, NMFS initiated a status review of coastal steelhead populations to identify the ESU that includes Illinois River winter steelhead. This status review has been completed and resulted in the identification of a Klamath Mountains Province ESU that includes steelhead from the Illinois River (Busby et al. 1994); NMFS has proposed listing this ESU as threatened (NMFS 1995).

Washington Trout (1993) petitioned NMFS on 21 September 1993 for ESA listing of Washington's Deer Creek summer steelhead. As was the case with Illinois River winter steelhead, NMFS determined that Deer Creek summer steelhead did not by themselves constitute an ESU (NMFS 1994b).

On 16 February 1994, Oregon Natural Resources Council and 15 co-petitioners asked NMFS to list all steelhead in Washington, Idaho, Oregon, and California as threatened or endangered under the ESA (ONRC et al. 1994). The petitioners identified 178 stocks of steelhead of special concern and included information on stock origin, stock status, and factors affecting their abundance.

Scope and Intent of the Present Document

This document addresses the ONRC et al. (1994) petition and presents environmental and biological information concerning steelhead populations in Washington, Idaho, Oregon, and California (Fig. 1). These will be collectively referred to in this document as west coast steelhead. The Klamath Mountains Province ESU of southwest Oregon and northwest California has been reviewed in detail elsewhere (Busby et al. 1994) and, therefore, will only be summarized in the present document.

Because the ESA stipulates that listing determinations should be made on the basis of the best scientific information available, NMFS formed a team of scientists with diverse backgrounds in salmon biology to conduct this status review. This Biological Review Team (BRT) discussed and evaluated scientific information contained in an extensive public record developed for west coast steelhead. This document reports conclusions reached by the BRT for west coast steelhead. These conclusions are subject to revision should important new information arise in the future.

Key Questions in ESA Evaluations

In determining whether a listing under the ESA is warranted, two key questions must be addressed:

1) Is the entity in question a "species" as defined by the ESA?
2) If so, is the "species" threatened or endangered?

These two questions are addressed in separate sections of this report. If it is determined that a listing(s) is warranted, then NMFS is required by law (1973 ESA Sec. 4(a)(1)) to identify one or more of the following factors responsible for the species' threatened or endangered status: 1) destruction or modification of habitat; 2) overutilization by humans; 3) disease or predation; 4) inadequacy of existing regulatory mechanisms; or 5) other natural or human factors. This status review does not formally address factors for decline, except insofar as they provide information about the degree of risk faced by the species in the future. A separate document (NMFS in press a) identifies factors for decline of west coast steelhead.

The "Species" Question

As amended in 1978, the ESA allows listing of "distinct population segments" of vertebrates as well as named species and subspecies. However, the ESA provides no specific guidance for determining what constitutes a distinct population, and the resulting ambiguity has led to the use of a variety of approaches for considering vertebrate populations. To clarify the issue for Pacific salmon, NMFS published a policy describing how the agency will apply the definition of "species" in the ESA to anadromous salmonid species, including sea-run cutthroat trout and steelhead (NMFS 1991). A more detailed discussion of this topic appeared in the NMFS "Definition of Species" paper (Waples 1991b). The NMFS policy stipulates that a salmon population (or group of populations) will be considered "distinct" for purposes of the ESA if it represents an evolutionarily significant unit (ESU) of the biological species. An ESU is defined as a population that 1) is substantially reproductively isolated from conspecific populations and 2) represents an important component of the evolutionary legacy of the species.

The term "evolutionary legacy" is used in the sense of "inheritance"--that is, something received from the past and carried forward into the future. Specifically, the evolutionary legacy of a species is the genetic variability that is a product of past evolutionary events and that represents the reservoir upon which future evolutionary potential depends. Conservation of these genetic resources should help to ensure that the dynamic process of evolution will not be unduly constrained in the future.

The NMFS policy identifies a number of types of evidence that should be considered in the species determination. For each of the two criteria (reproductive isolation and evolutionary legacy), the NMFS policy advocates a holistic approach that considers all types of available information as well as their strengths and limitations. Isolation does not have to be absolute, but it must be strong enough to permit evolutionarily important differences to accrue in different population units. Important types of information to consider include natural rates of straying and recolonization, evaluations of the efficacy of natural barriers, and measurements of genetic differences between populations. Data from protein electrophoresis or DNA analyses can be particularly useful for this criterion because they reflect levels of gene flow that have occurred over evolutionary time scales.

The key question with respect to the second criterion is, If the population became extinct, would this represent a significant loss to the ecological or genetic diversity of the species? Again, a variety of types of information should be considered. Phenotypic and life history traits such as size, fecundity, migration patterns, and age and time of spawning may reflect local adaptations of evolutionary importance, but interpretation of these traits is complicated by their sensitivity to environmental conditions. Data from protein electrophoresis or DNA analyses provide valuable insight into the process of genetic differentiation among populations but little direct information regarding the extent of adaptive genetic differences. Habitat differences suggest the possibility for local adaptations but do not prove that such adaptations exist.

The "Extinction Risk" Question

The ESA (section 3) defines the term "endangered species" as "any species which is in danger of extinction throughout all or a significant portion of its range." The term "threatened species" is defined as "any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range." NMFS considers a variety of information in evaluating the level of risk faced by an ESU. Important considerations include 1) absolute numbers of fish and their spatial and temporal distribution; 2) current abundance in relation to historical abundance and carrying capacity of the habitat; 3) trends in abundance, based on indices such as dam or redd counts or on estimates of recruit-to-spawner ratios; 4) natural and human-influenced factors that cause variability in survival and abundance; 5) possible threats to genetic integrity (e.g., selective fisheries and interactions between hatchery and natural fish); and 6) recent events (e.g., a drought or a change in management) that have predictable short-term consequences for abundance of the ESU. Additional risk factors, such as disease prevalence or changes in life history traits, may also be considered in evaluating risk to populations.

According to the ESA, the determination of whether a species is threatened or endangered should be made on the basis of the best scientific information available regarding its current status, after taking into consideration conservation measures that are proposed or are in place. In this review, we do not evaluate likely or possible effects of conservation measures. Therefore, we do not make recommendations as to whether identified ESUs should be listed as threatened or endangered species, because that determination requires evaluation of factors not considered by us. Rather, we have drawn scientific conclusions about the risk of extinction faced by identified ESUs under the assumption that present conditions will continue (recognizing, of course, that natural demographic and environmental variability is an inherent feature of "present conditions"). Conservation measures will be taken into account by the NMFS Northwest and Southwest Regional Offices in making listing recommendations (see NMFS in press b for a discussion of conservation measures for west coast steelhead).

Artificial Propagation

NMFS policy (Hard et al. 1992, NMFS 1993b) stipulates that in determining 1) whether a population is distinct for purposes of the ESA, and 2) whether an ESA species is threatened or endangered, attention should focus on "natural" fish, which are defined as the progeny of naturally spawning fish (Waples 1991b). This approach directs attention to fish that spend their entire life cycle in natural habitat and is consistent with the mandate of the ESA to conserve threatened and endangered species in their native ecosystems. Implicit in this approach is the recognition that fish hatcheries are not a substitute for natural ecosystems.

Nevertheless, artificial propagation is important to consider in ESA evaluations of anadromous Pacific salmonids for several reasons. First, although natural fish are the focus of ESU determinations, possible effects of artificial propagation on natural populations must also be evaluated. For example, stock transfers might change the genetic or life history characteristics of a natural population in such a way that the population might seem either less or more distinctive than it was historically. Artificial propagation can also alter life history characteristics such as smolt age and migration and spawn timing. Second, artificial propagation poses a number of risks to natural populations that may affect their risk of extinction or endangerment. These risks are discussed below in the "Assessment of Extinction Risk" section. In contrast to most other types of risk for salmon populations, those arising from artificial propagation are often not reflected in traditional indices of population abundance. For example, to the extent that habitat degradation, overharvest, or hydropower development have contributed to a population's decline, these factors will already be reflected in population abundance data and accounted for in the risk analysis. The same is not true of artificial propagation. Hatchery production may mask declines in natural populations that will be missed if only raw population abundance data are considered. Therefore, a true assessment of the viability of natural populations cannot be attained without information about the contribution of naturally spawning hatchery fish. Furthermore, even if such data are available, they will not in themselves provide direct information about possibly deleterious effects of fish culture. Such an evaluation requires consideration of the genetic and demographic risks of artificial propagation for natural populations. The sections on artificial propagation in this report are intended to address these concerns.

Finally, if any natural populations are listed under the ESA, then it will be necessary to determine the ESA status of all associated hatchery populations. This latter determination would be made following a proposed listing and is not considered further in this document.

Summary of the West Coast Steelhead Petition

The petition of February 1994 was filed by Oregon Natural Resources Council, California Sport Fishing Protection Alliance, Coast Range Association, Fish in Northwest Streams, Greater Ecosystem Alliance, National Wildlife Federation, Oregon Wildlife Federation, Pilchuck Audubon Society, Quilcene Ancient Forest Coalition, Rivers Council of Washington, Save the West, Siskiyou Audubon Society, Siskiyou Regional Educational Project, Trout Unlimited of Oregon, University of Oregon Survival Center, and Western Ancient Forest Campaign. The petition called upon the Secretary of Commerce to list "anadromous steelhead trout (Oncorhynchus mykiss)" in Washington, Idaho, Oregon, and California according to one of four alternatives presented: 1) all steelhead, 2) summer and winter "races," 3) each steelhead ESU, or 4) each of 178 individual stocks described in the petition.

Petitioners' "Definition of Species and Application to Steelhead Trout"

The petitioners focus on the anadromous form of O. mykiss, stating that "the common fish culture practice of keeping separate brood stock for steelhead and resident rainbow trout during captive breeding is the most obvious empirical proof for a genetic basis of anadromy" (ONRC et al. 1994, p. 7). Among anadromous steelhead, homing is seen as an effective mechanism for developing "distinct fish stocks" and "maintaining their genetic integrity" (ONRC et al. 1994, p. 7).

Summer steelhead--The petition states that summer steelhead are reproductively isolated from other steelhead temporally by migration and spawn timing and spatially through spawning "in small tributaries generally not used by winter run steelhead" (ONRC et al. 1994, p. 8). The petition states that "summer steelhead are evolutionarily significant because of time of migration, state of gonadal maturity at migration, and location of spawning" (ONRC et al. 1994, p. 8).

A- and B-run steelhead--The petition states that B-run summer steelhead from the Clearwater and Salmon Rivers, Idaho differ from A-run steelhead based on "greater body size at a given ocean age," ... "late run timing, older average ocean age, and long river migration," and, therefore are evolutionarily significant (ONRC et al. 1994, p. 8).

Half-pounders--The petition states that the half-pounder life history of steelhead from the Rogue, Klamath, Mad, and Eel Rivers is evolutionarily significant.

Southern steelhead--The petition states that southern steelhead (footnote 2) comprise an ESU based on the following: 1) "they are the southernmost distribution of native steelhead in North America," 2) they utilize seasonally warm rivers and streams that frequently have dewatered reaches, 3) they occupy habitat that is different from that north of San Francisco, 4) they have lower smolt age and older ocean age, and 5) "southern steelhead may breed as metapopulations that allow for recolonization of streams after prolonged drought" (ONRC et al. 1994, p. 9).

INFORMATION RELATING TO THE SPECIES QUESTION

In this section, we summarize biological and environmental information that is relevant to determining the nature and extent of west coast steelhead ESUs. Again, in this document, west coast steelhead refers to steelhead in the states of Washington, Idaho, Oregon, and California. We considered information on steelhead from other locations, such as Alaska and British Columbia, in addressing the species question. However, they are not included in the phrase west coast steelhead and, when discussed in this document, they are specifically mentioned.

Groupings of Oncorhynchus mykiss

The biological species Oncorhynchus mykiss is phylogenetically and ecologically complex. The diversity of morphology and life history within this presently recognized species has led to many classification schemes, including that of David Starr Jordan that included "32 full species, which are presently referable to the diversity within rainbow and cutthroat trout [O. clarki]" (Behnke 1992, p. 6). The volumes of work on this species have resulted in the recognition of several groups within the species and the development of terminology unique to O. mykiss. As these terms will be used extensively in this document, they are introduced here.

Phylogenetic Groups

Two major genetic groups of O. mykiss are presently recognized in North America: the inland and coastal groups, generally separated in the Fraser and Columbia River Basins in the vicinity of the Cascade crest (Huzyk and Tsuyuki 1974, Allendorf 1975, Utter and Allendorf 1977, Okazaki 1984, Parkinson 1984, Schreck et al. 1986, Reisenbichler et al. 1992). Both inland and coastal steelhead occur in British Columbia, Washington, and Oregon; Idaho has only inland steelhead; California is thought to have only coastal steelhead. These genetic groups apply to both anadromous and nonanadromous forms of O. mykiss; that is, rainbow (redband) trout east of the Cascades are genetically more similar to steelhead from east of the Cascades than they are to rainbow trout west of the Cascades. Behnke (1992) has proposed that the two forms should be considered subspecies and suggested the names O. mykiss irideus and O. m. gairdneri for the coastal and inland forms, respectively. Other subgroups of the species O. mykiss that may be involved in a discussion of west coast steelhead are the redband trout of the upper Klamath and upper Sacramento River Basins (O. m. newberrii and O. m. stonei, Behnke 1992), see Table 1.


Table 1. Proposed taxonomy of various forms (subspecies) of Oncorhynchus mykiss (Behnke 1992).
Scientific name Common name and comments
Rainbow trout of coastal basins
O. mykiss irideus Coastal rainbow trout from Alaska to California (anadromous form is called steelhead)
O. mykiss mykiss Kamchatka rainbow trout or mikizha (anadromous form is called steelhead)
Redband trout of northern inland basins
O. mykiss gairdneri Columbia redband trout of the Columbia and Fraser River Basins east of the Cascades, including Kamloops trout (anadromous form is called steelhead)
Redband trout of eastern Oregon basins
O. mykiss newberrii Upper Klamath redband trout (including Upper Klamath Lake)
(no name given) Oregon desert basin redband trout (other than Upper Klamath Lake)
Redband trout of the Sacramento Basin
O. mykiss aguabonita California golden trout
O. mykiss gilberti Kern and Little Kern River golden trout
O. mykiss stonei Sacramento redband trout (McCloud River subspecies)

Life History Variations

Oncorhynchus mykiss is considered by many to have the greatest diversity of life history patterns of any Pacific salmonid species (Shapovalov and Taft 1954, Barnhart 1986), including varying degrees of anadromy, differences in reproductive biology, and plasticity of life history between generations.

Reproductive ecotypes--Within the range of west coast steelhead, spawning migrations occur throughout the year, with seasonal peaks of activity. In a given river basin there may be one or more peaks in migration activity; since these runs are usually named for the season in which the peak occurs, some rivers may have runs known as winter, spring, summer, or fall steelhead. For example, large rivers, such as the Columbia, Rogue, and Klamath Rivers, have migrating adult steelhead at all times of the year. Through time, the names of seasonal runs have generally been simplified, especially in the Pacific Northwest (footnote 3) , to two: winter and summer steelhead. There are local variations in the names used to identify the seasonal runs of steelhead; in northern California, some biologists have retained the use of the terms spring and fall steelhead to describe what others would call summer steelhead.

Biologically, steelhead can be divided into two basic reproductive ecotypes, based on the state of sexual maturity at the time of river entry and duration of spawning migration (Burgner et al. 1992). The stream-maturing type (commonly known as fall steelhead in Alaska, summer steelhead in the Pacific Northwest and northern California) enters fresh water in a sexually immature condition and requires several months to mature and spawn. The ocean-maturing type (spring steelhead in Alaska, winter steelhead elsewhere) enters fresh water with well-developed gonads and spawns shortly thereafter. This document generally uses the terms summer steelhead to refer to the stream-maturing type and winter steelhead to refer to the ocean-maturing type.

In the Pacific Northwest, steelhead that enter fresh water between May and October are considered summer steelhead, and steelhead that enter fresh water between November and April are considered winter steelhead. Variations in migration timing exist between populations, although there is considerable overlap. Some river basins have both summer and winter steelhead; others have only one type. It appears that the summer, or stream-maturing, steelhead occur where habitat is not fully utilized by winter steelhead; summer steelhead usually spawn farther upstream than winter steelhead (Withler 1966, Roelofs 1983, Behnke 1992). In rivers where the two types co-occur, they are often separated by a seasonal hydrologic barrier, such as a waterfall. Coastal streams are dominated by winter steelhead, whereas inland steelhead of the Columbia River Basin are almost exclusively summer steelhead. Winter steelhead may have been excluded from inland areas of the Columbia River Basin by Celilo Falls, or by the great migration distance from the ocean. The Sacramento-San Joaquin River Basin historically may have had multiple runs of steelhead that probably included both ocean-maturing and stream-maturing stocks (CDFG 1995, McEwan and Jackson 1996, McEwan). Currently, the steelhead of this region are referred to as winter steelhead by the California Department of Fish and Game (CDFG); however, some biologists call them fall steelhead (Cramer et. al 1995). It is thought that hatchery practices and modifications in the hydrology of the basin caused by large-scale water diversions may have altered the migration timing of steelhead in this basin (McEwan) .

A- and B-run steelhead--Inland steelhead of the Columbia River Basin, especially the Snake River Subbasin, are commonly referred to as either A-run or B-run. These designations are based on the observation of a bimodal migration of adult steelhead at Bonneville Dam (Columbia River river kilometer (RKm) 235) and differences in age (1- versus 2-ocean) and adult size observed among Snake River steelhead. Adult A-run steelhead enter fresh water from June to August; as defined, the A-run passes Bonneville Dam before 25 August (CBFWA 1990, IDFG 1994). Adult B-run steelhead enter fresh water from late August to October, passing Bonneville Dam after 25 August (CBFWA 1990, IDFG 1994). Above Bonneville Dam (e.g., at Lower Granite Dam on the Snake River, 695 km from the mouth of the Columbia River), run-timing separation is not observed, and the groups are separated based on ocean age and body size (IDFG 1994). A-run steelhead are defined as predominately age-1-ocean, while B-run steelhead are defined as age-2-ocean (IDFG 1994). Adult B-run steelhead are also thought to be on average 75-100 mm larger than A-run steelhead of the same age; this is attributed to their longer average residence in salt water (Bjornn 1978, CBFWA 1990, CRFMP TAC, 1991). It is unclear, however, if the life history and body size differences observed upstream have been correlated back to the groups forming the bimodal migration observed at Bonneville Dam. Furthermore, the relationship between patterns observed at the dams and the distribution of adults in spawning areas throughout the Snake River Basin is not well understood. A-run steelhead are believed to occur throughout the steelhead-bearing streams of the Snake River Basin; additionally, inland Columbia River steelhead outside of the Snake River Basin are also considered A-run (IDFG 1994). B-run steelhead are thought to be produced only in the Clearwater, Middle Fork Salmon, and South Fork Salmon Rivers (IDFG 1994).

Half-pounders--The half-pounder (terminology of Snyder 1925) is an immature steelhead that returns to fresh water after only 2 to 4 months in the ocean, generally overwinters in fresh water, then outmigrates again the following spring. Half-pounders are generally less than 400 mm (Kesner and Barnhart 1972, Everest 1973). Half-pounders are only reported from the Rogue, Klamath, Mad, and Eel Rivers of southern Oregon and northern California (Snyder 1925, Kesner and Barnhart 1972, Everest 1973, Barnhart 1986); however, it has been suggested that as mature steelhead, these fish may only spawn in the Rogue and Klamath River Basins (Cramer et al. 1995). Various explanations for this unusual life history have been proposed, but there is still no consensus as to what, if any, advantage this life history affords to the steelhead of these rivers.

Rainbow and redband trout--As mentioned earlier, the species O. mykiss exhibits varying degrees of anadromy. Nonanadromous forms of the species are usually called rainbow trout; however, nonanadromous O. mykiss of the inland type are often called Columbia River redband trout. Another form occurs in the upper Sacramento River and is called Sacramento redband trout. Although the anadromous and nonanadromous forms have long been taxonomically classified within the same species, the exact relationship between the forms in any given area is not well understood. In coastal populations, it is unusual for the two forms to co-occur; they are usually separated by a migration barrier, be it natural or manmade. In inland populations, co-occurrence of the two forms appears to be more frequent. Where the two forms co-occur, "it is possible that offspring of resident fish may migrate to the sea, and offspring of steelhead may remain in streams as resident fish" (Burgner et al. 1992, p. 6; see also Shapovalov and Taft 1954, p. 18). Mullan et al. (1992, p. K-427) found evidence that in very cold streams, juvenile steelhead had difficulty attaining "mean threshold size for smoltification" and concluded that "Most fish here [Methow River, Washington] that do not emigrate downstream early in life are thermally-fated to a resident life history regardless of whether they were the progeny of anadromous or resident parents." Additionally, Shapovalov and Taft (1954) reported evidence of O. mykiss maturing in fresh water and spawning prior to their first ocean migration; this life history variation has also been found in cutthroat trout (O. clarki) and some male chinook salmon (O. tshawytscha).

Environmental Features

West coast steelhead are presently distributed across 15 degrees of latitude, from approximately 49°N at the U.S.-Canada border south to 34°N at the mouth of Malibu Creek, California. In some years steelhead may be found as far south as the Santa Margarita River in San Diego County. Climate and geological features vary greatly across this area, resulting in a variety of landforms and diverse patterns of vegetation, weather, soils, and water quality parameters that affect the distribution and ecology of plant and animal species, including fish.

West Coast Ichthyogeography

Geological events--Western North America, part of the Pacific Ring of Fire, is a geologically active region that experiences large-scale volcanic, tectonic, and glacial events. These events affect landforms, soil types, and, therefore, drainage patterns. Headwater transfer and stream capture events have provided dispersal opportunities for several species of freshwater fish in various drainages; Minckley et al. (1986) summarized several examples of these events in Oregon and California streams.

Landforms and aquatic species distribution in the Pacific Northwest were greatly affected by glaciation and flooding that occurred during the Wisconsin glacial age between 70,000 and 10,000 years ago (Porter 1983, Allen et al. 1986, Briggs 1986). In the late Wisconsin glacial age, the Cordilleran ice sheet covered parts of present-day British Columbia, Alberta, Washington, Idaho, and Montana. Although the Cordilleran ice sheet extended only to the Puget Sound region, it affected sea level and climatic conditions much farther south (Porter 1983). Thus, much of present-day patterns of landform and zoogeography in western North America evolved in the last 10,000 years.

Ecoregions--Omernik (1987) delineated 13 ecoregions within the freshwater distribution of west coast steelhead based on soils, land use, land surface form, and potential natural vegetation (Table 2). The ecoregions occupied by west coast steelhead can be grouped by climatic regions.


Table 2. Ecoregions (Omernik 1987) within the distribution of west coast steelhead.
Ecoregion Washington Idaho Oregon California
Coast Range
Puget Lowland
Willamette Valley
Cascades
Sierra Nevada
Southern and Central California Plains and Hills
Central California Valley
Southern California Mountains
Eastern Cascades Slopes and Foothills
Columbia Basin
Blue Mountains
Snake River Basin/High Desert
Northern Rockies

The north coastal region includes rivers and streams draining the Coast Ranges of Washington, Oregon, and northern California. Climate in this area is under maritime influence and, therefore, includes abundant precipitation (primarily in the form of rain), summer fog, and moderate temperatures (Jackson 1993). Vegetation in this region is dominated by conifers, Sitka spruce (Picea sitchensis) in the north and coast redwood (Sequoia sempervirens) in the south (Donley et al. 1979, Jackson 1993).

In the south coastal region, south of Point Piedras Blancas, coastal rivers and streams drain directly from the South Coast Range, and from the Transverse and Peninsular Ranges of southern California to the coastal plain. This area is much drier than the north coastal strip. Vegetation is dominated by chaparral, coastal scrub, and grassland (Donley et al. 1979).

The western lowlands include the Puget Lowland in Washington and the Willamette Valley in Oregon. These areas include the rain shadows of the Coast and Olympic Mountain Ranges and the foothills of the taller Cascade Range; they receive a moderate amount of precipitation compared to regions east and west of them. Vegetation within these valleys is primarily grassland, with oak woodlands occurring in the foothills and coniferous forest dominant at higher elevations.

The Central Valley of California is positioned between the Coast Range and the Cascade and Sierra Nevada Ranges. It is warmer and drier than the western lowlands. Native vegetation in the Central Valley was bunchgrass prairie (Donley et al. 1979). The intermountain valleys of the western lowlands and Central Valley of California are now productive agricultural areas.

The rivers and streams of the Columbia Basin ecoregion (Omernik 1987) are in the rain shadow of the Cascade Mountain Range. The vegetation in this zone includes pine, juniper, and sagebrush. Streamflow is provided by snowmelt and springs. Many rivers in this region experience extreme ranges in water temperature.

The northern Rockies zone includes the high elevation Clearwater and Salmon River Basins of arid north-central Idaho. The region is dominated geologically by the Idaho Batholith, which is composed of highly erosive granitic soils (see Matthews and Waples 1991 for a discussion on the effects on water quality and productivity).

Ichthyogeographical classification--Several authors have published ichthyogeographical studies for western North America (e.g., Snyder 1907, Moyle 1976, McPhail and Lindsey 1986, Swift et al. 1993). Within the range of west coast steelhead, five major freshwater ichthyogeographic regions have been described (Snyder 1907, Moyle 1976, McPhail and Lindsey 1986): Chehalis, Columbia, Klamath, Sacramento-San Joaquin, and South Coastal Drainages. Although anadromy provides steelhead with distribution opportunities not available to freshwater species, it is instructive to consider the distribution of freshwater species for evidence of potential mechanisms of reproductive isolation between steelhead populations, and for evidence of environmental parameters that address the question of ecological diversity of the species.

Marine and estuarine ichthyogeography--Along the U.S. Pacific Coast, there are two points where marine fish distribution and abundance markedly change: Cape Mendocino (Allen and Smith 1988) and Point Conception (Briggs 1974); both are in California. Environmental conditions that differ north and south of these points (e.g., ocean currents, upwelling, temperature, productivity) may affect anadromous fish as well as marine species.

Monaco et al. (1992) grouped west coast U.S. estuaries that have similar species assemblages. Their findings were largely consistent with ichthyofaunal distribution changes in the vicinity of Cape Mendocino and Point Conception. Monaco et al. (1992) also identified an assemblage within the inland estuaries of Puget Sound and Hood Canal in Washington (Fjord Group). Other estuary groupings are less clear geographically and seem to depend more on estuarine characteristics rather than on location.

Hydrology

Streamflow patterns show several geographic trends. Month of peak flow is delayed with decreasing latitude, shifting from December in Washington and northern Oregon to January from the Alsea River, Oregon south to Point Arena, California, to February from Point Arena south to Big Sur, and to March in southern California (Hydrosphere 1993). In northern Washington there are often two peaks in streamflow, the larger December peak caused by precipitation (often a rain-on-snow event) and a smaller peak in spring caused by snowmelt. Rivers in Oregon and California usually have one peak streamflow month (Hydrosphere 1993). Additionally, northern streams have greater discharge per watershed area, longer periods of peak flow, and more consistent base flow than southern streams. Many coastal streams from southern Oregon to southern California experience seasonal intermittent flows, including physical isolation from the ocean through formation of sand berms. When a sand berm forms, through a combination of low streamflow and ocean transport of sand, it functions as a dam, creating a lagoon in the lower stream reach. In periods of drought, these closures may persist for extended periods of time--even years (Snider 1983, Titus et al. in press). This affects access to salt water by juvenile steelhead and access to freshwater spawning areas by adult steelhead.


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