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

NOAA-NWFSC Tech Memo-24: Status Review of Coho Salmon
INTRODUCTION

Coho salmon (Oncorhynchus kisutch) is a widespread species of Pacific salmon, occurring in most major river basins around the Pacific Rim from central California to Korea and northern Hokkaido, Japan (Laufle et al. 1986). Recently published investigations have reported that a number of local populations of coho salmon in Washington, Oregon, Idaho, and California have become extinct, and that the abundance of many others is depressed (e.g., Brown and Moyle 1991, Nehlsen et al. 1991, Frissell 1993, Wilderness Society 1993). These declines have led several conservation groups to petition the National Marine Fisheries Service (NMFS) to list populations of coho salmon as threatened or endangered "species" under the U.S. Endangered Species Act (ESA, technical terms and abbreviations such as "ESA" are defined in the glossary, Appendix A). Under the ESA, the term "species" is defined rather broadly to include subspecies and "distinct population segments" of vertebrates (such as salmon) as well as taxonomic species.

The first petition to NMFS for coho salmon requested ESA protection for populations in the lower Columbia River, excluding the Willamette River and its tributaries (Oregon Trout et al. 1990). Following a status review, NMFS concluded that as a result of substantial and long- term stock transfers, habitat degradation, and overfishing, they were unable to identify any indigenous populations of coho salmon in the lower Columbia River that warranted protection under the ESA (Johnson et al. 1991, NMFS 1991a).

In March 1993, NMFS was petitioned by the Santa Cruz County Planning Department to list coho salmon in Scott and Waddell Creeks, California as a threatened or endangered species (SCCPD 1993). Before a status review was completed for this petition, NMFS received two additional petitions for ESA listing of coho salmon. Oregon Trout, the Portland Audubon Society, and the Siskiyou Regional Education Project asked NMFS for ESA protection for 40 coho salmon populations in Oregon (Oregon Trout et al. 1993). In response to these petitions, NMFS announced a coastwide review of coho salmon from California, Oregon, and Washington in a Federal Register Notice (58 FR 57770; 27 October 1993) (NMFS 1993). However, a week before this broader status review was announced, NMFS received a petition from the Pacific Rivers Council and 22 co-signers for ESA listing of coho salmon throughout their range in Washington, Oregon, Idaho, and California (Pacific Rivers Council et al. 1993). This petition thus included all populations covered by the previous three petitions.

In 1994, NMFS announced its determination that coho salmon from Scott and Waddell Creeks do not by themselves constitute an ESA "species", and therefore a listing was not warranted (Bryant 1994, NMFS 1994).

Scope and Intent of the Present Document

This document considers environmental and biological information for coho salmon populations in Washington, Oregon, and California (Fig. 1) and British Columbia. These populations will be collectively referred to in this document as west coast coho salmon. The scope of this document thus encompasses both the Oregon Trout et al. (1993) and the Pacific Rivers Council et al. (1993) petitions. In addition, we determine the boundaries of the "distinct population segment" that includes coho salmon from Scott and Waddell Creeks.

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 coho salmon. This document represents the findings and conclusions of the BRT on the status of west coast coho salmon under the ESA.

Key Questions in ESA Evaluations

An ESA status review involves answering two key questions: 1) Is the entity in question a "species" as defined by the ESA? and 2) If so, is the "species" in danger of extinction or likely to become so (the "extinction risk" question)? These two questions are addressed in separate sections in the text that follows.

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 criteria in listing decisions over the past decade. 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 1991b). A more detailed discussion of this topic appeared in the NMFS "Definition of Species" paper (Waples 1991a). 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 criteria, 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/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.

Artificial propagation--NMFS policy (Hard et al. 1992, NMFS 1993) 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 1991a). 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 'Extinction Risk' Question" section. 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.

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 spawner-recruit 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.

Summary of Information Presented by the Petitioners

This section briefly summarizes information presented by the petitioners (Oregon Trout et al. 1993, Pacific Rivers Council et al. 1993) to support their arguments that coho salmon in Washington, Oregon, Idaho, and California qualify as a threatened or endangered species under the ESA. We discuss this information and related issues in the following sections, and we evaluate the status of west coast coho salmon in the conclusions of the Assessment of Extinction Risk section.

Distinct Population Segments

The two petitioners differed in the approach they suggested NMFS use for listing coho salmon under the ESA. Oregon Trout et al. (1993) identified 40 coho salmon populations in Oregon that they believed comprised five "distinct population segments" from the following geographic areas: 1) south of Cape Blanco, 2) the Coquille and Coos Rivers, 3) the Umpqua River, 4) all coastal drainages north of the Umpqua River, and 5) the lower Columbia River, including the Clackamas River. Based primarily on evidence from a genetic analysis of mitochondrial DNA from coho salmon from Oregon (Currens and Farnsworth 1993), Oregon Trout et al. (1993) argued that these population groups qualified as ESUs under NMFS policy and recommended that each be listed as a separate "species" under the ESA.

In contrast, Pacific Rivers Council et al. (1993) did not focus on identifying distinct population segments or ESUs of coho salmon. Rather, they argued that a listing of all populations in California, Oregon, Washington, and Idaho was warranted because this region "comprises an ecologically, evolutionarily, economically, and culturally significant portion of the range of the species" (Pacific Rivers Council et al. 1993, p. 4). Under this scenario, west coast coho salmon could be listed as a single species, rather than as multiple ESUs.

In this review, we have focussed on identifying ESUs of coho salmon that can be considered for listing under the ESA. This approach has been taken for several reasons. First, it is consistent with NMFS policy and with the approach that has been taken with other ESA status reviews for Pacific salmon. Second, identifying ESUs provides biological information for the species on a scale corresponding to the smallest units that can be listed under the ESA. Finally, this approach would not preclude a broader listing under the ESA if it were determined that the biological species is threatened in all or a significant portion of its range. In fact, such an evaluation could most easily be made by considering the status of each of the species' distinct population segments, or ESUs.

Population Abundance

Pacific Rivers Council et al. (1993) and Oregon Trout et al. (1993) presented qualitative and quantitative information indicating that current abundance of west coast coho salmon populations have declined to small fractions of their historic levels and continuing declines and local extinctions are widespread within this range. Nehlsen et al. (1991) identified 35 stocks of coho salmon that are at short-term risk of extinction in Washington, Oregon, Idaho, and California and 15 stocks that are extinct in California, southern Oregon, and the Columbia River. Frissell (1993) estimated that coho salmon are extinct in the eastern half of their range in the lower 48 states and imperiled throughout the southern two-thirds of this range. The petitioners also referred to a report (Wilderness Society 1993) that estimated that coho salmon are extinct in 56% of their historic range, endangered in 13%, threatened in 20%, of special concern in 5%, and not known to be extinct, declining, depressed, or facing imminent threat in only 6.5% of their historic range.

The petitioners also provided region-specific estimates of current vs. historical population levels. For California coho salmon, Pacific Rivers Council et al. (1993) reported that Brown and Moyle (1991) estimated that naturally spawned adult coho salmon (regardless of origin) returning to California streams were less than 1% of their abundance at mid-century, and indigenous, wild coho salmon populations in California did not exceed 100 to 1,300 individuals. They further state that Brown and Moyle (1991) found that 46% of California streams, which historically supported coho salmon populations, and for which recent data were available, no longer supported runs.

Oregon Trout et al. (1993) argued that wild coho salmon spawner abundance along the Oregon coast declined between 1965 and 1975 and has fluctuated at low levels since then. According to Oregon Trout et al. (1993), escapement goals and maximum sustained-yield escapement levels have not been reached since 1986 and 1971, respectively. Pacific Rivers Council et al. (1993) used historical catch estimates (Lichatowich 1989) to calculate that the potential production of wild coho salmon in coastal Oregon rivers in the 1980s had decreased 86% from the turn of the century. Pacific Rivers Council et al. (1993) also cited Lichatowich and Nicholas' (in press) estimate that current production, including hatchery fish, in many coastal basins is less than 10% of historic levels. The petitioners expressed concern that the standard survey methods used to estimate Oregon coast coho salmon abundance overestimated population sizes (Oregon Trout et al. 1993, Pacific Rivers Council et al. 1993).

Discussion of Columbia River populations by Oregon Trout et al. (1993) was restricted to coho salmon from the Clackamas River. They suggested that this population should be listed as a separate species under the ESA because it is genetically distinct from other Columbia River populations and has undergone continuing declines in abundance. In contrast, Pacific Rivers Council et al. (1993) did not emphasize any particular stocks of Columbia River coho salmon. They cited several reports (Nehlsen et al. 1991, Chilcote et al. 1992, WDF et al. 1993) that show that coho salmon above Bonneville Dam are largely extinct, and the majority of populations below the Dam are endangered, depressed, or out of compliance with ODFW's wild fish policy.

Pacific Rivers Council et al. (1993) presented several estimates of current vs. historical coho salmon abundance in Washington rivers outside of the Columbia River Basin. They reported substantial declines (40-50%) in coho salmon populations in Puget Sound (Bledsoe et al. 1989), the Chehalis River Basin (Hiss and Knudsen 1992), and in the Queets and Quinault Rivers on the Olympic Peninsula (Houston 1983) between the earlier part of this century and the period from the 1970s to the present.

Causes of Decline for Coho Salmon

The petitioners identified many of the same factors discussed above, including habitat destruction, overfishing, artificial propagation, and poor ocean conditions, as the causes of decline for coho salmon. Both petitioners argued that the primary cause for decline has been habitat destruction (Oregon Trout et al. 1993, Pacific Rivers Council et al. 1993). Oregon Trout et al. (1993) also identified overutilization of the species for commercial and recreational purposes as an equally important factor for Oregon coho salmon, while the Pacific Rivers Council et al. (1993) identified deteriorating ocean conditions as a major cause for the general decline of west coast coho salmon. Both petitioners cited adverse effects of artificial propagation as an aggravating factor. Pacific Rivers Council et al. (1993) also identified intraspecific hybridization and interspecific hybridization with chinook salmon as an additional concern.

INFORMATION RELATING TO THE SPECIES QUESTION

In this section, we summarize environmental and biological information that is relevant to determining the nature and extent of ESUs for west coast coho salmon. This information was used to indicate possible ESU boundary locations, as a systematic means of dealing with the large area concerned, multitudes of coho salmon populations, and high variability in environmental conditions and biological characteristics. This process involved determining where significant changes in environmental and biological parameters occurred, and then identifying locations or zones where attributes changed in common. Areas where many attributes exhibited significant changes were identified as possible boundary locations for ESUs. Final ESU boundaries were determined by the BRT on the basis of the team's professional opinion of the value or weight that these attribute changes merited with respect the reproductive isolation and ecological/genetic diversity of west coast coho salmon.

Environmental Features

Environmental information was used to indicate where ESU boundaries might occur. We identified areas where the physical environment appeared to change based on environmental characteristics (i.e., river flow patterns, ocean conditions, water temperatures, climate, etc.), and on the distributions of other organisms. Areas with different habitat types may have different selective pressures, and may lead to local adaptations within specific areas. The distributions of organisms sympatric with coho salmon were considered because these distributions may reflect environmental, ecological, or historical processes that may also affect coho salmon.

Physical features of the freshwater environment

The following discussion includes climate data from the U.S. Department of Commerce (1968) and Farley (1979), calculations of river flow patterns using U.S. Geological Survey (USGS) data from Hydrosphere Data Products, Inc. (1993), and information from Forstall (1969). Riverflow data are presented in Figures 2-6 and Appendix Table B-1, water temperature data are presented in Figures 7-8 and Appendix Table B-2, and average annual precipitation is presented in Figure 9. Because coho salmon typically spawn and rear in small tributaries, run timing and spawning timing are particularly sensitive to patterns in river flow. In this respect, river flow patterns help define the temporal availability of, and access to, rearing and spawning habitat.

California and southern Oregon--California rivers having coho salmon, from Redwood Creek (in Humboldt County) southward, drain the 500-1,000 m-high Coast Range, an area underlain by easily eroded sedimentary rocks of the Franciscan Formation (California State Lands Commission 1993). To the north, the Rogue and Klamath River Basins cut through the Coast Range to drain the Cascade Mountains as well. Maximum elevations in this area are typically 1,000-2,000 m. Rivers from the Rogue River south to the Mattole River exhibit peak flow in late January or early February, while rivers farther south have peak flows in late February (Fig. 2). Duration of peak flows in rivers south of the Mattole are much shorter than in those farther north (Fig. 3), although both areas experience relatively low flows during the summer and early fall (Fig. 4). Annual precipitation levels are also much higher along the west side of the Coast Range in northern California and southern Oregon (160-200 cm) than they are farther south (60-160 cm) or in the dry interior along the east slope of the Coast Range (60-160 cm) (Fig. 9). Central California has a relatively short rainy season compared to regions farther north. Annual winter snowfall at higher elevations is also lower south of the Mattole River, averaging 60 cm or less, compared to 60-250 cm in the Klamath Mountains Province. Maximum summer stream temperatures (18-26°C) (Fig. 7) and average summer maximum and winter minimum air temperatures (around 21°C and 4°C, respectively) are similar along the California coast north of the San Lorenzo River through southern Oregon. However, winter stream temperatures in coastal river basins in central California (between Cape Mendocino and Monterey Bay) are generally warmer (8-12°C) than they are in northern California/southern Oregon (3-8°C) (Fig. 8, Appendix Table B-2). Finally, average annual sunshine along the coast in central California is higher than it is anywhere farther north, averaging 2,200-2,800 hours per year (h -1), while northern California/southern Oregon receives 2,000-2,200 h yr-1 of sunshine.

Oregon coast--North of Cape Blanco, all coastal Oregon rivers, with the exception of the Umpqua River, drain only the west side of the Coast Range. The Oregon Coast Range is relatively low, with peaks at 500-1,000-m elevations, in contrast to most Cascade peaks which are 1,000-2,000-m high. Seasonal river flows in this region follow a fairly consistent pattern, with a single peak in December or January (Figs. 2, 5) and relatively low flow (Fig. 4) in summer and fall. The Oregon coast receives high rainfall (120-240 cm yr-1) compared to areas east of the Coast Range (60-120 cm yr-1) or farther south (60-200 cm yr-1), but receives less rainfall than the extremely wet Olympic Peninsula farther north (>240 cm yr-1) (Fig. 9). Both air and stream (Figs. 7-8) temperatures are fairly consistent along the Oregon coast, with little latitudinal change. Minimum average winter air and stream temperatures are typically around 4°C and 4-8°C, respectively, while maximum average summer air and stream temperatures are typically around 21°C and 15-21°C, respectively. Because of the relatively low elevation, snowfall in the Coast Range is low, averaging 30-60 cm annually, while the higher Cascade Mountains receive 250-760 cm annually. On average, the Oregon coast receives more sunshine (1,800-2,200 h yr-1) than the wetter Olympic Peninsula (<1,800 h yr-1), but less than northern California and southern Oregon (2,000-2,200 h yr-1).

Columbia River Basin--Rivers draining into the Columbia River have their headwaters in increasingly drier areas moving from west to east, as the Columbia River cuts through the 500-1,000-m-high Coast Range/Willapa Hills and the 1,000-2,000-m-high Cascade Mountains farther inland. Flow patterns for rivers draining into the lower Columbia River are similar to those of coastal rivers immediately north and south of the Columbia River, with a single peak in December or January (Figs. 2 and 5) and relatively low flows (Fig. 4) in summer and fall. Columbia River tributaries draining the Cascade Mountains have proportionally higher flows (Fig. 4) in late summer and early fall than rivers on the Oregon coast, reflecting the greater contribution of snowmelt to these systems.

Precipitation levels in the Willamette Valley in Oregon (100-120 cm yr-1) are much lower than those on the coast (120-240 cm yr-1) or the Cascades (120-280 cm yr-1) (Fig. 9). Water and air temperatures also reflect the more extreme climate east of the Coast Range. Maximum water temperatures in rivers draining into the Columbia River are slightly warmer (13- 23°C) and minimum temperatures are slightly cooler (3-6°C) than those along the coast (Figs. 7-8, Appendix Table B-2). Similarly, maximum air (around 27°C) and minimum air (around -1°C) temperatures during the summer and winter are warmer and cooler, respectively, than along the coast. The Willamette Valley receives 2,000-2,200 h yr-1 sunshine, while the lower Columbia River receives less than 2,000 h yr-1.

Southwest Washington--Rivers in southwest Washington drain the Willapa Hills, an area characterized by relatively low elevations (500-1,000 m), with moderate amounts of rain (200-240 cm yr-1) (Fig. 9). These rivers flow either south into the lower Columbia River or west into the Pacific Ocean through Willapa Bay and Grays Harbor. However, many characteristics of rivers draining the Willapa Hills, such as water temperature (Figs. 7-8), a single peak in flow in December or January (Figs. 2, 5), and relatively low flows (Fig. 4), which occur during late summer and early fall, are similar regardless of the direction they drain.

The Chehalis River is the largest river flowing into Grays Harbor; it drains the south slope of the Olympic Mountains and a small area of the Cascade Mountains, in addition to the Willapa Hills. Although the Chehalis River is much larger and drains additional areas, it shares many of the characteristics of other southwest Washington rivers. Most striking is the similarity between the Columbia River estuary, Willapa Bay, and Grays Harbor; all three are characterized by extensive intertidal mud and sandflats and are very different from estuaries to the north or south.

Part of this similarity results from the shared geology of the area; the Chehalis River Basin was the northern-most area that remained ice free during the most recent glaciation (McPhail and Lindsay 1986), and the Chehalis and Columbia Rivers periodically had much higher flows during that time period, which greatly enlarged their respective valleys (Alt and Hyndman 1984, Allen et al. 1986). The Columbia River estuary, Willapa Bay, and Grays Harbor were all inundated as ocean levels rose following the last ice age. Material carried by the Columbia River has slowly been filling the lower Columbia River and has been transported northward along the coast to form Long Beach, which in turn has formed Willapa Bay. In addition, this material has created extensive sand beaches and dunes north and south of Grays Harbor (Alt and Hyndman 1984, Allen et al. 1986, Landry et al. 1989).

Olympic Peninsula--The Olympic Peninsula is much wetter (160-380 cm precipitation yr-1) than southwest Washington or areas farther east (Fig. 9) and receives considerable snowfall (over 150 cm yr-1) at higher elevations. This high precipitation results at least partially from the relatively high elevation of the Olympic Mountains (1,000-2,000 m) compared to the Willapa Hills or the Oregon Coast Range (both approximately 500-1,000 m high). Olympic Peninsula rivers derive much of their water from snowmelt that causes a second flow peak each year (Fig. 5). These rivers have relatively high flows even in summer (Fig. 4) and have the highest annual flows, given their drainage areas, of any of the areas discussed here (Fig. 6). Maximum and minimum air and water temperatures (Figs. 7-8) are cooler in the Olympic Peninsula than along the Oregon coast, reflecting both latitudinal effects and elevation. Annual maximum and minimum water temperatures are 10-14°C and 2-4°C, respectively, while annual maximum and minimum air temperatures are <21°C and around 2°C, respectively. Annual sunshine along the Olympic Peninsula coast is the lowest of anywhere in the continental United States, averaging less than 1,800 hours per year.

Coastal British Columbia--The very wet climate of the Olympic Peninsula continues north along the west coast of Vancouver Island and along the British Columbia mainland north of Vancouver Island. Limited hydrographic data (Farley 1979) indicate that river flow patterns in this area are similar to those on the Olympic Peninsula, with relatively high flows throughout the year. There is a general decrease in summer air temperatures with increasing north latitude--the Olympic coast is 3-5°C warmer than the southwest coast of Vancouver Island, which is 3-5°C warmer than the northwest coast and the mainland north of Vancouver Island.

Inland Waters--Precipitation rapidly decreases east of the Olympic Peninsula because of the rainshadow caused by the Olympic and Vancouver Island Mountains to the north, and Willapa Hills and Oregon Coast Range to the south. This rainshadow continues from the Willamette Valley through lowland Puget Sound, up the lowlands bordering the Strait of Georgia, to the south end of Queen Charlotte Strait. It receives less than 120 cm rain yr-1, with some areas receiving as little as 50 cm yr-1 (Fig. 9). Mountains on either side of this rainshadow receive high precipitation (up to 280 cm yr-1) (Fig. 9) and have an annual snowfall of 500-1,020 cm yr-1.

Because of snowmelt in their headwaters, rivers along the eastern Strait of Juan de Fuca, Puget Sound, and Hood Canal share many features with Olympic Peninsula rivers. All have relatively high flows in summer and two peaks of high flow, although the levels of flow relative to the basin area are not as large (Figs. 4-6). Limited data from British Columbia indicate similar river flow patterns (Farley 1979).

There appears to be a summer temperature cline within the greater rainshadow region: average maximum air temperatures in the Willamette Valley (around 27°C) are a few degrees higher than those in Puget Sound and Hood Canal (20-24°C), which in turn are slightly higher than in the Strait of Georgia (16-20°C) or areas inside Vancouver Island farther north (14-16°C). In contrast, winter air temperatures are more uniform and average 0-5°C throughout the area. Stream temperatures in the area are fairly cold, with a maximum of 12-20°C in summer and 0-4°C in winter (Figs. 7-8). The greater Puget Sound area receives 2,000-2,200 h yr-1 of sunshine.

Vegetation

Dominant vegetation types are a valuable indicator of relative precipitation, temperature, soil type, solar radiation, and altitude because of the specific requirements of different forest communities. Consequently, changes of vegetation types indicate changes in the physical environment, which may affect freshwater salmon habitat. The following discussion of vegetation was compiled from studies by Viereck and Little (1972), Franklin and Dyrness (1973), Barbour and Major (1977), Farley (1979), and Whitney (1985).

Sitka spruce zone--Coastal regions in Oregon, Washington, and British Columbia are forested with a Sitka spruce-dominated floral community: Sitka spruce, western hemlock, western red cedar, red alder, and Douglas fir are major species. This vegetation type is restricted to coastal regions and river valleys; only over coastal plains does it extend farther than a few kilometers inland, and it reaches elevations above 150 m only in areas immediately adjacent to the ocean. This vegetation type is typified by a uniformly wet and mild climate. Sitka spruce forests could be considered a variant of western-hemlock forests of higher elevations and inland areas, but they are distinguished by frequent summer fogs and proximity to the ocean (Franklin and Dyrness 1973).

Along the coast, Sitka spruce forests grade into redwood forests in southern Oregon and northern California and into western hemlock-dominated forests along the Strait of Juan de Fuca to the north. Sitka spruce forests also extend up the Columbia River to approximately the Clatskanie River (River Kilometer (RKm) 80), beyond which point the vegetation increasingly reflects the drier climate east of the Coast Range. The Columbia River passes through western hemlock forests in the Coast Range and Cascade Mountains, Oregon white oak forests in the Willamette Valley, and areas dominated by ponderosa pine or sagebrush in the arid interior east of the Cascade Mountains.

Redwood zone--Beginning in the Chetco River basin in southern Oregon, Sitka spruce and western hemlock are replaced by redwood forests, slightly inland and in river bottoms along the coast. This forest type forms the dominant coastal vegetation south to Monterey at elevations between 30 and 800 m. From the redwood zone along the coast, vegetation on the moist western slopes changes to Douglas fir/hardwood forests at lower elevations, followed by Shasta red fir and white fir, and finally mountain hemlock at higher elevations.

Vegetation in the upper basins of the Rogue and northern California rivers is adapted to a more arid climate than that of basins closer to the coast and, consequently, is distinct from upper-basin vegetation types either north or south. These vegetation types include forests dominated by Oregon oak, mixed evergreen, Klamath montane, coastal montane, blue oak-digger pine, and chapparal. South of the Mattole River, upper basins are not as arid, and the vegetation shows greater similarity to the coastal type--primarily redwoods with patches of mixed evergreens and mixed hardwoods, and coastal prairie-scrub around the San Francisco Bay area.

Western hemlock zone--Along the Washington and Oregon coasts, the western hemlock-dominated floral community replaces Sitka spruce at elevations above 150 m, and in the Puget Sound/Strait of Georgia area forms the dominant vegetation from sea level to 700-1,000 m. This zone includes western hemlock, Douglas fir, red alder, and western red cedar as major floral species. The transition point between Sitka spruce and western hemlock along the Strait of Juan de Fuca appears to be approximately the Elwha River on the U.S. side and Sooke Inlet on the Canadian side. South of the Columbia River, the western hemlock zone extends southward along the Coast Range to the Klamath Mountains and southward along the Cascade Mountains to the Umpqua River.

Forests in the Puget Sound area are often considered a special type of western hemlock forest. Because of Puget Sound's lower precipitation and glacial soils, drought-tolerant western white, lodgepole, and occasionally ponderosa pines are major species, whereas they are considered minor species elsewhere in the western hemlock zone.

Alpine and subalpine zones--The headwaters of rivers draining higher mountains, such as the Olympic and Cascade Mountains, and the British Columbia and Oregon Coast Ranges, begin in alpine meadows and subalpine parklands, before they change to western hemlock-dominated forests below 700-1,000 m. The higher, alpine regions are typified by a mosaic of meadows and tree patches with extended and deep snow cover. The subalpine zone is dominated by mountain hemlock and subalpine fir and is wetter and colder than areas at lower elevations but has less extended snow cover than higher alpine areas.

Analyses of vegetation types--In his factor analysis of western U.S. floras, based on the distribution of over 9,000 plant species, McLaughlin (1989) defined three floristic areas within the range of coho salmon: the Vancouverian, Sierra Nevada, and California areas. The Vancouverian area includes the Sitka spruce zone described above, the western hemlock zone excluding the central and southern Oregon Cascade Mountains, and the redwood zone from its northern boundary to approximately Cape Mendocino. The California floristic area is comprised of the redwood zone south of Cape Mendocino and the lower elevation portions of the Sacramento/San Joaquin Valley, while the Sierra Nevada area is defined by the central and south Oregon Cascade Mountains, the interior Klamath Mountain Province, and the Sierra Nevada Mountains. In a similar analysis based solely on Pacific coast beach vegetation, Breckon and Barbour (1974) identified a temperate eco-floristic zone, which extended from 54°N to 36°30'N. This zone was subdivided into northern North Coastal Zone and a southern Mediterranean Zone with the boundary at 43°30'N, approximately the Coos River, about 70 km north of Cape Blanco.

Upwelling

Ocean upwelling (the movement of cold, nutrient-rich subsurface water to the surface) along the coasts of British Columbia, Washington, and Oregon is primarily wind driven (Bakun 1973, 1975). Consequently, upwelling in the area is both seasonal and episodic, because winds that cause upwelling are more frequent in the spring and summer but do not occur uniformly even at those times (Smith 1983, Landry et al. 1989). Wind-driven upwelling also occurs within the Strait of Georgia, where it is similarly limited both spatially and temporally (Thompson 1981). One exception to this pattern has been observed off the southwest corner of Vancouver Island, where consistent and strong upwelling appears to occur throughout the year (Denman et al. 1981). Upwelling in this area is thought to be caused by current-driven as well as wind-driven events, leading to relative temporal and spatial stability.

South of Cape Blanco, (43°30'N), upwelling is much more consistent, less seasonal, and is stronger on average than in areas farther north (Bakun 1973, 1975). This strong upwelling area extends into central and southern California, beyond the southern distribution of coho salmon.

Zoogeography

Patterns of marine and freshwater species' distributions, like vegetation types, indicate changes in the physical environment which they share with coho salmon. These environmental differences may affect salmon habitat and provide different selective pressures in different areas to which salmon must adapt.

Marine fishes--There are two distinct faunal boundaries for marine fishes within the range considered in this status review: Point Conception (34°30'N) and the northern tip of Vancouver Island (approximately 50°N) (Allen and Smith 1988). Marine fishes north of 50°N are primarily coldwater, subarctic species; those between 50°N and 34°30'N are primarily temperate species; and those south of 34°30'N are primarily subtropical. Although not a distinct faunal boundary, Cape Mendocino represents the southern limit beyond which the presence of many northern species markedly declines (Horn and Allen 1978).

Marine invertebrates--The distribution of marine invertebrates shows transition points between major faunal communities similar to those for marine fishes (Hall 1964, Valentine 1966, Hayden and Dolan 1976, Brusca and Wallerstein 1979). Invertebrate faunal boundaries along the west coast of North America occur at approximately Dixon Entrance (directly west of Prince Rupert), Strait of Juan de Fuca, and Point Conception, with minor boundaries at Cape Mendocino and Monterey Bay (Hall 1964, Valentine 1966). The primary cause of this zonation is attributed to temperature (Hayden and Dolan 1976), but other abiotic (Valentine 1966) and biotic (Brusca and Wallerstein 1979) factors may also influence invertebrate distribution patterns.

Freshwater fishes--Freshwater fishes in south/central British Columbia, Washington, and most of coastal Oregon are of Columbia River origin (McPhail and Lindsey 1986, Minckley et al. 1986). Variations in the makeup of freshwater fish communities in these areas reflect the varied dispersal patterns of fishes between river basins. The Stikine River in northern British Columbia is the point at which freshwater fishes from the north displace the Columbia River fish fauna (McPhail and Lindsay 1986). Similarly, the Sixes River in southern Oregon marks the southern extent of the Columbia River freshwater fish fauna (Minckley et al. 1986). Freshwater fishes in the Klamath-Rogue ichthyofaunal region, which includes the Klamath and Rogue Rivers, differ from the Columbia River-dominated assemblages to the north and the Sacramento/San Joaquin River-dominated faunas to the south (Moyle 1976, Minckley et al. 1986). Freshwater fishes in north/central California between Redwood Creek and the San Lorenzo River are derived from the Sacramento-San Joaquin River system. However, many of the smaller basins have no exclusively freshwater species, but only those that can move readily through salt water (Moyle 1976). From the San Lorenzo River southward, freshwater fishes belong to the Pajaro-Salinas type (Moyle 1976). This faunal type is derived from the Sacramento- San Joaquin River system, but it has been isolated for some time, which has allowed for significant divergence from species of that system.

Estuarine fishes--Estuarine fishes also show regional differences based on presence or absence of species and can be roughly divided into five groups in Washington, Oregon, and north/central California (Monaco et al. 1992). Two groups were identified in Washington: one restricted to Puget Sound and Hood Canal, and a second consisting of Grays Harbor, Willapa Bay, and the Columbia River estuary. Two large groups with considerable geographic overlap extend from Willapa Bay in Washington to the Eel River estuary in California. The differentiation between these latter two groups appeared to be related to the size of the respective estuaries. A final group extends from Tomales Bay to Morro Bay in California.

Amphibians--Although most amphibians are not restricted to aquatic habitats and therefore have little direct habitat overlap with coho salmon, many amphibian species have very restricted distributions, suggesting preferences for specific habitat types and environmental conditions. Because of this sensitivity, patterns of amphibian distributions may serve as indicators of subtle differences in environmental conditions.

The distributions of many amphibians appear to begin and end at several common geographical areas within the range of west coast coho salmon. For example, the Strait of Georgia and Vancouver Island are the northern extent of many amphibian distributions including tailed and red-legged frogs, and Pacific giant, western long-toed, western red-backed, Oregon, and brown salamanders (Cook 1984). Southern Oregon, in the vicinity of Cape Blanco, is both the northern (southern long-toed, Del Norte's, and California salamanders), and the southern (western red-backed salamander) extent of some amphibian distributions (Stebbins 1966, Leonard et al. 1993), as is Cape Mendocino (northern endpoint of the southern red-legged frog, red-bellied newt, and the arboreal salamander, and southern endpoint of the northern red-legged frog and Del Norte's salamander distributions) (Stebbins 1966). In addition, several amphibians are restricted to the Olympic Peninsula (Olympic torrent and Van Dyke's salamanders), while other species occur in most areas in western Washington and Oregon except in the Olympic Peninsula (Pacific giant and Dunn's salamanders) (Leonard et al. 1993).

Other Ecological Factors

The U.S. Environmental Protection Agency has developed a system of ecoregions, based on the patterns of a combination of factors such as land use, climate, topography, potential natural vegetation, and soils (Omernik and Gallant 1986, Omernik 1987). Under this system, the range of coho salmon in Washington, Oregon, and California covers seven ecoregions, although only three border on salt water: the coast range ecoregion extends from the Strait of Juan de Fuca to Monterey Bay, from the ocean to approximately the crest of the coastal mountains; the southern and central California plains and hills ecoregion extends from the Sacramento/San Joaquin basin through the coast range ecoregion to the coast around San Francisco Bay; and the Puget lowland ecoregion begins at approximately the Dungeness River on the eastern end of the Strait of Juan de Fuca and extends through Puget Sound to the Canadian border. The remaining four ecoregions cover the upper basins of coastal rivers and were defined as the Willamette Valley, Cascades, Sierra Nevada, and eastern Cascades slopes and foothills ecoregions. There has generally been good correspondence between Omernik's ecoregions and the distribution of freshwater fish assemblages (Hughes et al. 1987, Lyons 1989).

The Washington and Oregon portion of the coast range ecoregion has since been subdivided (Thiele et al. 1992), primarily on the basis of elevation and geology. One interesting subregion, however, is the California coast range extension subregion, which begins at Cape Blanco and extends south into California, replacing coastal lowlands and mid-coastal sedimentary subregions to the north.


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