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

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

ASSESSMENT OF EXTINCTION RISK

Background

As outlined in the "Introduction" above, NMFS considers a variety of information in evaluating the level of risk facing an ESU. Aspects of several of these risk considerations are common to all coho salmon ESUs. These are discussed in general below; more specific discussion of factors for each of the ESUs under consideration here can be found in the following sections. Because we have not taken future effects of conservation measures into acount (see "Introduction"), we have drawn scientific conclusions about the risk of extinction faced by identified ESUs under the assumption that present conditions will continue. Future effects of conservation measures will be taken into account by the NMFS Northwest and Southwest Regional Offices in making listing recommendations.

Absolute Numbers

The absolute number of individuals in a population is important in assessing two aspects of extinction risk. For small populations that are stable or increasing, population size can be an indicator of whether the population can sustain itself into the future in the face of environmental fluctuations and small-population stochasticity; this aspect is related to the concept of minimum viable populations (MVP) (Gilpin and Soul 1986, Thompson 1991). For a declining population, present abundance is an indicator of the expected time until the population reaches critically low numbers; this aspect is related to the concept of driven extinction (Caughley 1994).

In addition to total numbers, the spatial and temporal distribution of adults is important in assessing risk to an ESU. Spatial distribution is important both at the scale of river basins within an ESU and at the scale of spawning areas within basins ( metapopulation structure). Temporal distribution is important both among years, as an indicator of the relative health of different brood-year lineages, and within seasons, as an indicator of the relative abundance of different life history types or runs.

Traditionally, assessment of salmon populations has focused on the number of harvestable or reproductive adults, and these measures comprise most of the data available for Pacific salmon. In assessing the future status of a population, the number of reproductive adults is the most important measure of abundance, and we focus here on measures of the number of adults escaping to spawn in natural habitat. However, total run size (spawning escapement + harvest) is also of interest because it indicates potential spawning in the absence of harvest. Data on other life history stages (e.g., freshwater smolt production) can be used as a supplemental indicator of abundance.

Because the ESA (and NMFS policy) mandates a biological review that focuses on viability of natural populations, we attempted to distinguish natural fish from hatchery produced fish. All statistics are based on data that indicate total numbers or density of adults that spawn in natural habitat ( naturally spawning fish ). The total of all naturally spawning fish ( total escapement ) is divided into two components (Fig. 43): Hatchery produced fish are reared as juveniles in a hatchery but return as adults to spawn naturally; Natural fish are progeny of naturally spawning fish.

Historical Abundance and Carrying Capacity

The relationship of current abundance and habitat capacity to that which existed historically is an important consideration in evaluating risk for several reasons. Knowledge of historical population conditions provides a perspective of the conditions under which present stocks evolved. Historical abundance also provides the basis for establishing long-term population trends. Comparison of present and past habitat capacity can also indicate long-term population trends and problems of population fragmentation.

Although the relationship of present abundance to present carrying capacity is important for understanding the health of populations, the fact that a population is near its current capacity does not in itself mean that it is healthy. If a population is near capacity, there will be limits to the effectiveness of short-term management actions to increase its abundance, and competition and other interactions between hatchery and natural fish may be important considerations because hatchery supplementation will further increase population density in a limited habitat.

Quantitative assessments of habitat are quite rare, although rough estimates of carrying capacity are frequently made for setting management goals. From the evidence available, it is clear that natural production of coho salmon is now substantially below historical levels for all ESUs considered here, although this decline has been offset by hatchery production in many areas. Although we have not attempted analysis of the proportion of total habitat lost due to blockages, we found significant blockages of freshwater habitat in every ESU. Freshwater and estuarine habitats are also degraded throughout the entire region considered, although the severity of degradation varies among ESUs.

Trends in Abundance

Short- and long-term trends in abundance are a primary indicator of risk in salmonid populations. Trends may be calculated from a variety of quantitative data, including dam or weir counts, stream surveys, and catch data. These data sources and methods are discussed in more detail below, under Approach. When data series are lacking, general trends may be inferred by comparing historical and recent abundance estimates, or by considering trends in habitat quantity or condition.

The role of artificial propagation (in the form of hatcheries) for Pacific salmon requires careful consideration in ESA evaluations. Artificial propagation has implications both for evaluating production trends and in evaluating genetic integrity of populations.

Waples (1991a, b) and Hard et al. (1992) discussed the role of artificial propagation in ESU determination and emphasized the need to focus on natural production in the threatened or endangered status determination. Because of the ESA s emphasis on ecosystem conservation, this analysis focuses on naturally reproducing salmon. A fundamental question in ESA risk assessments is whether natural production is sufficient to maintain the population without the constant infusion of artificially produced fish. A full answer to this question is difficult without extensive studies of relative production and interactions between hatchery and natural fish.

One method of evaluating this issue involves calculating the natural cohort replacement ratio, defined as the number of naturally spawning adults naturally produced in one generation divided by the number of naturally spawning adults (regardless of parentage) in the previous generation. Data for coho salmon are rarely sufficient for this calculation, and we have not attempted to estimate this ratio in this report. However, the ratio can be approximated from the average population trend if the degree of hatchery contribution to natural spawning can be estimated (Busby et al. 1994 appendix B). Where such estimates were available, the presence of hatchery fish among natural spawners was taken into consideration in evaluating the sustainability of natural production for individual populations.

Coastwide trends in coho abundance provide another method of evaluating relative production, though from a larger perspective. Coastwide abundance must be approximated from commercial and sport harvest data, which is tracked closely by government agencies. Commercial landings of coho salmon in Washington, Oregon, and California from 1882 to 1982 have been estimated by Shepard et al. (1985). These estimates show relatively constant landings since 1895, ranging mainly between 1.0 and 2.5 million fish, with a low of 390,000 fish (1920) and a high of 4.1 million fish (1971). Comparable recent estimates are not available, but ocean commercial troll and sport landings have been summarized from 1971 to 1994 by PFMC (1995). These data show a recent harvest decline from 4.3 million fish in 1971 to 290,000 fish in 1993, and less than 500 fish in 1994 (Fig. 44). However, this decline largely reflects reductions in allowable harvest, which were imposed in response to perceived declines in production.

Factors Causing Variability

Variations in the freshwater and marine environments are thought to be a primary factor driving fluctuations in salmonid run size and escapement (Pearcy 1992, Beamish and Bouillon 1993, Lawson 1993). Recent changes in ocean condition are discussed below. Habitat degradation and harvest have probably made stocks less resilient to poor climate conditions, but these effects are not easily quantifiable.

Threats to Genetic Integrity

In addition to being a factor in evaluating natural replacement rates, artificial propagation can substantially affect the genetic integrity of natural salmon populations in several ways. First, stock transfers that result in interbreeding of hatchery and natural fish can lead to loss of fitness in local populations and loss of diversity among populations. The latter is important to maintaining long-term viability of an ESU because genetic diversity among salmon populations helps to buffer overall productivity against periodic or unpredictable changes in the environment (Riggs 1990, Fagen and Smoker 1989). Ricker (1972) and Taylor (1991) summarized some of the evidence for local adaptations in Pacific salmon that may be at risk from stock transfers.

Second, because a successful salmon hatchery dramatically changes the mortality profile of a population, some level of genetic change relative to the wild population is inevitable, even in hatcheries that use local broodstock (Waples 1991b). These changes are unlikely to be beneficial to naturally reproducing fish.

Third, even if naturally spawning hatchery fish leave few or no surviving offspring, they still can have ecological and indirect genetic effects on natural populations. On the spawning grounds, hatchery fish may interfere with natural production by competing with natural fish for territory or mates. If they successfully spawn with natural fish, they may divert production from more productive natural-by-natural crosses. The presence of large numbers of hatchery juveniles or adults may also alter the selective regime faced by natural fish.

For smaller stocks (either natural or hatchery), small-population effects (inbreeding, genetic drift) can also be important concerns for genetic integrity. Inbreeding and genetic drift are well understood at the theoretical level, and researchers have found inbreeding depression in various fish species (reviewed by Allendorf and Ryman 1987). Other studies (e.g., Simon et al. 1986, Withler 1988, Waples and Teel 1990) have shown that hatchery practices commonly used with anadromous Pacific salmonids have the potential to affect genetic integrity. However, we are not aware of empirical evidence for inbreeding depression or loss of genetic variability in any natural or hatchery populations of Pacific salmon or steelhead.

Recent Events

A variety of factors, both natural and human-induced, affect the degree of risk facing salmon populations. Because of time-lags in these effects and variability in populations, recent changes in any of these factors may affect current risk without any apparent change in available population statistics. Thus, consideration of these effects must go beyond examination of recent abundance and trends. Unfortunately, forecasting future effects is rarely straightforward and usually involves qualitative evaluations based on informed professional judgment. A key question regarding the role of recent events is: Given our uncertainty regarding the future, how do we evaluate the risk that a population may not persist?

For example, climate conditions are known to have changed recently in the Pacific Northwest, and Pacific salmon stocks south of British Columbia have been affected by changes in ocean production that occurred during the 1970s (Pearcy 1992, Lawson 1993). Much of the Pacific coast has also been experiencing drought conditions in recent years, which may depress freshwater salmon production. However, at this time we do not know whether these climate conditions represent a long-term change that will continue to affect stocks in the future or whether these changes are short-term environmental fluctuations that can be expected to be reversed in the near future. Possible future effects of recent or proposed conservation measures have not been taken into account in this analysis.

Other Risk Factors

Other risk factors typically considered for salmonid populations include disease prevalence, predation, and changes in life history characteristics such as spawning age or size. We have not found evidence that any of these factors are widespread throughout any coho salmon ESU, except for the documented decline in body size of adult coho salmon previously discussed in the Coho Salmon Life History.

Approach

In considering the status of ESUs, we evaluated both qualitative and quantitative information. Qualitative evaluations considered recent, published assessments by agencies or conservation groups of the status of coho salmon stocks within the geographic area (Nehlsen et al. 1991, Higgins et al. 1992, Nickelson et al. 1992, WDF et al. 1993). These evaluations are summarized in Table 6. Nehlsen et al. (1991) considered salmon stocks throughout Washington, Idaho, Oregon, and California and enumerated all stocks that they found to be extinct or at risk of extinction. Stocks that do not appear in their summary were either not at risk of extinction or were not classifiable due to insufficient information. They classified stocks as extinct (X), possibly extinct (A+), at high risk of extinction (A), at moderate risk of extinction (B), or of special concern (C).

Nehlsen et al. (1991) considered it likely that stocks at high risk of extinction have reached the threshold for classification as endangered under the ESA. Stocks were placed in this category if they had declined from historic levels and were continuing to decline, or had spawning escapements less than 200 individuals. Stocks were classified as at moderate risk of extinction if they had declined from historic levels but presently appear to be stable at a level above 200 spawners (stocks in this category were considered by Nehlsen et al. (1991) to have reached the threshold for threatened under the ESA). Stocks were classified as of special concern if a relatively minor disturbance could threaten them, insufficient data were available for them, they were influenced by large releases of hatchery fish, or if they possessed some unique character. For coho salmon, Nehlsen et al. (1991) classified 50 stocks as follows: 15 extinct, 2 possibly extinct, 15 high risk, 16 moderate risk, and 2 special concern.

Higgins et al. (1992) used the same classification scheme as Nehlsen et al. (1991) but provided a more detailed review of northern California salmon stocks. Of the 20 stocks Higgins et al. (1992) identified as being at some risk of extinction, 7 were classified as at high risk of extinction, and the remainder were classified as of special concern.


Table 6. Summary of assessments of coho salmon stock status from recent reviews, grouped by evolutionarily significant unit (ESU) or geographic area.
Nehlsen Higgins Nickelson WDF et al. 1993
et al. et al. et al. Prod.
Basin or stocka 1991b 1992b 1992c Origind typee Statusf

Central California coast ESU
Ten Mile River C
Pudding Creek A
Noyo River C
Big River C
Albion River C
Navarro River C
Garcia River A
Gualala River A
Russian River A
Scott River A
Small Southern CA streams A
Southern Oregon/northern California coasts ESU
Elk River A D
Euchre Creek X D
Rogue River A D
Hunter Creek D
Pistol River A D
Chetco River A D
Winchuck River A D
Small Northern CA Streams B
Klamath River C
Trinity River C
Wilson Creek C
Lower Klamath River C
Redwood Creek C
Little River C
Mad River A
Humboldt Bay C
Eel River C
Bear River C
Mattole River A
Oregon Coast ESU
Necanicum River B D
Elk Creek B D
Nehalem River B D, SC
Tillamook Bay B D
Nestucca River B D
Salmon River B D
Siletz River B D
Yaquina River D
Beaver Creek B D
Alsea River B H
Yachats River B D
Siuslaw River B D
Siltcoos River H
Tahkenitch Creek H
Umpqua River B D, SC
Tenmile Creek D
Coos River B H, D
Coquille River B H, D
Floras Creek (New River) A D
Sixes River A D

Nehlsen Higgins Nickelson WDF et al. 1993
et al. et al. et al. Prod.
Basin or stocka 1991b 1992b 1992c Origind typee Statusf

Lower Columbia/southwest Washington ESU
Grays Harbor M C H
Willapa Bay A M C U
Sandy River A
Clackamas River B
Lower Columbia Tributaries A, A+ (Washougal) M C D
Olympic Peninsula ESU
Coast minor drainages C (Lake Ozette) N, MW, C U
Quillayute River N W, C H
Hoh River N W H, U
Queets River N, X C H
Quinault River M C H, U
Puget Sound/Strait of Georgia ESU
N. Puget Sound minor drainages N, M, U W U
Nooksack River A+ M C U
Samish River M C H
Skagit River N, U C D, U
Stillaguamish River N, M W D, U
Snohomish River M, X W, C H, D
Lake Washington M W, C H, D
Duwamish River M C H, D
Puyallup River M C H, D
Nisqually River M C H
S. Puget Sound minor drainages A (Chambers Cr.) M, XW, C
Hood Canal M W, C H, D
Strait of Juan de Fuca minor drainages A (Lyre R.) M W, C H, D, C, U
Dungeness River M C D
Elwha River A M C H
Upper Columbia Basin (not a designated ESU)
Klickitat River M C D
Hood River A
Middle Columbia Tribs. X
Tucannon River X
Spokane River X
Snake River X
Methow River X
Yakima River X
Wenatchee River X
Entiat River X
Snake River X
Grande Ronde River X
Walla Walla River X
Umatilla River X

aTributaries and minor drainages combined. Multiple status designations indicate different status for tributaries or minor drainages within basin.
bA+--possibly extinct; A--high risk, B--moderate risk, C--special concern, X-- extinct.
cH--healthy, SC--special concern, D--depressed.
dN--native, M--mixed, X--non-native, U--unknown.
eW--wild, C--composite.
fH--healthy, D--depressed, C--critical, U--unknown.


Nickelson et al. (1992) rated coastal (excluding Columbia River Basin) Oregon salmon stocks on the basis of their status over the past 20 years, classifying stocks as depressed (spawning habitat underseeded, declining trends, or recent escapements below long-term average), healthy (spawning habitat fully seeded and stable or increasing trends), or of special concern (300 or fewer spawners or a problem with hatchery interbreeding). They classified 55 coho salmon populations in coastal Oregon as follows: 41 depressed, 2 special concern, 6 healthy, and 6 unknown.

WDF et al. (1993) categorized all salmon stocks in Washington on the basis of stock origin (native, non-native, mixed, or unknown), production type (wild, composite, or unknown) and status (healthy, depressed, critical, or unknown). Status categories were defined as follows: healthy, experiencing production levels consistent with its available habitat and within the natural variations in survival for the stock ; depressed, production is below expected levels . . . but above the level where permanent damage to the stock is likely ; and critical, experiencing production levels that are so low that permanent damage to the stock is likely or has already occurred. Of the 90 coho salmon stocks identified, 37 were classified as healthy, 35 as critical or depressed, and 18 as unknown. Of the 37 healthy stocks, only 4 (all on the Olympic Peninsula) were identified as native and of wild production type.

One of the problems in applying results of these studies to ESA evaluations is that the definition of stock or population varied considerably in scale among studies, and sometimes among regions within a study. Identified units range in size from large river basins to minor coastal streams and tributaries. A second problem is the definition of categories used to classify stock status. Only Nehlsen et al. (1991) and Higgins et al. (1992) used categories intended to relate to ESA threatened or endangered status, and they applied their own interpretations of these terms to individual stocks, not to ESUs as defined here.

Nickelson et al. (1992) and WDF et al. (1993) used general terms describing status of stocks that cannot be directly related to the considerations important in ESA evaluations. For example, the WDF et al. (1993) definition of healthy could conceivably include a stock that is at substantial extinction risk due to loss of habitat, hatchery fish interactions, or environmental variation. A third problem is the selection of stocks or populations to include in the review. Nehlsen et al. (1991) and Higgins et al. (1992) did not evaluate (or even identify) stocks not perceived to be at risk, so it is difficult to determine the proportion of stocks they considered to be at risk in any given area.

Quantitative evaluations included comparisons of current and historical abundance of coho salmon. Historical abundance information for these ESUs is largely anecdotal, although harvest records are available for some areas back to the late 1800s from which minimum run sizes can be estimated. Time-series data are available for many populations, but data extent and quality varied among ESUs. We compiled and analyzed this information to provide several summary statistics of natural spawning abundance, including (where available) recent total spawning run size and escapement, percent annual change in total escapement, recent naturally produced spawning run size and escapement, and average percentage of natural spawners that were of hatchery origin.

Although this evaluation used the best data available, it should be recognized that there were a number of limitations to these data, and not all summary statistics were available for all populations. For example, spawner abundance was generally not measured directly; rather, it often had to be estimated from limited survey data. In many cases, there were also limited data to separate hatchery production from natural production.

Quantitative Methods

Information on stock abundance was compiled from a variety of state, federal, and tribal agency records. We believe these records to be complete in terms of long-term adult abundance records for coho salmon in the region covered. Principal data sources were commercial catch, dam or weir counts, and stream surveys. Specific problems are discussed below for each data type.

Data Types

Quantitative assessments were based on historical and recent run-size estimates and time series of freshwater-spawner and juvenile surveys, harvest rate estimates, and counts of adults migrating past dams. We considered this information separately for each ESU. Because of the disparity of data sources and data quality among different ESUs, data sources and analyses are described separately for each ESU.

Dam and weir counts are available in several river basins along the coast. These counts are probably the most accurate estimates available of total spawning run abundance, but they often represent only small portions of the total population in each river basin. As with angler catches, these counts typically represent a combination of hatchery produced and natural fish, and thus cannot be used as a direct index of natural population trends.

Stream surveys for coho salmon spawning abundance have been conducted by various agencies within most of the ESUs considered here. Survey methods and time-spans vary considerably among regions, so it is difficult to assess the general reliability of these surveys as population indices. For most streams where surveys were conducted, they are the best local indication we have of population abundance trends.

Computed Statistics

To represent current run size or escapement, we have computed the geometric mean of the most recent years reported. We tried to use only estimates that reflected the total abundance for an entire river basin or tributary, avoiding reliance on index counts or dam counts that represented only a small portion of the available habitat.

Where adequate data were available, trends in total escapement (or run size if escapement was not available) were calculated for data sets with more than 5 years of data. As an indication of overall trends in coho salmon populations in individual streams, we calculated average (over the available data series) percent annual change in adult spawner indices within each river basin. Trends were calculated as the slope (a) of the regression of ln(abundance) against years corresponding to the biological model N(t) = b eat. Slopes significantly different from zero (P < 0.05) were noted. The regressions provided direct estimates of mean instantaneous rates of population change (a); these values were subsequently converted to percent annual change, calculated as 100(ea-1). No attempt was made to account for the influence of hatchery produced fish on these estimates, so the estimated trends include any supplementation effect of hatchery fish.


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