A LARGE-SCALE, MULTISPECIES STATUS ASSESSMENT: ANADROMOUS SALMONIDS IN THE COLUMBIA RIVER BASIN

Abstract

Twelve salmonid evolutionarily significant units (ESUs) throughout the Columbia River Basin are currently listed as threatened or endangered under the Endangered Species Act; these ESUs are affected differentially by a variety of human activities. We present a standardized quantitative status and risk assessment for 152 listed salmonid stocks in these ESUs and 24 nonlisted stocks. Using data from 1980–2000, which represents a time of stable conditions in the Columbia River hydropower system and a period of ocean conditions generally regarded as poor for Columbia Basin salmonids, we estimated the status of these stocks under two different assumptions: that hatchery-reared spawners were not reproducing during the period of the censuses, or that hatchery-reared spawners were reproducing and thus that reproduction from hatchery inputs was masking population trends. We repeated the analyses using a longer time period containing both “good” and “bad” ocean conditions (1965–2000) as a first step toward determining whether recent apparent declines are a result of sampling a period of poor ocean conditions. All the listed ESUs except Columbia River chum showed declining trends with estimated long-term population growth rates (λ's) ranging from 0.85 to 1.0, under the assumption that hatchery fish were not reproducing and not masking the true λ. If hatchery fish were reproducing, the estimated λ's ranged from 0.62 to 0.89, indicating extremely low natural reproduction and survival. For most ESUs, there was no significant decline in population growth rates calculated for the 1980–2000 vs. 1965–2000 time periods, suggesting that the current population status for most ESUs is not solely a result of changes in ocean conditions, and that without other changes, risks will persist even during upturns in ocean conditions. However, estimated population growth rates for the Snake River spring–summer chinook salmon and steelhead ESUs were significantly lower during the longer time period. This difference may be due to a period of dam building on the Snake River during the 1960s and 1970s. For 33 stocks and seven ESUs, the probability of extinction could be estimated. The estimates were generally low for all ESUs with the exception of Upper Columbia River spring chinook and Upper Willamette River steelhead. The probability of 90% decline could be estimated for all stocks. The mean probability of 90% decline in 50 years was highest for Upper Columbia River spring chinook (95% mean probability across all stocks within the ESU) and Lower Columbia River steelhead (80% mean probability). We estimated the effects of two different management actions on long-term growth rates for the ESUs. Harvest reductions offer a means to mitigate risks for ESUs that bear substantial harvest pressure, but they are unlikely to increase population growth rates enough to produce stable or increasing trends for all ESUs. Similarly, anticipated improvements to passage survival through the Snake and mainstem Columbia hydropower systems may be important, but additional actions are likely to be necessary to recover affected ESUs.