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Historical Abundance of Puget Sound steelhead

Category: Newsletter Articles | Posted by: Jonathan | 7/6/12 | Comments: 0

Summary: The Puget Sound steelhead , Oncorhynchus mykiss, was listed as threatened under the U.S. Endangered Species Act (ESA) in 2007, prompting the initiation of recovery planning.  Recovery planning requires, ultimately, the identification of conditions under which the “evolutionary significant unit” (ESU) can be considered to have been recovered and subsequently removed from the list of threatened and endangered species (“de-listed”).
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By Nick Gayeski, Bill McMillan, and Pat Trotter

(Reprinted from the Wild Fish Journal with permission from the Wild Fish Conservancy)

Skagit River

Skagit River

The Puget Sound steelhead , Oncorhynchus mykiss, was listed as threatened under the U.S. Endangered Species Act (ESA) in 2007, prompting the initiation of recovery planning.  Recovery planning requires, ultimately, the identification of conditions under which the “evolutionary significant unit” (ESU) can be considered to have been recovered and subsequently removed from the list of threatened and endangered species (“de-listed”). A number of factors are considered when setting recovery goals and de-listing criteria, such as population sizes, life-history, and geographic and genetic diversity. Generally, in order to de-list a species, a sufficient number of self-sustaining populations within the ESU must be established. An important consideration is to identify conditions that, when attained, will afford each population a very high probability of persisting over the long-term in the face of all normal environmental variation.  In this context, some key questions are “How many populations are needed?” and “How big do they have to be?” and the correct answers undoubtedly lie somewhere between the conditions that existed immediately prior to the ESA-listing and some previous time when the abundance and diversity of the listed populations were much closer to pristine, pre-development conditions.

In this regard, recovery under ESA confronts many of the same problems and controversies that have arisen in discussions of the rebuilding of overfished marine stocks.  In a recent discussion of rebuilding depleted marine stocks, A.A. Rosenberg and co-authors (2005) noted that “…it is important to recognize that fisheries for key commercial species like cod were far more productive in the past.  As we attempt to rebuild these fisheries, our decisions should reflect real and realistic goals for management, not just recently observed catch levels.” We believe that the same perspective applies to the recovery of Puget Sound steelhead under the ESA.

History can provide the necessary baseline for comparing the current status of steelhead populations and can therefore be a useful tool for recovery planning.  However, the absence of high-quality quantitative data regarding population abundances and dynamics, of the sort recognized today as necessary for the proper fish population management, poses a considerable challenge. And historical information, when available, must be carefully analyzed before it can inform recovery goals.

We analyzed several kinds of available historical information for Puget Sound steelhead and developed a robust estimate of the abundance of several specific populations as well as Puget Sound as a whole immediately prior to the turn of the 20th century. Using 1895 as our benchmark year (the year of the peak commercial catch of steelhead), we combined data on the commercial harvest of winter-run steelhead and average weight of steelhead with historical information on Euro-American development and settlement of Puget Sound, which enabled us to estimate the number of steelhead “harvested” outside of the commercial fishery, the overall intensity of the harvest of Puget Sound winter-run steelhead, and the terminal run sizes of steelhead. We employed a statistical analysis to address the uncertainties associated with the estimation process and developed abundance estimates for four large northern Puget Sound rivers as well as the remaining aggregate of rivers and streams in Puget Sound.

The best estimate of total abundance ranged from 485,000 to 930,000, with a mode of 622,000.  Compared with the 25-year average abundance for all of Puget Sound of 22,000 for the 1980-2004 period, our results show that current abundance is likely only 1%-4% of what it was prior to 1900. Because we have lost no more than one-third of the lengths of streams potentially accessible to adult winter-run steelhead in 1895, the loss of freshwater habitat alone can account for this reduction in abundance only if there was an extraordinary decline in productivity. Currently accessible freshwater habitat is either under-occupied by juveniles and adults, or (in conjunction with other factors) is not capable of supporting the numbers of juveniles and adults that it supported near the turn of the twentieth century – or both.

In order to assure that Puget Sound steelhead actually recover (i.e. there is a reasonably low probability of having to re-list the fish in the subsequent 100 years), the abundance of wild winter-run adult steelhead populations throughout Puget sound may need to be considerably greater than it was in the years immediately prior to the ESA listing of Puget Sound steelhead. Such large populations were likely essential for the long-term persistence and resilience of Puget Sound steelhead in the face of past environmental variation, but today, environmental variation is only getting worse.

Reference

Rosenberg, A.A., Bolster, W.J., Alexander, K.E., Leavenworth, W.B., Cooper, A.B., and McKenzie, M.G. 2005.  The history of ocean resources: modeling cod biomass using historical records. Front. Ecol. Environ.  3(2): 78-84.

 

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