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Open-File Report 2009–1068

Prepared in cooperation with the Bureau of Reclamation

Minimum Pool and Bull Trout Prey Base Investigations at Beulah Reservoir—Final Report for 2008

Brien P. Rose and Matthew G. Mesa

ABSTRACT

Beulah Reservoir in southeastern Oregon provides irrigation water to nearby farms and supports an adfluvial population of threatened bull trout (Salvelinus confluentus). Summer drawdowns in the reservoir could affect forage fish production and overwintering bull trout. To assess the impacts of drawdown, we sampled fish, invertebrates, and water-quality variables seasonally during 2006–08. In 2006, the summer drawdown was about 68 percent of full pool, which was less than a typical drawdown of 85 percent. We detected few changes in pelagic invertebrate densities, and catch rates, abundance, and sizes of fish when comparing values from spring to values from fall. We did note that densities of benthic insects in areas that were dewatered annually were lower than those from areas that were not dewatered annually. In 2007, the drawdown was 100 percent (to run-of-river level) and resulted in decreases in abundance of invertebrates as much as 96 percent, decreases in catch rates of fish as much as 80 percent, decreases in abundance of redside shiners (Richardsonius balteatus) and northern pikeminnow (Ptychocheilus oregonensis) as much as 93 percent, and decreased numbers of small fish in catches. In the fall 2007, we estimated the total biomass of forage fish to be 76 kilograms, or about one-quarter of total biomass of forage fish in 2006. Bioenergetics modeling suggested that ample forage for about 1,000 bull trout would exist after a moderate drawdown, but that forage remaining after a complete dewatering would not be sufficient for a population one-fifth the size. Our results indicate that drawdowns in Beulah Reservoir affect the aquatic community and perhaps the health and well-being of bull trout. The severity of effects depends on the extent of drawdown, population size of bull trout, and perhaps other factors.

For additional information contact:
Director, Western Fisheries Research Center
U.S. Geological Survey
6505 NE 65th Street
Seattle, Washington 98115
http://wfrc.usgs.gov/

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Suggested citation:

Rose, B.P., and Mesa, M.G., 2009, Minimum pool and bull trout prey base investigations at Beulah Reservoir—Final report 2008: U.S. Geological Survey Open-File Report 2009–1068, 54 p.



Contents

Abstract

Introduction

Methods

Results

Discussion

Methods

Summary

Acknowledgments

References Cited

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