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The Problem
Bull trout were listed by the U.S. Fish and Wildlife Service as a threatened species throughout the Columbia and Klamath river basins in 1998, and Oregon has listed the North Fork Malheur River population "Of Special Concern". Reasons for the decline of bull trout in the Malheur River likely include habitat degradation, habitat fragmentation, and downstream loss through entrainment at dams. Agency Valley Dam was constructed on the North Fork of the Malheur River (central Oregon) during 1934-35 by the Bureau of Reclamation, creating Beulah Reservoir. The project is operated by the Vale Irrigation District for irrigation and downstream flood control. Although the project is not operated for fish and wildlife values, the reservoir supports a rainbow trout Oncorhynchus mykiss fishery and also seasonally harbors an adfluvial population of bull trout Salvelinus confluentus. Both subadult and adult bull trout likely reside in Beulah Reservoir for either all (subadults) or part (mature adults) of the year. During residence, bull trout are likely feeding invertebrates and fish, including stocked rainbow trout, and will be exposed to temperatures, dissolved oxygen, and other conditions that might change with season or reservoir operation. The construction of dams in the Malheur River drainage isolated formerly connected bull trout populations of the Snake River Basin. Migratory bull trout are important to the persistence and stability of the North Fork Malheur population
Current project operations at Agency Valley Dam do not include minimum pool requirements or consider water quality issues, and thus may pose a risk to the North Fork Malheur River bull trout population. The BOR has initiated an investigation of alternatives for creating a conservation fisheries pool in Beulah Reservoir. Water quality monitoring and modeling are underway to describe the seasonal distribution of dissolved oxygen and temperature relative to bull trout needs. General limnological information is also being collected on a regular basis to describe algal and zooplankton standing crops under wet and dry year conditions. A Beulah Reservoir sedimentation survey was initiated in 2000 to provide updated capacity data, and a bathymetric map for use in developing a conservation pool. Management agencies are particularly interested in the occurrence of subadult bull trout in Beulah Reservoir during summer and fall months when conditions may be poor for their growth and survival. Objectives The USGS began a study in 2001 to estimate the quality of habitat in Beulah Reservoir for bull trout growth and survival. Information will be used to assist managers in reservoir operations and perhaps in establishment of a minimum conservation pool. Studies include field work to characterize the prey base for bull trout, laboratory studies to improve parameter estimates of a bioenergetics model, and modeling analyses. Data will be synthesized with ongoing physical studies in the reservoir. Methodology
Field sampling was conducted during the summer and fall of 2001 and 2002. Fish in the reservoir were sampled with experimental gill nets fished on the bottom, and with Fyke nets fished near the shore. Gill nets were generally set during the evening and early night hours and fished for about one hour before retrieval. Fyke nets were set along the shoreline in about 1 m depth, and were fished for one or two nights. Fish captured were identified and measured (weight and length). Laboratory studies were begun in 2002 on the maximum consumption of bull trout at various temperatures (7 - 16o C). Bull trout ranging in size from < 80 g to over 400 g were fed preyfish in laboratory tanks. Bioenergetic modeling for bull trout was started, using an energy-balance model for congeneric lake trout (Salvelinus namaycush) as a template. Preliminary analysis of this model showed that growth and consumption by bull trout were sensitive to maximum consumption and respiration parameters. Highlights and Key Findings Although over 2,000 fish were collected during field work in 2001 and 2002, none of these fish were bull trout. Failure to capture bull trout in Beulah Reservoir during this first 2 years of sampling suggests a very low abundance of bull trout in the reservoir during summer and fall. Historical records suggest that bull trout were fairly common during spring sampling, but little data were collected during summer months. Adult bull trout that may be present in Beulah Reservoir during winter and spring, likely migrate to tributaries during summer, and return to the reservoir during late summer. A variety of other species were collected and measured, including northern pikeminnow, sucker species, mountain whitefish, sculpins, and redside shiners. These species may form the preybase for bull trout that occur in the reservoir during winter or early spring months.
An important result of field sampling was collection of white crappie, which had not been detected in Beulah Reservoir previously. White crappie are a non-native species that can become rapidly established in warm reservoirs, consuming much of the zooplankton and outcompeting native species. We collected few white crappie in 2001, but found many more during sampling in 2002. The appearance of small crappie suggests that there was successful reproduction in the reservoir. In an attempt to prevent the white crappie population from expanding and potentially causing problems, the management agencies drew all water out of Beulah Reservoir in August, 2002.
Where Are We Headed In 2003 Field sampling in 2003 will concentrate shift focus and will concentrate on detection of any white crappie that might remain within the reservoir. Sampling methods will be aimed at larval and juvenile fishes, in an effort to determine if any white crappie remained after the drawdown. Water quality sampling will be continued, and some effort will be continued on characterizing the other fish populations. Laboratory feeding experiments will be continued to collect data for parameterizing the bioenergetics model. An improved model will be used to estimate the "growth rate potential" for both adult and subadult bull trout in Beulah Reservoir under different operational scenarios and conservation pool levels. Project Contact Alec Maule Email: alec_maule@usgs.gov Publications |
