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Ecology

Bull Trout Studies, Lower Snake River and McNary Reservoir

PNNL conducted and prepared a synthesis report on bull trout in the lower Snake River and McNary reservoir within the mainstem from Hells Canyon Dam downstream to McNary Dam. Research studies also include the tributaries to the lower Snake River and McNary Reservoir including the Tucannon, Walla Walla, Touchet, Grande Ronde, Imnaha, Salmon, Umatilla, and Yakima rivers, and Asotin Creek. In addition to preparing a synthesis report, PNNL installed a 5-ft.-diameter rotary screw trap in the lower Walla Walla River 9.3 miles upstream from its confluence with the Columbia River and monitored it in the fall of 2003 and the spring of 2004 to determine if sub-adult bull trout migrate out of the Walla Walla River into the Columbia River.

The trap was operated from November 24 to the end of December 2003 and from March 17 to May 28, 2004. No bull trout were captured during either time period. The trap was fished 432 hours during the fall and 1190 hours during the spring for a total of 1622 hours. A total of 6536 fish were captured in the screw trap during the course of the study. During fall sampling, 2818 fish were caught at a rate of 6.5 fish per hour. During spring sampling, 3718 fish were captured at a rate of 3.1 fish per hour. A majority (4707 or 72%) of the fish captured were suckers (Catastomus spp.). Steelhead (Oncorhynchus mykiss) were the next most abundant species (612 captured, or 9.4%) followed by bass spp. (Micropterus spp, 451 captured, or 6.9%) and Chinook salmon (Oncorhynchus tshawytscha, 228 captured, or 3.5%).

Trapping efficiency varied among trapping weeks. Over the entire season, trapping efficiency was thus 5% for Chinook salmon and 4.8% for steelhead. Although no bull trout were captured due to low trap efficiency we can not conclude that bull trout do not migrate out of the Walla Walla system. Based upon these results it is recommended that trapping continue in subsequent years during the fall and early winter with a 5-ft.-diameter rotary screw trap. In addition, we suggest that a larger screw trap or perhaps multiple screw traps be used during the higher spring flows to increase trapping efficiency.

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