Monitoring the Use of the Mainstem Columbia River by Bull Trout from the Walla Walla Basin Annual Report 2007

Bull trout distribution, abundance and habitat quality have declined range wide and several local

extirpations have been documented. As a result, the Columbia River Distinct Population

Segment of bull trout was listed as threatened under the Endangered Species Act in June 1998.

Mainstem Snake and Columbia River dams have the potential to impact migratory bull trout.

Use of the Columbia River by bull trout from the Walla Walla Basin is unknown and the need

for further research is identified in the U.S. Fish and Wildlife Service Draft Recovery Plan. A

full stream width passive integrated transponder (PIT) detection array was maintained in the

lower Walla Walla River and used to monitor use of the Columbia River by bull trout from the

Walla Walla Basin from October 2006 through September 2007. A second detection array with

a different design was installed approximately 25 m upstream from the existing array in October

2006 and operated until May 2007. The U.S. Fish and Wildlife Service PIT tagged 12 bull trout

in the mainstem Walla Walla River from May through September 2007. Additional bull trout

tagged by other agencies in Mill Creek (tributary to the Walla Walla River), the South Fork

Walla Walla River, and the Touchet River were also available for detection. One PIT tagged

bull trout from the Walla Walla Basin was detected moving through the lower Walla Walla River

toward the Columbia River in January 2007. This is the first empirical evidence of Walla Walla

Basin bull trout using the Columbia River. Additional PIT tagged bull trout may have passed the

array undetected when detection efficiencies were relatively low. Since only a small proportion

of Walla Walla Basin migratory bull trout are PIT tagged, this single detection likely represents

some larger number of bull trout exhibiting a similar pattern of movement. No PIT tagged bull

trout were detected at McNary or Ice Harbor dams during the year.