Monitoring the Use of the Mainstem Columbia River by Bull Trout from the Walla Walla Basin Annual Report 2009

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.

Little is known about use of the Columbia River by bull trout from the Walla Walla Basin and

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

Plan. From October 2008 through September 2009, use of the Columbia River by Walla Walla

Basin bull trout was investigated by operating a passive integrated transponder (PIT) detection

array to monitor fish movements between the Walla Walla and Columbia rivers, tagging bull

trout in the Walla Walla River with PIT tags, and querying PTAGIS for detections at the mouth

of the Walla Walla River and at the Columbia and Snake river dams. The full stream width PIT

detection array was maintained near Oasis Road Bridge in the lower Walla Walla River. A total

of 210 bull trout from the middle and lower Walla Walla Basin were captured with screw traps,

beach seines, fyke nets, and by angling and were subsequently PIT tagged in 2009. Additional

bull trout PIT tagged by other agencies in Mill Creek (tributary to the Walla Walla River), the

South Fork Walla Walla River, and the Touchet River contributed to the pool of potentially

detectable fish. Thirteen PIT tagged bull trout were detected at the Oasis Road Bridge PIT

detection array during the year; twelve were detected moving downstream in the Walla Walla

River toward the Columbia River from November through February, and one was detected

moving upstream in the Walla Walla River past the array in June. This was the first empirical

evidence of a bull trout returning to the Walla Walla River after overwintering in the Columbia

River. Additional PIT tagged bull trout may have passed the PIT array undetected when

detection efficiencies were relatively low. Estimated detection efficiency at the PIT array was

relatively high (>80%) from October through December, 2008, but declined to <35% from

January through September, 2009 following damage to the array from high streamflow events.

Since only a small proportion of Walla Walla Basin bull trout are PIT tagged, these 13 detections

likely represent some larger number of bull trout exhibiting a similar pattern of movement.

Three bull trout PIT tagged in the Walla Walla River were detected at Columbia River dams in

2009. Two fish were detected at McNary Dam; one moving downstream in the juvenile bypass,

and one in the Oregon shore adult ladder. One fish was detected moving upstream through the

adult ladder at Priest Rapids Dam. These PIT detections at mainstem dams indicate Walla Walla

River bull trout are using the Columbia River as a migratory corridor, and for rearing and

overwintering.