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

Little is known about use of the Columbia River by bull trout Salvelinus confluentus from the

Walla Walla Basin. Mainstem Snake and Columbia River dams have the potential to impact

both the connectivity between bull trout Core Areas (metapopulations), and the connectivity

within migratory corridors. The need for research and monitoring of bull trout use of the

Columbia and Snake rivers is identified in the U.S. Fish and Wildlife Service Biological

Opinion. From April 2005 through December 2009, use of the Columbia River by Walla Walla

Basin bull trout was investigated by operating a Passive Integrated Transponder (PIT) detection

array at Oasis Road Bridge (ORB) near the mouth of the Walla Walla River to determine bull

trout abundance and describe the migration timing between the Walla Walla and Columbia

rivers. A quantitative estimate of the number of migratory Walla Walla Basin bull trout that

moved into the Columbia River over the duration of this study was developed by utilizing

empirical data consisting of monthly array detections in combination with physical detection

efficiencies and annual estimates of the proportion of the population that was tagged based on

hook and line sampling. Migration timing periodicity was described by examining detections at

the ORB detection array, observations from a screw trap operated near the mouth of the Walla

Walla River, and detections at mainstem dams. Migration timing was also examined for

temporal patterns of activity, including peak migration periods, using abundance estimates. Data

were insufficient to estimate migratory abundance during 2005 and 2006. We estimate a total of

192 bull trout emigrated from the Walla Walla Basin to the Columbia River from November

2007 through December 2009. Over the duration of the study, only one bull trout was detected

returning to the Walla Walla River from the Columbia River. We did not attempt to make any

estimates of the total number of returning adults for the single detection in June 2009. The

timing of migratory bull trout emigration from the Walla Walla River to the Columbia River

varied from year to year, but overall, occurred from October through May, peaking between

November and February. Variation in streamflow patterns across migration seasons appears to

influence emigration timing to a greater extent than changing water temperatures. The single

detection of a returning bull trout in June was similar to upstream migration timing observations

at other PIT detection arrays throughout the Walla Walla Basin, and other historic observations

of upstream migrants. Four Walla Walla Basin bull trout were detected at mainstem Columbia

River dams over the duration of this study. Detections in the juvenile bypass systems at John

Day and McNary dams indicated two of these bull trout were moving downstream. Detections in

the adult fish ladders at McNary and Priest Rapids dams indicated two of these bull trout were

moving upstream. Walla Walla Basin connectivity with the mainstem Columbia River, and

connectivity within the Columbia River migratory corridor is required to maintain genetic

diversity of the Core Area metapopulations in the Columbia River Distinct Population Segment,

and for re-colonization of areas where local populations have been extirpated by natural

stochastic events or impacts from human-related activities. Future work should include

monitoring efforts in the mainstem Columbia River to provide a more detailed understanding of

the temporal and spatial aspects of mainstem habitat use and migration by Walla Walla Basin

bull trout, and how they interact with the dams and reservoirs of the Federal Columbia River

Power System.