Evaluate Bull Trout Movements in the Tucannon And Lower Snake Rivers, 2004

We sampled and released 313 bull trout (Salvelinus confluentus) from the Tucannon River in 2004. Passive Integrated Transponder (PIT) tags were inserted in 231 of these individuals, and we detected existing PIT tags in an additional 44 bull trout. Twenty-five of these were also surgically implanted with radio-tags, and we monitored the movements of these fish throughout the year.

Ten bull trout that were radio-tagged in 2003 were known to survive and carry their tags through the spring of 2004. One of these fish outmigrated into the Snake River in the fall, and remained undetected until February, when it’s tag was located near the confluence of Alkali Flat Creek and the Snake River. The remaining 9 fish spent the winter between Tucannon River miles 2.1 (Powers Road) and 36.0 (Tucannon Fish Hatchery). Seven of these fish retained their tags through the summer, and migrated to known spawning habitat prior to September 2004.

During June and July, radio-tagged bull trout again exhibited a general upstream movement into the upper reaches of the Tucannon subbasin. As in past years, we observed some downstream movements of radio-tagged bull trout in mid to late September and throughout October, suggesting post spawning outmigrations. By late November and early December, radio tagged bull trout were relatively stationary, and were distributed from river mile 42 at Camp Wooten downstream to river mile 17, near the Highway 12 bridge. As in previous years, we did not collect data associated with objectives 2, 3, or 4 of this study, because we were unable to monitor migratory movement of radio-tagged bull trout into the vicinity of the hydropower dams on the main stem Snake River.

Transmission tests of submerged Lotek model NTC-6-2 nano-tags in Lower Granite Pool showed that audible detection and individual tag identification was possible at depths of 20, 30, and 40 ft. We were able to maintain tag detection and code separation at all depths from both a boat and 200 ft. above water surface in a helicopter. However, we lost detection capability from 40 ft. water depth when we passed 700 ft. above the water surface in a helicopter.

Two years of high tag loss, particularly after spawning, has prevented us from documenting fall and winter movements with an adequate sample of radio tagged bull trout. The high transmitter loss after spawning may be a reflection of high natural mortality for large, older age fish that we have been radio tagging to accommodate the longer life transmitters. Therefore, we reduced the size of the radio tags that we implanted, and delayed most of our collection and tagging of bull trout until after spawning. These changes are a new approach to try to maximize the number of radio tagged bull trout available post spawning to adequately document fall and winter movements and any use of the Snake River by bull trout from the Tucannon River.