North Fork Walla Walla River Bull Trout Occupancy and Habitat Use Assessment FY 2012-2013 Annual Report

Effective management and eventual recovery of the threatened bull trout (Salvelinus confluentus) requires a sufficient knowledge of their spatial distribution and the ability to assess seasonal movements and habitat use for all life-history stages and strategies. This assessment is a component of a broader effort to describe bull trout habitat use, distribution, migratory patterns and connectivity between bull trout populations within the Walla Walla River Basin. We employed a multifaceted approach to investigate bull trout use of the North Fork Walla Walla River (NFWWR) in northeastern Oregon. We used a full duplex PIT detection array near the mouth of the NFWWR to describe seasonal use by migratory bull trout between 18 May 2012 and 19 April 2013. A total of 16 individual PIT-tagged bull trout were detected at the PIT detection array from 24 May 2012 to 10 December 2012. An examination of PIT detection histories for these fish revealed that both adult and subadult bull trout entered the NFWWR during fall and winter months, likely to utilize overwintering habitat within the subbasin. We also found that some of the subadult bull trout that dispersed from their natal headwaters in the South Fork Walla Walla River during the spring subsequently entered the NFWWR instead of continuing to downstream rearing areas in the mainstem Walla Walla River. Detection histories also revealed that a portion of the adult-sized bull trout that typically migrate from overwintering areas in middle and lower reaches of the mainstem Walla Walla River entered the NFWWR during May and June instead of continuing with the rest of the migratory population to known spawning grounds in the South Fork Walla Walla River. These fish resided within the NFWWR subbasin from 11 to 30 days (average 21.3 days) before exiting on the declining limb of the hydrograph in late June and early July. Several of these fish subsequently moved rapidly to known spawning reaches in the South Fork Walla Walla River. We also conducted a single bull trout redd survey in the headwater reaches of the NFWWR to determine if spawning currently occurs within the subbasin. Four small, unoccupied redds were found on 17 October 2012. The redds may have been constructed by resident-sized bull trout, but this could not be confirmed. In addition to the PIT detection array and the redd survey, we applied Recovery Monitoring and Evaluation Group (RMEG) guidance on monitoring bull trout occupancy and distribution. Occupancy and distribution sampling was conducted using backpack electrofishing between 26 June 2012 and 28 June 2012. A total of seven 50 meter reaches were sampled within the NFWWR patch. No bull trout were captured or observed during this effort. Collectively, our findings help elucidate the importance of understanding bull trout use of tributary habitat and its role in bull trout recovery.