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

A significant gap in our knowledge of migratory bull trout Salvelinus confluentus life history is

associated with their use of the mainstem Columbia and Snake rivers. Few data are available

regarding movements within the mainstem, the use of various mainstem habitats, or bull trout

presence and passage at mainstem dams. We conducted our sampling effort for bull trout from

October 2010 through February 2011 during the time period when most of the emigration from

the Walla Walla Basin occurs. Twelve bull trout were captured between 9 November 2010 and

18 February 2011, all of which were subsequently tagged with PT-4 acoustic transmitters and

full duplex ISO 134 kHz PIT tags. Tagged bull trout ranged in fork length from 225 to 311 mm

and their weight varied from 135.0 to 281.1 g. Seven of the tagged bull trout were detected

entering the Columbia River from November through February. Two of these fish were located

during mobile tracking surveys between the mouth of the Walla Walla River and McNary Dam.

Both fish used mainstem habitats that ranged from 12 to 20 m in depth, with an average water

velocity of 0.37 m/s. Four of the seven bull trout that entered the Columbia River were

subsequently detected returning to the Walla Walla River between March and June 2011. No

acoustic tagged or PIT tagged bull trout from the Walla Walla Basin have been detected to date,

at or near any mainstem dams since tagging began in the fall of 2010. A PIT tag from one of the

bull trout tagged for this study was recovered on Foundation Island, apparently a consequence of

avian predation. Weather and river conditions limited our ability to conduct mobile tracking

surveys in the Columbia River to determine the extent of movements and distribution, and to

collect detailed habitat use data.