2016-2019 Report-Brook Trout in the Walla Walla River Basin - Assessing the Conservation Risks to Bull Trout

The presence of self-sustaining populations of nonnative Brook Trout (Salvelinus fontinalis) in stream systems can potentially threaten the long-term persistence of Bull Trout (Salvelinus confluentus). A crucial component of the Bull Trout Recovery Plan involves identifying potential threats and improving our understanding of how those threats may affect the species. The potential for the nonnative Brook Trout population in the spring branches of the East Little Walla Walla River to expand to neighboring tributaries is a concern because further upstream expansion could threaten ESA-listed Bull Trout strongholds in the South Fork Walla Walla River and Mill Creek. When the Recovery Plan was finalized, information was lacking to accurately assess the conservation risks Brook Trout posed to Bull Trout in the Walla Walla River Basin. However, Recovery Action 4.3.1 specifically called for additional research into the distribution of Brook Trout in the spring branch tributaries and to evaluate the need for control. In 2016, the Columbia River Fish and Wildlife Conservation Office, initiated a study to gain a better understanding of Brook Trout distribution, gather spawning information, and monitor movements within the spring branch tributaries and to the mainstem Walla Walla River. Results from distribution sampling indicated that Brook Trout of multiple age classes were widely distributed throughout the East Big Spring Branch, however, Brook Trout were also found in portions of the West Big Spring Branch, the East Little Walla Walla River and the mainstem Walla Walla River at least seasonally. Of the 161 Brook Trout we PIT-tagged, the majority did not emigrate from the East Big Spring Branch. However, those that did (6.2%), primarily used the East Little Walla Walla River during the winter months as habitat linkages to connect with other habitat or to enter the mainstem Walla Walla River. Detection histories suggest six (3.7%) of the fish entered the mainstem Walla Walla River. Due to the limited nature of our tagging efforts, PIT-tagged fish represented the movements of an unknown, but likely much larger number of untagged individuals. PIT detection data also indicated that juvenile Chinook salmon (Oncorhynchus tshawytscha), adult steelhead (Oncorhynchus mykiss), Bull Trout, and large numbers of juvenile steelhead used the East Little Walla Walla River and the spring branch tributaries seasonally, primarily between the months of November and April, but fish were either sampled or detected in those areas during all seasons. Redd surveys were only conducted in a small portion (16%) of the East Big Spring Branch. The seven redds found only partially characterized spawning within the entire study area. In summary, this Brook Trout population demonstrates connectivity to the mainstem Walla Walla River, heightening the risk of colonizing nearby tributaries or expanding upstream into Bull Trout spawning and rearing habitat.