Bull trout in the Boundary System: managing connectivity and the feasibility of a reintroduction in the lower Pend Oreille River, northeastern Washington

Many of the World’s rivers are influenced by large dams (>15 m high) most of which have fragmented formerly continuous habitats, and significantly altered fish passage, natural flow, temperature, and sediment fluxes (Nilsson and others, 2005; Arthington, 2012; Liermann and others, 2012). In the Pacific Northwest, dams on major rivers have been a major focus for fishery managers, primarily in regard to passage of anadromous salmonids (principally Pacific salmon and steelhead trout [Oncorhynchus mykiss], for example, Ferguson and others, 2011), but more recently other species, such as Pacific lamprey (Entosphenus tridentatus) and resident (non-anadromous) salmonids, are receiving more attention (Neraas and Spruell, 2001; Moser and others, 2002; Muhlfeld and others, 2012). In the case of resident salmonids, fish can adopt a wide range of migratory behaviors that often bring them into mainstem rivers where they can come into direct contact with large dams. When this occurs, some of the most important direct effects of dams on salmonids include barriers to upstream and downstream movement and mortality associated with entrainment within the dam or spill over dams. Biologically, these direct impacts can lead to (1) disruption of natural historical (pre-dam) genetic and demographic connectivity among local populations, (2) loss of access to historically used migratory destinations, (3) loss of individuals to the population through mortality associated with entrainment.

In this report, we address these issues for the case of Boundary Dam, located immediately south of the Canadian border on the lower Pend Oreille River in northeastern Washington (fig. 1). Specifically, we addressed the following objectives:

• Evaluate the justification for bull trout (Salvelinus confluentus) passing over Boundary Dam in the context of likely historical patterns of gene flow that occurred prior to dam construction, current patterns of movement, and status of existing populations.
• Assess the role of passage over Boundary Dam, in the context of other factors in the system that may influence the feasibility of establishing a self-sustaining bull trout population in the Boundary system.