Analyzing Tradeoffs Between the Threat of Invasion by Brook Trout and Effects of Intentional Isolation for Native Westslope Cutthroat Trout

Native fishes often face simultaneous threats from habitat fragmentation and invasion by nonnative trout. Unfortunately, management actions to address one may create or exacerbate the other. A consistent decision process would include a systematic analysis of when and where intentional use or removal of barriers is most appropriate. We developed a Bayesian belief network (BBN) as a tool for such analyses. We focused on native westslope cutthroat trout (Oncorhynchus clarkii lewisi) and nonnative brook trout (Salvelinus fontinalis). We considered the environmental factors influencing them, their potential interactions, and the effects of isolation on the persistence of local cutthroat trout populations. The tradeoffs between isolation and invasion were strongly influenced by the size and quality of the stream network to be isolated and existing demographic linkages within and among populations. A strength of our approach was that it captured interactions where effects would otherwise difficult to visualize. The model can be used to conduct site-level analysis of barrier management relative to other possible conservation actions, such as habitat improvement or angling restrictions, and to help prioritize actions among streams. By eliciting precise definitions of conservation priorities, the model can also help clarify management objectives and facilitate communication among biologists, managers, and the public. Introduction Conservation of inland cutthroat trout (Oncorhynchus clarkii spp.) can involve either the placement or removal of migration barriers to address threats from invading species and habitat fragmentation. There are important tradeoffs, because barriers that may limit invasion can also isolate a native population making it more vulnerable to local extinction through a variety of processes. Projects to install or remove barriers may proceed without a formal analysis that considers potential tradeoffs from addressing these competing threats. Because resources for conservation management are limited, effective prioritization is important. Tradeoffs may be relatively clear to biologists and managers with intimate knowledge of a particular system, and their efforts can be focused effectively. Elsewhere, the tradeoffs may be more ambiguous or the data and experience more limited, and the result may be a decision that is influenced more by personal philosophy or public pressure than by knowledge. When the differences in these choices cannot be clearly supported and articulated, the decision process can appear inconsistent and arbitrary to the public or the administrators who fund these projects. A consistent decision process would include an analysis of the relative risks associated with either action. Fausch et al. (2006) provided a synthesis of the science and theory relevant to this issue and proposed a framework that could help guide an appropriate analysis. The analysis could be complex because of the potential interaction of multiple physical and ecological processes. For example, the probability that brook trout (Salvelinus fontinalis) may invade and displace cutthroat trout from any stream may depend on the physical characteristics defining the suitability of stream habitat for either species (e.g. Peterson et al. 2004), the condition of that habitat (e.g. Shepard 2004), the size, connectivity and complexity of the available habitat network (Rieman and Dunham 2000), distance of potential source populations, fishing pressure, and their interactions (Figure 1). Many biologists may inherently understand those processes for systems they have studied in detail, but consistent evaluation of these processes across multiple populations and environments could be improved by a formal assessment tool. We explored the application of a Bayesian belief network (BBN) as a tool to facilitate such analyses (Cain 2001). We focused on westslope cutthroat trout (Oncorhynchus clarkii lewisi; hereafter WCT) and nonnative brook trout and current understanding of environmental factors