The Downside of Aquatic Connectivity

Much of the emphasis in watershed management involves restoring the connectivity in riverscapes that has been lost due to dams, road culverts or habitat degradation.  Improving connectivity helps restore migratory pathways important to many native species.  But the downside to enhanced connectivity is the increased opportunity for colonization by nonnative species, some of which can cause ecological harm.  Efforts to alter connectivity will be discussed in relation to a conceptual model that views invasions as a series of stages with varying probabilities of success.  A challenge for managers is to reduce the probability of a nonnative species spreading beyond its original beachhead and this generally means reducing the connectivity among aquatic systems. Options for maintaining connectivity for native species while simultaneously reducing connectivity for nonnative species can be considered in the context of non-selective versus selective fish movement barriers.  Non-selective barriers block movements of all fishes and are problematic when native species with migratory behaviors are present.  Such barriers can be made more selective by allowing fish to pass into a holding facility where manual sorting can be used to allow passage of native species while nonnative species can be culled.  Selective barriers rely on differences in fish swimming or jumping ability to allow passage of native species while preventing passage of nonnative species.  Examples of selective and non-selective barriers will be discussed with a focus on connectivity issues in the management of native cutthroat trout populations in the western United States.