Contemporary controls and future predictions of non-native smallmouth bass range expansion into salmon-rearing habitat

Non-native fishes have been stocked in freshwater ecosystems throughout the world to support recreational fishing opportunities. Since their initial introduction and establishment many of these species have spread, and future range expansions of non-native fishes in response to climate change are imminent. Although only a subset of non-native species have been shown to cause ecological harm, in some cases their impacts to recipient ecosystems are dramatic and span multiple levels of biological organization ranging from the genome to the ecosystem. Smallmouth bass (Micropterus dolomieu; hereafter referred to as bass), a cool-water fish native to central and parts of eastern North America, provide a compelling example of the potential ecosystem consequences of introducing a novel predator. Their introduction has resulted in reduced growth of other native predators and induced changes in the behaviour, distribution and at times caused the local extirpation of prey fish species. In the Pacific Northwest region of the United States, bass have been purposely stocked over the past century to promote recreational fisheries in rivers, lakes and reservoirs, where their potential to overlap with salmonids residing in headwater habitats was considered to be minimal. Many bass populations have greatly expanded from their initial introduction sites and there is increasing concern that they may now seasonally occupy upstream areas where endangered salmonids rear. Climate-induced stream temperature warming and land-use management practices are likely to enable range expansions of predatory bass farther into salmon rearing grounds, potentially adding more stress on an already highly threatened group of cold-water fishes. I combined a field observation dataset with correlative and mechanistic models to determine the contemporary controls on bass distribution, and predict the future range expansion of bass into salmon-bearing streams of the Pacific Northwest. In my study system, the John Day River (a tributary of Columbia River), I found the upstream expansion of bass into salmon rearing areas is largely controlled by water temperature. My distributional surveys revealed that bass overlap with juvenile Chinook salmon (Oncorhynchus tshawytscha) in a considerable amount of their early summer rearing habitat. Using a series a linked models, I predicted how the thermal regime of the John Day River is likely to change as a result of anticipated climate change, and in turn, how bass and juvenile Chinook salmon distributions are likely to respond to climate-induced stream warming. The results of this work suggest that bass will occupy an increasingly large portion of juvenile Chinook salmon rearing habitat in the future, and this pattern is likely to be repeated in many stream systems where these species co-exist. This is of broad conservation concern given that salmon have to deal with both the direct stress of warming streams, as well increasing spatial and temporal overlap with piscivorous bass. I also investigated the potential for riparian vegetation restoration to ameliorate some of the water temperature increases predicted due to climate warming, to provide a tool to maintain salmon rearing habitat and keep bass from expanding their range upstream. I found that restoration could provide this dual benefit and that spatially prioritized restoration will result in a greater return-on-investment in terms of reducing bass abundance while increasing thermally suitable habitat for rearing Chinook salmon in the face of climate change. In my third chapter I used physiology to examine the strong correlative relationship I observed between bass upstream extent and water temperature in Pacific Northwest streams. In this work I employed bioenergetic models, calibrated with input data from the study system, to show that bass upstream extent is determined by temperature-constrained growth of their earliest life history stage (i.e., young-of-year bass). I found that upstream, the scope for growth collapses in young-of-year bass because their ability to consume food declines rapidly as temperatures cool. Reduced growth of young bass, in turn, would lead to recruitment failure of bass populations if they spawned their young too far upstream due the size-selective overwinter mortality on age 0 bass. Adult bass exert a large degree of parental care for their young, so the distribution of the population as a whole is determined by conditions that are suitable for the growth of their progeny. By knowing which life-history stage determines bass distribution in temperate streams, managers can target strategies to diminish the growth performance of that stage to reduce the range expansion of bass into salmon habitat. Overall, this work (1) provides insight into contemporary controls on the distributions of a non-native fish in mid- to high-latitude river systems, (2) demonstrates that bass are moving into salmon habitat presently (a pattern that is likely repeated in many river systems in the Pacific Northwest where bass are broadly distributed), and (3) provides management relevant tools to anticipate and prevent the future range expansion of bass farther into salmon-bearing streams. In my final chapter I broaden my focus of freshwater conservation and assess the national scale potential of National Parks to serve as protected areas for native U.S. fish. I found that although most parks were not designed with freshwater conservation in mind, nearly two-thirds (62%) of native US fishes reside in national parks. Using a conservation planning approach that integrated fish diversity representation provided by parks and their current and future ecological threats and management challenges, I identified 50 parks that could serve as a foundation for a nationally comprehensive freshwater protected area system. While the NPS has limitations as the potential basis for an aquatic protected area network, I argue it provides considerable representation of freshwater fish diversity that should be taken into account during systematic conservation planning for freshwaters.