Freshwater fishes are highly vulnerable to human-caused climate change, resulting in rapid changes in status. Because quantitative data on status and trends are unavailable for most fish species, a rapid assessment approach that incorporates expert knowledge is needed to assess current status and future vulnerability. In this study, we present a method that allows systematic evaluation of potential climate change effects on freshwater fishes, using California as an example. The method uses expert knowledge of the authors, supported by literature reviews of status and biology of the fishes, to score ten metrics for both (1) current status of each species (baseline vulnerability to extinction) and (2) likely future impacts of climate change (climate change vulnerability to extinction). Baseline and climate change vulnerability scores were derived for 121 native and 43 nonnative (alien) fish species. The two scores were highly correlated with one another and were consistent among different scorers. Native species had both greater baseline and greater climate change vulnerability than alien species. Fifty percent of California’s native fish fauna was assessed as having critical or high baseline vulnerability to extinction whereas all alien species were classified as being less or least vulnerable. With respect to future climate change, 82% of native species were classified as highly vulnerable versus 19% for aliens. Our results show that predicted climate change effects on freshwater environments will dramatically change the fish fauna of California. Most of California’s native fishes will suffer population declines and become more restricted in their distributions; some will likely be driven to extinction if present trends continue. Fishes requiring cold water (<22°C) are particularly likely to go extinct. In contrast, most alien fishes will thrive, with many species likely to increase in abundance and range. However, some alien species will likewise be negatively affected through loss of aquatic habitats during severe droughts and physiologically stressful conditions in most waterways during the low flow summer months. The method presented here has high utility for predicting vulnerability to climate change of diverse fish species, so should be useful for setting conservation priorities.
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