Developing geospatial tools to identify refuges from alien trout invasion in Australia to assist freshwater conservation

Context: Introduced fish have caused significant range reductions for many native fish, with many threatened species now found in headwater refuges, protected by in-stream barriers such as waterfalls, weirs and culverts. Owing to the remoteness of such refuges, distribution of many native species is poorly understood despite the urgency of determining their distribution because of threats posed by the spread of introduced fish into these refuges. Aims: We investigated the application of emerging remote-sensing technology (LiDAR) to improve our ability to locate potential invasion barriers and identify headwater refuges. Methods: We used LiDAR-derived digital elevation models to find likely barriers, and conducted fish surveys to determine introduced trout passability and distribution in tributary headwaters. Key results: Trout were rarely observed upstream of waterfalls with a gradient of >0.82, whereas native galaxiids were found only in the absence of trout. Of 17 trout barriers surveyed, 9 supported a population of galaxiids upstream, whereas 8 were fishless. Implications: LiDAR-based analysis is an effective tool for preliminary site selection and prioritisation for freshwater fish conservation. Discovery of three new populations of galaxiids in this study demonstrates the potential of this technique to locate additional trout-free headwater streams, important for threatened galaxiids and other trout-sensitive aquatic species.