Nesting microhabitat use and hybridisation of endemic Bartram's Bass in Savannah River tributaries

Bartram's Bass Micropterus sp. cf. cataractae is a rare black bass endemic to the Savannah River of the south‐eastern United States and is threatened by habitat degradation and hybridisation with nonnative Alabama Bass Micropterus henshalli . While Bartram's Bass has been functionally extirpated from reservoirs, some populations persist in tributaries. Understanding the mechanisms affecting hybridisation in streams will help to inform conservation. Our objectives were to (a) compare spawning microhabitat use of Bartram's Bass between pure and hybridised individuals, (b) infer pure and hybridised spawning based on assessment of young‐of‐year (YOY) individuals, and (c) compare abiotic variables between sites where pure YOY Bartram's Bass were collected and where they were not collected. We snorkelled 27 sites in 2017 and 2018, recording microhabitat variables at each nest and at randomly selected paired locations. We then electrofished for YOY bass later in the summer. We found that Bartram's Bass selected for slower current velocities but not depths when spawning. Bartram's Bass nested on a variety of substrates associated with slow current velocity. Microhabitat variables did not differ between nests with pure individuals and those with hybrids; differences in tributary spawning habitat use are not likely to prevent hybridisation. We found widespread hybridisation in tributaries across the basin. Compared with sites where hybrid YOY were detected, sites with pure Bartram's Bass were in watersheds and local catchments that were more forested and less developed, and were higher in elevation with steeper gradients. Maintaining habitat integrity in locations where pure populations remain may help protect this rare species.