Growth and Recruitment of Nonnative Smallmouth Bass along the Upstream Edge of Its Riverine Distribution

Abstract Nonnative species have been widely introduced, and once established, often exhibit secondary spread to new areas. For instance, after its initial introduction in the John Day River, Oregon, smallmouth bass (Micropterus dolomieu) has expanded upstream into headwater habitats. Recruitment is a key component of successful range expansion and has been highlighted as a potential bottleneck to continued expansion by smallmouth bass. We explored growth, body lengths, and survival of young-of-the-year (YOY) smallmouth bass in the North Fork John Day River to better understand the recruitment dynamics near its invasion boundary. In 2014–2015, we collected YOY across the upstream 63 km of smallmouth bass distribution at the end of the first growth season and after a winter starvation period. We found that growth, body length, and survival showed varied correspondence with patterns in water temperature. Specifically, body lengths matched temperature predictions in upstream sites (after accounting for spawning delays) where smallmouth bass density is low. By contrast, individuals achieved smaller than predicted body lengths in downstream sites where density is relatively high. Model selection revealed that temperature and age ≥ 1 density were the most important predictors of body length. Additionally, individuals predicted to be too small to survive a winter starvation period were present. Our findings reveal nuanced recruitment dynamics at the invasion boundary, where departures from temperature-based predictions point to multiple mechanisms affecting growth and survival. Understanding mechanisms operating at invasion boundaries may help develop management strategies to prevent future spread of smallmouth bass into headwater salmon habitat.