Black Bullhead Removal from a Headwater Trout Stream in Southern California

Abstract

AbstractThe introduction of invasive species has resulted in negative impacts to aquatic ecosystems and is cited as the second leading cause of fish extinctions. In the southwestern United States, low diversity and high levels of endemism increase the deleterious effects of these introductions. Near the southern edge of their range, the last remaining population of native Coastal Rainbow Trout Oncorhynchus mykiss irideus in San Diego County, California, inhabits the West Fork San Luis Rey River (WFSLRR). The nonnative Black Bullhead Ameiurus melas was discovered in the WFSLRR in 2007, elevating concern over this small population of Coastal Rainbow Trout. Preliminary surveys found that baited minnow traps set overnight were effective at capturing Black Bullhead; therefore, a removal project was initiated. Crew members set baited traps at the upstream end of the Coastal Rainbow Trout distribution in the evening (maximum = 72 traps/trip) and checked the traps the following morning. Four two‐night trips were completed in 2016, removing 1,315 Black Bullhead. The initial Black Bullhead population was estimated as 1,361 fish by a depletion function. Follow‐up surveys in 2017 and 2018 captured no Black Bullhead. Prolonged drought conditions in California during the removal effort may have concentrated the Black Bullhead into pools, facilitating this rare complete eradication of an invasive fish species.