Field sampling, inquiry to fishermen, and DNA analysis were used to determine the distribution and origin of the bagrid catfish Tachysurus nudiceps in the Oita River system, central Oita Prefecture, Kyushu Island, Japan. Specimens were collected from only three adjacent localities (nine localities sampled) in the river system. Localities the species found were cramped and limited in the river system, despite the potential for a greater distribution range. Reproduction in the wild was inferred from the juveniles collected in October 2012 and June 2014. Capture records of the Oita River Fishermen’s Association suggested that the species first appeared in the Oita River system in 2004. Stocking records of the association indicated that Funa (Carassius sp.) seedlings from a pond in the Yakkan River system (within the native range of T. nudiceps; northern Oita Prefecture) were released in 2000, 2001 and 2003. Further inquiries also revealed contamination of the Funa seedlings with T. nudiceps and other species. Subsequent sampling of the pond showed the continued presence of T. nudiceps. A comparison of DNA data from Oita River specimens with published data for seventeen other localities in Japan were consistent with the scenario of accidental introduction of T. nudiceps together with Funa seedlings. These results suggest that T. nudiceps is not indigenous to the Oita River system, and its occurrence likely resulted from the introduction of contaminated Funa seedlings released in the early 2000s. Introduced T. nudiceps likely compete with native species for some resources, particularly with the sleeper Eleotris oxycephala for food and spawning sites in the Oita River system. The establishment of the former in the Oita River system and future ramifications for the freshwater fish fauna of northeastern Kyushu should be noted.
