Current status of the non-native Channel Catfish, <i>Ictalurus punctatus</i> in the Yahagi River: Re-evaluation of the species’ distribution incorporating the reliability of occurrence information

<p> Abstract: Recently, the distribution of the Channel Catfish, Ictalurus punctatus, an invasive alien species, has been expanding in several water systems in Japan. The species’ first occurrence in the Yahagi River was confirmed in 2005. Since then, a local institute and citizen groups have conducted collection surveys to eliminate the species, catching over 100 individuals in the middle reaches. However, no surveys of the species have been conducted in the river since 2015; therefore, its current status is unknown. Additionally, although several occurrence records of the species in the river have been published, including progress reports of the collection surveys, the contents and accuracy of information provided in these documents vary, with some discrepancies in descriptions that appear to refer to the same occurrence record. To eradicate the species, it is essential to assess the reliability of individual records, incorporate differences among them, and estimate the species distribution before planning a control program. This study used the following four methods to collect information about the spatial extent of the species’ occurrence (i.e., captures and sightings) in the Yahagi River system: (1) a literature review; (2) information collected in public databases; (3) re-examination of previous collection survey data; and (4) information collection targeting the public (e.g., residents and anglers). The database search covered the River Environmental Database, which collects data from the National Census on River and Dam Environments conducted by the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT). To collect information from the public, leaflets and cards calling for information on the species were designed and distributed; face-to-face and online interviews were conducted. Based on the information obtained, we have confirmed reliable occurrence records for 122 individuals collected in the area from the Azuri Dam reservoir (Toyota City, Aichi Prefecture, approximately 53 km upstream from the river mouth) to the backwater of the Meiji Weir (35 km upstream from the river mouth). Fourteen of these individuals (12%) were provided by citizens (e.g., fishermen and anglers). Capture records and sighting reports (with relatively low reliability) were collected in the area from the Yahagi Dam reservoir (approximately 70 km upstream from the river mouth) to the Yahagi-Furukawa River branching point (Nishio City, Aichi Prefecture, 13 km upstream from the river mouth). Obtained occurrence information was concentrated in the middle part of the river, likely due to sampling bias. Therefore, a systematic quantitative survey over a wide area at multiple sites will be required to identify the species’ distribution range in the future.</p>