Using the Judas technique to locate and remove wintertime aggregations of invasive common carp

Radio- and acoustic telemetry in three Midwestern lakes demonstrated that common carp, Cyprinus carpio L., aggregate as water temperatures descend below 10 degrees C. Particularly dense aggregations formed at temperatures <5 degrees C, and once located, these aggregations could be removed with an efficiency of up to 94% using seine nets. Carp aggregated just below the surface of the ice (approximately 1.5 m) and rarely descended to warmer waters, which extended down to 10 m. Although aggregations consistently formed close to shore, their locations could not be explained by temperature or dissolved oxygen. The aggregations also moved frequently, making radio-tagged fish invaluable to locate them. Coldwater aggregations of carp may reflect a type of shoaling behaviour and can be exploited with the aid of radio-tagged (Judas) fish to control this invasive fish effectively. Similar approaches might be developed for other gregarious invasive fishes.