Using the Judas Technique to Locate and Remove Invasive Common Carp

The Judas technique employs a small number of radiotagged individuals to locate and remove natural aggregations of invasive species. This technique has been successfully used to control invasive mammals and birds and in this study we present how it is applied to locate and remove seasonal aggregations of the common carp (Cyprinus carpio), world’s most invasive fish. Radio- and acoustic telemetry in three Midwestern lakes demonstrated that although carp are relatively dispersed during summers, they aggregate as water temperatures descend below 10°C. Particularly, dense aggregations formed at temperatures <5°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 in locating them. Coldwater aggregations of carp may reflect a type of shoaling behavior and can be exploited with the aid of radiotagged (Judas) fish to control this invasive fish effectively. Similar approaches might be developed for other gregarious invasive fishes.