Carassius auratus gibelio—The most successful invasive fish in waters of the Czech Republic

Carassius auratus gibelio is considered an alien fish species in the hydrographic system of the Czech Republic. Around 1976, this form immigrated from the River Danube into the confluence of the Morava and Dyje rivers and gradually spread over the hydrographic network over the course of the next 15 to 20 years. Helped by man, this species overcame the boundaries between particular drainage areas, expanding its range and colonising suitable habitats. C. a. gibelio has become fully naturalised and has produced numerous stable populations. This is the only non-indigenous fish species in the Czech Republic showing distinct characteristics of an invasive taxon. Initial populations of C. a. gibelio colonising the new area consisted solely of triploid females. The occurrence of males after 1990 started a process of transformation of the originally monosexual female (triploid) population into a mixed population containing both females and males. At present, males are predominantly diploid, with occasional triploid individuals. Females are predominantly triploid and less frequently diploid. Tetraploid individuals are quite rare. At the Morava-Dyje confluence, the population reproduces both gynogenetically and sexually. C. a. gibelio exerts important competitive, as well as destructive, effects upon the indigenous ichthyofauna. Native species of Carassius carassius and Tinca tinca , previously abundant have vanished from those localities dominated by C. a. gibelio. C. a. gibelio is fished for on hook and line in natural habitats in the Czech Republic, with an annual recorded catch of 25–50 t. Annual production in fishponds varies between 15 and 70 t, yet its marketable utilization is problematic.