Managing invasive alien crayfish in isolated water bodies: Evaluating control strategies for biodiversity conservation

Invasive alien crayfish pose significant threats to biodiversity in aquatic ecosystems. In isolated water bodies they impact local amphibian populations by predation, and altering water quality due to burrowing behaviour. Eight pools in either natural and urban areas were selected, where either signal crayfish, red swamp crayfish, or both species occurred. During six years, we assessed three methods to reduce crayfish abundances: biannual crayfish trapping and removal, trapping combined with drainage of pools, and trapping combined with introducing eels as predators. Trapping alone was insufficient to stagnate population growth of red swamp crayfish, though it slowed. Trapping + draining proved ineffective, since red swamp crayfish hid in burrows and aquatic vegetation. The trapping + eels method, reduced population growth and mean length of the red swamp crayfish. Furthermore, we observed a strong population decline in signal crayfish when the red swamp crayfish colonised a pool. Our findings demonstrate that crayfish negatively impact populations of several amphibian species. In natural pools, amphibian numbers increased when crayfish numbers decreased. Urban pools, which showed an increase in crayfish numbers, displayed a decline in amphibians. A combination of predator introduction and removal of crayfish could provide a sustainable solution for controlling crayfish populations and improving amphibian habitats.