Invasive catfish linked to the decline of freshwater crayfish in two Aotearoa New Zealand lakes

Abstract

The introduction of invasive species can have profound impacts on ecosystems, especially where endemic species have evolved in isolation. However, determining the influence of invasive species on native biota can be difficult because long-term data are lacking. Our study examines the impact of invasive brown bullhead catfish (Ameiurus nebulosus) on native kōura (freshwater crayfish, Paranephrops planifrons) populations in two North Island, Aotearoa New Zealand lakes (Rotoiti and Rotorua). The invaded lakes were compared with two control lakes (Ōkāreka and Rotokākahi) where catfish are absent using a quasi-BACI (Before-After, Control-Impact) design. Long-term monitoring data in the invaded lakes (2005–2024) show significant declines in kōura abundances and biomass with changes in size structure after catfish establishment. In contrast, kōura populations in control lakes showed no significant changes in abundance or biomass in 2009 compared to 2021. Environmental variables (water quality, dissolved oxygen, and invasive macrophytes) showed minimal change in the same period. Netting data showed catfish abundances increased as kōura numbers fell, supporting the conclusion that catfish predation—rather than broader environmental degradation—is the main driver of kōura decline. The impact of catfish on kōura has severe cultural ramifications, to the extent that customary Māori fisheries are no longer viable in Lake Rotoiti. Restoration of kōura populations will require catfish control, habitat enhancement, and ongoing monitoring to prevent further spread of invasive species. More broadly, integrated invasive species management and habitat restoration are crucial to protect culturally and ecologically significant native species in Aotearoa New Zealand’s freshwater ecosystems.