Ecological risks of using non-native freshwater fish for biological control

The use of freshwater fish as biological control agents has been widely implemented to manage populations of perceived nuisance species, ranging from disease vectors to aquatic weeds. While these introductions are often intended as a management tool, they can have unintended and severe ecological, economic, and social consequences. This review assesses the risks associated with using non-native fish for biological control, emphasising the challenges presented by their persistence, adaptability, and interactions with native species and ecosystems. Key case studies illustrate how species such as mosquitofishes (Gambusia spp.), grass carp (Ctenopharyngodon idella), and tilapias (mainly Oreochromis spp.) have inadvertently contributed to biodiversity loss, habitat degradation, trophic disruptions, and disease transmission, while often failing to effectively control the target species. We further explore the limitations of biocontrol strategies, including the context-dependent nature of species interactions and deferred negative impacts. Given the complexity and unpredictability of biological control interventions, rigorous impact assessments and long-term monitoring programmes are needed before and after introducing non-native fish as control agents, embedded within an adaptive management framework that allows for responsive decision-making based on observed ecological outcomes. Precautionary policies should prioritise habitat restoration and native species-based solutions over the introduction of non-native species to support more sustainable and ecologically responsible management decisions.