The Bycatch Paradox: Incidental capture of native semi-aquatic vertebrates as a recovery signal during non-native fish eradication in high-mountain lakes

Bycatch is generally regarded as an unintended negative consequence of wildlife exploitation. However, we hypothesized that, in the context of invasive species eradication actions, temporal changes in bycatch may provide indirect evidence of native fauna recovery. Here, we investigated bycatch dynamics in semi-aquatic vertebrates during multi-year removal campaigns of non-native fish in 31 high-mountain lakes across the Alps and Pyrenees. We predicted that, as fish densities declined, incidental captures of amphibians and semi-aquatic birds and mammals would increase due to competitive or predatory release. Our results supported this hypothesis for all amphibian species and for semi-aquatic birds, with bycatch events consistently increasing throughout the eradication process, whereas no clear pattern was detected for mammals. This general pattern contrasts with conventional fisheries, where bycatch is expected to decline with improved management. We refer to this pattern as the “Bycatch Paradox”, in which bycatch increases during restoration processes, potentially reflecting native fauna recovery. Nevertheless, mitigating bycatch remains important, especially for threatened species.