Mitigation measures to reduce seabird's interactions with bottom-set nets in southern Iberia

Bycatch poses a significant threat to seabird populations globally. On the southern coast of mainland Portugal, mitigation measures were tested to reduce seabird interactions with fishing vessels. Between 2021 and 2023, this study evaluated the effectiveness of a visual deterrent (a 'scarybird' device), an acoustic deterrent (megaphone broadcasts), alongside modification in fisher's behaviour discard management— avoiding discards during fishing operations to minimize seabird interactions. Observers monitored 166 fishing events, comparing control and experimental treatments. Discard management was the most effective deterrent, significantly reducing seabird abundance, particularly gulls (Larus spp.) and northern gannets (Morus bassanus). Model predictions indicated a 37 % and a 47 % reduction in the abundance of gulls and northern gannets, respectively, around commercial fishing vessels when discard management was used. This practice effectively kept a substantial proportion of birds away from the vessel during operations, thereby reducing their vulnerability to bycatch. The megaphone showed limited efficacy, with higher seabird abundance in treatment groups compared to controls. The scarybird device was largely ineffective in deterring gulls unless combined with discard management but showed potential in reducing interactions with northern gannets. Importantly, all mitigation measures did not negatively impact Landings Per Unit Effort (LPUE), which facilitated their acceptance among fishers. Our findings suggest that simple modifications in the fisher's behaviour, such as retaining fish discards and viscera onboard, could serve as a promising bycatch mitigation measure, potentially eliminating the need for additional less effective mitigation devices that fishers may be reluctant to adopt.