Upcycled glass and plastic bottles offer potential low-cost mitigation to megafauna bycatch in gillnet fisheries

Bycatch in gillnet fisheries represents the primary threat to cetaceans globally, yet mitigation has been hampered by a lack of effective, affordable solutions. Here we tested upcycled glass and plastic drink bottles as low-cost bycatch mitigation devices (mechanical pingers and acoustic reflectors, respectively) in gillnet fisheries across three regions. Observed trials were conducted in surface-set driftnet fisheries targeting teleosts and elasmobranchs off Zanzibar, East Africa, and Peru (2017–2021), and in bottom-set trammel net fisheries targeting teleosts off Brazil (2020–2022). Glass and plastic bottle treatments were tested against control sets across 1107 and 456 drift net sets in Zanzibar and Peru respectively, and 105 trammel net sets in Brazil. Catch differences between treatment and control sets were analysed using generalised linear hurdle and mixed models. Plastic bottles showed promise in Brazil, potentially reducing dolphin bycatch while increasing fish catch, though further trials are needed to confirm this. In Peru, glass bottles had no effect on dolphin, porpoise, or turtle bycatch but altered elasmobranch and teleost catches; plastic bottles similarly had no effect on dolphin bycatch but increased shark catch. In Zanzibar, no dolphin bycatch was recorded in any sets, though plastic bottles increased tuna catch. Overall, our results suggest plastic bottles may reduce dolphin bycatch in bottom-set fisheries, but neither device showed efficacy in surface-set fisheries. We encourage further trials of these and similar low-cost devices as accessible mitigation tools for small-scale fisheries.