An interview-based investigation of marine megafauna bycatch in the northern South China Sea

There is still limited information available regarding the patterns and extent of bycatch mortality in heavily-fished marine systems. To address this gap, we conducted an interview-based study to investigate the bycatch of five marine megafaunal species/categories in the northern South China Sea. Approximately two-thirds of the interviewed fishers reported encountering bycatch events, with sea turtles being the most commonly reported megafauna (33.6 % of respondents), followed by Indo-Pacific finless porpoises and whale sharks (both 12.4 %), and Indo-Pacific humpback dolphins (8.5 %). Dugongs were only reported by one respondent. Bycatch of these taxa was mainly associated with gillnets, trawl nets, and seine nets. Our estimates indicated a total of 7464 bycatch events annually, resulting in 1391 deaths for these five taxa. These numbers underscore the alarming impact of fisheries on marine megafauna, particularly on small cetaceans, which accounted for 66.1 % of the reported annual deaths, including 690 finless porpoises and 230 humpback dolphins. Lower bycatch levels were reported for all taxa in winter, potentially due to temporary reductions in fishery activity and/or animal migrations. Random forest model revealed that bycatch of humpback dolphins, finless porpoises, and sea turtles was primarily influenced by water depth and distance from the coast, while whale shark bycatch was influenced by administrative region. This study demonstrates that local ecological knowledge can provide a rapid means of obtaining basic information on the bycatch of marine megafauna, and serves as a sobering reminder that fisheries-related mortality contributes to the population declines of coastal small cetaceans.