Voluntary fishing logbooks are essential for unveiling unsustainable bycatch levels and appropriate mitigating measures: The case of sea turtles in the Gulf of Manfredonia, Adriatic Sea

1. Studying the impact of bycatch on marine megafauna, including sea turtles, is challenging for a variety of technical and biological reasons. The Mediterranean Sea has among the highest levels of turtle bycatch globally, notably of the loggerhead sea turtle (Caretta caretta), and bottom trawling represents a particularly relevant threat.2. Bottom trawlers from a recently discovered neritic foraging area, the Gulf of Manfredonia, reported 1,152 loggerhead turtles incidentally caught in the period 2015-2020. Capture locations were available for 497 turtles. These data were complemented by the distribution of fishing effort obtained by the vessel monitoring system.3. High bycatch rates were observed, leading to more than an estimated 5,600 annual captures and to the death of a minimum of 560 large individuals with high reproductive value. These findings are extremely concerning for the affected population and require immediate action. A strong seasonality of turtle bycatch was observed, with most captures occurring in November-March when fishing effort occurs in shallow waters where turtles concentrate all year round.4. Therefore, a seasonal approach (e.g. adopting measures only in the high-turtle bycatch season) can be pursued as long as the spatial distribution of fishing effort varies among seasons. However, such a spatio-temporal pattern should be monitored; then, in the case of change, although more difficult to implement, an area-based conservation approach should be pursued. Several recommendations are provided, including the urgent implementation of turtle excluder devices, at least on a seasonal basis.5. This case shows that identifying the best conservation approaches requires information on the actual spatio-temporal pattern of turtle occurrence and bycatch. Such information can only be derived from spatial distribution of bycatch and fishing effort obtained through a voluntary collaboration of fishers.