Turtle bycatch from trawlers: What modelling is telling us in the southern Adriatic sea

Abstract

The Southern Adriatic Sea (Geographical Sub-Area GSA18) is intensively exploited by bottom trawling and it is also an important foraging ground for both juvenile and adult loggerhead sea turtles (Caretta caretta). The risk of unwanted catches of sea turtles with trawling is therefore high in this area, demanding tailored insights on this issue. In this study, we investigated the patterns and potential drivers of loggerhead sea turtle bycatch in this area, using generalized linear and generalized additive models. We analyzed data collected by observers onboard and logbooks. Results indicated that the likelihood of turtle bycatch is significantly higher during the day, likely due to the turtles' diel foraging patterns. Seasonal variations revealed an increased bycatch rate in summer and autumn, coinciding with the turtles' southward migration to warmer waters. Additionally, the generalized additive model provided spatiotemporal insights, identifying two bycatch hotspots around the Gargano promontory and off the coast of Brindisi, areas recognized for their high suitability as foraging habitats. Depth and distance from the coast were also significant key factors, with most bycatch occurring in shallow and coastal waters. The drivers of bycatch identified in this study provide crucial insights for shaping initiatives to reduce bycatch of loggerhead sea turtles. These findings emphasize the need for tailored conservation measures to mitigate bycatch, such as temporal and spatial fishing restrictions.