Diet of loggerhead turtles in the Gulf of Manfredonia, South Adriatic Sea: evidence of winter feeding and anthropogenic impacts

Abstract

Investigating biological aspects of migratory marine animals like sea turtles is challenging. The Adriatic Sea is a key Mediterranean foraging ground for loggerhead turtles, yet certain feeding behaviors remain poorly known, including winter feeding and foraging within the neritic area of the Gulf of Manfredonia. With high fishing effort, this area experiences considerable turtle bycatch from trawlers during winter, representing an important conservation issue. Insights on how and to what extent the species interacts with anthropogenic threats such as trawlers and debris can be obtained from diet analysis. We examined feces and gut contents from 57 bycaught or stranded turtles to assess active feeding and digestion time during winter, identify and quantify prey items, evaluate feeding patterns among size classes, and ascertain the anthropogenic impact via debris and fishing discard ingestion. Our findings show that turtles feed actively during winter, primarily on benthic items, starting from a small size (32 cm Curved Carapace Length). No size effect was observed on dietary diversity or biomass percentage. We identified 37 Operational Taxonomic Units (OTUs) across 10 dietary groups, with crustaceans and mollusks being most prevalent. Osteichthyes and cephalopods, likely scavenged from trawl discard, were also common, implying intensive local trawling may attract turtles, increasing bycatch and mortality rates. The frequency of anthropogenic material ingestion was comparable to other Mediterranean regions and its presence in feces suggests non-lethal effects, if any. This study shows how information on seasonal feeding behavior can provide insights into how the relationship of the species with the environment and threats changes over time, ultimately steering conservation efforts.