Seasonal niche and spatial distribution modelling of the loggerhead (Caretta caretta) in the Adriatic and Ionian seas

Abstract

Although Caretta caretta is the most common turtle in the eastern Mediterranean Sea, its distribution is relatively difficult to assess due to the lack of data in offshore waters. The ecological niches in the central-southern Adriatic Sea and the north-eastern Ionian Sea were investigated for loggerhead turtles with body sizes ≥20 cm during two periods: winter (W), which includes months from October to March, and breeding–nesting (BN), from April to September. Differences in the spatial distribution of loggerheads were explored between periods, and the environmental predictors driving habitat selection were examined. Loggerheads mainly select habitats within the continental shelf (<200 m) differently, and exhibit: (i) a generalist behaviour in the Adriatic subregion during BN and a specific selection of environmental conditions during W; and (ii) a specialist behaviour in the Ionian Sea. The Adriatic niche appeared as an important foraging ground year-round, whereas the distribution of suitable areas in the Ionian Sea is patchy and limited to specific habitats close to shore. Extreme values of sea surface temperatures (SSTs) from October to March (W) influenced loggerhead distribution differently in the subregions: areas experiencing cold and warm SST peaks were selected, respectively, in the Adriatic and Ionian subregions. Moreover, SST in April largely contributed to defining the habitat suitability in BN for both subregions This study further confirms the importance of the Adriatic and Ionian seas for the conservation of loggerhead sea turtles and the need for seasonal investigations to establish effective conservation measures. Here, the use of a cost-effective method (fixed line transects from ferries) to inform the distribution of sea turtles in offshore waters is a valuable methodology for long-term seasonal investigations of the habitat use of sea turtles.