Young sea turtles of the pelagic Sargassum-dominated drift community: habitat use, population density, and threats

In the western Atlantic young sea turtles in their surface-pelagic juvenile stage, a stage often described as the lost year(s), have been hypothesized to occupy habitat dominated by pelagic Sargassum macroalgae. But despite substantial indirect evidence, there has been no direct study of sea turtles within this community. To determine the importance of Sargassum to young sea turtles, we transected surface-pelagic habitat, measured relative abundance of sea turtles, recorded their behavior and association with surface features, and assessed their diet. On vessel transects in the eastern Gulf of Mexico and Atlantic Ocean off Florida (USA), we recorded 1884 turtles of 4 species-loggerhead sea turtle Caretta caretta, green turtle Chelonia mydas, hawksbill Eretmochelys imbricata, and Kemp's ridley Lepidochelys kempii. Most (89%) were initially observed within 1 m of floating Sargassum. Turtles included both post-hatchlings (39 to 78 mm straight carapace length, SCL) and juveniles (130 to 280 mm SCL). Dive-profile data from overnight logs (18.5 to 23.1 h duration) of 3 juvenile Kemp's ridleys showed that they spent an average of 97% (day) or 87% (night) of their time within 1 m of the surface. Juvenile turtles from which esophageal lavage and fecal samples were obtained showed a diet composed principally of Sargassum-community associates, primarily marine animals. Other items included marine plants (predominantly pelagic Sargassum), synthetic material (e. g. plastics), terrestrial plants (mostly wood), and terrestrial animals (flying insects). Plastics in diet samples averaged 13% of dry mass. Data support a description of pelagic Sargassum as a transient hot spot for young sea turtles and a focal point for threats, including debris ingestion and petroleum.