Abstract
In-water behaviour and long-term movements of oceanic-stage juvenile sea turtles are not well described or quantified. This is owing to technological or logistical limitations of tracking small, fast-growing animals across long distances and time periods within marine habitats. Here, we present, to our knowledge, the first long-term offshore tracks of oceanic green turtles (Chelonia mydas) in western North Atlantic waters. Using a tag attachment technique developed specifically for young (less than 1 year old) green turtles, we satellite-tracked 21 oceanic-stage green turtles (less than 19 cm straight carapace length) up to 152 days using small, solar-powered transmitters. We verify that oceanic-stage green turtles: (i) travel to and remain within oceanic waters; (ii) often depart the Gulf Stream and North Atlantic Subtropical Gyre currents, orienting towards waters associated with the Sargasso Sea; (iii) remain at the sea surface, using thermally beneficial habitats that promote growth and survival of young turtles; and (iv) green turtles orient differently compared to same stage loggerhead turtles (Caretta caretta). Combined with satellite tracks of oceanic-stage loggerhead turtles, our work identifies the Sargasso Sea as an important nursery habitat for North Atlantic sea turtles, supporting a growing body of research that suggests oceanic-stage sea turtles are behaviourally more complex than previously assumed.
Highlights
Sea turtle life history is defined by ontogenetic shifts in habitat use as turtles mature
With the exception of flatback turtles (Natator depressus; [6]), most hard-shelled species are assumed to migrate from natal neritic waters to oceanic nursery areas, eventually returning to coastal waters as larger juveniles where they grow to maturity and through adulthood [1,5]
Juvenile North Atlantic green turtles return to coastal waters once they reach approximately 20 cm straight carapace length (SCL) [10]; yet, the turtles’ preceding oceanic period remains unknown as empirical observations of the turtles’ long-term movements, behaviour and habitat use are lacking [11,12,13]
Summary
Sea turtle life history is defined by ontogenetic shifts in habitat use as turtles mature (reviewed by [1,2,3,4]). Mansfield et al [7] observed that some turtles departed the currents associated with the NASG, displaying directional movements into the interior of the NASG and the Sargasso Sea—an unexpected deviation from the long-held hypothesis (hypothesis 2; [11,15]) that Atlantic 2 loggerheads travel around the Atlantic basin remaining within NASG currents until recruiting to near shore developmental habitats as large juveniles [11] Based on this prior neonate loggerhead tracking work, we question whether oceanic green turtles will behave to loggerheads and remain within oceanic waters, at the sea surface, solely within the currents of the NASG or whether they too, will enter into the Sargasso Sea. Wild-caught oceanic-stage green turtles in the Gulf of Mexico are known to associate with Sargassum spp. and other flotsam and are observed to occur at the sea surface [9,18,24]; turtles in the Gulf of Mexico probably originate from several rookeries including Florida and many outside of the US (Mexico, Costa Rica, the Caribbean) [32]. This subset represents the spatial range within which both green and loggerhead tracks occurred (maximizing available track data), the northern limit of the NASG, and includes the region tested for loggerheads from Mansfield et al [7] encompassing the artificial magnetic field locations used by Lohmann & Lohmann [17]—a region where laboratoryreared naive loggerheads oriented to the east northeast to theoretically remain within the NASG
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