Friday, November 12, 2010

Atlantic Leatherback Migration & Dives

The critically endangered Leatherback Sea Turtle, Dermochelys coriacea, occurs in all of the ocean basins and makes the longest migrations of any sea turtle. The Leatherbacks travels involve foraging for jellyfish which are patchily-distributed as well as reproduction. Sabrina Fossette of the Universite´ de Strasbourg, and colleagues have just published the results of a study combining data from 16 individuals from different populations. They attempted to understand intra- and inter-population variability and take it into account in the implementation of conservation strategies of this critically endangered species. They investigated the movements and diving behavior of 16 Atlantic Leatherback turtles from three different nesting sites and one foraging site during their post-breeding migration to examine the variability in migratory patterns.

The team used satellite-derived behavioral and oceanographic data and showed that turtles used Temporary Residence Areas (TRAs) scattered throughout the Atlantic Ocean. Nine were in the neritic (coastal waters less than 200 m in depth) domain and 13 in the oceanic domain. These TRAs were associated with large scale surface oceanographic features of different types (i.e., altimetric features and/or surface chlorophyll a concentration). Turtles also exhibited relatively similar horizontal and vertical behaviors when in TRAs, such as slow swimming velocities, a sinuous path, and shallow dives, indicative of foraging activity in these food rich regions. The migratory paths and TRAs distribution showed interesting similarities with the trajectories of passive satellite-tracked drifters, suggesting that the general dispersion pattern of adults from the nesting sites may reflect the passive dispersion initially experienced by hatchlings.

One female turtle was tracked for 632 days and moved a total of 17,614 km (about 10,700 miles), suggesting she was averaging about 28 km or 28 miles per day. Behavioral variability of these turtles may be linked with the initial hatchling drift scenarios and be greatly influenced by environmental conditions. This high degree of behavioral plasticity in Atlantic Leatherback turtles makes species-targeted conservation strategies challenging and stresses the need for a larger dataset (>100 individuals) for providing general recommendations in terms of conservation.

The team observed shallow dives at all TRAs at all latitudes. Oceanic TRAs dives were longer (20 min) than in neritic TRA dives and were concentrated in the epipelagic layer (50–80 m). Fossette et al. suggest that the diving behavior was shaped by local prey distribution and density. Periods of very short shallow dives and high use of surface waters were previously reported for foraging Leatherbacks at high latitude where gelatinous plankton is available at shallow depths. 

Below is a map showing the movements of the turtles in this study, with the original caption.

Reconstructed movements of 16 Argos-tracked leatherback turtles during their migration in the Atlantic Ocean from 2005 to 2008. Twelve SRDLs were deployed on gravid females nesting in Panama (n = 3, PAyear-ID), Suriname and French Guiana complex (n = 6, SUyear-ID and FGyear-ID, respectively), and Gabon (n = 3, GAyear-ID). Four others were deployed on leatherback turtles incidentally captured by Uruguayan fisheries (pelagic longlines and coastal bottom-set gillnets) in international waters of the Southwest Atlantic and in Kiyú, Uruguay, respectively (URyear-ID). For each turtle, transit and Temporary Residence Areas (TRAs) are identified by dotted and solid lines, respectively. Each TRA is identified by a number in black and white, for neritic and oceanic domains, respectively

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