Friday, January 31, 2014

Leatherback movements associated with food

Previous studies of leatherback turtles have analyzed surface movement patterns using only surface covariates. Since turtles and other marine predators spend the vast majority of their time diving underwater, an analysis of movement patterns at depth should yield insight into what drives their movements.

Photo credit DivingSeaSafari
In a new paper Scihick and colleagues (2013) analyze the movement paths of 15 post-nesting adult female Pacific leatherback turtles, which were caught and tagged on three nesting beaches in Mexico. The temporal length of the tracks ranged from 32 to 436 days, and the spatial distance covered ranged from 1,532 km to 13,097 km. The tracks were studied using a movement model designed to yield inference on the parameters driving movement. Because the telemetry data included diving depths, the authors extended an earlier version of the model that examined surface only movements, and analyze movements in 3-dimensions.

They tested the effect of dynamic environmental covariates from a coupled biophysical oceanographic model on patch choice in diving leatherback turtles, and compared the effects of parameters measured at the surface and at depth. The covariates included distance to future patch, temperature, salinity, meridional current velocity (current in the north–south direction), zonal current velocity (current in the east–west direction), phytoplankton density, diatom density, micro-plankton density, and meso-zooplankton density.

They found significant correlation between movement and the parameters for oceanic covariates in eight of the tracks. Of particular note, for one turtle they observed a lack of correlation between movements and a modeled index of zooplankton at the surface, but a significant correlation between movements and zooplankton at depth. Two of the turtles express a preference for patches at depth with elevated diatoms, and two turtles prefer patches with higher mezozooplankton values at depth. In contrast, four turtles expressed a preference for elevated zooplankton patches at the surface, but not at depth.

The authors suggest that our understanding of a marine predator’s response to the environment may change significantly depending upon the analytical frame of reference, that is whether relationships are examined at the surface, at depth, or at different temporal resolutions. They also tested the effects of accounting for ocean currents on the movement patterns and found that for 13 of the 15 turtles, the parameter governing distance to the next patch decreased.

Citation
Schick RS, Roberts J, Eckert  S, Clark  J, Bailey H, Chai F, Shi  L & Halpin  P 2013 . Pelagic movements of pacific leatherback turtles (Dermochelys coriacea) reveal the complex role of prey and ocean currents.  BMC Movement Ecology  1(11).

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