Abstract
We designed a novel aggregated methodology to infer the impact of ocean motions on the movements of satellite-tracked marine turtles adopting available oceanographic observations and validated products of a numerical oceanographic forecasting system. The method was tested on an 11-months trajectory of a juvenile loggerhead turtle (LT) wandering in the Tyrrhenian Sea (Mediterranean Sea) that was reconstructed with a high-resolution GPS tracking system. The application of ad-hoc designed metrics revealed that the turtle’s route shape, ground speed and periodicities of its explained variance mimic the inertial motions of the sea, showing that this methodology is able to reveal important details on the relation between turtle movements and oceanographic features. Inertial motions were also identified in the observed trajectory of a surface drifting buoy sampling the Tyrrhenian Sea in a common period. At each sampling point of the turtle trajectory, the sea current eddy kinetic energy (EKE) and a Sea Current Impact index were computed from a validated set of high-resolution ocean modeling products and their analysis showed the relevant effects of the highly variable local sea currents mechanical action. Specifically, the metric we adopted revealed that the turtle trajectory was favorably impacted by the encountered sea current advection for about 70% of its length. The presented oceanographic techniques in conjunction with high-resolution tracking system provide a practicable approach to study marine turtle movements, leading the way to discover further insights on turtle behavior in the ocean.
Highlights
Satellite tracking studies have repeatedly shown that marine turtles often spend long periods in offshore areas roaming over extended regions and usually following complex routes (Benson et al, 2011; Briscoe et al, 2016; Robinson et al, 2016)
Such a habit has been documented for turtle species that are normally recognized as pelagic wanderers, like adults of the leatherback turtle and for Hydrodynamic Controls on Marine Turtle Trajectories species and life stages that were generally thought to preferentially frequent neritic and coastal waters, like loggerheads (Godley et al, 2008; Luschi and Casale, 2014)
The new proposed aggregated methodology was tested on the study case following two successive steps: (1) the exploration of any common feature between the observed trajectory and velocities of a tracked loggerhead turtle (LT) and the path of a surface drifter (OBS) released in the same basin in a similar period, followed by a quantitative comparison with the sea surface currents obtained by ocean models application (MOD) along the LT and observed sea current velocity (OBS) trajectories; (2) the quantification of the impact of the sea current velocities, obtained from modeling products (MOD), on the LT trajectory through the adoption of ad-hoc designed indicators
Summary
Satellite tracking studies have repeatedly shown that marine turtles often spend long periods in offshore areas roaming over extended regions and usually following complex routes (Benson et al, 2011; Briscoe et al, 2016; Robinson et al, 2016) Such a habit has been documented for turtle species that are normally recognized as pelagic wanderers, like adults of the leatherback turtle and for Hydrodynamic Controls on Marine Turtle Trajectories species and life stages that were generally thought to preferentially frequent neritic and coastal waters, like loggerheads (Godley et al, 2008; Luschi and Casale, 2014). Rich biological environments can be created in pelagic areas by convergence and divergence areas, like gyres, meanders, or intense surface/deep streams, resulting from ocean physics and dynamics at different spatial and temporal scales These foraging opportunities attract juvenile turtles during their developmental phase (Mansfield et al, 2014) and older individuals during the non-breeding stage (Chimienti et al, 2020). Sea current advection has a direct effect on turtles moving offshore, with individuals that can take advantage, or are hampered in their movements for the continuous action of encountered currents (e.g., Gaspar et al, 2006; Lambardi et al, 2008; Mencacci et al, 2010)
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