Abstract
Predictions of organismal movements in a fluid require knowing the fluid's velocity and potential contributions of the organism's behaviour (e.g. swimming or flying). While theoretical aspects of this work are reasonably well-developed, field-based validation is challenging. A much-needed study recently published by Briscoe and colleagues in Proceedings of the Royal Society B compared movements and distribution of satellite-tracked juvenile sea turtles to virtual particles released in a data-assimilating hindcast ocean circulation model. Substantial differences observed between turtles and particles were considered evidence for an important role of active swimming by turtles. However, the experimental design implicitly assumed that transport predictions were insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the model. Here, we show that the magnitude of variation in physical parameters between turtles and virtual particles can profoundly alter transport predictions, potentially sufficient to explain the reported differences without evoking swimming behaviour. We present a more robust method to derive the environmental contributions to individual movements, but caution that resolving the ocean velocities experienced by individual organisms remains a problem for assessing the role of behaviour in organismal movements and population distributions.
Highlights
Understanding the mechanisms driving organismal movement has long-been viewed as an essential component to the conservation and management of species and ecosystems [1,2]
Ocean currents were presumed to dominate organisms’ movements, owing to limited swimming capacity relative to ocean velocity and/or a limited ability of animals to direct their swimming in the barren sensorial-environment of the open sea [2]
Briscoe et al [13] compared the movements and distribution of captive-reared loggerhead sea turtles (Caretta caretta), released off the coast of Japan, to ocean currents from a global ocean circulation model. They concluded that turtle velocities and distribution could not be the result of ocean currents alone, indicating the importance of directional swimming during the oceanic migration of juveniles. Implicit in their analyses were the assumptions that transport predictions are insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the ocean circulation model
Summary
Understanding the mechanisms driving organismal movement has long-been viewed as an essential component to the conservation and management of species and ecosystems [1,2]. They concluded that turtle velocities and distribution could not be the result of ocean currents alone, indicating the importance of directional swimming during the oceanic migration of juveniles Implicit in their analyses were the assumptions that transport predictions are insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the ocean circulation model. To assess whether ocean currents accounted for the turtles’ movements, Briscoe et al [13] released virtual particles at two locations in the surface layer of Global Hybrid Coordinate Ocean Model (HYCOM) output [28] using the particle-tracking software Ichthyop v. We assessed whether particle speeds were correlated with the speed of the drifter using Spearman’s rank-order correlation and whether particle directions were correlated with the direction of the drifter using Circular– Circular correlations
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