Oceanic dispersal of juvenile leatherback turtles: going beyond passive drift modeling

Abstract

The current paper presents the first detailed investigation of open-ocean dispersal of hatchlings and juveniles of the critically endangered western Pacific leatherback turtle Dermochelys coriacea populations nesting in New Guinea. Dispersal patterns were simulated by releasing particles drifting passively, or almost passively, into a state-of-the-art World Ocean circulation model. Analysis of the simulation results combined with sighting, genetic, bycatch, and adult satellite tracking information reveals that: (1) Hatchlings emerging from the main New Guinea nesting beaches are likely to be entrained by highly variable oceanic currents into the North Pacific, South Pacific, or Indian Oceans. Those drifting into the Indian Ocean likely suffer very high mortality. This suggests that, as ocean current variability determines the partition of hatchlings into different dispersal areas, it also largely influences juvenile survival rate at the population level. (2) Within 1 to 2 yr, most passively drifting juveniles reach temperate oceanic regions where the water temperature in winter drops well below the minimum temperature likely tolerated by such small individuals. This leads us to hypothesize that, after an initial period of mostly passive drift, juveniles initiate active swimming towards lower (warmer) latitudes before winter and back again towards higher latitudes, where food abounds, during spring. Such seasonal migrations would significantly slow the eastward progression of individuals circulating in the North Pacific current. This slower drift scenario better explains the size distribution of leatherbacks observed, or incidentally caught by pelagic fisheries, in the North Pacific. This dispersal mechanism combining passive drift with active habitat-driven seasonal migrations might well apply to many other sea turtle populations and deserves further study.

Predicting connectivity of green turtles at Palmyra Atoll, central Pacific: a focus on mtDNA and dispersal modelling

“…Large immature green turtles are known to actively swim among different FGs in the Atlantic [66], and swimming by young loggerhead turtles is also reported from Japan [67]. Linkages between Palmyra and the South Central RMU could highlight the importance of directed swimming by turtles of various stage-classes [2,66,68] to reach the small and isolated atoll.…”

Satellite tracking and stable isotope analysis highlight differential recruitment among foraging areas in green turtles

“…Several alternative and not mutually exclusive drivers could result in such a foraging area-specific increase in re cruitment. These include (1) an increase in the survival probabilities of juveniles and sub-adults as industrial fisheries are excluded (Casale 2011, Casale & Heppell 2016, (2) greater foraging re sources reducing the age to sexual maturity , (3) temporal oscillations in sea surface currents, such as those dictated by the Cyprus eddy (Zodiatis et al 2005), that can vary the distribution of pelagic-stage juveniles and thus the number of individuals recruiting to each foraging area (Gaspar et al 2012, Scott et al 2014 or (4) a shift in the ecological conditions within Lake Bardawil so that this site now provides a more suitable foraging resource (El-Bana et al 2002, Abd Ellah & Hussein 2009, Nada et al 2013. To expand on this latter hypothesis, evidence suggests that the reopening and ongoing maintenance of the 2 man-made channels in the western and central part of the lake have significantly reduced salinity levels (1970: 100 ‰, 2012: 46.1 ‰) (Abd Ellah & Hussein 2009, Nada et al 2013 and references therein) and allowed Cymo docea nodosa, the primary dietary item of the green turtle within the Mediterranean (Cardona 2010), to colonise and now dominate the shallow western basin (ElBana et al 2002, Abd Ellah & Hussein 2009).…”

Active dispersal in loggerhead sea turtles ( Caretta caretta ) during the ‘lost years’

“…Specifically, they have helped reconstruct timelines between empirical observations of adults with distant reproductive and foraging grounds [15]. Such studies have also given rise to imprint hypotheses (see [5]), which tie the role of passive dispersal in the evolution of active migration routes and habitat preference in later life-history stages [5,9,16].…”