Changes in diel diving patterns accompany shifts between northern foraging and southward migration in leatherback turtles

Abstract

Diel diving patterns have been widely documented among plankton-feeding marine vertebrates. In many cases, these patterns have been interpreted as a response to the diel vertical migrations of prey. The leatherback turtle, Dermochelys coriacea (Vandelli, 1761), is a large marine predator that exploits gelatinous plankton in disparate foraging areas. Individuals of this species spend extended periods at northern latitudes before moving southward through pelagic waters. To identify and compare potential diel patterns of diving behaviour in temperate areas, where foraging has been observed, versus during southward migration, 15 subadult and adult leatherbacks were equipped with satellite-linked time-depth recorders off Nova Scotia, Canada. We observed variation in nocturnal versus diurnal behaviour, both at northern latitudes and during migration; however, diel differences in both diving and surface activity were much less pronounced while leatherbacks were in the north. We interpret the difference in leatherback diel diving regimen to reflect a response to changing resource conditions at these times, with leatherbacks foraging throughout the day and night at high latitudes, then changing to a bimodal pattern of diving during southward migration, with generally longer, deeper diving occurring during the night versus during the day. By quantifying diel changes in leatherback behaviour, we provide the first surface time correction factors based on multiple individuals for use in estimating abundance from aerial surveys.Résumé : Les patrons journaliers de plongée chez les vertébrés marins planctonophages ont été bien étudiés. Souvent, les patrons sont interprétés comme des réactions aux migrations verticales journalières des proies. La tortue luth, Dermochelys coriacea (Vandelli, 1761), est un prédateur marin de grande taille qui exploite le plancton gélatineux dans des aires d'alimentation disparates. Les individus de cette espèce passent de longues périodes dans les latitudes nordiques avant de se dé-placer vers le sud dans les eaux pélagiques. Dans le but d'identifier et de comparer les patrons journaliers potentiels du comportement de plongée dans les zones tempérées où on observe de l'alimentation (par rapport aux patrons observés durant la migration vers le sud), nous avons muni 15 tortues subadultes et adultes d'enregistreurs de temps et de profondeur reliés à des satellites au large de la Nouvelle-É cosse, Canada. Nous avons observé une variation entre les comportements nocturne et diurne, tant dans les latitudes nordiques que durant la migration; les variations journalières d'activité en plongée et en surface sont, cependant, beaucoup moindres lorsque les tortues sont dans le nord. Nous interprétons les différen-ces dans le régime journalier de plongée des tortues luth comme des réactions aux conditions changeantes des ressources à ce moment; aux latitudes élevées, les tortues s'alimentent tout au long de la journée et de la nuit; durant la migration vers le sud, elles adoptent un patron bimodal de plongée, av...

Identifying leatherback turtle foraging behaviour from satellite telemetry using a switching state-space model

“…Indeed the turtles tend to spend more time at greater depths and dive durations are longer when transiting versus foraging. This pattern lends support to the idea that leatherbacks may make scouting dives while transiting as an efficient means for sampling prey density (James et al 2006a) and perhaps opportunistic foraging without greatly reducing travel rate. Moreover, the shallower dives associated with extended, and presumably more profitable, foraging suggest that the turtles focus on regions where prey are available at shallower strata perhaps to reduce the energetic demands of foraging at depth.…”

“…We used the diving statistics described earlier, which show relatively clear behavioural patterns (James et al 2006a), as a means of corroborating the estimates of b t obtained from the DCRWS model. Because b t is a discrete parameter -values can only be 1 or 2 -we used the means of the MCMC samples as a convenient way to visualize behavioural switches.…”

“…The leatherback turtle Dermochelys coriacea is a cosmopolitan marine species that undertakes a variety of migratory patterns (Luschi et al 2003, James et al 2005a. Leatherbacks tagged off Nova Scotia, Canada, make annual return migrations from tropical to sub-Arctic waters (James et al 2005a), with much of their time in northern waters presumably spent foraging on gelatinous zooplankton (James et al 2006a). Despite a growing awareness of the migratory strategies and behaviours (James et al 2005a, Jonsen et al 2006) and population characteristics (James et al 2007) of these animals, we know remarkably little about how they utilize foraging habitat, and even less about the ecology of their jellyfish prey (Witt et al 2007).…”

Water mass characteristics and solar illumination influence leatherback turtle dive patterns at high latitudes

“…In summer, the Atlantic coast of Canada hosts one of the largest seasonal aggregations of leatherbacks in the world (James et al 2006a). The turtles typically arrive in Atlantic Canada in late spring and early summer, with a portion of the population migrating along the Scotian Shelf to foraging areas off Cape Breton Island, southern Newfoundland, and the southern Gulf of St. Lawrence (GSL) (Appendix: Fig.…”

“…In contrast to the tropics, only general conclusions have been drawn regarding dive patterns at north temperate latitudes (e.g., James et al 2005bJames et al , 2006a. Nevertheless, there is some evidence that dives become progressively shorter and shallower as turtles migrate northward (Eckert 2006, Hays et al 2006, Fossette et al 2008, though lowresolution telemetry data indicate that dive patterns do not appear to vary predictably in Atlantic Canada until the onset of southward migration (James et al 2005b).…”