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

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.