Abstract
After hatching, juveniles of most sea turtle species undertake long migrations across ocean basins and remain in oceanic habitats for several years. Assessing population abundance and demographic parameters during this oceanic stage is challenging. Two long-recognized deficiencies in population assessment are (i) reliance on trends in numbers of nests or reproductive females at nesting beaches and (ii) ignorance of factors regulating recruitment to the early oceanic stage. To address these critical gaps, we examined 15 years of standardized loggerhead sighting data collected opportunistically by fisheries observers in the Azores archipelago. From 2001 to 2015, 429 loggerheads were sighted during 67,922 km of survey effort. We used a model-based approach to evaluate the influence of environmental factors and present the first estimates of relative abundance of oceanic-stage juvenile sea turtles. During this period, relative abundance of loggerheads in the Azores tracked annual nest abundance at source rookeries in Florida when adjusted for a 3-year lag. This concurrence of abundance patterns indicates that recruitment to the oceanic stage is more dependent on nest abundance at source rookeries than on stochastic processes derived from short term climatic variability, as previously believed.
Highlights
Of all loggerhead sea turtle life stages, the post-hatchling and oceanic juvenile stages are the least understood[8]
Between late June and early November each year, loggerhead hatchlings with a curved carapace length (CCL) of about 5 cm emerge from approximately 1500 km of nesting beaches from Florida to North Carolina[36] and enter the sea to be dispersed into the North Atlantic Gyre by the Gulf Stream[9,32]
This study provides the first assessment of relative abundance of oceanic-stage juvenile sea turtles
Summary
Of all loggerhead sea turtle life stages, the post-hatchling and oceanic juvenile stages are the least understood[8]. Contemporary population models for Northwest Atlantic loggerheads rely heavily on nest counts in the source rookeries, and assume minimal variation in annual survival rates within the oceanic realm and relatively stable recruitment from oceanic to neritic habitats after approximately the first decade of life[14,29]. These assumptions have been challenged by recent modeling studies that inferred a predominantly variable recruitment determined by highly variable neonate survival under influence of climate forcing[30,31]. These juveniles are primarily epipelagic[9] and appear to actively explore the vicinity of seamounts and peaks[39], which are common features in the region[40]
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