Movements and High-Use Areas of Western Pacific Leatherback Turtles

Abstract

The western Pacific leatherback (Dermochelys coriacea), one of three genetically distinct stocks in the Indo-Pacific region, has declined markedly during past decades. This diverse metapopulation nests year-round at beaches of several western Pacific island nations and has been documented through genetic analysis and telemetry studies to occur in multiple regions of the Pacific, including waters of the eastern and central North Pacific, the western South Pacific, the South China Sea, and the Sea of Japan. The objective of this study is to synthesize results of 126 satellite telemetry deployments conducted on leatherbacks at western Pacific nesting beaches and at one eastern Pacific foraging ground during 2000-2007, to provide a large-scale picture of movements, high use areas, and habitat associations and to support ecosystem-based management and conservation. Nesting beach deployments were conducted in Papua New Guinea (PNG) and Solomon Islands during boreal winter (December-February), and in Papua Barat, Indonesia (PBI) during boreal summer (July-August) and winter. Deployments at distant foraging grounds off California, USA were conducted during September. A Bayesian switching state-space model (SSSM) was applied to raw Argos-acquired surface locations to estimate daily positions for each turtle. The SSSM provided a posterior distribution of behavioral mode for each daily location, from which a probability of transit (Ptransit, t) was derived to infer behavior (transiting vs. foraging). Monthly areas of high use were identified for post-nesting periods using kernel density estimation. There was a clear separation of migratory destinations for boreal summer vs. boreal winter nesters, and leatherbacks used multiple large marine ecosystems (LMEs) as foraging destinations. Individuals nesting in PBI during boreal summer moved either to the temperate North Pacific Ocean, including the Kuroshio Extension or the California Current, or to tropical waters of the South China Sea, remaining north of the equator at all times. Foraging habitats included diverse pelagic and coastal regions exhibiting a wide range of mechanisms that are known or expected to aggregate leatherback prey, including mesoscale eddies, coastal retention areas, and current boundaries. In contrast, individuals tagged at the same PBI beaches during January-February and leatherbacks tagged in PNG and the Solomon Islands moved into tropical and southern hemisphere LMEs, including the East Australia Current, the New Zealand Shelf, and tropical Indonesian seas. These regions are also characterized by mesoscale eddies, stationary fronts, or coastal retention areas. Western Pacific leatherbacks thus exploit diverse ecological processes that can aggregate gelatinous prey in different ways throughout the Pacific basin. Use of the most distant, temperate LME (California Current) required a 10-12 month trans-Pacific migration and commonly involved multiple years of migrating between high-latitude summer foraging grounds and low-latitude eastern tropical Pacific wintering areas without returning to western Pacific nesting beaches. In contrast, tropical foraging destinations in the South China Sea and Indonesian seas were reached within 5-7 months and appeared to support year-round foraging, potentially allowing a more rapid return to nesting beaches. Based on these considerations, we hypothesize that demographic differences are likely among nesting females using different LMEs.