Persistent leatherback turtle migrations present opportunities for conservation

Abstract

Effective transboundary conservation of highly migratory marine animals requires international management cooperation as well as clear scientific information about habitat use by these species. Populations of leatherback turtles (Dermochelys coriacea) in the eastern Pacific have declined by >90% during the past two decades, primarily due to unsustainable egg harvest and fisheries bycatch mortality. While research and conservation efforts on nesting beaches are ongoing, relatively little is known about this population of leatherbacks' oceanic habitat use and migration pathways. We present the largest multi-year (2004-2005, 2005-2006, and 2007) satellite tracking dataset (12,095 cumulative satellite tracking days) collected for leatherback turtles. Forty-six females were electronically tagged during three field seasons at Playa Grande, Costa Rica, the largest extant nesting colony in the eastern Pacific. After completing nesting, the turtles headed southward, traversing the dynamic equatorial currents with rapid, directed movements. In contrast to the highly varied dispersal patterns seen in many other sea turtle populations, leatherbacks from Playa Grande traveled within a persistent migration corridor from Costa Rica, past the equator, and into the South Pacific Gyre, a vast, low-energy, low-productivity region. We describe the predictable effects of ocean currents on a leatherback migration corridor and characterize long-distance movements by the turtles in the eastern South Pacific. These data from high seas habitats will also elucidate potential areas for mitigating fisheries bycatch interactions. These findings directly inform existing multinational conservation frameworks and provide immediate regions in the migration corridor where conservation can be implemented. We identify high seas locations for focusing future conservation efforts within the leatherback dispersal zone in the South Pacific Gyre.

Figure 1. Map and Timeline of Leatherback Sea Turtle Tracking Data

(A) Satellite transmission positions for 46 leatherback turtles from 2004 (n = 27, orange), 2005 (n = 8, purple), and 2007 (n = 11, green), tagged at Playa Grande, Costa Rica, overlaid on bathymetry (in m). Prominent bathymetric features and island groups are labeled (EPR = East Pacific Rise). (B) Timeline of satellite transmissions for each tag (tag ID is the ARGOS-assigned transmitter number).

Figure 2. Large-Scale Oceanographic Characteristics and Leatherback Movements in the EP

Turtle median daily positions (black dots) generated with state-space model interpolation [21], overlaid on long-term mean. (A) Turtle median daily speed (blue line) and heading (black line), with corresponding standard-error envelopes, averaged in one-degree latitudinal bins. (B) EKE (in cm² s⁻²). (C) Near-surface CHL concentration (in mg m⁻³).

Figure 3. Leatherback Movements in Relation to Ocean Currents in the Migration Corridor Region

(A–C) Turtle tracks overlaid on MKE (in cm² s⁻²) for February–April periods for 2004, 2005, and 2007. Stippling shows the 2000-m isobath highlighting the Cocos Ridge. (D) Ratio of turtle meridional velocity to current zonal velocity in the migration corridor region. Points corresponding to turtle meridional velocities faster than −70 km d⁻¹ are colored in red. (E) Schematic of turtle migration corridor through the equatorial current system (current abbreviations are given in the text), based on the 75% home-range utilization distribution contour. (F) Current corrected turtle tracks from 2004 (orange), 2005 (green), and 2007 (purple), overlaid on contours of magnetic force (solid thin black lines) and magnetic inclination (dashed thin black lines). The force field has an intensity ranging from 38,314 nT in the north to 27,798 nT in the south, and contours are drawn every 420 nT. The inclination field ranges from 1.6° and 43.1°, and contours are drawn every 1.7°.

Figure 4. Combined Utilization Distribution by EP Leatherback Turtles from all Tracking Data

EP leatherback turtle home-range utilization distribution for all years combined (2004, 2005, and 2007). Boundaries of the ETPS, corresponding to the exclusive economic zones of Costa Rica, Panama, Ecuador, and Colombia, are shown as dashed blue lines. The green polygon comprises the region with the lowest climatological EKE (≤30 cm² s⁻²) in the South Pacific Gyre.