Foraging Ecology of the Leatherback Turtle

The leatherback turtle is a powerful ocean traveler that ranges from the Arctic Circle to the edges of the Antarctic convergence zone. This unique pelagic reptile spends most of its life at sea, but females haul out onto tropical beaches every two to four years to lay their eggs. Much has been learned about the animal’s reproductive biology from studies conducted on nesting beaches, where the females, eggs, and hatchlings are easily accessible, yet little is known about leatherbacks in the marine environment, where they remain elusive and difficult to study. The need to achieve a better understanding of this part of the animal’s life history has become increasingly urgent as Pacific populations continue to decline to the brink of extinction, despite intense conservation efforts on the nesting beaches. The Pacific Leatherback Recovery Plan (http://swfsc.nmfs.noaa.gov/PRD/Seaturtle/) has identified the need to identify forage areas, to determine marine habitat needs, and to describe migratory patterns among the highest priorities for action. Monterey Bay is one of the first index areas to have been established for in-water studies of this species.

Leatherback turtles aggregate in Monterey Bay in late summer. photo William J. Douros for NOAA/MBNMS

Leatherbacks have been known to occur in Monterey Bay for some time, but it was not until 2000 that we achieved an exciting breakthrough with our first attempts to capture foraging animals (see Ecosystem Observations 2000). Prior telemetry studies had been limited to post-nesting movements of females tagged on nesting beaches.

We attached satellite transmitters to two adult females in September 2000 and tracked the animals as they migrated west; we continued to receive data from one of the turtles for eighteen months as she crossed the Pacific to the Mariana Trench, just north of the main nesting beach in North Papua, when she turned and began swimming east toward the central Pacific. Despite making this long migration back to the nesting areas in Papua, this turtle appears not to have nested, raising questions about factors that might influence timing of nesting and migratory behavior.

This pilot study was expanded in 2001 and is now a permanent component of the National Marine Fisheries Service (NMFS) Southwest Fisheries Science Center’s Sea Turtle Research Program, which partners with the Monterey Bay National Marine Sanctuary, Moss Landing Marine Laboratories, University of California Santa Cruz, and Hubbs-Sea World Research Institute. Having established the capacity to capture animals at sea, we increased field effort in 2002 and have now deployed transmitters on a total of thirteen animals, including three males. All were adults, with the largest one weighing 580 kilograms. Genetic results from six samples analyzed so far indicate that the animals are from western Pacific nesting stocks, most likely North Papua, Papua New Guinea, or the Solomon Islands.

We are currently tracking eight turtles (six females and two males), which were tagged in September 2002. As in previous years, the turtles moved rapidly westward after release. One of the turtles has turned around approximately 800 kilometers offshore and recently returned inshore and is currently just south of Monterey Bay. One of the females tagged in 2001 traveled north and foraged around the Gulf of the Farallones for two months before she began a westward migration.

Satellite telemetry is one aspect of a multi-faceted approach to studying the foraging ecology of leatherbacks that involves genetic, biochemical, behavioral, ecological, and oceanographic studies. In addition to looking at long-range movement and pelagic migrations, we are beginning to gain new insights into how leatherbacks interact with the marine environment by focusing on the Monterey Bay ecosystem.

Monterey Bay is one of several areas along the central California coast where leatherbacks aggregate during late summer. Other areas where we have observed the highest densities using aerial surveys include waters off Point Reyes, south of Point Arena, and in the Gulf of the Farallones. These areas represent upwelling shadows or regions where larval fish, crabs, and gelatinous organisms are retained during upwelling relaxation.

Figure 1. Leatherback sea turtle sightings in Monterey Bay

We hypothesize that leatherback turtle abundance is linked to the hydrographic retention of zooplankton and subsequent concentration of scyphomedusan prey (jellies and similar animals) in these coastal areas during relaxation of upwelling-favorable winds. When upwelling diminishes at the end of summer, sea surface temperatures along the coast tend to rise markedly. Observations suggest that leatherbacks move into Monterey Bay along with the 14-15o C water. The frequency, duration, and relaxation of upwelling-favorable winds can influence food web development in this region, including the occurrence and concentration of leatherback prey, such as scyphomedusae. Observations suggest that leatherbacks seek out the sea nettle (Chrysora spp.) to feed on in Monterey Bay, even though they have several different types of jellyfish to choose from. A better understanding of the factors that influence distribution and abundance of this jellyfish may help shed light on the local movement and dive behavior of the leatherbacks in the bay. )

In 2001 locations where turtles were seen during fine-scale aerial surveys corresponded to the 50- to 100-meter depth contours throughout the bay (Figure 1). Local hydrographic features may have influenced prey distributions, and future work will attempt to map turtle’s behavior against a three dimensional matrix of physical and biotic factors that describe its forage habitat. In 2002 pop-up archival tags (PATs) were attached to four of the turtles in addition to the satellite-linked dive recorders previously used. These PATs were programmed to collect fine-scale dive and temperature data that are archived and transmitted to orbiting satellites once the PAT releases and pops to the surface. Once these data are analyzed we will be able to look at foraging behavior on a finer scale than has previously been possible.)

The sanctuary will continue to play a key role in the recovery effort for Pacific leatherbacks by providing a unique venue to study foraging animals. Monterey Bay is perhaps one of the best-studied marine ecosystems in the world, with a wealth of data now available from various projects monitoring the physical characteristics of the marine environment using remote observing systems, such as the deep-ocean moorings deployed by Monterey Bay Aquarium Research Institute that report subsurface temperature, salinity, and current information, or ship-based transect studies across the sanctuary. We will be able to integrate physical and biological oceanographic data from these studies with results from our telemetry and aerial survey work to understand better how the leatherbacks interact with their ocean environment. We will also be able to develop new models to predict their oceanic distribution in order to help formulate appropriate at-sea conservation measures to complement the ongoing efforts on the nesting beaches.

Peter H. Dutton1, Scott Benson(1), and Scott A. Eckert(2)
(1)NOAA-National Marine Fisheries Service, Southwest
Fisheries Science Center
(2)Hubbs-Sea World Research Institute