"Nobody is really out chasing jellyfish down," Shillinger said. "They are poorly studied organisms and there is very little data on them in the region of the gyre."
But the data that came back from the tagged turtles suggest there may be plenty of jellyfish on which to feast.
"We saw a distinct reduction in the swimming speed of the turtles as they entered the South Pacific Gyre," Shillinger said. "They were making more turns, diving more frequently and diving deeper. All those things suggest feeding behavior."
Another piece of evidence was the timing of the turtles' dives. Like many marine organisms, jellyfish appear to engage in daily vertical migrations, moving into shallower depths at night and returning to somewhat deeper depths during the day.
The turtles' dives mirrored those movements, with their nighttime dives averaging about 38 meters deep, while average daytime dives were around 65 meters.
"The deepest dives we had in the data set were in the daytime, including the longest one, which was over 900 meters," Shillinger said. "That dive was also one of the longest leatherback dives ever reported. It was about 84 minutes." The cause for these superlative dives remains a mystery, although seeking prey and avoiding predators are likely motivations.
"Understanding what sort of areas leatherbacks are likely to favor is a critical first step in protecting them in the open ocean," he said.
From 2004 to 2007, Shillinger and his colleagues tagged 46 female leatherbacks on the beach in Costa Rica with satellite tags that broadcast information on location, depth and water temperature for an average of 245 days, with one tag transmitting for 562 days. "Altogether, it added up to 13,038 days of turtle tracking," Shillinger said.
One of the biggest hazards leatherbacks face on t
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