Netting or grabbing the turtles could disturb their normal diving patterns after they are released, Dr. James said. So he has spent many hours in a small, homemade cradle under the bowsprit of a fishing boat, while the crew tracks the turtles’ paths as they dive. They estimate where a turtle will come up, then bring the boat close enough for the tagger to plant the device.

Dr. James said none of this would work without the cooperation and experience of the fishermen who run the boats and built the cradle. “Get a bunch of scientists on board, and it’s never going to happen,” he said.

In a paper published Monday in Frontiers in Ecology and Evolution, Dr. James, along with Bryan P. Wallace of Stratus Consulting in Boulder, Colo., and Duke University, and Michael Zolkewitz of Ecological Research Solutions in New Hope, Pa., presented video and other data, and their analysis.

They confirmed that the turtles made shallower and shorter dives while feeding in cold waters than they did elsewhere. And they also found that the turtles get a lot of their prey on the way up from their dives, suggesting that they may be silhouetting their prey against the sun. Any detailed knowledge of their behavior, Dr. James said, could help in protecting them from becoming entangled in fishing gear.

Leatherbacks are not your average turtle. They are the only sea turtle without a hard shell, thus the name; they are more comfortable in colder waters than other turtles; and they are heroic in size and abilities. They normally weigh 600 to 1,000 pounds, but the largest one on record weighed a little more than 2,000 pounds. They can spend an hour and half underwater and dive to 3,000 feet. Common feeding dives are more than 20 minutes long and deeper than 300 feet.