Leatherback sea turtle tracking data to help cut back on fishing-related deaths

Hatchling leatherback crawling from nest to sea (Credit: Nathan Robinson)

Leatherback turtles don’t swim through the water. They fly.

Their bodies have a teardrop shape that lets them shoot through the water at speeds up to 22 miles per hour. Massive pectoral muscles propel them along as their flippers encounter the same forces as airplane wings.

“We actually discuss what they do as flight,” said Stephen Morreale, a senior research associate and adjunct associate professor in the Department of Natural Resources at Cornell University. “You can do the same calculations for flying aircraft on their flying wings. Their flippers and body shapes are the same thing. They have attack angles. They have stall speeds.”

With such impressive hydrodynamics, the leatherbacks roam all corners of the world’s oceans. They were made for long-range movement and have been tracked on journeys from Indonesia all the way to the west coast of the United States, a 12,000-mile trek.

Morreale says all that traveling makes it difficult for fishing operations to avoid the turtles. The mostly commercial ventures use long-line fishing techniques. Lines are often more than 100 kilometers long and lined with meat-baited hooks that snag aquatic life indiscriminately.

Leatherback turtles get entangled in long lines as well as fish. But when an air-breathing turtle gets stuck on a hook, it suffers more because it can’t resurface. Frenzied fish along the line pull it down. And the thick bones that make up the turtle’s head contribute to one of two outcomes: either the hook goes completely through, which means it can’t be removed, or it stabs through the path of least resistance, the eye socket.

Hatchling leatherback Pacific green turtle hooked on a longline (Credit: Samuel Friderichs)

Morreale has seen these outcomes first hand and recently took part in a study that might help minimize them. It combined existing data on leatherback movement throughout the Pacific Ocean, one of the world’s largest fisheries, to find areas the turtles are most at risk of being accidentally caught in hopes of helping fishermen avoid them.

“(Tracking leatherbacks) gets complex. You’ve got to look at it like a movie,” said Morreale. “It’s not static. These aren’t like Caribou standing in a field. They’re always moving.”

So scientists work with their movements, attaching small lanyards behind their tails that flow in the creature’s slipstream. Other tracking structures include harnesses tied around its body. Most all include some type of transmitter to share the animal’s location.

All of the locational data used for the tracking study were collected through the Argos system of satellites.

Leatherbacks aren’t distributed randomly, or even uniformly, said Morreale. That makes finding areas where they’re more likely to be tricky.

“We pinpointed on a Pacific basin-wide scale — which is almost impossible to do — by putting all our data together,” Morreale said. “Just to see all the data up on the same screen is pretty phenomenal.”

He noted populations of leatherbacks nesting in Costa Rica and Mexico. A foraging area near Monterey Bay, Calif. supports other turtles that come in to feed on its jellyfish. Nesting beaches exist near Papau New Guinea and Indonesia. “We saw animals from both sides of the oceans, and you would think that each side would be of one group or another,” said Morreale. But leatherbacks from Central America didn’t mix with populations feeding in California.

Hatchling leatherback entering the sea (Credit: Nathan Robinson)

Feeding areas and nesting beaches are common spots for the turtles, but leatherback locations are also influenced by the time of the year. Morreale says there’s a seasonal likelihood for the occurrence of a leatherback turtle. By taking those figures and overlaying them with known industrial fishing zones, researchers came up with fisheries hotspots — places likely to have abundant prey populations that attract both leatherbacks and fishermen.

And then to quantify the risk of a leatherback turtle getting caught, a little math is required. In order to get the long-line probability, the number of hooks deployed annually in the Pacific Ocean is multiplied by the probability of just one leatherback being caught. “Multiply that probability by 760 million hooks and, even if it’s miniscule, tell me what you come up with. It’s just not even believable,” said Morreale.

So what can be done to bring the risk down?

“I’ve been with the long-line industry a lot and I think everyone would love a gear fix – some magic solution to keep them (leatherbacks) out and exclude them from bycatch,” said Morreale.

But there isn’t one. Could currently available models say when to close off areas? “That’s fine, that’s good,” said Morreale. “But it’s not pinpointed enough.” If there’s going to be something like that put in place, he says time-area closures, which prohibit fishing in regions during certain periods of likely high leatherback occurrence, are effective and easier to implement.

Leatherback hatchling swimming (Credit: Samuel Friederichs)

Morreale says solving a problem like this needs a constantly changing solution. Once leatherbacks are in the ocean, they are always on the move. Fishing boats move from area to area looking for abundant schools. And as many sailors know, the ocean is fickle.

“A good model will react to that. It has to be moving. It has to be multi-dimensional,” said Morreale. For example, he says, it would be simple to set up a system that would close areas to fishing if a leatherback turtle were caught nearby. If one is caught, then the likelihood of catching others is higher. “These are the types of things we can aim for.”

This comes from someone who has seen leatherback tracking devices go from large obstructions the size of one-liter bottles to mini packages clipped on in less than five minutes. Before telemetry caught up, Morreale says there wasn’t a basis to model where the turtles are and where they’re going. Now that it has, moving into higher-level, dynamic models may finally allow researchers to answer long-standing questions about leatherback movement and prevent bycatch.

“This is a big problem. We’re only at the beginning of it and we only talked about one species of sea turtle here,” said Morreale. Six other species are also at risk.

Top image: Hatchling leatherback crawling from nest to sea (Credit: Nathan Robinson)