USGS Turtle Trackers Work To Fill Data Gap On Dry Tortugas Loggerheads

By Daniel Kelly on September 4, 2015

Juvenile Green sea turtle captured August 2008 in the moat at Fort Jefferson, Dry Tortugas National Park. (Credit: Kristen Hart, U.S. Geological Survey)

In 2007, state and federal officials designated half of Dry Tortugas National Park in Florida as a no-take zone. The move, which means no fishing or anchoring is allowed there, was meant to help protect Dry Tortugas turtles, a threatened subset of Loggerhead turtles that make their home in the park.

This sort of ban sounds like a good idea on its surface, but scientists working around the park who actually study how the turtles live are concerned the ban may not cover all the areas that it needs to.

“The ban wasn’t really based on a knowledge of things there or figuring out the habitat use of these animals,” said Kristen Hart, a research ecologist with the U.S. Geological Survey. “My role as a USGS researcher is to provide scientific data to the National Park Service and NOAA (National Oceanic and Atmospheric Administration) who deal with the Florida Keys National Marine Sanctuary to make decisions, like which half of the park to protect.”

Hart is one of many scientists at the USGS who study turtles, and her work covers a lot of them. Along with the Dry Tortugas, she regularly encounters Green and Hawksbill turtles in the field. And she has an intimate knowledge of all the techniques used to wrangle them so that tracking devices can be attached to them.

These include finding turtles on beaches and “boxing them in” with wooden planks so that scientists can keep them in one spot long enough to do their work.

“When we’re on the beach, we’ll find a turtle walking, crawling or false crawling (when female turtles land on a beach to lay eggs and then decide not to). We’ll try to get them away from their nest and build a box around them,” said Hart. She mentions that researchers already have two halves of an enclosure built for the purpose. After they corral the turtle, they put both halves down and stake them together around the animal and then start on work to attach tags or take blood and tissue samples.

A nesting Green sea turtle bears a satellite tracking device on its shell in Dry Tortugas National Park. (Credit: U.S. Geological Survey)

From there, they commonly attach satellite tracking devices on the turtle’s shell. “We clean the site where the tracker will go, put a little epoxy on the tag and a little epoxy on the turtle and attach it,” said Hart. “It’s sort of like a pit crew.”

These land-based operations are a little more straightforward than the in-water alternative known as dip-netting. Hart calls it “rodeo capture,” and it involves finding a turtle in the ocean, diving in and grabbing it. In the Dry Tortugas National Park area, it seems to work well because the waters are clearer, she says. In the Everglades, everything is a bit muddier.

“When it all comes together, it’s pretty neat,” said Hart, describing the process that involves about four people to get a turtle aboard a ship. “You’re watching it and following it until you’re in the right position. I’m 5-foot, 4-inches tall and I think I caught the biggest turtle ever, about 450 pounds. You can’t be timid. You’ve got to just go for it.”

All of that tagging work helps her and others to track turtles all around the coasts of Florida, the Bahamas, Cuba and even into the Gulf of Mexico. Her team has put out around 20 tags this year around the park and there are some still out there transmitting from last year. She credits the success with the bigger tags that her group uses, as well as their epoxy application methods.

The data are especially good for the loggerheads, she says, and it’s possible to track their movements quite precisely with satellite tags. Hart points out a discovery that female loggerheads from the park sometimes travel as far south as the northern Bahamas. She and others simply had no idea they went that far. A similar find was made when researchers tracked a Green turtle that had bolted all the way to the Yucatan Peninsula, where they guessed he must’ve been pursuing a potential mate.

“Some males are residents. We saw green turtles that stayed there and lived there year-round,” said Hart. “That park needs to know there’s a segment of the population that’s non-migratory.”

But there are also morbid discoveries, Hart says, telling about a “harvested” turtle that was captured by humans.

“A harvest is really disappointing. When it (the tracker) crosses land and goes through a toll booth,” it’s not hard to tell what’s happened, said Hart. “When Google Earth shows it’s in a village somewhere, it doesn’t make sense. You can actually learn a lot through satellite tracking.”

Kristen Hart releases a juvenile Green sea turtle after workup and tagging, August 2008, Dry Tortugas National Park. (Credit: Keith Ludwig, U.S. Geological Survey)

Insights like those are what inform studies into how turtles use the region, including the Dry Tortugas National Park. To back up tracking data, researchers are also employing modeling techniques to show where most of the benthic cover that turtles rely on for food is located.

Hart says Green turtles mostly eat sea grass while Loggerheads, like the Dry Tortugas, rely on conches or lobsters. Hawksbills like to eat sponges. Basically, by looking at where those foods are, you can see if a certain area would be an appropriate zone for a turtle species to live.

Hart says her team is also partnered with others doing satellite work to map what sort of food sources are available and where. “A lot of times, researchers are good at seeing where a turtle went,” said Hart. “But what’s in the water, that’s the important thing.”

In a breeding season, she says turtles will lay eggs about every two weeks but then they go back to foraging areas. It’s important to know both where the breeding and foraging sites are to protect them.

The satellite data are particularly telling when it comes to seeing how turtles go back and forth between these areas.

“It’s like when you’re in a food court trying to find a place to sit. It’s based on analyzing tracks, points and information. You can almost see visually where they’re trying to settle,” said Hart. “It lets us test for site fidelity, if they’re more likely to settle somewhere. We can bring it all together and then we can delineate the boundary of that zone. It gives managers something to hang their hats on.”

That could mean more realistic regulations for visitors to Dry Tortugas National Park, she says. Certainly, it’s not ideal to keep people out of the park entirely. And there should be restrictions around nesting sites, since those are vulnerable areas. But leaving other areas open, she says, could provide neat opportunities for average people to see these majestic turtles up close.

“I hope that what we provide will help them make more realistic rules for where people should be during different times of the year, to make realistic regulations that wouldn’t discourage people from going to the parks,” said Hart. “We want to see that park managers can make more realistic rules or rethink why half of the park is closed off. It might be that the closed area isn’t the right size or that the closure isn’t enacted in key areas where it should be.”

Top image: Juvenile Green sea turtle captured August 2008 in the moat at Fort Jefferson, Dry Tortugas National Park. (Credit: Kristen Hart, U.S. Geological Survey)