From the deck of a small research boat, Rob Downs, a sonar expert with NOAA's National Ocean Service, lowered an automated underwater vehicle into the waves. The AUV was bright yellow, about 6 feet long, and shaped like a torpedo. Like the AUV that is currently searching the bottom of the Indian Ocean for Malaysian Airlines Flight 370, this one was equipped with side-scan sonar. But its first-of-a-kind mission was to find something much smaller than an airplane. It was searching for sea turtles.
All species of sea turtles found in U.S. waters are listed as threatened or endangered under the Endangered Species Act, and NOAA Fisheries scientists need to keep tabs on their populations. Larisa Avens, who leads sea turtle research at the NOAA Fisheries lab in Beaufort, North Carolina, is one of them. “Sea turtles are often surveyed from the air,” Avens said, “but flights can be expensive, and you only see the turtles when they surface to breathe.” Avens and Downs, along with their academic and state agency research partners, hope to help solve that problem using sonar.
Will Sea Turtles Stand Out on Sonar Images?
Scientists have been using side-scan sonar to search for marine animals for at least a decade. But they've been doing that with sharks, sturgeon, and other animals that travel near the bottom but not on it. That makes them easier to spot by the acoustic shadow they cast on the seafloor. Turtles, on the other hand, are often down in the mud, where they can resemble a pile of rocks.
Downs said the sonar images look like a charcoal drawing of the seafloor. “Metal and concrete are very strong acoustic reflectors,” he said, so man-made objects stand out. “Sea turtles give a weaker acoustic return.”
So for their first go at estimating sea turtle abundance using sonar, scientists chose an ideal spot. Cape Lookout Bight, at the very southern end of North Carolina's Outer Banks, is a small body of water formed by a hook in a barrier island. It's only about a half-mile square and 30 feet deep, but every May it fills up with sea turtles. Scientists aren't sure why these usually solitary creatures congregate there, but whatever the reason, they provide plenty of opportunities to test out the sonar.
Downs programmed the AUV to run tight transects at different depths and using different sonar frequencies. Low frequencies have a longer range, but they produce low-resolution images. High frequencies produce much clearer images, but can’t see out as far. Back at the lab, scientists will determine which combination of settings produces the best balance of image resolution and area covered.
Hands-On Techniques Will Always Be Needed
Even the highest possible resolution won't give scientists all the information they need. So while Downs and his team were running their underwater robots, Avens and a team of biologists were capturing a sample of turtles using more traditional techniques.
They set a 100-meter mesh entanglement net in the Bight and kept a close eye on the buoys atop the net. Whenever the buoys started shaking, they rushed to work.
"We want to clear the net as quickly as possible if anything is entangled," Avens said, since sea turtles need to come up for air. They also checked the length of the net every 30 minutes to make sure they didn't miss anything.
They caught 57 sea turtles over 4 days. The largest—an adult male loggerhead—weighed about 350 pounds and took a team of six to haul onto the deck of their small research boat.
Once they had an animal onboard they applied microchip and flipper tags and measured the turtle’s carapace to determine whether it was a juvenile or adult. Satellite and acoustic tags were attached to a subset of turtles so scientists could track their migrations. They then took tissue samples for DNA analysis that would show which population the turtle was from. Chemical analysis of stable isotopes in the tissue would reveal what the animals were feeding on and where.
Conservation Strategies Require Good Data
They also took blood samples. Until they mature, sea turtles have no external sexual characteristics, and blood testosterone levels are the only way to determine sex.
We're very interested in monitoring sex ratios because sex determination in sea turtles is temperature-dependent," Avens said. "The warmer the incubation temperature, the more females you'll get in a nest."
This may be a problem in the future if the climate continues to change. Sex ratios for many species are already female-biased, with about two to three females for every male. This can still be a functional ratio, since females nest on average every 2 to 3 years, whereas males may mate more frequently. "But if temperatures continue to rise, the populations might become male-limited, and that would affect their reproductive potential," Avens said.
Climate change is one threat sea turtles face, but they’re also confronted with more immediate dangers, such as being struck by boats or caught in fishing gear. NOAA Fisheries works to reduce these threats by coordinating a stranding network that rehabilitates injured turtles, for example, and by developing gear modifications that allow turtles to escape fishermen’s nets. For these strategies to be effective, scientists need to keep close tabs on sea turtle populations.
That should become easier with side-scan sonar. Hands-on data collection will always be necessary, but abundance estimates using sonar will help scientists extrapolate the results of their hands-on methods to the larger population more efficiently and accurately.
“We need a lot of data to protect these species,” Avens said, “and we’re always looking for better ways to get it.”
