Biology researchers from the College of Sciences ' UCF Marine Turtle Research Group studied the dispersal movements of four juvenile sea turtle species, revealing that they may be active swimmers, rather than passive drifters, during their early life stage known as the "lost years." These findings challenge existing hypotheses and provide important data for assessing risks from human activity and informing conservation efforts.
The study, funded largely in part by Florida RESTORE Act Centers of Excellence Program, was published this week in the journal Proceedings of the Royal Society B , representing the largest satellite tracking dataset of wild-caught juvenile sea turtle behavior from the Gulf of Mexico during this life stage, spanning from 2011 to 2022.
"One of the main findings is where these sea turtles are and where they go in this life stage because we haven't known much about it," says Katrina Phillips '22PhD, who led the study alongside Nathan Putman and Kate Mansfield . Phillips says understanding these movement patterns among juvenile sea turtles will help guide conservation efforts to protect critical habitats for these species.
After hatching, sea turtles are known to leave their nests on land and enter the ocean where they spend their early years. This shift from terrestrial to oceanic habitat marks a critical transition in their life cycle to a life stage that has been understudied.
According to Mansfield, co-author of the study, professor of biology at UCF, and director of the UCF Marine Turtle Research Group, we are still learning about this life stage and it's more complex than assumed.
"We don't know what they're eating, about their health, if and when they associate with floating algae called sargassum, which provides some protection," Mansfield says.
The team of researchers tagged 131 juvenile sea turtles — 94 green turtles, 28 Kemp's ridleys, five loggerheads, and four hawksbills — and tracked their movements using satellite-equipped, solar-powered platform transmitter terminals. These movements were compared with those of oceanographic surface drifters, floating objects used to study how sea turtle movements are influenced by ocean currents.
Researchers believe juvenile sea turtles swim offshore as an adaptive behavior to avoid predators such as birds, sharks and other fish, which are more abundant near the shoreline. Their small size makes them particularly vulnerable, so offshore waters can provide a safer refuge.
"One of the longstanding assumptions, is that juvenile sea turtles stay far offshore. Researchers call this the 'oceanic life stage,' which means off the continental shelf in waters deeper than 200 meters," Phillips says. "However, what we found was that the turtles in this life stage are crossing over the continental shelf into neritic zones a lot more than we expected."
A continental shelf is the gently sloping, shallow underwater area that extends between the shoreline and the continental slope, where the seabed drops steeply into the deep ocean at the shelf break. This shelf includes the neritic zone, which is the part of the ocean closest to the coast, characterized by nutrient-rich waters and a high concentration of marine life.
Phillips says the sea turtles were found crossing over to shallower waters and closer to shore, but it did not appear that they were transitioning to their next life stage, where they typically move to shallow habitats and feed off the bottom. Instead, the turtles seemed to approach the shore, then turned to avoid it.
"That was interesting because we had these passive drifters that we released with them and many of them washed up shore and none of the turtles did," Phillips says.
She adds that if the turtles don't behave like passive particles drifting with the currents and can actively swim and control their position, then existing movement models could consider both factors to correct errors in projections.
Existing hypotheses about the early life stage of most sea turtle species suggested they live exclusively in oceanic environments, drift passively with ocean currents and typically do not return to their previous habitat once they transitioned to a new one. However, these assumptions lack research into actual movement behavior.
"Historically, all our information about this young life stage has been limited to opportunistic sightings of these little, hard-to-see animals from boats passing by, tracking work on hatchlings in the first 24 hours after leaving nesting beaches, or laboratory studies," Mansfield says.
Previous work also focused on the North Atlantic and on loggerheads, a species that commonly nested on the east coast of the U.S.
"I think it's important to get data from different places and put the puzzle together to get a bigger picture of what's going on," Phillips says. "Researchers tracking this species were finding that they were staying offshore. But now that turtles are tracked from more places, we are finding that there are more nuances to what goes on. Loggerheads, for instance, we found stay off the continental shelf located in the west coast of Florida."
Mansfield says sea turtle tracking can be costly, labor intensive, and the technology has limitations.
"It's really hard to follow and manually track a little turtle over time," Mansfield says. "You have to fuel a boat with researchers who have a strong stomach to go into the ocean. Historically, technology just wasn't there to put a tag on a turtle and use satellites to be able to remotely track where they went. Tags were battery powered and as big as a brick."
Prior to her time at UCF, Mansfield figured out a method to safely tag and effectively track small turtles, thanks to more reliable tagging technology, which played a role in conducting this study and achieving its results. She also credits their partnership with Inwater Research Group in helping to catch and track smaller sea turtles.
This research into sea turtle movement during the "lost years," provides data for conservationists to assess and manage risks from human activity.
"The Deepwater Horizon oil spill in 2010 was a bit of the origin story of this project," Mansfield says. "If we have another oil spill, we need to know whether these animals [will be] transient through an area, stuck there due to currents, or if they'll end up somewhere else."
Data from this study is already driving conservation efforts, including a proposal for critical habitat designation under the Endangered Species Act for green sea turtles. This designation would complement earlier tracking data led by Mansfield, which established critical habitat for loggerheads — the sargassum algae nursery.
Mansfield and Phillips say if assumptions are that these animals are strictly oceanic, then they may not be protecting them completely or addressing what they need for their eventual recovery.
"If sea turtles are occurring on the continental shelf, we suggest renaming this life stage to 'dispersal stage' to account their behavior," Mansfield says. "This is important nuance in their life history, and the new terminology reflects a better understanding of sea turtle behavior, revealing more about these lost years."
Funding information
Funding and support for this research was provided in part by the NOAA Oil Spill Supplemental Spend Plan, NOAA Southeast Fisheries Science Center, Florida RESTORE Act Centers of Excellence Program administered through the Florida Institute of Oceanography, National Fish and Wildlife Foundation, Friends of Gumbo Limbo Gordon J. Gilbert Grant, Microwave Telemetry Christiane Howey Rising Scholar Award, U.S. National Science Foundation Graduate Research Fellowships Program, UCF Boyd Lyon Memorial Fellowship, National Research Council Research Associateship Program, and the University of Central Florida.
Researchers' credentials
Phillips, doctoral graduate, integrative and conservation biology, UCF; postdoctoral researcher, University of Massachusetts Amherst
Katherine Mansfield, professor, Department of Biology, UCF; director, Marine Turtle Research Group; and Davis-Shine Endowed Professorship in Conservation Biology
Nathan Putman, senior scientist, LGL Ecological Research Associates
