MARINE BIOLOGY Q
Turtles swim
north or south depending on when they were born
TRACKING TURTLES M
Biologists from the University of Central Florida, who have been satellite- tracking young sea turtles in the South Atlantic Ocean, say they have made some surprising discoveries about the reptiles’ ‘lost years’
arine scientists have discovered that South Atlantic sea turtles do not passively ride prevailing currents, but instead actively swim and orient themselves to remain offshore. And, depending on when they hatch within the sea turtle hatching season, they travel in varying, sometimes opposing, directions – including into the Northern Hemisphere.
“From a conservation perspective, it is vital to understand where the
youngest sea turtles go and how they interact with their environment,” explains professor Kate Mansfi eld of the University of Central Florida (UCF), who led the recent study. “Knowing that they disperse in different directions, depending on
changes in ocean currents, will help us get a better sense of where and when we need conservation efforts to ensure their continued survival.” This study was a collaboration between UCF, Brazilian sea turtle conservation group Projeto TAMAR, and the National Oceanic and Atmospheric Administration Fisheries. The study is helping to build up new knowledge about the sea turtles’ ‘lost years’ – the time after hatching then heading out to sea but before they return closer to shore as much larger juveniles several years later. “Young turtles have to survive several years out in the open ocean
and then a couple decades in coastal waters before they fi nally reach maturity and can contribute to their populations,” says Mansfi eld. “Sea turtles are very late-maturing and long-lived creatures, which makes fi nding out about their ‘lost years’ very challenging indeed.” The team tracked 19 turtles for more than 120 days off the coast of
Brazil. Turtles in the study were released early in the hatching season (when prevailing currents are to the south), in the middle of the hatching season (as currents begin to switch from south to north), and late in the hatching season (when offshore currents head mostly to the north). Passive fl oats, or oceanographic drifters, were released with the satellite-tagged turtles to test for their swimming behaviour. The fl oats also served as a comparison between the turtles’ behaviour and known passively drifting objects that would move with the currents.
The team found that early in the hatching season the majority of the
turtles followed the strong southward currents. The turtles released in the middle of the season initially moved southward and three eventually veered to the north as the currents shifted. All drifters accompanying these turtles beached within a month, yet all turtles remained offshore. Late in the hatching season, when currents fl owed to the north and
west, turtles uniformly moved north and north-west across the Equator. “Active orientation combined with swimming behaviour helps the
turtles remain seaward of the Brazilian continental shelf instead of being driven towards land like the drifters,” explains Mansfi eld. “Biologically, it is a fascinating phenomenon.” Mansfi eld’s original study in 2014 looked at the behaviour of North
Atlantic loggerhead turtles. The fi ndings gave researchers the fi rst insights into the turtles’ ‘lost years’. In that study, many of the tracked turtles unexpectedly left the currents associated with the North Atlantic Subtropical Gyre, a large, circular current system in the North Atlantic Ocean, and entered the Sargasso Sea, an area inside the gyre. A similar gyre, the South Atlantic Subtropical Gyre, exists in the
South Atlantic, but the sea turtles in the south did not act the same. Co-author Nathan Putman, a senior scientist with ecology research
company LGL Research, believes that the observed movement patterns may be “like money invested in a diverse suite of stocks, which allows Brazilian turtles to spread risk for their offspring between two, largely independent, ocean basins, fostering a resilient population. “Unlike some other populations that have undergone dramatic
declines or increases,” he adds, “these seem to be pretty steady – much like money invested in a diverse suite of stocks.” Mansfi eld concludes: “As we track more young turtles in different
ocean basins under different oceanographic conditions, we are fi nding that long-held assumptions and hypotheses about sea turtle ‘lost years’ behaviour are too limited – one hypothesis can’t explain it all.” Improving technology and further miniaturization of the satellite tags
are expected to help marine biologists and ecologists fi nd out much more about young sea turtles in the near future.
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