Nuclear clues to a turtle's decline

Sea turtles drift in mysterious currents. How long do they live? How fast do they grow? When do they reach sexual maturity?

Answers to these and other basic questions elude researchers, stymieing efforts to better understand and protect the world’s seven marine turtle species, all but one of which are endangered. Now scientists in Hawaii may have resolved some of the unknowns by turning to an unlikely environmental marker: nuclear fallout.

As described in the journal Proceedings of the Royal Society B, the researchers used bomb radiocarbon, which winds up in the turtles’ shells through their diet, to estimate the birth dates, growth rates and age of reproductive maturity of critically endangered hawksbill sea turtles. In doing so, the team also stumbled upon clues to two critical factors plaguing the turtles: late maturity and poor food sources. “Coming into this, we didn’t expect to raise all of these questions and red flags, but I’m happy it turned into a more complex story than one simply about growth and ageing,” said Kyle Van Houtan, a conservation ecologist at Duke and the National Oceanic and Atmospheric Administration.

Pretty little things

Hawksbill turtles are named for their beaky, bird-like faces, but their most eye-catching feature is their shells, ornately patterned in honey, chocolate and mahogany hues. Collectors have long taken note: In the 20th century alone, millions of hawksbills were killed so their shells could be carved into bracelets, hairpieces, jewellery boxes and other trinkets.

Hawksbills were listed as an endangered species in 1970, and international trade in them was banned a few years later. Yet in many parts of the world, including the United States, hawksbills have continued to decline. In Hawaii, the turtle’s numbers have remained stubbornly low despite a dedicated conservation programme. Roughly 100 adults live there, with fewer than 20 females nesting per year, making Hawaii’s hawksbills the world’s smallest population of sea turtles. Researchers believe a simple lack of information has been holding back conservation of the turtles. “We know very little about hawksbills, even compared to leatherbacks, loggerheads and green turtles,” Kyle said.

Bomb radiocarbon turns out to be one way to do that. In the mid-20th century, the United States and several other countries began conducting open-air nuclear tests that significantly increased concentrations of carbon 14, a radioactive isotope, in the atmosphere. Bomb radiocarbon began tapering off only in the 1970s, with the end of widespread nuclear testing. Carbon 14 does not harm plants and animals, but it does accumulate in their tissues at varying levels. Researchers figure out how much carbon 14 existed at a particular moment in history by using so-called reference specimens. A coral collected from a certain reef in Hawaii in 1975, for example, would provide a marker of carbon 14 levels for that place and year.

Because radiocarbon decays at a fixed rate, comparing levels in a reference material with those in a shell or bone of unknown age allows researchers to determine when it formed — a method that scientists regularly use for estimating the ages of sharks or other long-living fish. Until now, however, the method had not been tried on sea turtles. Kyle and his colleagues combed museums and government archives to find shell samples from 36 hawksbills — 23 from Hawaii, the rest from other Pacific sites. All of the turtles’ death dates were known, spanning 1962 to 2013.

The researchers then used coral samples to build a computer model of background levels of radiocarbon in the Hawaiian archipelago from 1900 to 2012. Because the turtles accrue tissue in their shells throughout life, concentrations of radiocarbon in the most recently formed layers can be compared with the corals’ carbon 14 records for the same year. Those values, however, don’t correspond exactly, because coral acquires carbon directly through the water while turtles do so through diet. The team was able to adjust for the difference and thus estimate each turtle’s date of birth.

In a final step, the researchers applied the radiocarbon results, along with known measurements from captive and wild-caught turtles, to a mathematical model that could estimate an animal’s growth rate. The results gave Kyle and his colleagues an idea of how old their turtles were when they hit maturity. The age at which sea turtles begin breeding depends on how well they are fed. Not surprisingly, the team found that animals raised in captivity grew the fastest, reaching maturity at 12 years. Wild turtles from places besides Hawaii hit that benchmark at 12 to 15 years. The Hawaiian turtles, though, were extremely late bloomers, holding out for an average of 29 years before they began breeding. “This means something in the environment in Hawaii is suboptimal, and significantly so,” Kyle said.

To discover what that “something” might be, he and his colleagues took a closer look at the radiocarbon levels in corals and turtles. The difference should have been constant over the years. But in recent decades, the researchers discovered, levels found in corals and turtle shells have become strikingly similar, indicating a shift in the turtles’ diet toward the bottom of the food chain. Hawksbills are normally omnivores, eating sponges, crustaceans, algae and more, but it appears the population in Hawaii is going vegetarian — a change the researchers suspect is linked to the decline of Hawaii’s coral reefs over the last century. “Decline of the reef means declines of other species in that ecosystem that hawksbills rely on for food,” Kyle said.