Every year, thousands of loggerhead sea turtles hatch on the beaches of Florida and flap desperately toward the sanctuary of the surf.
Thirty-one years later, the surviving females return for the first time, to lay eggs and begin the cycle anew. The intervening three decades are fraught with peril, from natural predators to oil spills to entanglement in fishing gear. And yet, according to a new study in the online journal PLoS One, by far the biggest factor determining how many turtles survive to sexual maturity is none of those. It is, simply, the conditions in the ocean in that very first year, when the hatchlings make it into the water and begin their battle for survival.
“Like fish, sea turtles have high offspring productivity but no parental investment,” Kyle van Houtan of the National Oceanic and Atmospherics Administration (NOAA) told Discovery News. “What ensures the hatchlings survive the first year of life? Unlike mammals, they have no parent guarding or nourishing them.”
The answer, he said, is that, “The climate is their parent. If the ecosystem is productive, there will be more food, and a greater chance of survival. But if the ocean regions where juveniles congregate are unproductive, then most of the hatchlings might not survive.” So even if the year is good for turtle mommas making tons of nests, the ocean dynamics that year could be murder on the hatchlings.
Van Houtan and John Halley of the University of Ioannina in Greece plotted loggerhead nesting counts in the Atlantic and Pacific against natural climatic cycles lasting one or more decades – the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. They developed a model that showed that, over time, nesting counts reflected changes in these patterns, and that the strong year-to-year swings that frequently occur in nesting counts can be tied to inter-annual temperature variations three decades previously.
“What we’re saying on one level sounds crazy: You put a thermometer in the ocean, and you know 30 years later how many sea turtles are going to show up to breed. It’s kind of amazing,” admitted van Houtan. But, he pointed out, the same theory has been a fundamental tenet of fisheries biology for over twenty years; the difference, of course, is that commercial fish species may take only a year – or, in the case of, say, rockfish, eight years – to reach maturity, substantially less than is the case with loggerheads.
The ocean temperature itself is not necessarily the determining factor, van Houtan explains, but is rather an indicator of underlying processes.
“What temperature reveals in the Pacific Ocean is wind shear coming off the north Asian continent,” he said. “That means that the surface water is being sheared off, and the cooler, nutrient-rich water from below is coming to the surface, and that just basically causes a trophic cascade: boom time for plankton and then up through the system. In the Atlantic, we believe the temperature is indicating differences in the Gulf Stream itself. And that would be both mechanistic – getting the juvenile turtles from Florida to the North Atlantic Gyre – but could also have something to do with productivity itself.”
Interestingly, temperature changes appear to affect Atlantic loggerheads differently than Pacific loggerheads, van Houtan notes:
The fact that there is such a strong correlation between climate and turtle survival does not, van Houtan is at pains to point out, mean that anthropogenic factors such as fishing mortality or egg poaching are at all unimportant. It is simply a matter of scale: In any year class, the greatest number of turtles are going to be those in the first year after hatching, and that is the year at which mortality will be highest. Because of the sheer numbers of turtles involved, anything that can affect the rate of mortality in that first year by, say 10 or 20 percent, is going to have immense consequences.
“We’re not in any way exonerating anthropogenic factors,” he said.
Top: Loggerhead turtle hatchlings heading for the coast. Photograph by Hila Shaked, via Wikimedia Commons.