Forecasting Turtles

A complex conservation problem just got even more complicated. A new study finds that shifting climate and ocean conditions appear to have played a dominant role in recent worldwide declines of endangered loggerhead sea turtles. The conclusion suggests that efforts to protect nesting beaches and keep turtles out of fishing nets will be just part of the solution to saving turtles. But the study also finds that future ocean climate shifts might bring good news for turtles living in the Atlantic Ocean.

“The long-term variability of marine turtle populations remains poorly understood, limiting science and management,” Kyle S. Van Houtan of the U.S. government’s Pacific Island Fisheries Science Center in Hawaii and John M. Halley of the University of Ioannina in Greece note in PLoS One. Although many loggerhead populations appear to have crashed in recent decades, for instance, the reasons weren’t clear. But some researchers suspected that large-scale climate oscillations might be playing a role – especially because the turtle declines often paralleled fisheries crashes that have been linked to periodic, decade-long swings in ocean currents and water temperatures.

To test that idea, Van Houtan and Halley compared climate records from the Atlantic and Pacific Oceans to key indicators of turtle population health, including the number of nests found on beaches in Japan and Florida. The past pattern was clear: Turtle populations in the North Pacific and Northwest Atlantic were “strongly correlated to ocean conditions — such that climate models alone explain up to 88% of the observed changes over the past several decades.” Sea surface temperatures, for instance, are often closely correlated with nesting numbers, and ocean conditions also appear to play a big role in determining the survival of juvenile turtles, which tend to aggregate in open ocean “hotspots.” In years when prey is scarce, many of these young turtle die – meaning they aren’t around to reproduce decades later, when they reach sexual maturity. Indeed, it appears that “climatic factors in the hatching year are the single most important variable” in determining turtle populations.

The researchers also examined what future climate changes might mean for the turtles, and found a mixed forecast. The news isn’t very good in the Pacific: “Available climatic data suggests the Pacific population will be significantly reduced by 2040,” they write. In the Atlantic, however, the “population may increase substantially” due to favorable shifts.

The results “do not exonerate” so-called “top-down” human impacts, such turtles killed in fishing nets and nesting-beach destruction, the authors conclude. But they do highlight the significance of “bottom-up oceanographic processes” in shaping populations. And climate models, they add, “provide new insights into the historical, current, and future populations of marine turtles” that could help conservationists figure out the best ways of helping this ancient species survive. – David Malakoff | April 29, 2011

Source: Van Houtan, K., & Halley, J. (2011). Long-Term Climate Forcing in Loggerhead Sea Turtle Nesting. PLoS ONE, 6 (4) DOI: 10.1371/journal.pone.0019043

Image © Sheldonso |