Green sea turtles may stop basking on beaches by 2100 due to rising ocean temperatures according to a new study. Researchers analyzed sea turtle surveys and satellite data over several years and found that turtles bask more when sea surface temperatures are cooler, around 23 degrees Celsius. As climate change continues to warm oceans, the study predicts turtles will no longer need to bask on land to regulate their body temperatures globally by 2100 or in Hawaii as early as 2039. Without basking, turtles' immune systems and digestion may be negatively impacted.

1. Warming Seas Decrease Sea Turtle Basking
Link: http://nicholas.duke.edu/news/warming-seas-decrease-sea-turtle-basking
Friday, January 23, 2015
featuring:
 Kyle Van Houtan
Contact:
Tim Lucas, 919-613-8084, tdlucas@duke.edu
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By Kati Moore, MEM ‘16
Nicholas School Communications Student Assistant
DURHAM, N.C. -- Green sea turtles may stop basking on beaches around the world within a century
due to rising sea temperatures, a new study suggests.
Basking on sun-warmed beaches helps the threatened turtles regulate their body temperatures and
may aid their immune systems and digestion.
By analyzing six years of turtle surveys and 24 years of satellite data, researchers from Duke
University, the National Oceanic and Atmospheric Administration (NOAA) Pacific Islands Fisheries
Science Center and the University of Ioannina in Greece have found the turtles bask more often each
year when sea surface temperatures drop.
If global warming trends continue, this behavior may cease globally by 2102, the study projects. In
Hawaii, where the study was primarily focused, green turtles might stop basking much earlier, by
2039.
The scientists published their peer-reviewed findings last week in the journal Biology Letters.
“By comparing turtle basking counts with sea surface temperatures, we found that green turtles tend
not to bask when local winter sea surface temperatures stay above 23 degrees Celsius,” said lead
researcher Kyle Van Houtan, adjunct associate professor at Duke’s Nicholas School of the
Environment.
To conduct the study, Van Houtan and his colleagues used six years of turtle count data collected
daily by the Hawaiian nonprofit Mālama na Honu on Laniakea Beach in Oahu. The counts showed
regular, seasonal fluctuations in the number of turtles basking on the beach. These fluctuations
correlated with sea temperatures at Laniakea, indicating that sea turtles bask more when waters are
cooler.

2. The scientists then compared these fluctuations in temperature and basking to growth marks in the
humerus bone of several green turtles. They found that the growth lines occurred at the same time of
year when turtles bask more, between February and April.
The turtles’ growth lines are similar to tree rings in that they indicate periods of stress for the
organism, said Van Houtan, who is also a scientist in NOAA’s Turtle Research Program. In trees,
growth rings can indicate winter, dry seasons, or periods of drought. In green turtles, the lines seem to
reflect periods when seas are colder and body temperatures are consequently lower, prompting the
turtles to haul out on beaches to warm in the sun.
More research is needed to fully understand the importance of basking and the effect climate change
will have on basking behaviors of green turtle populations around the world. said Van Houtan.
Not all green turtles bask on land, he noted. Though the turtles are found in tropical and subtropical
oceans around the world, beach basking has only been observed in Hawaii, the Galapagos Islands
and Australia. Sea surface temperatures at these sites have been observed to be warming at three
times the global average rate.
It is not yet clear whether populations that currently bask on land during cooler months will adapt to
warming sea temperatures and begin to bask exclusively in the water, as do some other populations
around the world.
“When looking at climate change, which is this vast geopolitical issue, you have to drill down to
specific climate variables impacting specific aspects of an organism’s life,” said Van Houtan. “The next
step for us is to look at how turtles are storing climate data in their bodies -- in their tissues, shells,
and bones, and how we can tease that out.”
The research was funded by the National Oceanic and Atmospheric Administration and by a
Presidential Early Career Award in Science and Engineering.
John M. Halley of the University of Ioannina, and Wendy Marks of the NOAA Marine Turtle Research
Program were co-authors of the study.
###
CITATION: “Terrestrial basking sea turtles are responding to spatio-temporal sea surface temperature
patterns,” Kyle S. Van Houtan, John M. Halley, and Wendy Marks. Biology Letters, January 14, 2015.
DOI: 10.1098/rsbl.2014.0744
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