Solving the mystery of the “lost years”
Ever tried to track a baby sea turtle? It’s not easy. They hatch on the beach and immediately head out to sea to go—where?
For years, scientists have put forth various theories about where the baby turtles go on their maiden voyage, but there’s been no hard data. Until scientists came up with a lightweight tracking device, there was no way to follow the turtles’ movements from the time they hatched until they returned to the same beach years later to mate.
Now we have some data and new insights, thanks to Kate Mansfield, a marine biologist at the University of Central Florida, and a team of scientists from the UCF, Florida Atlantic University, University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science, and University of Wisconsin. The team tracked 17 loggerhead turtles for 27 to 220 days in the open ocean, using small, solar-powered satellite tags.
The solar-powered tags were originally designed for terrestrial bird species. They don’t rely on batteries for power, so they’re small and lightweight. All they require to function is light and air (the tags only transmit data when exposed to air). It was a smart choice: sea turtles tend to stay near the sea surface, where there’s plenty of both.
Harnesses, epoxy, or silicone?
Settling on the tracking technology created another problem to solve: how to attach the satellite tags to the turtles so they stayed on in salt water and didn’t hamper the turtles’ movement or growth.
Dr. Mansfield and her team tested a number of methods for attaching the tags to the turtles’ shells. Small harnesses that carried the tags proved to be problematic: although the harnesses stayed on the turtles over time, they didn’t adjust for the turtles’ growth, which caused the turtles’ shells to become misshapen (luckily, a temporary effect that was reversed 1-3 weeks after the harnesses were removed). Breakaway harnesses also didn’t work, as they failed to release as the turtles outgrew them.
The team turned to a different strategy: attaching the tags directly to the turtles’ shells with adhesive. They tested various adhesives and methods until they found a neoprene-silicone attachment that lasted the longest, with the fewest side effects for the turtles.
The turtles find their sweet spot
The scientists hypothesized that the turtles would stay within the currents of the North Atlantic subtropical gyre, circling the Atlantic. The data suggests otherwise: it appears that the turtles drop out of those currents and into the middle of the Atlantic or the Sargasso Sea, where they stay on the surface, eat, and bask in the heat. After several years of this halcyon life, the turtles head to nearshore waters, and ultimately back to their beaches to mate*.
*Loggerheads reach sexual maturity between ages 17-28, (their average lifespan in the wild is 47-67 years). They are endangered largely due to the loss or disruption of their nesting grounds. The female turtles have a low reproductive rate, and any disruption to their nesting grounds further reduces their population. Efforts are underway to restore and protect their nesting habitats.
About the technologies
(from the published report in the Proceedings of the Royal Society B, April 22, 2014)
“Microwave Telemetry’s PTT-100 9.5 g solar-powered satellite transmitters were attached to the turtles’ shells. Using the Argos satellite data processing system and Kalman filtering algorithm, transmitter data were filtered based on accuracy of transmission using Argos location codes (LC) 3–0, A and B.”
“Mean daily ambient temperature…and solar cell charge…were collected from transmitter sensor data. Bathymetry data and MODIS 9 km resolution daily sea surface temperatures (SSTs) were extracted using the Satellite Tracking and Analysis Tool (STAT). Additional SST and bathymetry data were derived using the Global Hybrid Coordinate Ocean Model and 2 min Gridded Global Relief Data (ETOPO2 v. 2).”
Detailed—and surprisingly interesting— information on the methods explored for attaching the satellite tags to the turtles may be found at http://www.int-res.com/abstracts/meps/v457/p181-192/