George Shillinger in Monterey, CA -- Did the leatherbacks’ harnesses fall off, or have their tags broken?
One of the divas of the Great Turtle Race, Stephanie Colburtle, hasn’t sent us a message for more than 100 days. We’re a little concerned about her and three other turtles: Windy, Champira and Drexelina. The other seven turtles are well on their way to their distant feeding grounds off Peru and Chile. This week, we’re looking at all the possibilities of what could have happened to the missing turtles.
TAG FAILED: "The chance of a tag simply stopping working one day is very small," notes TOPP researcher Phil Lovell, who heads instrument development at the Sea Mammal Research Unit at the University of St. Andrews. "The rate of failure follows the 'bathtub curve' like most electronic items: i.e. the chance of failure is highest during manufacture due to faulty assembly or components. Once tags have passed their initial functional and pressure tests, the failure rate is very low, and then after several years there is an increase in vulnerability due to ageing of the tag components."
TAG SLIMED: If seaweed got caught on the tag contacts or slimy algae covered the contacts, then the tag would think it is always "wet", and wouldn’t transmit.
“We've had tags go offline for as much as six months until the turtle moved to colder latitudes where the hitch-hiking critters presumably died and allowed the tag to restart,” notes TOPP researcher Scott Eckert. Eckert is director of science for the Wider Caribbean Sea Turtle Conservation Network (WIDECAST) located at Duke University's Nicholas School of the Environment Marine Laboratory, in Beaufort, N.C.
In fact, Saphira's tag was quiet for nearly all of June and July, but started talking again during the end of July. During the last week, her tag has transmitted several times. This is a sign that her tag may have been biofouled -- but that it is now clear.
Other possibilities: barnacles might grow over the contacts. In this case, the tag thinks it is dry and it would transmit constantly. The Argos satellite, which circles the Earth pole-to-pole, wouldn’t pick up most of the transmissions.
Near shore is where researchers see most of the biofouling, says TOPP researcher Mike Fedak, a professor at the Sea Mammal Research Unit at the University of St. Andrews, one of the leading satellite-tag makers in the world. Their tags have been attached to several species of seals, turtles, dolphins, whales and sharks world-wide.
Further out in the ocean, “it’s like a desert out there,” says Fedak. The Sea Mammal Research Unit is working on developing anti-fouling coatings to cover solar cells, which will reduce the size of the tags and increase their reliability and transmittal time, the longest of which is about 18 months now.
The tags measure "dryness" of the circuitry, so when there's less than 100 percent, it can indicate fouling. Lovell analyzed the data, which shows that Champira and Drexelina's tags stopped suddenly. They weren't biofouled. Windy's tag was probably not biofouled. And Stephanie's tag data isn't clear -- her tag might have biofouled, which means there's a chance we'll hear from her again. Here's Champira's track and her last position.
HARNESS: Since Eckert developed the first leatherback harness in 1983, about 2,000 leatherbacks have worn them. The harness links, even the ones that are designed to corrode so that the harness falls off after the tag has stopped transmitting, have been tested to last one to two years.
“On some occasions we're able to make educated guesses as to what might have gone wrong,” when a turtle is found dead, says Eckert. “For example a leatherback I was tracking was lost off the coast of New Jersey. Three days later it stranded (dead) on the beach. When I reviewed the data record I noticed that the night before it disappeared it reported that it had been submerged longer than was normal for that turtle (in that area) and that it had remained at a depth close to that water depth for the area of its last location. The next uplink reported a very short surfacing early in the morning - but the quality was too poor (too few uplinks) to predict a location. With a bit more looking, I found that a bottom gillnet fishery operated in this areas, so theorized that the turtle had become entangled, drowned and when brought to the surface early in the morning the fisherman cut the netting and harness off the turtle to untangle it. The tag probably sunk and the turtle gradually drifted in......but there is no assurance that this is actually what happened.
“We've also had a few cases where turtles return during the nesting season without the transmitter or harnesses. Clearly in those cases the harness fell off or was knocked off by aggressive males.”
Losing a harness could happen to Stephanie. But to Windy, Drexelina and Champira, too? What do you think?