Satellite Tracking Sea Turtles
by Catherine McClellan
Sea turtles have been around a very long time—since the dinosaurs. And until the 1900s turtles were numerous enough to support a commercial fishery in North Carolina’s inshore waters. Today, however, all sea turtles are listed as threatened or endangered under the Endangered Species Act. Much of their decline has been attributed to humans. Despite their geologic age and use as a commodity around the world, we know very little about the natural history or abundance of these seven species. In part, this is because they spend almost their entire lives at sea, travel great distances, and remain submerged for substantial periods of time. Sea turtle populations are particularly vulnerable to harvest (on purpose or by accident) because they are slow growing, long-lived, and slow to mature. This means that the consequences of removing turtles from the wild may not be noticed for as long as thirty or more years. Recovery may take even longer. Simple questions—such as how many males and females exist, at what age a turtle matures, how turtles use habitat, or where they go in the winter—still baffle turtle biologists.
Researchers at Duke Marine Laboratory are using a unique method to answer some of these questions. We are teaming up with commercial fishermen to look at fishery interactions with sea turtles. Fisheries are among the greatest threats to sea turtle populations and gill nets, in particular, are highly lethal to the animals. Gill nets are long, submerged nets, usually made of clear monofilament line, anchored to the sea floor. Floats at the top stand the net up into the water column and sometimes to the surface depending upon the depth of water where the nets are set. Fish are caught in the net by the gills when they try to swim through it. Many gill nets are set together in a line, or in rows one after another, to collect all the fish that may pass by. If a turtle becomes entangled it is likely to drown before the fisherman retrieves his net. In North Carolina, the most active period for these interactions occurs during the fall when flounder are caught in gill nets and pound nets throughout the state's inshore waters.
The flounder fishery is worth over a million dollars each year to the state of North Carolina and provides income to a large number of fishers and fish retailers along the coast. Flounder fisheries are small scale and simple in design, but can have large impact on turtle populations.
Our project was funded by Fisheries Resource Grant program of North Carolina Sea Grant to try to better understand interactions between sea turtles and flounder gill nets in Pamlico Sound. To do so, we are focusing on the larger of the unknowns: the turtles themselves. Where they are, where they go, how they get there, and how long they remain in these areas are all questions we are looking into by following the turtles.
As stated earlier, sea turtles are difficult to follow. So how do we do it? We are using satellite transmitters to track the animals and to understand how they are using Pamlico Sound and surrounding areas. The transmitters relay information on each turtle’s location and water temperature to a satellite each time the turtle surfaces to breathe. That information is then sent to Duke Marine Lab by email so that we can retrieve, plot, and analyze the data (see Current Sea Turtle Locations for the most recent location data).
We are involving fisherman as a way to catch turtles. Pound nets are used in the summer and fall months to catch fish from Albemarle Sound through Core Sound. These nets have long leads with small, stiff mesh that direct fish, and turtles, to an impoundment. The animals are free to surface and swim around in the pound until the fishermen come to empty the net. Turtles actually seem to enjoy these nets and many fishermen have caught the same turtle time and time again. The pounds and leads are often strung one after another from a shallow area out into the deeper water. In fact the pound nets form a maze strung throughout the sounds and is not surprising that they are effective at catching fish as well as turtles.
By tracking the turtles, we can find out where the turtles are spending their time in relation to the location of gill nets. We are analyzing correlations between turtle movements and fishing effort, and environmental factors such as water temperature. Moreover, we can follow the turtles as they migrate out of Pamlico Sound, discover which inlets may be of most importance to their movements, and see where they go for the winter months. We are currently tracking three species of sea turtles: the loggerhead, the green, and the Kemp’s ridley. We have 45 satellite transmitters and tagged 21 turtles in 2002. We will deploy the remaining 24 transmitters in 2003.
Once we retrieve a turtle from the pound net, we bring it aboard our research boat, the Proteus. We scrub the turtle’s carapace to remove any mud or algae and take off any barnacles. Then we allow the carapace to dry. Once dry we sand it lightly and wipe it with alcohol (several fishermen have commented on the spa-like treatment we give these turtles). We use a two-part epoxy to secure the transmitter to the carapace. Usually the best placement is on the first or second vertebral scute (forward on the carapace) so that when the turtle comes up to breathe, the antenna will be out of the water.
We have two sizes of transmitters, one for large turtles and one for small turtles. The large ones can theoretically last up to a year, while the small ones, limited by battery size, can last up to four months. The larger tags are further secured by two layers of fiberglass cloth and resin. Once dry, the attachment materials are sanded smooth and the turtle is ready for release. In the end, the transmitter is not much more than a large barnacle on the carapace that records and sends data to us via satellite. Several of our turtles have been recaptured in pound nets and have appeared to be in good condition.
We hope that the information gained from this research will provide insight to help local fisheries and sea turtle populations co-exist in the sounds of North Carolina, as well as further improve our understanding of their mysterious travels.
About the author:
Catherine McClellan is a doctoral student at Duke University
in Durham, North Carolina.
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