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Eeliad Project Tracks Eels' Odyssey

Scientists are studying the migratory patterns of European eels in the North Atlantic, using transponders that detach from the fish in order to transmit a record of their journey.
By Claire Swedberg
Nov 10, 2009A team of researchers is employing wireless sensors to gain insight into the virtually unknown world of European eel migration. By tracking the animals' course during their annual travels from Europe to the Sargasso Sea (in the middle of the North Atlantic), scientists at the National Institute of Aquatic Resources, at the Technical University of Denmark, hope to understand the conditions of the waterway in which the eels travel, and thereby fathom why their numbers are dwindling so dramatically.

The tests are part of a larger scientific research project known as Eeliad, focused on gaining a better understanding of European eels, in order to help conserve their numbers. The project includes tracking the animals at sea, as well as research and monitoring projects in freshwater and brackish water, to develop an understanding of the fish's life cycle.

The researchers select the 22 biggest eels, shown here in a holding tank in Ireland.
The Danish scientists had speculated that eels travel approximately 3,000 miles to the Sargasso Sea by spring, after leaving the European coast in the fall. No migrating eels have ever been caught in the ocean, however, so there has been no way to know their exact course, or to verify that the Sargasso Sea is even their destination point. To gain some insight into that migration path, the researchers wanted to measure not only where and how far the eels traveled, but also the depth of water in which they swam, and the temperatures to which they were exposed.

But little technology is available for tracking animals underwater. "We had a very interesting problem there," says senior researcher Kim Aarestrup. RF or satellite signals would not pass through saltwater, he says, so the sensor and transmitting devices would need to come to the surface to be read. Eels, however, can travel at a depth of thousands of meters under water, and don't surface at all while migrating. In fall 2006, the researchers decided to try the battery-powered X-tag pop-off satellite tag (PSAT), manufactured by Maryland animal-tracking products firm Microwave Telemetry. The X-tag measures about 4 inches in length and attaches to an eel's back.

Researchers chose 22 of the largest individuals from a pool of approximately 100,000 eels, which were caught in Ireland by commercial fishermen during the animals' downstream freshwater migration. The eels chosen weighed about 5 pounds (a typical eel weighs about 1 pound), in order to ensure they would be strong enough to carry the sensors. Scientists attached a tag to each eel by inserting metal wires through the skin on the animal's upper back—in front of the dorsal fin—and placing protective pads around them, then attaching the transponders to these pads. The eels were then released.

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