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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
Sensors on the tag collect depth, temperature and light measurements every two minutes, and the tag stores that data on its 64 megabytes of flash memory. The tags were programmed to activate a release mechanism from the eels after various time periods at sea—some after a one-month interval, others at two, three or four months. The release mechanism was also programmed to open and release the tag from the eel in the event it descended to 1,250 meters below the water's surface (greater depths could permanently damage the tag, rendering it inoperable), or after four days of readings at the same depth, which would indicate the tag was no longer attached to a live eel.

Microwave Telemetry's X-tag contains a satellite transponder, as well as depth, temperature and light sensors and 64 megabytes of flash memory.
Once released from the eel, the tag floats to the water's surface, where it begins transmitting a 401.65 MHz signal encoded with its unique ID number and sensor data. The signal is picked up by Argos low earth orbiting satellites, launched by NASA and other agencies to support applications devoted to monitoring and protecting the planet's environment. To calculate a tag's location, the system measures frequency shifts in a signal received by a satellite passing over a tag—a phenomenon known as the Doppler effect. The Argos system then makes the data available on its global network, accessible to the scientists with a user name and password. It could also send e-mail messages to scientists, indicating a sensor had surfaced.

The results of the study did not provide scientists with an understanding of the eels' entire migratory process. However, it did enable the animals to be tracked up to 1,300 kilometers (808 miles) from the point of departure. Fourteen of the 22 tags actually transmitted, proving that the fish were following Atlantic drift currents. The eight tags that did not transmit may have been stuck on the bottom of the sea or in the stomach of a predator, or may have failed to transmit.

The eels seemed to travel between 5 and 25 kilometers (3 to 16 miles) per day, Aarestrup says (the distance was calculated by dividing the distance traveled by the days at large). This was considerably less than the 35 kilometers (22 miles) per day required to reach the Sargasso Sea by April. The tags could have created so much drag that they slowed the fish down, he notes. If not, he adds—if the eels just naturally take longer to get there—researchers would have to reconsider their estimation of the distance the animals travel while migrating. "We can't rule anything out at this point," he states.

The data also showed that the eels swim at depths of approximately 650 feet at night, descending to about 2,000 feet during the day. The descent to colder waters, the researchers suggest, delays the eels' sexual maturation until they reach the spawning ground.

Researchers tagged only 22 eels because of the cost of the technology being used, Aarestrup says. The tags cost about $4,000 each. The scientists intend to begin tagging more eels departing from Sweden in fall 2010, but hope to find a smaller, less expensive tag they can use. The team has not yet determined how many tags it will utilize. In the meantime, the researchers are placing non-transmitting pop-off tags on the eels. The non-transmitting versions have sensors that record temperature and depth, but lack a satellite transponder, relying instead on people to find the tags when they wash up on shore, and then mail them back to the researchers. That process is less expensive than employing PSAT tags, and the scientists are still in the process of receiving retrieved tags.

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