To do the study, McMichael's team used an elaborate fish-tagging system known as the Juvenile Salmon Acoustic Telemetry System that he and others have developed over the last 10 years. Ecologists implanted tiny, battery-powered tags smaller than a pencil eraser into more than 8,159 fish migrating down the Columbia, the nation's fourth-largest river, and released those fish at one of four sites anywhere from about 140 to 245 miles upstream from the ocean. As the fish swam, the acoustic tags inside them emitted tiny, unique beeps that were picked up by underwater receivers that collect and store the data.
The team set up several arrays of receivers along the Columbia, at its mouth, and also in the ocean. There, 20 detectors were dropped near the ocean floor, each about 1.5 miles from the next, about nine miles from the mouth of the Columbia. Ultimately, 1,701 of the original fish, or about 21 percent, were subsequently detected as they entered the ocean. Most of the rest of the fish likely went undetected through gaps in the sparse detection system.
The tags the team used are battery-powered "active" tags that emit signals every few seconds for at least 30 days. The closer to the detector that a fish swims, the more likely it is to be detected; depending on the noisiness of the environment, the signals can be detected up to 250 meters away in the ocean. That's much further and provides much more information than "passive" tags, where fish must swim within a few feet of a detector.
The latest study would have been difficult to do with a passive tag, since such systems detect just a tiny fraction of the fish compared to the JSATS system. Passive systems also require an extensive infrastructure which would be nearly impossible t
|Contact: Tom Rickey|
DOE/Pacific Northwest National Laboratory