Acoustic fish tags are a new technology used to monitor and track fish movement. Each fish tag is a transmitter that produces an unique sound. By listening with hydrophones, the location and depth of each fish can be determined. Some fish tags also record information about the water column, including, temperature, pressure, and ambient light levels. Knowing light levels can help scientists determine location based on sunlight.

         Courtesy of David Welch, Pacific Ocean Salmon Tracking (POST)

     Acoustic fish tags are either implanted (surgically or gastrically) or attached to the fish. They have a coating to prevent the fish's immune system from rejecting the transmitter. These transmitters then send out a "ping" at a high frequency (unable to be heard by the human ear) which can be heard as far as half a kilometer away. Hydrophones are strategically placed around the area of interest to listen for these pings. As pings are heard and recorded, travel time can be calculated and the fish's location can be pinpointed.

     Unlike radio tags, which only detect within the first 10m of the surface, or active acoustic fish finders, which only detect a fish when it is in the acoustic beam, acoustic fish tags can detect fish movement anywhere in the region of interest. The accuracy of this technology depends on the position of the tag relative to the hydrophone, the background noise level relative to the frequency of the ping, and the accuracy of the assumed velocity of sound in water.