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Tracking the Silver King

In an ideal world, biologists could assess the fate of every fish that has fought on hook and line and been released, but that would require the impossible: physically or visually following every fish for long periods. So they rely upon one of the best alternatives available: acoustic tagging and tracking. Many wildlife and fisheries scientists use this tool to follow and record observations of a species, be it a bird, bear, manatee, or fish.

Acoustic tracking was the primary technology used to assess the effects of catch-and-release fishing on the survival of tarpon (Megalops atlanticus) in an FWC study from 2002 to 2007. After tarpon were caught and by the side of the boat, they were sonically tagged and released. Biologists then tracked the tarpon by following signals emitted from the tags' internal transmitters for up to six hours.

Every few seconds, an acoustic transmitter emits a signal or "ping" set on an ultrasonic frequency. Biologists can hear this ping via a directional hydrophone (underwater microphone) and a receiver set to the corresponding frequency. A directional hydrophone receives the strongest or loudest signal when it points directly at the transmitter and, therefore, the tarpon. The signal fades as the hydrophone points away from the transmitter. These fluctuations in the signal strength would guide biologists trying to follow a tagged tarpon. The battery life of these tags allowed them to be heard for 14 to 21 days, but they often would last several days longer.

With the directional hydrophone mounted on the side of a boat, the operator could steer toward the strongest signal. When a fish remained very close (100 feet or less), the signal could be heard equally well in all directions. If the fish started to swim away, then the signal could be heard only when the hydrophone was held at certain angles relative to the fish's distance and direction of movement. Based on testing in Tampa Bay, the tags' signal range under ideal conditions in open water was determined to be about 1 mile. However, the air in a tarpon's swim bladder, boat traffic, currents, waves, and the presence of metal, concrete, or other sources of acoustic interference could lessen the range.

Every 15 minutes, biologists tracking a tarpon recorded its position and bearing relative to currents and other tarpon. Water temperature and salinity were recorded every hour. Based on where the tarpon was going and whether it moved against the current or passively drifted, biologists could deduce whether it was healthy, tired, or recovering from a fight.

The float attached to the sonic tag sometimes allowed observers to see a fish rolling at the surface or swimming just below the surface. One fish tagged in Boca Grande Pass appeared to several other anglers throughout the day in this manner.

If the signal remained stationary for some time, biologists might suspect that the fish was distressed or that the sonic tag had fallen out and was unable to float to the surface. In these instances, they deployed a remotely operated vehicle (ROV)--or, in previous years, a drop video camera--to assess the condition of the fish and determine the cause of the stationary signal.

Using geographic information system software, FWRI analysts plotted the tracks of tarpon fitted with acoustic tags. Data showed that the tarpon generally either remained in Boca Grande Pass (Figure 1), swam into Charlotte Harbor (Figure 2), swam north or south along the Gulf beaches (Figure 3), or swam out of the pass, headed offshore (Figure 4). Researchers hope the ROV will prove to be an invaluable tool in future marine fisheries research endeavors.