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Tracking the Silver King

By plotting data with geographic information system (GIS) software, biologists are able to follow the movement of the mighty silver king after it is released.

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In an ideal world, biologists would be able to assess the fate of every fish that has fought on hook and line and then been released, but to do so properly would require the impossible: physically or visually following a fish for long periods.  So biologists rely upon one of the best means available, acoustic tagging and tracking.  Acoustic tracking is a tool used by many wildlife and fisheries scientists to follow and record observations made of the species being studied, be it a bird, bear, manatee, or fish.

Tracking a tagged tarpon on Tampa Bay near the Sunshine Skyway Bridge. The hydrophone receiver unit is inside the black case on the deck and the hydrophone is submerged at the end of the pole on the left. The tagged tarpon is under the bridge among the pilings.

In a current FWC study, acoustic tracking is being used to assess the effects of catch-and-release fishing on the survival of tarpon, Megalops atlanticus.  After a tarpon has been landed at the side of the boat, sonically tagged, and then released, the tracking phase begins.  Biologists will track the signals emitted from the tag's internal transmitters for  a maximum of six hours.  Every few seconds, each tag emits a signal or “ping” that is set on a unique ultrasonic frequency. Biologists can hear this ping via a directional hydrophone (underwater microphone) and a receiver that is set to the correct frequency.  A “directional” hydrophone is one that receives the strongest or loudest signal when the hydrophone is pointing directly at the transmitter and, therefore, the tarpon.  The signal will fade to the listener as the hydrophone points too far to the left or right of the transmitter. These fluctuations in the strength of the signal guide biologists trying to follow the tarpon.

With the hydrophone mounted stationary on the side of the boat, the operator can drive the boat in the direction of the strongest signal.  When a fish is very close to the boat (about 100 feet or less) the signal is heard equally in all directions, which means that the fish is remaining within a relatively small area. If the fish starts to swim away, then the range of angles of degrees in which the signal can be heard will narrow relative to the distance that the fish is from the boat.  Based on testing performed in Tampa Bay, the audible range of the tags under ideal conditions in open water is one mile.  However, the level of air in the tarpon’s swim bladder, boat traffic, currents, waves, and the presence of metal, concrete, or other acoustic anomalies could lessen the range to less than one mile.  The battery life of these tags allows them to be heard for 14-21 days after the fish are released, but they often last several days longer.

To best judge the fate of a tarpon during the tracking period, biologists also record the position and bearing of the fish every 15 minutes, as well as the fish’s position relative to the currents and other schools of tarpon.  Water temperature and salinity are recorded every hour during the tracking period. Although the tarpon cannot be seen throughout the tracking period, it can be closely followed via the data being recorded by the biologists. The  data can help biologists  logically deduce,  for instance,  where the tarpon is going, if it is healthy, if it is able to move against the current, if it is passively drifting -- perhaps  tired from a fight, if it recovered from a fight, etc.

If the signal remains stationary for some time and there appear to be no other schools of tarpon nearby, one must assume that the fish may be distressed or that the sonic tag may have fallen out and is unable to float to the surface.  An ROV (Remotely Operated Vehicle) is then deployed so that the state or health of the fish can be assessed and the cause of the stationary signal can be determined.  In 2003, FWC biologists were able to use a drop video camera to determine why the signal from a fish that had been tagged earlier that day was stationary.

Some tarpon have been tracked visually as well as acoustically.  The float attached to the sonic tag not only enables the tag to float to the surface if it detaches from the fish, but it also allows the biologist to see the fish rolling at the surface or swimming just below the surface as seen in this video clip (6.3 MB).  This visual aspect can help biologists  track the fish,  especially in difficult conditions.  One of the fish tagged in Boca Grande Pass went right back to the school of tarpon in the pass and was seen rolling at the surface by several other anglers throughout the day. 

With the help of FWRI geographers using geographic information software (GIS), biologists were able to plot the tracks of tarpon fitted with acoustic tags. Data showed that each tarpon seemed to react in a unique way after the fishing event, but in general they did one of four things: 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).

Figure 1 - Geographic information software (GIS) was used to plot the tracks of tarpon fitted with acoustic tags. This track shows a tagged and released tarpon that stayed within Boca Grande Pass.

Figure 2 - The plotted track of a tagged and released tarpon that swam into Charlotte Harbor.

Figure 3 - The plotted track of a tagged and released tarpon that swam parallel to the shoreline a distance of more than 10 miles. Six days later the signal from this fish was heard again back in Boca Grande Pass.

Figure 4 - The plotted track of a tagged and released tarpon that headed offshore into the Gulf of Mexico.

Next year a pilot study is being proposed in which several tarpon will be tagged and then released into the schools during the stronger afternoon tides and moon phases in May and June to see if they head offshore.  We hope to follow these tagged fish for 24 hours or more to observe them and learn as much as possible about their reproductive nature and biology. May and June are known as the peak spawning season for tarpon in Florida waters, but little is known about their spawning behavior or habitat.    We hope that the ROV will prove to be an invaluable tool in this and other tarpon research endeavors.  In Boca Grande Pass, FWC biologists were able to deploy the ROV and visually verify that  a large school of tarpon was present in the pass  even though only a few fish were seen at the surface (View ROV video footage, 8.7MB). 

Perhaps in the near future biologists will find these schools of tarpon offshore and further the knowledge of the mysterious and mighty silver king.