Juvenile and sub-adult lemon sharks are caught as described in the gillnetting overview and long-lining overview. Some of these sharks receive active, continuous acoustic transmitters that are surgically implanted in the shark's body cavity. To implant these transmitters, sharks are placed in tonic immobility. This is done by turning the shark over onto its back where it enters a state of torpor making it much easier to handle. The transmitter is inserted through a small incision made on the shark's underside near the pelvic fin which is then sutured closed. Transmittered sharks are kept in pens until the incision has sufficiently healed. Then the sutures are removed, and the shark is released.
Implanting a transmitter
Tracking crew in the field
Lemon sharks with transmitters are tracked using a hydrophone and receiver from small flat-bottomed Carolina Skiffs with 25 hp or 50 hp outboard motors (Mercury, Inc.). Sharks are tracked using teams of 2 or 3 people. Two types of tracks are done. The first type, searching and locating, consists of a team going out and getting locations for all sharks in that study site. The second type, tracking, consists of 8, 16, 24, 48, or 72-hour tracks where teams track for 6 or 8 hour shifts and then are relieved by another crew. These tracks are continuous so we track throughout the night and across all tides. Tracking crews remain 30-100 meters from the shark depending on water depth and ocean conditions. One person sits at the bow of the skiff with the hydrophone in the water listening for the shark. This person tells the driver of the boat the direction and distance of the signal. Every 5th minute of tracking, we record the GPS location of the boat, the bearing to the shark, and an estimated distance to the shark. The bearing and estimated distance allows us to calculate more accurate locations for the shark. Every 6th location or every 30 minutes, we record additional environmental data including: water temperature, water depth, salinity, dissolved oxygen, current, surface conditions, tidal stage, and wind speed. These allow us to determine if certain environmental factors affect the shark's movements.
All shark locations are then plotted on a map of the waters surrounding Bimini using a GIS program. This allows us to visually see the areas the shark is using during different times of the day and at different tides. It also lets us look at patterns of movement among and within individual sharks.