Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

Effects of Tidal Current on the Movement Patterns of Juvenile Bull Sharks and Blacktip Sharks

Pascale A. Steiner and Marcel Michel


Abstract.—Ultrasonic telemetry was used to study the influence of tidal current on the activity patterns of bull sharks Carcharhinus leucas and blacktip sharks C. limbatus within their nursery grounds. This study was conducted in the estuary of the Ten Thousand Islands, Florida. Twenty-eight sharks (8 bull and 14 blacktip sharks) were tracked for at least one tidal cycle. The two species showed significantly different swimming patterns relative to the tidal current. Blacktip sharks all showed a similar pattern of moving towards the open waters during outgoing tide and returning to the backwater bays with the incoming tide. It was found that their swimming direction was significantly influenced by the tidal current. The swimming direction of bull sharks showed no significant dependence on the tidal stage. In addition, the index of linearity as a measurement of re-use of an area confirmed a significantly straighter swimming pattern for blacktip sharks during both incoming and outgoing tides, as compared to bull sharks.