Part of understanding dolphin behavior is learning how these animals interact with their predators.  Large sharks are the only known natural predator of dolphins in the Sarasota Bay area; about 31% of Sarasota’s dolphins bear shark bite scars.  Bull, tiger, and dusky sharks are the most likely dolphin predators, based on historic shark stomach content data.  Beyond a paper published by Mote’s Dr. Eugenie Clark and Kay von Schmidt back in 1965, very little is known about the distribution, abundance, or ranging patterns of large sharks in this area, especially within Sarasota Bay.  Previous efforts have been unable to locate large sharks inshore, yet there have been several instances where dead dolphins have been found deep inside the bay with fresh shark bite wounds.

Figure 1.  Bull shark caught by Mote Marine Laboratory scientists.

Further evidence of shark presence inside the bay came on the night of 15 August 2007 when a New College student was bitten by a shark while swimming off the school’s seawall along the eastern shore of Sarasota Bay. A shark expert at Mote Marine Laboratory believed the bite pattern was that of a 2 m (6 ft) long bull shark (Figure 1). This project, planned well in advance of the attack on the coed, was designed to gain a better understanding of large (>2m) shark abundance and habitat use within Sarasota Bay waters.
       Baited hooks were attached to drum lines which are designed to select for large sharks.  Hooked sharks are able to swim easily and relatively unharmed until retrieved by researchers.  Over nine field days, 280 drum lines were set for about two hours each.   On 21 August, fishing effort was concentrated in the area around the 15 August shark incident near New College.  Two hooks came up without bait and with cut lines, but no sharks were caught.  The cut lines were considered reasonable evidence of large sharks taking bait, and the two locations were noted as probable areas of shark use.  Overall, three animals were caught: two nurse sharks that were not considered potential dolphin predators, and one 2.3 m bull shark (Figure 2).   An ultrasonic tag was attached to the bull shark in order to track its movements.  Unfortunately, the tracking boat lost the shark’s signal almost immediately after release.  Over the next few weeks several attempts were made to reacquire the bull shark using the ultrasonic receiver, but no positive signals were acquired.  This study has, however, shown that large sharks do inhabit Sarasota Bay but more effort is needed to further understand shark use in this area.

Figure 2.  Locations of shark attack and fishing activities in 2007.

      Mote’s Shark Research Center, especially Armando Ubeda and Beau Yeiser, was integral to the project with initial advice and hands on field assistance.  This project was supported with funding from NOAA’s Fisheries Service.