They are hardly found anywhere else in the country, but their presence in the Tubbataha Reefs makes it possible to generate greater understanding of their kind. A team of researchers from the Large Marine Vertebrates Project – Philippines (LAMAVE) and the Tubbataha Management Office successfully tagged two tiger sharks (Galeocerdo cuvier) in the Park during Expedition Shark 2 on 11 to 19 June.
The tiger sharks, listed as Near Threatened (NT) in the IUCN Red List of Threatened Species, were both caught in successive days in the South Atoll off the Delsan Wreck dive site. A baited fishing line about 60 meters deep with a barbless hook at the end was used to catch the sharks. Both the captured tiger sharks measured more than three meters long and each are estimated to weigh more than 340 kilograms (www.FishBase.org).
Dr. Alessandro Ponzo from LAMAVE – Philippines embedded an acoustic tag in one of the tiger sharks. This type of tag will send ‘pings’ to three acoustic receivers strategically moored in different spots around the park once the tagged individual comes within a 500 to 800-meter radius. A fin-mount tag was installed on the dorsal fin of the second tiger shark. While the acoustic tag works only within the park, the fin-mount tag will send signals to a nearby satellite once the tagged tiger shark surfaces anywhere in the world. Both of the tags are expected to produce information on the location and movement of the sharks. These information will be useful in understanding the dynamics of how the sharks use Tubbataha as a habitat, their range, and the potential areas of concern in the protection of the species.
After embedding the tags, tissue samples were collected from the pelvic fin of the sharks. These will be used for genetic connectivity studies in order to contribute to an ongoing global research on sharks. Visible parasites were also removed and collected. The sharks were measured and their gender verified before being released from the hook and ropes which secured them beside the patrol boat. These were carried out on a time span of between 15 to 25 minutes, depending on the size and behavior of the shark. Aside from the two tiger sharks, five grey reef sharks (Carcharhinus amblyrhynchos) were also hooked and embedded with acoustic tags. All of the sharks showed normal activity after release as great efforts were made to reduce the stress of the animals during the procedure.
The team also carried out Underwater Visual Surveys (UVS) for elasmobranchs. Last year’s UVS revealed that Tubbataha has the highest density of whitetip reef and grey reef sharks in the world. Sharks and other apex predators are vital to functional marine food webs and to the health of the marine environment in general. By maintaining the normal abundance of species below their trophic level, sharks facilitate balanced competition between lower groups, therefore improving the species diversity within an area. The research team cruised the Sulu Sea onboard WWF’s research vessel, M/Y Navorca. The crew of M/Y Navorca were most helpful in ensuring the success of the study.