March 12, 2009, San José, Costa Rica—Shark researchers from the Costa Rican organization Pretoma made a startling observation last week while studying bull sharks (Carcharhinus leucas) at the mouth of the Sirena River, in Corcovado National Park—specimens over 4 meters in length (12+ feet). The researcher’s long-line fishing gear, rigged to catch smaller pelagic sharks, proved inefficient in catching the larger coastal bull sharks.

“We didn’t expect to encounter sharks this size”, said Randall Arauz, the expedition’s Scientific Director.  “Many of the hooks and steel leaders were mangled, destroyed, or simply bitten off by the large animals, only one of which came close enough to our fishing vessel for us to take a glimpse of its amazing size, before it too broke free”, said an astonished Arauz.

An acoustic tag is placed on a juvenile bull shark. Mouth of the Sirena River

An acoustic tag is placed on a juvenile bull shark. Mouth of the Sirena River

As part of the research, the team of scientists took a smaller boat into the Sirena River where they observed schools of juvenile bull sharks, swimming at seemingly random intervals up and down stream.  Using hand lines, and under the scrutiny of the local crocodiles, the researchers caught 5 sharks, all females measuring around 1 meter in length.  Acoustic tracking devices were fastened to 4 of the animal’s dorsal fins, and their movements will now be monitored via an acoustic receiver the team placed on a buoy they anchored in the river.  Hooks were removed prior to release using dehooking devices.

The bull shark is a coastal species, common in shallow waters.  Adults are found in muddy, murky waters near estuaries and river mouths where they hunt.  Bull sharks commonly venture deep into fresh water habitats.  Juveniles also inhabit river mouths, where they feed and seek protection from marine predators in the brackish waters.  They owe their name to their “bulky” body types.  In Spanish, the bull shark is commonly called the “muddy shark”, due to its foraging habits. They are classified as “Near Threatened” by the International Union for the Conservation of Nature (IUCN).

By learning more about the bull shark, researchers hope to strengthen the case for the expansion of marine protected areas along Costa Rica’s southern Pacific coast.  “Corcovado National Park only enjoys a 500 meter no take zone, which obviously isn’t enough to provide efficient protection for bull sharks”, said a worried Allan Bolaños, a Pretoma fisheries observer.  “Our preliminary results show that these waters are essential habitat for a number of shark species, not just bull sharks”, concluded Bolaños.

During the 4 day expedition (26 February-1 March), scientists observed 13 sharks of 3 different species, including nurse sharks (Ginglymostoma cirratum) and black tip sharks (Carcharhinus limbatus).  All sharks caught were measured, sexed, and a tissue sample was taken for DNA analysis.  Curiously, all specimens were female.

A next expedition is planned for May, 2009, and the researchers will be better equipped to catch, handle and tag, the large adults.  More acoustic receivers will also be deployed in the surrounding waters to track the animals for further distances.

The Bull Shark Tagging Project is funded by the Whitley Fund for Nature (WFN), the BBC Wildlife Fund, and Pretoma’s Corporate Membership.  To learn more about Pretoma’s Corporate Membership, which allows businesses to partner with the organization and work towards the common goal of conserving Costa Rica’s marine resources, visit our website at www.pretoma.org or contact Andy Bystrom at andy@pretoma.org.

Share it! / ¡Compartir!