Bull Shark Hotspots Mapping & Acoustic Tagging

April 29, 2018 | Howard Rickard

Tagging large marine megafauna such as sharks & marlin is a powerful way to learn more about migration patterns, or local & regional movements to then apply adequate management measures in e.g. National Parks.

The Bazaruto Archipelago National Park (BANP) is home to one of the largest hotspots of sharks in the world, especially of bull sharks (Carcharhinus leucas), locally known as “Zambezi” or “Zambi”. This species, which is a very aggressive animal is responsible for numerous human attacks every year. They come to breed in the Bazaruto Archipelago very close to inshore waters taking advantage of the warm water temperatures during the summer and spring months. They also follow the bait and tuna schools that start arriving in great numbers around April to feed on them.

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Bull sharks are typically around 100 to 150 kg but in the Bazaruto Archipelago area they easily reach 200 to 300 kg. This makes BANP one of the few areas in the world with localized breeding bull shark populations easily reachable for scientific study. Very few information exists in Mozambique concerning sharks in general and in particular bull sharks. Despite the numerous attacks on people during the years it is unknown their movements, ranges, hotspots and behavioral patterns.

To this end, Ph.D candidate Calum Murie joined BCSS Ocean Observatory during April 2018 to deploy several acoustic tags on +200 kg bull sharks between 2-mile and 5-mile reef. Working with Dr. Mario Lebrato and Sean Lange they were able to get 6 bull sharks from 200 to 300 kg, but only managed tagging 2 individuals. The rest of the fish either broke the gear or cut the metal wire used to bring them to the boat.

The reality of bull sharks in BANP is that they are among the biggest in the world thus they grow to overwhelming sizes that are difficult to handle. BCSS mapped all shark hotspots from 2-mile reef to Lighthouse Reef to further understand where they stay. The conclusion was that bull sharks are really active under tuna shoals and they go in schools themselves, but then there are isolated very large individuals, always shallower than 30 to 40 m. Deeper than this the team did not find any bull sharks.