The whale shark, Rhincodon typus, is the world’s largest fish reaching more than 20 meters (65 ft.) in length. They are cartilaginous fish, not mammals, and their name refers to their size and diet of plankton. Despite their spectacular size they are very docile and do not harm humans. The only predators of whale sharks are orcas, white sharks and humans. They are actively fished to be sold in Asia but not in Tanzania. Whale sharks reach sexual maturity at about 7 to 8m in length (after 20 to 30 years) and are ovoviviparous, which means that they lay eggs but retain these in their uteri until the eggs hatch and the fetuses mature and live pups are “born”. A mature female may have more than 300 eggs and fetuses, at various stages of development, in her two uteri at any one time. It is postulated that the females may mate very few times and may be able to store sperm.
Whale sharks are most often seen in tropical/sub tropical regions. They are solitary and travel alone but aggregate in a few locations around the world, where they can be seen in larger numbers, such as the Yucatan, Ningaloo, Bahrain, Mozambique, the Phillipines and a few other sites. Mafia Island is one of these special locations, however with the longest lasting aggregations and thus one of the best places to study these animals.
Ongoing research and indigenous knowledge has proven that the sharks are in the Mafia channel all year round. More than one hundred animals have been identified that feed here on surface plankton from September to March, when they are easily seen. The whale sharks aggregating in Mafia are most commonly not yet sexually mature, being typically less than about 8 meters in length and 80% of these are male. The reason for this gender imbalance and size prevalence is still unknown but it is seen amongst similar aggregations of sub-adult whale sharks everywhere else, except the sub-adult whale shark aggregation in the Red Sea and the adult aggregations in the waters of the Galapagos islands.
Kitu Kiblu and the founders of Kitu Kiblu, who are also founders of the Mafia Island Whale Shark Conservation Society, have been associated with WWF Tanzania-funded whale shark research since 2007 and continue to support whale shark research and conservation.
In the 2016/2017 season we will be hosting Dr Jesse Cochran, who specializes in the study of cartilaginous fishes, including sharks and rays. He earned his PhD in marine science from the King Abdullah University of Science and Technology where he studied Red Sea whale sharks. On Mafia, he studies the local whale shark aggregation using a variety of photography and telemetry based methods to describe the sharks’ movement patterns and population structure.
Kitu Kiblu interns assist Jesse with his research.