Whale sharks are the biggest fish in the ocean. They breathe water through gills and get their name from their large size and ability to feed on plankton like some whales. Whale sharks are members of a group of fish called the elasmobranchs, which have skeletons made of cartilage instead of bone. Estimates for maximum size are upwards of 18+ meters.
Whale sharks feed in a different way to most sharks. They prey on tiny plants and animals called plankton, which float in the water column. Using their vast mouths, they are able to suck in enormous quantities of water. Plankton is trapped on special organs that work like filters located in front of their gills. Their docile demeanor and lack of “bite” have led to them being nicknamed “Gentle Giants”.
Whale sharks have a very distinctive pattern of spots. Each individual has a spot pattern as unique as a human’s fingerprint. They may use the colouration as camouflage, as the spots mimic how light dapples through the plankton-rich waters they usually inhabit. Their biggest form of defense is their size, which when fully grown leaves them with no natural predators.
Over the past few years, whale sharks have been observed aggregating off the western side of Mafia Island, particularly in Kilindoni Bay. The nutrient outflow from the Rufiji River creates an environment suitable to plankton blooms. The blooms ride the wind and currents and may become trapped and concentrated by Mafia Island, creating a feast for the whale sharks. A tourism initiative has been established whereby visitors are able to swim with the sharks.
Whale sharks are extremely vulnerable to boat traffic since they spend significant amounts of time at the surface. Fishing vessels in Mafia have been known to collide with the sharks as fishers often follow the sharks to locate their target prey. Increased boat traffic from tourism also poses a threat to the whale sharks in Mafia and it is critical that any growth in the industry is well regulated to protect the welfare of the sharks.