More recently it has become a significant tourist destination for viewing whale sharks Rhincodon typus, the world’s largest fish. These can be huge. The largest scientifically confirmed specimen was 12.5m and weighed 21.5t; they probably grow much larger, perhaps up to 20m - unconfirmed records of such animals are common. They live for about 70 years.

Whalesharks evolved about 60mya, and are filter feeders. They have an extremely efficient filtering system and unlike the smaller basking sharks found around the UK they can both ram and gulp - i.e. ram feeding by swimming along with their mouths open like basking sharks, and gulp feeding by suddenly expanding their throat cavity by muscular action, sucking in large volumes of water. The filters are presumed to be modified gill rakers. They feed on plankton, and fish spawn, and sometimes small fish. Much of their lives is mysterious - they have never been observed mating, and are only presumed to be ovoviviparous (giving birth to live young from eggs that hatch in the body, like the UK’s viviparous lizard) on the basis of the catch many years ago of a female with highly developed young inside her.

Feeding on plankton brings them to the surface in the daytime, with the plankton. A coincidence of sea currents off NW Australia causes an upwelling of nutrients during the ‘winter’ (such as it is round here) and therefore lots of plankton followed by the sharks. The animals here are usually reasonably small subadult males, for reasons that no-one yet knows.

The one that we saw well was about 5-6m which is quite big enough. Some animals are quite remarkably tolerant of swimmers, paying them no mind at all and carrying on their slow measured swim up and down the coast. Others are much more timid. Whale swimming is rightly highly regulated to minimise disturbance; clearly successfully because the animals keep coming back. 

The technique is that a spotter plane (soon to be replaced by a much cheaper drone I expect) flies along the reef seeing the sharks as silhouettes against the shallow sandy sea floor. The pilot then guides in dive boats, one at a time, and the snorkelers jump in on command and swim with the shark. Always to one side and behind the head (and keeping well away from the tail). Whalesharks conveniently swim in economy cruise mode at the same speed as a moderately fit human with fins, so a really good view can be obtained.      

Here is the VS crew, post dive, with obligatory glass of Australian sparkling wine.

Video of ourl whale shark is on Youtube at:  https://youtu.be/KD6dZ6g7UAk

Another video, same fish, with swimmer in the background for scale at:      https://youtu.be/kxy6-6kc_BQ