Wildlife Column: Whale Sharks
The Biggest Fish on Earth
An adult is bigger than a bus (up to 20 metres long) and can weigh 20 tonnes, but the biggest fish in the sea – and the biggest non-cetacean creature on Earth – is a gentle giant who appears not to care about a gaggle of puny humans swimming alongside him.
And chances are the shark you see will be a ‘him’ – three out of four of the whale sharks we see on Ningaloo are sub-adult males – ie teenagers, even though they’re between five and nine metres long! We sometimes see females, but they are usually quite a bit smaller, at around four metres.
The only obvious way to tell the difference, by the way, is that males have two distinctive ‘claspers’ between their pelvic fins.
Whale sharks earn their moniker for their size and eating habits, not their genealogy. Although they have around 3000 tiny teeth, they feed on plankton and small fish, sifting them through filter plates and gills in a similar fashion to the baleen whales, their only competitors for size. But they are, in fact, a type of carpet shark – a relative of the harmless Wobbegong – with a skeleton of cartilage rather than mammalian bones.
Spots and Stripes
Just like a human fingerprint, the pattern of spots and lines on each Whale shark is unique and can be used to identify individuals. Marine scientists map the sharks’ ‘fingerprint’ using technology originally developed by NASA to map galaxies.
Based on years of monitoring, it appears that around March to July Whale sharks visit Ningaloo each year – the largest reliable aggregation on Earth – their arrival coinciding with the annual coral spawning on Ningaloo Reef in March/April. They stay around until August…but then what?
Well, nobody really knows. Satellite tracking has confirmed that they are a migratory species, capable of covering thousands of miles each year and diving to 1800 metres. They are found in all of the world’s tropical and temperate oceans where sea surface temperatures are between 21 and 25 degrees Celsius. But beyond that, there’s a lot of mystery still surrounding the huge spotty fish. For example, adult females are rarely seen and neither mating nor pupping has ever been witnessed. The length of gestation, how often whale sharks breed, and where, still remains unknown.
Nobody knows how long a Whale shark lives, though it’s thought to be over 100 years. It’s thought that they take up to 30 years to reach sexual maturity and it’s known that they are ovoviviparous: eggs remain in the female’s body and the shark gives birth to live offspring. Perhaps the most amazing thing is that the female appears to be able to control the development of her embryos: a pregnant whale shark was found to be carrying 300 embryos at different stages of development.
When they’re so readily seen at Ningaloo, it’s hard to imagine they are under threat as a species. But the IUCN lists them as Endangered: they are vulnerable to ship strikes and plastic waste. They’re also hunted – along with so many other shark species – for their fins, to make shark fin soup.
Thankfully, they are protected in Australia, and you’re in for an unforgettable experience with these extraordinary, enormous and beautiful creatures.
*If you’d like to be a citizen scientist and help build our knowledge of whale sharks, visit Wildbook for Whale Sharks to learn how to take an ID photo and submit it to the international database of Whale shark celebrities.