Whale sharks gather at a few specific locations around the world – now we know why

The largest fish in the world gathers in just a handful of places. Better understanding of whale shark movements could help prevent further population loss in a species that has already experienced a 63% population decline over the past 75 years.

Recently, a group of scientists tracked the 12,000 mile journey of one whale shark across the Pacific from Panama to the Philippines.

Anything from ten to 500 whale sharks may gather at any one time in areas off the coasts of Australia, Belize, the Maldives, Mexico and more.

Approximately 20 hotspots have been identified. In some cases the sites are linked to a specific phenomenon – such as the spawning of land crabs at Christmas Island in the Indian Ocean.

A new global study shows that whale sharks congregate in shallow water, next to steep slopes that quickly give way to areas much deeper water (usually between 200 metres and 1,000 metres).

We identified three main reasons. First, the deep water is used by whale sharks for feeding. Studies have shown the sharks diving to depths of almost 2 kilometres to feed on zooplankton and squid.

Second, the steep slopes are known to bring nutrients up to the surface from the deep, which in turn increases the abundance of plankton and attracts large numbers of filter feeding species. And finally, in shallow water, as well as feeding on coral and fish spawn, the sharks are able to thermoregulate, warming themselves back up after their dives into deep water which gets as cold as 4?.

Coastal gatherings are predominately made up of immature male sharks, usually still just four or five metres long. So where are all the girls? And where do whale sharks mate and give birth? Mating and pupping have never been seen in the wild – but, intriguingly, up to 90% of the whale sharks passing through the Galapagos marine reserve are female and thought to be pregnant.

Could this be a key labour ward for the world’s whale sharks?