The water is so warm, even whale sharks are heading south for summer

Whale sharks usually swim in tropical or subtropical seas, making them a rare sight in the normally crisper waters off the NSW South Coast.

Tourists spend thousands of dollars to swim with the gentle giants at Ningaloo Reef in Western Australia, one of the few places they are almost guaranteed to be found.

But a few lucky Bermagui locals on a fishing boat had a free encounter closer to home, courtesy of a marine heatwave and strong East Australian Current bringing warmer waters further south.

“It’s a bit warmer off the east coast than you’d expect, but it’s still very unusual to find one down that far south,” said Professor Culum Brown, a shark expert at Macquarie University.

Fishing shop Bermagui Bait & Tackle posted footage of the endangered species on its Instagram account in a post that has attracted more than 5000 likes.

“Check out this Whale Shark that popped up next to Mick’s boat this morning in 70m of water just out the front of Bermagui. Now that’s pretty cool,” said the post.

Whale sharks or Rhincodon typus are the world’s largest living fish, growing up to 18 metres and 34 tonnes. They can live for 100 years, according to the Australian Institute of Marine Science.

A whale shark swimming at Ningaloo Reef in Western Australia.Credit: Getty Images

The institute’s scientists have mapped and tracked the animals using advanced satellite tags and found they can dive more than a kilometre deep and migrate to the waters of countries thousands of kilometres of away.

Brown said the sharks, like baleen whales, fed on plankton and were harmless to humans.

He said whale sharks preferred water at 23 to 25 degrees and no colder than 21 degrees. However, cooler waters had the benefit of producing more plankton, and some plankton species cannot withstand hot water.

Australia has been in the grip of marine heatwaves for several summers in a row.

The water temperature in the Coral Sea off the North Queensland Coast is 1 to 2 degrees warmer than average, which could have exacerbated the flooding rains that drenched the region over the weekend.

At Ningaloo Reef, the water temperature on Tuesday was 30.7 degrees – well above the 26.8-degree average for February. The WA government identified “prolonged thermal stress” from an escalating marine heatwave as the likely cause of more than 30,000 fish being washed up dead in the Pilbara in January.

The ocean off Bermagui was 21.1 degrees and expected to rise to 22 degrees later in the week. Sea temperatures rarely drop below 20 degrees at this time of year.

At a latitude of 36.25 degrees south, Bermagui is 388 kilometres south of Sydney and 142 kilometres north of the Victorian border via the Princes Highway.

Whale sharks are listed as vulnerable under Australian environmental laws and endangered by the International Union for Conservation of Nature.

Humane Society International marine campaigner Lauren Sandeman said threats included vessel strikes, including from tourism boats, entanglements in fishing nets and bycatch from fishing, and climate change and overfishing reducing food availability.

“Whale sharks are slow growing and they have low levels of reproduction, so it takes a while for their populations to recover,” she said.

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