Humpback whales make 'wrong turn' into crocodile-infested river in Australia's Kakadu national park

A humpback whale has been spotted for the first time in Kakadu’s East Alligator River, and experts are concerned it may get stuck.

It is believed three whales initially entered the river and that one has remained, although it is not clear if the others have actually left or are merely staying underwater.

The unprecedented sighting has stunned locals and authorities, with scientists suggesting the whales may have just made a “wrong turn”.

“It’s never occurred before, we’ve had no records of this in the past. We’ve collaborated with the traditional owners of East Alligator River, and they’ve also never seen humpback whales in any of the alligator rivers,” Dr Carol Palmer, a senior scientist at the NT Department of Environment and Natural Resources, said.

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Palmer said it was a mystery how the whales turned up in the river, with humpback whales normally migrating back to Antarctica around this time of year.

“How it has ended up in the river, no one can be certain. It sort of feels like they’ve just made a wrong turn.”

A marine ecologist, Jason Fowler, and his friends saw the whales on 2 September during an eight-day fishing trip along the river and reported the sighting to the NT government.

He told Guardian Australia he spotted a “huge blow” about one kilometre from their boat. “In the back of my mind I thought, that’s no dolphin, that’s huge,” Fowler said.

He has spent years sailing the coast between Broome and Darwin and has taken part in whale and dolphin surveys.

In a report of his sighting, sent to Guardian Australia, Fowler wrote: “I knew I was looking at humpback whales, I had the pics of Kimberley whales on my laptop to prove it, but my mind struggled to believe what I was seeing and my crew weren’t even entertaining the idea. Humpbacks don’t live in a croc-infested, muddy, tropical river in tepid, 27C water with the consistency of a thick café latte! Yet, they’re here!”

Fowler sent his GPS co-ordinates to the NT government for them to check the sighting.

He agreed it was possible the whales – two adults and a large juvenile – just took a wrong turn and then became stranded, reluctant to swim back across the shallow mouth of the river.

Fowler wrote: “The other possibility is food. The standard consensus amongst cetacean biologists is that humpback whales feast on krill over summer in Antarctica and cease feeding once they migrate north for winter.”

The marine ecologist said he had previously witnessed humpbacks “lunging” at baitfish and the river was known to host a lot of fish. He said as humpback numbers had grown in recent years “some are bound to be discovered in the most unexpected places.”

Authorities have set up a 30km boat exclusion zone in the river to try and prevent fishers going upstream.

Palmer said the exclusion zone was to protect both fishing boats and the whale, given they would not be expecting to bump into each other.

“You don’t want them zipping up there in their boats and hitting a whale, or the whale just knocking them over by accident.”

Although the remaining whale does seem to be safe and breathing well, there are some concerns it could be stranded if it moves further upstream.

“They’re about 20km up the East Alligator, and they’ve picked a spot where there’s still water even when it’s a low tide. But it is, from all that we can gather, quite shallow,” Palmer said, noting the whale was not touching the riverbed, but it was close.

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“It’s a very tricky place they’re in, but they look OK.”

If the whale doesn’t leave the river, Palmer said they may move in and try to push it back out to the ocean.

“The way that it’s worked in different situations is that they’ve used loud noises and banging sounds, with a number of boats, and just pushing them out that way.”

The East Alligator River is usually known to be infested with crocodiles, but experts believe they don’t pose a threat to the whale.

In fact, the local crocs seem entirely unfazed by the sudden intrusion of the ocean mammal.

“When we saw the crocodiles from the helicopter, there was no interest whatsoever in the humpback whale.”

Authorities continue to monitor the situation.