Sammy Hitzke, of Sammy Hitzke Fishing, told Newsweek he and his fellow fishermen had set bait at a depth of around 1,500 feet in the hope of hooking a swordfish. He said, initially, the shark they hooked felt very much like a swordfish: “We figured it was either just a really big one or foul hooked.”

The fishermen battled with the shark throughout the afternoon, eventually getting a look at their catch after 4.5 hours. “As any fisherman would agree, you can’t just give up mid-way through the fight, particularly when it could be the fish of a lifetime!”

When they finally got their catch close to the boat, they realized they had been reeling in a large shark: “We were both a little bit shocked,” Hitzke said. “Neither of us had seen a thresher shark in person so it was our first time laying eyes on one. Then obviously the fact that it had been half-eaten as well was very eye opening. The remains were probably a bit over 100kg [220lb].

“It was pretty difficult lifting it onto the boat because Shaun isn’t very strong, so I had to lift most of the shark myself. There was a period there where I didn’t think we would be able to get it in at all.”

The waters off the coast of Australia are home to several large marine predators, including great white sharks, bull sharks, tiger sharks and giant squid. In March, commercial fisherman Tony Walker was working in the Coral Sea off the coast of Australia when he pulled up three large fish covered in strange bite marks. He said he believes the fish had been attacked by a giant squid.

Giant squid can reach around 43 feet and use huge suckers on their arms to drag prey to their beaks, where they slick the meat into bite-sized chunks. Walker said he had always suspected giant squid were in the area but had never seen one.

Hitzke’s footage of the catch showed how most of the shark’s torso had been eaten, with just the head, fins and tail remaining. “I think it probably started getting eaten by the mystery attacker about three hours into the fight,” he said. “It was definitely alive when we hooked it. It was eaten during the fight.”

He said he believes their catch was attacked by other sharks or a giant squid: “I guess we’ll never know for sure unfortunately … I’ve been lucky enough to catch plenty of big fish and tons of different species, but I’ve never seen something like that happen.”