Kevin Finch managed to get the porbeagle shark near enough to his 17 foot boat in order to get a decent idea of how much it weighed, but it wasn't an easy task for him.
Finch said that he fought with the powerful animal for around an hour before he was eventually able to catch it.
The 30-year-old said that he didn't really know how big it actually was until he managed to reel it towards his boat.
It was 10 feet long, with a girth of around eight feet, and had a calculated weight of 500lbs, which is about 35 stones.
It's a big fish, basically.
In fact, this porbeagle - which is a relative of the great white - is one of the largest ever captured in British waters, although it can't be verified officially because he chose to release the shark.
In order to be considered for a record, the shark must be landed and weighed, meaning that it has to die. However, many fishermen don't want to do that, and therefore attempt to calculate the weight whilst the shark is still in the water.
Kevin travelled down to Cornwall from his home in Kent to head out fishing, and his journey was quickly validated when he caught the shark after about an hour on the water.
He explained: "When I caught the shark it probably took about 300 metres of my line before I could even think about reeling it in.
"When I did it was like trying to fight a lorry with a stick. I was fighting it for an hour. It was man against beast or in this case a man eating shark. I saw it come up through the water and I said 'that is a big fish'."
"When I got it to the side of the boat it was thrashing about with its big jaws. I couldn't believe my luck.
"I was just gobsmacked afterwards and had a massive sense of achievement. I have still got sore arms and an aching back from it now."
After measuring up the shark, they removed the hook and let it go.
The biggest porbeagle shark ever caught near the UK was a 507lbs monster caught off the Orkney Isles in 1993 by a fisherman called Chris Bennett.
However, that's one that was killed and brought for official weighing.
In 2012, two men called Graeme Pullen and Wayne Comben believe they captured the unofficial record holder, having bagged a 550lbs shark off the Cornish coast.
Whilst porbeagles are huge and look terrifying, they're not usually a threat to humans as they rarely come anywhere near the shore.
Featured Image Credit: Kevin Finch/BNPS