Plunging a knife in again and again, diver Craig Clasen grapples with a 12ft tiger shark to protect a friend.

For two hours he wrestled with the giant, spearing it seven times, even drowning the beast before eventually finishing it off with a knife.

Mr Clasen was hunting yellow fin tuna with fellow fisherman Cameron Kirkconnell, photographer D.J Struntz and film maker Ryan McInnis in the Gulf of Mexico when the encounter took place.

Free diver Craig Clasen was forced to fight head-on with a 12-foot tiger shark when the predator turned on his friend during a spearfishing trip

The group were about to leave the deep waters south of the Mississippi River's mouth, when Mr McInnis found himself alone in the company of a tiger shark.

With no time to lose, Mr Clasen grabbed his speargun and swam to his stranded friend, who was being circled by the giant predator.

'I positioned myself between Ryan and the shark and I tried to watch it for a second, hoping it would pass us by,' explained 32-year-old Mr Clasen.

'I noticed that the shark was getting tighter and tighter and just kept trying to get a back angle on us and behaving in an aggressive manner.

'The shark made a roll and looked like it was going to charge us so I just went ahead and took the conservative route and put a shaft through its gills.

'Cameron and I have been around sharks for years and we all have a lot of experience with them but this encounter had a different feel to it.

'Down in my core I really felt the shark was there to feed. I didn't want it to come to that.'

Mr Clasen spent nearly two hours wrestling with the giant 12ft shark, spearing it seven times and even attempting to drown the beast before eventually finishing it off with a long blade knife.

The experienced spearfisherman spent nearly two hours wrestling with the shark, spearing it seven times

'Once I shot it in the gills I felt a moral obligation to finish the job,' says Mr Clasen.

'I didn't want it to go on any longer than it had to. I shot the fish like I would do any other fish and worked it up closer and did my best to kill it as humanely as possible.

'I speared it in the gills which I knew would kill it and from that I tried to put a shaft into its brain as quickly as possible.

'I shot it six times in the head with a spear and I wasn't having much luck - it was a slow drawn out process.

'Sharks are so resilient and so tough from millions of years of evolution they are just survivors.

'The best way and quickest way to finish the job and kill the shark and recover it was to get a rope around its tail, drag it from the back of the boat and attempt to drown it.

'In the end we had put a knife its skull once I got lose enough to it and use a long blade knife even after trying to drown it.'

Mr Clasen, who usually sees 50-100 sharks on such dives, said that such an attack was rare

Mr Clasen has been free diving and fishing since an early age. Hailing from Mississippi, he was brought up in a fishing family, and is an expert in all fishing disciplines. Despite his experience, Mr Clasen took no pleasure in disposing of the giant shark.

'This was one of the most remorseful moments I have ever had in all of my years in hunting, gathering and fishing,' explains Mr Clasen.

'Personally I never shoot anything or kill anything that I am not going to eat.

'We saved the tail and the head, cut a giant chunk out of it and ate a piece.

'I wasn't there to hunt the shark, it was a defensive move for me and I would do it again. Unfortunately it had to be done and its not something I was proud of. It was a situation that presented itself to us. This was one of those rare instances where we had to protect ourselves.

'I have so much respect for sharks in general. With the amount of time that we spend out there we are exposed to so many potential risks.'

The divers had been hunting with spear guns for yellow fin tuna when the tiger shark attacked

Spearfishing is a form of fishing that has been popular throughout the world for centuries. Considered to be the most selective way of fishing, the amount of fish taken by spearfishermen accounts for just 0.1 per cent of fishing globally.

Today spearfishermen use effective elastic- or pneumatic-powered spearguns and slings to strike the hunted fish using free-diving, snorkelling or scuba-diving techniques.

Regarded by many as two of the world's best free diving spearfishermen, Mr Clasen and Cameron Kirkconnell have come into contact with thousands of sharks.

Watching from the boat, Mr Kirkconnell is sure Mr Clasen the right decision. 'We had been in the water all day and had caught lots of tuna,' he explained.

'But every dive we do is a shark dive and at certain times of the year, especially in Louisiana, we expect to see between 50-100 sharks from 7-12 ft.

'This encounter was so rare though. This shark might have been part of a feeding frenzy and still fired up and thought this was an easy kill.

'Tiger sharks have no problem eating whole sea turtles, 150lb tuna and even dolphins. It wasn't a split second decision on a whim, Mr Clasen has had hundreds of hours of experience.