There are three types of shark...

I've had a fascination - and slight fear - of sharks ever since I watched Jaws, and then began diving. I've been lucky enough to dive in some pretty exotic places over the years, and have had some close encounters with hammerheads, blue sharks, silvertips, bull sharks and a tiger. Never a great white. Not sure I want to see one of those...

Sharks are finely-honed predators, and they can be pretty smart. I remember a bull shark in Sharm el Sheikh (Egypt) splitting off a female diver from the rest of our group, and herding her away from the reef out into the blue. She was fascinated by the shark, and didn't realise what was going on. It was a hell of a finning episode trying to get to her on the other side of side of the shark in order to rescue her.

I also remember being caught between two groups of four-metre hammerheads in Sipadan, at some considerable depth, and deciding that these were beautiful but completely scary demon-fish (okay, there may have been some narcosis involved). In Palau there is one of the best dives on the planet called Blue Corner, where the visibility can easily be fifty metres, and you can watch a dozen sharks hunting in the morning, zipping in and out of massive shoals of fish. While watching this spectacle, and clinging on to a rock because of the intense current, a shark sidled up to me, and came within arms' length, watching me with its beady eye. I had to decide what to do, so I let go, wondering if it would whip around and have a bite, but it just ignored me as the current swept me away. My buddy asked later, 'what took you so long to let go?'

In my latest diving thriller, 37 Hours Jake is a British diving instructor abroad on a tropical island, and here he explains how I feel about sharks, as he puts them into three simple categories, depending on whether they will run away from you, mawl you if provoked, or kill you if you're bleeding...

‘There are three types of shark.’

Jake was in dive instructor mode. Nadia wasn’t averse to it. He’d asked if she’d dived with sharks before, and she’d replied no. But she didn’t like being passive. She held up one finger, the second one, and gave him her blandest smile.

It didn’t put him off his stroke. Several other divers plonked themselves onto the bench. Dominic – the lanky, foppish-looking chief instructor – hustled his diving group over to listen. From his grin, clearly he knew Jake, and had heard this particular lecture before.

Jake caught Dominic’s eye, nodded, and continued. ‘First, there are reef sharks, about four feet long. They’re more afraid of you than the other way around, but they can nip you, if you harass them, or box them into a corner.’

‘How do you know if you’re harassing them? How close can you get?’ One of the British divers. The way he’d said it, it was a challenge.

Dominic tossed Jake a whiteboard marker. Jake neatly snatched it out of the air, turned to the whiteboard, and drew a crude side view of a shark with a thin body. He pointed to the pectoral fins. ‘These will drop down, move closer together, and…’ he sketched the same shark as seen from above ‘…the shark’s body will move from side to side.’ He added little arrows, and Nadia imagined the shark dancing, its body gyrating. ‘If that happens,’ he said, ‘back away fast.’

‘What if you’re in a cave?’ The dude again, pressing Jake.

‘Stick to the sides or the ceiling,’ Jake replied, zero antagonism in his voice. ‘Don’t block the entrance. Point is, even if they bite you, it’s a defence mechanism. They want to get away, or get you away from their nest. You can add to this class the slightly larger nurse sharks and leopard sharks, because they’re really not interested in us.’

Nadia held up two fingers, adding the forefinger, in a victory ‘V’, because he was winning this.

‘Second type is longer, six to eight feet, sometimes local, like grey reef sharks and black-tips, sometimes ocean-going – pelagic – like silver-tips. The first two are often in groups.’ He drew a longer and broader shark. ‘If you get cut around these sharks, they’ll attack, and the sheer numbers mean you won’t make it. Other predator fish like trevally, known colloquially as Jack, will arrive almost simultaneously, and all you’ll see is a whirlwind of silver, and every half-second one will dart in and tear off a piece of your flesh.’

‘Ever seen that?’ The Brit again.

Jake nodded to Dominic.

Dominic took a sip of his tea. ‘We occasionally do shark-feeding here, with chain-mail arm protection, using chum – that’s chopped-up fish intestines or heads – as bait.’ Several divers immediately sat up, their eager faces swivelling towards Dominic. He held up a hand. ‘Not very often, and only with advanced divers and instructors. It attracts the bigger ones to the reef, and they begin to associate humans with food, and then, as Jake already mentioned, there’s the trevally. They get pretty antsy. They’re just too unpredictable, too fast.’

Nadia added her ring finger.

Jake resumed. ‘Third are big, lone sharks. Bulls, tigers, the blue shark, and the great white.’

‘Ever seen a great white?’

This guy was a pain. Harmless, but a pain.

Jake didn’t take the bait. ‘There’s a saying amongst divers. The first time you see a great white…’ He flourished an open palm to Dominic.

‘Is the second time it’s seen you.’

Jake drew three flattened circles. ‘This is what you see when a shark is heading towards you. This one…’ he pointed to the reef shark ‘…can bite you. This one…’ he pointed to his type two ‘…can kill you, but it usually takes a few of them. And this one…’ He put down the pointer. Stared at the divers one by one. ‘Is out there. Fifty metres from where you’re sitting right now. If you swim away from the reef, just fifteen metres away, you’ll see him materialise out of the blue. A face, the mouth, the eyes. He’ll be coming straight towards you. It won’t be coincidence he’s heading your way. If this happens,’ he said, leaning forward on his knuckles on the bench, ‘DO NOT head for the surface. DO head straight back to the reef. NEVER lose sight of the reef. The really big sharks won’t approach the reef unless there’s already blood in the water.’

It was deadly quiet. Dominic grinned. The Brit piped up. ‘Bullshit. There’re no sharks that big just out there.’ Other divers turned to him, then to Jake.

‘You’re welcome to find out. We call it Anspida Roulette. See how long you can stay off the reef.’

In reality, we did play Sipadan Roulette (Anspida is an anagram of Sipadan), and that's where I saw my blue shark and a tiger, and swam like hell back to the reef. Needed a few beers later. In the book, though, since it's a thriller, there is a shark attack, as Jake is stabbed by another diver, so there is a lot of blood in the water. I'll save that for the book. It was quite a harrowing scene to write.

I'll still keep diving with sharks. Last time was two years ago in Mauritius, in the Passage St Jacques, with a dozen reef sharks in very murky waters. 

I think sharks are amazing creatures, and I'm still fascinated by them. But I'll never turn my back on them.

37 Hours is available here