IN JUST OVER AN HOUR he had caught three bonitos. Their plump bodies were piled up in the stern, glistening in the sun.

Ucañan felt jubilant. That was more than he'd caught during the past four weeks. There was no real need for him to stay any longer, but now that he was here he might as well wait. The day had got off to an excellent start: perhaps it would end even better.

In any case, he had all the time in the world.

The caballito was drifting leisurely along the edge of the underwater rocks. He let the rope slacken and watched as the buoy skittered away. Every now and then he scanned the water for the lighter patches where the lava reached the surface. He needed to keep the net out of danger. He yawned.

He felt a gentle tug on the rope.

A split second later the buoy disappeared in a flurry of motion. Then it shot up, danced wildly on the surface and was wrenched under again.

Ucañan seized the rope. It strained in his hands, tearing his palms. He cursed. Within seconds the boat was tilting dangerously. He wobbled and let go. Deep beneath the surface of the water, the buoy flashed red. The rope hung vertically below it, taut as a wire, dragging the caballito down by its stern.

Something must have swum into the net. Something big and heavy. A swordfish, perhaps. But swordfish were faster than that. A swordfish would have sped off, taking the caballito with it. Whatever was trapped in the net seemed determined to dive to the seabed.

Ucañan made a grab for the rope. The boat jolted again, pitching him into the waves. Spluttering, he rose to the surface – in time to see the caballito disappear underwater, its bow pointing upright into the air. The bonitos drifted into the sea. He seethed with rage – he couldn't even dive after them. He had to save himself and his boat.

The morning's catch, all wasted.

The paddle was floating a short distance away from him, but Ucañan didn't have time to go after it now – he could fetch it later. He flung himself over the prow but the boat was being pulled inexorably into the depths. In a frenzy he hauled himself towards the stern. With his right hand he fumbled for what he needed. Blessed St Peter – his knife hadn't been washed away and neither had his diving mask, his most precious possession, apart from the calcal.

He sliced through the rope, and the caballito shot to the surface, spinning giddily. Ucañan saw the sky circle above him and his head plunged back into the water. Then he lay coughing on the little reed craft. The boat rocked gently in the waves as though nothing had happened.

He sat up in confusion. The buoy was nowhere to be seen, but the paddle was drifting nearby. With his hands he steered the caballito towards it, hauled it on board and laid it in front of him. Then he looked around him.

There they were: light patches of lava in the crystal water.

He'd drifted too close to the underwater rocks, and his calcal had snagged. No wonder he'd gone under – he shouldn't have been daydreaming. Now he knew where the net and the buoy were: they were tied together, so if the net was caught on the lava, the buoy couldn't rise to the surface. Yes, that was what had happened. Still, Ucañan was shocked by how violently he'd been pulled underwater – he was lucky to have survived. But he'd lost his net.

Paddling swiftly, he steered the caballito to the site of his accident. He peered into the depths, straining to see the net through the clear blue water. There was no sign of it or the buoy.

Was this really the right spot?

Ucañan had the sea in his bones – he'd spent all his life on the ocean – and even without technical equipment he knew that this was the place. This was where he'd cut the rope to save his boat from being ripped apart. His net was down there somewhere.

He had to go after it.

The thought of diving filled him with trepidation. He was an excellent swimmer but, like most fishermen, he had a deep-seated fear of the water. Few fishermen loved the sea, even though they took to it every morning. Some had fished all their lives and couldn't live without it – yet they had trouble living with it too. The sea sapped their strength – making them pay with their lifeblood for every catch they brought home – and leaving them washed up in the ports, withered figures hunched silently at bars, with nothing left to hope for.

But Ucañan had his mask. It had been a present from a tourist he'd taken out on his caballito last year. He leaned back and pulled it out, spat on the lens and rubbed it carefully so that it wouldn't cloud over. Then he dipped it into the water, pressed it over his face and pulled the strap round the back of his head. It must have cost a lot of money: the fittings were made of soft latex that moulded to his face. He didn't have any breathing equipment but he wouldn't need it. He could hold his breath for long enough to dive down and untangle the net from the rocks.

The sharks in these waters weren't usually a threat. Hammerheads, shortfin makos and porbeagles occasionally plundered fishing nets, but that was much further out. It was almost unheard of to sight a great white. Besides, he wouldn't be swimming in the open: he'd be near the rocks and the reef, which offered some protection. In any case, whatever had ruined his net, it hadn't been a shark, he thought.

It was his own fault for not being more careful.

He filled his lungs, dived into the water and sped away from the surface, body vertical, arms pressed to his sides. From the boat the water had looked forbidding, but now a welcoming bright world opened up around him. He had a clear view of the volcanic reef, which stretched into the distance, dappled with sun. There were few fish, but he wasn't looking for them. He scanned the reef for the calcal. He couldn't stay down too long or his caballito might drift away. He'd give himself a few more seconds, then go up and try again.

He'd make ten trips if he had to. He didn't mind if it took all day. He wasn't going back without his net.

Then he spotted the buoy.

It was hanging ten to fifteen metres below the surface, suspended over a tip of jagged rock, the net directly below it. It seemed to be caught in several places. Tiny reef fish were swarming around the mesh, but they dispersed as he swam over. He straightened up, treading water as he tried to free it, his shirt billowing in the current.

The net was in tatters and he stared at it in disbelief. It had taken more than the rocks to do that. Something had been on the rampage. What, in God's name, had been here?

And where was it now?

Ucañan felt uneasy as he fumbled with the net. It would take days to repair. Now he needed to breathe. He would go back up, check on the caballito, then dive again.

Before he could move, a change took place around him. At first he thought the sun had disappeared behind a cloud. The light stopped dancing over the rocks; the reef and weeds no longer cast a shadow…

His hands, the net, everything around him was losing its colour and turning a murky grey. He dropped the calcal and looked up.

Gathered just beneath the surface of the water was a shoal of shimmering fish, each as long as his arm, stretching as far as he could see. The shock made him gasp and bubbles rose from his mouth. Where had a shoal of that size come from? He'd never seen anything like it. It seemed almost stationary, but now and then he saw the flick of a tail-fin or a flash of silver as a fish darted forward. Then, as a unit, the shoal changed course by a few degrees. The gaps between the bodies closed.

It was normal shoaling behaviour, but something wasn't right. It wasn't so much what they were doing that unnerved Ucañan: it was the fish themselves.