Counting fish in Mozambique

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Twenty five metres below the sparkling waters of the Indian Ocean the rock pinnacle sticks out from the reef like a talon. Behind me a coral wall stretches north and south as far as the eye can see, and its sheer face plummets into an underwater chasm 500m deep.

Hanging above the dark space I am one of six divers armed with clipboards and pencils. At a signal from marine scientist Nick Hill we begin to count fish. He gives us just ten minutes, and it is hard not to concentrate on the biggest creatures; 23 grey reef sharks that circle the pinnacle, but retreat when we get too close. Like us, they congregate here because the fish life is extremely rich.

Returning to the surface after an hour underwater, we sit on the dive boat and compare notes. Nick laughs when he sees I have