Don’t flush the fish

John Whitfield

  • Coral: A Pessimist in Paradise by Steve Jones
    Abacus, 242 pp, £8.99, July 2008, ISBN 978 0 349 12147 5
  • A Reef in Time: The Great Barrier Reef from Beginning to End by J.E.N. Veron
    Belknap, 289 pp, £22.95, February 2008, ISBN 978 0 674 02679 7

Tens of thousands of years ago, the arrival of people in the Americas, and in Australia and New Zealand, was followed by a wave of extinctions, particularly of the largest species, which made the most attractive game. More recently, rats, cats and goats have eaten their way through the native plants and animals of small and not so small islands; and California is home to four hundred introduced plant species, which have almost entirely displaced the native prairie. But in the next hundred years or so, we are likely to see something new, as human activities cause the disappearance of ecosystems on a global scale. Species living on mountain-tops are going to find their habitat disappearing, as warmer climates rise up to engulf them. And Steve Jones and J.E.N. Veron warn that climate change may well bring about the end of coral reefs – if overfishing, disease, invading species and pollution don’t get them first.

The surface waters of the open tropical ocean are poor in nutrients and almost lifeless because the warm upper layer does not mix with the cool water below; as a result, everything edible sinks, never to return. In shallow coastal waters, corals get round this by symbiosis. Reefs are built by polyps (tentacled creatures related to the sea anemone) that secrete limestone and live on the growing pile of rock. Polyps take living algae into their cells, which photosynthesise, and provide most of their hosts’ energy, as well as giving them their colour. In return, the algae get protection, and feed off the polyps’ waste nitrogen and carbon dioxide. Polyps in symbiosis with algae can grow up to a hundred times more quickly than a lone polyp. In waters where an efficient nutrient cycle prevents materials from washing away, reefs can be the most productive environments on earth, laying down carbon at twice the rate of an equivalent area of rainforest, and producing several centimetres of rock per year.

A hectare of tropical reef can support two tonnes of fish, and a quarter of all known fish species live on reefs. Reef fisheries account for less than 5 per cent of global catches (reefs cover only 0.1 per cent of the sea’s area), but they provide employment, and protein, to people in developing countries who would otherwise struggle to obtain them. More than half a billion people live within 100 kilometres of a coral reef, and conservation groups estimate that hundreds of millions, mostly in Asia, have some dependence on reefs for their welfare.

Like nearly all other fisheries, reefs are severely overexploited. Last year, British and Canadian researchers compared the actual catch in reef fisheries with an estimate of how much fishing they can support, based on the rate at which fish populations grow. They found that the one million tonnes of fish caught from the world’s reefs is three times more than is sustainable. Put another way, we would need four more Great Barrier Reefs to support current levels of fishing without depleting stocks.

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[*] ‘The Problem with Biodiversity’ (LRB, 10 May 2007).