NEW agricultural crops do not appear that often, but the past 30 years have seen one blossom fruitfully. Though people have raised fish in artificial ponds for millennia, it is only since the 1960s that the practice has been extended to shrimp. And farming these crustaceans has rapidly become big business. Some 1m tonnes (one in every four shrimp sold) are cultivated each year. The retail value of these cultivated shrimp is $15 billion, of which the farmers themselves see about $6 billion. And the industry is also a big employer, mainly in poorer countries adjoining the Pacific ocean. In Ecuador, for instance, 230,000 people earn their livings working for shrimp farms, and a fifth of all the households in the country's coastal provinces are dependent on the farms for their income.

It might thus be thought that shrimp farming is an unalloyed good, but the industry has picked up a bad reputation among environmentalists during its short lifetime. This year, therefore, the AAAS organised a small symposium on the subject, inviting academics, environmentalists and some of the farmers to what proved to be an encouraging meeting of minds.

The most persistent accusation against shrimp farmers is that they are wrecking the mangrove forests that fringe much of the sea in tropical climes. This turns out not to be the case. Mangrove forests were, indeed, destroyed during the early days of the industry, but the rate at which the trees are being felled has dropped considerably. This, it turns out, is for a good, non-altruistic reason. Land where mangroves have grown has proved less than ideal for raising shrimp. It is too acidic for them, so that ponds dug in former forest land have to be heavily (and expensively) limed to stop the crop from dying. Since those ponds cost $10,000-50,000 a hectare to build, siting them on the wrong soil is an expensive mistake. Besides, leaving the mangrove forests intact is now recognised to yield positive benefits, for the forests help to deal with the second environmentalist charge against the industry—that waste from its ponds pollutes coastal waters.

Again, this was true during shrimp farming's early days, but since farmers are replenishing the water in their ponds from those self-same coastal waters, they have come to realise that discharging untreated waste is often tantamount to fouling their supplies. The cheapest solution is usually to put a farm just inland from a mangrove forest and trickle the effluent into it. Like reed beds, mangrove forests are superb natural filters for organic matter. What reaches the sea is more-or-less pristine brine.

The forests themselves therefore seem to be at little risk from shrimp farming. The lands that have suffered most from the shrimp farmers' attentions are salt flats, a habitat of less concern to conservationists. In Honduras, for example, the Pacific coast (where most of the country's shrimp farms are located) was 65% forest, 20% salt flats and 12% shrimp farms in 1987. By 1995, shrimp farms occupied 34% of the coast and salt flats 4%. The forest had shrunk relatively little, to 58% of the total coastline.

A third accusation against shrimp farming—that it damages other local fisheries—is more finely balanced. The alleged damage is caused because many of the shrimp raised in farms are grown from larvae caught in the wild. That in itself does not matter, but the shrimping nets the collectors use also haul in a considerable “by-catch” of other species—frequently the larvae of economically or environmentally important fish. However, the consequences of this are unclear. The natural attrition rate of fish larvae is horrendous. Some species lay hundreds of thousands of eggs for every individual that survives to adulthood. There is no clear evidence that the fishermen actually reduce the number of adult fish around.

The general message from the symposium, therefore, was that the environmental mistakes that have given shrimp farming a bad name were often the result of ignorance by farmers who were, after all, feeling their way into a new line of business. Now those farmers have a better idea what they are doing, the problems are diminishing. And, sadly for people whose instinct is to champion the little guy against the giant corporation, the evidence also suggests that large farms pollute the sea less than small ones, usually because they are better managed. Something to chew on over your next prawn cocktail.