Prawns/Shrimp Farming:
AAAS Symposium Report

A ProMED-mail post

[The American Association for the Advancement of Science recently held its annual meeting in Philadelphia, during which they had a symposium on shrimp farming, which was reviewed in the Economist, from which the following edited version has been taken. - Mod.MHJ]

It is only since the 1960s that shrimp have been farmed intensively and this has rapidly become big business. Some 1m tonnes (one in every four shrimp sold) are cultivated each year with a retail value of $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.

Because of its bad reputation among environmentalists during its short lifetime, this year, 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 the sea in the tropics. This turns out not to be the case. Mangrove forests were destroyed during the early days of the industry, but the rate at which the trees are being felled has dropped considerably. 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 per hectare to build, siting them on the wrong soil is an expensive mistake. Leaving the mangrove forests intact is now recognised as yielding 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, the latter was true during shrimp farming’s early days, but since farmers are replenishing the water in their ponds from those coastal waters, they have come to realise that discharging untreated waste is often tantamount to fouling their supplies. The cheapest solution is frequently 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.

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 from 65% to only 58% of the total coastline.

A third accusation against shrimp farming — that it damages other local fisheries — is more difficult. The alleged damage is caused because many farm-raised shrimp are grown from larvae caught in the wild. Unfortunately, the shrimping nets the collectors use also haul in a considerable “by-catch” of other species — frequently larvae of economically or environmentally important fish. However, the consequences of this are unclear as some species lay hundreds of thousands of eggs for every individual that survives in nature to adulthood. At present there is no clear evidence that the fishermen reduce the number of adult fish.

The general message from the symposium was that the environmental mistakes that have given shrimp farming a bad name were often the result of ignorance by farmers feeling their way and developing a new business. Now as those farmers have a better idea what they are doing, the problems are diminishing. And, the evidence suggests that large farms pollute the sea less than small ones, usually because they are better managed.

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