|We are moving from hunting to farming|
"The latest figures of the UN Food and Agriculture Organization (FAO) say it is close to 50%," observes Patrick Sorgeloos, an aquaculture specialist from the University of Ghent, and an advisor to the FAO. "It is about 60 million tonnes; and the FAO is indicating that by the year 2030 we will need an extra 40 million tonnes just to satisfy the need."
Given the parlous condition of many of the world's fish stocks, it is tempting to see aquaculture as the answer - a source of healthy protein which the oceans can no longer guarantee. Governments and regional authorities from the industrial might of Norway to rural Africa see it as the way ahead.
"The potential to expand the catching of fish is getting limited," notes Sloans Chimatiro, fisheries advisor to the New Partnership for Africa's Development (Nepad), an initiative of the African Union. "But also the products that come out of the lakes and oceans, because of the cost involved in catching them, become very expensive and so unaffordable for the local community. So growing fish is going to enable the continent to produce fish that is affordable."
It is not a view shared by everyone.
"Aquaculture has got a big potential value for food security, it's revenue for communities and it's an alternative food source," concedes Simon Cripps, director of the marine programme at the conservation group WWF. "But the thing to remember is that it isn't the solution to the problem of wild-caught fisheries."
Environmental groups lodge a long list of grievances against aquaculture.
Shrimp ponds destroy mangrove forest, a natural shoreline protector, and leave land too saline to use again. The waste excreted by concentrated masses of fish or shellfish wreak havoc with local ecosystems.
Fish, especially salmon, escape from their cages and breed with their wild cousins, weakening their genetic stock and ability to survive and reproduce. Carnivorous species such as salmon and cod have to be fed on food made from wild fish such as anchovies, depleting stocks still further.
Connemara in western Ireland is a cragged, peaty landscape where time seems to hang in the air like a fine mist. As a holiday destination, the first attraction is fishing.
"Tourism is easily the most important industry here now," says Peter Mantle, "and a crucial part of that industry was the fishing that derived from the clean waters and the unpolluted countryside."
The rivers here are famed for their salmon. But there are fewer, he believes, than in former years; and he believes salmon farms are the culprit.
Salmon naturally carry a parasite, the sea louse. Packing them into cages gives the lice a permanent home; overcrowding stresses the fish, making them still more vulnerable to infestation.
There is a wealth of evidence now, some from scientists in Europe and more from Canada, showing that lice harboured in farms materially damage wild stocks - something that Peter Mantle believes is happening before his eyes.
In spring 2006, he released a number of baby salmon from his hatcheries. All were tagged; half were treated with an anti-louse chemical before release, half were not.
"This year we're beginning to get some of the adults back, and we've just finished reading the tags of the first 80 to return," he relates. "Sixty fish came from the treated group, and 20 from the untreated group. So even the most elementary maths implies that two out of three of the untreated fish that would otherwise have made it back have been killed by sea lice."
Faced with criticism over their environmental record, some arable and livestock farmers have responded by moving to organic methods. Aquacultural farmers are following suit.
"When we sell our fish as organic, we are charging a very big premium for that," says Gerry O'Donohue from the Mannin Bay Salmon Company, also in Connemara.