But this success story of penning salmon, sea bream, sea bass, haddock, flounder, cod, turbot and tuna has been marred by deepening worries about environmental damage.

One big criticism is that to produce these upmarket fish, aquaculture firms are indirectly abetting the collapse of the world's fisheries.

The carnivorous, penned fish are fed from two main sources -- pellets of ground-up small fish that are often trawled up accidentally as so-called bycatch, but also food-grade fish such as mackerel, herrings and pilchard.

For the 10 species of carnivorous fish that are the mostly commonly farmed, around 1.9 kilos (3.8 pounds) of wild fish are needed for every one kilo (2.2 pounds) of fish that are raised, according to a benchmark 2000 study.

Nearshore table fish represent only around a 10th of the world's aquaculture production, but in profit terms they are the fastest-growing and most alluring sector in an industry that has expanded by 11 percent annually over the past decade.

"They're not in large quantities yet, but the way the marine fish prices are going up, particularly in Asia, people will tend to use food-grade fish to feed [penned] fish," said Rohana Subasinghe, a senior fishery resources officer at the UN's Food and Agriculture Organisation (FAO).

"It's a vicious circle, and believe me the demand for these things will never go down, it will always increase," he told AFP.

The good news is that scientists are getting smart at tailoring diets to the specific needs of a species in order to ensure maximum growth.

And they are figuring out ways of finetuning feeding so that pellets are not dumped haphazardly into cages but delivered in measured, timely amounts.

Fishfeed that ends up rotting on the seabed is not just a waste of the ocean's resources but also an environmental risk, because in waters where there is poor circulation, it can cause oxygen starvation.

Thanks to smarter diets, the 1.9-to-1 ratio of feed input to fish growth of 2000 has been reduced in some species to 1.5:1.

Sadavisam Kaushik, a researcher leading a team from France's Bordeaux University, National Institute for Agronomic Research (Inra) and French Research Institute for Ocean Exploitation (Ifremer), says that improved delivery systems means that as much as 97 percent of fishfeed is eaten.

One of the most encouraging areas of progress is in finding vegetal substitutes such as soybean, corn gluten and wheat flour as a substitute for fish protein while at the same time weeding out their cellulose and starch, which the caged fish do not digest, says Kaushik.

In the case of farmed salmon, fishmeal today now represents only 35 percent of the diet compared with 60 percent in 1985, says FAO fisheries nutrionist Mohammad Hasan.

Even so, salmon and other fish-eating species cannot go completely vegetarian. They need a long-chain highly unsaturated fatty acid called omega-3.

This is famously found in fish oil and cannot be totally replaced. But some canny research has helped to push back the limits more and more, using rapeseed oil, linseed, soybean and other oils.

In Britain, only 10 percent of the oils in salmon diet comes from vegetal sources, although a producers' association in Scotland, Scottish Quality Salmon (SQS) has recently allowed its farmers to take this to 25 percent while in Norway, up to 50 percent of dietary lipids are of plant origin, said Hasan.

In March, University of British Columbia researchers reported that chinook salmon thrived on a diet where three-quarters of the fishoil was substituted by rapeseed oil.

At the moment, the world produces around 50 million tonnes of aquaculture products a year, most of which comes from freshwater sources.

"By 2030, we will need another 40 million tonnes," said Subasinghe.

"Captive fisheries will not increase much more over this time, which means the extra 40 million tonnes will have to come from production. A tremendous amount of work has to be done to figure out how we are going to do this."

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