OEYGARDEN (AFP) - Tucked away in the corner of an enchanting fjord, 600,000 baby trout frolick in underwater cages as they wait their turn to end up on dinner plates: fish farming is booming in Norway, under the watchful eye of environmentalists.

In Oeygarden near the western Norwegian town of Bergen, the Blom family's fish farm consists of a building constructed on the water and three submerged basins where the fish are raised.

It is just one of the 800 fish farms dotting the coastline in the Scandinavian country, where three times more salmon and trout are produced than meat.

But while fish farming helps ease the pressure that industrial fishing is putting on the planet's fish stocks, it is not without its own slew of problems.

Farmed fish that escape, rampant illnesses and a debate over feed have tarnished the reputation of a sector whose exports totalled 2.5 billion euros (3.5 billion dollars) in Norway last year.

"In some areas the problems are so big that we cannot certify that it is (environmentally) sustainable. We should chill down the growth and make sure that all the main problems are under control," says Geir Lasse Taranger, research program manager at Bergen's Institute of Marine Research.

Heavy concentrations of fish encourage the spread of diseases and parasites that can have a disastrous effect on stocks, such as Lepeophtheirus salmonis, a louse that attacks fishes' skin and mucous membranes.

In Chile, the second-biggest producer of farmed salmon after Norway, a virus that emerged earlier this year devastated stocks and halved production, putting 20,000 people out of work, according to the international environmental group Pure Salmon Campaign.

The problems are not limited to the fish farms. Many of the farmed fish escape from their cages and contaminate the wild salmon, thereby weakening the stocks' genetic makeup.

"The wild genotype will disappear if there are too many escapes," Taranger said.