But a breakthrough last year made it possible to provide a stable supply to the market in the coming years, the Agriculture, Fisheries and Conservation Department said.

The aim is to bring 300,000 of the freshwater species to the market each year starting with the first batch of 3,000 in mid-May, and the Fish Marketing Organisation expects them to sell well despite the price.

'Even when the supply was unstable, we got calls from the public asking when the fish would be available again,' the organisation's market manager, Henry Yip Hin-wai, said.

The fish is high in Omega-3 fatty acids, which are essential to normal growth and, according to some studies, can help reduce prostate tumour growth.

The department first bred the fish successfully in 2007 in co-operation with several fish farms, but many were eaten by birds and other fish before they reached marketable size.

John Lee Hong, who has farmed pearl perch for a couple of years, said they cost 10 times more to rear than other freshwater varieties, and farmers were still looking for ways to reduce the costs. He said they ate a lot and were picky about their food.

'In the two weeks after birth, they need to be fed constantly for 18 hours a day, and they only eat live food,' Lee said.

The fish also had to be taken care of round the clock in the first few weeks, because they would kill each other even when they were only two days old.

But they grow quickly. While a grey mullet needs a year to grow to a marketable size of 600 grams, a pearl perch needs only half that, fish farmer Lai Loi-chau said.

Lai said about a dozen farms were now growing the fish and about 100 were needed to provide a stable supply. 'We can achieve that by the end of the year,' he said.

Net gain

The pearl perch will sell for a relatively high price per catty (600 grams), in Hong Kong dollars, of up to: $70