The chef, who usually trawls wet markets for his supply of live fish, became intrigued when his sister-in-law introduced him to Marine Culture Technology (MCT), an Australian company that is farming reef fish on the 14th floor of a Chai Wan industrial building. A sampling of the coral trout from MCT convinced him to add it to the menu, although the reef fish aren't commonly used for sashimi.

'The taste is subtle, so people who don't like strong-tasting raw fish may also enjoy it. The meat is tender and delicate but firm,' Yu says.

He hadn't considered serving coral trout until now because there was no assurance of a constant supply at stable prices, but the farm addresses all his concerns. At HK$600 per kilogram, the coral trout is at least 20 per cent cheaper than the netted variety he buys from wet markets and supply is regular.

Similar considerations have also led major buyers such as food chain Maxim's and the InterContinental group to turn to farmed reef fish, and MCT is also getting nibbles from the Venetian Macao.

It's a welcome result for managing director Lloyd Moskalik, whose company relocated its fish farm from Sydney to Hong Kong five years ago.

'We want to be as close to the customers as possible because it's very expensive to ship live fish by air,' Moskalik says. 'You need to ship four times the volume of water. Hong Kong being one of the largest global fish markets, it's quite obvious why we are here.'

'The only problem is that Hong Kong doesn't have the luxury of cheap land, so the only choice was to move inside an industrial building. By stacking the fish tanks on each other, we are able to save a lot of space.'

MCT raises fish from fingerlings produced at a three-hectare hatchery on Flores, an island in eastern Indonesia. It uses six coral trout, 12 mouse groupers and more than 25 tiger groupers originally caught from the sea as breeding stock.

'The water quality in east Indonesia is very good - it's clear and clean, and it's not a place with a lot of fish farming, which means the risk of contamination from other fish farms is low,' says MCT marine biologist Pierre Buray.

The millions of eggs laid by the parent fish every full moon are collected and incubated to yield about 20,000 fingerlings. These are kept in tanks for about three months until they reach about 10cm in length before being shipped to the Chai Wan facility, where they're kept in two 1,500-litre tanks.

Before releasing the fingerlings into the tanks, scientists test the water's oxygen content, pH, temperature and salinity, and make adjustments to avoid a sudden change of water quality, which may cause some weaker fish to die from shock.

So far, the fingerlings have been shipped just twice a year because they take up to 12 months to mature and there are only two tanks at the Chai Wan site.

However, that will change as the company opens a new facility in March in a Yuen Long industrial building, where it will have 200 tanks.

Computers regulate the temperature and levels of oxygen, ammonia and nitrite through multiple sensors placed in the water to maintain optimum conditions all year round.

Buray says the fish are fed pellets made of organic fish meal, fish oil, vegetable meal and vitamins. What's more, the tank system is a self-contained environment, with the water constantly recycled by filtering it through beds of bacteria that digest waste produced by the fish, including ammonia and nitrites.

'There's no exchange with the sea, which means no contamination, no heavy metals, no diseases and no chemicals,' says Buray.

(While the fish are maturing, MCT also imports net-caught species - grouper from Southeast Asia, lobster from North America, for instance - to generate quick cash. However, because fish are usually anaesthetised for easier transport, the company retains the imports in their system for about two weeks to allow the fish to excrete the drugs and recover from the stress before selling them.)

With fish populations in nearby waters decimated by decades of overfishing (species caught around Hong Kong now weigh an average of 10 grams or less), environmentalists might be expected to welcome the venture.

Although Dr Guillermo Moreno, who heads the WWF's marine programme, acknowledges that indoor fish farming has potential, he worries about its shortcomings.

'The problem is a lot of aquaculture fish are carnivorous, which means sometimes you have to use 5kg of fish to feed 1kg of fish.'

The energy that fish farms consume and the way the imported fish are caught should also be taken into account when gauging whether the venture is sustainable, he says.

'Without imports, Hong Kong wouldn't have enough seafood. We can't afford to lose this essential food source. We need sustainable alternatives,' he says.

Not having sampled the farmed reef fish, Lee Choi-wah, president of the Hong Kong Chamber of Seafood Merchants, couldn't comment on its quality yet but he queries the future of such farming methods.

'Production is limited, operating costs are high and the fish takes time to grow. Besides, people may not be willing to pay as much for farmed fish as they are the wild variety,' Lee says.

Nevertheless, he concedes that farmed reef fish offers the benefits of stable supply and prices.

'When there's bad weather, such as a typhoon, fish prices can double because of limited supply,' he says.

At his restaurant, Yu says his customers welcome his newly introduced coral trout dish. The sashimi, which is served with pomelo vinegar to bring out the delicate taste of the flesh, sells at HK$300 for a 20-slice portion.

'The fish is drug-free and isn't tainted by marine pollution,' he says.

Even so, Yu orders just one fish daily, which is freshly delivered every afternoon.

'Hong Kong people prefer sashimi made with more intense tasting fish such as mackerel or salmon. But I would love to provide alternatives in my restaurant,' he says. 'I like the fact that we're using ingredients that are organic and sustainable.'