Capture brought no quick death for this or dozens of other large exotic fish crammed into tanks around Hong Kong. Too old and costly for the tastes of most diners, some spend months or even years being gawped at by weekend visitors with little prospect of ending up on a dinner plate any time soon.

One giant grouper measuring around 1.5 metres long was this particular restaurant's prize draw for more than 12 months before it fell ill and died in its tank around Lunar New Year.

'It was very, very old,' the waiter said. 'After it died, we sliced it up and served it in the restaurant. You wouldn't believe how much meat there was on it. It's true that many people prefer to eat younger fish because they think the taste is better, but some of our older customers do like to eat these very big fish. They usually choose them to eat on special occasions, and that is when some of the bigger fish you see here will go.'

This weekend, as Easter crowds pour into seafood hotspots, Hong Kong's taste for exotic fish appears defiantly recession-proof. Falling fish stocks and rising price have if anything, it seems, sharpened people's appetite.

However, the practice of enticing customers with the display of huge fish in small tanks is troubling animal welfare experts, with Society for the Prevention of Cruelty to Animals (SPCA) executive director Sandy Macalister likening it to the way caged leopards or shackled elephants were displayed in the streets of colonial Hong Kong half a century ago.

Writing about the Sai Kung groupers in the SPCA's magazine Paw Prints, Mr Macalister said: 'These wonderful animals, which since the 1940s have lived and bred in the coral depths, now lie behind thick distorting glass in a narrow tank on the footpath. How many times have we walked past such horrific living conditions for these animals without a second thought? Is it because we consider them to be 'just fish'? If a passer-by or a restaurant patron knew that these magnificent creatures were more than 65 years old, would that make a difference?'

Mr Macalister believes that Hong Kong's laws should be changed to stop big fish being put on public display in cramped conditions by restaurants.

'They have sophisticated brains, and animal welfare science shows that they are feeling things we never knew they felt,' he said. 'Some of those fish you see outside restaurants have probably been around since the 1940s. They are used to swimming around freely in the depths. The next thing they know, they are in a tank on a footpath in Sai Kung. It's cruel and it must be terrifying for them.'

Mr Macalister said research suggested that despite common misconceptions, fish had memories and feelings similar to other animals, meaning that being kept for months or years in a restricted space amounted to a form of torture for a mature, adult fish.

'The only thing with a fish is it can't express it,' he said. 'People assume that means they have nothing going on. But when you consider what a fish does in its daily life - it can tell where it is, identify things and make decisions - it is clear there's far more going on than anyone suspects. They learn, and they have memories, and they can identify people. They feel stress and they feel pain. People used to believe fish couldn't remember anything for longer than three seconds, but we know now that isn't true.'

Mr Macalister said that as the law stood, it was very difficult to bring prosecutions.

'The issue is defining what is too small in terms of a tank,' he said. 'It's similar to a dog being kept in a cage. If the fish has clean water and he has got the space to move around, then it's not prosecutable under law. Attitudes, as well as the law, have to change. It's an issue of education. You can't blame people for not knowing, but there has to be a good look at the law, taking into account what is now known with animal welfare science and upgrading the law.'

Marine biologist Yvonne Sadovy, of the University of Hong Kong's school of biological science, said the notion that fish felt pain and stress was becoming increasingly accepted in academic circles.

'There has been a big question over whether fish feel pain and how they respond,' she said. 'Fish are vertebrates like us. They have a backbone, and a lot of the biology and physiology have some similarities to us. The nervous system and hormonal system in some ways are very similar. I think most biologists would say there is absolutely no reason to believe they would not feel pain. How they perceive it is obviously incredibly difficult to know, but you pick up a fish and take it out of water and put a hook in its mouth and it struggles. There is something clearly uncomfortable and not right and that fish is perceiving stress in some way.

'There have been studies of fish in mariculture environments where stress levels are measured by hormones when they are crowded and not fed properly, and chemicals associated with stress are very high. So there is no reason to think that they don't feel pain, In fact, that would have to be the assumption, unless it can be shown that they do not.'

Professor Sadovy has examined how large fish are treated in Sai Kung and said: 'The way they are handled is pretty awful. They are there for the spectacle and to attract people to the businesses. Sometimes, they will take a big one out and slice it up in a very public way to draw attention to the restaurant.

'What I see when I go and look at the tanks is that some of them are very damaged on the front of their faces. They often have abrasions. I guess when they are shipped and often moved over large areas, they get banged around.'

A worse and lingering fate sometimes awaits them in the tanks that sit outside the seafood restaurants.

'I've actually seen these fish physically thrown from one net to another, Professor Sadovy said. 'People stand on the tanks and pick one up in a net and throw it to another tank like a game of lacrosse.

'These fish have already been shipped over very long distances. They might be picked up in the Solomon Islands and shipped for weeks. They are often not fed for long periods of time so they are sometimes in pretty poor shape when they get here. They come from an astonishing range of places, right from the Maldives or the central Indian Ocean to the central Pacific and Australia - they are coming from all of these places and, with the exception of Australia, the fishing is not managed.'

Professor Sadovy draws a parallel with the way other animals used to be treated before popular ideas changed. 'We used to put lions and tigers in tiny cages in the past,' she said. 'With fish, we still don't treat them like we do other animals that we have come to have more respect for. Fish always get left until last.'

Existing animal cruelty laws do apply to fish, a spokeswoman for the Agriculture, Fisheries and Conservation Department confirmed. 'All animals in confinement, including fish, should be given enough space to move around freely,' she said.

However, the wooliness of that definition appears to be a significant bar to prosecutions. Asked if any cases had been brought to court, the spokeswoman said: 'There have been no prosecutions for keeping fish in cruel conditions in the past three years.'

As far as Mr Macalister is concerned, a public debate and a review of the law on the treatment of fish is urgently needed.

'We should start the discussion today,' he argues in his article. 'Otherwise will future generations wonder, 'How could we have been so inhumane?''