In a turquoise pool, far from the sea, the only surviving member of a species performs before a crowd of spectators. Emitting odd sounds and making spectacular leaps is Peter Bloom, the last person in Britain training dolphins to perform in public.

There is a sharp whistle and the three female bottlenose dolphins of Flamingo Land, a zoo and fun park in Malton, North Yorkshire, swim upside down, then leap five metres out of the water to smack a suspended plastic ball.

The dolphins themselves are not threatened with extinction. But dolphinariums are a vanishing attraction in Britain. Flamingo Land's last remaining rival, Windsor Safari Park, is being bought by Legoland, and its dolphin pool is now closed to the public.

In the Sixties heyday there were 15 dolphinariums, including half a dozen mobile ones, which Mr Bloom says deserved to close. A couple bowed to pressure from conservationists, others shut down from a lack of investment, but most moved to the Continent, to tap the foreign holiday market. Flamingo Land survives because it is popular - in the season the show attracts three full houses a day - but the dolphinarium costs pounds 40,000 a year to run and Mr Bloom says he is not in it for the money. 'If I wanted to get rich I'd be a stockbroker.'

Above the whistles and clicks picked up on hydrophones around the pool, Mr Bloom shouts to the crowd through a microphone: 'Lotty, Betty and Sharky are helping to save the lives of millions of their wild relatives. Dolphins and porpoises get caught in the man-made nets used by every fishing nation and they drown.

'We are using our dolphins' echo-location system to develop sonar reflectors to attach to nets so dolphins and porpoises can detect them and take avoiding action.' The dolphins lift their beaks to demonstrate their sonar clicking sounds.

Lotty, Betty and Sharky gather at Mr Bloom's feet like adoring labradors and he tosses three whiting from a zinc bucket. There are screams from the splash zone.

Mr Bloom is a director of the net research programme, which involves Aberdeen, Loughborough and Cambridge universities. Sea trials are being organised on the Moray Firth to test the sonar reflectors that could save the lives of dolphins in the wild. He explains: 'The dolphins' reactions are filmed and recorded using hydrophones and sonar buoys, and go back to the labs for analysis.'

Mr Bloom has a degree in fishery science, marine biology and trawler management. He is a member of Greenpeace and the Marine Conservation Society and gives papers on dolphin jaw geometry and net entanglement at symposiums across Europe and the US.

He is not the type of person you would expect to be preyed upon by protestors. But every Sunday animal welfare activists gather outside the Flamingo Land gates. 'They hold placards saying 13 dolphins have died here in 14 years,' says Mr Bloom. 'If it were true I'd be out there with a placard. We've got the same three dolphins. There's no difference in the lifespans of wild and captive dolphins. If they want to ignore the science, fine.

'Groups such as Into the Blue want to release dolphins at all costs. They want to release the Windsor dolphins despite the fact that they are all geriatric, pregnant or young, none of which can travel. The stress would kill them.'

One dolphin released from Windsor Safari Park was Rocky, set free into the Caribbean by Into the Blue two years ago. Mr Bloom fears that Rocky is dead. 'There's been one photo since he was released and he was very thin.'

He heard a children's television presenter say that if a dolphin dies in captivity the owners sneak in another at night. But Mr Bloom says there is no supply of substitute dolphins. 'None has been imported since 1983. If a dolphin dies here there's a post-mortem and the results are lodged with the Department of the Environment.

'We follow a preventive health programme using two good marine mammal vets, so we have no problems.'

Mr Bloom has plenty of academic visitors, though I am the first journalist, 'despite the thousands of words written'.

The job has evolved since he started with his father, Reg, at the Clacton Pier Show, cutting up fish and picking up litter. Reg Bloom was a zoo director before he caught the dolphin bug. 'He started at the top and ended up a floor-scrubbing dolphin trainer. We had four dolphins and a killer whale called Cuddles.'

Peter Bloom's own curriculum vitae includes the post of assistant killer-whale trainer at Ocean Park, Hong Kong. Nowadays he lectures to European trainers on the need to bring issues such as sea pollution into the spiel.

He is most angry about claims in leaflets distributed at the gates that the Flamingo Land dolphins belong to an endangered species. 'Untrue. This is an animal welfare, not a conservation issue.'

But surely some of the objectors' points are valid. The pool is small. Dolphins flicking footballs out of the water to Match of the Day music do lack dignity. 'I know the pool doesn't look much but it's deep. Next year it will be 6m deep and much larger, twice the UK standards. A concrete pool can't mimic the wild. If we put in seaweed it would clog the filters.

'These dolphins are family and we do something with them every day. Research is part of our daily routine, especially out of season.'

Back at the pool: 'I want a volunteer,' Mr Bloom shouts above the squeals, and a small girl with a surprised face is whirled round the pool in a dinghy pulled by one dolphin. 'The tail is the dolphin's outboard motor. It packs 80hp, the same as a small family car.'

As the dolphins demonstrate porpoising - jumping in unison over a long pole - Mr Bloom packs in as much conservation and education as the splashes and squeals allow: 'Only buy tuna from supermarkets that bother to label their tins 'dolphin-friendly'. Unscrupulous fishermen set their nets on surface-feeding dolphins, hoping that tuna are feeding on the same shoal of smaller fish below.'

Next year he hopes to start a breeding programme, 'Not because this is an endangered species but so we don't take any more dolphins from the wild. We're expanding this pool and building an adjoining one, so we'll have plenty of space.'

Downstairs after the show the dolphins are ogling the audience through the underwater viewing window. Girls are buying Be Choosy About Tuna stickers and Mr Bloom is distributing Cetacean Rescue Cards, instructions in credit-card format for dealing with a stranded dolphin: 'Keep it cool and moist, taking care no water enters the blowhole.'

Dave Goodson, senior experimental officer at Loughborough University's department of electronic engineering and a member of the net testing programme, says captive dolphins are vital for research: 'You can't solve everything using wild dolphins. You have to go out into the wild, then return to the dolphinarium to look at your results in fine detail. We would have terrible problems if every UK dolphinarium closed.'

Mr Goodson estimates that more than a million and possibly as many as six million dolphins die annually in nets.

Chopping mackerel at the huge double sink, Mr Bloom describes the 'curtains of death' fishing nets which trap wild dolphins in the eastern Pacific. 'They are up to 30 miles long. The dolphins leap in a frenzy but they can't break through the nylon.'

He talks about Hector's dolphins, threatened by drift nets off the New Zealand coast. And 500 remaining Indus River dolphins in Pakistan are threatened by pollution, damming and deliberate killing. 'Professor Liu, who came here recently, has established that there are fewer than 200 Chinese River dolphins. He came to see us because he wants to take some into captivity and start a breeding programme.'

The show is over but a dolphin is tossing a yellow ball at people and catching it. Mr Bloom nods to a pile of teaching packs: 'All the teachers say that seeing animals means more than any number of projects. My feeling is that if they can be kept successfully a few of each species should be kept. Three dolphins for 60 million people isn't over the top. At that level I can sleep quite happily at night.'

(Photographs omitted)