GLASSY seas off Sai Kung ruffle white when hit by a gusty breeze. But apart from the slap and fizzle of parting waves as the yacht Delerium Tremens ploughs southwards, the water seems serene. Beneath the blue-green surface, however, a commotion of creatures feeding, mating, fighting and dying creates an ear-splitting cacophony. A sizzling crackle like chips frying in hot oil fills the headphones transmitting the underwater babble to human ears, normally deaf to the marine world. The fat of the sea is giving Swire Institute of Marine Science researchers aboard the 37-metre ketch, a headache - literally and scientifically. 'It's shrimp-fantastic,' shouts Lindsay Porter above the din, one ear glued to the headset while she scans the horizon for vessels. Millions of tiny clamorous prawns, as if riotously celebrating their survival in Hong Kong's overfished seas, threaten to hijack the underwater acoustic device designed to track marine mammals. 'That's why we should eat more shrimps, they are good for you and . . .' she breaks off as a tumult of static and buzzing erupts . . . they will not confuse researchers tracking one of Hong Kong's most elusive mammals: the finless porpoise. Jet black and blunt nosed, the porpoises bear little resemblance to their famous cousins, the Chinese white dolphins. But as Hong Kong's other resident marine mammal - both are locally and internationally protected - they share part of the coastal habitat and suffer similar pressures of development and pollution. 'We are not sure about the abundance of finless porpoises. We know they have been sighted in the east and south parts of Hong Kong but how many there are we don't know,' says Agriculture and Fisheries Department marine parks officer, Dick Choi Kwong-chuen. Record numbers of strandings in recent years have raised fears that polluted waters are taking their toll on marine mammals. So far this year, five finless porpoises have been washed up dead. All were juveniles. Last year eight were stranded, while in 1996 there were 15. In 1996 the first tests on tissue and milk samples revealed high levels of polychlorinated biphenyl (PCBs) and the pesticide DDT, an organochlorine banned in Hong Kong and China. Without knowing how many finless porpoises exist in Hong Kong waters, it is impossible to assess the impact of mortality on the population, argue conservationists, and that is why a thorough study is needed urgently. That is about to happen with an $8-million 30-month study, due to start in July. 'For three years we have been studying Chinese white dolphins but we have not studied finless porpoises,' says Mr Choi. 'We don't want to repeat the same thing as the Chinese white dolphins - when we tried to build the new airport we did not have a lot information about them. We learnt from that previous experience that we need some basic information.' The question is, how best to study these elusive creatures? Unlike the Chinese white dolphins, whose bright pink colouring and habit of frolicking makes them easily observable, finless porpoises as their name suggests, lack a dorsal fin - the first thing human watchers usually spot. 'They are quite shy and difficult to observe,' Mr Choi says. An underwater acoustic device, developed in Britain under the auspices of the International Fund for Animal Welfare, could prove the key to studying them. During a pilot project by World Wide Fund for Nature Hong Kong, researchers trailed the hydrophone from the yacht, Delerium Tremens, covering 900 nautical miles. Beneath the deck, on plastic seats sticky with salt and humidity, sits the 'porpoise box', the world's first automatic device for tracking marine mammals. It is connected to a conventional laptop beside which lounges researcher Connie Lau Hong-yu, earphones clamped firmly to her head, occasionally glancing at the screen across which zig-zag red and blue lines. It could be a computer programme charting a company's profit and loss except for the words 'dolphin detector' on the left-hand corner of the screen. The porpoise box translates ultrasonic sound detected by the hydrophone and splits it into three channels before displaying it on the computer. 'Is there a boat?' shouts Ms Lau, appearing through the hatch. Apart from animal chatter, the hydrophone picks up human-induced noise within a two-kilometre radius, mainly the throbbing engines of passing boats. 'It is such a pain,' says Ms Lau, who is studying dolphin acoustics for a Phd at the Swire Institute of Marine Science. 'You can imagine how dolphins and finless porpoises feel in a stressful environment. We don't know how it will affect them,' she says of the underwater thunder. The graph's red lines represent the ultrasound of the porpoises; their distinctive 'clicks' are the key to the equipment devised by biologist Dr Jonathan Gordon and electronics engineer Oliver Chappell of Oxford University's Department of Zoology. Because they use a narrow range of ultrasound - not usually audible to humans - the device can distinguish porpoise communication from dolphins' lower frequency whistles. It can also separate the shriek of shrimps from porpoise 'clicks' - heard once briefly that day close to Boulder Point on Lamma, before the call was drowned by the throbbing engines of tugs nearby. 'It is the first time it's been tested in Hong Kong waters and it works perfectly,' says Ms Porter. 'Definitely it picks up finless porpoise noises very well.' Ms Porter and Ms Lau have identified a very rapid 'click' which they believe indicates feeding. 'We hear lots of feeding noises near the Sokos and Shek Kwu Chau. We could see them chasing fish and hear them at the same time,' says Ms Porter. After 150 hours of acoustic tracking, the study has revealed much about porpoise behaviour, their range and preferred feeding areas. 'For the first time we are actually observing - through sound - what the porpoises do, how they interact and what they use localised parts of the environment for, rather than merely where they occur,' says Ms Porter. The pilot project, backed by the Environment and Conservation Fund, also suggests acoustic tracking may be a better way of estimating population than traditional boat surveys. 'For every porpoise we have seen, we have heard three,' says Ms Porter. 'We miss two-thirds of possible encounters purely because we cannot see them, either because of environmental conditions or human error.' It was 20 years ago when studying sperm whales, that Dr Gordon realised the potential of an underwater listening device. Together with Mr Chappell, they applied it to harbour porpoises, which although common in the North Atlantic are threatened by human activity in their coastal habitat. 'Porpoises have characteristic vocalisations - the very narrow band pulse and all the other sources of noise out there are broadband so we realised it was quite an easy job to make something which automatically detected porpoise clicks,' says Dr Gordon, director of research of IFAW's vessel, Song of the Whale. 'Harbour porpoises are very like finless porpoises in that they have a lot of conservation problems,' says Dr Gordon. 'We could see there was a real need to be able to monitor the porpoise population and study them in other ways. The visual methods had a lot of shortcomings.' It was Mr Chappell's computer wizardry which produced the world's first automatic tracking and logging device - an invention Dr Gordon believes will become essential equipment for cetacean researchers. The device has also been used in the Gulf of Mexico to study the vaquita - which vies with the baiji as the world's most endangered dolphin - and is available for other 'useful conservation projects'. As yet, cetacean researchers have not realised its significance, says Dr Gordon. 'Biologists tend to forget that but these animals live in an environment which is dominated by acoustics. A large whale can hardly ever see its tail - light just doesn't travel that well in oceans but sound travels even better through water than it does through air. It is an important part of what these animals are about. They are very vocal,' he says. Acoustic tracking is one of the methods proposed in the government study, along with boat and helicopter observation. Another suggestion, although only 'provisional' according to Mr Choi, is radio tracking, controversial because of effects it may have on the animals. 'I worry about that sort of stuff - scientifically very interesting - but the tags actually have quite a bit of effect on the animals just in terms of the drag of the tags,' says Dr Gordon. WWF conservation officer Carmen Lee remains unconvinced of the scientific validity of results obtained this way. Mr Choi acknowledges the fears. 'We are quite concerned about this method because it involves capturing the finless porpoises and if you capture, you have the possibility of hurting them,' he says. 'We respect members' comments on this.' While there is no single infrastructure project looming over the porpoises as Chek Lap Kok did the pink dolphins - by smothering a prime portion of their habitat - the cumulative impacts of coastal development and pollution are cause for concern, say conservationists. The jury is still out on the environmental impact of Hongkong Electric's proposed extension of their Lamma Island power plant. But according to the company's pre-emptive year-long study of marine mammals, Lamma's southwestern tip 'has the highest concentration of finless porpoise sightings'. Results from the government study should provide a clear idea of threats to porpoises and point to conservation measures. If the study pinpoints areas often frequented by finless porpoises, says Mr Choi, 'it is a logical step to gazette some marine parks to protect them - that is why we need the information.' The research should also raise awareness and perhaps bring the finless porpoise some of the fame enjoyed by the Chinese white dolphins. 'They are quite cute,' Mr Choi says.