UNDERWATER sonar equipment has been used for the first time to detect sharks in Hong Kong waters, after a false alarm sparked a major safety alert at a Sai Kung beach yesterday. Yesterday's ultimately futile hunt was launched after late night revellers enjoying a beach barbecue told police they saw a three-metre long shadow lurking underwater inside the Silverstrand Beach shark net at 2.30 am. The beach, which has recorded the highest number of fatal shark attacks in past years, was immediately closed by police, preventing early-morning swimmers from taking their daily dip. As a shark warning flag was hoisted, lifeguards peered out to sea and a few visitors began to arrive hoping for the rare sight of a fin. The all-clear was given at 4.45 pm, after sonar equipment worth $220,000, operated on a small craft by Maritime Mechanic - contractor for the shark nets - took just a few minutes to make a three-dimensional sweep of water using sound waves. Despite the early warning, some children paddled at the water's edge in the morning - but the majority kept away. And although bathers have been returning to Silverstrand since last year's fatal attack there, some say the memory is a permanent deterrent. ''I am always too worried about the sharks to swim and I won't let my daughter go in,'' said taxi driver So Chi-tung, from Junk Bay. Raymond Kwong Yiu-man, 32, said: ''I think the net makes it better but I never go in.'' At his supposed peril, one man did go for a quick dip about two metres offshore outside the net, but lifeguards raced to warn him off. ''No, I'm not scared,'' said the 45-year-old taxi driver, who said his nickname was Silver Shark. As an astonished crowd gathered to watch him, he said: ''I have been swimming here for more than 30 years. I don't think there are sharks here in the daytime. If I see one I'll just stay very still.'' Every centimetre of the shark net, installed in May by the Regional Services Department, was checked and reported to be in perfect condition, indicating no shark could possibly have gone through. The net is secured to the sea bed by concrete anchors and provides a complete below-water barrier at all tide levels. Following a specially convened inter-departmental meeting to assess the situation, the authorities reported a false alarm, but the Urban Council decided to put all 11 of its beaches on shark alert. Last June an elderly man was killed in an attack at Silverstrand just 10 days after a woman was killed by a shark at nearby Sheung Sz Wan beach. In January this year the Regional Services Department began a two-year shark prevention scheme.