Hong Kong authorities on Tuesday urged the public to remain alert after a shark sighting prompted a one-day closure of popular beaches across the territory at the height of summer. Officials closed 12 packed beaches on Sunday, during an extended holiday weekend, after a swimmer reported seeing a shark off the south of the Chinese territory. Inspections found \"no big fish\" inside shark nets at the beaches, and the nets were intact, the government said. The beaches were reopened on Monday. \"We urge the public to remain vigilant, however, and swim in the enclosed zones of the shark prevention nets,\" a spokesman for the leisure and cultural services department told AFP. Shark sightings are rare in Hong Kong and some experts said the closures may have been an overreaction. Reports suggested the sighting may have been of a harmless whale shark. The last fatal shark attack in Hong Kong was in 1995, according to the Shark Attack File website, which monitors such incidents worldwide.