Australian police were last night trying to unravel the mystery of a Taiwanese-owned 'ghost' fishing boat which was found drifting in the Indian Ocean with a hold full of rotting fish but no sign of its crew. The abandoned vessel, the High Aim 6, was intercepted last week by a Royal Australian Navy frigate, HMAS Stuart, about 300km west of the coastal town of Broome, in a remote part of Western Australia. It had first been sighted by a surveillance aircraft from Australia's Coastwatch service, triggering concern that it was carrying illegal immigrants. But when sailors boarded the 20-metre vessel, they were stunned to find it was unmanned and carrying tonnes of putrid fish, mostly mackerel. Although flying an Indonesian flag, Australian fisheries authorities said the boat was registered in Taiwan and is believed to have been fishing illegally. Officials said the boat would have needed a crew of around 12, but an aerial search of more than 7,000 square nautical miles has failed to find any trace of shipwrecked sailors or life rafts. The search was called off on Friday. A spokesman for the Australian Fisheries Management Authority said the boat was in good condition, but may have run out of fuel. As the boat was towed to Broome, Australian police launched an international investigation into what might have happened to its crew. Police are contacting authorities in Taipei and Jakarta in a bid to discover their fate. Immigration Minister Philip Ruddock said if the crew had reached the Australian mainland, he would expect them to have been found. 'Our experience has been that people who land off vessels in the northwest of Australia are quickly found and if they are not, it is more likely that they are at some risk,' he said. In the past two years, Australia has clamped down hard on people smugglers, excising a number of offshore islands from its jurisdiction, increasing naval and air patrols and sending asylum seekers to camps on Nauru and Papua New Guinea under its so-called Pacific Solution. Authorities say, however, that illegal fishing is on the rise as pirate boats deplete fish stocks in Indonesian waters and turn their attention south. The fisheries authority said 111 foreign fishing boats were intercepted last year, the most since 1997.