Olympians on shark alert

"Is that a dorsal fin I see?"

A spate of shark attacks in Sydney Harbour has prompted Olympic officials to consider deterrents to prevent world-class athletes becoming fish-food.

Recent sightings and attacks have raised concerns about holding the swimming leg of the Olympic triathlon event in the harbour.

A report says the risk of shark attack is minimal

Sydney Water Police issued a shark alert on Monday, warning boat and other water sports enthusiasts to be on guard.

But marine experts played down the risk to Olympians, saying the sharks would be long gone by September.

There has not been a fatal attack in the area in at least three decades.

The Sydney Morning Herald reported that Dr John Paxton, who co-authored a report on the risk of shark attack during the games, advised Olympic officials to ensure inflatable boats with outboard motors buzzed around the swimmers.

Sharks tend to avoid the electric fields created by outboard motors.

Rowers attacked

The warnings came after a 2m shark rammed a boat of schoolboy rowers last Friday on the Parramatta River, upstream from the harbour and close to the Olympic site.

Yachts will race in the harbour

No one was injured.

A shark had bitten a rower's oar the day before: "He had his mouth around the oar, the cleaver part of the oar and was shaking it," rower Al Hattersley said.

Earlier this month, a shark bit an elderly man swimming off a harbour-side beach.

This season is the biggest time of the year for shark warnings - warm water attracts schools of fish into the harbour, which in turn brings the dorsal-finned predators to Sydney's waterways.

Dr Paxton and netting expert Jim Lumb suspected the sharks causing the panic could be tropical bull sharks.

"They are the worst sharks in the world. They are more deadly than the whites and the tigers put together," Mr Lumb said.

The police, the Sydney Waterways Authority and the Department of Fisheries were considering calling in professional shark hunters.