A man has climbed on top of a whale carcass surrounded by several tiger sharks and a white shark off the West Australian coast.

It is believed the man dived off his boat and climbed onto the dead whale between Fremantle and Rottnest Island on Saturday afternoon.

In a tweet, Surf Life Saving WA said several tiger sharks and a white shark were spotted feeding on the whale carcass about 1:30pm (AWST).

It is not yet known why the man chose to board the carcass.

Surf Life Saving WA tweeted several shark sightings from the Westpac lifesaver helicopter throughout the day.

Among them was a five-metre white shark detected four nautical miles off Rottnest Island.