The man, who was uninjured, reported to Water Police just before 11am that a large shark had bitten his fins while diving near Little Island off Hillarys.
He said he saw a large dark shape when returning to the boat and then noticed parts of his fins missing.
Department of Fisheries shark response unit manager Lisa Clack said that the department’s senior shark scientist had assessed the fins.
‘There are some marks which could have been caused by a shark bite but other marks we would expect to see from a shark bite are absent,’ she said.
‘While this could have been a shark, other large marine animals, particularly seals and sea-lions, are known to occasionally bite divers’ fins.’
She said officers advised water users in the area of the incident and asked them to apply additional caution and the Surf Life Saving WA helicopter flew over but did not see a shark.