One of Perth's best known dolphins is believed to have died after becoming entangled in fishing line.

Fingers, who for the past two decades was a regular visitor in Cockburn Sound, has not been seen since July 11 and researchers believe he may have died from infected wounds or been victim to a shark attack.

Murdoch University dolphin expert Hugh Finn said Fingers had taken to begging for food from river users, causing his long-time best friend Backpack to leave him, before he was injured by a boat propeller and then permanently entangled in fishing line in February.

"We think he had started to beg and was hanging around boats and just got caught in some line," Dr Finn said.