Researchers fear a well-known Swan River dolphin will die after fishing line became wrapped around its tail.
"Fingers" was found late last month trailing the line and Murdoch University dolphin expert Hugh Finn said it would continue to cut into his fluke, possibly severing the fin.
Algae could build up on the line and cause drag until Fingers could not catch fish or avoid sharks. He could also die from a chronic infection.
"The line doesn't break down, it just creates heaps of scar tissue and keeps cutting into the tissue until it gets down to bone," Dr Finn said.
Researchers have followed Fingers since 1993, when he was in his late teens or early 20s, and believe he was born in Cockburn Sound.
Male dolphins form strong bonds and Fingers and friend Backpack are almost inseparable after 20 years.
Both were recently spotted begging for fish from boats after people had been illegally feeding dolphins in Cockburn Sound.
"Fingers probably has this entanglement because he's been hanging around boats," he said.
Dr Finn called for biodegradable line to reduce "death traps" for marine life.A Murdoch University report in January said the Swan River's dolphins were vulnerable, with only four adult females left after three died in a spate of deaths in 2009.
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