A killer whale found dead on the Scots island of Tiree drowned after getting entangled in fishing line.
Expert vets who carried out the post-mortem examination said it is likely she was entangled for several days.
It is believed the mammal eventually drowned as a result of the entanglement.
The carcass was found on 3 January and identified by specialists as "Lulu", a female member of Britain's only known resident population of the mammals.
The charity believes there now may be as few as eight individuals remaining in this killer whale group.
The post-mortem examination was carried out by Dr Andrew Brownlow and his team of vets from Scotland's Rural College (SRUC), who co-ordinate the Scottish Marine Animal Stranding Scheme (SMASS).
They believe that Lulu got caught up in fishing gear or creel lines.
Dr Brownlow advised that the experts had found "convincing evidence" that she had become chronically entangled in "10-15mm rope, likely still attached to gear of some sort".
He said the rope was wrapped around her tail, and would have dragged behind her.
This would have made normal swimming and foraging very difficult for her, he said.
The vets found seawater in the mammal's stomach.
Dr Brownlow added: "Although there were no ropes or fishing gear left on the carcass, the lesions are very similar to those we see from creel rope entanglement in baleen whales."
He said the rope had left deep wounds and abrasions on the animal's skin.
Although this is the first killer whale which the SMASS team has seen entangled, they believe there has been an increase in the number of whales, dolphins and porpoises who have become entangled in the past year.
The case, Dr Brownlow said, was "particularly tragic", but added that the SMASS team would carry out further research on Lulu to help "learn more about a poorly understood population".