An examination has revealed a bottlenose dolphin which died in Adelaide's Port River area last December had been shot.

The mammal's body was retrieved from the Barker Inlet but only examined this month by staff at the South Australian Museum.

Dr Mike Bossley of the Whale and Dolphin Conservation group said the cause of death was now apparent.

"When we found the body before Christmas it was hard to know why it died, that's why they did a necropsy," he said.

He said death might not have been immediate but the dolphin probably died within a short time of the shooting.

The museum team found four shotgun pellets in the adult female, named Graze.

Dr Bossley had been tracking her behaviour since his first sighting of the mammal in the middle of 1992.

Back then she had a big wound across her back, he recalled. Dr Bossley said Graze survived that injury.

Sightings over the years that followed indicated her home range was the Adelaide Dolphin Sanctuary, an area of the Port River, Barker Inlet and stretching north to Middle Beach.

He said the shooting last December presumably happened somewhere in that marine zone.

Two other dolphins were killed in the sanctuary area in 1998 and one of their calves later died.

Dr Bossley said shotgun pellets also had been found over the years in other dolphins beyond the sanctuary zone, despite both dolphins and whales being protected by law.

Port River dolphin Graze with an injury when first spotted decades ago. ( Supplied: Whale and Dolphin Conservation )

"It is clear despite the implementation of the Adelaide Dolphin Sanctuary these dolphins are still under threat," he said.

"It is important that more resources are put into the sanctuary to improve its effectiveness."

The mammal researcher urged a boost in patrols by rangers and that members of the public watch more keenly for problems.

"I think the Government has lost a bit of interest in the Adelaide Dolphin Sanctuary. I think it needs to look at the way it's resourcing ... and it's just not on to have a sanctuary and have dolphins being shot in it," he said.

Dr Bossley said he was keen to see an interpretive centre about the region's dolphins set up as a way to increase public knowledge about the mammals and their local habitat.