Every dolphin calf born in Adelaide's Port River this summer breeding season has died, sparking concerns that the whole population could disappear.

Key points: A dolphin calf was found dead on Sunday in the Port River

A dolphin calf was found dead on Sunday in the Port River It is the 11th death out of 13 dolphin calves born in the past two years

It is the 11th death out of 13 dolphin calves born in the past two years Speed limit regulations will be introduced in the coming weeks

Over the past two years, 11 of the 13 calves born in the Port River have died — many of them after being hit by boats.

The South Australian Government plans to introduce a 7-knot speed limit within part of the Adelaide Dolphin Sanctuary.

Mike Bossley from the Whale and Dolphin Conservation Society told ABC Radio Adelaide that a calf called Merlin had been discovered dead on Sunday.

He said it was the fourth calf to die in the area near Port Adelaide this summer, after a calf called Sparkle died last week.

"If this death rate for calves keeps going, eventually the population will die out and that's really sad for the dolphins and for the people who love the dolphins," Dr Bossley said.

"But it's also quite a significant economic issue because there's quite a lot of tourism built up around the dolphins and they've become a real selling point for Port Adelaide locally and even internationally."

About 50 dolphins live in the Port River, which leads to Port Adelaide's inner harbour.

Dr Bossley said they were "the most reliably present urban dolphins in the world".

He said the dolphins were attracted to Port Adelaide despite its perils because it was a fish breeding area and was protected from sharks.

Dolphin Dinah with her calf Merlin, who was found dead on Sunday. ( Supplied: Marianna Boorman )

Speed limit being widened

In November, Transport Minister Stephan Knoll announced his department would introduce a 7-knot speed limit into much of the Barker Inlet east of the Port River, most of which currently has no speed limit.

The same limit is already in place in the Port River.

Unlimited speeds would still be allowed around the Adelaide Speedboat Club at Gillman.

A spokesman for Transport Minister Stephan Knoll told the ABC that regulations to introduce the speed limit would be introduced in the coming weeks.

Dr Bossley said boat speeds were one of the few threats to dolphins that could be fixed.

"Some of the other threats are sickness and pollution and so forth, which are very difficult to mitigate but certainly boat speeds are something the [State] Government can very easily do which would help the situation," he said.

In January, the Environment Protection Authority (EPA) approved Flinders Ports to dredge the Port River to widen the shipping channel.

Flinders Ports this week said it planned to start work in June.

Port River dolphin Ripple tail-walking in front of a ship last year. ( Supplied: WDC/Sue Holman )

Calf died from bleeding

A post-mortem on a one-day-old dolphin calf found dead on March 3 revealed it died from severe bleeding, but scientists do not believe it was hit by a boat because there was no damage to the outside of its body.

The calf was the third belonging to a dolphin called Ripple to die in infancy between 2015 and 2019.

South Australian Museum senior research scientist Cath Kemper said very little was known about birthing problems in dolphins, which made it difficult to confirm why a newborn had died.

"Follow-up investigations are required to confirm the cause of death," Dr Kemper said.