The carnival’s general manager Trent Goulding said on social media that “huge shark activity in a nearby area” had forced organisers to pause and later cancel the event.

Swimmer Edwina Harrison had waded into the water off Shelly Beach to look at the dolphin,which she said had flesh hanging off its tail and a gash on its side.

“It wasn’t in distress so much as it was a bit slow in movement,” she said. “It was slowly swimming in a large circle,looked injured and tired.”

Ian Forster was about to begin his daily swim with Manly’s Bold&Beautiful swim squad when a diver came running out of the water.

“She said,‘don’t go in. There’s a bull shark following an injured dolphin’,” he said.

A Taronga Zoo spokeswoman said the common bottlenose dolphin had sustained severe shark bite wounds to its tail and abdomen. The animal died on the beach from its injuries before Taronga Wildlife Hospital staff arrived.

“The deceased dolphin has since been transported to the Australian Wildlife Registry based at Taronga Wildlife Hospital to undergo a necropsy[animal autopsy],” the spokeswoman said.

ORRCA vice president Jools Farrell said rescuers hoped the necropsy would reveal the cause of death:“At this stage,we’re not confident that the death was caused by the sharks.”

Footage shows the injured dolphin struggling to swim before lifeguards and swimmers pulled it onto the beach.

Macquarie University wildlife scientist Vanessa Pirotta said dolphins were known to be preyed on by sharks – particularly if they have sustained prior injuries or have compromised health.

“Footage of the dolphin today shows it had wounds which were not fresh,suggesting the animal was likely in poor health before today’s interaction and a vulnerable target for other animals,” she said. “There was clear evidence of teeth marks on the animal’s tail which looked like white flesh,which might be aged.”

Pirotta said the incident was a natural process,but that the removal of the dolphin lessened the chance of more sharks appearing in the area.

“It’s all part of a natural ecosystem,where if an animal is unwell and an easy target,other animals in the ocean will likely take advantage of that,” she said. “But a decomposing or unwell animal may draw in sharks.”

The website Dorsal Watch,which lists shark sightings around the world,said Surf Life Saving NSW reported three 2.5 metre bull sharks were spotted off Shelly Beach shortly before 9am.

Authorities have not yet confirmed the species of shark that was spotted.

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