A dolphin has died after it was mauled by at least one suspected bull shark on Sydney's northern beaches, suspending the Manly Open Surf Carnival.
Three bull sharks were spotted near Shelly Beach in Manly, where the attack occurred this morning, according to Surf Lifesaving NSW.
The dolphin could be seen circling in the water with its tail injured, before it beached on the shore.
Swimmers were cleared from the ocean about 7am on Saturday and large crowds gathered at the water's edge.
The beach carnival was suspended shortly after.
Beaches between Shelley and Queenscliff have been closed as a precaution and are being monitored for further shark sightings.
Tracey Hare-Boyd from Surf Life Saving NSW said a diver spotted the marine creatures underwater.
"The shark just literally swam straight past them and headed for the dolphin," she said.
Ms Hare-Boyd is unsure whether the dolphin was preyed upon because it was unwell.
"We're not sure whether [the dolphin] was ill and died of natural causes or was attacked and died of those injuries," she said.
"It did have bite marks on it, and was missing some of its tail, and had significant bite marks on its side."
Two bull sharks about 2.5 metres being are currently being monitored in the area.
The dolphin is being transported to Taronga Zoo for further assessment.
Ingrid Vonmoltke had travelled from Maleny in Queensland hoping to snorkel in Cabbage Tree Bay this morning.
"So we arrived early and we saw this dolphin swimming around in the bay which we thought was absolutely wonderful," Ms Vonmoltke said.
"And the next thing … hang on there's been a shark spotted and we can't go into the water."
"We came back and saw the dolphin was on the beach which was really incredibly sad."
Are shark attacks on dolphins unusual?
Dr Vanessa Pirotta, a wildlife scientist and marine predator researcher at Macquarie University, said such interactions were not uncommon.
"It can be hard to know how often these negative interactions do occur. Dolphins are known to be part of the natural diet of some sharks," she said.
"The fact that this took place in front of 100s of people on a busy Saturday morning in clear view is just evidence these interactions do happen from time to time.
"Around this time of year, around the summer and autumn months, bull sharks are known to be in this area."
Ms Pirotta said this could be for a number of reasons, including warmer sea temperatures.
Underwater photos showed the animal had old wounds and was unwell for some time before today, Ms Pirotta said.
The dolphin's injuries will be assessed by scientists to determine if it was bitten by a bull shark, or another species.
They will also examine the shape of its dorsal fin in case it matches a marine creature already observed in the harbour.
