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Sydney dolphin had prior fishing injury

Before being pursued by sharks and dying on a popular family beach in Sydney, an adult dolphin appears to have been seriously injured by fishing gear, zoologists say.

The adult common bottlenose dolphin died on Saturday on Manly's Shelly Beach after being spotted with noticeable injuries and sharks nearby.

Preliminary autopsy results show it had a reduced body condition, a high parasite burden and evidence of a severe and ongoing infection of the tail.

While the ultimate cause of death won't be known for a few weeks, not-for-profit wildlife organisation Taronga said the lesions on the dolphin's tail suggested a previous traumatic injury to the tail.

The wound likely occurred within the past year but not in recent weeks.

"Based upon the appearance of the lesions, the injury ... is consistent with an entanglement in fishing gear," wildlife health project officer Jane Hall said.

"All of these findings indicate that this dolphin was likely immunosuppressed and that these health issues had been present for some time.

"This would have predisposed it to the more recent shark encounter, which would be considered a normal ecological process."

The sharks following the 2.8-metre dolphin forced the postponement of a surf lifesaving contest and the closure of several beaches.

But their multiple attacks on the mammal were "relatively superficial", Ms Hall said.

"Despite looking serious, (they) would not have caused the death of this individual alone," she said.

"A healthy dolphin is very robust, and the sharks were fulfilling a natural role in the ecosystem by predating on a sick and injured animal and their interest in the injured animal just happened to take place in a very public space."

The incident and the likely prior fishing gear entanglement should serve as a reminder of humans' impact on wildlife, Ms Hall said.

A study published in academic journal Science in October estimated nearly two per cent of all fishing gear is lost to the ocean annually.

That included enough longline mainline to wrap the Earth 18 times and one pot or trap for every Australian adult and child.

The dolphin's autopsy was conducted at Taronga's Australian Registry of Wildlife Health, which operates in addition to the organisation's two wildlife hospitals which rehabilitate more than 1500 wild animals each year.