Update: Trigg, Scarborough and Mettams Pool have all been reopened after hundreds of people flocked to the beaches this morning to witness the spectacle of up to 16 sharks feeding on a dolphin carcass just 30 metres from the shore.
Lifeguards suggest swimming with caution.
Sharks were swimming into the shallows just 10 metres from South Trigg beach and were clearly outlined in the water as they struck at the carcass.
The pack appear to have been attracted by a dolphin carcass that washed up on shore earlier this morning.
A city of Stirling employee, who asked not to be named, said the stranded dolphin had washed up near the water’s edge about 5:30am.
A group of surfers tried to push it back into deeper water where it flailed around but ultimately started to drift back in.
A Department of Fisheries boat has been sitting offshore near the pack, which includes at least one Tiger shark about 1.5 - 2 metres long, as well as a mass of smaller sharks of indeterminate species.
The sharks are believed to range in size from one to three metres.
Cottesloe man Richard Wands, 33, managed to fight off two attacks by a two-metre shark before making it ashore and raising the alarm.
He hit the shark with his board then yelled to alert others in the water, before running about 300 metres up the beach and alerting a surf life saving officer.
Trigg and Scarborough beach will remain closed indefinitely while the Department of Fisheries monitor the situation, City of Stirling beach services coordinator John Snook said.
Helicopters are circling overhead and City of Stirling officers are patrolling the beaches to prevent people entering the water.
As of about midday the carcass seemed to have been completely consumed, but beaches remained closed as sharks continued to infest the area.
Department of Fisheries regional manager Tony Cappelluti said while feeding in this way was not unusual for this species, it was rare they swarmed like this in a crowded metropoloitan area.
He said the department had no plans to try to hunt the sharks, as the beaches had been successfully cleared and no one was at risk.
Surf Life Saving WA community safety manager Chris Peck urged the public to listen to authorities at the beach and stay out of the water.
“We hope people understand why we do closures and heed the advice of the authorities that are undertaking that because we are doing it for the right reasons,” he said.
“When sharks get up to that size, about 3 metres, is when they are capable of doing catastrophic damage.”
The last fatal tiger shark attack in WA was at Cottesloe Beach in 1925.
In August, Newcastle resident Jon Hines, 34, survived a suspected tiger shark attack at Red Bluff in the Gascoyne.
Eagle Bay will remained closed overnight after a four metre white shark was spotted in the area.