UPDATE: 3.47pm: Both Swanbourne and North Cottesloe beaches have been reopened.
Earlier, Surf Life Saving WA closed Swanbourne and North Cottesloe beaches after a shark was spotted 50m offshore.
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READ NOWThe organisation tweeted at 3.06pm that an unknown species of shark, measuring 3m, had been sighted by the SLS Westpac helicopter and was tracking in a southerly direction.
The incident comes just 10 days after Mandurah teen Laeticia Brouwer was killed by a shark while surfing with her father at Kelp Beds, near Esperance.
There have been 15 fatal shark attacks in WA since the year 2000.
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