Update, 9.30am: Scarborough Beach has been reopened following a shark sighting which delayed the Davey Sunset Coast Swim this morning.

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A Whitfords Volunteer Sea Rescue crew keeps watch over swimmers at Scarborough Beach. Picture: Nic Ellis/The West Australian

Update, 9.30am: Scarborough Beach has been reopened following a shark sighting which delayed the Davey Sunset Coast Swim this morning.

The swim at was postponed this morning after a shark was spotted off the coast at about 8am.

The 3m tiger shark was seen from the Westpac Rescue Helicopter about 200m off Brighton Beach, moving north.

The sighting forced the closure of both Brighton and Scarborough beaches and delayed the start of the swimming event while a helicopter sweep of the ocean was carried out.

Competitors had been expected to take part in 5km, 2km and 1km swims, with the first event starting at 8am.

The event was delayed by about an hour and Whitfords Volunteer Sea Rescue crews watched over swimmers entering the water.

The shark forced the closure of Trigg Beach after it was spotted 300m offshore at about 8.50am. The beach has since been reopened.