Justin Chapman knew he had hooked a big one while fishing off Claremont, but he never expected to haul in a monster pink snapper.

The 10.6kg fish was displayed to gasps of amazement from other keen anglers at the Swanfish 2017 event in Victoria Park yesterday.

Premier Colin Barnett, who was at the event to announce an election commitment to install five floating fishing platforms on the Swan, said the size of the fish was evidence of the river’s health.

Recfishwest chief executive Andrew Rowland said while pink snapper were caught often in Cockburn Sound, he had never seen one that big caught in the Swan River.

Mr Chapman, from Cloverdale, said he took about 15 minutes to bring in the fish on a light bream rod at high tide near midnight on Saturday.

“I thought it was a good chance it was a mulloway because it was very heavy,” he said. “You get juvenile pink snapper in the river, but to find one that big, that’s a once in a lifetime opportunity.”

Mr Barnett said he had caught a few tailor near Claremont, but never a big snapper. The $500,000 investment in floating platforms would make it easier for families with young children or people with disabilities to enjoy fishing, he said, as well as protect river bank vegetation.

Mr Barnett said a re-elected Liberal government would also restock the Swan and Canning rivers with recreational fishing species such as prawns and mulloway.