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An Auckland fisherman fled for safety when a shark in a feeding frenzy lunged at his boat, smashing off a small outboard motor bolted to the stern.

Kelvin Travers said he was trolling lures for tuna near the Alderman Islands, about 22km out from Tairua and Pauanui on Saturday night.

"I was coming up to a school of fish being worked by birds and I saw this big fin.

"It went for the fish teasers [attractors] attached to the stern and it hit the boat with an almighty thump, knocking me across the cockpit.

"I started madly trying to pull in the lures and the next thing it suddenly launched itself at the back right corner of the boat and I thought it was coming in. I dived for the engine controls and floored the throttle out of there.

"When I looked round the auxiliary outboard motor had been ripped off the back."

Mr Travers said he had not got a good look at the shark because his focus was getting from the stern to the controls as quickly as possible.

But he thought it might have been a mako shark that was in a feeding frenzy in the school.

The small outboard had been about 30cm clear of the water on the 7.2m vessel. All that was left of it was the handle, with gouge marks on it.

He headed for port just after 7pm.

He returned to the islands yesterday - "cruising not fishing".

Tony Lanzi, of Dive Tairua, said last night the fish sounded like a mako because they were aggressive sport fish and leapt like marlins.

"It might have gone for the teasers and missed and hit the motor."

On January 10, the Herald reported that a 3m mako attacked an outboard motor in Hawke Bay and left tooth marks on it.

Department of Conservation marine ecologist Clinton Duffy said sharks were attracted to outboard motors because a metal part generated an electric field that sharks mistook for prey.

Sharks have jelly filled receptors on their snouts which detect small electric fields.