Having sailed out of Westport, the fishing boat Jewel was off the coast not far from Karamea when the big fish bit.

Standing on the back deck, McCowan fought with, and reeled in, the 325kg Pacific blue fin tuna.

McCowan said he'd been fishing for about 20 years and this fish was the big catch.

Jewel skipper Ian Bain said finding the right spot had been the hardest task that morning.

"We followed the Korean boats for a while and just nothing. It was behind the Russian boats we found the tuna".

The tuna are attracted to the hoki fishing boats to eat the cut-offs cast overboard.

McCowan said the crew had caught two other Pacific blue fins that morning, but had released them.