The mystery of how four marlin heads washed up on Muriwai Beach is being investigated by the Ministry for Primary Industries.
Auckland fisherman Darrin Collett was heading out for a spot of fishing on Sunday when he saw the heads, minus their distinctive bills and bodies, washed up along the beach in a line.
The number of fish heads on the beach and the cuts along the base of the fish heads and where the bills had been removed suggested commercial fishers had caught the marlin, Mr Collett told the Herald.
It is an offence for commercial fishers to land marlin -- dead or alive. If a commercial fisher mistakenly catches a marlin, it must be returned to the sea, fully intact.
Mr Collett, who has been sport fishing for nearly 50 years, said the cut along the neck of the fish looked like it had been done with a professional tool.
"You could see where they cut the bill of the marlin off, with something that looks like a saw ...
"The cut behind the head is not done with a knife, it looks like it's been done with an air pressure saw or something."
The fact there were four heads also made Mr Collett suspect commercial fishers had caught the marlin, cutting them up at sea, dumping the heads overboard and leading to them washing up at Muriwai.
"I find it very hard to believe a recreational fisher would catch four marlin and cut the bills and the rest of the body off all the same way with something that's not a knife."
It was frustrating as a recreational fisher to think commercial fishing boats might be breaching the country's fishing laws, he said.
"If that is a commercial fish head that's had the body cut off and the head thrown over the side, where was it caught and why was it done? I thought they weren't allowed to catch inside our limits."
Mr Collett had not yet reported the find to MPI, which said it was investigating how the heads ended up on Muriwai Beach following questions from the Herald this afternoon.
An MPI spokesperson said removing marlin's bills for a souvenir was consistent with recreational caught marlin, but MPI could not definitively say where the heads had come from at this point.
"We encourage people to report any suspicious fishing activity or potential fish dumping incidents to our 0800 4POACHER (0800 476 224) so fishery officers can go to the site and investigate," the spokesman said.
"We have had no calls around this sort of activity on the West Coast over the past few days."
MPI is in the process of making further inquiries about this incident.