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.