The Ministry for Primary industries (MPI) believes that illegal recreational set-netting is to blame for the death of a Hector's dolphin on the West Coast earlier this year.
An investigation involving MPI and the Department of Conservation (DOC) was launched after a member of the public found the dead dolphin on Blaketown beach in Greymouth in March.
Under the Marine Mammals Protection Act, anyone who kills or injures a marine mammal must report the event.
The penalties for a commercial fisher who fails to report a capture of this nature are a fine of up to $100,000 and, for a recreational fisher, a fine of up to $10,000.
MPI manager of Compliance Investigations, Gary Orr, says the Ministry's investigation covered interviews with a number of parties including commercial fishers, local service stations, net makers and sellers, and local iwi.
"Our inquiries were very thorough. We made sure that no stone was unturned. After a nearly 3-month-long investigation, reviewing the pathology report and examining data and environmental considerations, we have concluded that this animal's death was very likely to have been the result of the dolphin becoming fatally entangled in an illegal recreational set-net.
"The type of deep and narrow lacerations on its body are consistent with monofilament net which is used by recreational fishers rather than trawl mesh which is used by commercial fishers.
"Additionally, only one commercial vessel set-nets close to the area and it had not been fishing there at the time the dolphin died. We are confident this is not linked to any commercial fishing activity."
Mr Orr says the pathology report shows a knife had been used to remove the dolphin's tail after death.
"There are anecdotal reports that suggest fishers in some countries open up the abdominal cavity of mammal bycatch in an attempt to make them sink when they return them overboard. This may well have been the case here. That particular action is also an offence, even after death.
"Unfortunately, despite the fact we spoke to a large number of people, we have not been able to identify the individual or individuals responsible for this.
"We are appealing to anyone who may have seen anything suspicious around the Taramakau River mouth from the evening of Friday 10 February to the morning of Saturday 11 February, to contact us. The area where the dolphin was found is subject to a total ban on set-netting. Any recreational set-netting in this area is illegal.
"Someone may even have overheard conversations about this incident. Any information that is useful and relevant would be welcomed.
"We would like to thank everyone we've spoken to including the local community for their cooperation in this investigation as well as the person who reported finding the dolphin back in March. Everyone has been understanding and excellent to deal with."
MPI is continuing to investigate the death of a Hector's dolphin that was found near Banks Peninsula in March.
To report illegal or suspicious fishing activity call: 0800 4 POACHER, that's 0800 476 224.