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The Department of Conservation is reviewing surveillance footage and appealing for help from the public after the striped dolphin was mutilated less than 24 hours after washing ashore at Karioitahi Beach.
Biodiversity ranger Olivia Keane said tampering with dead marine animals was an offence under the Marine Mammals Protection Act.
"We received a report that a washed-up dolphin was on the beach. Washed-up marine mammals are not uncommon, and we expect people to leave them alone," she said.
"What we don't expect is for people to mutilate this treasured species in a disrespectful way for commercial or personal gain.
"We are actively investigating this incident including reviewing surveillance footage of the area and would appreciate any information from the public."
The decapitated dolphin is believed to be a striped dolphin (Stenella coeruleoalba) and similar in size to a common dolphin and can grow to about 2.5m long.
They are found throughout tropical and temperate seas worldwide and can be seen in large groups of up to thousands of animals.
In New Zealand they are often seen in association with common dolphins and prefer offshore waters.