A killer whale found dead on a West Auckland beach this week was the same orca, dubbed Nibble, that temporarily stranded itself in Whangarei Harbour last year, scientists believe.
But, while chasing stingray was the likely reason for its stranding last September, there's no evidence to indicate that's what led to Nibble's demise this month.
The orca, discovered this week on Whatipu Beach, 40 kilometre west of downtown Auckland, was analysed yesterday with a necropsy by Massey University researchers.
While the post-mortem was inconclusive, Dr Karen Stockin of Massey's Coastal-Marine Research group said it did offer critical information relating to the whale's death and helped rule out some theories that had been put forward.
Although some damage to the whale's head and pectoral flipper had been observed, Stockin said the areas of reported bruising were small and relatively localised.
"No evidence of extensive blunt force trauma was observed during necropsy, indicating the whale unlikely died as a consequence of boat strike."
Secondly, the necropsy revealed no evidence of trauma in the blubber indicative of a live stranding, suggesting the whale most likely died at sea and washed ashore.
"This refutes earlier speculation that the whale possibly live-stranded after feeding in the shallows."
Further, examination of the stomach contents revealed no evidence of any prey items, suggesting the whale would not have recently fed.
"This further supports our conclusion the whale wasn't chasing prey and didn't live strand."
Despite being refuted, the accidental stranding theory was nonetheless reasonable given the whale's earlier stranding.
"Images of the dorsal fin suggest the whale dissected yesterday, is we believe, known to the Orca Research Trust as 'Nibble'.
"If this indeed is confirmed by the trust, Nibble has got a prior history of live stranding when chasing prey, as observed in Whangarei Harbour in 2016."
While the state of decomposition prevented examination of most internal organs, blubber and teeth samples were still taken to determine the whale's age and correlate age against pollutant burden.
As orca were at the top of the food web, they were particularly susceptible to pollution via bio-accumulation, the accumulation of toxins through the food chain.
According to the Department of Conservation (DoC), threats facing orca around New Zealand include disturbance caused by boats, which are known to disrupt the animals' normal behaviour, particularly resting, and underwater noise may disrupt echolocation signals and other communication.
Marine mammals in New Zealand are legally protected under the Marine Mammal Protection Act and anyone who accidentally kills or injures one must report it to a fishery officer or DoC within 48 hours.
The Marine Mammal Protection Regulations covered commercial whale- and dolphin-watching activities, and incidental recreational interaction.
Under these regulations vessel skippers must avoid rapid changes in speed and direction and not exceed speeds faster than the slowest mammal within 300m.