The death of a young bottlenose dolphin who was cut to the bone by a propeller in the Hauraki Gulf has prompted pleas for boat and jetski operators to slow down around marine mammals.
Known to Massey University marine researchers as TM007, the dolphin and its mother had been regular visitors to Great Barrier Island and the inner gulf for 2 years.
However, in late September 2012, Massey PhD student Sarah Dwyer saw it swimming slowly with its mother in a pod of about 50. She saw the dolphin had a large open wound behind its dorsal fin and two more wounds on the tail that were consistent with propeller strikes.
"Considering the severity of the wounds, it was surprising that these injuries were not immediately fatal," said Ms Dwyer in a paper published in this month's New Zealand Journal of Marine and Freshwater Research.
Over the next 23 days, the condition of the wounds deteriorated.
The last reported sighting, in the shallow bays of Great Barrier Island, was on October 21, 2012. A few months later, its mother was seen without her offspring.
It was highly unlikely the mother would have abandoned her calf if it was still alive therefore it was assumed it had died. Its carcass was not recovered for a necropsy to establish cause of death.
Coastal-Marine Research Group director Dr Karen Stockin said some mammals died as the result of blunt-force trauma - where a vessel struck hard enough to kill. Other dolphins and whales had wounds or scars from propeller strikes.
"It's difficult to work out how many marine mammals are killed or injured by vessel collision or propeller strike, mainly due to the lack of reporting when collisions occur as well as difficulties in recovering fatally struck animals."
Dr Stockin recalled a common dolphin dying in the Hauraki Gulf in 2011 as a result of blunt-force trauma caused by a jetski.
These craft projected less noise underwater than other boats, which made it even harder for marine mammals to detect and avoid them, she said.
A skipper for Wavedancer charters in the gulf, Grant Bittle, said boat propellers were like "circular saws" running across the top of dolphins coming up for air.
"I know of a big common dolphin I call 'Finless' - he had a fin taken off three years ago - and there are a couple of other dolphins with propeller or skeg marks. There are also a couple of marked Bryde's whales out there, one with a dorsal fin chopped.
"The injuries could be caused by a shark but it is likely to be a boat.
"When boaties see these fish work-ups they charge straight over to it and through it," Mr Bittle said.
"On any boat fizzing along at 20 to 30 knots, people should remember six to 200 dolphins could be there, and maybe a whale too. They ... won't see what a boat is doing."
• Avoid rapid changes in speed and direction and do not exceed speeds faster than the slowest mammal within 300m.
• Travelling at over 15 knots is more likely to kill a whale or a dolphin if it gets hit.
• Severe harm can be caused if travelling over 5 knots, or at no wake speed.