Dolphin deaths remain a mystery

05:10, Sep 24 2009

The deaths of eight dolphins in the Hauraki Gulf look set to remain a mystery, but they do not appear to be linked to dog deaths on Auckland beaches.

An inter-agency team led by the Auckland Regional Council conducted initial investigations into the dolphin deaths, as part of an investigation into the cause of recent dog deaths.

But tests for possible causes of the dolphin deaths have all come negative, the Department of Conservation (DOC) said today.

Independent laboratory tests had come back negative for domoic acid – an algal toxin known to be a common cause of dolphin deaths.

The team quickly ruled out any link to brodifacoum, the rat poison used by DOC in its recent restoration programme on Rangitoto and Motutapu islands, after autopsies on the dolphins showed no signs of internal bleeding.

To reassure the public that brodifacoum was not involved, DOC commissioned independent laboratory tests, which came back negative for brodifacoum, confirming earlier findings by veterinary scientists and pathologists.


Tests for tetrodotoxin – the toxin found in sea slugs that was responsible for recent dog deaths – also came back negative.

Investigations into dolphin deaths were sparked after eight dolphins died over a three-week period in the Auckland region.

Strandings data analysed by Massey University from the 1950s onwards suggested that single stranding events of common dolphins normally occurred about once or twice a month for the entire New Zealand coastline.

DOC acting Auckland area manager Phil Brown said today all lines of inquiry had been exhausted.

"Post-mortems on the dolphins have given us no further clues as to what else we could test for, so unfortunately we may never know how these dolphins died," he said.

Massey University marine biologist Dr Karen Stockin agreed.

"We may never get to the bottom of what happened to these dolphins," she said.

Dr Stockin said that the spike in dolphin strandings now seemed to be over.

"The number of carcasses has returned to what we would normally expect at this time of year, which suggests to me that whatever caused this issue in the dolphins is no longer a threat."

She said Massey University had conducted autopsies of dolphins as part of a long-term research programme since 2002.

"Monitoring of common dolphin mortality will continue as part of our research," Dr Stockin said.

Meanwhile, in Christchurch a small container full of toxic pellets was found washed up on New Brighton beach yesterday.

Christchurch City Council said tests of the pellets indicated they were a highly poisonous fumigant, which released a toxic chemical when exposed to water.

The council's enforcement compliance team and park rangers have been searching the beach and the nearby area to see if any more containers have washed up.

Anyone finding a container should not touch it, but immediately contact the council or the Fire Service.

The grey container was about 25cm high, with grey pellets inside.