Independent laboratory tests on dolphins that died recently in the Hauraki Gulf have come back negative for domoic acid — an algal toxin known to be a common cause of dolphin deaths.
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 suggest that single stranding events of common dolphins normally occur approximately once or twice a month for the entire New Zealand coastline.
An interagency team led by the Auckland Regional Council led initial investigations into the dolphin deaths, as part of an investigation into the cause of recent dog deaths.
The team quickly ruled out any link to brodifacoum — the rat poison used by the Department of Conservation 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 that specifically looked for the presence of brodifacoum. These tests 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.
“We have now exhausted all lines of enquiry. 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,” says DOC’s acting Auckland Area manager Phil Brown.
Massey University marine biologist Dr Karen Stockin says that the spike in dolphin strandings now seems 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. We may never get to the bottom of what happened to these dolphins,” says Dr Stockin.
Massey University has conducted post-mortems of dolphins as part of a long-term research programme since 2002.
“Monitoring of common dolphin mortality will continue as part of our research,” she says.