Many marine deaths in Rodney remain a mystery.
A suspected herpes virus has been ruled out as causing the deaths of thousands of pichards which washed up from Martins Bay to Whangaparaoa in July. Now testing for domoic acid – an algal toxin known to be a common cause of dolphin deaths – has also been dismissed.
Tetrodotoxin in sea slugs and puffer fish was the cause of illness and death in dogs after visiting North Shore and inner Waitemata Harbour beaches, but the toxin hasn’t been linked to other deaths.
Independent testing on pilchards, dolphins, mussels, pipis, and penguins showed the rat poison brodifacoum used on Rangitoto and Motutapu Islands in two of the dead penquins with the level too low to have killed them. Starvation is the main cause of death among penguins.
Dead dolphins have appeared well fed, but tests for various poisons have been negative too.
"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," says dolphin expert Dr Karen Stockin of Massey University. "We may never get to the bottom of what happened to these dolphins."