On 11 October 2002 tens of thousands of fish were reported dead on Orewa beach north of Auckland. They included a whole range of shallow, midwater, and bottom-dwelling estuarine species (e.g., parore (mangrove fish), flounder, yellow-eyed mullet, eel, goby, and spotty). About 8500 paua at a marine farm at Kennedys Bay on the eastern Coromandel Peninsula were reported killed, even though seawater fed into the land-based aquaculture facility was pre-filtered to remove algal cells. What was causing the mass deaths of all these marine animals?
Water samples collected between 9 and 16 October from several areas around Orewa, Waiwera, north of Waiheke Island in the Hauraki Gulf, and also at Kennedys Bay outside the Gulf, showed the presence of several species of algae, Karenia and Gymnodinium. Karenia cell concentrations ranged from 1.8 to 10.7 million cells per litre around Orewa and Waiwera, up to 32.1 million cells per litre near Waiheke Island, and 6.7 million cells per litre in Kennedys Bay. While Karenia mikimotoi was the dominant species, there were also small numbers of Karenia brevisulcata, Gymnodinium pulchellum, and a possibly new Gymnodinium species.