Parasites and bacteria responsible for mass cockle deaths at Whangateau Estuary

21 August 2009

After months of detective work, MAF Biosecurity New Zealand (MAFBNZ) has unravelled the cause of mass deaths of cockle shellfish in an estuary north of Auckland earlier this year.

The population of New Zealand cockles (Austrovenus stutchburyi) in the Whangateau Estuary near Omaha/Point Wells took a massive hit between January and May with local people reporting vast numbers of dead shellfish in the tidal estuary.

The estuary is an important recreational fishery and the community, including a locally-based marine scientist and the local Ministry of Fisheries staff, were helpful in supplying samples to MAFBNZ over a period of several months to help find the cause.

Principal Adviser and Aquatic Animal Disease Scientist Colin Johnston says after a complex series of scientific tests, MAFBNZ has now determined the mass mortality was the result of a number of coinciding factors.

"It appears the cockle population in the area may have been initially weakened by an environmental event such as heat stress. It was a particularly long hot summer last year. This may have left them susceptible to infection by two separate pathogens – one a parasite infecting their gills and the other a mycobacterium," says Dr Johnston.

"The parasites involved, coccidian parasites, are not uncommon in shellfish, although it is unusual to find mycobacterium in molluscs," he says. Mycobacteria are free-living bacteria common in marine and aquatic environments. They are occasionally associated with skin lesions in people who have handled fish or cleaned fish tanks. People should take normal precautions to cover any open cuts and wounds when handling fish and shellfish, and consult a doctor if they notice skin lesions or wounds on their arms and hands that aren't healing, particularly if they have been handling shellfish from the Whangateau Estuary. The coccidian parasite is not transmitted to people.

Dr Johnston says the cockle event appeared to end in April or May, and there is no indication that other estuaries were affected.

He also points out that the cockle deaths occurred at the beginning of the year and are highly unlikely to be in any way related to the current Auckland beach poisoning issue.

The Ministry of Fisheries (MFish) is now considering future action to help the decimated cockle population recover.

John Taunton Clark, Inshore Fisheries Manager says MFish will be working with the community and local tangata whenua to determine possible management options for the harbour.

For further information, please contact:

Lesley Patston, Senior Communications Adviser, MAFBNZ
Phone: 029 8940163

Barbara Crocker, Communications Services Manager, Ministry of Fisheries
Phone: 021 388659

Contact MPI

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0800 00 83 33