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Bonamia Ostreae and Sanford’s Oysters

Bonamia Ostreae and Sanford’s Oysters

You may have heard about the outbreak of the oyster parasite Bonamia ostreae that’s been detected in the waters off Stewart Island. Sanford operates several aquaculture farms including an oyster farm in Big Glory Bay on Stewart Island in a joint venture with a company called Tio and some of those oysters have returned a positive test for Bonamia ostreae.

The most important thing for oyster lovers to know is that Bonamia is not harmful in any way to humans. It is perfectly safe to eat an oyster that has it. However we are currently doing everything we can to stop the spread of this pest, as it is very harmful to our oysters and that means we are not taking any shellfish from the farm, so these oysters will not be in our supply chain at present.

The frequently asked questions below should help you understand more about Bonamia and what it all means. If you have any other questions, feel free to write to us at

What is Bonamia ostreae (pronounced Bon-aim-ee-a os-tree-ay)?

It is a parasite that attacks flat oysters, which includes Bluff oysters but not the cupped Pacific oyster. It was first detected in New Zealand in 2015 and has been found up till now in the Marlborough Sounds. No one knows how or when it got into New Zealand. It was confirmed to be in Stewart Island waters by MPI (the Ministry for Primary Industries) on Wednesday 31st May. A related parasite, Bonamia exitiosa, has been in the waters of Foveaux Strait since the 1960s and has caused two significant episodes of oyster mortality in the wild population.

How did it get to Stewart Island?

We don’t know and we may never know. It could have come to Stewart Island in water currents, in boat movements or any number of ways. It spreads very easily. It is important to note that Sanford keeps all its boats up to standard with antifouling measures etc.

Could this parasite spread between oyster populations?

At this point, we have stopped all movements of oyster and mussel spat, stock and equipment from our farm. Mussels aren’t affected by Bonamia but some think they may be a carrier, so we have shut down all mussel movements also. Unfortunately there is nothing we can do about the tide and Bonamia spreads in the water column through natural water movements.

How dangerous is it?

It is important to note that there is no risk whatsoever to humans. MPI’s advice is that you will not suffer any ill effects if you eat oysters that contain the parasite and there are no food safety issues. It says “fresh, good quality New Zealand oysters continue to be safe to eat.”

It is very harmful to the oysters though. In places where it has already impacted oysters stocks, such as Marlborough, mortality rates can be very high, but how high very much depends on the age of the oysters, how densely they are growing and whether they have spawned recently. It tends to be more harmful to older oysters and is most damaging to them when they are spawning.

Where is it exactly?

MPI has found it on two farms in Stewart Island. One of these is run by Sanford and Tio in a joint venture. MPI says “Extensive sampling to date has not found Bonamia ostreae in Bluff oysters in Foveaux Strait, however, we are continuing our sampling and increasing surveillance.”

What is Sanford doing about it?

We are doing everything and anything that MPI asks of us, which means that right now we have stopped all movements of shellfish in or out of our Big Glory Bay farm. We are acting under the requirements of a Controlled Area Notice which MPI put out for Stewart Island on Wednesday May 31st.

Where can I go to if I have more questions?

Check out MPI’s advice on Bonamia which you can find here: