Toxic algal bloom could recur
A species of toxic algae that has caused mass mussel farm closures might become an annual issue.
More than 100 mussel farms in Kenepuru Sound and inner Pelorus Sound have closed after the toxic algal bloom Alexandrium catenella was found.
With 122 farms closed and another 128 voluntarily ceasing their harvest while the toxins clear up, it is believed to be the largest number of closures since 1993.
Ministry for Primary Industries specialist adviser seafood Brian Roughan said the ministry was ‘‘on a bit of a learning curve’’, as it was the first time this particular species had been detected in the inner Sounds.
There had been related species of algae that spread from Onapua Bay, through Tory Channel, to other mussel farms in East Bay in Queen Charlotte Sound, but it was a waiting game to see if the latest bloom was a one-off or a sign of things to come, he said. ‘‘Because they sit on the seabed, the right conditions might occur again and away they go next season.’’
The algal bloom had happened later in the year than was typical, but the ‘‘stars had lined up’’ in terms of nutrient levels, temperature and water conditions, Roughan said.
Scientists at Nelson’s Cawthron Institute were researching whether algal blooms could be predicted, he said.
‘‘It’s about farming accordingly and knowing when it typically happens. But it can be unpredictable.’’
It was thought the species had spread through tidal movements. ‘‘There is lots of water that flows in and out, and we think the bloom started in Nydia Bay and it’s just spurted out into the rest of the Sounds.’’
Once Alexandrium catenella stopped reproducing and shellfish began eating other forms of phytoplankton, they could get the toxin out of their systems ‘‘reasonably quickly’’, he said. Two clear samples taken from mussels, 48 hours apart, were needed before the area would be reopened.
Aquaculture New Zealand said the algae was detected during routine monitoring.
Because mussels naturally cleared the algae, no stock losses were anticipated, it said, but the farm closures would delay harvests.
Aquaculture company Sanford’s general manager of aquaculture, Ted Culley, said that while 60 of its farms were affected by temporary closures, staff were continuing to harvest at unaffected locations around the outer Sounds.
‘‘We work in a natural environment, and these things happen.’’
Culley said a good chain of communication in relation to the bloom’s emergence had enabled Sanford and other farmers to act quickly.
‘‘We have a really good system to alert industry around potential toxin blooms, to make sure our customers are safe, and we don’t produce any mussels that go to market with toxins in them.’’
Based on previous experience, Culley estimated it would be 20 to 30 days before the mussels would be ready to harvest, depending on the level of Paralytic Shellfish Poisoning (PSP) toxins found.
‘‘We think the bloom started in Nydia Bay and it’s just spurted out into the rest of the Sounds.’’
Brian Roughan, Ministry for Primary Industries