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Modern technology is ensuring Northland oyster farming keeps its clean, green image.

Bacterial pollution from run-off into Whangaroa Harbour forced the drastic measure of a voluntary harvest closure for part of last year.

However, the National Institute of Water and Atmospheric Research has found a way to solve the problem - placing salinity detectors directly beside the oysters.
Oysters, like other filter feeders, reflect the condition of the water. If left alone when polluted, they will clean themselves as the water quality improves.

The key is to predict when it is safe to harvest. Water sample tests for bacteria can take 24 hours, by which time weather conditions may have changed and the results are meaningless.

Institute regional manager Ken Grange says that by necessity, most farmers rely on basic predictions.

"I think most of them have just got a rain gauge in their back yard and they just look at it 8 o'clock every morning and see how much rain there's been."

Unpredictable run-off channels, and the fact it can be raining in one part of Whangaroa Harbour but not the other, make this system unusable.

Previous attempts to use salinity buoys to measure salt levels, and thus identify when fresh water run-off hits the harbour, were also not accurate enough.

Bacteria-laden fresh water can sit on top of the salt water, contaminating the oysters as the tide goes out.

After nine months of rigorous sampling, during which time harvesting was greatly reduced, the institute decided salinity detectors worked.

Grange says it is now possible to accurately determine a safe time to harvest. To ensure timely disclosure of this information to farmers, the sensors return data 24 hours a day.

Farmers who previously received harvesting notices twice a day now receive them every 15 minutes via text messages and pagers.

Northland Health and the Ministry of Agriculture and Forestry have been satisfied with the results and Whangaroa Harbour has been conditionally approved for direct harvesting.

The system, costing less than $10,000 a year, is paid for by the farmers and run from the institute's Nelson office.

Grange says the technology will continue to be developed and does not have to be confined to New Zealand.

"We could run the same system in Australia or the United States."