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Production Systems

Most farmed Pacific oysters are grown on sticks or in trays and netting bags on intertidal oyster farms in northern North Island bays and harbours.

These intertidal farms use a series of wooden racks to support sticks, trays, or bags of growing oysters.  Farms are built in shallow, sheltered waters between the high and low tide marks. The racks are built so the oysters sit just above the water level at low tide and are covered by one to two metres of water at high tide, depending on the tidal range.

Some New Zealand Pacific oysters are grown in deeper waters, in trays beneath the surface, or on longlines supported by plastic floats - similar to the way Greenshell™ mussels are grown. The ‘Japanese longline’ culture method and elevated rack system described above have less environmental impact than some other bottom culture methods used around the world.

Longline farming generally leads to faster growth because the oysters remain under water all the time. The oysters, however, can have more fouling and poorer shell development.

Oysters grown on intertidal farms have a slower growth rate, but the shells and adducter muscles are stronger. This produces a live oyster with a better shelf life as well as a better half-shell product.

Farmers with both intertidal and subtidal sites can transfer oysters between deep water and intertidal sites to help optimise condition at harvesting.

A typical Pacific oyster farm occupies about four hectares of water space.

Pacific oysters are farmed in the following New Zealand regions:

• Northland
• Auckland
• Bay of Plenty
• Waikato
• Marlborough

Source of Stock

Since the 1970s, wild Pacific oysters have colonised many areas of New Zealand and now can be found in the wild from Northland to the Marlborough Sounds (at the top of the South Island).

Most New Zealand oyster farmers source their spat (young oysters) from the wild. Wild spat is ‘caught’ during the summer time, often on specialised spat-catching farms.

Some New Zealand farmers source their oyster spat from hatcheries, where the spat is selectively bred for desirable characteristics. Such hatcheries need to be compliant with New Zealand’s environmental management legislation - the Resource Management Act (1991) - and with other building and operating codes. This ensures any environmental effects are minimised (avoided, remedied or mitigated).

To catch spat from the wild, farmers leave materials in the water that oyster larvae will settle on and attach themselves to. Typically 1.5m long wooden sticks are used, often dipped in concrete slurry to help catching, and packaged into bundles of 30 sticks.

Some farming areas are good for spat-catching, so farmers can catch spat on their own farms (for example, Mahurangi Harbour). But not all places are good for spat catching so many farmers use spat that has been caught in other regions, or grown in hatcheries.

Many North Island farmers get their spat from spat-catching farms in the Kaipara Harbour. Spat for South Island farms are caught in Croisilles Harbour.

Stocking Densities

Oyster farms are stocked at levels that optimise their growing conditions, while still maintaining the integrity of the environment, including water quality.

Adverse water quality effects from oyster farming are highly unlikely in the New Zealand growing environment. However, these may result where farms are overstocked and sited in poorly flushed environments. This can be avoided by appropriate site selection, and by ensuring farm structures have a minimal effect on flushing processes. Both these factors are taken into account when planning oyster farm developments or extensions in New Zealand.  

Growing and Harvesting

Once the spat are caught, it takes 12-20 months on an intertidal farm for the Pacific oysters to grow big enough to sell.  Subtidal farms can achieve faster oyster growth.

On intertidal farms, the spat is either on-grown on the sticks they settled onto, or else stripped from the sticks to be on-grown in mesh bags, trays or baskets. The latter method is called ‘single culture’ as the oysters are not attached to each other or to a stick.

On subtidal farms, oysters are on-grown on longlines - either suspended in bags, trays, baskets or directly on the dropper ropes.

Feed is not added at any stage in the farming of Pacific oysters. As oysters are filter feeders, they filter natural plankton (small free-floating plants and animals) out of the water column.

Oyster farmers usually work on farms in the two hours either side of low tide, or otherwise they use motorised barges to access their sites. They generally use motorised barges to transport oysters and gear between the farm and their land base.

On intertidal farms, farmers often walk around the racks in the water while doing maintenance or harvesting. On longline farms the water is too deep to stand in, so all operations are done from the barge.

Only crop from growing areas with clear sanitation status and where there is no evidence of biotoxin accumulation is considered for harvest (see Food Safety section for regulations and details).

This means all Pacific oysters harvested in New Zealand meet the highest raw food consumption standards (including compliance with EU and US standards).