Glenhaven Oyster Hatchery

August 2009

Selective breeding of oysters improves harvest speed and size for oyster farmers

Cutting production costs and increasing efficiency are two of the first things farmers do when the going gets tough. And for hundreds of years, primary producers have been turning to selective breeding to help them grow more crop for less effort. This is good for commodity products but what happens when you’re dealing with a delectable high value product like a Pacific oyster?

One option is to try and grow them cheaper but New Zealand will always be up against the costs of shipping export products into overseas markets. The other option is to add value and make the product more attractive. Aesthetic considerations are important for an oyster buyer, and people are often prepared to pay more for an oyster that looks good and tastes even better.

Selective breeding of the Pacific oyster was started in the USA in the 90s. As is common with many breeding programmes, the initial priority was to increase yield. In the case of oysters, this was achieved through selecting for faster growth and increased survival. In many countries, oysters take up to 3 years to mature, and survival may be well below 50% so improvements to growth rate and survival have big economic benefits to oyster farmers.

Things are a little different in New Zealand. In many cases oysters reach harvest size in only one year and survival commonly exceeds 90%, so the economic benefits to New Zealand oyster farmers of faster growth and increased survival are not that great. However breeding for growth rate and yield are straightforward and it has been done successfully overseas so the first step in a Pacific oyster breeding programme at the Cawthron Institute has been to replicate the results from overseas. This has been very successful, and commercial oyster lines now have growth rates more than 20% higher than the average crop from wild spat.

Many farmers grow oysters on sticks but over the past five to eight years some have been working to convert their farm from this traditional style to the long-line system, growing single seed oysters in a plastic basket suspended on a plastic long-line. Stick culture requires a lot of labour whereas this system lends itself to mechanisation.

Cawthron supplies farmers with single seed spat. All that is required is an email message to the hatchery. The spat arrive in poly bins big enough to go into 6mm mesh baskets.

Farmer feedback is that mortality has been minimal, and the oysters generally have a better shape than oysters grown on sticks. They also say that single seed allows them to manipulate the crop to suit their needs and to optimise the use of our farms. Farmers can adjust the stocking density, move oysters between farms or within farms, and adjust the height of the bags in the tide. These stock management options allow them to grow a uniform crop with good overall efficiency regardless of the uncontrollable environmental conditions like food availability.

With the selective breeding well underway, Cawthron's attention has shifted to more challenging and commercially interesting breeding goals. The Pacific oyster industry in New Zealand already markets high value product into overseas niche markets under the Jemco brand. Companies exporting under Jemco have been quick to recognise the potential of selective breeding to improve product quality. Some selection is still made for fast growth and good survival but the priority now is for traits that increase the percentage of oysters meeting the highest paying Jemco grades. These traits include improved shell shape and meat to shell ratio.

For the past 10 Cawthron has been running a selective breeding programme for Pacific oysters with support from Pacific Marine Farms Ltd. This has involved breeding oyster families, where one male is crossed with a female to produce a few million offspring who are all brothers and sisters. Up to 64 families are produced in the hatchery at one time and are placed out onto commercial farms for on-growing to harvest. Once at harvest size and condition, the families are measured and the best families selected for commercial spat production and for another generation of breeding. Selective breeding is completely natural and uses the same traditional methods that plant and livestock farmers have been using for millenia.

Breeding for fast growth is easy. Cawthron has achieved a 20% increase in oyster green weight with each generation. Growth rate is easy to breed for because it is easy to measure; just weigh a bag of oysters and you have a measure of their performance. Measuring ‘quality’ is not so easy and so far the focus has been on visual assessment for traits like shell shape and meat condition.

Another technique has been to assess the oyster families against the Jemco grade standard. Oysters within a family are graded and the number going into each of the Jemco grades is recorded. There are big differences in the value of the different grades and so a small difference in oyster biology like a slightly different shell shape can make a big difference to the grading and hence the price paid for the oysters in the overseas market.

Consistency is another easily achieved benefit of selective breeding. Ask anyone who processes plant or animal products what makes the biggest improvement to their processing efficiency and they will probably mention raw material consistency. Consistent size means you can delay harvesting until a maximum proportion of your product is at the optimal size. Consistent quality makes life a lot easier for the graders and puts most of the product into the grades being targeted. In general, oysters produced by crossing two families are much more uniform than oysters grown from wild spat that has come from a large number of parents.

Commercial oyster spat grown in the nursery system rely on a constant supply of algae produced by the ponds to achieve excellent growth rates and spat quality. Recently, experiments involving fattening oysters with algae from the ponds have produced very encouraging results.

There are a number of benefits about the Cawthron nursery site.

(1) The land lies below spring high tide, which means that they can fill and empty the ponds by gravity, saving on energy costs.

(2) The land is adjacent to Tasman Bay and distant from any major river mouths or pollution so the water is of good quality.

(3) It is in one of the sunniest spots in New Zealand, which means the algae get plenty of energy to grow.

(4) There is a dense clay pan at the right soil depth so the ponds are naturally water-tight.


When mussel and oyster larvae first settle and become spat they are less than 1mm long. The tiny and fragile spat is moved from the hatchery into a nursery system to grow several mm before it is big enough to transfer onto farms. During this time their food requirements increase dramatically, but can be satisfied by algae produced in the ponds. The ponds enable Cawthron to produce enough food for commercial quantities of spat at an acceptable cost. At the Nelson site, 2 ha of pond area is enough for a commercial-scale mussel nursery.

Evidence suggests that pond water can successfully condition shellfish broodstock for spawning, even when wild stock are in poor condition. This makes the year-round production of shellfish seed possible without complicated and expensive broodstock conditioning regimes. Rapid re-conditioning of spent broodstock is also possible, which greatly reduces the number of broodstock required for commercial-scale spawnings.

Ponds can be used to improve the meat condition and marketability of shellfish. An impressive example of this is practiced in France where over 40,000 tonnes of Pacific oysters are fattened every year in shallow seawater ponds before being marketed as premium ‘Claire’ oysters. Initial trials of this concept at the Glenhaven Aquaculture Centre have provided very encouraging results. There is exciting potential to produce distinctive, fresh, premium seafood through pond fattening in New Zealand.

Zooplankton feed on micro-algae and are naturally abundant in the ponds. These tiny marine animals represent nutritionally rich food parcels themselves and can be fed to larger animals such as seahorses and small fin fish. Pond produced zooplankton could be used as a cheap source of live feed for operations such as seahorse farms and fin fish nurseries.