Pacific oyster

About Pacific oysters

Scientific name: Crassostrea gigas

Māori names: Tio

Pacific oysters were introduced in New Zealand and have a moderately complex life-cycle. In the wild, they can live for several years and grow to 20 cm or more (shell length) but most have become reproductively active by the time they reach 6-8 cm in 12-20 months. Adults release their gametes into the open water. Eggs, sperm and subsequent larval stages are planktonic. They will drift in the sea for approximately three weeks and prefer to settle on horizontal surfaces. Newly settled individuals (spat) are small (0.5 mm shell length). Once settled, oyster spat cannot re-attach and will continue to grow on their chosen settlement surface.

Why farm Pacific oysters?

Shellfish are a nutritious food: high in trace elements, protein and poly-unsaturated fatty-acids, but low in cholesterol, saturated fats, and trans-fats^3,4,5. Shellfish production imposes a relatively small burden upon the local environment and the industry’s carbon and freshwater footprints are small in comparison with many other protein crops^2,3. Oyster farms can also provide habitat for some fishes when the tide is in.

How are Pacific oysters farmed?

The New Zealand oyster industry historically started outgrowing the native rock oyster (Saccostrea commercialis)¹. Rock oysters were revered and ‘gardened’ by early Māori and hand-picked to unsustainable levels by the early European fishery. In the 1960s, the faster growing Japanese or Pacific oyster (Crassostrea gigas) became introduced - some say with shipments of sections of the Auckland Harbour bridge.

Pacific oysters ‘broadcast’ spawn and their larvae drift in the sea for several weeks before settling. Oyster farmers catch the spat on bundles of wooden sticks at preferred sites. The wooden sticks are then nailed onto wooden racks, set out at the correct intertidal level to minimise fouling, parasites and pests. Once grown, the oysters are then simply harvested for processing. The ultimate quality (size and uniformity) of the oyster crop is strongly influenced by the initial density and evenness of the settled spat – which are difficult to regulate in this traditional growing system. Thus, some growers have recently moved to growing ‘single-seed’ oysters by either stripping wild-caught spat off the sticks at an early age and reseeding, or by growing spat that have been produced in a hatchery.

Single-seed oysters require more labour and mechanisation but produce a more even-shaped product and can benefit from breeding programs. Single-seed oysters are grown in several different systems including trays, baskets, long-line and cages in subtidal or intertidal areas. Subtidal oysters can have issues with fouling, pests and parasites, and can be moved for a time in an intertidal area to “harden-off” to increase their value.

How is NIWA research helping sustainable Pacific oyster farming?

NIWA’s research aims to promote the continued development of an aquaculture industry that is both environmentally sustainable and economically viable. For Pacific oyster farming, research and consultancy services have:

Tested the optimum intertidal level for maximising growth whilst reducing fouling, and pests like “mudworm” that cause the oyster to produce unsightly “mudblisters” in their shells
Evaluated methods to avoid and treat predation by “flatworms” or “wafer-worms”. These cryptic predators can consume oyster spat and adult oysters alike
Monitor environmental factors affecting productivity in oyster growing areas
Provide real-time monitoring equipment for salinity of growing waters to ensure oysters are healthy to consume

References

Curtin, L. (1971) Oyster farming in New Zealand. N.Z.Mar.Dept., Fish.Res.Tech.Rep.Wgtn., 72: 75-78.
Meyhoff-Fry, J. (2012) Carbon footprint of Scottish Suspended Mussels and Intertidal oysters, SARF078: 55. http://www.sarf.org.uk
MPI (2013) Overview of ecological effects of aquaculture: 81. http://www.mpi.govt.nz/news-and-resources/consultations/marlborough-salm...
PFR, P.F.R., NZMOH, M.o.H. (2016) New Zealand food composition data. Plant and Food Research & The Ministry of Health. http://www.foodcomposition.co.nz
Venugopal, V., Gopakumar, K. (2017) Shellfish: Nutritive Value, Health Benefits, and Consumer Safety. Comprehensive Reviews in Food Science and Food Safety, 16(6): 1219-1242. 10.1111/1541-4337.12312