Other common names: Pacific king oyster, Pacific rock oyster

Introduction

Pacific oysters are endemic to Japan, but have been introduced into a number of other countries including Australia. Most of these introductions have been for the purposes of aquaculture, with Pacific oysters the most widely cultured shellfish species worldwide.
Pacific oysters were first introduced into south-eastern and western Australian waters for aquaculture. They later found their way into NSW waters, where they have spread and invaded intertidal habitats of many waterways.
Pacific oysters are a hardy species with fast growth and high reproductive rates. This has allowed them to establish dense populations in some areas, often displacing native intertidal species.
Although Pacific oysters are the basis of an important aquaculture industry in Port Stephens, elsewhere they have caused significant problems for oyster farmers who culture native Sydney rock oysters (Saccostrea glomerata). As the two species live and spawn in the same locations, Pacific oysters can settle on and (due to their faster growth rate) smother farmed Sydney rock oysters.

Natural distribution & biology

Pacific oysters are bivalve molluscs belonging to the family Ostreidae. They are native to northeast Asia (including Japan), but have been translocated and spread widely throughout many countries (including the UK, France, USA, Canada, Korea, China and New Zealand) for the purpose of aquaculture.
Pacific oysters have a fairly thin shell with no hinge teeth on the inner, upper shell (unlike Sydney rock oysters). The adductor muscle (which holds the two shells together) is purple or brown in colour, whilst the edges of the mantle (the tissue which secretes and lines the shell) are black.

Adult Pacific oysters are sessile and will settle on any hard substrate in the intertidal and shallow subtidal zones, to a depth of about 3 metres. They favour brackish waters in sheltered estuaries, although they tolerate a wide range of salinities and water quality and can also occur offshore.

Pacific oysters have very high growth rates (they can grow to over 75 mm in their first 18 months) and high rates of reproduction.

Like most oyster species, Pacific oysters change sex during their life, usually spawning first as a male and subsequently as a female. Spawning is temperature dependent and occurs in the summer months. Pacific oyster females can produce between 30 to 40 million eggs per spawning, often giving the surrounding water a milky appearance. Fertilisation takes place in the water column.

The larvae are planktonic and free swimming, developing for three to four weeks before finding a suitable clean hard surface to settle on. Although they usually attach to rocks, they can also settle in muddy or sandy areas (where they attach to small stones, shell fragments or other debris) or on top of other adult oysters. A very small percentage of oysters survive this phase; those that do are called "spat".
Pacific oysters can live up to 10 years and reach an average size of 150-200 mm.

Pacific oysters are plankton feeders that filter minute marine algae and other microorganisms out of the water.

Where are they in NSW?

Pacific oysters have established breeding populations in most NSW estuaries south of the Macleay River and some offshore areas. However, few Pacific oysters are found north of Port Stephens.
Pacific oysters have been commercially cultivated in Port Stephens since 1991. Production is increasing each year with strong demand bringing high prices.

How did they get here?

Pacific oysters were introduced deliberately into Western Australia and Tasmania in 1947, into Victoria in 1953 and into South Australia in 1969. Oysters planted in Tasmania and Victoria successfully spawned although those introduced to Western Australia eventually died out.

Although Pacific oysters were not brought legally into NSW, it is suspected that small numbers were introduced and they soon formed small populations in some NSW estuaries. Once established, Pacific oysters spread into other areas, possibly in part by mixing with native rock oyster spat which was moved between estuaries by oyster farmers.

What are their impacts?

Once introduced into an area, Pacific oysters are generally impossible to contain if environmental conditions are suitable. Their planktonic eggs and larvae facilitate natural dispersal, allowing them to greatly expand their original range.

Pacific oysters have the ability to develop high density populations within the intertidal zone. This has led to the problem of competition between Pacific oysters and other species for food and space. In many areas Pacific oysters have become the dominant oyster species, displacing native species such as Sydney rock oysters.

Native Sydney rock oysters are a prized seafood species and are the main focus of oyster production in NSW. The spread of Pacific oysters, and their ability to displace (or even smother) native species, is a major concern for NSW oyster farms which cultivate Sydney rock oysters.

This potential change in the species balance also has the potential to impact on non-oyster species, through a modification of their habitat.

What is NSW DPI doing?

Pacific oysters havebeen listed as a Class 2 noxious species in all NSW waters except Port Stephens under the Fisheries Management Act 1994. The possession and sale of Pacific oysters may only occur under a specific permit issued by NSW DPI.

This listing recognises the fact that farming of Pacific oysters is a major and well-established industry in Port Stephens, worth around $1.8 million annually.

To minimise the spread of Pacific oysters in NSW, NSW DPI has implemented a Pacific oyster closure which includes strict criteria for the movement of oysters between estuaries, with movement of oysters into some areas prohibited. Look on the fishing closures page for more information.

How can you help?

Like many aquatic pests, once they are established the elimination of Pacific oyster populations is extremely difficult or impossible.

Oyster farmers can assist in preventing the further spread of Pacific oysters by following NSW DPI's guidelines for inspections of oyster leases and complying with rules regarding the movement of oysters between estuaries.

There are several steps you can take to help prevent other marine pests from entering or spreading further in our waterways. For example:

• If you have visited an area known to be infested with a noxious aquatic species, inspect anchors, ropes and chains before leaving the area, and wash your boat and gear down in wash down bays (where provided) or an area away from water bodies and stormwater drains.
• When diving or fishing in marine waters, keep a lookout for new species.

If you find a new species that you suspect is not native to the area, freeze it whole as soon as possible and Report it!

References

Medcof JC, Wolf PH. (1975). Spread of Pacific Oyster worries NSW culturists. Australian Fisheries, 34(7): 32-38.
Pollard DA, Hutchings PA. (1990). A review of exotic marine organisms introduced to the Australian Region: II Invertebrates and Algae. Asia Fisheries Science, 3: 223 - 250.
Thomson JM. (1951). The acclimatisation and growth of the Pacific Oyster (Gryphaea gigas) in Australia. Australian Journal of Marine and Freshwater Research, 3-4: 65-73.
Zibrowius M. (1991). Ongoing modification of the Mediterranean marine fauna and flora by the establishment of exotic species. Mesogee, 51:83-107.