The Ecology of Crassostrea gigas in
Australia, New Zealand, France and Washington State

Introduced Pacific Oysters in Australia
Peter Ayres

Introduction

This presentation considers the introduced Pacific oyster, Crassostrea gigas, in Australia and in particular in New South Wales (NSW) where it is considered by many to be undesirable and to pose potentially serious ecological, social and regulatory problems. In NSW, Pacific oysters have appeared in the midst of a century-old established oyster industry based on culture of the indigenous species, Saccostrea commercialis (Sydney Rock Oyster). As a result, they have been declared "Noxious Fish" by the state government and must be removed and destroyed as found. It is important to stress however that in two states, Tasmania and South Australia, the species has been introduced to create an oyster industry where none existed before.

History of Introductions

There is a certain irony in the fact that it was the Commonwealth Scientific and Industrial Research Organization (CSIRO), a federal government agency, which after several attempts, succeeded in introducing Pacific oysters from Japan. In a country where the often well intentioned but naive introduction of non-indigenous fauna and flora has led to serious ecological consequences and the extinction of some native species, reference to the Pacific oyster in NSW as a "marine rabbit" seems appropriate. Table 1 summarizes the original introductions of Pacific oysters into Australia and Table 2 its subsequent appearance in NSW.

The first recorded importation of Pacific oysters in 1940 was seized and destroyed in Sydney on the recommendation of the then superintendent of NSW State Fisheries who had earlier expressed concern about ecological consequences of its introduction. However, further consignments were imported in 1947,1948 and 1952 and relaid in Tasmania, South Australia, and Western Australia. Only those in Northern Tasmania survived and these spawned successfully in Pittwater and Port Sorrell. It appears likely that larvae from those stocks were transported by tidal currents some 30 km eastwards to the Tamar estuary in Tasmania and became established as a self-sustaining population while all others died out. Stock transferred from Tasmania in 1955 northwards to Mallacoota Inlet in Victoria on the Australian mainland were still surviving in reduced numbers three years later. In 1967 five Pacific oysters were found 70 kms north at Pambula, NSW and again it is possible that ocean transport of larvae was responsible. On a state-by-state basis the situation may be summarized thus:

Tasmania

Pacific oyster spat observed in the Tamar River in 1958 developed into a self-sustaining population which by 1967 had multiplied to such an extent that large reefs of oysters approaching nuisance proportions had become established on foreshores. However, spatfalls were erratic in subsequent years and market size oysters were found to contain excessive levels of heavy metals, especially zinc, rendering them unfit for human consumption.

In 1977 the Tasmanian government and prospective oyster farmers established a pilot scale commercial Pacific oyster hatchery at Bicheno on the east coast. The success of this and later private hatcheries, producing close to 100 million spat annually, forms the basis of the Tasmanian Pacific oyster industry today. The original local industry based on the native mud oyster ( Ostrea angast) fell into decline early this century so Pacific oysters did not conflict with an existing species. The fishery is now totally reliant on hatchery produced spat and in 1989-90, 3.5 million dozen oysters worth 13.7 M$Aus were marketed. Freak warm weather conditions in 1990 resulted in some natural spawning and catch but generally Tasmanian waters are too cold and saline for reliable recruitment of Pacific oysters to occur.

South Australia

Although earlier CSIRO introductions were unsuccessful, in the early 1980's a hatchery and grow out facility was established in salt ponds in South Australia. This operation closed down and only recently have Pacific oyster spat from Tasmanian hatcheries been introduced to establish an industry. The 1989-90 production of 0.47 million dozen oysters will certainly increase. Temperatures in the area should be sufficient for larval production but near oceanic salinities suggest it is unlikely that populations will be self-sustaining.

Victoria

Stock transferred by CSIRO to Mallacoota survived until 1958 and perhaps longer but is not evident now. Attempts to emulate the South Australian experience in salt ponds eventually failed because of lack of funds. Prospective oyster farmers requested the Victorian government to permit the introduction of Pacific oyster spat from Tasmania. Assessing the potential risks and benefits, the government refused permission and decided to foster cultivation of the native mud oyster and the mussels ( Mytilus edulis and Mytilus planulatus).

New South Wales

Pacific oysters were first found here in 1967 but not reported again until 1973 when they appeared in a number of estuaries (Table 2). Conceivably larvae could have been transported to these areas by ocean currents or on the hulls of Tasmanian scallop boats, but the mechanism involved is not understood. In response to the 1973 reports the Fisheries Department issued warning notices to oyster farmers to assist in locating and destroying any Pacific oysters that might be found. Surprisingly, no reports were documented until 1985 when large numbers of Pacific oysters were found on commercial rock oyster leases in Port Stephens, 160 km north of Sydney.

Port Stephens is a large tidal inlet approximately 25 km long and 10 km wide, with a narrow entrance to the ocean and a promontory that separates the upper and lower Port areas. The Port is fed by numerous freshwater creeks but has only one major freshwater input of significance, the Karuah River to the west. Port Stephens has a mean annual salinity between 25 and 28 ppt but salinities may range from less than 10 to 35 ppt towards the ocean depending on rainfall and season. Summer (Nov-Feb) water temperatures generally exceed 25°C and in winter may fall to around 13°C. Because it is a large open expanse of water, Port Stephens is generally unsuitable for tray culture of oysters. However tidal and climatic conditions are such that rock oyster spatfalls are the heaviest and most consistent of any area in NSW and so Port Stephens has traditionally supplied spat and stock for other estuaries. Overall, the rock oyster in NSW produces some 10 million dozen oysters worth 36 M$Aus annually.

How Pacific oysters got into Port Stephens in the 1980's remains a matter of conjecture but the evidence suggests they were deliberately introduced, probably as spat from Tasmania around 1982-83. The area where they were discovered in 1985 is heavily cultivated, and close to a number of oyster depot areas; it seems improbable that no one knew Pacific oysters were present before 1985. Since 1985, Pacific oysters have spread rapidly throughout Port Stephens, initially throughout the upper area and finally in 1990 became established in the outer Port also. The nature of the spread implicates a number of factors. Initially, water movements and wind kept distribution to the southern shores but as the number of mature animals increased, accidental and deliberate transfer with rock oyster stocks spread the species to other areas. Large scale survival and settlement in the outer Port followed exceptionally heavy rain and lowered salinity in 1990 and now Pacific oysters occur throughout the entire area.

Movement of Port Stephens stock to other areas north and south along the Australian east coast has led to populations becoming established in many other estuaries, at worst in nuisance proportions but generally under control. Because the 1500 km of NSW coastline has such a wide range of climate and estuarine conditions, the southern, cooler areas seem to favour Pacific oyster survival, whereas the northern areas are too warm. A local combination of temperature, salinity and estuarine type seem to be the critical factors in survival and reproduction of Pacific oysters in NSW. Port Stephens is located almost midway along the coast and so has average conditions, but I believe the important factor here is a failure to control Pacific oyster numbers in the early years after 1985. One very clear lesson is that, once introduced, Pacific oysters are impossible to eradicate. However, control of the population is effective and achievable if both government and industry are committed to the task.

The Australian Experience - Summary

1. The Pacific oyster is a rapidly growing, highly fecund, and adaptable species that account counts for some 80% of the recorded edible oyster production in the world.
   
   
2. In Australia the species has been successfully and deliberately introduced into Tasmania and South Australia but in neither case has it (a) replaced an existing commercial species because none existed before and (b) spawned in local waters on a reliable basis. All the industry needs are met by hatchery production which permits selection and manipulation (e.g., triploidy) to produce a uniform oyster generally superior to wild-caught oysters.
   
   
3. The major limiting factors in both areas appear to be low water temperature and high salinity.
   
   
4. The Pacific oyster has a limited shelf life as a live product and must be processed soon after harvest. In contrast, the NSW rock oyster will remain alive without loss or deterioration out of water for up to 3 weeks and can be safely transported long distances.
   
   
5. The NSW rock oyster industry is over a century old, utilizes a unique indigenous oyster, produces 36 M$Aus of oysters annually and is a major and important contributor to local employment and economies along the NSW coast.
   
   
6. More than 95% of NSW's commercial oyster farmers have consistently voted to retain the rock oyster as the species of choice and have spent six years controlling Pacific oysters.
   
   
7. Government regulations have not been enforced because of lack of resources and political will.
   
   
8. Oyster farmers in Port Stephens who spent six years fighting the spread of Pacific oysters have effectively been abandoned by the government and many businesses will collapse.
   
   
9. Those who have done little or nothing have been rewarded by the subsequent legalization of sale and cultivation of Pacific oysters (in Port Stephens).
   
   
10. The parallel cultivation of two oyster species is already fraught with problems and a worse-case scenario where neither can be grown and harvested economically would spell the death of the industry and the dereliction of thousands of hectares of oyster leases. This would be an environmental disaster.
    
    
11. Pacific oysters neither grow nor survive uniformly throughout NSW and it is unlikely they will ever be grown as the major species of commerce except in specific estuaries, and even then may not be economic.
    
    
12. Oceanographic experts confirm that Pacific oyster larvae could be carried up to 50 km up and down the coast from Port Stephens with possible distribution up to 200 km at certain times. Government experts say it cannot happen.