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

Aquaculture of Crassostrea gigas in France
Philippe Goulletquer and Maurice Héral

Introduction

Commercial oyster landings by French oyster farmers were a record 150,000 metric tons valued at $ 210 million in 1990, close to the production of the USA of 158,000 metric tons. Two species are cultivated in France, namely the native flat oyster, Ostrea edulis, and the introduced cupped oyster, Crassostrea gigas. The latter provides 92% of the landings, enough to satisfy the French demand. Less than 3% is exported to foreign countries. Oyster culture is concentrated in bays and estuaries, where it is often the most important activity in the coastal economy. More than 35,000 people are employed in the oyster industry, which occupies 50,000 acres of state leasing grounds. Production is dependent almost entirely upon natural spatfall, which is collected on various type of spat collectors. Hatchery production is limited to marginal markets (e.g. remote setting, triploid oyster, flat oyster spat).

Historical Background

Although oyster culture represents a large business in France, it suffered from overfishing in the past. More recently, diseases and pollutants caused large upheavals and the need for new management techniques to restore and sustain production. Beginning about 1860, spat of cupped oysters, Crassostrea angulata, were imported by French farmers from natural beds in Portugal to compensate for the lack of flat oyster spat collected on overfished beds. However, the introduction of C. angulata was mostly the result of an accident in 1868: due to a storm, a boat was forced to throw its load of supposedly dead Portuguese oysters into the Gironde estuary. Environmental conditions were favorable for reproduction in the southwest of the country up to the Loire River. After 1920, production increased when spat-collectors began to be used systematically. Production reached 85,000 metric tons of C. angulata and 2,000 metric tons of O. edulis in 1960 in spite of a massive mortality of the flat oyster during the 1920's and a lack of spat settlement of C. angulata in the 1930's. However, an increase in the mortality rate and a decline in growth rates were both reported in the main culture areas (Bay of Marennes-Oleron, Bay of Arcachon) in the early 1960's. From 1966 to 1969, the first outbreak of "Gill disease" reduced C. angulata production. Drastic mortalities from a second viral disease occurred between 1970 and 1973 caused the total disappearance of this species from the French coast. The crisis affected 5,000 oyster farmers and the economic loss was estimated to be at least $90 million a year. Therefore, an urgent solution was required to prevent unemployment.

Introduction of the Japanese Oyster Crassostrea gigas

Spat of C. gigas were first imported from Japan in 1966 by an oysterman looking for growth rate improvements at a time when the C. angulata production was reduced by overstocking. The non-official aspect of these imports and the concomitant increased mortality of C. angulata prompted the research institute MREMER to call for their ban. Investigations were conducted to evaluate the quality of the oyster beds in Japan with regards to pathology, fouling organisms, and oyster quality. Experts concluded that C. gigas appeared healthy and no relationship was established between the pathology observed in France, the mortality of C. angulata, and the status of oysters in Japan. The introduction of C. gigas was approved since the whole industry was on the verge of collapse. The introduction was performed through large imports in two stages aimed at (1) building sanctuaries and (2) supplying the oystermen with spat.

Sanctuaries - "Resur" Operation

After an evaluation of the sanitary quality of C. gigas beds in British Columbia (Canada), adult oysters were imported to constitute broodstocks. Histological control was used for each transfer to ensure the oyster quality. This operation lasted from 1971 to 1973. An additional batch was introduced in 1975 to the Bay of Marennes-Oleron. Populations were successfully established in three areas, all located south of La Rochelle.

Spat Supply

From 1971 to 1977, spat was imported from Japan to sustain the aquaculture activity. Each imported batch of spat was inspected and certified for origin, sanitation, and presence of predators. For disease, histological analyses were performed and an index of meat quality was established. Each batch was immersed in freshwater to reduce the risk of importing species such as the flatworm Pseudostylochus.

Reasons for the Success of the Introduction

The extensive introduction of C. gigas was so successful that production exceeded the record for C. angulata in less than ten years and is still increasing. There are several possible reasons for this rapid success:

Biological

C. gigas is resistant to any disease affecting C. angulata (viral diseases), O. edulis ( Marteilia and Bonamia), and clams ( Perkinsus).

Environmental conditions in the southwest of France favored C. gigas to such an extent that the spat supply is sufficient for the entire industry. However, C. gigas dominates only a few intertidal zones outside the breeding areas (e.g., Gironde estuary), which reduces the ecological cost of this introduction. The sanctuaries and the growth of the cultivated broodstock provided a spat supply as early as 1975. In the Bay of Marennes-Oleron, spat recruitment has been successful since the introduction, except that abnormally low temperatures were responsible in 1972, 1981 and 1986 for a low spatfall. Recently, the yearly production has been estimated to be 15 trillion spat in Marennes-Oleron and Arcachon bays.

No competition occurred between the Portuguese oyster C. angulata and C. gigas nor with other species. Extensive mortalities caused the disappearance of the C. angulata. With a low population (e.g. 15,000 tons of C. angulata and 8,000 tons of C. gigas in 1972), the hybridization observed experimentally between the two species was not a constraint to the growth of the C. gigas population.

The carrying capacity of the estuarine areas was large, allowing for optimal growth and reproduction of C. gigas. At that time, a total weight of 70g was reached in 18 months and the yield by aquaculture was maximal. In addition, oyster habitat was fully available for settlement because an almost monoculture of C. angulata had been practiced in these bays. Accordingly, the extension of the C. gigas population was facilitated. In spite of the relatively small number of introduced individual oysters that participated effectively in the building of the population, no inbreeding problem has been observed. Genetic variation is still high according to electrophoretic analyses.

Several exotic species that have accompanied C. gigas have survived in limited areas in spite of the practice of immersing spat collectors in freshwater. These are the cnidarian Aiptasia pulchella, the mollusc Anomia chinensis, and the cirripeds Balanus amphitrite and Balanus albicostatus. Two seaweeds are also associated with this introduction on the Mediterranean coast, namely Laminaria japonica and Undaria pinnatifida. Their limited biomass and distribution do not represent an ecological problem. Culture of Undaria pinnatifida is currently being assessed.

Human Component

The introduction schedule was well prepared and preliminary scientific investigations in Japan maximized benefits while limiting the associated risks. Many sanitary precautions were taken to control the introduction.

Because the C. angulata and C. gigas share many similarities (reproduction, recruitment, etc.) the aquaculture practices in France were directly applicable to C. gigas without drastic changes. Thus, the skills of oyster farmers were sufficient to handle the modification. Additional advice by IFREMER scientists to optimize recruitment (e.g. timing for spat-collector deployment) maximized the success of the operations.

Eventually, the new oyster benefited from an overall acceptance by consumers. They found a high quality product, with a specific taste per breeding area, similar to that for C. angulata. Oyster consumption has increased along with production.

Aquaculture of C. gigas

Currently, the aquaculture of the Japanese oyster is widely distributed along the French coastline, in estuarine areas, bays, and lagoons. This distribution, mainly extended by aquaculture activities, is well beyond what would have been a "natural" population.

Reproduction

Gametogenesis is initiated when temperature reaches 10°C, and ripening accelerates above 15°C. In southwest France increasing temperatures are accompanied by a concomitant salinity increase (25-35 ppt). Temperature is critical for the distribution of C. gigas on the Atlantic coast and the English Channel. In Brittany, no spawning occurs even though ripening is observed. In contrast, in the Mediterranean open sea and lagoons, high salinity (37 ppt) associated with high temperatures (22-30°C) impedes successful reproduction. Reproduction of C. gigas occurs only on the Atlantic coast in summer (July-August) when seawater temperature is above 18°C. Thus, spat production relies totally on breeding areas located South of La Rochelle; Bay of Marennes-Oleron, Gironde estuary and Bay of Arcachon.

Production of gonads and gametes has been estimated to comprise 7% of the body weight for yearling oysters, 60% for two year-olds and 80% for three-year olds. A single spawning occurs per summer and optimum spat settlement is observed at 21- 22°C after a 2-week pelagic larval stage.

Growth and Biology

In contrast to the conditions required for spat survival, C. gigas will grow in various environmental conditions. Oyster growth is observed in upper estuaries until salinity declines to 15 ppt (range of yearly variability 2-25 ppt). This euryhaline species is also deployed in hypersaline ponds (45-50 ppt); mortality occurs above 50 ppt. C. gigas displays high filtration activity, even at 5°C (4 liters per g dry meat weight from October to May and 5 liters from June to September). This is particularly important with regard to overstocking and ecosystem carrying capacity. Growth rate declines and higher mortality rates have been observed with increased stocking biomass (e.g., the market size that was reached in 18 months in 1972 now requires more than 4 years in the Marennes-Oleron Bay), resulting in new management techniques being required. Summer mortalities (20%) have been observed in intertidal areas when temperature peaks and are related to a physiological disorder during reproduction (i.e., glycogen deficit).

Special Problems

Contaminants. No spatfall occurred in Arcachon Bay between 1977 and 1981. At that time, experiments demonstrated the high toxicity of TBT acetate used in antifouling paints on veligers and phytoplankton (a 0.1 micrograms/L concentration affects larval development). Shell abnormalities were also observed on adults at 0.01 micrograms/L. The 1982 regulation restricting TBT use lead to a prompt spatfall recovery.

Anoxic Waters. In 1975 and 1986, severe dystrophic crises in the Thau Lagoon on the Mediterranean coast resulted in anaerobiosis and high oyster mortality.

Trophic Competition. Recently, increased populations of the gastropod Crepidula fornicata in specific areas have been responsible for disturbance of water circulation patterns (e.g., sedimentation) and trophic competition with the oyster. Annual management, based on population dynamics studies, limits the effect of this gastropod.

Deterioration of Shellfish Grounds. Oyster overstocking has induced sedimentation of large quantities of biodeposits. Other farming structures (e.g., iron tables to hold shellbags) have also affected water circulation and sedimentation patterns. So far, management practices and regulations (e.g., timing for structure deployment) have limited the effect on the ecosystem.

Literature Reviews

Anonyme, 1986. Evolution et état du cheptel ostréicole dans le bassin de Marennes-Oléron: intérét d'une régulation. IFREMER DRV 86-06-AQ/TREM., 35p.

Anonyme, 1987. Données de base sur la génétique de l'hultre plate Ostrea edulis et de I'huitre creuse Crassostrea gigas. Rapport final, IFREMER/DRV/RA, 120p.

Anonymous, 1991. Fisheries of the United States, 1990. NOAA Current Fisheries Statistics, N* 9000,112p.

Deslous-Paoli J.M., M. Héral, P. Goulletquer, 1987. Evolution saisonniére de la filtration de bivalves intertidaux dans des conditions naturelles. Oceanis, 13: 575-579.

FAO, 1991. Fisheries statistics in 1989, 68, 516p.

Grizel H. and M. Héral, 1991. Introduction into France of the Japanese oyster Crassostrea gigas. J. Const. int. Explor. Mer., 47:399-403.

HéraI M., 1990. Traditional oyster culture in France. In: Aquaculture; G. Barnabé edit., Ellis Harwood Limited, Chichester, 46p.

HéraI M. and A.C. Drinkward, 1990. Management of oyster and mussel culture. Report of the panel session held during "Aquaculture Europe 89", Bordeaux, France, p. 147-162.

HéraI M. and J.M. Deslous-Paoli, 1991. Oyster culture in European -countries. In: Estuarine and marine bivalve mollusc culture; W. Menzel edit., 59p.

Héral M, C. Alzieu, J.M. Deslous-Paoli, 1989. Effects of organotin compounds (TBT), used in antifouling paints, on cultivated marine molluscan populations: a literature study. In: Aquaculture, a biotechnology in progress. De Paw et al. edit., EAS, p.1081-1089.

Maurin C. and J. LeDantec, 1979. The Culture of C. gigas in France. In: "Exotic Species in Mariculture", Edit. R. Mann; Symposium on Exotic Species in Mariculture, Woods Hole Oceanographic Institution, 1978, p. 106-172.

Sornin J.M., 1981. Influence des installations conchylicoles sur l'hydrologie et sur la morphologie des fonds. Rev. trav. Inst. des Peches Marit., 45 (2):127-139.