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

Ecological and Biological Considerations for the Introduced Pacific Oyster to the West Coast of the United States
Kenneth K. Chew

Introduction

Recent statistics from the U.S. Department of Commerce indicate that the 1990 oyster industry on the west coast (5 million kg. of meat) now ranks as the largest in comparison to those in other coastal regions of the United States. This is due primarily to the production of the Pacific oyster ( Crassostrea gigas) which was originally introduced from Japan.

At the turn of the century the main oyster fishing on the west coast was for the Olympia or native oyster ( Ostrea lurida). With the decline of the native oyster industry, the east coast or American oyster ( Crassostrea virginica) was introduced for cultivation primarily in the State of Washington and parts of British Columbia. Survival was not good for this species and ultimately the Pacific oyster was introduced and became the mainstay of the present west coast oyster industry. There are several published accounts of this introduction (Schaefer 1938; Quayle 1988; Chew 1984,1990). Although experimental introductions were positive in terms of adaptation to the oyster growing areas of Washington State, it was not until the 1920's that large quantities of Pacific oyster seed were introduced from Japan to the Pacific coast. This oyster has adapted well to a wide range of environmental conditions and is probably the most globally widespread and ubiquitous oyster species in the world (Chew 1990).

Brief History

The spread of the oyster throughout the north of New Zealand has come about as a result of farming practices adopted for the native Rock oyster, Saccostrea glomerata, and initiated in 1967. Farmers relied on natural spatfall and set out cuItch in one or two main spat collecting areas during the summer. They then shifted the spatted cultch to growing areas. As Pacific oysters began to settle in increasing numbers on spat collectors in the 1970's, they were effectively distributed to all other areas along with the Rock oyster spat. The Pacific oyster began to gain ascendancy over the native oyster from 1975 onwards and farmers found it impossible to grow the two species together or selectively. As a result, the Pacific oyster became the dominant farmed oyster from 1977 on.

Reproduction and Spawning

It is apparent that the Pacific oyster is not only the most adaptable oyster to a wide range of hydrographic and environmental conditions, but it is a very fecund species of bivalve. During the breeding season the reproductive organs may form at least 50% of the body volume (Quayle 1988). The sexes of this species are separate and can only be determined by examination of reproductive tissues. Although there are indications that protangery does exist, environmental conditions apparently have considerable influence on determination of sex in the oyster. There appears to be a tendency for females to change to males where and when the food supply is poor, and for males to become females where and when supply of food is good. Thus, in areas with good food supply the sex ratio in older oysters shows a predominance of females, whereas the reverse is true in areas of low food supply.

Conditioning for spawning of Pacific oysters has been well established in hatcheries along the Pacific coast. Generally oysters can be brought into the hatchery during late winter or early spring and conditioned for varying lengths of time with gradual increases in temperatures and adequate food. Spawning can be generally initiated at an average of 21-22°C, although in some cases spawning can be initiated below this temperature. Quayle (1988) has observed natural spawning at temperatures as low as 15°C, but this may have been a result of chemical stimulation. Initiation of spawning may be accomplished not only by increased temperature, but also by chemical stimulation, or a combination of both. The presence of sex products is enough to stimulate spawning, making it possible to force large numbers of oysters to spawn simultaneously.

Although the average optimal salinity range for spawning seems to be between 25 and 3 ppt, Japan has reported that the salinity range for development (breeding) is between 11 and 32 ppt, with the optimum between 20 and 25 ppt. Various attempts have been made to cross Crassostrea gigas of the Miyagi variety from northern Japan (the normal oyster cultured along the Pacific coast of the United States) with other species of Crassostrea, including C. virginica, C. rivulatis, and C. gigas of the Kumamoto variety. Crosses on the west coast between C. virginica and C. gigas have only been moderately successful, and much is still not understood concerning the genetic makeup between these two species. However, crosses between the Pacific oyster and C. rivularisshow promise and more experiments are being conducted at the University of Washington School of Fisheries, Experimental Shellfish Hatchery by Sandra Downing. It appears from her work that a cross of a C. gigas (female) by C. rivularis (male) is the most successful. Crosses between the two varieties of C. gigas (Miyagi and Kumamoto) seem to attain moderate to good success.

Commercial hatcheries are breeding Kumamoto varieties because they are highly desired for of their deep cup morphology. Although the Miyagi variety is the main hatchery oyster used by west coast oyster growers, experimental plantings from hatchery stocks of C. rivularis and C. gigas of the Kumamoto variety are being made on a limited scale. Both of these latter species have positive attributes for market. It should be noted that for unknown reasons (although pollution is suspect), Kumamoto stocks from the southern island of Kyushu in Japan have essentially disappeared and many Japanese oyster growers have switched to nori seaweed culture.

Competition with the Native Oyster - Ostrea lurida

As indicated above, the Pacific oyster was introduced in the early part of the century to augment the declining native oyster industry, especially in the State of Washington. Since its introduction, the Pacific oysters has essentially taken over most intertidal areas that have potential for growing oysters. There are a few native oyster beds available in southern Puget Sound, but their production is very limited.

Relative to the question of competition with the native oyster, the Pacific oyster has essentially dominated. It has been postulated that there is a lack of adequate food to sustain native oysters or that there may be some physiological effect, namely, metabolic competition maybe taking place. This has been proven, although in Willapa Bay there has been in most years a good natural set of native oysters on Pacific oyster shells. However, once they grow to about 1 cm, the native oysters died. At that time it was thought that C. gigas' metabolites, or feces and pseudofeces, were affecting survivorship of the native oysters, or that there were problems related to available nutrients from phytoplankton that had been greatly reduced by the more intensive feeding and filtering of the Pacific oyster. This has never been proven. There are some small reefs of native oysters still available in isolated locations in southern Puget Sound as well as Willapa Bay in the state of Washington.

Disease

Generally the Pacific oyster is hardy. Up until the late 1960's, the Pacific oyster had very low mortalities compared to C. virginica stocks in the Gulf or mid-Atlantic coasts. I ran tests documenting yearling single oysters from a common stock of oysters distributed in trays at California's Humboldt Bay and Oregon's Yaquina Bay from the State of Washington in the 1960's. Less than 10% of these yearling Pacific oysters placed in various experimental situations succumbed during two years of monitoring. Mortalities occurred in British Columbia (Quayle 1988) in 1960 when a disease outbreak lead to loss of 30% of the Pacific oysters. This was referred to as the Denman Island disease, in which pustules on the oyster meat were characteristic. The disease developed in early spring during April when temperatures were just beginning to rise to 9°C from the winter low of about 7°C. The disease apparently disappeared by mid-July when water temperatures had reached 18°C. This cyclical disease existed for only a couple of years in British Columbia, before subsiding with no major mortalities noted since. The causative organism was never identified.

There was a major mortality of Pacific oysters in the State of Washington beginning in the mid 1960's through the early 1970's during the late summer months. As much as 60-80% of some groups of oysters growing into their second summer were dying on specific shellfish beds. This mortality was usually associated with a bed that had warmer temperatures than normal, as well as poor circulation. After extensive sampling and histological studies, no disease organism was ever related to this major mortality which occurred in parts of various bays in Washington as well as Humboldt Bay in California. Later research by the University of Washington tended to indicate that perhaps these mortalities related to physiological stress. Experiments by Perdue et al. (1981) indicated that when the mortalities occurred, the oysters were usually fully ripe. Sampling of the tissues of those families that had the highest mortalities revealed virtually no glycogen as an energy reserve. There was speculation that this lack of glycogen was a consequence of the high fecundity of C. gigas, accomplished at the expense of stored energy necessary to keep the individual oysters alive. The mortality of Pacific oysters along the west coast declined greatly by the mid to late 1970's.

Predation and Parasitism

Unfortunately, two major predators were introduced with Japanese oyster seed over the years. These are the Japanese oyster drill ( Ocenebra japonica) and the turbellarian flatworm ( Pseudostylochus ostreophagus). These two species have become well adapted in various oyster growing bays in the State of Washington as well as Humboldt Bay in California Further, the flatworm, which attacks primarily young spat, is a major problem now in southern Puget Sound.

Relative to predation by indigenous organisms, these might be as follows: (1) Red rock crab ( Cancer productus). This crab is quite common in the states of Washington and California where it consumes large numbers of oysters. (2) Cancer oregonensis. This small crab has been identified as a serious predator on juvenile oysters under 45 cm. (3) Starfish. Several species of starfish attack oysters, but usually on those grown low in the intertidal zone. (4) California bat stingray (Holorhynus californicus). This stingray has been linked to consuming oysters growing in the intertidal zone. (5) Birds. It is generally understood that ducks, particularly the lesser scaup, or bluebill ( Aythya affinis) and the surf and white winged scoters ( Mellanita) frequent oyster beds in large numbers in winter. At times a considerable portion of the birds' diet may be made up of clams, mussels, and in some cases young Pacific oysters up to 2 cm in diameter.

Success in Aquaculture

It is no secret that oyster culture for the west coast of the United States is very important. Of all the oyster meat produced from the Pacific coast, it would be safe to estimate that over 98% of its production is for C. gigas. Because of innovations on remote setting of eyed larvae, dependence on natural catches of seed is now minimal. Almost all of the oysters that are being cultivated on the west coast come from hatcheries and unquestionably this has helped to fuel the success of West Coast oyster culture. This process of remote setting is continually being updated and refined and it is generally understood that many countries and the east coast of the United States are looking for this avenue of producing hatchery eyed larvae to supplement the supply of seed.

Literature Cited

Chew, K.K. 1984. Recent advances in the cultivation of molluscs in the Pacific United States and Canada. Aquaculture 39:69-81.

Chew, K.K. 1990. Global bivalve shellfish introductions: Implication for sustaining a fishery or strong potential for economic gain? World Aquaculture 21(3): 9-22.

Perdue, J., J.H. Beattie, and K.K. Chew. 1981. Some relationships between the gametogenic cycle and the summer mortality phenomenon in the Pacific oyster ( Crassostrea gigas) in Washington State. Jr. Shellfish Res. 1 (1):9-16.

Quayle, D.B. 1988. Pacific oyster culture in British Columbia. Canadian Bull. of Fish. and Aquatic Sciences No. 218:241 pp.

Schaefer, M.B. 1938. Preliminary observations on the reproduction of the Japanese common oyster, Ostrea gigas, in Quilcene Bay, Washington. Wash. Dept. Fish., Biol. Rept. No. 365:36 pp.