The Ecology of Crassostrea gigas
in
Australia, New Zealand, France and Washington State
Introduced Pacific Oysters in New Zealand
Parameswar (Mani) Dinamani
Introduction
The Pacific Oyster, Crassostrea gigas, was introduced accidentally into New Zealand and was first discovered and positively identified in 1971. Its origin, method and time of arrival are not clear but it is probable that (a) the oyster may have been in New Zealand for a number of years before it was first discovered and (b) its introduction(s) may have been facilitated through a series of events from 1950 onwards.
In brief, the New Zealand stock of Pacific oyster probably came mainly through spawnings larvae/spat of:
- oysters on ships' hulls in Auckland Harbour, particularly a few
Japanese vessels that were moored for long periods in 1966/67, close to
the main spatting area of native oysters on the East Coast;
- introduced Pacific oyster stock breeding in the 1950's in
Australia, with the larvae drifting across the Tasman Sea to the West
coast of northern New Zealand;
- oysters on hulls of a large number of Korean and Japanese squid boats that fished the northwestern waters of New Zealand for long periods during the southern summer (late 1960's -early 1970's).
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.
Competition with the Native Species
Pacific oysters bred successfully in New Zealand waters over the same time period as the Rock oyster and they competed vigorously for settlement space: in 1972 the ratio of Rock oyster to Pacific oyster on a standard spat collector was 1000 to 1; by 1977 it was 1:1 and in 1978 it was 1:4 in favour of the Pacific oyster in the main spat catching area of Mahurangi. Pacific oysters grew to marketable size (~100g) in 15 to 18 months, compared with 36 months for the Rock oyster (~70g) on the same substrate. Large Pacific oysters outgrew the native on cultch surfaces and in many farms oversettlement of Pacific oyster spat smothered the smaller native oyster. As a result of its faster growth, greater size, and regular and ample spatfalls, the Pacific oyster superseded the Rock oyster within a decade of its arrival in the farms.
Environmental Requirements
In major growing areas, mean sea temperature ranges from 14 to 22°C and salinity is nearly oceanic at 33 to 35 ppt. In smaller inlets and upper reaches of bays and in large west coast harbors salinity fluctuates from 22 to 35 ppt depending upon the tidal state and incidence of freshets and rain.
Pacific oysters settle below the zone of Rock oysters in the intertidal area of rocky shores; denser settlement occurs along the lower parts of the shore in less saline waters, where dense beds form on level or gently sloping shores. As a result of oversettlement the oysters grow in clumps, die during hot spells and are easily detached in rough weather. Heavy build-up of silt occurs along the foreshore in these areas. Pacific oysters also selectively settle on upper reaches of inlets where waters are less saline (20 to 22 ppt).
Pacific oysters grow well in subtidal or suspended cultures in nearly oceanic water. Larvae and spat are optimally reared in the hatchery at 22 to 26°C and salinity of 33 to 35 ppt.
Reproduction and Spawning
Pacific oysters ripen and spawn over the same period as the native Rock oyster during the southern summer: the gonad begins to proliferate late in August when water temperature is about 14°C and some oysters spawn in September/October. The first major spawnings occur in December when water temperature rises to above 20°C; several spawnings occur throughout the summer months at temperature peaks of 22 to 24°C. Spawnings may extend to April in some years. A small percentage of oysters appear to be active throughout the breeding season.
Oysters 9 to 10 months old are fully ripe, and spawn as males; the percentage of females rises appreciably in 1+ year-old oysters, and 2 to 3-year-old oysters are predominantly female. The gonad passes through an inactive/resting phase from April through July/August in most oysters. The larval period lasts for 21 to 24 days at mean water temperature of 20°C during December/February, but may last up to 30 days later in the season. There are normally two major spatfalls in one season.
Pacific oysters established themselves successfully throughout the 1970's with large spatfalls, in the main spatting area followed by invasion of several inlets and small bays. Spatfalls began to decline from 1982 onwards, leading to near failure in 1983 and scant falls through to 1985. The causes of this decline were not clear, as successful breeding and settlement continued to occur in other areas, particularly in less saline bays and upper reaches of inlets.
Genetics and Interbreeding
Odd looking oysters were detected and reported by farmers from 1974 through 1977 and were suspected to be hybrids. Experiments were carried out to test whether the Rock and Pacific oysters would hybridize. Gametic compatibility was observed in many matings, and when eggs of Rock oyster were fertilized with sperm of Pacific oyster, a higher percentage was fertilized and completed larval development. Several batches of larvae were raised; some were raised through settlement and reared as spat. However, genetic tests carried out on a small sample a year later-failed to establish that they were -true hybrids.
When spatfalls declined, analyses were carried out in the New Zealand population to examine whether there has been a reduction of genetic variation as a result of inbreeding or directional selection from a possibly small founder stock The results showed that the New Zealand population has the same type and range of variation as the Pacific oyster populations in Japan. Also, genetic differences were found between oysters on the west and east coasts of New Zealand.
An important finding was the presence of two rare alleles in New Zealand Pacific oysters which have been recorded previously only in the Rock oyster. This suggests that some hybridization has occurred in nature, with the introgression of S. glomerata alleles into C. gigas during the history of the evolution of the species in New Zealand.
Diseases
No known cases of seasonal or sporadic mortality have occurred during the decade that the Pacific oyster has been actively farmed in New Zealand. Disease surveillance programs carried out since 1979 showed the presence of several associated organisms such as copepods, nematodes, ciliates and polychaetes in the mantle cavity or shell, but no pathogen or disease causing organism was detected. Spionid polychaetes (mudworm) are particularly of concern to farmers and occur in oysters at low tidal levels and cultures close to the bottom. They render the oyster difficult to market, and in large numbers cause shell blisters and stress to the oyster but infestations can be avoided by good farm management.
In the last year or so, two incidents of disease have come to light. In one, investigations of mortalities in hatchery-reared larvae in 1991 showed the presence of iridiovirus in the velum of settling larvae similar to OVVD type recorded in C. gigas larvae in Washington State by Elston. In the other, extremely emaciated oysters growing on the west coast in dense clumps on the shore were discovered to be showing symptoms of nocardiosis, similar to "focal necrosis" or "multiple abscesses described elsewhere. A disease watch is being kept up by the Fisheries Research Division with regular sampling and examination.
