Pacific oyster aquaculture in South Australia
The first attempts to cultivate oysters in South Australia were those by Chief Inspector Randall
in Proper Bay from 1910 to 1912 (Grove-Jones 1986). These attempts it seems, were
instigated by poor catch rates of the native, mud or flat oyster, Ostrea angasi and an attempt to
re-stock natural populations.
Ostrea angasi stocks had been commercial exploited in Coffin Bay and surrounds since
the1860′s. This lead to the Bay being closed in 1885 to assist recovery of the species. In 1910
Chief Inspector Randall attempted to collect spat and on-grow the native oyster although death
rates in the spat were very high and further development was hindered (Grove-Jones 1986).
Native oyster farming was attempted again in the 1960′s, however this too failed due to poor
spat survival. At this time it was decided that a hardier species such as the Pacific oyster,
Crassostrea gigas, which was being farmed in Tasmania, was required.
The Pacific oyster is a native species of Japan which was first introduced to Tasmania in the
1940′s. By the early 1960′s Pacific oysters were being farmed in Tasmania and by 1969 the
first shipment of Pacific oysters arrived in SA from Tasmania.
Present day Pacific oyster farming activities are based in five major areas in South Australia;
Murat Bay, Smoky Bay, Streaky Bay, Coffin Bay and Franklin Harbour, as well as on the
eastern side of Yorke Peninsula and the north- eastern side of Kangaroo Island.
Today, the oyster farming industry is a major economic contributor of seafood in South
Australia. In 1996/97 $5.8 million of Pacific oysters were produced in South Australia, an
increase of approximately 48% from 1995/96 values. Demand for oysters currently outstrips
supply, and by the year 2000 the South Australian industry expects to be producing 3.5 milliondozen oysters a year, worth around $12.5 million.
Life history
Pacific oysters are protandrous hermaphrodites, that is, they begin life as males before
becoming functional females. They have a simple reproductive system consisting of gonads
(the creamy-white area of the oyster) which hold the gametes.
Pacific oysters begin their life as a fertilised egg, formed by the union of gametes which are
released by sexually mature oysters and are distributed by ocean currents. Spawning (release
of gametes) is induced by a rise in water temperature (December in South Australia).
Hatching of the fertilised egg occurs approximately 24 hours after fertilisation and results in the
formation of a free-floating larvae. After approximately 3 weeks in this free-floating phase the
larvae enter the fixed stage of their life-cycle known as the “spat” or juvenile stage (see below).
In order for larvae to develop into oysters they must attach themselves to a suitable substrate
and undergo metamorphosis into spat.
Quick Facts
Pacific Oysters will grow in temperatures above 10oC although they prefer temperatures
between 15 to 18oC. Pacific oysters will also survive between salinities of 14 to 15 ppt although
the salinity for optimal growth is between 25 to 32 ppt.
Filter feeders
Oysters are filter feeders, they consume a variety of species of phytoplankton (or microalgae),
bacteria, detritus and viruses from the surrounding water. Adult oysters (80 to 100mm in length)
may filter up to 10L of water per hour. Oysters prefer golden-brown microalgae and diatoms
because of their size, digestibility and nutritional value.
The Pacific oyster’s life-cycle
1=Embryo 2=Embryo division 3=Free swimming
veliger 4=D-shaped veliger 5=Umbo-veliger 6=Spat
(juvenile oyster) 7=Adult oyster.
Pacific oyster culture
Hatchery
Hatcheries are land-based facilities which serve two functions:
• spawn sexually mature oysters; and
• culture larvae and spat until they reach a size suitable for on-growing on a intertidal or
subtidal nursery site.
Pacific oyster larvae do not survive in sufficient numbers to use as spat because of the high
salinity levels (sometimes above 40ppt) which occur in most oyster growing parts of the State.
Consequently the main benefit of a hatchery is so that farmers do not have to rely on wild spatfall
for stock.
Hatcheries are typically operated by personnel who have knowledge and skills in the areas of
spawning, larval rearing, larval settlement and microalgae culture.
Preferred techniques and equipment vary widely between hatcheries. The following techniques
are examples of how one oyster hatchery may operate.
The anatomy of an oyster
Broodstock
Broodstock is the term used to describe sexually mature oysters which are spawned in
hatcheries. Broodstock are usually selected on the physical characteristics of the oyster. For
example a good shaped cupped-shell and high meat to shell ratio. Selecting broodstock on this
basis will ensure that these traits will be passed on to the oysters progeny.
Spawning
Pacific oysters spawn naturally in the wild, however in a hatchery oysters are induced to spawn
in a variety of ways. One of the most effective and commonly used methods of spawning is by
temperature cycling.
Temperature cycling involves placing sexually mature oysters in water at a certain temperature
then slowly increasing the temperature of the water over a period of time. An example of this
would be oysters which are exposed to 20oC water with a slow increase of 1oC every 5 minutes
until the water reaches 28oC. In addition to temperature cycling, individual hatcheries have their
own methods of inducing spawning. These methods include water cycling and the presence of
gametes or microalgae in the water.
Once spawning is induced and gametes are released into the water they are mixed together to
fertilise. Once fertilised the eggs are then placed into hatching tanks at approximately 80 to 100
eggs /ml water.
After approximately 4 hours the eggs have hatched and develop into shelled larvae which move
around inside the larval rearing tank in search for food. This stage of the oysters life cycle lasts
for 2 to 3 weeks during which time the hatchery operator must feed the larvae microalgae each
day.
Oyster larvae have a voracious appetite therefore the hatchery must culture their own
microalgae. This is a labour intensive procedure which requires the hatchery to grow large
volumes (up to 500L bag cultures) of high quality microalgae from small pure-strain samples. At
various stages of it’s life cycle the oyster prefers different species of microalgae so the hatchery
may have a number of different microalgal bag cultures growing at once.
Settlement
Settlement refers to the process whereby oyster larvae attach to a suitable substrate (usually
where there is lots of food). This allows the larvae to undergo metamorphosis into a juvenile
oyster (spat). Settlement occurs 1 to 3 weeks after hatching. While in the larval rearing tank the
hatchery operator must check frequently for larvae settlement. As soon as some larvae begin
to settle they are placed in special tanks which contain cultch; crushed shells to which the
larvae can attach.
Once all the larvae have all settled, they are sieved out of the tanks and transferred to an indoor
nursery tank system. Here they are fed high quality microalgae until they reach 3 to 15mm in
length. Once the spat reach this size they can be sold to farmers for on-growing in intertidal or
subtidal sites.
At present South Australian oyster growers can obtain their Pacific oyster spat from the South
Australian Oyster Hatchery at Louth Bay, near Port Lincoln, or from hatcheries in Tasmania.
Grow-out
Techniques and equipment
There are a number of different methods used for intertidal oyster culture in South Australia.
These methods have been developed to best suit the physical factors of the site as well as the
Pacific oyster aquaculture in South Australia 4
size of oysters transferred on to the farm.
a) Intertidal racks and baskets
The rack and basket method of oyster culture has been used since the inception of oyster
farming in SA.
Rectangular shaped baskets, handmade from tough polypropylene mesh, are secured to the
racks by two wooden sticks (approximately 1m length). The oyster racks are made of wooden
posts (uprights) which are drilled vertically into the seabed and act as the main supporting
structure. Attached to these are more wooden posts which run parallel to the seabed and hold
up the baskets.
Farmers position the posts and oyster baskets according to the water depth and tides
experienced at the farm site.
b) Intertidal racks and trays
Spat, straight from a hatchery need greater protection than adult oysters when on the farm site.
For this reason trays are generally used for spat smaller than 10mm. Trays allow good water
exchange but also protect them from excessive currents which can cause the spat to fall out.
An oyster tray used to hold oyster spat. They are made of wooden frames with mesh covering
secured to racks. They are usually separated into compartments to prevent oysters “clumping”
up at one end.
c) Intertidal Long-lines
In some areas of South Australia intertidal longlines and bags are used to on-grow oysters. The
bags are made of polypropylene mesh; cylindrical in shape, 90cm length x 15cm diameter, and
hung on a wire suspended by wooden uprights. The benefit of this system is that the height of
the wire can be adjusted so the oyster bags can be lowered or raised depending on the tide.
The intertidal bag method. Similar to baskets, these bags are made of various mesh sizes
according to the size of the oysters. The bags are designed to hang on a wire and move with
the water currents. This rumbles the oysters and removes excess shell growth and improves
meat to shell ratio.
d) Subtidal Long-lines
In some parts of SA, farmers have access to sub-tidal sites where sub-surface or surface
longlines are used to suspend a series of stacked trays (approximately 1.5 x 1.5m). These
longlines are moored to the sea-bed and supported by buoys.
Stocking densities
The number of oysters per basket (stocking density) can have a major impact on the growth of
oysters. Although immobile, oysters compete for space and food and if stocking densities are
too high or too low it can affect the rate of growth and shell shape of the stock. Appropriate
stocking densities are an individual matter; the farmer evaluates the best densities according to
growth of the oyster, the production system being used and environmental conditions of the
area.
Grading
Grading of oysters is a daily occurrence on oyster farms during the production season
(approximately March to December). Oysters of similar size must be kept together to ensure
that larger oysters do not out-compete smaller oysters for space and food. In addition, grading
Pacific oyster aquaculture in South Australia 5
and sorting oysters assists the farmer to keep stock movement records and coordinate
harvesting. A batch of oysters may be graded every 6 to 8 weeks during their life-time.
Harvest and post-harvest handling
South Australian oysters are known for their creamy texture, fresh sea smell, pearly white inner
shell and their “frilled” exterior. Together with a high meat to shell ratio, presentation of SA
Pacific oysters is highly appealing. In order to maintain this image oyster growers must not only
care for their product while on the farm but also from the time when the product leaves the farm
until it reaches the consumer. This involves a whole series of steps which oyster growers in SA
have documented as their Industry Code of Practice.
Harvesting for market occurs prior to spawning when the oysters are in peak condition (in SA
this is in December when water temperatures rise). Oysters are brought in from the farm and
graded into various shell sizes including bistro (50-60mm), plate (60-70mm), standard (70-
85mm), large (85-10mm) and jumbo (100+mm).
The oysters are then packed into hessian bags (45 dozen oysters per bag for 50-60mm oysters)
and transported in refrigerated trucks directly to the wholesaler or retailer. Delivery of the
oysters must be within 24 hours of harvest. Oysters stored at 5oC has a maximum shelf-life of
10 days.
Quality assurance
Water quality
The oyster industry are actively involved in a water quality assessment program which aims to
provide public health protection for consumers of SA shellfish. This program, called the South
Australian Shellfish Quality Assurance Program (SASQAP) meets standards set down by the
United States Food and Drug Administration, the benefits of this being the potential for export to
otherwise restrictive overseas markets.
In addition to SASQAP, the industry together with SARDI Aquatic Sciences, are undertaking an
environmental monitoring program to evaluate and monitor the area surrounding oyster farms in
South Australia.
http://www.pir.sa.gov.au/__data/assets/pdf_file/0004/33979/oyster.pdf
