Rock oyster aquaculture

Market and industry

The majority of oyster production occurs south of Hervey Bay, with most oyster areas in Moreton Bay. Seasonal risk of Marteiliosis disease (also known as QX) in south-east Queensland waters restricts which tidal oysters can be viably produced, and limits the growing season.

Site selection

Oyster farming is divided into 3 stages:

• spat collection and placement
• growing the spat to a mature or semi–mature stage (depending on market demand)
• maturing and fattening the oysters.

These 3 stages are interrelated, and each stage may require different conditions or a different location.

Before issuing a resource allocation authority, we assess oyster areas to determine any likely environmental impacts on oysters. If the site is inclined to be influenced by sewage, stormwater run-off, industrial pollutants or other influences that are considered detrimental, the authority will not be issued.

Site conditions

No single set of conditions can dictate the suitability of a site for oystering. Access to a suitable growing area may pre–determine the methods used for culture.

For example, sites in:

• estuarine areas often use racks and trays
• offshore areas with vast tidal flats are more suitable to BST longline systems.

Oyster banks should not be exposed to strong currents or wave action under normal tidal conditions or during floods. Areas that are prone to shifting sand or sediment can place additional stress on the oyster furniture, damage oysters, and make working these areas difficult.

A balance must be found between currents and tidal flows so oysters have suitable water and a nutrient–rich food source. The substrate oysters are placed in should be firm enough to walk over and work on, if possible.

Offshore areas can be challenging if they are not close to a suitable land-based facility if farmers need to travel long distances to place or maintain stock. Nearby access to markets is also crucial, particularly when moving live oysters for processing.

Maturing areas

Oysters are filter feeders, so selecting ideal maturing grounds is important to avoid possible contamination. The oyster grounds in the western part of Moreton Bay are the best areas for fattening oysters, particularly estuaries.

Estuary systems are ideal for producing the populations of phytoplankton that oysters predominantly feed on. Commercial oyster culture is managed under strict environmental guidelines designed to ensure that oysters are only grown in areas with appropriate water quality.

Collecting and importing oyster spat

The first step in oyster aquaculture is finding a good source of spat (oyster larvae). The successive steps of growing and maturing depend on the quantity and quality of the spat.

Importing spat

Queensland oyster farmers need to source supplies of stock that are both economical and contaminant–free. Queensland farms import spat from New South Wales for on-growing into Queensland waters. Small amounts of spat are still collected in Queensland, but the amount is small compared to spat sourced from New South Wales.

An oyster is considered spat up to the age of 12 months, so size can vary considerably. Many growers ask to see a sample of the stock before they buy.

Policy for the transshipment of oyster spat

Restrictions are in place to prevent importing the Pacific oyster (Crassostrea gigas) into Queensland. This introduced species competes with rock oysters for food and space.

Any import of oyster seed (spat) stock from interstate into Queensland waters must follow the policy for the transshipment of oysters:

• notify Fisheries Queensland when stock is arriving in Queensland
• keep each oyster consignment separate and identifiable
• comply with minimum periods for holding oysters from interstate before they can be sold for human consumption.

Locating spat

Known spat–catching areas in southern Queensland are:

• the Great Sandy Strait
• Pumicestone Passage
• leeward sides of Moreton
• North Stradbroke and South Stradbroke Islands.

Spatfall occurs throughout the year, but peaks in Moreton Bay between November and March.

Collecting spat

When you collect spat:

• choose a collection area where spat are already known to be present in the water
• develop a suitable substrate for the spat to settle on.

Stick placement is one of a variety of proven methods used for collecting spat. In this method, 6–8 sticks are nailed to cross–beams to form frames that are then grouped to form batches. The batches are placed in the mid to lower part of the tidal range, where the spat settle on the sticks.

The tide, wind and currents will influence where spat collection devices are placed. The artificial substrate must be attractive to the larvae to encourage them to settle and attach.

Relaying oysters

Relaying is the movement of shellfish from one oyster area to another for the purposes of natural cleansing (depuration).

The policy for the relaying of oysters applies to holders of aquaculture licences for oysters in Queensland:

• when and where relaying can take place
• minimum periods for relaying oysters before they can be sold for human consumption.

Culture and production systems

Stick cultivation

Stick cultivation keeps the sticks used for spat collection, removing excessive spat and leaving the remaining oysters to mature. The sticks used for spat collection are separated and laid out more extensively on the racks. Sticks are spaced 15–20cm apart and then fixed to the racks.

A disadvantage with this system is that if the oysters are not thinned out they will grow in clumps and produce smaller and irregular–shaped oysters. Moving the oysters from the sticks to an alternative method can help as the oysters tend to reach a larger and more regular size.

To avoid young oysters attaching to the crop (overspatting), place the sticks in the water column above or below the optimum levels for spat settlement. This can be determined by simple experimentation.

Tray cultivation: single seed

Tray culture uses wood, aluminium or plastic frames with bases of galvanised wire or plastic mesh. The mesh size is determined by the size of the oyster. The tops of the trays are usually covered with a wire mesh or netting, which you can remove to inspect the stock.

The tray is designed so that the height of the frame is equivalent to the height of one oyster only. This maximises the water flow to each oyster in the tray and reduces the likelihood of the oyster being flipped over.

As with the stick method, overspatting should be avoided by placing the trays in the water column above or below the optimum settlement region for spat.

BST adjustable longline: single seed

A line is tensioned between two anchoring posts with intermediate posts to keep it above the bottom. From this line, a series of PVC mesh bags are suspended with clips. You can quickly change the position of the line by adjusting the height of the intermediate posts. Bags can be removed or attached using stainless steel or plastic clips. Bags of any size can be made up to suit, depending on the size and quantity of oysters farmers wish to place in them.

Rack and basket system: single seed

Bags of PVC mesh with two sticks running lengthwise through either end are suspended between a rack structure. They look somewhat like a stretcher.

Sub–tidal system

Oysters are suspended in the water and remain below the water at all times. Various furniture may be used to accommodate the oysters, including trays suspended on buoys or pyramid–like structures with horizontal trays.

An advantage of this system is that the oysters are able to feed uninterrupted. However, as they are not pre–stressed to being out of the water, as with other methods, their shells tend to be thinner and care must be taken post–harvest to ensure the shells are not damaged, and the oysters remain alive and in sound condition.

Culling

Culling reduces the spatfall that may have accumulated on an oyster's shell and helps grade oysters into various sizes.

Regular inspections are necessary and the culling process can be time consuming and labour-intensive. Recent advances involve covering the oysters in a special coating that prevents spat and unwanted species from attaching to the oyster's surface.

The most common practice is immersing oysters in a hot water bath of 82°C for 3 seconds. This kills off all small spat that can’t tolerate high temperatures due to their smaller size and thin shell. The immersion method is easier on aquaculture farms that use the BST longline system, trays and sticks.

In many cases, it can be done on the water by placing a pre-heated immersion tank on the barge. Oysters are removed from the longline or racks, placed directly in the hot water, then returned to their original positions.

Harvesting

Oysters are generally large enough to market in 2–3 years.

The qualities that determine the grade of an oyster are size and condition of the meat. Rock oysters are marketed as 'bottle size' at 29–40g (whole weight) and as 'plate size' at 40–67g. Oysters less than 5cm are returned to banks or trays for further growth.

Bottled oysters are 'shucked' (opened) with a knife and the body of the oyster removed by cutting the muscles joining it to the shell. The oysters are rinsed in fresh water and are bottled in clean fresh water with salt added.

Plate oysters are sold either in full or half shell. Shells are cleaned of silt and any surface growth prior to sale using a scrubbing brush or some form of tumbler.

Predators, pests and diseases

Both spat and mature oysters can fall prey to predators and pests. Modifying farming techniques and implementing control measures can significantly reduce losses.

Mudworm

The mudworm (Polydora sp.) is a segmented worm that infects oysters grown on, or near, the ground.

Mudworms do not directly harm oysters, but they compete with the oyster for food. Infected oysters not only endure reduced food but they spend a large amount of energy secreting shell to isolate the invader, which gives them less energy to grow. The major problem with infected oysters is that their market value drops because of unsightly brown blisters.

To prevent infestation with mudworm, oysters are placed above the mid–tide level. Although this reduces feeding time for the oysters, it also reduces the chance of mudworm larvae entering them.

Oyster drills

Drills attack oysters by boring a hole in the oyster's shell using their hard radular (or teeth). Once the drill reaches the soft tissue of the oyster it secretes digestive enzymes into the oyster, killing it. The drill then digests the oyster meat, sucking up the digested fluids.

Drills can be controlled for a prolonged time by dunking oysters in freshwater, or for a shorter period by dunking oysters in hot water. Alternatively, drills can be separated from oysters by drying the oysters out and placing them on 3cm mesh where the drills will fall through.

Barnacles and mussels

Barnacles and mussels stop oyster spatfall by taking over the spat collecting areas first, where they compete with the oysters for food and space. Avoid barnacles by placing the spat catchers out at the right time and height, to keep the catchers away from main barnacle and mussel spatfalls.

Mussels are restricted to the subtidal zone and low in the intertidal range but barnacles usually occur over the full intertidal range, and so are more of a problem. Once these pests are established, the only option is physical removal of the barnacles and mussels by hand picking or the use of a culling iron. However, care must be taken not to damage the young oysters during this procedure.

QX disease

QX oyster disease is caused by the microscopic parasite Marteilia sydneyi. This spore-like parasite invades the oyster's digestive system and multiplies in the digestive gland or 'liver'. Spores become so numerous that the oyster cannot absorb food and can starve to death within 40 days of infection.

Oyster fatality in an infected bank can be as high as 95%. Once infected, oysters cannot be cured of the disease. Although QX is usually fatal to oysters, there is absolutely no evidence it is harmful to consumers.

A sign of QX infection is a mass mortality of oysters. A thorough and correct diagnosis can only be obtained by microscopic examination of the digestive gland for the detection of QX spores.

Samples of oysters suspected of QX must be sent away for examination.

QX is known to occur in patches from southern Queensland to the Georges River in southern New South Wales. The parasite commonly occurs in the western areas of Moreton Bay, including Pumicestone Passage and the Southport Broadwater.

It is still possible to farm oysters in QX–susceptible areas within certain constraints. The risk of infection is highest in the summer months (from December to March). Many operators in Moreton Bay import large spat or bottle–size oysters from disease-free areas in central New South Wales and put these out on trays around April. The oysters are then harvested as bottle–size or plate–size by Christmas when at optimum condition and market demand is high. This leaves the summer months free for equipment maintenance.