Blue mussel - aquaculture prospects

Background

Mussel farming is an international aquaculture industry established in many parts of the world, including Scandinavia, Spain, New Zealand, China and Canada.

In Australia, mussel farming is a relatively new venture undertaken in embayments of the southern states. A number of species are cultured around the world, the blue mussel (Mytilus galloprovincialis) is the only marine mussel species farmed in Australia. Although the blue mussel in Australia (Mytilus galloprovincialis) is similar to and share the same scientific name with the one from southern Europe, it is native to Australia and has been found in ancient Aboriginal middens.

This information examines the potential for mussel farming in New South Wales and some factors to be considered when selecting a site.

FARMING METHODS

There are two commonly used mussel-farming systems used successfully for mussel farming, namely

Longlines
The longline system consists of two suspended horizontal headlines made of 12-15 cm polypropylene rope about 100m-220m in length and anchored at both ends. Single headlines are sometimes preferred as the gear required to lift and harvest a single line is far more economical and less labour intensive. The headlines are connected to large floats (usually of plastic, each with about 300kg displacement), which together support a large number of 5m culture droppers (ropes). The number of droppers used varies with hydrological and biological conditions but is normally in the order of 400-500 per (double headline) longline.

Secure anchoring is necessary to prevent the longlines shifting in heavy seas or strong currents. In soft bottom sediments traditional concrete or steel anchors of 250-500kg are used. Depending upon the amount of movement encountered one or two anchors are placed at the end of headlines. At exposed sites a mid-anchor may be needed to control movement and stretch; screw anchors may also be used to maintain the position of the longline.

On large mussel farms reseeding and harvesting are becoming increasingly automated. Continuous seeding of entire loops of culture rope over 200m long - rather than 5m lengths - is now common. Harvesting has also become mechanised; the one machine lifts and strips the ropes, washes the mussels, separates and packs them into bags.
Raft Culture
Raft culture is similar in many respects to longline culture, except that the raft commonly swings on a single mooring. As with longline culture, ropes are suspended from the raft. This technique has far less scope for mechanisation; most harvesting is done by hand and the operation generally smaller in scale than longlining. However, the raft does provide a valuable depot structure.

In New South Wales, longlines are preferred over rafts as they are less obtrusive and may be sunk to further reduce aesthetic objections. The aquaculture area can then be shared with recreational fishing and pleasure boats.

Selecting a site

Although mussels can be grown in full oceanic conditions, embayments are generally more productive because they have higher nutrient levels, higher water temperatures and offer a more sheltered location. The selection of a suitable site is crucial to the success or failure of a mussel farm and must be based on biological and economic factors, namely:

marketing
availability of mussel spat
predation and fouling
water user conflict
shelter
water quality
nutrient supply and water currents

These factors are discussed below to show how they may affect the success of a mussel farm. To evaluate a site properly you may need to do growth trials over several seasons.

Marketing

The production of mussels in Australia and the importation of mussels from New Zealand is anticipated to increase over the next few years. It is interesting to note that New Zealand greenlip mussels currently make a significant proportion of the Sydney market. Because of the limited market size in Australia prices cannot be expected to rise dramatically. The wide price range reflects the varied marketing strategies of the producers. Higher prices are received when a product of high quality is produced for niche markets, such as exclusive restaurants. This strategy avoids the commission of wholesalers but often requires more labour in presentation and grading the mussels. Increased sale prices are often offset by high production costs. An alternative to domestic sales is to consider exporting mussels but not before carefully analysing the market and securing Department of Agriculture and Water Resources requirements. For current price & production figures see the NSW DPI aquaculture production reports.

Availability of mussel spat

If good quality spat can be collected locally it will be sure to suit the growing site. The quality of spat can only be determined through trial and error. Often there are surprising differences in growth rate and survival between different batches. In New South Wales, Twofold Bay is the only known reliable area for spat settlement, although episodic settlement has occurred as far north as Port Stephens.

Importing mussel spat from other States or even other embayments within New South Wales is strongly discouraged because of the possibility of importing toxic dinoflagellates (free-swimming microscopic organisms). Check with NSW DPI if you are considering importing spat as a permit is required for any interstate or inter-estuary movement of spat.

Predators and fouling

The mussel farmer can expect a proportion of the stock to be lost through predation by marine life and theft by humans. Fish are the major predators, as they are particularly fond of newly settled spat. In addition, the longlines create a floating reef habitat that attracts a variety of fish species.

The seasonal occurrence of macroalgal blooms can cause problems for mussel farmers. During these fouling events regular inspection and maintenance of the farm is the best form of action. Another problem is the settlement and growth of encrusting organisms such as other bivalves, barnacles and tubeworms. Severe encrustation requires the affected structures be lifted from the water and scraped down or replaced. How often this needs to be done depends on how bad the encrustation is. The stock may also have to be passed through specialised equipment to remove encrusting organisms, or otherwise manually scrubbed.

Water user conflict

New South Wales has a large population and relatively scarce areas of sheltered coastal water suitable for mussel farming. Aquaculture is one of many coastal activities that has to compete for this limited space with already established resource users and so the process of obtaining an aquaculture permit and lease over an area involves extensive public consultation. Suitable sites that may be available are extremely limited.

Shelter

Although mussels need some current and wave action to ensure they get sufficient food, mussel farms are relatively fragile and easily damaged in heavy seas and strong currents unless they are securely anchored. A good knowledge of prevailing weather conditions and a well engineered platform and anchoring systems required to ensure the crop will survive even the most severe conditions.

Water quality

A viable mussel farm requires an abundant supply of contaminant free water. Australia has an envious reputation for its clean waters and this may prove to be an increasingly important selling point in the marketplace, particularly the export market as traditional mussel growing areas overseas have suffered from water quality degradation causing the loss of consumer confidence in the marketplace.

Human pollution is usually not an issue with mussel farms as they are often positioned well offshore and away from point sources of effluent. However, mussel cultivation is subject to the direction of the New South Wales Shellfish Quality Assurance Program, which is administered by the NSW Food Authority. The program is designed to ensure bivalve organisms taken for human consumption are of a merchantable quality and free of contaminants and infection from viruses.

High freshwater input can also cause high nutrient loading and a drop in salinity. Mussels may cease feeding during these periods; this causes metabolites to build up inside them, making them capable of causing disease if eaten. It is common, as for example in the oyster industry, to stop harvesting for a short period after floods.

Of greater concern is the potential for a toxic algae bloom to affect mussel farms. This has happened in Port Phillip Bay, Victoria, in recent years. The mussel farmer has no defence against this problem but can minimise the threat to public health by regular monitoring of water quality and restricting sales of mussels if any threats of toxic algae have been identified.

Nutrients supply and water currents

Water currents interact with the seasons to control the amount of food, in the form of phytoplankton, available to the mussels. The farm should be sited to take best advantage of the food resources. The availability of food and prevailing water currents will determine how you space and orient the longlines, how you space the culture ropes, and the density at which mussels can be grown.

Farmers can use spacing and orientation of the culture ropes to control growth and condition of the crop. In the same way, the potential for 'no growth' areas (areas in which water movement is not sufficient to replace the food being consumed) can be minimised.

Applying for a fish farm lease

Contact NSW DPI, Port Stephens Fisheries Centre for details about applying for an aquaculture lease over public water land on 02 4916 3900.

Conclusion

Mussel farming (using the methods described in this paper) has proved viable worldwide. The blue mussel (Mytilus galloprovincialis) is a similar species to those farmed overseas. The economics of mussel farming in Australia are likely to be dependent on keeping production costs down and achieving a good market price. Therefore, before investing any money on establishing a mussel farm, the full market potential should be investigated. To do this properly you should conduct a business planning exercise to estimate the expected outlay verses returns.