The growth rate of mussels on long lines varies according to the density of the mussels on the line, the depth, an the availability of phytoplankton. Decreased growth is experienced at high stocking densities, greater depths and lower phytoplankton levels.
Wild mussels live in dense beds on the bottom or in crowded conditions on jetty pylons. As cultured mussels are kept at relatively low densities on long lines, they generally have a faster growth rate than wild mussels.
In Cockburn Sound, cultured mussels reach about 40-46 mm in six months and 65-85 mm, which is the preferred market size, in 12-14 months.
Harvesting of blue mussels for market requires that the product be removed from the long lines and the shells cleaned of external fouling before sale.
This process is usually automated and involves a washer-tumbler machine in which the mussels are rotated and rub against each other to dislodge small mussels and barnacles and other fouling organisms. The mussels are then cleaned, graded and bagged for sale.
Like most bivalves (molluscs with two shells), mussels are filter feeders. This means they feed by pumping water through a set of gill filaments which filter out small particles such as phytoplankton, zooplankton and other organic material. Sediment is discharged while food is retained and passed into the stomach where it is digested.
Mussels are able to obtain all of their nutritional requirements from the environment. As a result of this feeding mechanism, farmed mussels do not require additional feeding and as an aquaculture operation, are considered to be relatively environmentally benign, with any effects being very localised (see Jeffs et al. 1999).
Husbandry and Health Status:
In Western Australia, mudworms (Polydora spp) which bore into the shell of the mussel have been reported. These worms have been found in up to 20 per cent of bottom grown mussels in and about six per cent of longline cultured mussels.
The mudworm bores into the shell and the mussel secretes a flexible layer or a blister to cover the point where the worm comes into contact with its flesh. The mudworm lives in this blister and accumulates mud and food particles. The tunnels built by the mudworm affect the aesthetic value of the shell and if the blister bursts, mud may be introduced into the mussel.
A microscopic parasite known as mussel egg disease (Steinhausia spp) occurs in Western Australian mussels every few years. This microsporidian parasite lives inside the eggs of some mussels and causes the mussel meat to have a lumpy appearance when cooked.
Mussel egg disease may alter the look of the mussel and also may reduce the number of eggs released during spawning. Mussel egg disease has absolutely no effect on human health or the taste of the mussel.
Mussels are susceptible to predation by fish and growth can be slowed by competition for space with other organisms such as barnacles, tunicates, tube worms and bryozoans, which are also filter feeders and reduce the food available to mussels.
Mortality on long lines may result from a lack of food (phytoplankton), or mussels being physically removed from long lines by extreme wave action in more exposed locations.
In liaison with the shellfish industry, the WA Shellfish Quality Assurance Program (WASQAP) was established jointly by the Department of Fisheries and the Health Department of WA in 1991 to ensure the safety and quality of shellfish grown in WA waters.
The quality assurance program for the shellfish growing sites in Cockburn Sound and the Albany Harbours includes sampling of shellfish for the presence of faecal bacteria, algal biotoxins and heavy metals.
The quality assurance program is overseen by way of a tripartite Memorandum of Understanding between the Health Department, the Department of Fisheries and the Australian Quarantine and Inspection Service (AQIS). The main goal of the testing program is to ensure the quality of the mussel product being harvested for both the domestic and export market. The Health Department also conducts regular water sampling, independent of the Shellfish Sanitation Program and these results are used as additional information for the WASQAP.
Results obtained since the inception of the program demonstrate that the water quality in Cockburn Sound and Albany Harbours is safe for the growing of mussels and that the main growing areas meet the standards required for both export and the domestic market.
While Western Australia’s mussel production is minor relative to the world production, the local demand for fresh product favours local industries. If the industry was to expand several fold it may need to sell more processed product and experience competition from large volume producers such as New Zealand, which produces 65 000 tonnes of green mussels (Perna canaliculus) annually.
These industries will suffer higher freight costs but are highly mechanised. There is also a significant potential for New Zealand farmers to begin farming blue mussels which would further increase competition for Western Australian farmers.
Having noted these potential limitations, the local mussel industry has been very successful with establishing mussels as a very popular part of local seafood cuisine, particularly in highly popular chilli mussel dishes.
The blue mussel industry in Western Australia produced 679 tonnes of product in 1998/99 worth over $1.8 million. With the advent of new farming technology and the possibility of value adding of mussel product, it has been predicted that in the next few years this figure could increase substantially through increased production and profitability on existing mussel growing sites. See Cole (2000) for monthly production trends.
A national profile of exports is provided by Brown et al. (1997) and a review of the economic viability of mussel farming in Australia has been produced by Treadwell, McKelvie and Maguire (1991). Note that this analysis is highly sensitive to the size of boat used.
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