Mussel farming has a long history that dates back to the thirteenth century. Mussels are farmed in many areas of the world with the most common species cultured being the blue mussel, Mytilus edulis.
The main producers of mussels are countries such as China, Korea, Spain, The Netherlands, Denmark, France and New Zealand. In 1997, 1.1 million tonnes of mussels were produced worldwide, with most production occurring in China (nearly 400,000 tonnes).
The Australian mussel industry is relatively small compared with world standards and in 1998/99, the total Australian production of mussels was 1,297 tonnes.
Since last reviewed by Lawrence (1995), the Western Australian industry has shown good growth, increasing from 127 tonnes in 1990/91 to 506 tonnes in 1996/97 to 679 tonnes in 1998/99.
Thirteen species of native mussels occur within Western Australia, but these are all small species with low or no market value.
The Western Australian commercial mussel industry is based on the farming of the blue mussel Mytilus edulis although the actual identification of the species is a matter of scientific debate.
Recent evidence suggests that it is in fact the European mussel species, Mytilus galloprovincialis. In the absence of a definitive answer to the taxonomic question, (see Gosling 1992) the name M. edulis is used here to be consistent with the Department of Fisheries legislation.
The origins of the blue mussel in Australia are unknown. While it is possible the species occurs naturally in Australian waters, it is also possible that it came to this country on the hulls of early sailing ships when the continent was colonised by Europeans. In either event, the species has occurred along the coastline of southern Australia for a considerable period of time.
Distribution:
The blue mussel is found in the temperate waters of both the northern and southern hemispheres and grows best within a temperature range of 16-22o C. As a result of this requirement, the range of the species in Western Australia is limited to areas south of Geraldton.
Wild mussels tend to occur in bays and estuaries which have elevated levels of nutrients from land runoff, causing an increase in phytoplankton (microscopic plants) which is the main food for the filter-feeding mussels.
A large population of blue mussels developed in Cockburn Sound during the 1970s and 1980s due to an increase in the nutrient levels in the area. However the significant reduction in nutrients since the mid-1980s has resulted in a decline in this wild population.
In Western Australia the major mussel farming area is Cockburn Sound, but commercial mussel farming has also been established in King George Sound and Warnbro Sound (Figure 1).
Habitat:
Blue mussels occur naturally in marine environments on shallow sandy bottoms in large communities attached to one another, known as beds. Mussels also occur as dense aggregations attached to submerged or partially submerged structures, such as jetty pylons.
They are shallow water organisms, occurring to a maximum depth of 10 metres, although they are most abundant at or near low tide level, where the food supply of phytoplankton is most concentrated.
Although mussels grow best in marine embayments, they will also colonise estuaries or river mouths. However, given that mussels do not tolerate freshwater for extended periods of time, growth rates in these areas can be variable and serious mortalities occur when these areas are subject to substantial fresh water runoff.
General Biology:
Mussels reproduce by releasing either eggs or sperm directly into the water in a process known as spawning. Hermaphrodite mussels (containing both sperm and eggs) are rare. Mussels first spawn when they are over 3 cm in length at approximately 11 months of age.
In Western Australia, mussels primarily spawn during the winter months, which contrasts to the spring/summer spawning reported for the colder regions in the northern hemisphere. Mussels in spawning condition are ‘fat’ and can be recognised by eye at this stage. During spawning, a mussel may produce up to eight million eggs, each of which is 0.07 mm in diameter.
Spawning first occurs with a lowering of water temperatures to between 14o C and 20o C from late April to July. Generally, a number of animals may spawn a second time after a 4-5 month period. Once fertilisation has taken place, a free-swimming larva is formed.
The larvae are ‘planktonic’ meaning they simply drift with the currents. Larvae usually spend a few days in this planktonic stage, however the length time depends upon environmental factors, such as temperature. After this time, the larvae will settle and attach onto a surface such as a jetty pylon with their byssal threads. Once mussels have settled they are no longer called larvae, but are now called ‘spat’.
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