Supply of Juveniles:
Mussel spat have been produced by Tasmanian hatcheries, however the successful grow out of these spat has been limited by fish predation (Daintith et al. 1997). In other Australian states, most spat are supplied by natural collection on specially designed collection ropes.
Although artificial hatchery production of mussel spat is technically possible, the relatively low value of mussels and he ready availability of wild spat means that hatchery production in Western Australia is generally not necessary, or commercially viable.
The existing mussel aquaculture industry in Cockburn Sound utilises collection of wild spat from within the existing farm sites at the Kwinana Grain Terminal in Cockburn Sound. Spat are collected using ‘Christmas tree’ collector ropes.
These ropes provide a suitable settlement place for the mussels at the end of their planktonic larval stage. The ropes are usually placed in approximately eight metres of water.
As the mussels grow, overcrowding of spat may result in stunted growth and may also result in excess mortalities.
For this reason, juveniles are generally removed from spat collectors when they are 12 mm in length and then transferred to long lines at lower densities as described below.
In Cockburn Sound, blue mussel populations have been reported to generate up to six periods of larval settlement (spatfalls) per year, with the result that this area is recognised as a particularly good area for collecting spat.
Culture Techniques:
Several culture techniques for growing mussels have been used worldwide. A selection of these are described below:
Long lines:
In Western Australia, all mussels are currently produced by long line culture. A rope is stretched horizontally near the water surface and maintained 1-2 m from the surface with buoys. Mussels are grown on vertical ropes known as ‘droppers’ which hang from the horizontal rope for a length of 4m.
The droppers are placed a minimum of 0.5 m apart and have at least 4 m of free space from the bottom. In deeper waters the gap between the bottom of the line and the sea floor is greater.
Mooring lines extend from each end of the horizontal rope to anchors buried in the mud of the bottom. As the ropes are kept taut, there is no movement around the anchor to disturb the bottom as occurs when boats are anchored.
Small mussels stripped from the collection ropes are inserted into a continuous sock-like cotton tube, which is approximately 17.5 cm in width. This cotton sock is then wound around the dropper. The mussels grow and attach to the ropes using their byssal threads and the cotton sock slowly disintegrates and falls away.
The density at which mussels can be cultured on long lines could be about 300 per metre, but depends on the food availability, which varies from site to site. Mussels grown on longlines can become smothered by naturally settling juvenile mussels and other fouling organisms.
For this reason, most farmers prefer to position their farms away from heavy spat settlement areas to avoid layers of spat attaching to larger mussels.
Raft Culture:
Raft culture utilises similar principles to long line culture in that the mussels are suspended on droppers but these are suspended from the raft instead of the long lines.
The raft itself is anchored to the seabed removing the need for several anchoring systems. Long line culture is usually preferred to raft culture in Western Australia as it creates less of a visual impact, and the droppers can be spaced farther apart to maximise the use of the available phytoplankton.
Raft culture is more suited to areas of dense phytoplankton and to smaller operations, as there is less scope for mechanical harvesting. This method of culture is used in Saldahna Bay in South Africa but has been abandoned by the New Zealand industry in favour of long lines.
Intertidal Pole Culture:
Intertidal pole culture, also known as the bouchot technique is utilised in France. In this method, ropes with spat attached are wound around large vertical poles (bouchots) in the intertidal zone. A mesh netting is used to cover the mussels to prevent them being detached and lost.
A barrier is placed at the bottom of the pole to prevent predators such as crabs from reaching the mussels. This method of culture requires large tidal ranges, in order to supply the densely packed mussels with food. This method is not practiced in Western Australia as the tidal range in the mussel growing region is not sufficient. Other farming procedures are described in Gosling (1992), Jamieson (1989), and Mackenzie et al. (1997).
Information Sourced From:
