Farming Blue Mussels

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 14^oC and 20^oC 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'.

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.