Grow-out Phase

Once the juveniles have reached 12 mm in size on the spat collectors, they are ready to be re-seeded on to grow-out longlines. Reseeding is a process whereby the juveniles are “thinned-out” to encourage further growth of the mussels.

The commercial stocking rate for mussels on grow-out lines is generally between 200 and 400 mussels/m of rope. It is important to reseed mussels when they are still juvenile spat, or when mussels get larger than 40 mm, they do not attach as readily to the rope. This means that slippage will occur with mussels slipping down and forming clumps in the bottom of the sock. This results in a reduced growth rate and poor shell shape. The stockings and rope are skewered and tied at 0.5 m intervals to ensure that the mussels are evenly distributed.

Ideally, grow-out farms are situated away from heavy spat settlement areas to avoid layers of spat attaching to larger mussels. Longlines require culling to remove fouling and smothering by naturally settling juvenile mussels. Mortalities of mussels can occur if there is a lack of food in the area or if they are exposed to extreme wave action.

Selection of farm site

Areas sheltered from strong wave action and free from both industrial and sewage pollution are suitable for mussel culture. Clear water with good phytoplankton production and a moderate current to bring in the food and carry away the waste products are required. Heavy silt affects the growth of mussels. The depth and nature of substratum are important for a given type of farming method. For on-bottom culture, hard substratum is needed and both intertidal and subtidal regions upto 5 m depth are suitable. Muddy substratum is not a limiting factor for various types of suspended culture. In shallow waters stake and rack methods are suitable and high tidal amplitude is not a bar for these methods. Under rough sea conditions longlines are preferred over rafts.

Both the green and brown mussels thrive well under marine conditions; a salinity range of 30-35 ppt is preferred. Sudden lowering of salinity to below 20-25 ppt may result in mortality. Ambient water temperature varies from 21°C to 30.5°C and dissolved oxygen from 3.80 ml/l to 5.25 ml/l.