Grow Out - Maximising Seed Performance

HATCHERY TO PALATE

Grow Out - Maximising Seed Performance

Seed purchases are a large cost input for oyster farms, so it makes good sense (and good business practice) to maximise survival and growth of seed purchases

1.  On opening the box ....

The first area of focus is on how you manage the seed during the first few weeks or months on your farm. When the seed arrives it should be clean, moist, smell fresh and have no gaping shells. Check the quality assessment report from the hatchery to ensure that weights and counts correspond with your own; and that you are happy with the overall quality and health of the seed. If there is a problem you should advise the hatchery immediately.

2.  Loading and deployment ...

When loading the seed into trays, tubes or bags, be careful not to overstock; and ensure there are no holes or damaged gear where seed can inadvertently escape, or predators such as crabs can get in. Make sure the containers are well secured to the racks or lines - it can take just one decent wind to lose hundreds of thousands of seed

3.  Flow rates ...

Seed requires maximum water flow to access the available phytoplankton in the water. This is particularly important if the phytoplankton levels in the water are low. Maximising flow will help you maximise survival and growth. For those farmers buying small seed (< 5000 screen), it is absolutely critical to regularly check and clean the mesh. The frequency of this will depend on your particular environment, but those farming in silty water will need to pay particular attention to this.

4.  Density management ...

Once the seed can rest on a 6mm mesh, you will find that the mesh aperture will maintain sufficient flow rates between grading intervals. The next key management step is to make sure the seed or juvenile oysters don't become 'overstocked' as they grow. We've all seen the so-called "bricks", where growth takes off and before you know it, the tube is full and solid like a brick.  In this case you will end up with large size variation, elongation, curl back, soft shells, unhealthy stock, and possibly mortality. Flow will be restricted tot hose oysters on the outer edges of the tube and the stock in the middle will get none.

Remember - it's all about density management !

5.  Growing heights ...

Setting the correct growing height is important to get the ocrrect balance between growth and hardening of the shell. Hardening promotes a thickening of the shell, making the oysters more robust during grading and tolerant of extreme weather. It can also influence the shape of the shell. To promote hardening of the shell, growing heights should be set to achieve 15 - 30% air exposure.

Hardening of the shell can also be achieved by air exposure. Therefore it is essential to establish a balance between air exposure and height within your particular site.  You may also find that some locations on your farm receive more wave energy than others - for example the outside racks. These can also be utilised to harden shells - but care must be taken not to sacrifice too much growth in the process

6.  Weather watch ...

It is important to always keep an eye on extreme weather conditions of excessive heat, cold or storms; and check your stock immediately after such events.

During summer when there are weather predictions of several days of extreme temperatures, lowering the heights of your racks to reduce the air exposure can reduce the risk of mortalities. Afternoon low tides in summer can be deadly for stock of all sizes, and it pays to have a management plan in place to deal with these. For example, some farms in NSW use spray irrigation to cool the air around their stock.

7.  Grading and handling ...

Ideally, stock should be brought in during early morning, graded and returned to the lease that night or the following morning. This is particularly important during the summer months. The best way to grade seed is in the wtaer, as this method reduces shell damage, stress and trauma. Regular grading will help you to avoid over crowding (density management) and promotes good shape and uniform growth.

8.  Management system ...

A good system for stock movement is a  'must have' for any farm. There are many systems available from whiteboards to spreadsheets to database programs. Whatever system you adopt, make sure you can track the batch, stock size, numbers and rack locations accurately and quickly. The more sophisticated programs can also help you track or predict growth rates, monitor survival, and set grading and maintenance schedules.

Many farms are growing a mix of 'standard'  and  'spawnless' oysters. In such circumstances it becomes very important to have stock correctly labelled and segregated to avoid mixing up. If stock does becomeaccidentally mixed then the advantage of being able to sell 'year round' is lost. A good management system will reduce this risk.

 

source :   Mr Kerry Wells,  Shellfish Culture

 

Setting out seed trays at Ceduna (South Australia)
Setting out seed trays on the Little Swanport lease (Tasmania)
Racks of seed trays at our Little Swanport lease (Tasmania)