A question we are often asked is what is the optimum bag density to on-grow gigas oyster seed. It’s a tricky question to give a straight answer to as there is so much variation between growing sites but a few general principals can be applied.

When the seed first arrives from the hatcheries you can put them into the bags at densities of up to 5000 units initially. It is important to get them back into the water as quickly as possible but you can leave them like this for up to 10 days which should allow you the time to redistribute them. It is advisable not to leave seed too high on the shore when you get them first so that they get ample water coverage.

If you are short of bags, you could consider a maximum first stage growing density of 4000/p bag. You may notice that at 4000 the growth appears to be faster: this is because the oysters have less space to roll around with tide action and therefore the frill (new growth) is not being knocked off.

At a lower seed density, with the frill being broken off more regularly, you should see a difference in the quality of the shape of the seed, producing a rounder oyster with a deeper cup. The biggest advantage of this is that you end up with a hardier oyster which should be more resistant. That nice round shape should follow through to the adult oyster as long as good husbandry is maintained throughout the growing cycle.

Oyster Seed Sizes Supplied

GRADE OF SEED (mm)	UNIT WEIGHT (average)	AGE (approximate)
G3	0.015g	2 months
G4	0.04g	3 months
G5	0.07g	4 months
G6	0.14g	6 months
G7	0.2g	6-7 months
G8	0.31g	7 months
G10	0.5g	8 months
G12	0.85g	10 months
G13	1.0g	11 months

In France oyster seed is described using T for ‘taille’ or ‘size’ (so T6, T8 etc). In English we use G for grade and the number represents the size of the oyster seed in mm.

Moving on to the Next Stage

Once oysters are big enough to sit on a bigger mesh, you can cut them down to 4 to 600 per bag, cutting down for the final growing stage to 150 oysters per bag (some go as low as 120 per bag). If your site is rich in nutrients, 120 oysters per bag is the recommended density for the highest meat content.

A few considerations to bear in mind when bagging up your seed:

• We would recommend trying to grow seed at different levels on the shore: lower down and you should achieve faster growth; mid-tide and higher level on the shore will produce a hardier oyster.
• Higher densities present a potential risk if you have diseases or red tides coming into a bay: the higher the density, the faster the mortality could spread in a bag (not always the case, but the majority of time).
• If you are willing to compare results with different densities, we would suggest 1000 / 2500 and 4000 per bag to see what works best on your site and with your unique growing conditions.
• If you are experimenting with different levels of seed density don’t put all the low density bags at the end of a trestle. Try alternating end and middle of trestle for comparisons as the nutrients available to the growing oysters will be higher at the ends of the trestle.
• Lower densities mean more bags to turn and more work to do! Turning is an essential step to avoid seed setting in the mesh.
• Make sure you use the correct size mesh for your seed so that you don’t lose seed slipping through the mesh. For example if you have fewer in a bag they will roll around more and it can happen that with the frills knocked off a G10 can ‘size down’ to a G9 so keeping on top of the mesh size you use is important.

Oyster Bag Mesh Sizes (suggested)

MESH SIZE	BAG TYPE	SUITED TO OYSTER SIZE
2MM	Pouch	G3 / G4 Seed
4MM	Pouch / Zip	G6 / G7 Seed
6MM	Pouch / Zip	G8 Seed
8MM	Zip only	G10 Seed
9MM	Pouch	Half-grown / Adult
10MM	Zip only	Half-grown / Adult
14MM	Pouch / Zip	Half-grown / Adult
18MM	Pouch	Adult
23MM	Pouch	Adult

It is important to remember that these are just general rules and each grower will have to find the method that works best on their site. For instance some growers choose to move their seed from a 4mm to a 6mm and then on to a 9mm bag whereas other growers skip that middle step entirely. Available equipment, manpower and site conditions will all play their part in these decisions.

Give us a call to discuss ordering seed and for advice to chat about bagging up your seed.

Read our 7 Essential Tips for Successful Oyster Seed Growth Blog to learn how to raise healthy meaty oysters from your seed.