Bag and Cage Culture
Around the world culture systems utilizing semi-rigid plastic mesh bags and cages are being successfully employed to produce single oysters and rear juvenile clams. In France this is the primary method of oyster production. Growers in Washington state use them extensively for near-bottom intertidal oyster growing on racks. In British Columbia they are used in on-bottom intertidal oyster and clam nursery and grow-out culture systems. These bags and cages are also being adapted to off-bottom methods by suspension from longlines and rafts.
The bags and cages most widely used for this type of culture operation are made from extruded polyethylene material. This is frequently referred to by the trade name Vexar�. Other manufacturers make similar extruded plastic mesh (e.g. Durethene�). Suppliers of this material will usually have it available in a variety of forms, mesh configurations and mesh sizes. It can be ordered in single layer rolls, tubular form (in rolls) and as purpose-built cages and sealed or unsealed bags from suppliers such as ADPI Inc. in Philadelphia (manufacture Durethene�), and InterNet Inc. in Minneapolis (supply Vexar�).
Another alternative is to use rigid materials for the construction of cages for intertidal or off-bottom culture of oysters. The material has been successfully used for such devices such as lobster traps and has now been adapted for the construction of shellfish culture cages. A recently developed type of this material is known as “Aquamesh� and is a “galvanized after welding” wire mesh with a PVC coating. It is available from Riverdale Mills Corporation in Northbridge, MA, USA.
Bag and Cage Culture Systems: Intertidal and Deep Water
The polyethylene mesh may be sold in rolls of tubular material which the user must cut into desired lengths. It is also commonly sold in standard lengths of material that can be purchased as bags or fold-out cages. For bags, one or both ends have to be folded over and fastened by some means (e.g. plastic ties or PVC pipe cut longitudinally to slide over the folded end) to create the “bag”. In some cases, the bottom of the bag will be sealed by the manufactures leaving an opening at one end only for putting in or taking out stock. Some growers are now finding suppliers for already constructed bags and cages in standard dimensions and mesh sizes. The “cages” constructed of this material fold out to form rigid-sided boxes and the ends are cut, folded and pinned to seal the ends. It is important to consider handling efficiency in these systems which can be greatly increased by using pins that can be set or removed quickly.
The bags are used in a variety of ways and for different stages of culture. Bags can function as intertidal nursery rearing systems for clam or oyster seed of sufficient size. For rearing clams, the 1/4″ mesh bags are sometimes put directly onto the substrate and held down with re-bar pins. This is usually done only on firm gravel substrate. Sediment is allowed to penetrate the bags and cover the clam seed. The bags must be flipped regularly to control fouling of the upper mesh surface. The mesh cages (4.5″ deep) are reported to be successfuly used for rearing juvenile clams either up to 25mm or up to harvest size. The cage is pinned onto the substrate and gradually sinks into it until it is half filled. The clams bury themselves in the substrate but are still completely protected from predators. At harvest the cages are pulled up and the substrate falls through the mesh. It is important to ensure good closure on bags or cages on or near the bottom. Small predators such as crabs may find an entrance if there are small gaps in the folding on the ends of the cage.
If these bags are to be used for rearing oyster seed, then a situation of silt and sediment buildup in the bag must be avoided, particularly in areas where there is a soft mud substrate. With heavier siltation and more extensive mud bottom, growers extensively use racks to hold bags off the bottom. These racks are usually made of re-bar and can be fashioned into any form the grower wishes by utilizing a re-bar cutter-bender and welding the pieces together.
The juvenile oysters may be put into spat bags which are then put into the mesh bags or cages. Larger animals can be put directly in the cages and bags. A typical program may involve splitting densities as the oysters grow and using progressively larger mesh bags or cages.
A simple rack can be constructed by cutting and bending 2 lengths of rebar int U shapes (i.e. the legs of the rack) and welding several pieces of straight rebar (e.g. 6-7feet) to equivalent spots on either U-bar to form a “top”. Two or three bags may be strapped to each rack.
To increase culture density, racks capable of holding bags or cages several layers deep, such as the one below, can be constructed. These will require power-assisted handling.
Growers have experimented with hanging bags such as these in deep water. Spat bags with 2-3mm or larger clam or oyster seed can be put into these larger bags which can then be strung in “stacks” by using short PVC spacers between each bag to allow for water circulation. This type of system requires increased maintenance. If it is allowed to foul up, water flow will be reduced, and in circumstances where there are mussel sets, the byssus threads can entangle seed, spat bag and outer bag so that removing seed for grading becomes a slow and difficult process.
Grow-out of single oysters is now being done successfully using these bags on intertidal areas where the bags are anchored yet there is some means of allowing the bag to flip or turn with the ebb and flow of the tide (e.g. by attaching a float to an anchored bag). Other growers are now experimenting with a grow-out system for single oysters in which the bags are suspended from a longline and where sufficient water movement can keep the oysters rolling over in the enclosures.