The method of shellfish aquaculture mainly include raft system, longline system and intertidal system.
- Raft System
Shellfish farmers use rafts at deepwater sites to suspend culture systems used for different stages in the rearing of oysters, clams and mussels. For example, many shellfish farmers suspend tray systems that are used as nurseries for juvenile oysters and clams as well as for oyster growout. This method of “off bottom” farming is considered one of the most productive in the world.
Rafts must be built to withstand the severest weather, hold the weight of hundreds of dozens of mature oysters, and serve as safe work platformsfor workers handling product. Improtant considerations in the design of a raft system include flotation, flexibility, stability,functionality, durability and capacity.
The raft system must be securely anchored to prevent movement and / or drift. Rafts are usually roped together and securely tied at three points on each raft and then anchored at each end. Anchor ropes will sway in the currents and slacken at low tides.
- Longlines system
Longlines are used worldwide to grow everything from scallops to seaweed. These systems are preferred in high exposure areas. They are productive and flexible enough to handle a variety of shellfish species as well as a range of culture systems. In a longline system, the farmer anchors a lenghth of line at both ends, attaches a flotation and hangs culture system on the line.
Nursery rearing as well as grow-out can be accomplished on longlines. Trays, tube modules and bags or cages can be hung in deep water for nursery rearing of clams, oysters or scallops. Seeded lines or socks (with adequate predator protection) are commonly suspended from longlines. Scallops are frequently grown-out on sunken longlines, in suspended lantern nets, or ear-hung directly on a down-line.
Layout of a longline system depends on site characteristics. The most significant feature, from a security and stability perspective, is availability of shore to anchor one end of the longline. In some locations, both ends can be fixed to the shore. Anchoring both ends in deep water may be done at sites where shore anchoring is not possible or desirable.
- Intertidal system.
Intertidal farming refers to systems in which shellfish are exposed to air during the low tide of each tidal cycle. These systems include both bottom (beach) and near-bottom (epibenthic) techniques.
Managing and maintaining productive intertidal growing areas is no different than land agriculture. The substrate will be cleared and prepared for planting. Both oyster and clam farming may require substrate improvement to reach acceptable levels of productivity. The area will be seeded and, in many cases, the seeded plots are protected from prodators by overlaying them with mesh that is then secured into place. The plots will be tended regularly.
Farmers will develop and maitain an inventory control system to know what was planted when and how it is performing.
*clams perform best in a substrate composed of a mixture of mud, sand, pea gravel and some shell fragments. Improvement for silted beaches may mean gravelling while for other sites it involves debris and rock removal.
*In oyster farming, intertidal grow-out systems include beach distribution of seeded shell cultch as well as oysters grown on stakes, racks and intertidal longlines. Near-bottom methods have been adopted on sites where bottom conditions are not suitable for growing oysters; e.g., soft mud,silt. Nursery rearing of oysters on shell cultch or tubes may also be done intertidally.