Clams are bottom-dwelling mollusks, thus growout systems are designed to place seed clams in the bottom substrate and provide protection from predators. The system must allow substantial water flow to provide both oxygen and natural food, phytoplankton (micro algae), for growth. In the mid-Atlantic and Northeastern states, where there are large intertidal areas, clam growers use bottom net covers. Since Florida’s inshore coastal waters have a small tidal range, most Florida growers rear their crops subtidally in bottom bags. A bottom bag is made from woven polyester mesh material that is sewn in the shape of a bag with a spout on one end. Typically, the bag is 4’ by 4’, or 16 square feet, in size. The flexible bag contours to the bottom substrate and can be staked down on each corner, allowing natural accumulation of sediments to fill the bag. Usually several bags are connected together with cable ties to form a line, or belt, that is staked to the bottom with either PVC pipe or fence post wire. The bag also serves as a harvesting device. Since dredging is not allowed on shellfish aquaculture leases, clam harvesting is a matter of pulling the bag from the bottom by using a winch from the boat. Bags can be easily patched and cleaned and reused for several crop periods.
Florida growers incorporate a field-based nursery step into their production process, so two types of bags are used - a field nursery bag with mesh openings varying from 3 to 4 mm in size and a growout bag with mesh openings varying from 9 to 12 mm. Clam seed as small as 4 to 6 mm in shell length (SL) are planted in nursery bags at around 10,000 per bag. After 3 to 6 months, depending upon the season planted, 12 to 15 mm SL seed are moved into growout bags at lower numbers. Stocking rates used in the growout bags vary from 800 to 1,400 clams per 16 ft2 bag, or 50 to 85/ft2, and are dependent upon the productivity of the lease area and food availability. Clams are harvested 10 to 18 months latter when the majority of the crop reaches the preferred littleneck market size, or 1” shell thickness or width.
In recent years, growers have needed additional predator protection to increase clam survival. Cover netting, such as chicken wire or plastic “bird” netting, is used to cover the rows of bags. In addition, some growers are coating their bags with net dips, which stiffen the fabric and prevent rays and drum from sucking the bag material into their mouths. Net coatings must be approved by the Florida Department of Agriculture and Consumer Services for specific use on clam bags