The nursery is a critical link in the hard clam grow-out process. Placing seed clams from the hatchery directly into the field for grow out can cause a high level of mortality.
The nursery provides a controlled, intermediate step, whereby the hatchery-reared seed clams are nurtured to a size less vulnerable to the stress and predation found in the field grow-out phase.
It would not be cost effective to grow seed to the size required for the grow-out stage within an intensive hatchery environment using cultured algae. So, natural seawater is generally used in these systems. Natural feed is provided by seawater as it moves through the nursery system.
The oldest method of nursery culture is called the land-based raceway method. The raceway system typically uses long, shallow, wooden trays lined with plastic or covered with epoxy resin or other protective coating.
A thin layer of sand covers the bottom of each tray, and the seed clams are distributed over it. Raw seawater is pumped into one end of the tray at a prescribed rate to establish a horizontal flow across the seed clams.
Another method of nursery culture is the land-based upweller method. The upweller system pumps seawater to reservoir tanks and upflow cylinders, which provide vertical flow for the seed clams.
The flow of water can be forced up through the seed clams or pulled down through the seed clams. The seed clams rest on a fine-mesh screen. The movement of water removes waste and prevents the seed from being suffocated by any accumulated silt.
Floating upwellers are a recent innovation. These are similar in design to land-based upwellers, but can be powered (pumped) by airflow, submerged pumps or tidal flow. The fourth method of nursery culture is the field-based bottom plant system, which involves placing seed clams directly from the hatchery into the submerged bottom setting. Seed clams smaller than 3 to 4 mm should be used.
Traditional designs employ subtidal and intertidal trays made of wood that contain a layer of gravel or sand and have a protective cover to discourage predators. More recent innovations include bottom bags and systems of bags held together in long belts.
The series of bags significantly reduces maintenance and labor expenses. All of these field-based nursery techniques are carried out in protected, shallow water areas so the threat of poaching can be minimized.
These nursery systems vary considerably in terms of investment cost, operational expenses and required management skill. The land-based system requires waterfront land and an investment in pumps, whereas the field-based system is located on leased bottom without the need for controlled water movement.
The floating upweller systems are intermediate to land-based and true field bottom plants. Energy requirements are much higher for the land-based systems. Maintenance costs are lower for the upflow systems than for raceways. Replacement and maintenance costs are higher for the floating upweller systems because of their location in the water and the greater risk of damage by predators, fouling and wave action.
Survival rates may be higher for the land-based systems because there is greater control over water quality and a lack of predators, but the production cost per clam seed may be less with the in-water systems, particularly if waterfront property is not available.
Grow-out:
The time needed for grow-out will largely depend on water quality, food availability and temperature. Grow-out to 50-mm (2-inch) market clams from 7- to 15-mm nursery clams may require 12 to 24 months.
There are various grow-out culture systems for raising hard clams for the market. Grow-out systems are stocked at a density of 50 to 70 seed per square foot for final grow out.
Although landbased grow-out methods such as raceways and tanks have been developed, the field-based growout methods are more economical and better suited to hard clam production.
Grow-out operations use some form of hard structures (pens, trays) or soft structures (bags, net covers) to reduce predation and ease harvesting. Pens are harvested by hand rakes or with mechanical harvesters, where legal.
A lifting apparatus is required to lift trays from the water. Covers (bottom plants) are placed over the seed clam planted area and staked down to discourage predators. To harvest clams, the net is rolled from the planted areas and the clams exposed.
Soft bags resemble 4-foot by 4-foot laundry bags; usually a number of bags are connected together to form a line that is staked to the bottom. Harvesting soft bags is a matter of pulling up the bags because the clams are confined in the bags.
To increase the survival rate, seed clams longer than 6 mm should be used, even though larger seed are more costly and are in limited supply. Clam farmers in certain counties in four states—Florida, South Carolina, Virginia and Massachusetts—may participate in the first USDA Pilot Crop Insurance Program in aquaculture.
Clams 10 mm and larger are insurable against natural losses in excess of state average mortalities, and farmers may purchase crop insurance with differing deductibles to cover insurable losses and reduce their financial risk.
Authors:
Jack M. Whetstone, Leslie N. Sturmer and Michael J. Oesterling
