Hard Clam Hatchery and Nursery Production

SRAC Publication No. 4301

VI September 2007 PR

Nancy H. Hadley1 and Jack M. Whetstone2 Biology of the hard clam remove food particles, primarily small phytoplankton, bacteria and DOM. phytoplankton (single-celled algae), The ciliated velum, which gives the Hard clams of the genus Mercenaria from the water. An adult clam filters veliger its name, is used for both loco- are found from the Gulf of St. an average of 7 to 8 liters (about 2 gal- motion and feeding. The length of the Lawrence to the Gulf of Mexico and lons) per hour. larval period depends largely on tem- have been introduced to other areas of perature and food supply. After 7 to the United States, notably the coasts Hard clams usually reach sexual maturity at a shell length (SL) of 21 days the veliger larva develops a of California and Washington. They foot and is called a pediveliger (Fig. 1). also have been introduced to Puerto about 35 mm (about 1.4 inch) (Fig. 1). The sexes are separate but are exter- This stage is brief. The clam soon Rico and Great Britain. There are two loses its swimming organ (velum) and species, Mercenaria mercenaria and M. nally indistinguishable. Clams are protandric, maturing as males at an develops siphons. This is referred to as campechiensis. M. mercenaria is distrib- settlement, and newly settled clams uted primarily in the more northerly early age and changing sex in subse- quent years to spawn as females. may be called “set” or “post-set.” Post- latitudes, while M. campechiensis is the set clams assume the sedentary more southerly species. Both species When stimulated by appropriate environmental conditions (normally high lifestyle of the adult. The term post-set are found on the west coast of Florida is informal and usually refers to clams and in Georgia, South Carolina and water temperatures), clams release gametes (sperm or eggs) into the that have completed the larval stage North Carolina, and some hybridiza- but are still housed in a hatchery. tion between the two may occur. A water. The presence of gametes in the subspecies, M. c. texana, is found in water stimulates other clams in the Juvenile clams less than 35 mm (about the western Gulf of Mexico. This fact immediate vicinity to begin spawning. 1.4 inches) SL are called seed. As seed sheet refers specifically to M. merce- Fertilization occurs in the water col- grow to market size they are classified naria because it is the major cultured umn. Clams usually spawn intermit- into commercial categories, which are species, although some of the general- tently from May through October. A arbitrary classifications that may vary izations may apply to all hard clams. female clam can release several mil- from state to state and dealer to deal- lion eggs in a single spawning, but rel- er. In most states the smallest legal The hard clam is rarely found where atively few survive the larval period size for wild-caught clams is a little- salinities average less than 20 parts to become juvenile clams in the wild. neck, usually defined as 1 inch (25 per thousand (ppt). Hard clams occu- mm) thick. This generally corresponds py intertidal and subtidal habitats, Fertilized eggs (zygotes) undergo rapid cell division and within 12 hours to a shell length of 45 to 50 mm (1 to where they burrow into the substrate 2 inches). Slightly larger clams (1.25 to various depths (normally less than develop into free-swimming tro- chophore larvae. Within another 12 inches or 30 mm thick, 50 to 60 mm 20 cm or 8 inches). They are found in or 2 to 2.5 inches SL) are referred to a variety of substrates, including sand, hours, bivalve shells have formed and the larvae are in the veliger, or as topnecks. Cherrystones are approxi- mud, shell and mixtures of these mately 1.25 to 1.5 inches (32 to 38 materials. Clams are filter feeders and straight-hinge, stage. Veligers are often referred to as “D” larvae because their mm) thick and 65 to 79 mm (2.5 to 3 shape resembles a capital letter “D.” inches) long. Anything larger than that is a chowder clam. Littlenecks com- 1 Marine Resources Research Institute, South Before the veliger stage the larva is Carolina Department of Natural Resources, sustained by lipids stored in the egg mand the highest price of the com- Charleston, South Carolina mercial categories. In some states mar- 2 and by dissolved organic matter Baruch Institute of Coastal Ecology and Forest icultured clams may be sold at smaller Science, South Carolina Sea Grant Extension (DOM) absorbed from the surround- Program, Clemson University, Georgetown, ing water. The veliger feeds on small sizes and some companies have creat- South Carolina ed their own names for these size Straight hinge Veliger Maintenance and conditioning where fertilization takes place. Age range: 1-5 days of broodstock Spawning is induced by alternately Length range: 90-140 μm heating and cooling the clams in a Adult clams are brought into the water bath. Sperm or eggs from a sac- Length hatchery for “conditioning” several rificed clam also can be used to stimu- 110 µm weeks before spawning. Conditioning late spawning. If controlled breeding Straight is the process of inducing gametogene- is desired, the clams are spawned in hinge sis, or the ripening of gonads, to make individual beakers so fertilization can Height clams ready for spawning. In the be controlled. If the objective is sim- 87 µm southern U.S. clams become naturally ply to produce larvae and the parent- ripe in the spring and remain intermit- Velum age is unimportant, it is easier to tently ripe into the fall. To get them to “mass spawn” the clams in a common ripen at any other time of year, one Umboned veliger container. Sperm from spawning must simulate early spring conditions Age range: 3-15 days males will stimulate other clams to of cool temperature (18 to 20 °C) and Length range: 140-220 μm spawn also and the eggs will be fertil- ample food. Clams will usually condi- ized in the common spawning con- tion in 2 to 8 weeks, but this varies Length 175 µm tainer. Early with the time of year and the physio- umbo logical condition of the clams. A clam Larviculture that is already ripe can be maintained Height in this condition for a long period (up Zygotes (fertilized eggs) are main- 160 µm to 6 months) by keeping it in cool tained in clean, filtered seawater at water (18 to 20 °C) and providing relatively high densities (30 or more abundant food. Partially ripe clams per milliliter of culture water) for 24 will be ready to spawn in 2 to 4 hours or until the veliger stage is weeks under these same conditions. It reached. At that point the larvae are is much more difficult to condition thinned to 5 to 10 per ml and given Pediveliger clams that have recently spawned. So food (algae). The veliger stage lasts 7 Age range: 8-20 days for year-round operation it may be to 21 days, depending largely on tem- Length range: 170-230 μm advisable to collect naturally ripe perature and food quality (Table 2). During this time the larvae grow from Length 220 µm clams in early spring and keep them in conditioning tanks to maintain a 100 µm to 180 to 250 µm and are Umbo population of readily spawnable indi- gradually thinned to a final density of viduals. Naturally ripe clams also can 1 per ml. Larvae are kept in static cul- be collected in the fall, although fewer tures, the water is changed frequently Height wild clams are ripe at this time than (daily if possible), and food is added 188 µm in the spring. The general require- daily. Some hatcheries aerate larval ments for maintaining and condition- cultures, but this is not usually neces- ing broodstock are listed in Table 1. sary. Water for larval culture must be filtered (1 to 10 µm) to remove silt Spawning and native plankton. Depending on Foot the quality of the seawater, it may When clams spawn they release eggs also need to be treated with charcoal Figure 1. Veliger and pediveliger larvae. or sperm into the water column,

Table 1. General culture requirements for maintaining and condition- classes. In the southern U.S. wild ing broodstock clams in closed system raceways. clams grow to littleneck size in 2 to 4 Variable Requirement years, while farm-raised clams may reach this size in about 1 to 2 years. Salinity 25-35 ppt Temperature 18-20 °C for gradual conditioning/maintenance Hatchery culture 22-23 °C for rapid conditioning/pre-spawn priming There are five main processes in the Water pretreatment 25 µm filtration hatchery culture of the hard clam: 1) UV and/or charcoal maintenance and conditioning of broodstock; 2) spawning; 3) larval cul- Water change at least 3 times per week ture; 4) post-set culture; and 5) food Stocking density 1 clam/4 liters water production (algal culture). These Feed mixed phytoplankton activities are supported by a water Feed rate 1-3 x 10 9/clam/day distribution and treatment system, air distribution system, freshwater wash- Feed method batch, continuous drip, discontinuous metering down facilities, supplemental lighting Duration 2-8 weeks for conditioning for algal culture, and test equipment up to 6 months to maintain conditioned clams and instrumentation. to remove dissolved organics and ster- veliger stage is fairly short; within a Downwellers (also known as “silos”) ilized (usually with ultraviolet light) to few days the velum disappears entire- are open-ended cylinders, usually con- kill bacteria. ly and a siphon is formed. This meta- structed of plastic pipe, suspended in a Larval culture containers may be morphosis is known as “setting” and reservoir. The bottom of the cylinder is made of fiberglass or plastic and are the young clams are referred to as covered with a fine mesh that supports usually no more than 1 meter deep (3 “set” or “post-set.” the clams (Fig. 2). Before they can be feet). Container size is dictated by the moved to silos, the clams should be Post-set culture amount of larval production desired. large enough to stay on a 150-µm Plastic buckets (20-liter, 5-gallon) can Pediveligers are often removed from mesh. Smaller mesh will clog too readi- be used to grow small quantities of the larval culture system and kept in a ly, obstructing the flow of water. Water larvae (20,000). Large hatcheries have separate post-set culture system until (and food) is circulated through the fiberglass tanks that may hold several metamorphosis is completed. This silos with airlifts (Fig. 3). In a down- thousand liters of water and millions improves survival because pedi- weller, the airlift is positioned outside of larvae. Containers can be flat-bot- veligers, although still capable of the cylinder and moves water from the tomed or conical-shaped. Conical bot- swimming, spend most of the time reservoir into the silo. Downwellers are toms facilitate draining and cleaning. crawling on the bottom of the tanks used for early post-set that might be Tanks may have valved drains at the where waste products and dead larvae sucked up the airlift. When the post-set bottom or may be drained with siphon are concentrated. Post-set clams may reach a size of about 0.5 mm SL, the hoses. When the water is drained be cultured in a variety of systems. silos can be converted to upwellers by from the containers, the larvae are Many hatcheries use shallow trays in a moving the airlift to the inside of the captured on a fine-mesh nylon or gentle flow of seawater, augmented cylinder, drawing water up through the polyester screen. with cultured algae. These trays can be clams. The silos are suspended in a reservoir (a container accommodating Toward the end of the larval period, stacked in tiers to save floor space. An alternative is a recirculating culture one or more silos). The reservoir vol- usually at a size of 200 to 250 µm, the ume should provide at least 0.5 ml larvae begin the metamorphosis to the system with downwellers and/or upwellers. This system is easier to water for each clam. Food is added to juvenile form. The first indication of the reservoir either in batches or by this metamorphosis is the appearance clean than a tray system, makes it easier to control water quality and feeding continuous delivery. All the water in of a foot. This stage is called “pedi- the reservoir is changed regularly— veliger” because the larva has both a rations, and supports a large number of clams in a small space (Table 3). daily if possible, but at least three velum and a foot (Fig. 1). The pedi- times a week. Post-set clams are usually maintained in a hatchery until they are at least 1 mm Table 2. General culture requirements for larval hard clams in closed or can be retained on a 710-µm mesh. system tanks. At that point, the clams may be trans- Variable Requirement ferred to a nursery culture system. Salinity 25-35 ppt Food production Temperature 24-28 °C optimal, 20-30 °C acceptable Broodstock, larvae and post-set are fed Water treatment 1 µm filtration, UV and/or charcoal recommended phytoplankton produced in the hatch- Aeration optional gentle aeration, especially if tanks are deep ery. There are three basic methods of Water change daily if possible, every other day at least algal production: the “Wells-Glancy” Stocking density Stage Density (number/ml) method, the “brown water method,” and the “Milford” method. The Wells- fertilized eggs 20-30 Glancy method and the brown water early veligers 5-10 method are relatively low-tech and late veligers 1-2 inexpensive, but are not as reliable as Food type small flagellates (e.g., Isochrysis); for older the Milford method for producing con- sistently high-quality food. They are veligers add diatoms (e.g., Chaetoceros) most often used in seasonally operated, Feed rate 10,000-25,000 cells/ml of culture water as back- low-budget hatcheries or as a supple- ground + amount below ment to the Milford method. Stage Feed (cells/clam/day) The Wells-Glancy and brown water early veliger 1,000-5,000 methods both rely on the native phyto- mid-veliger 5,000-10,000 plankton available in the seawater supply. The water is filtered (5 to 15 µm) to late veliger 15,000-30,000 remove zooplankton and used immedi- pediveliger 30,000-50,000 ately as culture water for larvae (the Feed method batch, continuous drip, discontinuous metering brown water method) or allowed to Duration 7-21 days bloom for 24 to 48 hours before being used. Fertilizer can be added to encour- Table 3. General culture requirements for post-set hard clams raised in downwellers housed in reservoir tanks. The system is converted to upwellers as clams grow to 0.5 mm SL. Variable Requirement Water quality 200-500 µm—1 µm filtered with UV or charcoal if needed 500 µm-1mm—supplement above with slow flow of ambient water Salinity >25 ppt Temperature 24-28 °C optimal, 20 to 30 °C acceptable Water exchange 200-500 µm—every other day >500 µm—add flow-through gradually to acclimate Stocking density Size clams/cm2 ml/cm2 up to 600 µm 575 0.03 700 µm 285 0.03 1 mm 140 0.04 Food type mixed diatoms (e.g., Chaetoceros) and flagellates (e.g., Isochrysis) at about a 50:50 ratio Feed rate 25,000-50,000 cells/ml of culture water as background + amount below Stage Size range Feed (cells/clam/day) early post-set 300-400 µm 50,000-75,000 mid post-set 400-600 µm 75,000-150,000 late post-set 600 µm-1mm 150,000-500,000 Feeding methods batch, continuous drip, discontinuous metering Duration 4-6 weeks

PVC cylinder Overhead view (4 in-12 in diameter) Distribution line Tap Inflow

PVC coupling through Reservoir wall of cylinder tank (1 in-2 in diameter) Silo Polyester mesh Gutter Retaining ring

A. Cylinder construction Support rod

Air line Cross-section Drain pipe Water level Elbow Silo Water level Cylinder Tank wall wall Couplings Retaining ring

Clams Gutter Mesh B. Upweller Cleanout Mesh

Water level Trough

Figure 3. Flow-through upwelling nursery system.

age a denser bloom. Shallow tanks are um, although artificial lighting may usually used for this method so that also be used. C. Downweller light can penetrate. Tanks should be The Milford method of algal culture is Figure 2. Downwelling and upwelling aerated gently. This type of culture is a sequential process in which a single post/set culture cylinders. usually done in a greenhouse or solari- species of phytoplankton is grown in batch cultures. Phytoplankton for cul- field grow-out increases with seed mented. However, predators are ture can be obtained by isolating native size. However, it is difficult to keep excluded, fouling organisms are con- species, but they are usually purchased clams larger than 10 mm in a nursery trolled, and conditions necessary for from another hatchery or from a labo- because of space and water flow limi- rapid growth are provided. Clams can ratory that specializes in isolating and tations. Some Florida growers plant be introduced into a nursery at a size producing phytoplankton clones. Most their seed at smaller sizes (6 to 7 mm), of 1 mm or larger and are usually hatcheries grow two or more different but survival varies. grown to a size of at least 8 mm before species of phytoplankton to provide There are many types of nursery sys- they are transferred to field grow-out. the varied rations required by clam lar- tems. These can be roughly divided Clams can grow as much as 2 mm per vae, post-set and broodstock. into land-based and field-based sys- month. Therefore, it will take 3 to 4 The quantity of phytoplankton need- tems. Common land-based systems months (at least) for a 1-mm clam to ed depends on the seed production are raceways and upwelling systems. reach 8 mm, and another month for goals of the hatchery. Algal consump- Field-based systems use a variety of each 2 mm of growth beyond that size tion rates for different size clams are on-bottom and off-bottom containers, The two basic types of onshore or presented in Table 4. Fifty broodstock floating rafts, and floating upwellers. land-based nursery systems are race- (a minimum for a spawning) will Also included under field-based sys- ways and upwelling systems. Both are require about 1.5 x 1011 algal cells tems are nurseries located in protect- energy-intensive, requiring the continu- each day; that will be 30 to 50 liters of ed areas such as impoundments and ous pumping of large volumes of high- dense algal culture containing 5 x 106 ponds. quality estuarine water. Both require cells per ml. One million veliger lar- considerable capital investment for 9 Field-based systems are relatively vae will consume 1 x 10 cells each inexpensive to construct and operate waterfront property and infrastructure. day (1 liter); a similar number of set- but have high maintenance require- Both are labor-intensive, requiring ting size larvae will consume 50 times ments, offer limited protection from daily cleaning and frequent monitoring as much. A million post-set clams will predators, may be subject to environ- of the seed. Despite these drawbacks, require three to four times as much mental damage, and have unreliable onshore nurseries are popular because algae as the broodstock (200 liters per production. Land-based systems are they promote rapid growth and have day). Algal culture is probably the more expensive to construct and oper- high seed survival rates. most time-consuming and challenging ate but provide almost complete Raceways element of the hatchery operation, predator control, ease of access, and but it is essential to its success. near-optimal conditions for the growth Raceways are shallow, rectangular and survival of seed clams. trays that may be stacked in tiers. Nursery culture Water is introduced at one end, flows When post-set clams reach a size of 1 Land-based systems over the seed clams, and exits at the mm SL (retained on a 710-µm screen) A land-based nursery is a semi-con- other end through a drain. Ten to 20 they can be transferred to a nursery trolled environment for culturing juve- liters of water per minute should be system. The purpose of a nursery is to nile clams. Water is usually not treat- supplied for each liter of large seed provide a protected environment for ed except for gross (200 µm) filtration clams (8 mm). Although raceways pro- small seed until they reach a size (8 to to reduce the influx of fouling organ- duce rapid growth with good survival, 10 mm) suitable for field grow-out. isms (e.g., barnacles and oysters), and they are not well-suited to areas where Studies have shown that survival in the natural food supply is not supple- the water supply has a high load of fine silt. Silt settles in the raceways and creates labor-intensive maintenance Table 4. Estimated consumption rates and recommended feeding rates problems. If not cleaned daily, small for different life stages of hard clams fed a diet of Isochrysis galbana. seed clams suffocate in this fine silt. Cleaning is particularly difficult if the Feeding rate seed are very small (less than 4 mm). (concentration Another drawback with raceways is Consumption rate in culture that clams near the drain end receive Life stage Age/Size cells/clam/day tank: cells/ml) only water that has already passed over all the other seed and is largely Veliger 1-2 days 1,000-5,000 10,000-75,000 devoid of useable food. Thus, the seed 3-5 days 5,000-10,000 must be rotated in the raceway to 5-8 days 10,000-15,000 achieve uniform growth. Finally, raceways cannot support large biomasses 8-14 days 15,000-30,000 of seed. A typical raceway can support 2 Pediveliger 14-21 days 30,000-50,000 about 0.2 L of large seed per 0.1 m (1 ft2). This is about 1,400 seed at 8 mm Early post-set 300-400 µm 50,000-75,000 25,000-100,000 SL or 700 at 10 to 12 mm SL. Race- Mid post-set 400-600 µm 75,000-150,000 ways probably should not be used for seed smaller than 4 mm SL because of Late post-set 600 µm-1mm 150,000-500,000 the cleaning difficulties. Seed need Broodstock >35 mm 1-3 x 109 50,000-200,000 about 2 months to grow from 4 to 8 mm in raceways. Upwelling systems ern U.S. Two of these are powered by the obvious advantage of low operat- airlifts and one is tidal-powered. ing costs (maintenance only), but is Upwelling systems are probably the Operation and maintenance is similar vulnerable to storm damage. most widely used land-based nursery to that for a land-based system. Floating systems. Although they, too, have high upwellers have been used with very Bottom culture capital costs and energy expense, small seed (1 mm), but their success upwellers promote the rapid, uniform Various bottom culture nursery sys- depends on proper site selection. growth of seed; they are easier to tems are in use in the southern U.S. maintain than raceways and make Both airlift systems are designed for These include trays supported off the more efficient use of space. An use in protected waters such as salt- bottom on racks in subtidal areas, upwelling system consists of a reser- water ponds or impoundments. They trays planted on or just above the voir (usually rectangular and about 2 can also be moored to a dock in a bottom in intertidal and shallow sub- feet deep) that contains several culture tidal creek if they will not be exposed tidal areas, and “soft” bags or pens. units (Fig. 3). Each culture unit (“silo”) to heavy wave action. One airlift sys- All bottom culture systems require has a screen bottom that supports the tem uses a raft of 4-inch PVC pipe to frequent maintenance to remove silt, seed clams. The silos are placed so that support the silos and an air manifold. fouling organisms and predators. their sides project above the water The manifold is pressurized with a The subtidal rack and tray system level and the bottom screen is several small air blower. Silos are suspended must be deployed in deep water so it centimeters above the reservoir bot- within the raft and water is circulated will not be a navigation hazard. tom. A drainpipe near the top of the through the silos with airlifts (Fig. 4). Therefore, a large boat is needed to silo extends through the silo wall and The second airlift system consists of a deploy, maintain and harvest the trays. the reservoir wall and empties into an reservoir (tank) that is floated in a Trays are made of wood frames with external common drain. Water is pond or adjacent to a dock. Water is screening top and bottom. Several pumped into the reservoir, rises up pumped into the reservoir with a trays are supported on one rack in a through the screens and the seed large airlift or with a submersible tiered arrangement, with space clams, and exits out the drains near the pump and flows out the silo drains as between the trays for water circula- top. The vertical water flow supports in a land-based system. tion. A rack and tray system is labor- much larger biomasses of clams than A tidal-powered upwelling nursery intensive to maintain but, if properly the horizontal raceway flow. Each 0.1 consists of a raft with a scoop on one sited, promotes good growth and sur- m2 of upweller screen can support 1.0 end and an interior chamber where vival even for very small seed (1 mm to 1.5 liters of planting size seed (7,000 upwellers are housed (Fig. 5). The raft and up). Small seed (1 to 2 mm) can to 10,000 seed at 8 mm SL). The water is tethered from the scoop end so that be planted at very high densities flow requirements for upwellers are it swings to point the scoop into the (50,000 to 150,000/m2, about 5,000 to similar to those for raceways. Each current. Water is directed by the 15,000/ft2) initially, but must be liter of large (7-mm) seed requires a scoop into the silo chamber and flows thinned as they grow. The fine mesh flow of 10 to 20 liters per minute; very out through sideports at the water that must be used to retain very small small seed (less than 3 mm) should level. In pilot testing, this system seed has to be cleaned weekly to receive ten times that much (Table 5). appeared to foster more rapid growth remove fouling organisms and silt. than a land-based nursery. It offers Field-based systems Field-based nurseries are not as reliable as land-based systems, but they Table 5. Flow requirements and stocking densities for seed clams in require less capital investment and an upwelling nursery. are less expensive to operate. They do Size Velocity Flow ratio Density Density not protect clams from predators as mm cm/sec L/min:L clams ml/cm2 clams/cm2 well as land-based systems and are more susceptible to environmental 1 0.6 240 0.15 450 damage (e.g., from storms). They are 2 0.6 90 0.4 120 less accessible and consequently more difficult to maintain, but the mainte- 2 0.5 100 0.3 90 nance is done less frequently. The 2 0.4 110 0.2 60 water flow (and thus food supply) to 3 0.4 70 0.3 30 the seed may be variable, which results in less uniform growth. 3 0.2 80 0.15 15 However, some floating systems rival 4 0.2 40 0.3 12 land-based nurseries in production 5 0.2 30 0.4 8 capability and are relatively inexpensive to build and operate. 6 0.2 20 0.6 7 7 0.2 15 0.8 7 Floating upwelling systems 8 0.2 12 1 7 Three floating upwelling systems have 9 0.2 10 1.2 6 been used to some extent in the south- 10 0.2 9 1.4 4 Smaller trays can be deployed in out predators. Pens have removable Pond culture intertidal or shallow subtidal tops made of the same soft mesh or areas if substrate is firm (sand or of lighter weight plastic netting. The Ponds and impoundments can be shell). The trays are buried so that lid must have a mesh size small used as nurseries to some advan- their tops are level with the surround- enough to exclude most crabs and it tage. They protect clams somewhat ing substrate; then they are filled must fit securely over the edge of the from environmental disturbances with sand and covered with screening pen so that there is no gap where and predators and are usually more (6-mm or 0.25-inch openings). If the crabs can enter. These systems are accessible than field sites, while trays are buried too deep they may not suitable for small seed (less than 4 being less expensive to operate than be covered over by shifting sand or mm) because the mesh size required land-based nurseries. However, it is silt. If they are not buried deep to retain such little seed would foul very expensive to construct a pond enough, they may create scouring rapidly and restrict water flow. so this method is probably an currents that will wash out the tray. Banks that have sandy substrate are often subject to strong currents that Top view may wash substrate out of the trays. Screens must be checked often and PVC Air Cross Support any silt removed from the top screen. pipe line Air support Airlift rods Site selection is important because Silo flow tube heavy silt can kill clams. These trays are probably not suitable for very small seed (less than 2 mm). Trays or polyethylene mesh bags may be placed on racks in intertidal or shallow subtidal areas. Racks may be con- structed of wood, rebar or PVC. Trays Hose from PVC Air Support with solid bottoms are filled with sand air blower Water pipe flow PVC rod or similar substrate. Bags will not level Silo retain substrate and will work only in pipe well-protected sites with no waves or strong currents; bags and are not well- suited to seed less than 4 mm. Seed “Soft” bags are used extensively in Florida for nursery culture and have Side view (partial) Airlift been tried farther north with mixed success. In Florida, cages with several Mesh Water flow tiers of bags have also been tried. Predation of small seed clams (less Cross-section than 7 mm SL) by crabs and rays has been a problem in “soft” bag nurseries. Figure 4. Floating upwelling nursery system. The bags are made of woven polyester in an appropriate mesh size to contain the seed. Bags are anchored to the bot- Working deck Seed bins tom with a weighted rope, rebar or (partially cut away) stakes. A float inside the bag holds the upper side above the seed. In very silty areas, the float is often not ade- quate to prevent silt from collecting on the top of the bag. To mooring A modification of the “soft” bag is the “soft pen.” These pens are construct- ed of the same soft, woven material but have a rigid frame that supports Flotation the upper edge 25 to 36 cm (12 to 18 (partially cut away) inches) above the substrate. The frame is made of PVC pipe and Outflow trough placed on the inside or outside of the (front section cut away) mesh. A recent development is the Tidal Bin screen use of a vinyl covered wire (14-gauge, flow 0.5-inch openings) skirt to support the Tidal scoop soft pen. The wire fencing helps keep Figure 5. Tidal-powered upwelling nursery system. option only where a pond already Chanley, P. 1972. Laboratory cultivation of Manzi, J. and N. Hadley. 1988. Recent advances exists. assorted bivalve larvae. pp. 297-318 in W.L. in nursery culture of bivalve mollusks in North With pond culture the natural food Smith and M.H. Chanley, eds., Culture of America. NOAA, Tech. Rep. NMFS 70, NMFS. supply can be enhanced by managing Marine Invertebrate Animals. Plenum Press. Mook, W. 1988. Guide to construction of a tidal for algal blooms. However, pond man- New York. upweller. Mook Sea Farm, Inc. Damariscotta, agement is complex and poorly under- Hartman, M. 1989. Manual for the design and ME. stood, particularly as it relates to the operation of a low budget hatchery for the hard Mook, W. and A.C. Johnson. 1988. Utilization Mercenaria mercenaria requirements of bivalve culture. In clam in Florida. of low-cost, tidal-powered floating nurseries to addition to the food supply, the pond Aquaculture Report Series, Florida Dept. of rear bivalve seed. Mook Sea Farm, Inc. manager must consider many other fac- Agriculture and Consumer Services, Division of Damariscotta, ME. tors that will affect clam growth and Marketing. (To get a copy, write Aquaculture survival, such as dissolved oxygen, pH, Program, Room 425, Mayo Building, Pfeiffer, T.J. and K.A. Rusch. 2001. Integrated temperature and waste products. Tallahassee FL 32399-0800 or call 904-488-4033. system for microalgae nursery seed clam culture. Global Aquaculture Advocate 4(5)p.33-35. Various sizes of seed have been plant- Free or nominal cost.) Vaughan, D. and L. Creswell. 1990. Field grow- ed in a variety of containment sys- Nursery culture tems, including airlift-driven out techniques and technology transfer for the upwellers, trays and bags supported Baldwin, R.B., W. Mook, N.H. Hadley, R.J. hard clam Mercenaria mercenaria. Florida on racks, trays and bags placed on the Rhodes and M.R. DeVoe. 1995. Construction Department of Agriculture and Consumer bottom, and pond-side upwellers to and Operations Manual for a Tidal-Powered Services, Aquaculture Report Series. Tallahassee, which water is pumped. Seed have Upwelling Nursery System. South Carolina Sea FL. Grant Consortium. also been planted on window screen- Algal culture and diets ing placed directly on the pond bot- Castagna, M. 1984. Methods of growing tom. In pond culture, containers main- Mercenaria mercenaria from postlarval to pre- Benemann, J.R. 1992. Microalgae aquaculture ly facilitate the handling and harvest- ferred-size seed for field planting. Aquaculture feeds. Journal of Applied Phycology 4(3): 233-245. ing of seed and are less important for 39:355-359. Guillard, R.L. 1983. Culture of phytoplankton predator control, as predators are Hadley, N. and J. Manzi. 1984. Growth of seed for feeding marine invertebrates. pp. 108-132 in physically excluded from the pond. clams (Mercenaria mercenaria) at various densi- C.J. Berg, Jr. ed. Culture of Marine Invertebrates. Hutchinson Ross Publishing, References ties in a commercial scale nursery system. Aquaculture 36:369-379. Stroudsburg, PA. Hadley, Nancy H., J.J. Manzi, A.G. Eversole, Malinowski, S. 1988. Variable growth rates of Miyachi, S., O. Nakayame, Y. Yokohama, M. R.T. Dillon, C.E. Battey and N.M. Peacock. seed clams in an upflow nursery system and Ohmari, K. Komagata, H. Sugawara and Y. 1997. A manual for the culture of the hard clam the economics of culling slow growing animals. Ugawa, eds. 1989. World Catalogue of Algae. Mercenaria spp. in South Carolina. South Journal of Shellfish Research 7(3):359-366. Japan Scientific Societies Press, Tokyo. Carolina Sea Grant Consortium. Malinowski, S. and S. Siddall. 1989. Passive Ogle, J.T. 1982. Operation of an oyster hatchery Manzi, J.J. and M. Castagna, eds. 1989. Clam water reuse in a commercial-scale hard clam utilizing brown water culture technique. Journal Mariculture in North America. Elsevier, NY. upflow nursery system. Journal of Shellfish of Shellfish Research 2:153-156. 461 pp. Research 8(1):241-248. Walsh, D.T., C.A. Withstandley, R.A. Krause Hatchery culture Manzi, J.J. and M. Castagna. 1989. Nursery and E.J. Petrovitz. 1987. Mass culture of select- culture of clams in North America. pp 127-148 ed marine microalgae for the nursery produc- Brown, C. 1983. Bacterial disease in bivalve lar- in Manzi, J.J. and Castagna, M. ed., Clam tion of bivalve seed. Journal of Shellfish Research val cultures and their control. pp. 230-242 in Mariculture in North America. Elsevier, NY. 6:71-77. C.J. Berg, ed. Culture of Marine Invertebrates. Illustrations © John Norton Selected Readings. Hutchinson Ross Pub. Co. Manzi, J., N. Hadley, C. Battey, R. Haggerty, R. (www.johnnortonart.com) Stroudsburg, PA. Hamilton and M. Carter. 1984. Culture of the northern hard clam in a commercial-scale, Castagna, M. and J. Manzi. 1989. Clam culture upflow, nursery system. Journal of Shellfish in North America: Hatchery production of nurs- Research 4(2):119-124. ery stock clams. pp 111-126 in Manzi, J.J. and M. Castagna, eds. Clam Mariculture in North America. Elsevier, NY.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have not been revised are considered to reflect the current state of knowledge.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 2004-38500-14387 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.