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Several species of clams are commonly cultured and consumed in the U.S., depending to some extent on what region of the country the farming operations and markets are located. The Hard clam, or quahog, is perhaps the most recognizable of these species. The Northern quahog, (Mercenaria mercenaria) is found along the Atlantic coast from Florida as far north as southern Canada. Northern quahogs have been introduced and established in the Gulf of Mexico and various locations along the Pacific coast of North America, as well as in Great Britain and France. Hard clams are found mainly in open waters with flat bottoms of sand or mud, burrowed just below the surface. The closely related Southern quahog (M. campechiensis) ranges from Florida to the southeastern portions of Mexico, in habitats similar to those of M. mercenaria.
The Soft clam (Mya arenaria) is also found along the Atlantic coast. Its natural range is from Eastern Canada southward to North Carolina. It has been introduced to the Pacific coast of North America (from San Francisco to Alaska), Europe (Norway south to Portugal) and the Mediterranean.
Historically, most clam commerce in North America centered on the northern quahog, even before Europeans arrived on the continent. Native Americans used quahogs for food and also fashioned their shells into tools and jewelry. After centuries of local consumption along the Atlantic coast, the advent of rail transport and cold storage on ice greatly expanded markets inland during the 1900’s. Markets for the Soft clam, (Mya arenaria) also expanded rapidly at that time.
On the West Coast of the U.S., clam culture is primarily focused on the Manila clam, (Venerupis philippinarum). The Manila clam is a fast growing non-native species that was probably introduced inadvertently with oysters from Japan in the 1930’s. The native littleneck Protothaca staminea is also farmed on the Pacific coast. Seed production for this species is mostly dependent on wild stocks – historically there has been little to no hatchery seed apart from a small amount produced in Alaska. The geoduck (Panope abrupta), is also cultured in some areas along the Pacific coast.
Spawning of hard clams, and many other species, occurs throughout the warmer months of the year. Hatchery spawning relies on the ability to fluctuate water temperatures (~21o C to ~29o C) in order to trigger the release of eggs and sperm. Fertilized eggs are then filtered out of the water and transferred to other tanks, where they hatch quickly. Larval rearing, ideally at about 26o C, begins with a diet of natural algae supplied by pumping through upwellers, downwellers or traditional tray-like tanks.
Hatchery methods for Manila clam were first developed in the late 1970’s, and grow-out methods have advanced since then. Procedures are generally similar to those outlined for hard clams. Most nursery production utilizes upwelling systems, but intertidal trays are also common.
Algae production is a major cost factor in many clam hatchery operations, especially where natural supplies are not available or inadequate. In general, clam hatcheries require significant investment and professional staff with a high degree of specialization.
Following development in the hatchery stage, remote setting is possible with Manila clams. But once hard clams and several other commercial species attain a size of 1+ mm nursery rearing begins, often on land using upflow/downflow cylinders or shallow trays with pumped seawater to provide oxygen and natural food and to remove wastes before they can accumulate. Other nursery methods involve floating trays or submerged mesh bags in open water. Both approaches require direct access to high-quality seawater, and each involves trade-offs between costs and yields. Fouling can be a problem during this phase of production, and early research trials suggested superior results were obtained from on-bottom strategies. In general, the nursery phase results in growth to as much as 7-15 mm, then young clams are moved to grow-out beds.
Grow-out takes place in natural habitats near the shore. Young clams should be at least 5 - 7 mm in size when grow-out begins. Clams are typically planted in a sandy or muddy substrate, covered with a protective mesh to discourage predators, and then left to grow naturally until they are harvested after 1.5 to 3 years, or occasionally longer periods of time. Juvenile clams burrow into the sand/mud bottom, then extend their siphons up to the water to obtain oxygen and filter out natural feed like algae. If the predator mesh is excessively fouled, clams will suffer and/or die from lack of food and oxygen. Some species are also grown in soft mesh bags buried at sufficiently shallow depths to allow the clams to access the water above the sediment.
Access to and use of these beach-side water bottoms is subject to various regulatory requirements, and these vary from state to state. Leases from the state are often the most appropriate legal mechanism, although outright ownership can be a consideration in some states. Local regulations may also apply, so the mechanisms for securing the use of water bottoms for clam production and harvest should be investigated prior to starting a clam grow-out business.
Depending on the scale of the operation and applicable regulations, when sediment-planted clams are ready for harvest they can be retrieved with rakes or hydraulic dredging. When producing quahogs, smaller clams are generally more valuable than excessively large ones. Little necks is the common term for hard clams between 1 and 2 inches in shell height, while clams that have reached 2 to 3 inches are referred to as cherrystones. Larger sizes are generally referred to as chowders, and command significantly lower prices.
Clams are relatively resistant to pathogens commonly found in oysters, but mortalities associated with protozoans can be substantial in certain circumstances. Harmful algal blooms can also result in temporary, localized impacts. These can include significant reduction in growth, poor meat quality, mortality and reduced or halted reproduction.
The 2018 USDA Census of Aquaculture reported a total of 312 clam farming operations in the U.S. (all species included), with combined sales of $136,153,000. Hard clam production included 218 farms, 198 of which produced market size clams with a value of $52,096,000. A total of 66 farms produced Manila clams, with a combined value of $28,841,000. The bulk of Manila clam production and sales was attributable to 59 operations reporting sales of market size clams. Clam farming was reported from 17 states, led by Florida with 95 farms, Washington with 79, and Virginia with 43.
A number of regulatory agencies are involved in production and marketing of clams. Natural resource agencies and health departments are among the most important in most situations. As with many other aquacultured species that share the market with wild-harvested product, regulations are generally focused on protecting wild stocks from illegal harvests. Sales of wild clams fraudulently represented as farmed product can cause problems for consumers and producers. Regular reporting and communication with state and local departments of health must also be practiced to ensure compliance with procedures for monitoring and certifying the safety of harvested shellfish. License and permit requirements often depend on whether an operation simply harvests and sells to a licensed dealer, or buys, ships, shucks and packages product.
Paralytic and amnesic shellfish poisoning are significant threats in some regions, so it is critical to follow all monitoring, reporting and certification procedures. Closures due to harmful algal blooms or other health concerns can disrupt harvesting and marketing, as well as cash flow. Similar impacts were felt by clam producers as a result of restaurant closures during the recent pandemic. Poaching can also be a problem for some producers, but a combination of regulatory reporting requirements and word of mouth within the various available market chains tends to minimize these losses in most instances.
Depending on the species and region, as well as applicable regulatory requirements, various market channels are available for most clam growers. Some utilize direct sales, some sell to retailers or wholesalers, and many sell to processors, who in turn sell clam meats to wholesalers, retailers, restaurants and consumers. In every instance the post-harvest emphasis should be on offering a consistently high quality product. Clam farmers should consider using purging tanks or cages, especially if selling directly to retailers, restaurants or consumers. However, many direct sales strategies will require special permits and licenses, including HACCP training.
Once clams are purged, quality control continues to be an important marketing consideration. Sudden or drastic temperature changes can reduce product quality or lead to high mortality. It is important to avoid direct contact of clams with ice, but clams should always be gradually cooled to approximately 40o F after purging. Moist, fresh air is essential during storage and transport, but standing water should be avoided in storage areas or containers.
A reliable supply of seed is essential in order to accurately program production and marketing. Without regular, consistent product, sales will be on a commodity level and growers are forced to take whatever price is offered. For their part, many hatcheries require advance orders for sales of seed clams. Fortunately, the relative slow growth of most clams in traditional growing regions allows for more flexibility in scheduling harvests than would be the case with many other aquatic species. Although growers should avoid slow-growing seed, in some circumstances it may make sense to request clam seed that includes a mixture of individuals with both fast and average growth rates.
- Small Business Administration Guide to Licenses and Permits https://www.sba.gov/business-guide/launch-your-business/apply-licenses-permits U.S. SBA.
- Selling Fish and Shellfish in Washington State. https://cms.agr.wa.gov/WSDAKentico/Documents/DO/RM/RM/33_SellingFishAndShellfish.pdf Washington State Dept. of Agriculture.
- Marketing Your Shellfish: A Resource for Shellfish Harvesters and Growers in Rhode Island http://www.dem.ri.gov/riseafood/documents/rishellfishmktguide012017.pdf Rhode Island Sea Grant.
- Shellfish Handling https://portal.ct.gov/DOAG/Aquaculture1/Aquaculture/Shellfish-Handling Connecticut Department of Agriculture.
- Interstate Certified Shellfish Shippers List https://www.fda.gov/food/federalstate-food-programs/interstate-certified-shellfish-shippers-listU.S. Food and Drug Administration.
- Association of Food and Drug Officials http://www.afdo.org/
- Interstate Shellfish Sanitation Conference http://www.issc.org/
- National Shellfish Sanitation Program http://www.fda.gov/Food/GuidanceRegulation/FederalStateFoodPrograms/ucm2006754.htm
- The Maine Shellfish Handbook https://www.maine.gov/dmr/shellfish-sanitation-management/documents/MaineShellfishHandbook-AccessibleVersion.pdfUniversity of Maine Sea Grant.
Prepared by C. Greg Lutz, Louisiana State University Agricultural Center