People have been enjoying clams, oysters, and mussels for thousands and thousands of years. In fact, the ancient Romans actually farmed oysters. Today, almost all of the molluscan shellfish (clams, oysters, and mussels) sold come from farms or aquaculture where the shellfish are raised under semi-controlled conditions.

Molluscan shellfish farms can operate in a variety of ways, using several different production methods. In general, shellfish are grown either on-bottom, where juvenile shellfish are planted directly on the sediment, or off-bottom, where shellfish are placed in racks, bags, or cages that are suspended or off-the-bottom lifted. In some situations, those structures are located in the intertidal zone, which means the shellfish are covered during high tide and are exposed during low tide. This makes it easier to perform routine maintenance. While shellfish larvae and juveniles can begin their lives in tanks, final grow-out to market size typically occurs in natural water bodies in coastal areas.

Microscopic shellfish larvae can be collected from the wild, but shellfish growers more commonly rely on hatchery-produced seed. In the hatchery, sperm and eggs from spawning adults are mixed. Larvae continue to grow in nursery systems, where they are fed high concentrations of cultured microalgae.

When the animals are large enough, they are transplanted into approved growing waters. Since they are filter feeders, they feed on the natural algae in the water column. No additional feed, fertilizers, drugs, or chemicals are used. Once transplanted, farmers often implement a variety of methods to protect them from predators like starfish, crabs, snails, and finfish.

Shellfish positively impact the environment by removing algae from the water column. This improves light penetration and incorporates the excess nitrogen and phosphorous, which promotes algae growth, into their tissues and shells. Too many algae in the water column result in lower available oxygen. Overall, the removal of excess nutrients and algae improves water quality, and the structures created by large shellfish beds provide habitat and hiding places for juvenile fish and other marine organisms. This adds to biodiversity, which is a cornerstone of a healthy ecosystem.

According to the Woods Hole Oceanographic Institute, an adult oyster can filter as many as 50 gallons of water per day, while an adult clam can filter up to 24 gallons per day, depending on environmental conditions.¹ In many areas, such as Chesapeake Bay and New York Harbor, environmental groups are actively planting shellfish beds to restore wild populations and improve water quality. According to the Chesapeake Bay Foundation, the oysters in the Bay could once filter a volume of water equal to that of the entire Bay (about 19 trillion gallons) in a week. Today, it would take the remaining Bay oysters more than a year.²

During the 19th and early 20th centuries, oysters were extremely popular in many cities, where they were enjoyed by the rich and poor alike. They were sold by street vendors much like hot dogs are sold today. City streets were dotted with popular oyster cellars. Business was booming, but then public health officials noticed an uptick in the number of reported cases of illness associated with the consumption of raw shellfish. Shellfish are filter feeders and may concentrate microorganisms (bacteria and viruses), as well as natural toxins and chemicals, if they are present in the growing waters. This can pose serious risks to consumer health and safety, since oysters are often consumed raw and whole.

In 1925, the Surgeon General of the U.S. Public Health Service called a meeting to develop control measures to protect the public. That meeting led to the development of rules to improve sanitation in the shellfish industry. Those rules gave rise to the present National Shellfish Sanitation Program (NSSP).

The current NSSP dictates uniform requirements that every state must meet with federal oversight provided by the U.S. Food and Drug Administration (FDA). Several other agencies provide advice and consultation including the Environmental Protection Agency (EPA), the Fish and Wildlife Service, the National Oceanic and Atmospheric Administration (NOAA), the U.S. Department of Agriculture (USDA), and the Army Corp of Engineers.

The NSSP requires states to maintain minimum sanitation standards addressing issues such as water quality monitoring, harvest area enforcement, training of harvesters and dealers, processing, shipping, and handling. These controls were established to create standard best practices for growing and harvesting shellfish to ensure safe consumption and distinguish shellfish harvested as food from those planted for environmental restoration.

Shellfish Safety Programs

All shellfish must be sold through a licensed dealer, and every dealer is required to have appropriate training in the principles of Hazard Analysis and Critical Control Points (HACCP), develop a HACCP plan in accordance with the Seafood HACCP Regulation (21 CFR 123), and adhere to Good Manufacturing Practices (GMPs). The HACCP regulation requires all producers to develop and implement a preventive food safely plan (HACCP plan) that identifies and controls potential hazards associated with their shellfish and how they are handled and processed.

GMPs ensure that dealers follow dozens of specific requirements covering issues such as facility water quality and use; employee hygiene; shellfish plant sanitation; equipment sanitation, use, and maintenance; plant construction and maintenance; appropriate storage of potential chemical contaminants; and control of pests. Shellfish harvesters must be licensed by the shellfish control authority in each state. The authority is responsible for ensuring that HACCP and GMP requirements are met under the guidance of the NSSP model ordinance. That license ensures that shellfish are harvested from approved waters and are handled in a safe manner.

The HACCP System

The harvest, holding, transport, and sale of shellfish are tightly regulated to reduce the risk of foodborne illness to consumers. All seafood processors, including those who hold and transport shellfish, are required to be in compliance with the Seafood HACCP regulation (21 CFR 123). All producers are required to develop and implement a HACCP plan, which is a preventive food safety plan meant to proactively identify potential hazards and establish controls to prevent them from occurring.

Dealers are required to conduct a hazard analysis to identify potential food safety hazards for each type of shellfish they handle and identify points in their process where those hazards can be controlled. The hazard analysis should identify hazards that can occur both inside and outside the processing facility, including before, during, and after harvest.

Every HACCP plan includes seven components:

1. Hazard analysis: Production, harvest, and processing methods are analyzed to identify all potential food safety hazards that could occur in a facility, species, and the products produced
2. Critical control points: Producers must identify specific points in their process where the potential hazards can and will be controlled
3. Establishment of critical limits: Minimum or maximum thresholds necessary to control a hazard must be established
4. Monitoring procedures: Monitoring procedures must be instituted to continually ensure that identified critical limits are met
5. Corrective actions: Actions are identified to address deviations from critical limits
6. Recordkeeping systems: Producers must keep records of all monitoring, verification, and corrective actions taken
7. Verification procedures: Producers must implement verification procedures to validate that the selected controls are effective (e.g., equipment accuracy checks and calibration).

Time and temperature considerations are another important component of maintaining shellfish safety and a crucial part of any HACCP plan. Small levels of bacteria are naturally present in shellfish and do not cause a health hazard. However, when shellfish are handled improperly, those small amounts could grow and result in human illness. Preventing significant bacterial growth is achieved through temperature control.

The HACCP system is not a standalone program. Dealers must also comply with prerequisite programs to ensure safe and sanitary food production. All food processors are required to abide by current Good Manufacturing Practices (cGMPs). Dealers are also required to develop adequate sanitation procedures and keep records of their sanitation practices.³

Food Safety Modernization Act

The Food Safety Modernization Act (FSMA) of 2011 expanded the food safety requirements specific to seafood producers. GMPs were updated to include more controls related to allergens, cross-contamination, training, and training records for staff. In addition, FSMA's Final Rule on Sanitary Transportation of Human and Animal Food was published in the Federal Register in 2016. This rule focuses on ensuring that all foods are transported safely and emphasizes the importance of training programs and training records.

National Shellfish Sanitation Program (NSSP)

Since shellfish are often consumed raw, extra precautions have been established to help ensure that they are safe for consumption. In addition to the mandatory FDA Seafood HACCP program, the National Shellfish Sanitation Program (NSSP) is the federal and state cooperative program overseen by FDA and the Interstate Shellfish Sanitation Conference (ISSC) for the sanitary control of shellfish sold for human consumption. FDA provides formal oversight to ensure that all states meet uniform minimum guidelines.

The ISSC provides a formal structure for state regulatory agencies. Participants in the ISSC include state agencies, FDA, EPA, NOAA, and the shellfish industry.The ISSC publishes and regularly updates the NSSP Model Ordinance, which provides minimum standards to help ensure that shellfish have not been adulterated during cultivation, harvest, processing, shipping, or handling.⁴

The model ordinance helps ensure uniform regulation of shellfish production and harvesting across the nation. Each state designates a shellfish control authority that is responsible for monitoring and enforcing the model ordinance. State agencies, in cooperation with FDA, administer a certification program requiring wholesale shellfish dealers to harvest, handle, process, and ship shellfish under sanitary conditions and maintain records verifying that the shellfish were harvested from approved waters. Strict temperature control regimes, maintaining shellfish below 45 °F (7.2 °C), are outlined in the NSSP model ordinance and required of all shellfish dealers.

Shellfish Harvesters

Shellfish harvesters must be licensed by the designated state shellfish authority. Harvesters are required to complete a training program that covers safe practices for the harvest, handling, and transport of shellfish to prevent contamination, deterioration, and decomposition. Harvesters who pack shellfish for sale and distribution are also considered dealers. Dealers are required to acquire a separate certification, which can be obtained only after an onsite inspection is completed and the inspector confirms that the dealer has an effective HACCP plan and sanitation procedures in place. Dealer certifications must be renewed annually. They are also responsible for appropriate tagging of shellfish and must sell only to a certified shellfish dealer. They must provide that dealer with trip records that include time and temperature information.

Interstate Shellfish Shippers List

To ensure that shellfish on the market in the U.S. are harvested and handled in accordance with NSSP guidance, FDA manages and publishes a list of certified dealers monthly. The Interstate Shellfish Shippers List⁵ indicates those companies certified by recognized regulatory authorities to ship shellfish in accordance with the uniform sanitation requirements outlined in the NSSP model ordinance.

Shellfish Growing Waters

Approved growing waters are designated by the state shellfish authority—the agency responsible for regulating shellfish harvest and aquaculture. Approved waters are monitored regularly to ensure adequate water quality with low concentrations of pathogens and other toxins. This is crucial to ensure that shellfish are safe for consumption, as they are filter feeders and can pick up toxins and bacteria from the water.

Growing waters are routinely inspected by state agencies and can be classified as approved, conditionally approved, or restricted. Shellfish from approved waters can be marketed directly. Conditionally approved waters are open when water quality conditions permit. For example, waters may be closed during periods of heavy rain and increased runoff. Shellfish harvested from restricted waters must undergo either a relay or a depuration program to help ensure that pathogens and toxins are removed before they can be marketed. In a relay program, the shellfish are transplanted to approved waters for a specified amount of time before they can be harvested. In a depuration facility (Figure 1), the shellfish are held in a strictly regulated artificial environment until they are thoroughly purged of any potential chemical or bacterial hazards that may be present. States routinely patrol shellfish areas to help ensure that product is being harvested appropriately by licensed shellfish growers and harvesters.

FIGURE 1. Shellfish Depuration Facility

Shellfish Tags

All bivalve molluscan shellfish (clams, oysters, and mussels) must be tagged with information about when they were harvested, who harvested them (name and address), and where they were harvested. The tag must also include the shellfish shipper's number. This helps ensure that shellfish are harvested from approved waters by licensed harvesters. It also allows investigators to trace the source of any shellfish implicated in an illness or outbreak. An outbreak is defined as two or more illnesses traced to the same source.

Traceability is a key component of the shellfish sanitation program. Tags must remain attached to the carton, bag, or container until it is emptied or retagged. Once the container is empty, the tag must be kept on file for an additional 90 days. This system allows for traceability in the unlikely event of a shellfish-related illness. The shellfish tag also contains a warning message for high-risk individuals.

If someone becomes ill from consuming shellfish, then the tagging requirements outlined in the model ordinance allow states to quickly identify the waters from which those shellfish were harvested. The potentially contaminated harvest area can be closed immediately for testing, and other potentially affected shellfish can be traced, recalled, and removed from the market or destroyed. All buyers must ensure that their shellfish are properly tagged and come from a certified dealer.

High-Risk Groups and Raw Molluscan Shellfish

Molluscan shellfish, including clams, oysters, and mussels, are filter feeders and can accumulate marine bacteria and viruses. Vibrio parahaemolyticus and Vibrio vulnificus are two of the most common marine bacteria that can cause gastroenteritis. Both are naturally occurring in the marine environment and are not associated with pollution. They are often found in warmer southern waters and during the summer. Vibrios can also be carried by finfish, shrimp, and crabs, but they are easily killed by cooking. These organisms can cause fever, chills, vomiting, abdominal pain, nausea, and other gastrointestinal symptoms. In a few high-risk individuals, the symptoms may be more severe or even life-threatening.

Although these pathogens occur naturally in the marine environment, they are often in low concentrations. However, once shellfish are harvested, improper handling—specifically exposure to temperatures above 45 °F (7.2 °C)—can result in rapid growth of harmful bacterial pathogens. The strict regulatory oversight governing the harvest and sale of shellfish in the U.S. helps ensure that this does not occur, but it is also important for buyers to understand and properly handle shellfish.

High-risk individuals include those who may have compromised or weakened immune systems due to a variety of health conditions, as well as people over 65 years of age, pregnant people, and young children. Those individuals should not eat raw or partially cooked finfish or shellfish. Since thoroughly cooking oysters, clams, mussels, and finfish will destroy bacteria, vulnerable populations can continue to enjoy these seafoods in fully cooked preparations.

Harmful Algal Blooms (HABs)

Algal blooms occur when colonies of specific, naturally occurring microalgae grow rapidly. Algae make up the bottom of the food web and are an important source of food for marine animals. While not all algal blooms are harmful, some algae produce harmful toxins. When these toxin-producing algae grow to high concentrations, the toxins produced rise to high levels. At these levels, they can harm marine life and cause illness in humans.

Since the population of algae is so large during a bloom, the water can change color. The color varies depending on the species growing, with red, green, and brown being the most common. Cooking does not destroy the harmful toxins produced by some algae, so this can even be a concern in cooked shellfish. When HABs are identified, growing areas are immediately shut down.

In summary, clams, oysters, and mussels have an impressive nutrient content and can play an important role in a healthy diet. They are naturally low in calories, high in good-quality protein, and an important source of vitamins and minerals. In the U.S., extensive shellfish safety regulations help ensure that shellfish is safe for human consumption.

References

1. Koenig, Erin. "Can Clams and Oysters Help Clean Up Waterways?" Woods Hole Oceanographic Institute. January 22, 2018. https://www.whoi.edu/oceanus/feature/can-clams-and-oysters-help-clean-up-waterways/.
2. Chesapeake Bay Foundation. "Saving the Bay through education, advocacy, litigation, and restoration." https://www.cbf.org/.
3. U.S. Food and Drug Administration. "Managing Food Safety: A Manual for the Voluntary Use of HACCP Principles for Operators of Food Service and Retail Establishments." April 2006. https://www.fda.gov/food/hazard-analysis-critical-control-point-haccp/managing-food-safety-manual-voluntary-use-haccp-principles-operators-food-service-and-retail.
4. U.S. Food and Drug Administration. "National Shellfish Sanitation Program (NSSP)." Current as of October 29, 2020. https://www.fda.gov/food/federalstate-food-programs/national-shellfish-sanitation-program-nssp.
5. U.S. Food and Drug Administration. "Interstate Certified Shellfish Shippers List." https://www.fda.gov/food/federalstate-food-programs/interstate-certified-shellfish-shippers-list.

Linda J. ODierno is Outreach Specialist with the National Aquaculture Association.

Michael Ciaramella is Seafood Technology Specialist with the New York Sea Grant, part of the Cornell Cooperative Extension.

Bob Rheault is Executive Director of the East Coast Shellfish Growers Association.