More Information on Classifying Oyster and Clam Harvesting Waters in Maryland

Shellfish are filter-feeding organisms that strain the surrounding water through their gills which trap and transfer food particles to their digestive tract. If the water is contaminated with disease-causing bacteria, the bacteria are also trapped.  For this discussion, the term shellfish does not include crabs, lobsters, or shrimp, and does include all bivalve mollusks such as the clams, oysters, and mussels. Because shellfish pump large quantities of water through the gills each day, they have the potential to concentrate human pathogens.  Some of the most common illnesses associated with consumption of shellfish from polluted waters include norovirus, hepatitis, and salmonella.  While cooking is an effective way to minimize the risk of these pathogens, shellfish are often eaten raw or partially cooked, and illness may result. Therefore, it is mandatory that shellfish be harvested only from approved harvesting waters.

Because shellfish harvested from polluted water may cause human illness, the sanitary control of the shellfish industry is necessary. Maryland is a member of the Interstate Shellfish Sanitation Conference (ISSC), a voluntary, cooperative association of states, the in shellfish industry, U.S. Food and Drug Administration (FDA), National Oceanic and Atmospheric Administration (NOAA), and Environmental Protection Agency (EPA). State responsibilities include adopting laws and regulations for the sanitary control of shellfish and the shellfish industry, formulating comprehensive shellfish harvesting area surveys and adopting control measures to ensure that shellfish are grown, harvested and processed in a safe and sanitary manner. FDA reviews methods for shellfish sanitation inlcuding for classification and management of shellfish areas proposed by the ISSC, and incorporates those methods consistent with standard health practice into the National Shellfish Sanitation Program (NSSP) Model Ordinance. FDA is also responsible for the annual audit of each state shellfish control program to determine conformity with the NSSP standards and guidelines. NOAA and EPA provide technical assistance and comment to the ISSC. The shellfish industry responsibilities include participation in the ISSC Conference, meeting the requirements of the NSSP Model Ordinance.  These responsibilities include obtaining shellfish from safe sources, maintaining sanitary operating conditions and making records available that document location of harvest and sale of all shellfish. FDA, MDE and shellfish industry fulfill their responsibilities to a high degree, thus ensuring shellfish harvested in Maryland are safe and wholesome. 

In Maryland, responsibility for the sanitary control of the shellfish industry is shared by three state agencies: the Department of the Environment (MDE), the Department of Health and Mental Hygiene (DHMH), and the Department of Natural Resources (DNR). MDE is responsible for identifying and eliminating pollution sources affecting Maryland's shellfish harvesting waters, as well as classifying shellfish harvesting waters so that shellfish harvested are safe for human consumption. DHMH is responsible for any food control measures necessary to ensure that shellfish are harvested, processed, packaged, and transported under sanitary conditions. DHMH also regulates shellfish dealers to assure compliance with the required sanitary standards. DNR is responsible for managing the fishery, enforcing sanitary harvesting practices and harvest restrictions, posting areas restricted to shellfish harvesting and for patrolling/enforcing these areas to prevent illegal harvesting.

NSSP/MDE Standard

Approved: (direct harvesting permitted)
Median <14 MPN/100ml 90th Percentile <49

Conditionally Approved (direct harvesting permitted only as outlined under heading Shellfish Water Classification below):
For conditionally approved Median <14 MPN/100 ml and no more than 10% can exceed 49 when the area is in the open status.

Restricted: (relay required)
Median <88 MPN/100ml 90th Percentile <300

Prohibited: (no growing or harvest permitted)
Median >88 MPN/100ml 90th Percentile >300 or an area that has a high risk for human pathogens to be present.​

Relay of Shellfish from Restricted Shellfish Waters to Approved Shellfish Waters

Documented research and studies with Eastern Oysters grown on natural bars have demonstrated that when the oysters are transplanted from moderately polluted to clean environments they will naturally attenuate accumulated indicator bacteria and associated pathogens over a 14-day period if environmental conditions are favorable for active filtering and feeding by the oysters.

Shellfish Waters Classification

Shellfish waters are classified by MDE based on sanitary, hydrographic, meteorological and bacteriological surveys. Sanitary surveys identify waters where contaminants may be present in amounts that present a health hazard; hence, should not be open to harvest. The bacteriological survey identifies waters meeting NSSP fecal coliform standards. A comprehensive shellfish harvesting area survey is written for each shellfish harvesting area to document the methods and findings of these surveys, as well as proposed changes in classification and management. NSSP guidelines require that these reports be updated annually. MDE monitors these areas monthly and reevaluates all areas as needed, including aquaculture sites.  Areas that do not comply with sanitary requirements are to be immediately reclassified or closed.  Once an aquaculture site is identified for commercial harvest MDE will establish water quality monitoring stations and begin sampling and conduct a shoreline survey to properly classify the aquaculture site.  These sites require a minimum of 18 months to properly classify the waters.  The area is classified as approved, conditionally approved, restricted, or prohibited.  See above for MDE/ NSSP standards for each type of shellfish water classification.  Please note that an area classified as *Conditionally Approved must meet the Approved standard for those days that the area is open to harvest.  Direct harvest is permitted from Conditionally Approved areas except for the three days following a rainfall event of greater than one inch in 24 hours. Daily information regarding conditional closure is available via a daily recording, (800) 541-1210 or on MDE’s web siteThe most recent monitoring data and current classification of all Maryland’s harvesting areas are on MDE’s web site.

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