Shellfish beds are closed when they are subject to accidental releases of untreated sewage or hazardous materials such as petroleum products. Beds in close proximity to wastewater treatment plant outfalls and marinas are permanently closed. Areas where no sanitary survey has been conducted are closed as a safety precaution. Additionally, if the sanitary survey is not kept current through mandatory three-year updates, the area must be closed. Incidences of Red Tide, which can cause Paralytic Shellfish Poisoning, will also cause closures.
Red tide is caused by blooms of microscopic algae which produce a potent neurotoxin.
When these algae populations reach high densities, the toxin can be concentrated in shellfish, which in turn pose a potential health risk to shellfish consumers. Once levels of toxin in shellfish reach an established threshold, shellfish harvesting is stopped in affected areas. Some shellfish beds are closed for resource conservation for some or all of the summer. Rains can cause temporary shellfish bed closures because stormwater runoff is known to transport pollution into surface waters. Some shellfish beds will be closed after a rainfall event until adequate flushing of the shellfish growing areas has occurred and the shellfish themselves have had a chance to purge themselves of pathogens.