There is a mussel quarantine for wild mussels throughout the coast. There's an old saying to never eat shellfish in the months that do not have an "R" in them. There's something called a red tide. This fine plankton blooms in hot weather and mussels take it in and become so toxic they give you paralytic shellfish poisoning (PSP). This can cause anything from a tingly quality on your lips to a serious health threat.
You should not harvest mussels in the summer through October 31st.
Commercially grown mussels are grown in protected areas and monitored constantly throughout the summer.