The definition of commercial shellfish varies from state to state. Generally it is meant to apply to oysters, blue mussels, hard and soft shell clams, scallops and whelk. Some states include blue crabs and lobsters under their shellfishing regulations for simplicity's sake.
Hard shell clams are also known as quhogs. Soft shell clams are the variety known to most Americans as simply clams.
Dredging, a form of trawling, is considered to be the most common method of commercial shellfishing. Trawling is the towing of a funnel shaped net through the water. It can be used to catch both bottom dwelling groundfish and mid-depth fish. Weighted nets are towed along the ocean floor. Trawlers catch groundfish such as pollock, sole, cod, flounder and shrimp. Bottom trawling in unforgiving on the ocean floor's terrain and is not an environmentally friendly method.
Trawling with a heavily weighted frame on the sea floor is called dredging. Dredging is used to harvest shellfish such as hard and soft shelled clams, scallops and oysters.
Because shellfish are bottom dwelling species they have historically been impacted by pollution. Presently that impact continues, as many shellfish beds are periodically closed during times of heavy rain which carries runoff pollution such as fecal coliform.