FISHING CREEK, Md.—Music blared as 21 Mexican women methodically cracked open steamed crabs piled high inside a cool, bright room. They picked out the meat and packed it into 1-pound containers that their employer, G.W. Hall & Sons, ships to wholesalers in the mid-Atlantic and as far away as Canada.

A half-mile down Old House Point Road, the picking room at a competing company, Russell Hall Seafood, was silent, no workers to be seen. Bare metal tables, normally heaped with crabs this time of year, gleamed.

The...