At the time, it seemed an unlimited resource, but as bargeloads of mussels were plucked daily from the river bottoms, mussels began to decline rapidly. In the 1940s, however, plastic was invented. The shell button industry eventually folded, and the mussels began to recover. Cultured pearls begin as blanks cut from the shells of freshwater mussels. With a fishing license, you may take five non-endangered mussels a day for your use.
Native clams are tough and tasteless, anyway.
Popular in Pacific Rim countries, Asian clams may be taken and possessed in any number, although you still need a license. Because they are filter feeders they should be taken only from unpolluted waters and cooked thoroughly. Use only clams that have tightly closed shells. Put live clams in a bowl with fresh water to cover and let stand for 12-24 hours to clear them of sand and possible pollutants before cooking. Asian clams are easily distinguished from native species by the concentric ridges on their shells.