The dredge is a basket attached to a toothed bar. Dragged over an oyster reef, the bar lifts of the oysters, which are caught by the basket. It can scoop up as many as 200 oysters. Tongs seldom get more than 30 at a time.
Oystering in Chesapeake Bay started out with tongs, later replaced by dredges. The "oyster wars" were fought between tongers and dredgers, marked by violence.
The harvest today in Chesapeake Bay is estimated to be 1/100 of what it was in the late 19th century. Chesapeake

Oysters kept at cool temperatures (not freezing) may remain alive for weeks. Even in the days of the westward expansion, restaurants in some of the mining towns offered this delicacy. The railroads, as lines expanded, brought oysters to almost the entire country. In the 1930s, in Santa Fe, NM, I remember fresh oysters were available, brought in on the Santa Fe railroad. Air transit was not necessary.