Why does USMCA create a free trade area as opposed to a customs union or a common market? Explain the difference by giving examples

Alaska king crab fishing in the 1960s and '70s was a dangerous but rich fishery. boats from as far away as california and japan braved the treacherous gulf of alaska crossing to reach the abundant king crab beds in cook inlet and bristol bay. suddenly, in the early 1980s, the fishery crashed due to over fishing. all crabbing in those areas ended. to this day, there is no crabbing in bristol bay or cook inlet. a. how would an economist explain the decline of the alaska king crab fishery