SEOUL—"There have been no buyers yet," said fish trader Choi Mi-ja as the clock ticked toward 3 p.m., some 10 hours after her store at the Noryangjin Fisheries Wholesale Market opened for the day.

"In 26 years in this business, I've never seen anything like this before," Ms. Choi added, standing by tanks where live, locally bred flatfish and Russian-imported king crabs were displayed.

Ms....