MARATHON, Fla. — Like other commercial fishermen along the east and west coasts, Ethan Wallace had been waiting 18 months for China — the world’s largest importer of live lobster — to lift its crushing retaliatory tariffs on American seafood that had whittled down his profits.

This week, that moment came: Beijing started allowing Chinese businesses to apply for tariff exemptions. But for Mr. Wallace, it no longer mattered.

Tariffs or not, no one in China is buying. The coronavirus outbreak meant the Lunar New Year banquets and wedding parties that feature a fresh lobster on every plate, a symbol of good fortune, were canceled. In several cities, restaurants are shuttered and public indoor gatherings are prohibited. And even if they weren’t, many of the planes that ferry live lobsters aren’t flying to China.

“Boom! Coronavirus,” said Mr. Wallace, 28, after he had steered Piece of the Pie, his 43-foot Torres boat, into the Keys Fisheries marina in Marathon. Although the season continues through the end of March, he and his crew that day took home more lobster traps than pounds of lobster from the Gulf of Mexico.