Donald Trump has upended global trade relationships, promising that temporary disruption will end in better terms for American businesses. Tell that to the Maine lobster industry that his policies are putting at a major disadvantage in Europe and China.

These should be halcyon days in lobstertown. Maine harvests more lobster than any other U.S. state or Canadian province. Last year it landed nearly 111 million pounds—its fourth-largest annual haul—which it sold for $450 million. The lobster industry accounts for 2% of Maine’s...