A plan to protect corals in the Gulf of Mexico is close to becoming a law, drawing cheers from environmental groups who believe leaving the corals alone would help vulnerable ocean ecosystems to grow.

The plan would create 21 protected areas off the coasts of Texas, Louisiana, Alabama, Mississippi and Florida. Thirteen of the areas would carry new commercial fishing restrictions, and that has attracted the attention of fishing groups, who want the government to take a cautious approach.

Pew Charitable Trusts has characterized the plan as a way to protect nearly 500 square miles of slow-growing...