It’s been more than three decades since the U.S. government first tested the waters of offshore fish farming. In 1990 a company received a federal permit to raise nearly 50 million pounds of Atlantic salmon a year in pens anchored some 50 miles off Massachusetts. But the project became mired in legal questions and environmental concerns and never materialized. Other proposals have hit similar snags, and today the industry is limited to inland and state coastal waters. Not a single farm operates in federal waters, which typically begin three miles offshore. That may soon change. Proponents of aquaculture, which in its many forms provides more than half of global seafood supplies, say farming more fish offshore is a low-impact way to feed a growing, protein-hungry human population. In the United States, which imports roughly 85 percent of its seafood, supporters argue that it would boost food security and rural economies while helping Americans eat more sustainably; not all...