What fish's roe is commonly sold as masago?

The fish whose roe is commonly sold as masago is the capelin.

Capelin are small forage fish in the smelt family, with the scientific name Mallotus villosus. Their eggs are harvested and sold as masago, a familiar ingredient in sushi and other Japanese dishes. Masago is often seasoned or colored, including with wasabi flavoring, and may be used as a less expensive alternative to flying-fish roe.

Masago and tobiko are easy to confuse because both are small, brightly colored roe used as toppings. Tobiko comes from flying fish and generally has larger, firmer eggs with a more pronounced crunch. Masago eggs are smaller and usually softer. Neither should be confused with ikura, the much larger orange roe of salmon.

Capelin are ecologically important beyond the culinary trade. They feed largely on plankton, krill, and other small crustaceans, while serving as major prey for Atlantic cod, whales, seals, seabirds, squid, and mackerel. Their seasonal spawning migrations can bring enormous schools close to shore.

Capelin are found in Arctic and northern Atlantic and Pacific waters. They typically mature and spawn between roughly two and six years of age, making their populations important to both marine food webs and fisheries management.