Miami, with its tropical climate, might not seem the most obvious place for farming a cold-water fish. But just outside the city, in a 9-acre building equivalent in size to the Boston Red Soxs home stadium almost 50 million salmon are projected to be grown from eggs each year .

Within this climate-controlled building, which looks like a large aircraft hangar, fish will be moved from small to large tanks of both fresh and salt water as they grow. Once they reach about 20 months, theyll be killed and sent out to the U.S. market. The first fish are due to be harvested in May.

Atlantic Sapphire, the Norwegian company that owns this farm, says its indoor operation will be far more sustainable than traditional offshore salmon farms. It claims that within a decade, it will be able to meet half of the current U.S. demand for farmed salmon.

While we are becoming much more aware of the downsides of the intensive farming of chickens and pigs with a constant stream of stories about their environmental impact and the overuse of antibiotics , and about animals being mistreated and forced to live in cramped conditions we hear far less about how our fish is produced and what effect it has on our planet.

But thats beginning to change. There is an increasing recognition of the threats posed by fish farming, or aquaculture, including marine pollution, antibiotic overuse and significant greenhouse gas emissions. Miamis indoor salmon megafarm is pitching itself as a solution to these problems. Its one of a number of new land-based salmon farms around the world that are forecast to produce half a million tonnes of salmon a year within a decade which would be equivalent to around 20% of global salmon production today .

The owners of the Miami farm claim its land-based facility can serve as a much more sustainable source of salmon that wont pollute the sea. But not everyone is convinced.