This slightly astonishing question was the focus of a study by the food researchers Astrid Scholz, Ulf Sonesson and Peter Tyedmers. The result is even more astonishing and counterintuitive: when considering environmental impact, it is better to buy and eat frozen salmon. This is why:
Most salmon consumers live far from where the fish was caught or farmed, and the majority of salmon fillets they buy are fresh and shipped by air, which is the world’s most carbon-intensive form of travel. Flying fillets from Alaska, British Columbia, Norway, Scotland or Chile so that 24 hours later they can be served “fresh” in New York adds an enormous climate burden, one that swamps the potential benefits of organic farming or sustainable fishing. (“To Reduce Carbon Emissions, Buy Your Salmon Frozen” by Astrid Scholz)
So, enjoy fish sticks.
The Sceptical Economist 