The New York Times reports:
At issue for many is where the two fish swim. Most Asian salmon spend the bulk of their lives in saltwater. Rainbow trout are often cultivated in water tanks or ponds, which could expose them to freshwater parasites that could infect humans if the fish is eaten raw. The United States Food and Drug Administration warns of potential parasite hazards from eating freshwater fish. In Hong Kong, a Chinese city that operates under its own laws, serving freshwater fish raw is illegal.
“It’s not only the issue of rainbow trout being substituted for salmon, but whether freshwater fish should be used for sashimi at all,” Dr. Kevin Kwok, an assistant professor in the department of applied biology and chemical technology at the Hong Kong Polytechnic University, said.
Fish is often mislabeled around the world in order to fetch higher prices, such as labeling yellowtail as mahi mahi or sea bass as halibut. Regulators in the United States label these substitutions as economic deception.
That goes for salmon, too. Officials in the United States forbid fish sellers from labeling steelhead trout — essentially, rainbow trout that swim in saltwater — as salmon. Generally, salmon spawn in freshwater but return to the sea.
(click here to continue reading In China, Salmon is Salmon, Even if It’s Trout – The New York Times.)
I wouldn’t be surprised if this and similar substitution happens frequently in the US as well. Budgets of regulatory agencies have been sliced over the last decade, so who really monitors what kind of fish gets delivered to restaurants?