HONG KONG — For years, fish sellers in China have labeled something other than salmon as salmon, according to a local media report that outraged sushi lovers across the country.

Now, Chinese fish authorities have responded: That's perfectly O.K. with them.

Chinese regulators said this week that rainbow trout can be sold as salmon, according to new standards set by a government-affiliated fish association and 13 commercial fisheries. To justify the change in definition, officials cited biology: Salmon and rainbow trout belong to the same fishy family. They also required sellers to note the exact type of fish elsewhere on the label.

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Still, the fuzzy definition touched a nerve in a country with a long history of food labeling issues and a vast population of increasingly sophisticated consumers. Thousands took to the internet to blast regulators for lowering food standards instead of fixing the issue. Some declared they would never eat salmon again.

Even patrons at a sushi restaurant said that they could no longer trust salmon enough to eat it raw.

"If I come across especially cheap salmon that is hard to tell apart from rainbow trout, I probably will not order it," Ma Xinyi, a 20 year-old college student, said. She has eaten only cooked salmon since news of the salmon-trout switching began to circulate three months ago.

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.

Regulators elsewhere take a dim view of substituting one kind of fish for another.

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.

In China, the salmon labeling controversy follows a litany of food labeling problems that have angered consumers and cast doubt on the country's ability to enforce safety standards. Melamine, a chemical used to make plastics, was found in dairy products that sickened tens of thousands of children in 2008. In 2013, police accused traders of selling rat and mink as mutton. In 2014, Wal-Mart Stores Inc. recalled meat labeled as donkey after testing showed it contained fox meat.

The new rules in China came three months after state media recirculated a video segment profiling a large freshwater fishery at the Longyangxia Reservoir in Qinghai Province. The company, it said, supplied one-third of China's salmon. That piqued the interest of other media in China, as Qinghai is an inland province far from the ocean. Subsequent reports said a considerable amount of fish labeled as salmon in China was actually rainbow trout.

The industry argued that the fish are essentially the same — and Chinese regulators this week agreed. The rules issued this week by the China Aquatic Products Processing and Marketing Alliance, an industry group affiliated with China's Ministry of Agriculture, said that though rainbow trout can be marketed as salmon, markets and restaurants also have to list the species of the fish and its origin. For example, a label might read "salmon (Atlantic salmon)" or "salmon (rainbow trout)."

Higher-cost rainbow trout are often raised in fiberglass tanks or in cages in oceans. Some fish breeders add salt into the water, though its effect on parasites has not been widely studied, said Dr. Kwok, the fish farming expert. He said Atlantic salmon also carry parasites, but they are highly unlikely to be infectious to humans.

China's fish industry defends the safety of its freshwater production, saying that the waters at China's fish farms are carefully controlled.

"Whether salmon has parasites does not depend on whether it is bred in seawater or freshwater, but rather on whether its growth can be safely monitored," said another industry group, the China Fisheries Association, in an announcement that has since been removed from its website.

In fact, the group added, consumers might be pleased with rainbow trout instead of salmon. In countries like Norway and Chile, it claimed, "many locals prefer rainbow trout, and the price of rainbow trout is higher than that of Atlantic salmon."

Chinese internet users ridiculed claims about the rainbow trout's higher value, saying that they would rather just have the regular salmon, please, because they could not afford to carry parasites.