The scientific consensus is that genetically-altered salmon is safe. But that hasn't stopped Congress from voting to ignore some inconvenient truths.

Last week the House of Representatives approved an amendment [PDF] to the agricultural appropriations bill. The amendment, co-sponsored by Reps. Don Young (R-Alaska) and Lynn Woolsey (D-Calif.), prohibits funding for U.S. Food and Drug Administration (FDA) approval of biotech salmon developed by AquaBounty Technologies in Waltham, Massachusetts. The vote was applauded by a motley collection of "consumer" and environmental activist groups. Opponents assert that the biotech fish, dubbed "frankenfish," threaten wild salmon and human health.

First some background: AquaBounty's faster growing biotech salmon eat 10 to 25 percent less feed than do conventional Atlantic salmon. AquaBounty salmon grow twice as fast as wild Atlantic salmon as the result of the installation of two genes, a promoter gene derived from the eel-like ocean pout and a growth hormone gene from Chinook salmon. People have long eaten both species.

Aquaculture already provides 50 percent of the fish that people eat around the world. Enhancing farmed fish production could relieve pressure on the world's already way overfished [PDF] wild fisheries. So, on the face of it, it would appear that biotech fish are a win for consumers and the environment.

But it's been a long slog for AquaBounty. The company created the genetically enhanced salmon back in 1989, and alerted the FDA that it planned to seek permission to commercialize the fish in 1995. FDA approval is required for all genetically engineered animals under its authority to regulate "new animal drugs." A new animal drug is defined as "an article intended to alter the structure or function" of an animal, which, in this case, means the two added genes.

Last September, the FDA finally held a public meeting to discuss the possible health and environmental consequences of the biotech salmon. With regard to human health issues, the agency's Veterinary Medicine Advisory Committee found no biologically relevant differences between conventional and biotech salmon on nutrition and allergenicity. Consequently, the committee concluded that the biotech salmon "is as safe as food [PDF] from conventional Atlantic salmon, and that there is a reasonable certainty of no harm from the consumption of food from this animal."

What about possible deleterious effects on the environment? The main activist concern is that some AquaBounty salmon might escape and interbreed with wild populations. As evidence for the harm that such an escape might cause opponents cite the "Trojan gene effect" scenario devised by Purdue University animal science professor William Muir. That scenario suggested that such interbreeding could lead to the extinction of wild populations. The idea is that biotech fish might have a mating advantage but nevertheless be less adept at surviving long term in the wild.

Muir's work with schools of tiny freshwater fish called Japanese Medaka assumed that the chief mating advantage is that biotech fish would be bigger than wild ones. As it happens AquaBounty's fish are not bigger; they just grow faster. In any case, recent research [PDF] has found that genetically modified fish are actually at a selective disadvantage to wild fish. Similarly, another recent study reported that genetically modified coho salmon fared badly against wild ones when it comes to reproduction. Muir himself testified at the FDA hearing in September that "the data conclusively shows that there is no Trojan gene effect as expected. The data in fact suggest that the transgene will be purged by natural selection. In other words the risk of harm here is low."

To make the risk even lower, AquaBounty salmon are sterile triploids, that is, instead of having the usual two sets of chromosomes, their fish have three sets. In addition, the company has devised a process that make essentially all of their fish females, so there are no males available to supply sperm even if the fish were fertile. Finally, the company plans to raise their biotech salmon in freshwater tanks in Panama. Panama has no salmon, and if the fish escape into the tropical waters they will die from the heat.

Given that the scientific evidence currently suggests that the biotech salmon pose no health risks to people and do not significantly endanger the natural environment, why would Congress vote to override an approval process based on science? According to the Associated Press, Alaska's Rep. Young "argued that the modified fish would compete with wild salmon in his state." In other words, plain old-fashioned special interest pleading trumps science.

Yesterday, the non-profit scientific advisory organization, the Council for Agricultural Science and Technology (CAST), issued a commentary, "The Science and Regulation of Food from Genetically Modified Animals." The CAST commentary notes that "all technologies are associated with some form of risk, but a critical and often overlooked issue is that all risks are relative to alternatives." The commentary points out that wild fish stocks are being depleted unsustainably now and that interbreeding between conventional farmed fish and wild fish already poses genetic risks to wild stocks. "Thus, the comparative risk of sterile transgenic salmon is likely to be less than that of fertile, selectively bred Atlantic salmon," reckons the CAST commentary. The CAST report points out that in the absence of unique risks, it doesn't make any scientific sense to subject genetically enhanced animals to different regulatory standards.

The CAST report understatedly concludes that the current regulatory approach "has resulted in an inhibitory effect on commercial investment in the development of genetically engineered animals for agricultural applications with ramifications for U.S. agriculture and food security." Everybody, including activists, should be wary of this kind of vote. In future conflicts, Congress may well vote to overrule science that the activists believe support policies they prefer.

Last week, Ronald Stotish, the president of AquaBounty, issued a blistering statement warning [PDF], "Whether or not you support this transgenic salmon, we should all agree these types of shenanigans have no place in a complex scientific debate. These actions threaten the fundamental basis of a science-based regulatory process. Americans deserve better from their elected representatives."

Yes, we do.

Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.