Even the Best Farmed Fish Can Cause Problems Too much of a good thing can be bad. A new study of marine aquaculture around the world finds that even the most efficient operations—think industrialized salmon farms—can cause large environmental harm if there are too many of them. The new index could interest seafood buyers and policymakers evaluating how farms are regulated, experts say. The environmental impact of seafood has been evaluated in many ways before. Nonprofit groups, such as the Monterey Bay Aquarium, have produced "watch lists" that list species to be preferred or avoided. For their part, industry groups have created certification schemes that promote operations that use best practices. But these methods are not very quantitative, nor do they allow policymakers an opportunity to compare the impact of aquaculture by species or country-of-origin, says John Volpe, an ecologist at the University of Victoria, British Columbia, in Canada. With funding from the Lenfest Ocean Program, Volpe and colleagues spent 2 years creating a global performance index. To make the job easier, Volpes team narrowed its scope to marine finfish species such as cod, sea bass, and salmon. The 20 species made up 90% of marine finfish harvested in 2007. Then they combed through the scientific literature and other databases to find information on 10 aspects of environmental performance. These include the amount of inputs, such as diesel fuel and feed; discharges, including antibiotics and parasiticides; and biological impacts, such as pathogens that can escape from pens and harm wild fish. Incorporating all 10 metrics, the team then gave two overall scores (between 0 and 100) to each species produced in a particular country. One score measured the environmental performance per unit of fish produced. Efficient and higher-tech operations, such as salmon farms, came out looking good. But the other score—cumulative impact—revealed some distinctions. Chinook salmon grown in New Zealand, for example, scored much better than Atlantic salmon from Chile or Norway, where much more fish is produced—and more pollution released. The new index "provides some hard numerical reality of the fact that if we do too much of something, it has impacts," says George Leonard of the Ocean Conservancy, who is based in Santa Cruz, California, and was an adviser to the project. The results could help those buying farmed salmon, for example, to refine their choices, he says. George Chamberlain of the Global Aquaculture Alliance, an industry group, agrees that the index will be useful for comparing environmental impacts. But he worries about some of the assumptions, such as that operations always use as many chemicals as legally permissible. "Clearly, this is a work in progress that has the potential to provide important insights and to stimulate productive debate."