The most environmentally friendly protein sources include clams, sardines, and cod. Not surprisingly, beef has the worst environmental impact. Somewhat surprisingly, farmed salmon is relatively innocuous; but catfish is not. These are the conclusions of a recent study that compares the environmental impacts of both seafood and livestock in what the authors say is unprecedented detail.

Many studies have analyzed the environmental impacts of livestock or seafood. But not many have compared them side-by-side—and none has examined as wide a range of environmental impacts as the recent analysis nor particular types of seafood, says lead author Ray Hilborn, an ecologist at the University of Washington.

The work, published in Frontiers in Ecology and the Environment, shows that the environmental impact of a given animal protein depends on a myriad of interdependent factors. Researchers like Hilborn argue that instead of focusing on specific animals or impacts, policies aimed at mitigating environmental harm need to take a more holistic approach.

The researchers compiled data from 148 studies, focusing on industrialized, mass-produced animal food production systems. They analyzed the environmental cost of producing 40 grams of a given animal protein by taking into account energy consumption, greenhouse gas emissions, contribution to acid rain, and the production of excess nutrients that can wreak havoc on ecosystems.

“There are very clear winners,” Hilborn says. Farmed shellfish such as clams, mussels, and oysters had the lowest impact—or among the lowest—across almost every metric. They don’t need to be fed and they actually consume excess nutrients. Small pelagic fish, such as anchovies, sardines, herring, and mackerel, also ranked low in terms of impact. Because they swim in huge schools, catching them is not resource intensive. Whitefish, such as pollock, cod, halibut, and hake, leave minimal environmental impacts as well.

Beef is one of the worst offenders, demanding lots of energy and leaving a big carbon footprint. Also energy-intensive are invertebrates like crabs and lobster, Hilborn says. Their high selling price incentivizes fishermen to guzzle a lot of fuel to hunt the crustaceans.

But there were surprises. Despite some public perception to the contrary, farmed salmon turns out to be relatively benign—primarily because salmon aquaculture has become increasingly efficient. Catfish and tilapia, often considered sustainable fish, aren’t so good. The reason, Hilborn explains, is that most of the fish are farmed in Asia, where the energy required to circulate the water in their tanks comes from coal-fired power plants.

Because environmental harm depends on such varied factors, effective policy is a challenge. “The key policy takeaway is that there’s a real imbalance in regulatory and policy frameworks for different food productions,” Hilborn says. For example, farmed shellfish are the clear winners, yet shellfish producers often face a strict permitting process that restricts expansion. Beef farmers generally don’t face this sort of barrier, says Hilborn.

Environmental organizations and policymakers need to consider these animal proteins as part of a complex production system. “In each of these arenas, you’ve got groups not thinking about connections to other groups,” he says. ”The big environmental groups need to start to get their act together and think globally and not locally.”

Halley Froehlich, a marine fisheries and aquaculture ecologist at the University of California, Santa Barbara, who wasn’t part of the study, agrees. “This type of research really starts leading down that path of how we create a policy system that’s integrated,” she says.

What can the individual consumer do? “Eat mussels, small pelagics, and whitefish,” Hilborn says. But on a global scale, it may not be that simple. For one, you can’t just catch more sardines. Many of the world’s fisheries are nearly maxed out, Froehlich notes, a key point that the study didn’t mention.

Not everyone has access to sustainable seafood, either. “That’s a luxury developed nations have,” Froehlich says. And most of the population growth—and rising demand for protein—will be in the developing world, in Africa as well as in China’s rising middle class.

And especially in the developing world, animals don’t just provide protein, but also nutrients, says Ermias Kebreab, an animal scientist at the University of California, Davis. A more complete comparison needs to account for the nutritional value of animals. Also important to include in such studies are comparisons with dairy products, which also provide animal protein, he says. “Not having dairy in the paper is to me a major weakness.”

Still, the study helps inform how to tackle the complex questions surrounding food and environment. “Without these types of research papers,” Froehlich says, “we can’t even begin.”