Which seafood causes the least damage to the planet? It's complicated.

“We looked at five environmental pressures for aquatic foods: greenhouse gas emissions, nitrogen runoff, phosphorus runoff, freshwater use, and agricultural land use,” says Jessica Gephart, assistant professor of environmental science at American University.

Gephart and her colleagues examined data from 1,690 farms and 1,000 fishery records worldwide.

Their findings:

Wild-caught seafood

For wild-caught seafood, greenhouse gas emissions are the key concern.

“The biggest factor is fuel use for fishing vessels,” says Gephart. “And that depends on how the species are caught and how easy they are to catch.”

Among the winners: “Small fish like sardines, anchovies, and herrings are low on greenhouse gas emissions. So are cods, haddocks, and hakes.”

Among the losers: Lobsters and the flounder-halibut-sole group. Why?

“Both lobsters and flounders are bottom-dwelling species, which are often fished with bottom trawls that require a lot of energy,” says Gephart.

In contrast, she adds, “a lot of the cod, hake, and haddock group is represented by Alaskan pollock from U.S. fisheries that often use less energy-intensive gear and are well managed, so they’re getting good catches for each unit of effort.”

Farmed seafood

“For aquaculture, greenhouse gas emissions depend on what it takes to grow the feed and how much feed is required,” explains Gephart.

Among the questions Gephart and her colleagues asked: “Was land deforested to grow the feed? How much fish gets to market for each pound of feed? How much energy is used for pumps or aerators on the fish farm?”

Again, there were some clear winners.

“Farmed bivalves like clams, mussels, oysters, and scallops are produced in coastal areas, so their feed comes from filtering wild phytoplankton in the water, rather than requiring feed inputs,” says Gephart.

“Salmon and trout are also pretty low on greenhouse gas emissions. That’s because they’ve been bred to be efficient at turning feed into meat.”

Tilapia, shrimp, and catfish have higher emissions. “Tilapia falls close to chicken,” says Gephart.

Fishing for greener seafood

seafood chart
Aim for seafood with low greenhouse gas emissions. Farmed fish have other costs like nitrogen and phosphorus runoff, but they’re typically lower than chicken’s.
Source: Nature 597: 360, 2021.

How does wild compare with farmed?

It depends.

“Greenhouse gas emissions for salmon and trout are similar, whether you’re talking about wild or farmed,” says Gephart, “whereas shrimp and bivalves have lower emissions if they’re farmed than if they’re wild-caught.”

That said, she adds, her study didn’t look at other concerns like lost biodiversity, shrinking fish populations, antibiotic resistance, and bycatch—when other fish, dolphins, or other animals get caught in fishing gear.

So what can consumers do?

“If you don’t have a retailer who’s working with farmers or fishers, the Monterey Bay Aquarium Seafood Watch is a great place to look,” says Gephart. Its website divides seafood into “Best Choice,” “Certified,” “Good Alternative,” and “Avoid” categories.