He and his colleagues examined how the two species are raised for consumption. The researchers focused on dynamics between land and sea.

“Chickens are fed fish from the ocean just as are salmon, and salmon are fed crop products such as soy, just as are chicken,” Halpern said about farmed broilers and farmed salmonids such as salmon, trout and char.

In addition to land-based crops, chickens are fed fishmeal and fish oil. Salmon, which typically eat other fish, are farmed with land-based feed, such as soybeans and wheat.

The researchers found that 95 percent of the cumulative environmental footprint of the two products – greenhouse gas emissions, nutrient pollution, freshwater use and spatial disturbance – is concentrated on less than 5 percent of the planet, with 85.5 percent spatial overlap. That’s due mostly to shared feed ingredients.

Total cumulative pressures from chicken production are greatest in the United States, China and Brazil, according to the study.The greatest cumulative pressures for fish are found off the coasts of Chile, Mexico and China, with some pressure on land due to salmon aquaculture.

The researchers found that while chicken has nine times the environmental footprint of farmed salmon, it has 55 times more production than salmon. The efficiency is due largely to the fast reproductive cycle of chicken – six weeks to eight weeks to reach slaughter weight versus one year to two years for salmon.

Within that 5 percent of the planet that bears the environmental pressures of chicken and salmon production, there are variations in the farming methods’ environmental efficiencies. In the case of chicken, for example, the United States – the world’s largest producer of chicken – and Brazil – the second largest producer – are more efficient than China – the third largest producer.

There also are variations between environmental pressures relative to the amount of salmon produced that differ by geography. That indicates opportunities to improve efficiencies while minimizing environmental impacts.

Chicken and salmon are among the most popular sources of protein and are relatively environmentally efficient in comparison to other protein production such as beef and pork, according to the researchers. But the magnitude of their production and their overlap in terms of environmental footprint raise questions about the connections between marine and land-protein production. That could provide opportunities for promoting sustainability. At the same time the study underscores the importance of integrating food policies across realms and sectors to advance food-system sustainability, according to the researchers.

“I knew from past research that what we feed animals is a key part of what determines their environmental footprint, but I really didn’t expect chicken and farmed salmon to be so similar,” Halpern said.

The study recently was published in Current Biology. Visit cell.com for “farmed chicken and salmon” for more information.