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The world’s aquaculture industry is booming. And it’s no wonder—overfishing in the open ocean has caused populations of many commercially caught fish to plummet. To keep up with the world’s growing demand for seafood, aquaculture, or the rearing of aquatic plants and animals for food, is the obvious alternative.

According to the 2024 United Nations State of the World Fisheries and Aquaculture report, the amount of seafood produced in 2022 from aquaculture exceeded the amount produced from capture fisheries. Within aquaculture, 38 percent of seafood is raised in marine systems, known as mariculture, and the other 62 percent comes from land-based, mostly freshwater systems. Despite aquaculture’s prevalence, the environmental impacts of this industry are still murky.

“To date, climate assessments have not accurately quantified the climate impacts of mariculture, especially with respect to the direct methane and nitrous oxide emissions from aquatic environments,” says Lu Shen, an environmental scientist at Peking University in China.

In a new study, Shen and his colleagues evaluate how marine aquaculture affects the climate, taking into account greenhouse gas emissions from the breakdown of fish feed and the energy used to operate fish farms. They found that the carbon footprint of mariculture is 40 to 50 percent lower than that of freshwater aquaculture. The variance mainly comes down to microbes that produce different greenhouse gases in distinct environments.

In most aquaculture systems, fish get more food than they can eat. The leftovers sink to the seafloor along with plankton, fish poop, and other detritus. Then, the naturally occurring microbes that digest this detritus emit huge amounts of nitrous oxide and methane—potent greenhouse gases. Marine systems differ from freshwater ones because they’re so salty. The dissolved sulfates stimulate the growth of a specific type of bacteria that uses the salt and competes with methane-producing microbes. According to the study, the saline water may also be less hospitable to methane-producing microorganisms.

To arrive at these conclusions, the scientists used satellite data to analyze how greenhouse gas emissions vary between 107 aquatic and marine aquaculture sites around the world. First, they measured how much carbon was available for greenhouse gas–producing microbes to consume at these sites. Then, they estimated how much methane and nitrous oxide these microbes release. They found that in marine environments, microbes convert a minuscule 0.07 percent of available carbon to methane, compared with freshwater environments, where between one and 10 percent of the available carbon is converted to methane.

Different seafood species also contribute different carbon footprints: farming carnivorous fish, such as salmon and sea bass, produces more greenhouse gases because they require more protein in their feed, compared with omnivorous fish, such as carp. Bivalve and seaweed farms had the lowest carbon footprint.

Despite mariculture having lower greenhouse gas emissions than other types of aquaculture, the industry can still come with adverse environmental impacts. “Offshore mariculture poses elevated threats for habitat disruption and wildlife interaction,” says Ramin Ghamkhar, a sustainability consultant who studied aquaculture for his doctoral research at the University of Wisconsin–Madison.

Fish farms in the ocean could spread diseases and parasites to wild fish, and large mariculture sites could squeeze out native flora and fauna. These potential harms need to be fully assessed before new sites are set up, says Ghamkhar.

According to the Food and Agriculture Organization of the United Nations, global demand for fish is expected to increase by up to 17 percent each year through 2050. Supplying more of this important protein via mariculture could reduce the seafood industry’s greenhouse gas emissions. This latest research, Shen says, could help ensure that fish farms of the future are as green as possible.