Here’s how fish farms could double as a climate solution

According to a new research model, adding iron to fish farms could capture at least 100 million tonnes of carbon dioxide a year, in aquaculture-intensive countries. This could be enough to offset most of the carbon impact of aquaculture operations, which contribute between 5 and 7% of the global emissions linked to agriculture and livestock farming each year.

Ironically, part of this solution lies in another polluting problem that fish farms create: mounting quantities of hydrogen sulfide. This is a toxic gas byproduct that builds up when microbes in low-oxygen environments like ocean sediments feast on organic matter. Industrial fish farms provide this in abundance, via fish feces and excess feed that sink to the bottom of the enclosure.

Even at low levels, the hydrogen sulfide gas that the feasting microbes produce can cause massive fish mortality on farms, and trigger ripple effects in the wider lake and ocean ecosystems where fish farms reside.

But as the researchers explain in their new Nature Food study, iron ore reacts naturally with hydrogen sulfide gas to form iron sulfide, and this mineral can then be sequestered in the sediments beneath a fish farm, from where it can’t poison fish. Crucially for this study, increased iron sulfide in an environment can also increase the water’s alkalinity. And, more alkaline waters are able to absorb more CO2, and convert it into stable forms.

“Higher alkalinity allows the water to absorb more CO2 from the atmosphere, converting it into bicarbonate and carbonate, which are stable and stored in the water long term,” says Mojtaba Fakhraee, a postdoctoral associate at Yale University, and lead author on the new study.

Based on this interesting dynamic, the researchers developed a model to simulate what would happen if the world’s fish farms were sprinkled with iron ore to tackle the twin challenges of fish-killing hydrogen sulfide, and greenhouse gas emissions. Their model simulated carbon, iron, and sulphur cycling in marine sediments, under a range of environmental conditions, and looked at aquaculture in several different countries to reach their conclusions.

Their central discovery is that adding iron ore could sequester up to 100 million tons of CO2 annually in an aquaculture-intensive country like China, alone. Generally, sprinkling in this ingredient would sequester between 2 and 10 tons of carbon per hectare of fish farm per year, the researchers calculate. China, combined with other major farmed fish-producing nations such as India and Indonesia, could collectively lock away between 25 and 140 million tonnes of carbon with iron ore, the study finds.

Depending on the country and context, the iron-enrichment method could in fact sequester the equivalent of 50%, on average, of aquaculture’s current emissions output, but as much as 100%. “This approach has the potential to turn aquaculture into a carbon-neutral—or even carbon-negative—industry, which is crucial for addressing climate change and ensuring sustainable food production,” says Fakhraee.

The method could also create an extremely low-carbon source of meat protein. A sprinkling of iron ore could reduce carbon output per gram of protein to between 0 and 20 grams in farmed fish—significantly less than the 240 grams of CO2 generated for every gram of protein from beef. Adding iron also doubles as a way to keep waters cleaner and healthier for fish, by controlling the hydrogen sulfide that otherwise sends thousands, annually, to their deaths.

Mining and transporting iron ore costs money. But when the researchers factored this into the price of carbon removal, they calculated that it would cost a relatively small $100 to $300 for each ton of carbon removed from fish farms. That’s in line with the cost of other carbon removal technologies, they say. Crucially, the researchers add that this amount is also lower than the social cost of carbon.

There are caveats to iron enrichment, and field research is needed to explore its possible environmental impacts, the study says. But fish farms are also in a unique position to make a positive climate impact. Aquaculture is growing globally, and is expected to expand by 22%—almost a quarter—by 2030. At that stage, the sector will provide 53% of all the fish we consume. Along with this, its emissions will grow—as will the potential to capture it in the ballooning quantities of hydrogen sulfide. Farms are also under our direct control and so interventions in these environments could have tangible, measurable effects on the health of this food system and the planet.

Ultimately, says Fakhraee, the findings “present a dual solution: reducing the carbon footprint of aquaculture while also benefiting the environment.”

Fakhraee and Planavsky. “Enhanced sulfide burial in low-oxygen aquatic environments could offset the carbon footprint of aquaculture production.” Nature Food. 2024.