Letting Carbon Sink with the Fishes

Fish fall to the seafloor when they die, sequestering carbon in the deep. Our penchant for catching big fish is breaking the cycle.

by Amorina Kingdon, Kelly Fretwell, and Meigan Henry

October 13, 2021 | 

Fisheries generate their share of environmental concerns, but carbon emissions are rarely among them.

Gaël Mariani, a PhD candidate in marine ecology at the University of Montpellier in France, was wondering if ocean fisheries might emit more carbon than one might think. Specifically, he was researching natural marine processes that policymakers might leverage to sequester excess carbon, when he became curious if catches were short-circuiting one such process: the carbon pump that kicks in when fish die naturally in the ocean, instead of snagged in nets and on hooks.

Most marine corpses, including fish, fall to the seabed. (Dead whales are referred to as “whale falls,” and smaller bits of decayed organisms fall as “marine snow.”) This movement channels some carbon out of the upper ocean and sequesters it in the deep for hundreds, even thousands, of years. But what if the fish is caught instead?

Mariani and his colleagues studied global catches of large fishes such as rays, tuna, billfish, and sharks from 1950 to 2014. They found that the sheer biomass of all those fish corresponded to 37.5 million tonnes of carbon that has ended up in the atmosphere. If those fish had remained in the ocean, even though some would have been eaten and their biomass would have remained in the upper ocean, more than half of their constituent carbon would instead have been stored in the seabed. (If you’re wondering, one tonne of carbon dioxide would fill a cube about the size of a three-story house.)

This short animation breaks down the process.

Critically, Mariani also found that nearly half of these fishes were taken by fisheries that aren’t profitable without government subsidies.

Boris Worm, a marine biologist at Dalhousie University in Nova Scotia, says that while Mariani’s totals aren’t that large compared with the carbon emissions of other sectors, the subsidies signify to him that the costs and benefits of the deep-sea fishing industry should be reconsidered, both ecologically and economically. “It’s not feeding the world,” he says.

Worm points out another reason beyond carbon sequestration to let fish fall. “It also brings carbon to a community that is also incredibly food starved,” he says. That’s important because climate change is reducing the nutrients that reach the deep sea through global trends like a shift to conditions that favor smaller plankton species, and increased stratification among ocean layers so that changes in salinity and temperature become physical barriers to sinking nutrients.

So, while accounting for fish falls can’t replace the benefits of emissions cuts, tallying the carbon in tumbling tuna or senescent sharks could inform better fisheries policy, and result in a bit less carbon in the air and a few more nutrients in the deep.