Cold fish: the global cooling effect of ocean life

Krill, fish and whales capture carbon and lock it into the ocean, a raft of new research shows

A school of sardines swimming in the Pacific Ocean

A school of sardines swimming in the Pacific Ocean. Given the potential of small fish to lock away carbon into the deep sea, researchers and policymakers are looking for ways to harness their power to mitigate climate change. (Image: Alamy)

What’s the value of a fish? You might think about its market price or, given its role as a primary source of protein for three billion people, its contribution to food security. You’re much less likely to think about how it mitigates climate change.

Last year, a study published in Science Advances calculated that since 1950, commercial fisheries of large species, such as tuna and billfish, have released an estimated 730 million metric tonnes of carbon dioxide into the atmosphere. Some of those emissions came from fishing vessels burning fuel, but a large share was released by the bodies of the fish extracted from the sea. If they had instead been left to follow their natural course, they would have locked that carbon into the ocean.

Fisheries are on the frontline of warming oceans that threaten the abundance and diversity of marine life. But the Science Advances study is part of a growing body of research looking at the other side of the equation: the potential of marine animals to capture carbon and keep it in the ocean. And it’s not just big fish that matter: increasingly, research also points to the importance of large schools of smaller fish in locking carbon away in the deep sea. As the evidence for this grows, researchers and policymakers are beginning to ask, how do we support the power of fish to fight climate change?

“This is one of the ways to capture carbon – a new way we didn’t know about, but about which the science is revealing more and more,” says Rashid Sumaila, director of the Fisheries Economics Research Unit at the University of British Columbia’s Institute for the Oceans and Fisheries.

ocean life carbon capture diagram
The flow of carbon in ocean creatures starts with the food web. As phytoplankton grow on the surface, they capture CO2 through photosynthesis and convert it into organic carbon. This carbon is passed on and accumulates in the bodies of zooplankton such as krill when they eat the phytoplankton, and then again in the bodies of fish and other animals that eat the zooplankton. When these animals defecate, this carbon-infused organic matter either falls to the seabed, where it gets trapped in the sediment, or is consumed by bacteria and other microbes. (Graphic: James Round / China Dialogue Ocean)

Like all living things, fish accumulate carbon as they grow. “A fish, whether it is little or big, contains between 10% and 15% carbon,” says Gaël Mariani, a PhD student from the University of Montpellier, France, and lead author of the Science Advances study. When fish defecate, and when they die, the carbon contained in that organic matter is consumed by predators, scavengers and microbes in a cycle that locks carbon into the food chain. A small percentage of the carbon-infused organic material also reaches the seabed as particulate matter, where it gets trapped in sediment.

But the largest share of sequestered carbon is likely to occur through respiration, by which CO2 is dissolved into the ocean. If respiration occurs below a depth of about 800 metres, the CO2 may remain trapped there, explains Grace Saba, assistant professor at the Department of Marine and Coastal Sciences, Rutgers University. “All sources of carbon – either particulate, dissolved or respired – can be sequestered for long periods of time, as long as they reach depths deep enough not to be impacted by large-scale seasonal… ocean mixing events,” says Saba, who investigates oceanic carbon flows. Particulate matter like faeces or flesh that ends up on the seabed “can be sequestered on the scale of millions of years”, she says.

A bigger fish carries more carbon in its body. That’s why these species have so far been the focus for researchers like Mariani, whose study considered the lost sequestration potential caused by fishing for shark, tuna, mackerel, billfish and other big species.

whales carbon capture diagram
The larger the ocean animal, the more carbon in its body. When these animals die, their bodies are consumed by scavengers, keeping the carbon in the food web. Some of the carbon is also sequestered in seabed sediment. (Graphic: James Round / China Dialogue Ocean)

Researchers have estimated the carbon contributions of whales, the largest ocean inhabitants of all. When whales die their bodies hold an estimated 33 tonnes of CO2, which is then taken up by scavenging sea creatures or sequestered in the deep sea, compared to the roughly 22kg a tree sequesters each year, the International Monetary Fund (IMF) reports.

But even big fish and whales can’t eclipse the value of schools of small fish to global carbon cycles: research published in Nature Communications showed that tiny crustaceans called krill are main players in a “biological pump” that shifts carbon from the surface to the deep sea, and ultimately sequesters up to 12 billion metric tonnes of carbon a year. Krill contribute to this system by consuming vast amounts of phytoplankton, which capture carbon via photosynthesis at the ocean’s surface. They then sequester the consumed carbon by respiring it at depth, and through their faeces which sinks to the bottom of the ocean. Their central importance to this carbon cycling process raises concerns about intensive commercial krill fisheries in the Southern Ocean.

Overall, Saba’s recent research, published in the journal Limnology and Oceanography, estimates that fish contribute about 16% of the carbon that ultimately sinks into the ocean’s deeper layers. If fish are such a prominent carbon sink, a natural store lowering the concentration of CO2, isn’t protecting them important to efforts against climate change?

diagram showing how respiration of ocean creatures helps capture carbon in the seas
When ocean creatures respire, the flow of carbon continues as they emit CO2. In shallow water, this CO2 can escape back into the atmosphere, but at lower depths, it can be trapped, sometimes for long periods of time. Many marine animals, including krill and mesopelagic fish, travel between surface waters, where they feed, and deeper layers of the ocean, where they rest and avoid predators. This creates a quite literal “carbon sink”, as they respire in the deep water and trap the carbon down there. (Graphic: James Round / China Dialogue Ocean)

That question was among many explored at a symposium in March run by non-governmental organisation Our Fish, which brought together fisheries and climate change researchers, activists and European politicians. Part of the event explored whether research findings could feed into fisheries policies that more proactively protect fish in order to help tackle climate change.

Several aspects of current fisheries management were flagged for intervention. For instance, researchers presented a study published in Nature, which showed that bottom-trawling releases as much carbon from the seabed as the entire aviation industry. This could be another reason to get behind marine protected areas (MPAs), which currently cover only 2.7% of the ocean floor, the researchers say. MPAs could also increase fish populations that will go on to sequester more carbon – and by building up fish stocks, they could simultaneously boost fishery yields and food security.

Other research (currently under review) revealed that the north-east Atlantic Ocean is one of the world’s largest carbon sinks, yet simultaneously has the highest fishing intensity on the planet – underscoring the need to tackle overfishing in European seas.

Researchers also flagged the potential of fishing subsidies to threaten the carbon-sequestering capacity of fish. Mariani’s research reveals that 43.5% of the “blue carbon” – stored in marine ecosystems – which was extracted by fisheries between 1950 and the present day came from areas of the ocean that would have been unprofitable to fish without subsidies. Removing the subsidies could protect these resources without impacting food security. “If we try to relocate these subsidies into something more sustainable, it would both limit overfishing, promote stock recovery, and maybe promote carbon sequestration by fish,” Mariani suggests.

A trawler in Egypt, used to catch small fish species. Research shows that bottom trawling releases as much carbon from the seabed as the entire aviation industry
A trawler in Egypt, used to catch small fish species. Research shows that bottom trawling releases as much carbon from the seabed as the entire aviation industry. (Image: Colin Munro / Alamy)

These were just a few areas where there was a clear potential for policy to leverage the blue carbon capacity of fish. “One of the things we hope to do is to bring another ecosystem service to the table, so that when we make decisions – whether that’s government, individuals, NGOs or industry – we know that fish are not there only to be eaten,” says Sumaila, who helped bring together several researchers to present at the conference. Yet, the message from the politicians in attendance was clear: to drive policy changes and civil society action, there needs to be more research into the contribution that fish make to marine carbon sinks. “It’s always easier to convince stakeholders when you have an evidence base,” said Virginijus Sinkevičius, Commissioner for the Environment, Oceans & Fisheries at the European Commission, who spoke at the symposium.

The complexity of carbon cycles already presents a considerable research challenge. In fluctuating ocean environments, weather extremes, temperature, depth and habitat can all affect how carbon cycles work in the deep sea. “This is new science. It’s not like trees and forests. People have been looking at those forever, and so they’ve come into the mainstream. But this research has yet to be mainstreamed,” Sumaila says.

Also vital will be determining exactly how much carbon different species sequester in the sea, and that means looking beyond just the big fish. “In my opinion, what would be most useful right now to policymakers would be to obtain a biomass-specific carbon flux estimate for different types of fish — small pelagics [living in the upper layers of the open ocean], large pelagics [such as tuna], migrating mesopelagics [living at depths of 200–1,000 metres],” says Saba. Understanding the blue carbon potential of all fish species is a research focus of Mariani too. “The next step is to estimate how much carbon is sequestered each year by all the species of fish in the ocean, based on different climate scenarios and different fishing intensity scenarios,” he says of his upcoming research.

group of Goldband Fusilier or Pterocaesio Chrysozona, a sea fish with a bright yellow stripe
To fully understand how ocean carbon cycles work, researchers say it is vital to determine how much carbon different species sequester beneath the waves. (Image: Alamy)

In the next few months, a group of about 25 researchers will be contributing to a body of papers on this general theme, which is being spearheaded by Sumaila and will be fully published later this year. The eventual goal is to build up enough research to give fish conservation a foothold in climate policy, Sumaila explains.

It’s difficult to tally up the true value of a fish – but the accumulated research suggests there’s one benefit of their existence that we’ve been overlooking for too long: instead of simply being the victims of climate change, fish could be powerful forces against it. “We need to harness all the ways we can reduce greenhouse gas emissions. And here, the science is telling us that fish bodies sequester a big portion of the CO2 we have in the atmosphere. We need to bring that to the table, with all our other efforts to take down climate change,” Sumaila says.