Krill, fish and whales capture carbon and lock it in the ocean, new research shows

[By Emma Bryce]

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

Last year, a study published in Science Advances calculated that since 1950, commercial fisheries for large species, such as tuna and billfish, have released an estimated 730 million metric tonnes of carbon dioxide into the atmosphere. . Some of these emissions came from fishing boats burning fuel, but much of it was released by the bodies of fish taken from the sea. If they had taken their natural course instead, they would have trapped this carbon in the ocean.

Fisheries are at the forefront of ocean warming which threatens the abundance and diversity of marine life. But the Science Advances study is one 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 the big fish that matter: Increasingly, research is also highlighting the importance of large schools of small fish for trapping carbon in deep water. As the evidence for this accumulates, researchers and policymakers are starting to ask, how do we support the power of fish to fight climate change?

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

Like all living things, fish accumulate carbon as they grow. “A fish, whether small or large, contains between 10 and 15% carbon,” explains Gaël Mariani, doctoral student at the University of Montpellier, France, and lead author of the Science Advances study. When fish defecate and die, the carbon in this organic matter is consumed by predators, scavengers and microbes in a cycle that traps carbon in the food chain. A small percentage of the carbon-permeated organic matter also reaches the seabed as particles, where it is trapped in the sediment.

But most of the sequestered carbon is likely to occur through respiration, where CO2 is dissolved in the ocean. If breathing occurs below a depth of about 800 meters, CO2 can get trapped there, explains Grace Saba, assistant professor in the Department of Marine and Coastal Sciences at Rutgers University. “All sources of carbon – whether particulate, dissolved, or inhaled – can be sequestered for long periods of time, as long as they reach depths deep enough not to be affected by large-scale seasonal events … oceanic mixing, ”says Saba, who investigates oceanic carbon flow. Particles like feces or flesh that end up on the seabed “can be sequestered on a scale of millions of years,” she says.

A larger fish carries more carbon in its body. This is why these species have so far been the focus of attention of researchers like Mariani, whose study examined the potential for lost sequestration caused by fishing for sharks, tuna, mackerel, and door fish. -sword and other large species.

The researchers estimated the carbon contributions of whales, the largest inhabitants of the oceans. When whales die, their bodies contain around 33 tons of CO2, which is then absorbed by sea creatures or sequestered in deep water, compared to around 22 kilograms that a tree sequesters each year, reports the International Monetary Fund (IMF).

But even big fish and whales cannot eclipse the value of schools of small fish for global carbon cycles: research published in Nature Communications has shown that tiny crustaceans called krill are the main players in a “biological pump”. which moves carbon from the surface to the surface. on the high seas, and ultimately sequesters up to 12 billion metric tonnes of carbon per year. Krill contributes to this system by consuming large amounts of phytoplankton, which capture carbon through photosynthesis at the ocean surface. They then sequester the carbon consumed by breathing it in depth, and through their faeces which sink to the bottom of the ocean. Their central importance in this carbon cycle process raises concerns about the intensive commercial krill fishery in the Southern Ocean.

Overall, recent research from Saba, published in the journal Limnology and Oceanography, estimates that fish contribute about 16 percent of the carbon that ends up sinking into the deeper layers of the ocean. If fish are such an important carbon sink, a natural reservoir lowering the concentration of CO2, isn’t protecting them important to efforts against climate change?

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

Several aspects of current fisheries management were discussed. For example, researchers presented a study published in Nature, which showed that bottom trawling releases as much carbon from the seabed as the aviation industry as a whole. This could be another reason to support Marine Protected Areas (MPAs), which currently cover only 2.7% of the seabed, researchers say. MPAs could also increase fish populations which will sequester more carbon – and by building up fish stocks, they could simultaneously increase fishing yields and food security.

Further research (currently under review) has revealed that the Northeast Atlantic Ocean is one of the largest carbon sinks in the world, but at the same time has the highest fishing intensity. highest on the planet, highlighting the need to combat overfishing in European seas.

Researchers have also pointed to the potential of fisheries subsidies to threaten the carbon sequestration capacity of fish. Mariani’s research reveals that 43.5% of the “blue carbon” – stored in marine ecosystems – that was extracted by fishing between 1950 and today came from areas of the ocean that would not have been profitable to fish without subsidies. Removing subsidies could protect these resources without affecting food security. “If we try to reallocate these subsidies to something more sustainable, it would both limit overfishing, help rebuilding stocks, and maybe help fish sequestrate carbon,” suggests Mariani.

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

The complexity of carbon cycles already presents a considerable research challenge. In fluctuating ocean environments, extreme weather, temperature, depth, and habitat can all affect how carbon cycles work in the high seas. “It’s new science. It’s not like trees and forests. People have always looked at them, and so they have become mainstream. But that research has not yet been integrated, ”says Sumaila.

Determining exactly how much carbon different species are sequestering in the sea will also be critical, and that means looking beyond the big fish. “In my opinion, what would be most useful at this time for policymakers would be to get an estimate of the biomass-specific carbon flux 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 also a focus of Mariani’s research. “The next step is to estimate the amount of carbon sequestered each year by all fish species in the ocean, based on different climate scenarios and different fishing intensity scenarios,” he told about his upcoming research.

Over the next few months, a group of around 25 researchers will contribute to a body of papers on this general topic, which is being led by Sumaila and will be fully published later this year. The end goal is to develop enough research to give fish conservation a foothold in climate policy, Sumaila explains.

It’s hard to calculate the true worth of a fish – but accumulated research suggests that there is one benefit of their existence that we have overlooked for too long: instead of simply being the victims of climate change, the fish could be powerful forces against him. . “We must use all means to reduce greenhouse gas emissions. And here science tells us that fish bodies sequester a lot of the CO2 we have in the atmosphere. We need to bring this to the table, along with all of our other efforts to tackle climate change, ”Sumaila said.

Emma Bryce is a freelance journalist who covers stories on the environment, conservation and climate change.

This article is courtesy of China Dialogue Ocean and can be found in its original form here.

The opinions expressed here are those of the author and not necessarily those of The Maritime Executive.