Oceans: Fish, an invisible carbon sink threatened by overfishing and climate change
The oceans play a major role in carbon sequestration, particularly through the biomass they support. The life cycle of fish thus helps to permanently sequesterCO2 in the deep ocean, but industrial fishing has weakened this essential mechanism, which is also threatened by climate change. Restoring marine populations in the open ocean could strengthen this natural carbon sink while minimizing conflicts with food security.
Gaël Mariani, World Maritime University; Anaëlle Durfort, University of Montpellier; David Mouillot, University of Montpellier and Jérôme Guiet, University of California, Los Angeles
When we talk about natural carbon sinks—those natural systems that absorb more carbon than they emit—we tend to think of forests and soils rather than the oceans. Yet the oceans are the second-largest natural carbon sink.
The impact of human activities (and in particular fishing) on ocean carbon storage had previously been little studied, even though marine macrofauna (especially fish) account for about one-third of the organic carbon stored by the oceans. Our research, recently published in the journals Nature Communications and One Earth, aimed to address this gap.
Our findings show that fishing has already reduced carbon sequestration by fish by nearly half since 1950. By the end of the century, this decline is expected to reach 56% due to the combined effects of fishing and climate change. This makes a strong case for more sustainable ocean management that takes into account the impact of fishing on carbon sequestration.
Why is carbon sequestration in the oceans important?
The Intergovernmental Panel on Climate Change (IPCC) states this explicitly in its reports: to meet climate goals, we must first drastically and immediately reduce our greenhouse gas emissions (human activities emit approximately 40 billion metric tons of CO₂ equivalents each year), and then scale up nature-based climate solutions.
These measures encompass a range of actions aimed at restoring, protecting, and better managing carbon-sinking ecosystems, such as forests. These measures could capture 10 billion metric tons of CO₂ equivalents per year and must be implemented in conjunction with policies to reduce emissions.
However, the carbon stored by these ecosystems is increasingly threatened by climate change. For example, forest fires in Canada emitted 2.5 billion metric tons of CO₂ equivalents in 2023: as a result, the forest is no longer a carbon sink but has become a source of emissions.
In light of this, the scientific community is now turning its attention to the oceans in search of new solutions that would allow for greater carbon sequestration there.
However, for this to be possible, we must first understand how marine life interacts with the carbon cycle, as well as the impact of climate change on the one hand and fishing on the other.
What role do fish play in this process?
The vast majority of the 38 trillion tons of carbon stored by the ocean is sequestered through physical processes. However, ocean biomass also contributes to this total, accounting for approximately 1.3 trillion tons of organic carbon. Fish account for about 30% of this carbon stock.
This is made possible by their contribution to what is known as the biological carbon pump—that is, the series of biological processes that transport carbon from surface waters to the seafloor. It is a major component of the carbon cycle.
This biological pump begins with phytoplankton, which is capable of convertingCO₂ into organic carbon. When it dies, some of this carbon sinks to the ocean depths, where it is permanently sequestered, while the rest is consumed by predators. Once again, it is when this carbon sinks into the depths (fecal pellets, carcasses of dead predators, etc.) that it is permanently sequestered.
Fish play a key role in this process: their carcasses and fecal pellets, being denser, sink much faster than those of plankton. However, the faster carbon sinks into the depths—and moves away from the atmosphere—the longer it will take to return to the atmosphere: the carbon will thus be stored for a longer period of time.
Our study, which focused specifically on commercially important fish species (i.e., those targeted by fisheries), estimates that these species were capable of sequestering 0.23 billion tons of carbon per year in 1950 (equivalent to 0.85 tons ofCO2 per year).
A vicious cycle caused by climate change
But since 1950, things have changed. First, due to climate change: as food resources become scarcer (less phytoplankton) and environmental conditions shift (temperature, oxygen levels, etc.), the more severe climate change becomes, the more the biomass of commercially important species—and, by extension, their ability to sequester carbon—will decline.
- In a scenario where the average temperature increase is limited to 1.5 °C (the Paris Agreement compliance scenario), biomass would decline by about 9% by the end of the century, representing a reduction in carbon sequestration of about 4%.
- In a business-as-usual scenario where temperatures rise by 4.3 °C, this decline would reach approximately 24% for biomass and nearly 14% for carbon sequestration.
So we’re dealing with what’s known as a positive feedback loop—in other words, a vicious cycle: the more severe climate change becomes, the less carbon fish will sequester, which will in turn exacerbate climate change itself. It’s a vicious cycle.
Carbon sequestration has already been cut in half by fishing
The impact of climate change under a 1.5°C warming scenario (which we are on track to exceed) therefore remains limited, but the effects of fishing are already evident.
Today, commercial fish species already sequester only 0.12 billion tons ofCO2 per year (compared to 0.23 billion tons of carbon per year in 1950), a decrease of nearly half.
This is especially true given that the effects of fishing vary depending on the carbon sequestration pathway in question. Since 1950, fishing has reduced carbon sequestration via fecal pellets by approximately 47%. For the pathway involving carcasses, this decrease is approximately 63%.
This is because fishing targets the largest organisms—those with the fewest predators—and thus those most likely to die of old age and have their carcasses sink into the depths.
This decline also means that less food is reaching the deep sea, as carcasses are a particularly nutritious source of food for the organisms that live there.
However, we know very little about these deep-sea ecosystems, with millions of species still to be discovered. So far, we have explored only 0.001% of the total area of these ecosystems. We may therefore be starving a multitude of deep-sea organisms that we barely know.
To protect the climate, restore fish populations?
Our study shows that if fish populations were restored to their 1950 levels, this would enable the sequestration of an additional 0.4 billion tons ofCO2 per year—a potential comparable to that of mangroves. With one key advantage: this carbon would be sequestered for approximately six hundred years—longer than in mangroves, where only 9% of the trapped carbon remains sequestered after one hundred years.
However, despite this significant potential, climate solutions based on the restoration of marine macrofauna would, if implemented on their own, have only a minor impact on the climate, given the 40 billion tons of CO₂ emitted each year.
Especially since this is a relatively new field of research, several uncertainties remain. For example, our studies do not account for trophic relationships (i.e., those related to the food chain) between predators and their prey, which also contribute to carbon sequestration. However, if predator biomass increases, prey biomass will automatically decrease. Thus, while carbon sequestration by predators increases, that of prey decreases, which can offset the impact of measures aimed at restoring fish populations to sequester carbon.
Thus, our results should not be viewed as sufficient evidence to consider such measures a viable solution. They nevertheless illustrate the importance of studying the impact of fishing on carbon sequestration and the need to protect the ocean to limit the risks of depleting this carbon sink, while taking into account the services the ocean provides to our societies (food security, jobs, etc.).
Conflicts between fishing and carbon sequestration, particularly on the high seas
In fact, marine organisms play a direct role in carbon sequestration, while also benefiting the fishing industry. This sector is a major sourceof jobs and economic income for coastal communities, contributing directly to maintaining and achieving food security in certain regions.
Conflicts between carbon sequestration and the socioeconomic benefits of fishing could therefore theoretically arise. If fishing increases, fish populations and their ability to sequester carbon will decrease, and vice versa.
However, we have shown that only 11% of the ocean’s surface is potentially at risk of such conflicts. These are areas where both fishing effort and carbon sequestration are high.
Furthermore, a majority (about 60%) of these potentially contentious areas are located on the high seas, where catches make a negligible contribution to global food security. Furthermore, deep-sea fishing is known for its low profitability and massive government subsidies (amounting to $1.5 billion, or more than €1.2 billion, in 2018).
These government subsidies have come under heavy criticism because they threaten the viability of small-scale coastal fisheries, encourage fuel consumption, and widen the gap between low- and high-income countries.
Thus, our findings provide further support for the protection of the high seas. In addition to preventing numerous negative socioeconomic impacts, this would also help protect biodiversity and, at the same time, preserve the oceans’ ability to sequester organic carbon.
Gaël Mariani, Ph.D. in Marine Ecology, World Maritime University; Anaëlle Durfort, PhD candidate in marine ecology, University of Montpellier; David Mouillot, Professor of Ecology, MARBEC Laboratory, University of Montpellier and Jérôme Guiet, Researcher in marine ecosystem modeling, University of California, Los Angeles
This article is republished from The Conversation under a Creative Commons license. Readthe original article.