Oceans: fish, an invisible carbon sink threatened by fishing and climate change
The oceans play a major role in carbon storage, notably through the biomass they harbor. The life cycle of fish contributes to the long-term sequestration ofCO2 in the deep sea, but industrial fishing has weakened this essential mechanism, which is also threatened by climate change. Restoring deep-sea marine populations could strengthen this natural carbon sink, while limiting conflicts with food security.
Gaël Mariani, World Maritime UniversityAnaëlle Durfort, University of MontpellierDavid Mouillot, University of Montpellier and Jérôme Guiet, University of California, Los Angeles
When we talk about natural carbon sinks- natural systems that trap more carbon than they emit - we tend to think of forests and soils rather than oceans. Yet the oceans are the second largest natural carbon sink.
The impact of human activities (particularly fishing) on ocean carbon storage has been little studied until now, and this despite the fact that marine macrofauna (especially fish) account for around a third of the organic carbon stored by the oceans. Our research, recently published in Nature Communications and One Earthhave set out to remedy this situation.
Our results show that fishing has already reduced carbon sequestration by fish by almost half since 1950. By the end of the century, the combined effect of fishing and climate change is expected to reduce carbon sequestration by 56%. All the more reason to call for more sustainable ocean management, taking into account the impact of fishing on carbon sequestration.
Why take an interest in ocean carbon sequestration?
The Intergovernmental Panel on Climate Change (IPCC) explicitly states this in its reports: to achieve climate goals, we must first drastically and immediately reduce our greenhouse gas emissions (every year, human activities emit around 40 billion tonnes of CO₂ equivalent), and then develop nature-based climate solutions.
These incorporate all measures to restore, protect and better manage ecosystems that trap carbon, such as forests. These measures could capture 10 billion tonnes of CO₂ equivalent per year, and must be implemented as a complement to emission reduction policies.
However, the carbon stored by these ecosystems is increasingly threatened by climate change. For example, forest fires in Canada emitted 2.5 billion tonnes CO₂ equivalent in 2023: the forest is then no longer a carbon sink, but becomes a source of emissions.
Faced with this situation, the scientific community is now turning its attention to the oceans, in search of new solutions to sequester more carbon.
But for this to be possible, we first need to understand how the life sheltered by the oceans interacts with the carbon cycle, and the influence 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 38,000 billion tonnes of carbon stored by the ocean is stored through physical phenomena. However, ocean biomass also contributes, accounting for around 1,300 billion tonnes of organic carbon. Fish account for around 30% of this carbon stock.
This is made possible by their contribution to the so-called biological carbon pump, i.e. the series of biological processes that transport carbon from surface waters to the seabed. This is a major component of the carbon cycle.
This biological pump starts with phytoplankton, capable of transformingCO2 into carbonaceous organic matter. When phytoplankton die, some of this carbon sinks to the depths of the ocean, where it is permanently sequestered, while the rest is ingested by predators. Once again, it is when this carbon sinks to the depths (faecal pellets, carcasses of dead predators, etc.) that it is permanently sequestered.
Fish play a key role in this process: their denser carcasses and fecal pellets sink much faster than those of plankton. And the faster carbon sinks to the depths - and away from the atmosphere - the longer it takes to return to the atmosphere: carbon is thus stored more sustainably.
Our study, which looked specifically at fish species of commercial interest (i.e. targeted by fishing), estimates that they were capable of sequestering 0.23 billion tonnes of carbon per year in 1950 (i.e. 0.85 tonnes ofCO2 per year).
A vicious circle of climate change
But things have changed since 1950. Firstly, due to climate change: because of the increasing scarcity of food resources (less phytoplankton) and changes in environmental conditions (temperature, oxygen, etc.), the stronger the climate change, the more the biomass of species of commercial interest - and by extension, their capacity to sequester carbon - will decrease.
- In a scenario where the average rise in temperature is limited to 1.5°C (the scenario of compliance with the Paris Agreement), biomass would decline by around 9% by the end of the century, representing a reduction in carbon sequestration of around 4%.
- In a business-as-usual scenario, with temperatures rising by 4.3°C, this drop would be around 24% for biomass, and almost 14% for carbon sequestration.
We're therefore dealing with what's known as a positive feedback loop - in other words, a vicious circle: the greater the climate change, the less carbon fish will sequester, which will reinforce climate change itself. It's like a snake biting its own tail.
Carbon sequestration already halved by fishing
The impact of climate change in the 1.5°C warming scenario(which we are well on the way to exceeding) therefore remains low, but the effects of fishing are already visible.
Today, commercial fish species already sequester only 0.12 billion tonnes ofCO2 per year (compared with 0.23 billion tonnes of carbon per year in 1950), a reduction of almost half.
All the more so as the effects of fishing are not the same depending on the sequestration pathway considered. Since 1950, fishing has reduced carbon sequestration via fecal pellets by around 47%. For the carcass pathway, the reduction is around 63%.
This is because fishing targets the largest organisms, those with the fewest predators - and therefore those most likely to die of old age and see their carcasses sink into the abyss.
This reduction also means less food reaching the abyss, since carcasses are a particularly nutritious resource for the organisms that live there.
Yet we know very little about these abyssal ecosystems, with millions of species yet to be discovered. To date, we have only observed 0.001% of the total surface area of these ecosystems. We may therefore be starving a multitude of abyssal organisms that we barely know.
To preserve the climate, restore fish populations?
Our study shows that if fish populations were restored to their historical levels of 1950, this would sequester an additional 0.4 billion tonnes ofCO2 per year, a potential comparable to that of mangroves. With the added bonus that this carbon would be sequestered for around six hundred years, longer than in mangroves, where only 9% of the carbon sequestered remains after one hundred years.
However, despite this notable potential, climate solutions based on marine macrofauna restoration, if implemented alone, would have only a minor impact on the climate, compared with the 40 billion tonnes of CO₂ emitted every year.
Particularly as this is a recent field of research, many uncertainties remain. For example, our studies do not take into account trophic relationships (i.e., linked to the food chain) between predators and their prey, which also contribute to carbon sequestration. If we increase predator biomass, prey biomass will mechanically decrease. So, if predator carbon sequestration increases, prey carbon sequestration decreases, which may neutralize the impact of measures aimed at restoring fish populations to sequester carbon.
Thus, our results should not be seen as sufficient evidence to consider such measures as a viable solution. They do, however, 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 rendered by the ocean to our societies (food security, jobs...).
Conflicts between fishing and carbon sequestration, especially on the high seas
Marine organisms play a direct role in carbon sequestration, while also benefiting the fishing industry. This sector is a major source ofemployment and economic income for coastal populations, contributing directly to maintaining and achieving food security in certain regions.
Conflicts between carbon sequestration and the socio-economic benefits of fishing can therefore theoretically arise. If fishing increases, fish populations and their capacity to sequester carbon will decrease, and vice versa.
However, we have shown that only 11% of the ocean surface is potentially exposed to such conflicts. These are areas where fishing effort and carbon sequestration are both high.
What's more, a majority (around 60%) of these potentially conflict-ridden areas are located on the high seas, where catches make a negligible contribution to overall food security. Also, deep-sea fishing is known for its low profitability and massive subsidization by governments (to the tune of $1.5 billion, or more than 1.2 billion euros, in 2018).
These government subsidies are strongly criticized for threatening the viability of small-scale coastal fisheries, promoting fuel consumption and increasing inequalities between low- and high-income countries.
Our results provide a further argument in favor of protecting the high seas. In addition to avoiding multiple negative socio-economic effects, it would also protect biodiversity and, at the same time, preserve the oceans' capacity to sequester organic carbon.
Gaël Mariani, PhD in marine ecology, World Maritime UniversityAnaëlle Durfort, PhD in marine ecology, University of MontpellierDavid 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. Read theoriginal article.