New research across several marine science disciplines is revealing the profound and previously underestimated role that fish play in the ocean’s ability to store carbon. Far more than passive actors in the marine food web, fish actively contribute to global climate regulation through a variety of biological and behavioral processes. These mechanisms, from biochemical excretions to daily migrations and seafloor interactions, are now understood to be significant drivers of oceanic carbon sequestration, influencing both the chemistry of the water and the long-term burial of atmospheric carbon.
These findings collectively illustrate that fish are a critical component of the global carbon cycle, responsible for transporting and sequestering billions of tonnes of carbon annually. Studies have identified four primary pathways: the excretion of carbonate minerals, the sinking of fecal matter as part of the biological pump, the active transport of carbon to deep waters via daily migrations, and the burial of carbon through sediment disturbance. Understanding these processes is not only reshaping our knowledge of marine biogeochemistry but also highlighting the urgent need to protect fish populations as a crucial, natural climate mitigation service.
The Carbonate Counterweight
One of the most surprising discoveries involves the chemical waste produced by fish. To manage the salt content in their bodies through a process called osmoregulation, marine fish precipitate and excrete carbonate minerals from their intestines. For decades, scientists were unsure if this process, observed in labs, had a meaningful impact in the vast open ocean. A breakthrough came from a long-term study in the North Atlantic, led by scientists from the Woods Hole Oceanographic Institution, who analyzed samples collected in sediment traps over 30 years. Researchers had consistently noted the presence of unusual “blue particles” that they could not identify.
The study, published in Global Biogeochemical Cycles, provided the first direct evidence that these particles were fish-derived carbonates. These carbonates are chemically different from those produced by plankton, notably containing high levels of magnesium. When released, these particles sink and were thought to dissolve quickly, increasing the alkalinity of the surrounding seawater and aiding in the ocean’s ability to absorb atmospheric carbon dioxide. The discovery of these particles intact at depths of several hundred meters challenges previous assumptions and opens new questions about which species produce them and how they survive the journey to the deep sea. This mechanism represents a previously hidden biological process that influences ocean chemistry on a global scale.
An Engine of the Biological Pump
Beyond producing carbonate minerals, fish are a major contributor to the ocean’s biological carbon pump, the vital process that moves organic matter from the surface to the depths. Through their daily activities of feeding, respiring, and excreting, fish package carbon into dense particles that sink rapidly. A 2021 study estimated that the carbon contained in fish feces, respiration, and other excretions accounts for approximately 16% of the total carbon that sinks below the ocean’s upper layers.
This contribution amounts to an immense scale: about 1.5 billion tonnes of carbon each year. To put this figure in perspective, it is equivalent to twice the annual carbon dioxide emissions of all 27 member states of the European Union. As phytoplankton at the surface convert dissolved CO2 into organic matter through photosynthesis, fish consume this carbon. Their subsequent waste, primarily in the form of fecal pellets, is much heavier than the original organic matter and thus sinks more efficiently into the deep ocean. Once in the depths, this carbon is effectively locked away from the atmosphere for hundreds or even thousands of years, mitigating the pace of global warming.
The Twilight Zone Taxi
Perhaps the most active role fish play in carbon transport is performed by the inhabitants of the mesopelagic, or “twilight zone,” an ocean layer between 200 and 1000 meters deep. This region is home to a vast biomass of fish, such as lanternfish, which undertake a daily journey known as diel vertical migration. Every night, billions of these fish swim from the dark depths to the food-rich surface waters to feed on plankton. As dawn approaches, they return to the relative safety of the twilight zone to avoid predators.
Active Carbon Transport
This daily commute acts as a carbon taxi service. After feeding near the surface, the fish transport that carbon deep into the ocean. At these depths, they release the carbon by breathing out carbon dioxide, excreting waste, and being eaten by other deep-sea predators. This process actively moves carbon away from the surface and into layers where it can be sequestered for much longer periods. Research led by scientists at the University of Washington is working to quantify this effect, though it remains a complex task with significant uncertainty. Early estimates suggest the amount of carbon transported by a single lanternfish can vary six-fold, making global calculations challenging but highlighting the mechanism’s enormous potential impact.
Seabed Engineers and Carbon Burial
The influence of fish extends all the way to the ocean floor. Many species, including commercially important ones like Atlantic cod and European eels, directly engineer the seabed through their natural behaviors. This process, known as bioturbation, involves disturbing and reworking sediments as the fish forage for food, build nests, or burrow for protection. A comprehensive study of 185 fish species on the UK’s continental shelf revealed that these activities are critical for burying organic carbon in the seabed.
By stirring the sediments, fish help mix carbon-rich organic matter from the surface into deeper layers of the seafloor, where it is less likely to be decomposed by bacteria and released back into the water column. This burial effectively sequesters carbon. However, the study, published in Marine Environmental Research, warns that this vital climate-regulating service is under threat. Many of the most impactful bioturbating species, such as the common skate and Atlantic cod, are vulnerable or critically endangered due to decades of overfishing. Their decline jeopardizes the ocean floor’s capacity to act as a stable, long-term carbon sink.
Implications for Climate and Conservation
The growing body of evidence on the role of fish in carbon storage has profound implications. Current climate and ocean models largely overlook these biological processes, meaning we may be underestimating the ocean’s true carbon sequestration capacity. Integrating these fish-driven carbon fluxes is essential for creating more accurate projections of future climate change and for understanding the full value of marine ecosystems.
Furthermore, these discoveries transform the conversation around fisheries management and marine conservation. Protecting fish populations is no longer just a matter of preserving biodiversity or food security; it is a direct climate action. Practices like bottom-trawling are doubly harmful, as they decimate fish stocks while also disturbing the very seabed sediments that help store carbon. Limiting destructive fishing practices and establishing marine protected areas can help restore fish populations, thereby boosting the ocean’s natural ability to draw down atmospheric CO2. Recognizing fish as allies in the fight against climate change provides another powerful argument for their conservation.