Keeping large fish in the ocean increases natural carbon sequestration

By Kristen Kessler. Kristen has a background in elementary education, and has been an environmental activist for several years.

Overfishing occurs when fish are taken out of the ocean faster than the population can naturally recover. This destructive method of fishing has been the norm for decades. Overfishing of any species is harmful, but taking too many species of large fish is especially detrimental due to its repercussions on the climate as well as ocean ecosystems. 

Large fish such as tuna, mackerel, sharks, and billfish, along with whales, protect the planet from climate change in several ways. These apex predators keep the populations of smaller fish, which are naturally more abundant, in check. Because of their larger populations, small fish produce more carbon dioxide (CO2) through the process of cellular respiration. The oceans are already taxed with absorbing atmospheric CO2, so higher levels of CO2 in the water simply compounds the problem. Letting large fish live so they can prey on smaller ones lowers CO2 levels in the water, which allows the ocean to absorb more CO2 from the air. It also keeps marine food webs balanced and reduces acidification.

Additionally, when sharks and other large fish die, they store carbon in their bodies. Their heavy carcasses sink to the bottom of the ocean floor, where the carbon their bodies are made of is trapped beneath the sediment for thousands of years. This long term storage of carbon is known as sequestration. Conversely, when large fish are caught, processed and consumed en masse, that carbon is digested and subsequently released into the atmosphere through human respiration, excretion, and waste treatment.

One study found that over the past five decades, commercial fishing extracted 318.4 million metric tons of large fish from the ocean, causing approximately 37.5 million metric tons of carbon to be released into the atmosphere. Of that amount, at least 21.8 million metric tons of carbon would have been naturally sequestered through the bodies of those fish sinking to the bottom of the ocean had they not been removed from the sea. That’s the equivalent amount of CO2 generated by  4.7 million cars in one year.

The study also found that the release of this blue carbon, another term for the carbon stored in ocean ecosystems, was largely due to the fishing of tuna and mackerel species, which together made up 87.4% of the emitted carbon. This underscores that the overfishing of large fish like tuna, several species of which are listed as either vulnerable or endangered, is a climate change issue as well as an issue of conservation and biodiversity. 

Fisheries provide jobs for tens of millions of people around the world, and provide food for billions. But with 90 percent of world fish stocks already categorized as fully exploited or overfished, and the human population set to reach 10 billion by 2050, it is clear that a more sustainable relationship with the ocean is needed in order to maintain food security and ocean health while meeting climate change goals. 

Photo by Jakob Owens on Unsplash

The reimagining of the human relationship with the oceans can start with commercial fishing, which is heavily subsidized by governments—an estimated 43.5% of global oceans would be unprofitable to fish without subsidies. Governments could reallocate commercial fishing subsidies, which currently perpetuate harmful and unsustainable practices, towards investments in sustainable fishing methods and programs that help the fishing industry adopt those practices. 

Some governments have taken steps to promote sustainable fishing by working with regional fisheries and non-profit groups. The Southeast Asian Fisheries Development Center has used a multi-pronged approach to working with countries in the region. With their guidance, catch traceability systems have been implemented to prevent overfishing. Illegal fishing has been reduced, and local communities are included in the management of small fisheries. 

Globally, there needs to be strong consensus among nations to implement sustainable fishing practices at the scale necessary to make a substantial impact. Nations already commit to a quota, or limit, of the amount of fish that they take. However, enforcing the rules can be a challenge. Much of the open ocean is considered “high seas” and isn’t ruled by one unified authority. 

Despite these challenges, some progress has been made towards restoring dwindling fish populations. The International Committee for the Conservation of Atlantic Tunas (ICCAT) has taken steps to limit overfishing by establishing quotas for Atlantic Bluefin tuna. Though still vulnerable, its population has gone from endangered to least concern over the past four decades. In the United States, the National Oceanic and Atmospheric Administration (NOAA) has been successful in addressing bycatch of Atlantic Bluefin tuna. NOAA has also protected white marlins and sharks from being overfished in international waters.

On an individual level, consumers have the power to advocate for sustainable fishing practices by making informed choices. The Monterey Bay Aquarium’s Seafood Watch advises consumers on which seafood to buy and which to avoid, and The Marine Stewardship Council certifies seafood that is sustainably caught. 

Dr. Sylvia Earle, a well-known marine scientist, says we should go further. She recommends that we stop eating seafood until vulnerable populations have a chance to recover. She says that “every fish counts at this point,” and is particularly concerned about the plight of tunas.

Continued progress towards sustainable fishing, coupled with consumer awareness promotes healthier fish populations. Leaving fish alone will store carbon, keep marine ecosystems in balance, and mitigate the effects of climate change.