Fish poo alters ocean chemistry and helps mitigate climate change


Daniele Bianchi, a researcher at the University of California, Los Angeles, has a message for climate scientists everywhere: Pay more attention to fish poop.

Fish and their droppings play a hugely important and vastly underappreciated role in ocean chemistry and the carbon cycle that shapes Earth’s climate, according to a new study edited by Bianchi and published in the journal Scientific advances.

The story goes something like this: Tiny marine organisms called phytoplankton absorb carbon from the water and air around them. As the plankton is eaten by larger and larger creatures, the carbon then moves up the food chain and ends up in the fish. These fish then dump much of it back into the ocean through their poop, much of which sinks to the sea floor and can store carbon for centuries. The scientific term for carbon storage is sequestration.

“We believe this is one of the most efficient carbon sequestration mechanisms in the ocean,” Bianchi told Vox. “It reaches the deep layers, where the carbon is sequestered for hundreds or thousands of years.”

The carbon that is stored in the deep sea is carbon that is do not making the oceans more acidic or trapping heat in the atmosphere. In other words, fish poo could be a bulwark against climate change.

The problem is that commercial fishing has reduced the world’s fish population to a fraction of its previous level. While scientists understand the importance of fish poop, they also recognize a new danger from large-scale fishing. Beyond endangering ecosystems, the industry is disrupting major nutrient cycles and perhaps eating away at an important carbon sink.

How much carbon do fish release?

About quarter of carbon dioxide emissions from cars, factories and farms end up in the ocean every year, making it one of the largest carbon sinks in the world. Much of this carbon is taken up by phytoplankton, which is then eaten by other marine organisms, which are then eaten by fish. It’s food chain 101.

What Bianchi and his co-authors wanted to know is how much of this phytoplankton (and the carbon it contains) ends up in the fish, and where it goes from there. The researchers focused their analysis on the ocean before industrial fishing began in the 19th century, and during a period of “maximum catch”, at the turn of the 20th century. Maximum catches, Bianchi notes, have led to overfished oceans of the type we see today.

A phytoplankton bloom in Sagami Bay in the Pacific Ocean.
Getty Images

The team had reliable data on commercial fish, such as tuna and cod, which have been widely studied by the fishing industry. According to their analysis, these fish alone absorbed about 940 million metric tons of carbon per year, or 2% of all the biomass produced by plankton, before pre-industrial fishing. “Two percent may seem like a small number, but it’s actually huge,” Bianchi said. By way of comparison, the United Kingdom emitted 326 million metric tons of carbon dioxide last year.

That 940 million number jumps to 1.9 billion metric tons of carbon per year, or 4% of total phytoplankton biomass, when the authors estimated the impact of all fish, not just those harvested by industry. some fishing.

Meanwhile, during peak catches – when there were about half as many fish in the ocean as before the Industrial Revolution – fish populations digested a much smaller portion of the world’s carbon. Commercially fished species absorbed about 1% of the total phytoplankton biomass, Bianchi said.

It’s similar to what’s happening in the oceans today, he explained: Fish are taking up about half the biomass and carbon than before, simply because there are far fewer of them.

Why fish poop is so important to the planet

When fish deposit carbon to the ocean floor, less is left to warm the planet.

That’s where poo comes in. About a fifth of the biomass eaten by fish “returns to the environment as fecal pellets,” the authors write. Because these pellets are relatively large and compact compared to the excrement of smaller organisms, they quickly sink into the deep ocean. This is the key to long term storage.

“When you think about carbon sequestration, a very important metric is how deep the carbon is going into the ocean,” Sasha Kramer, a researcher at the University of California, Santa Barbara who doesn’t have participated in the study. “Deeper particles are sequestered on longer timescales.”

According to Bianchi, commercial fish sequester about 10% of the carbon in the deep ocean and stay locked up for about 600 years, meaning fish poop is a significant cache of carbon.

A fisherman unloads walleye pollock in Primorye Territory, Russia.
Yuri SmityukTASS via Getty Images

Peruvian anchovies being processed at a fishmeal factory in Lima, Peru.
Ernesto Benavides/AFP via Getty Images

Fish can also sequester carbon when they die and sink to the ocean floor, says another recent study in Scientists progress. A single fish contains around 12.5% ​​carbon, Gaël Mariani, the study’s lead author, told Vox. This carbon can be locked up in the deep ocean, assuming the fish carcasses remain there.

In contrast, when fish are caught, the carbon they contain is partially released into the atmosphere a few days or weeks later, according to the study. This means that a large fishing operation can release a lot of carbon that could otherwise be stored. According to the paper’s estimates, fishing fleets harvested about 320 million metric tons of large fish – like shark and mackerel – between 1950 and 2014, which ‘saved’ about 22 million tons of carbon from being released. sequestered.

“We need to think about the interplay between fisheries management and carbon management,” said William Cheung, a professor at the University of British Columbia and co-author of the sinking fish study. “When we manage our fisheries and set targets, we also need to think about how this will affect the ocean’s ability to store carbon.”

The impact of fish and their excrement goes beyond carbon. For example, falling pellets provide food for some deep sea creatures, which consume oxygen as they feed. This affects the amount of oxygen available at those shallow depths, the authors say, some of which are already oxygen starved. Climate change play with the delicate balance of oxygen in the deep sea as well, Kramer said.

Excrement of a blue whale.
Getty Images

Fish aren’t the only sea creatures shaping the chemistry of the oceans. A study from 2010, for example, suggests baleen whale feces are rich in iron, which can seed phytoplankton blooms in the Southern Ocean. that, in turn, helps reduce carbon.

If baleen whale populations retrieve in the Southern Ocean, this could cause the populations of certain marine organisms in those waters to skyrocket, the authors write. “This food chain serves to keep more iron in surface waters where it is useful to phytoplankton, so [it] maintains productivity,” Stephen Nicol, a researcher at the University of Tasmania and lead author of the study, told Vox.

How commercial fishing affects ocean chemistry and climate change

Just as humans have industrialized agriculture with big AI-powered tractors and sprawling monocultures, we have also discovered how to harvest massive amounts of fish with large nets, trawls and dredges. In one year, fishing boats can catch more than 80 million tonnes of seafood. Today, more than half of the oceans are covered by industrial fisheries, search foundand from 2017, a third global marine fish stocks were overexploited.

The problems of overfishing go beyond damage to important species like sharks and rays and charismatic and endangered species like the vaquita porpoise. Researchers like Bianchi show that they also extend to climate.

Contrasting today’s depleted oceans with a theoretically “unfished” ocean, Bianchi and his co-authors show what kinds of benefits a fully stocked ocean provides.

“The authors hypothesize that a fishery-free ocean would have potentially combated some of the impacts of anthropogenic climate change,” Kramer said. If the ocean weren’t so overfished, the authors imply that “much more of this carbon would have been absorbed,” she said.

This is not to mention the carbon extracted by bottom trawling or the greenhouse gases emitted by ships. In 2016, for example, industrial fishing vessels released an estimated 159 million metric tons of CO2 emissions, a study estimates. It is roughly equivalent to the emissions from the netherlands Last year.

Ending industrial fishing would not be easy. Seafood provides protein to some 3 billion people worldwide and supports some 60 million jobs. And as a marine biologist Daniel Pauly argued in response to Netflix controversy Marine suction documentary, giving up seafood altogether isn’t feasible either. “It is a position that only a small fraction of the population of the wealthiest countries will adopt,” he writes.

But the industry can improve in many ways, and better understanding its impact on Earth’s climate should be part of that change. What Bianchi hopes others take away from the sinking study is that fish are critical to the chemistry of our oceans. “We changed their biomass,” he said, “and that has consequences.”

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