Fish poop alters ocean chemistry and helps mitigate climate change

Daniele Bianchi, a researcher at the University of California, Los Angeles, has a message for climatologists around the world: Be careful with fish droppings.

Fish and their droppings play an extremely important and vastly underestimated role in ocean chemistry and the carbon cycle that shapes Earth’s climate, according to a new study led by Bianchi and published in the journal Scientists progress.

The story goes like this: Tiny marine organisms called phytoplankton take up carbon from the water and air around them. As plankton are eaten by larger and larger creatures, the carbon then travels up the food chain to the fish. These fish then release much of it into the ocean through their droppings, much of which sinks to the seabed 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 carbon is sequestered for hundreds or thousands of years.”

The carbon that is stored in deep water is carbon that is not make the oceans more acidic or trap heat in the atmosphere. In other words, fish poop 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. As scientists discover the importance of fish droppings, they also recognize a new danger in large-scale fishing. Beyond endangering ecosystems, industry disrupts major nutrient cycles – and perhaps ingests an important carbon sink.

How much carbon do fish remove?

About a quarter of the carbon dioxide emitted by cars, factories and farms ends up in the ocean each year, making it one of the world’s largest carbon sinks. Much of this carbon is sucked up by phytoplankton, which is then eaten by other marine organisms, which are then eaten by fish. This is 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 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 “peak catch” period at the turn of the 20th century. Peak catches, Bianchi notes, have led to overexploited oceans as we know them today.

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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 tonnes of carbon per year, or 2 percent of all biomass produced by plankton, before pre-industrial fishing. “Two percent may seem like a small number, but it’s actually huge,” Bianchi said. For comparison, the UK emitted 326 million metric tonnes of carbon dioxide last year.

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

Meanwhile, during the peak harvest period – 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. Species fished commercially absorbed about 1 percent of the total phytoplankton biomass, Bianchi said.

It’s similar to what happens in the oceans today, he explained: Fish take up about half of 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 on the ocean floor, less is left to warm the planet.

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

“When you think about carbon sequestration, a very important measure is the depth at which the carbon enters the ocean,” said Sasha Kramer, a researcher at the University of California at Santa Barbara who did not participated in the study. “Deeper particles are sequestered over longer time scales.”

According to Bianchi, commercial fish sequester about 10 percent of the carbon in the deep ocean, and it stays locked up for about 600 years – meaning fish droppings are a major cache of carbon.

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Fish can also sequester carbon when they die and sink to the ocean floor, another recent study finds Scientists progress. A single fish contains around 12.5% ​​carbon, Gaël Mariani, the study’s lead author, told Vox. This carbon can be trapped in the depths of the ocean, assuming 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 document’s estimates, fishing fleets harvested around 320 million metric tonnes of large fish – such as sharks and mackerel – between 1950 and 2014, which “prevented” the sequestration of around 22 million tonnes of fish. carbon.

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

The impact of fish and their droppings goes beyond carbon. For example, falling dumplings provide food for some deep-sea creatures, which consume oxygen when feeding. This affects the amount of oxygen available in these shallow depths, say the authors, some of which are already oxygen starved. Climate change is also likely to upset the delicate oxygen balance in deep water, Kramer said.

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

If baleen whale populations recover in the Southern Ocean, it could cause populations of some 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 for phytoplankton, so [it] supports productivity, ”University of Tasmania researcher and lead author Stephen Nicol told Vox.

How commercial fishing impacts ocean chemistry and climate change

Just as humans industrialized agriculture with large AI-powered tractors and sprawling monocultures, we also figured out 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 fishing operations, according to research, and in 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 endangered species like the vaquita porpoise. Researchers like Bianchi show that they also extend to climate.

By contrasting the depleted oceans of today with a theoretical “non-fished” ocean, Bianchi and his co-authors show what kinds of benefits a well-stocked ocean offers.

“The authors hypothesize that an ocean without fishing would have potentially combated some of the impacts of anthropogenic climate change,” Kramer said. If the ocean had not been overexploited, the authors imply that “much more of this carbon would have been absorbed,” she said.

Not to mention the carbon that bottom trawling dredges, nor the greenhouse gases emitted by transport vessels. In 2016, for example, industrial fishing vessels emitted around 159 million metric tonnes of CO2 emissions, according to a study. This is roughly the equivalent of the emissions from the Netherlands last year.

Ending industrial fishing would not be easy. Seafood provides protein to some 3 billion people around the world and supports some 60 million jobs. And as a marine biologist Daniel Pauly argued in response to the Netflix controversy Seaspiracy documentary, abandoning seafood altogether is also not feasible. “It is a position that only a small fraction of the population of rich countries will take,” he writes.

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