Scientists show the importance of fish feces to ocean chemistry

According to a new study published in the journal Science Advances, fish and their droppings play a hugely important but underappreciated role in the ocean chemistry and carbon cycle that shape Earth's climate.

The biodiversity crisis has a message for climate scientists everywhere: Pay more attention to fish droppings , lead study author Daniele Bianchi, PhD, in Ocean and Atmospheric Sciences said.

The story goes like this: Tiny sea creatures commonly known as phytoplankton absorb carbon from the water and air around them. When phytoplankton are eaten by larger organisms, the carbon goes up the food chain and into the fish. Those fish then release a lot of carbon back into the ocean through their feces, much of which sinks to the seafloor and can store carbon for centuries. The scientific term for carbon storage is carbon sequestration or sequestration.

'We think this is one of the most efficient carbon sequestration mechanisms in the ocean. "It reaches deep layers where carbon is sequestered for hundreds or thousands of years," Vox quoted Daniele as saying.

Fish excrete carbon-rich feces.

Carbon stored in the deep sea is carbon that doesn't make the oceans more acidic or trap heat in the atmosphere. In other words, fish feces can be a bulwark against climate change.

The problem is that commercial fishing has made the global fish population pie smaller than it used to be. As scientists discovered the importance of fish droppings, they also realized a new danger was large-scale fishing. In addition to putting ecosystems at risk, the industry is disrupting major nutrient cycles and perhaps eroding an important carbon sink.

How much carbon do fish emit?

Every year, about a quarter of the carbon dioxide emitted from cars, factories and farms goes into the ocean, turning the ocean into one of the world's largest carbon sinks. Much of that carbon is taken up by plankton, which is eaten by other marine life, which is then eaten by fish. It's food chain 101 .

What Daniele and his co-authors want to know is how much of that phytoplankton, how much carbon in them ends up in the fish's belly, and where the carbon from the fish goes. The researchers focused their analysis on the ocean before industrial fishing began in the 19th century and during the 'peak fishing' period around the early 20th century. ' Peak fishing' led to the The oceans were all overfished in the way we recognize them today, Daniele notes.

Phytoplankton in the sea.

The team has reliable data on commercial fish such as tuna and cod that have been studied extensively by the fishing industry. According to their analysis, before pre-industrial fishing, these fishes alone consumed about 940 million tonnes of carbon each year, or 2% of the total biomass produced by plankton.

'Two per cent sounds like a little but it's actually a huge number ,' Daniele said. For comparison, the amount of carbon dioxide the UK emitted last year was 326 million tonnes.

The 940 million figure above adds up to 1.9 billion tonnes of carbon or 4% of total phytoplankton biomass annually, when the authors estimate the impact of all fish species, not just those caught. by the fishing industry.

Meanwhile, during peak fishing periods - when the number of fish in the oceans was about half what it was before the Industrial Revolution - the amount of carbon digested by fish populations relative to the world as a whole is a fraction of the total. much more. Commercially caught species account for about 1% of the total phytoplankton biomass, Daniele said.

What's happening in the oceans today is similar: Fish are absorbing one and a half times more biomass and carbon than before, simply because there are so many fewer fish.

Why is fish poop important to the Earth?

As fish trap carbon on the ocean floor, there is much less gas left to warm the planet. That's where poop comes in. About one-fifth of the biomass consumed by fish "returns to the environment as fecal matter," the authors write. These pellets quickly sink into the deep ocean because they are relatively larger and more compact than the droppings of smaller organisms. That's the key to long-term storage.

'In terms of carbon sequestration, one metric that really matters is the depth of carbon in the ocean'. "The deeper molecules are isolated for longer periods of time ," researcher Sasha Kramer (who was not involved in the study) told Vox. And according to Dr. Daniele, commercial fish isolates about 10%. carbon in the deep ocean, and carbon is 'locked up' for 600 years or so - that is, fish droppings make up a sizable carbon memory.

According to a recent study in the journal Science Advances, fish can also sequester carbon as they die and sink to the ocean floor. Talking to Vox, Gaël Mariani, lead author of this study, said that a fish contains about 12.5% ​​carbon. That carbon could be trapped in the deep ocean, assuming the fish remains there.

In contrast, when fish are harvested, some of their carbon is released back into the atmosphere a few days or weeks later. That means a large fishing operation will release a lot of carbon that could be stored if the fishing activity did not happen. According to the study's estimates, between 1950 and 2014, fishing fleets harvested about 320 million tons of large fish such as sharks and mackerel, "preventing" about 22 million tons of carbon from sequestration.

'We have to think about the interaction between fisheries management and carbon management'. "As we manage fisheries and set targets, we should also think about how that will affect the ocean's ability to store carbon ," Vox quoted the University of British professor verbatim. Columbia, William Cheung, co-author of the aforementioned sunken fish study.

The impact of fish and their droppings is not just carbon. For example, fecal pellets provide food for some deep-sea creatures. When they eat meat, these organisms use up oxygen, which affects the amount of oxygen available in hazy depths, some of which are already hypoxic, according to the authors. Climate change could also upset the fragile oxygen balance in the deep sea, says researcher Sasha.

The droppings of a blue whale.

Fish are not the only marine organisms that form ocean chemistry . For example, a study from 2010 found that baleen whale droppings are rich in iron, which can cause plankton in the Southern Ocean to bloom. This, in turn, helps reduce carbon.

According to this third study, if baleen whale populations in the Southern Ocean are restored, this could cause populations of some marine life in those waters to emerge. 'This food chain helps keep more iron in the surface waters, where it's useful for phytoplankton, thus maintaining productivity,' said the study's lead author, Stephen Nicol of the university. Tasmania.

The baleen whale (Mysticeti) is a whale that filters food from the water with baleen instead of teeth, unlike the toothed whale (Odontoceti).

The Southern Ocean is the water around Antarctica that has just been officially recognized by the US National Geographic Society as the 5th ocean in the world on June 8, 2021.

How does commercial fishing impact ocean chemistry and climate change?

Just as humans industrialized agriculture with large AI-powered tractors and vast monocultures, we've also found a way to harvest fish in bulk with nets, trawl nets and large dredgers. In a year, fishing vessels can catch over 80 million tons of seafood. Today, more than half of the ocean is covered by industrial fishing, and as of 2017, a third of the world's marine fish stocks are overfished, according to statistics in the study.

The problems of overfishing go beyond damage to important species such as sharks and rays and endangered species such as the vaquita (California dolphin, a species of porpoise that lives only in the southern part of the country). Northern Gulf of California). Researchers like Daniele are demonstrating that these problems also extend to climate. By contrasting today's dwindling oceans with a theoretical 'untapped' ocean, Daniele and his co-authors are demonstrating the kinds of benefits that a full ocean can offer. provided.

'The authors are hypothesized that an ocean without fishing would be more resistant to some of the effects of anthropogenic climate change' . If the ocean wasn't overfished, the authors imply that "much more of that carbon would be used , " Sasha said. That has nothing to do with the carbon that bottom trawls pick up or the greenhouse gases emitted by shipping vessels. For example, it is estimated that in 2016, industrial fishing vessels emitted about 159 million tons of CO2, roughly equivalent to the emissions of the Netherlands last year (2020).

It is not easy to end industrial fishing. Seafood provides protein for about 3 billion people worldwide and supports about 60 million jobs. And as marine biologist Daniel Pauly argued in the controversial Netflix documentary Seaspiracy, it's also not feasible to give up seafood entirely. 'This is a position that only a fraction of the population of richer nations would take,' Daniele wrote.

But there are plenty of ways industry can improve, and a better understanding of how industry impacts Earth's climate will be part of that change. What Danielle hopes others will learn from fish feces research is that fish are essential to the chemistry of our oceans. 'We changed their biomass', 'and that has consequences , ' Daniele concludes.

Dr. Daniele Bianchi - who led the first study in this article - is currently an assistant professor in the Department of Ocean and Atmospheric Sciences at the University of California at Los Angeles (UCLA).