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Carbon is the chemical element found in the greenhouse gas COâ‚‚ emitted from fossil fuels burned in cars, factories and other sources. When carbon is stored in the ocean instead of remaining in the atmosphere, it helps to reduce climate change on land.

"In deep seas, without fish, carbon will remain at the surface, where it easily returns to the atmosphere. Fish function as transporters that help to get carbon down to the depths where it can be stored for a long time," says Angela Martin. But not much is known about the role of coastal fish.

In her from the University of Agder (UiA), she studied the feces of several coastal fish species. Here, it emerges that they affect carbon storage in the ocean in several different ways. The findings can inform how we manage ocean resources.

"People think of fish as something we can harvest. But they have many other roles that we don't see or think about," says Martin, who now works as an adviser for British environmental authorities.

Measured carbon content

Martin developed new methods for studying fish poop. She studied poop from 10 Norwegian fish species: whiting, pollack, Atlantic cod, shorthorn sculpin, mackerel, corkwing wrasse, ballan wrasse, rock cook, goldsinny wrasse and black goby.

The species were collected in the Skagerrak and on the west coast of Norway between 2018 and 2020. The researcher analyzed feces from all 10 species to measure carbon content.

"Estimating the carbon content of fish feces helps us learn about how important fish might be for transporting and storing carbon in coastal ecosystems," she says.

Martin also measured how fast the poop sank for four of the species; Atlantic cod, ballan wrasse, corkwing wrasse and black goby.

Sink rate says something about the likely fate of the carbon in the feces. It indicates whether it will quickly sink to the sediments where it can stay for a long time, or if it slowly drifts at the surface where it can be broken down and returned to the atmosphere in a very short time.

"These four species are relatively easy to work with and therefore I chose to focus solely on them as I was developing methods to collect feces," she says.

The poop from Atlantic cod, ballan wrasse, corkwing wrasse and black goby contained between 17% and 38% carbon. At the same time, she saw that the poop sank rapidly down to the seabed—between 2,550 and 6,390 meters per day.

"I discovered large variations. The same fish could produce completely different poop depending on what it had eaten. Fish that had eaten algae produced feces which sank more slowly than fish that had eaten organisms with shells," says Martin.

Martin explains that the shell material may function as extra weight and make the poop sink rapidly.

Four ways fish affect the climate

The research shows that fish influence carbon storage in four main ways:

• Direct storage: Fish store carbon in their bodies and produce carbon-rich poop that sinks to the seabed.
• Transport: Fish transfer carbon between different ocean areas through migration and daily movements between shallow and deep waters.
• Ecosystem control: Predatory fish regulate grazer populations preventing them from destroying carbon-rich areas such as kelp forests.
• Nutrient pump: Fish transport nutrients that enable plankton and plants to grow and capture carbon.

Martin also found that the coastal fish she studied produced much more poop in summer than in autumn. This shows that fish contribute to carbon storage in different ways throughout the year.

"It's much more complicated than we thought. Fish store and release carbon at the same time. They breathe out COâ‚‚, but they also produce poop that stores carbon," says Martin.

The research can have practical consequences. Martin now participates in a working group in the International Council for the Exploration of the Sea (ICES). ICES does research into fisheries management and gives advice on how much fish can be caught.

"We work to ensure that fisheries management doesn't just consider how much fish we can catch, but also the role fish play for the climate," she says.

For the Skagerrak, the research can have an important impact. Several places in the Skagerrak are very deep. Here researchers see high occurrences of carbon, but they don't know where the carbon comes from.

"It could be that fish from shallower areas contribute to transferring carbon to the deep places," says Martin.

Martin's method for collecting feces is now being used by researchers at the University of Oslo and in Ireland. A Finnish researcher and a Norwegian master's student are working on similar studies of cod.

"I tried and failed on many things along the way. It's good if others can learn from my mistakes," says Martin.