A new study published in Science Advances has some news for the scientific community: We should be paying more attention to fish poop. While not a glamorous subject, the study finds that fish and their feces play an underappreciated role in ocean ecosystems. 

Why fish? Phytoplankton in the ocean absorbs carbon from the water and air around them. These creatures are then eaten by fish, moving the carbon up the food chain. Fish then release this carbon in their poop, sending it back down to the seafloor.

“We think this is one of the most effective carbon-sequestration mechanisms in the ocean,” said University of California Los Angeles researcher and study leader Daniele Bianchi. Unfortunately, as commercial fishing has slashed the size of global fish populations, there is less of this natural carbon sequestration taking place. How much less? Let’s take a look.

The researchers looked at ocean conditions from the 19th century, before the rise of industrial fishing, as well as high catch conditions at the turn of the 20th century. Looking at data from two key species, tuna and cod, the researchers estimate that these species alone took up 940 million metric tons of carbon per year before industrialization. That’s two percent of all biomass produced by phytoplankton. When adjusting estimations for all fish, that figure jumps to 1.9 billion metric tons of carbon per year or four percent.

For comparison, the entire UK emitted 326 million metric tons of CO2 last year. Unfortunately, following industrialization, the amount of sequestration by fish dropped to just one percent of phytoplankton biomass, meaning today’s fish are taking up half the CO2 they once did, even as we pump more into the atmosphere.

Another factor that makes fish so important is the way they sequester carbon. Fish excrement pellets are dense, so they sink to the seafloor quickly, making up about 10 percent of the carbon in the deep ocean. This carbon is stored for longer (roughly 600 years), making it an especially efficient carbon sequestration method. Fish also store carbon when they die and sink to the seafloor. In contrast, when fish are harvested, this stored carbon in their bodies is quickly released into the atmosphere.

Researcher William Cheung co-authored a study on fish death and carbon and told Vox, “When we manage our fisheries and set targets, we should also think about how that will affect the capacity of the ocean to store carbon.”

Source study: Science Advances – Estimating global biomass and biogeochemical cycling of marine fish with and without fishing