Deep ocean fish sequester carbon in seafloor sediments for thousands of years

Study author Clive Trueman and an orange roughy (Credit: Clive Trueman)

The creatures that swim and scuttle on the ocean’s bottom seem to defy nature, living in perpetual darkness with few apparent food sources. For years, scientists believed that these denizens of the deep subsisted off of particles drifting down from above, but new research shows that a far more complex and impactful transfer of energy is occurring.

“We know very little about the biology and ecology of deep water ecosystems … but at the same time we are increasingly exploiting these depths for fishing and also for energy and mining,” said Clive Trueman, associate professor in marine ecology at the University of Southampton.

Lead author of a study conducted with researchers from the University of Southampton and the Marine Institute, Ireland, Trueman and his team spent two weeks on the R/V Celtic Explorer studying aquatic communities that live between 500 and 1,800 meters deep in an unusual place known as the continental slope.

Tucked between the continental shelf and the abyssal plain in a deep underwater twilight, the continental slope is an aquatic interstate for a variety of species traveling to and from the surface.

“We typically think of the deep sea as being an empty environment, but in fact on the continental slopes there are highly diverse and abundant communities,” Trueman said. “So there is a need to understand how the communities on the continental slope function.”

The black scabbardfish was among the species sampled by the study (Credit: Clive Trueman)

The team spent long days sampling deep ocean fish off the coasts of Ireland and Scotland with a trawling net. The field study, however brief, was no cakewalk. Catching, sorting, measuring and weighing the fish was a full-day ordeal that began at 7 a.m. Trueman said the researchers found rest during deep sea trawls, where nets were cast down to 1,800 meters and raised back up — taking 2 hours each way.

As it turns out, at least half of all the fish in the continental slope gain energy by eating other fish that swim vertically throughout the water column. The predators’ waste is deposited on the seafloor, and the carbon it contains is stored there in the sediments for thousands of years.

Carbon isotope analysis and data modeling revealed that bottom-dwelling fish remove 1 million tonnes of carbon dioxide each year from the North Atlantic water column. Under European Union carbon trading prices, that’s worth up to £14 million ($23 million). But when estimated at the societal cost of removing carbon dioxide from the atmosphere, the deep ocean fish are performing a job worth £100 million ($169 million) annually.

The R/V Celtic Explorer (Credit: Clive Trueman)

When asked if the study could impact real-world management policies, Truman sounded optimistic.

“I hope so,” Trueman said. “Valuing marine resources in ways other than the market price of commercial fish is difficult — but I think [it’s] important when you are trying to manage ecosystems that are, up to now, relatively unspoiled.”

Top image: Study author Clive Trueman and an orange roughy (Credit: Clive Trueman)