There’s a New Reason to Save Life in the Deep Ocean

Porter Fox

To most of us, the ocean is a no man’s land — a vast, bottomless and uncharted void. Three-quarters of the ocean has never been seen by humans, and only a quarter of its floor has been mapped in detail, which means we have a better understanding of the surface of Mars than we do of the seas on our own planet. It is this lack of exploration and appreciation — particularly of the layer of cold, dark water that begins where light fades, known as the ocean’s twilight zone — that has led us to a very precarious place.

Recently, scientists at the Woods Hole Oceanographic Institution wrapped up the most comprehensive study of the twilight zone in history, helping to establish that some 11 billion tons of microorganisms, crustaceans, squid, fish and gelatinous animals that live there are helping to draw down a third of the carbon dioxide emitted by human activity, likely saving us and our planet from catastrophic climate change.

Just as we are learning to appreciate the extraordinary service of creatures in the twilight zone, companies that manufacture feed for industrial fish farms, fertilizer and omega-3 supplements are preparing to exploit it. Right now nations are considering authorizing commercial fishing fleets to grind life in the twilight zone into fish meal, fertilizer and plant food. Before they move forward with these plans, it would be wise to hit pause so we can understand how that decision will affect our planet.

Mass migration of life in the twilight zone — which can be found in the deepest parts of the ocean, between 650 feet deep and 3,300 feet deep — was first discovered in World War II, when sonar operators on the U.S.S. Jasper recorded an acoustic signature of what looked like the seafloor rising up. After studying the signature further, they realized the layer was alive, and that it rose and fell with the cadence of Earth’s rotation. What they were witnessing was the largest migration of animal life on the planet: trillions of creatures (copepods, bioluminescent lanternfish and basking sharks) swimming to the surface at night to feed, then sinking at dawn to hide in the depths.

The biological foundation of the twilight zone is algae on the ocean’s surface, which uses photosynthesis to break down carbon dioxide, the main gas contributing to climate change, into organic carbon and oxygen forms. This process creates both half of the oxygen and lots of carbon-rich food for twilight zone microorganisms. Animals such as zooplankton and krill feed on the algae, then larger animals feed on them. Apex predators like sharks and whales devour, digest and release the accumulated carbon as waste, after which much of it sinks to the deepest depths and can stay there for hundreds if not thousands of years. “We call it marine snow,” Heidi Sosik, the scientist who led the Woods Hole Ocean Twilight Zone study, told me.

There are a few other ways that carbon can reach the seafloor, sinking algae, dead plankton and bacteria among them. Scientists call this great system the biological carbon pump, and ocean temperatures, currents and climate change itself can influence it. Another takeaway from the institution’s Ocean Twilight Zone project: If you fish out the zone or shut down the pump, you would likely see double the amount of carbon dioxide that humans have already added to the atmosphere, raising global surface temperatures by six to 11 degrees Fahrenheit — a doomsday scenario in which most of the planet becomes uninhabitable for humans and life in the ocean dies off.

With a third of the world’s fish stocks already over-exploited and fish populations in decline, it may be hard to turn fishing fleets away from what they see as a potential windfall. Most twilight zone fish are unpalatable for humans, but their high nutritional content makes them a good choice for fish meal. Aquaculture is the fastest expanding food sector worldwide, accounting for half of all fish harvested for consumption. With current fish meal sources stretched thin, the $240 billion industry is eyeing the protein-rich twilight zone.

For now, countries are still assessing the resources and feasibility of fishing at these depths: Norway issued its first trial fishing permits for this portion of the ocean back in 2016 and European Union scientists are studying whether the creatures can be caught, processed and sold without disturbing the carbon pump or food webs.

At the same time, the private sector is working quickly to overcome obstacles to catching fish in the zone by devising new nets and trawling rigs that can operate a half-mile under the water’s surface. One firm from Norway has already constructed a processing plant onboard a ship to mill twilight zone creatures into liquid.

When commercial fishing operations do eventually make it to the twilight zone, they won’t be subjected to many fishing regulations because much of the zone is part of the so-called high seas, beyond the jurisdiction of a single nation. A new United Nations agreement to preserve marine biodiversity in these areas was signed in June 2023, but most member nations, including the United States, have yet to ratify it — and likely won’t for years. In the meantime, a temporary moratorium on fishing in the zone, enforced by the U.N. or regional fishery management organizations, should be put in place.

One might think that fishing out 11 billion tons of sea life would be impossible, but one would be underestimating the ingenuity and profiteering instincts of humankind. Let’s not forget that whalers killed nearly three million of their prey in 100 years — including 90 percent of all blue whales.

The mystery of the deep is an old trope, but nonetheless true. Despite a persistent lack of funding and resources, marine scientists recently unraveled several of those mysteries. We would do well to listen to what they learned: Wipe out all of the lantern fish in the twilight zone, and we may well see a spike in global surface temperatures from which we can’t recover.