The UN should protect the ocean’s twilight zone

United Nations delegates will gather in New York next week to discuss the future of the global ocean. This will be the UN’s third session in less than a year aimed at developing a new international agreement governing how to sustainably manage marine life in the open ocean, beyond coastal waters managed by individual member States.

So far, these negotiations have had a critical omission: the vast stretch of ocean known as the twilight zone. 

Initial discussions in 2018 and 2019 about “Biodiversity in areas Beyond National Jurisdiction” have focused on regions for which we have the most scientific data: surface waters and the deep seabed. We believe it is equally imperative for delegates to include the ocean’s midwater — the twilight zone — and to recognize its vital role in supporting ocean food webs and regulating global climate.

Well below the sunlit surface, the twilight zone extends throughout the ocean in an immense, globe-spanning layer from 200 to 1,000 meters (660 to 3,300 feet) deep. Its life forms are fascinating and diverse. They range from tiny bristlemouth fish — most smaller than your little finger — with gaping, tooth-filled jaws (the most numerous vertebrates on the planet), to giant siphonophores, whose gelatinous, tentacled chains can extend as much as 40 meters (130 feet) — approximately the length of three school busses — making them the longest animals on the planet. 

We are only beginning to appreciate the diversity and magnitude of benefits that these animals provide. We do know that every night, many twilight zone species participate in the largest animal migration on our planet, rising to feed near the surface and returning to deeper waters before daybreak. Through their movements, and by eating and being eaten, they support complex food webs. They also transport carbon from surface waters to the deep ocean, keeping it out of the atmosphere and playing an important role in regulating Earth’s climate.

We still have much work to do to assess the profusion of life in this vast region, but recent estimates suggest it could contain 10 times more fish by weight than previously estimated — on the order of 10 billion tons.

It is this biological abundance that makes the ocean twilight zone such an attractive — and vulnerable — target.

Surface fisheries are declining and aquaculture is rapidly expanding, resulting in a commensurate increase in demand for fish feed. In response, a few commercial interests are already harvesting twilight zone species to produce fish meal for aquaculture and to extract oils for nutraceutical markets.

Now, technologies are being developed to extend their reach even farther into the twilight zone. But the region remains largely unregulated, and even with protections in place, fisheries managers would not be able to determine sustainable catch limits without robust scientific information about twilight zone ecosystems.

When it comes to the natural environment, we have a long history of repeating our mistakes. We discover new resources and then exploit them to near depletion, usually before we fully understand them or the essential services they provide.

Marine fisheries are a case in point. The United Nations Food and Agriculture Organization estimates that one third of major commercial ocean species are being fished at biologically unsustainable levels. Some formerly productive fisheries have been exploited to near or total collapse, among them Northwest Atlantic cod, bluefin tuna, Atlantic halibut, and many commercial species in the South China Sea. The combined pressures of overfishing and ocean warming could threaten fish stocks on a global scale, and with them, the vital ocean food webs and processes they help to support.

With the twilight zone, we have an opportunity to transform how humanity relates to little known — yet potentially indispensable — ecosystems and resources. The ocean’s midwater is remote enough that it has mostly remained shielded from human impacts. We do not have to make another mistake of epic proportions and find ourselves trying to repair the damage done by overexploitation when it is already too late.

The United Nations Sustainable Development Goal 14 calls on member States to conserve, and sustainably use, the ocean and its resources. The UN — and the global citizenry it represents — must rise to the challenge presented by this new frontier, and act now to sustain its future. In so doing, we can promote discovery and international cooperation, become better stewards of our ocean planet, and avoid yet another tragedy of the unmanaged commons.

Mark Abbott, Ph.D., is the president and director of Woods Hole Oceanographic Institution and a biological oceanographer whose work focuses on the interaction of biological and physical processes in the upper ocean.

Chris Scholin, Ph.D., is the president and chief executive officer of the Monterey Bay Aquarium Research Institute and a marine biologist whose work focuses on developing autonomous, ocean-going robots to detect and quantify marine organisms using molecular probes.