The Ocean Twilight Zone

Can Science Promote Sustainability? The ocean's twilight zone is in the spotlight, as researchers explore what lives in this vast, mid-water world.

Bonnie Waycott

The ‘twilight zone’ is a huge expanse between the ocean’s surface and perpetually dark regions. Also known as the mesopelagic, referring to depths between 200 and 1000 metres, it plays a fundamental role in regulating Earth’s climate through the long-term ‘export’ of carbon from greenhouse gases in the atmosphere to the deep ocean and seafloor.

What lives there also supports life in the surface waters, including many commercially important species such as swordfish, tunas and sharks, as well as whales, seals and penguins.

Sloan's Viperfish

Technological challenges mean that few fishing nets have journeyed to the mesopelagic. But this could change in response to growing markets for aqua feed and nutraceuticals. Scientists want to improve our knowledge of this relatively unknown ecosystem to help improve the prospects for the sustainable conservation and management of mesopelagic fisheries.

Researchers at the Woods Hole Oceanographic Institution (WHOI) are currently leading a six-year Ocean Twilight Zone (OTZ) project (under the ‘Audacious Projects’ housed at TED) designed to transform our understanding of the mesopelagic, with scientific studies focused primarily in the slope waters off the Northwest Atlantic continental shelf.

Woods Hole researchers concentrated on slope waters off the Northwest Atlantic continental shelf

The project has drawn considerable attention, with a TED talk by Dr. Heidi Sosik, OTZ project lead at WHOI, which has been viewed over a million times.

´Our knowledge of life in the twilight zone comes from a few widely scattered research expeditions, localised trips with submersibles or targeted trips by research vessels containing specialised sonar systems,’ Heidi Sosik said.

Those expeditions hint at a remarkable abundance of fish that, if accurate, challenge our understanding of productivity and ecological processes in the open ocean. I also believe that any consideration of the harvesting of fish must first take into account their fundamental role in the ocean’s ecological processes.’

Snipe eel

The mesopelagic is said to contain as much as ten times the amount of fish biomass as there is in all of the world’s surface fisheries combined.

Although these fish could be a potential resource, substantial research efforts will be required to understand environmental pressures, the resilience of stocks to fishing and the potential effects on the ocean ecosystem if a reduction in biomass and biodiversity were to occur.

Sergia robusta

Most of the twilight zone lies beyond national boundaries on the high seas, making it all the more urgent to study the twilight zone and how it is connected to other parts of the ocean, to ensure that its resources are managed sustainably. The WHOI is seeking to determine whether scientific discovery and analyses can be substantially accelerated to gain an understanding of the ecosystem services provided by the twilight zone before it’s too late.

‘The fishing industry may have opportunities to engage and even collaborate with scientists to ensure that future harvests of twilight zone fish proceed sustainably and this resource remains available for generations,’ said WHOI OTZ Program Manager Philip Renaud.

Acoustic data collated on board NOAA research vessel Henry B Bigelow

Mesopelagic species are unlikely to appear on our plates any time soon. Instead, their most likely markets could be fishmeal and fish oils for aquaculture or terrestrial livestock, nutraceuticals or cosmetics. They would also need to compete on the basis of cost with other factors in the production of fishmeal, including other reduction fisheries, fish by-products (frames, off-cuts and offal) and plant-based oilseeds.

At present, some twilight zone fisheries exist in Antarctica and the Northeast Atlantic for krill and other crustaceans, including copepods, while regions of the Indian Ocean and Northwest Atlantic have also been explored.  But could mesopelagic species be fished sustainably?


‘Because of their ecological role, they shouldn’t be considered a single-species target,’ said Porter Hoagland, a marine policy specialist with the WHOI project.

Management should involve a broader ecosystem-based approach, which would enable caution in the choice of quotas and their allocation and the possible establishment of fishery reserves in some areas, designed to account for the three-dimensional environment inhabited by mesopelagic fish.’

Read more about the ocean’s Twilight Zone