Deep below the ocean’s surface, in a dimly lit region known as the twilight zone, millions of fish are being caught every year. Scientists say the consequences are largely unknown.
The twilight zone, or mesopelagic zone, stretches from about 200 to 1,000 meters below the surface. It is home to species such as opahs, snake mackerels, and lancetfish that play an important but poorly understood role in ocean ecosystems.
Researchers from America’s Woods Hole Oceanographic Institution (WHOI) recently warned that these fish form a marine “dark web” — a hidden network of mid-level predators and prey that helps connect ocean food chains and may contribute to regulating the planet’s carbon cycle. The term dark web refers to their largely unseen ecological role, not anything sinister.
“There is a huge swath of the open ocean that we thought was not yet fished,” said Martin Arostegui, a research associate at WHOI and lead author of the study. “But it turns out the tactics that different fisheries have used over the years have made the deep-water large fishes of this ecosystem more and more susceptible to being captured.”
The researchers found evidence that industrial fishing has been reaching into these ecosystems for decades, often without attracting much attention from scientists or policymakers. Many mesopelagic species are not covered by mandatory catch reporting, making it difficult to know exactly how many are being removed from the ocean. Chinese fishing boats are notorious for going out in the thousands, and trawling around what should be protected areas, like the Seychelles. There are currently no superpower or group of countries regulating China.
Over-fishing the Dark Web matters because twilight-zone fish do more than simply feed larger predators. They also move carbon and nutrients through the ocean as they migrate between deep and shallow waters.
“We know surprisingly little about these fishes despite their likely importance to ocean ecosystems,” said WHOI oceanographer Camrin Braun. “That knowledge gap makes it difficult to understand how fishing pressure may already be affecting food webs and ocean carbon storage.”
Scientists have estimated that mesopelagic fish make up one of the largest animal biomasses on Earth, although the true figure remains uncertain. They are also an important food source for tuna, swordfish, sharks, marine mammals, and seabirds. The concern is that large-scale harvesting could disrupt both marine food webs and the ocean’s ability to store carbon.
“This study highlights how much remains to be learned about life in the twilight zone and how urgently we need better monitoring and management,” Arostegui said. As interest grows in harvesting deep-ocean fish for fishmeal, aquaculture feed, and other products, researchers say understanding these hidden ecosystems should come before expanding fishing pressure.
The ocean’s twilight zone may be out of sight, but scientists warn that what happens there could have consequences that reach all the way to the surface. There are similar concerns about deep sea mining, an issue we have covered extensively on Green Prophet.
