Industrial fishing has been depleting midwater fish for decades, new WHOI study finds
With fishing efforts being directed progressively deeper, catches shifted heavily toward midwater species like pomfrets and opahs. (©NOAA Fisheries) May 8, 2026
Woods Hole, Mass. (May 8, 2026) — A new study led by scientists at Woods Hole Oceanographic Institution (WHOI) finds that industrial-scale fishing has been removing substantial biomass from the ocean’s “twilight zone” for decades, challenging the common assumption that this vast midwater ecosystem remains largely unexploited.
The research focuses on a poorly understood group of larger midwater fishes that the authors call the “dark web,” species, such as pomfrets and snake mackerels, that live in or move through the mesopelagic zone, roughly 200 to 1,000 meters below the surface. Unlike smaller fishes often sampled in research nets, larger species are frequently missed by traditional scientific surveys but are regularly caught in commercial fisheries.
“Much of the discussion around the ocean twilight zone has assumed large-scale fishing there has not really begun,” said Martin Arostegui, lead author of the study and research associate at WHOI. “Our study shows that for these larger midwater fishes, that is simply not the case.”
Through an analysis of decades of catch records from the Hawai‘i-based longline fishery and other published examples from around the world, researchers found that catches of these midwater species have grown substantially and, in some fisheries, now exceed catches of traditional target species such as tuna and swordfish.
Lancetfish are among the species discarded in staggering numbers, which rarely survive release. (Camrin Braun, ©Woods Hole Oceanographic Institution)
Starting over 100 years ago, the Hawaiians' longline fishery that primarily targets tuna and swordfish provides one of the most thorough long-term data sets. What was once a small-scale fishery began dramatically expanding in the 1990s with the development of more effective fishing techniques and valuable export markets. With fishing efforts increasing and being directed progressively deeper, catches shifted heavily toward midwater species like pomfrets and opahs, and the prices of which soared. Unwanted species, such as lancetfish, were discarded in staggering numbers.
“In many fisheries, so-called discard species are thrown back because they have little to no market value, but studies show that roughly 80 to 100% of these ‘dark web’ midwater fish die after release due to capture stress and injury, meaning they are effectively returned to the ocean already dead and simply sink out of the system,” said Simon Thorrold, a co-author of the study and fish ecologist at WHOI.
The study identified evidence of heavy mesopelagic fishing pressure globally, including declining abundance, shrinking fish size, and pervasive under-reporting that complicates management.
“We know surprisingly little about these fishes despite their likely importance to ocean ecosystems,” said co-author Camrin Braun, an oceanographer and ecologist at WHOI. “That knowledge gap makes it difficult to understand how fishing pressure may already be affecting food webs and ocean carbon storage.”
Twilight zone species play important roles in the broader ocean system. Many serve as prey for commercially valuable predators such as tunas and swordfish, helping sustain ocean food webs, fisheries, and dependent communities. Some also migrate daily between deep and surface waters, a behavior that may help move carbon into the deep ocean as part of the biological carbon pump and, thereby, contribute to the mitigation of climate change.
The authors call for improved catch reporting and better integration of these species into fisheries management frameworks. They note that existing institutions could provide a foundation for more effective oversight if supported by better data and coordinated policy action.
“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.
###
About the Woods Hole Oceanographic Institution
The Woods Hole Oceanographic Institution is a private, nonprofit research and higher education organization dedicated to advancing knowledge of the ocean and its connection to the Earth system. WHOI scientists and engineers work around the globe to explore the ocean, understand its changing environment, and develop solutions to critical challenges facing the planet.