Fish that live in the ocean's 'twilight zone' are the missing links between sharks and their prey

Large sharks spend long stretches in the deep, midwater “twilight zone” of the ocean – the mesopelagic zone between 650 and 3,300 feet – in search of other fish to eat. That layer holds enormous animal biomass, and an open ocean analysis suggested it may be far higher than earlier estimates.

The bigscale pomfret (Taractichthys longipinnis) sits right in the middle of this action. It is not tiny plankton and not a top predator, yet it may carry energy up and down the water column every single day.

Tracking midwater fish

New WHOI research tracked bigscale pomfret using satellite telemetry, revealing how they move through the midwater.

Lead author Martin Arostegui, a research associate at Woods Hole Oceanographic Institution (WHOI), directed the tagging effort alongside collaborators at sea.

“The data shows bigscale pomfret are permanent residents of the ocean’s twilight zone, and follow the pattern of diel migration,” said Arostegui.

The team used pop up satellite archival transmitting tags that log depth and temperature. Those tags later transmit summaries from far offshore, keeping the work going long after a vessel heads home.

Commercial partners made the logistics possible. A longline captain joined the author list and helped put tags on fish that usually slip past scientists in ordinary surveys.

Daily midwater fish movement

The twilight zone is a low light band that sits below the sunlit surface, and many midwater animals move within it every 24 hours in a behavior called diel vertical migration.

This daily movement is a widely observed pattern that helps animals feed while balancing the risk of being seen.

Bigscale pomfret followed that rhythm in the new work. They stayed deep by day, then rose toward shallower waters at night to hunt.

The vertical swings covered hundreds of feet. Those shifts lined up with the idea that animals often track a preferred light level while they commute.

These commuters do not stay put in one spot either. The tags recorded movements across different water masses, which set the stage for a second surprise.

Light changes depth of water

Light penetration can reset where midwater life spends its time, and earlier work showed that clearer water can push the deep scattering layer deeper.

The pomfret data echoed that idea as fish crossed from the Slope Sea into the clearer Sargasso Sea.

Fish choices changed when water conditions shifted. That likely altered which prey fish could reach and which predators could reach them.

Clarity is not a small detail in a fish’s life. When light goes deeper, the daily commute goes deeper with it, and that can rewire who meets whom in the midwater.

A single species cannot explain the entire midwater, but it can anchor a key link. This species linked lower and upper layers on a schedule set by the sun and the water itself.

Impact on sharks and fishing

Sharks hunt where food is available, and they use many of the same depths these fish occupy. When mid-sized predators like pomfret shift up or down, the energy pipeline that feeds bigger animals shifts too.

“These mesopelagic fish are doing the hard work of connecting the deep ocean to the surface food web,” said Camrin Braun, senior author and principal investigator of WHOI’s Marine Predators Group (MPG). Sharks, tunas, and other hunters feel those connections during their foraging hours.

Fisheries also intersect with these movements. When and where lines or nets meet a commuting species can raise bycatch risk, which makes knowledge of timing and depth more than an academic point.

Managers want to reduce waste and protect threatened animals. Understanding how light and water clarity shape vertical habitat could inform smarter rules about timing or gear that lower unintended catch.

Fish in wider waters

The twilight zone stores and moves carbon, feeds predators, and shelters vast numbers of animals. When many species ride the night elevator, they carry energy up and down and pull predators along with them.

Sharks and other large animals can spend hours off the surface, and that reality changes how we think about their time budgets and risks. Oil and gas activity, shipping, and fishing often overlap with these same depths.

Tracking more midwater species will sharpen the map. Tagging technology, acoustics, and optical measurements together can show how different fish slice the water column in different places and seasons.

The new pomfret study nudges the field toward the missing middle. With more tags and careful ocean optics, scientists can test which changes are general rules and which are local quirks.

The study is published in the Marine Ecology Progress Series.

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