Scientists at the Woods Hole Oceanographic Institution (WHOI) discovered that mid-sized fish, such as the bigscale pomfret, serve as a crucial link between the deep ocean and surface food webs, explaining why large predators like sharks spend significant time in the ocean's twilight zone. Researchers tracked these fish, which inhabit the mesopelagic zone during the day and ascend to feed at night, using satellite tags for the first time, revealing their pivotal role in the marine ecosystem. The study, conducted in December 2025, highlights how these fish transfer energy from the depths to the surface, impacting the entire food chain.

The mesopelagic zone, also known as the twilight zone, spans depths of 200 to 1,000 meters (650 to 3,300 feet) and is characterized by limited sunlight. This research sheds light on the complex interactions within this dimly lit environment. "We've long suspected that mid-sized fish play a vital role in connecting the deep and surface ocean," said Dr. Emily Carter, lead researcher at WHOI. "But tracking their movements has been a significant challenge until now."

The team's innovative use of satellite tags allowed them to monitor the pomfret's movements and feeding patterns. These fish undertake daily vertical migrations, spending daylight hours in the relative safety of the deep and ascending at night to feed on plankton and smaller organisms near the surface. This behavior effectively transports nutrients and energy from the deep ocean to the upper layers, sustaining larger predators.

The study also found that the pomfret's movements are influenced by water clarity. In clearer waters, the fish tend to stay deeper to avoid predators, while in murkier waters, they may venture closer to the surface. This adaptability has significant implications for the ocean's food web, as changes in water clarity due to climate change or pollution could alter the pomfret's behavior and, consequently, the distribution of energy throughout the ecosystem.

"Understanding these intricate connections is crucial for predicting how the ocean will respond to environmental changes," added Dr. Carter. "The mesopelagic zone is a critical component of the global carbon cycle and supports a vast array of marine life. Disruptions to this ecosystem could have far-reaching consequences."

The research team plans to expand their study to include other mid-sized fish species and investigate how their interactions with the environment are changing. They also aim to develop AI-powered models to predict the long-term impacts of climate change on the mesopelagic zone and its inhabitants. These models will incorporate data from satellite tracking, oceanographic surveys, and ecological studies to provide a comprehensive understanding of this vital ecosystem.