Seals Can Serve As ‘Smart Sensors’ to Know Fish Populations in Ocean’s Twilight Zone: Study

UC Santa Cruz marine biologists have made a groundbreaking discovery, revealing that northern elephant seals can serve as “smart sensors” for monitoring fish populations in the ocean’s twilight zone. The study, led by researcher Roxanne Beltran and published in Science, suggests that tracking the foraging success of these marine mammals could revolutionize our understanding of deep-sea ecosystems and sustainable fisheries management.

For the past 60 years, scientists at UC Santa Cruz have closely monitored elephant seals migrating to Año Nuevo Natural Reserve. With an extensive dataset comprising over 350,000 observations on more than 50,000 seals, researchers have gained invaluable insight into the behavior, foraging success, and population dynamics of these marine giants. Now, this long-term research is shedding light on an oceanic region that remains largely unexplored: the twilight zone.

The twilight zone, located between 200 and 1,000 meters below the ocean’s surface, is a critical but poorly understood ecosystem. It harbors the majority of the planet’s fish biomass, yet current ocean monitoring tools—ships, floating buoys, and satellites—struggle to provide comprehensive data from these depths. Beltran’s study demonstrates that elephant seals, which dive into this zone to feed, can offer real-time insights into fish abundance and distribution, presenting a potential game-changer for marine science and conservation.

“Given the importance of the ocean for climate regulation, carbon sequestration, and food security, it is urgent that we develop new ways to measure changes in marine ecosystems,” said Beltran, an assistant professor of ecology and evolutionary biology at UC Santa Cruz. “Our research shows that elephant seals are not only top predators but also exceptional ecosystem sentinels.”

Each elephant seal embarks on a remarkable journey spanning 6,000 miles over seven months, making an average of 75,000 foraging dives. Tracking just 14 seals per year could provide fish population estimates across a staggering 4.4 million cubic kilometers of ocean. By measuring the weight fluctuations of these seals, researchers can also assess long-term changes in prey abundance, offering valuable data to fisheries managers as commercial fishing extends deeper into the ocean.

This research holds profound implications as discussions intensify around harvesting twilight zone fish to meet the growing demand for protein-rich food. With little known about the potential ecological consequences, experts warn that overfishing this hidden realm could disrupt food chains and impact economically significant species.

“The fish in the twilight zone are crucial prey for commercially valuable species, yet our best estimates of their abundance vary by a factor of ten,” Beltran explained. “If their populations decline, the entire marine ecosystem, including species relied upon by humans, could suffer.”

In addition to its scientific significance, this study also highlights the power of education and collaboration. Fourteen undergraduate students co-authored the paper after participating in an immersive field course at UC Santa Cruz, where they analyzed six decades of elephant seal data. Students conducted research, developed hypotheses, and presented findings, making real contributions to marine science.

“We want students to feel like they are part of a scientific community,” said Allison Payne, a graduate student in Beltran’s lab and teaching assistant for the course. “This experience builds confidence and provides invaluable hands-on training.”

The study also builds on decades of research led by distinguished professors Burney LeBoeuf and Dan Costa. Their work previously uncovered elephant seals’ long-distance migrations and the critical role of maternal foraging success in seal pup survival.

Costa emphasized that only a long-term dataset and a multidisciplinary team—including oceanographers, modelers, and marine biologists—could have achieved this breakthrough. “This research connects elephant seal behavior thousands of miles at sea to their breeding success on land,” he said.

Beltran’s study also demonstrated that elephant seal foraging success aligns with broad-scale oceanographic indices detected by satellites, allowing scientists to estimate fish population trends over the past 50 years and even project them into the future.

“This research provides a crucial ecological baseline for sustainable fisheries and helps assess the impact of human-driven environmental changes,” Beltran concluded.

With the potential to revolutionize marine conservation efforts, the findings underscore the value of long-term ecological research and the extraordinary role elephant seals play in unveiling the ocean’s mysteries.