Elephant seals provide a window into the ocean’s twilight zone

For six decades, marine biologists at UC Santa Cruz have closely monitored northern elephant seals that make their way to Año Nuevo Natural Reserve. 

With thousands of seals gathering on the beach for breeding and molting, researchers have been able to collect over 350,000 individual observations on more than 50,000 animals. 

This long-term study has provided invaluable insights into the fitness, foraging patterns, and population dynamics of these massive marine mammals.

Seals as smart sensors

Now, a new study suggests that elephant seals can serve as “smart sensors” for monitoring fish populations in the ocean’s twilight zone – the deep, dimly lit layer between 200 and 1,000 meters below the surface. 

The research was led by Roxanne Beltran, assistant professor of ecology and evolutionary biology at UC Santa Cruz.

The study demonstrates how tracking the seals’ foraging success can offer a novel way to estimate fish abundance across vast ocean regions, a task that conventional monitoring tools have struggled to accomplish.

Elephant seals in the twilight zone

The twilight zone is a critical region in the ocean, containing the majority of the planet’s fish biomass. However, studying it presents significant challenges. Ships and buoys can only sample limited areas, while satellites cannot penetrate below the ocean’s surface. 

Because the twilight zone is where elephant seals hunt for food, their movements provide a previously impossible method to assess fish populations on a large scale.

“Given the importance of the ocean for carbon sequestration, climate regulation, oxygen production, and food for billions of people, there is an urgent need to measure changes in marine ecosystems,” Beltran said. 

“Our research shows that the vast foraging extent and millions of feeding attempts by elephant seals make them a fantastic ecosystem sentinel, both for fish populations and top predators in the open ocean.”

Each elephant seal makes approximately 75,000 foraging dives over a seven-month period, covering nearly 6,000 miles across the Pacific Ocean. 

The researchers estimate that tracking just 14 seals per year could provide fish abundance data across an astonishing 4.4 million cubic kilometers of ocean volume.

Seal foraging success and oceanic changes

Beyond simply tracking where seals search for food, weighing them before and after their foraging migrations provides crucial information about prey availability. This offers a long-term record of fluctuations in fish populations, which is particularly important as commercial fisheries push into deeper waters in search of new protein sources. 

Sustainable management of these fisheries requires reliable assessments of fish stocks and their responses to environmental changes.

Beltran and her colleagues integrated data from decades of elephant seal research to analyze how their foraging success correlates with oceanographic conditions. 

The findings suggest that changes in seal pup survival rates over the past 40 years are tightly linked to variations in the ocean environment, reinforcing the seals’ role as natural indicators of ecosystem health.

Decades of data on elephant seals

In addition to its scientific implications, the study highlights the importance of hands-on research training for students. The paper includes 14 undergraduate co-authors, all of whom participated in a field-based course led by Beltran and Año Nuevo Natural Reserve Director Patrick Robinson. 

Through this immersive program, students analyzed six decades of data, developed scientific questions, and contributed directly to the study’s findings.

“We want the students to feel like they are part of a community of scientists,” said Allison Payne, a graduate student in the Beltran Lab who served as the course’s teaching assistant. 

“It’s an incredible opportunity for the students and instructors alike to collaborate on real-world science, and it builds students’ confidence in navigating the scientific process.”

Biological oceanographic research

One of the undergraduate co-authors, Madi Reed, examined how oceanographic conditions influence seal pup survival. She found dramatic fluctuations in elephant seal reproductive success that strongly correlated with changing ocean conditions. 

“It was really exciting to directly experience how the concepts we learned in our classes could be applied to real research,” Reed said. “This discovery has hugely inspired my current drive to pursue a career in biological oceanographic research.”

Beltran’s lab has also published a related perspective in Ecology Letters, discussing the importance of long-term studies in providing inclusive research opportunities for students.

Seals and twilight zone conservation

The twilight zone plays a crucial role in marine food webs, serving as an essential feeding ground for economically valuable species like tuna and squid. 

However, scientific estimates of twilight-zone fish populations remain highly uncertain, with a 10-fold range of possible values. This uncertainty presents a major challenge for fisheries management, especially as discussions about commercially harvesting twilight-zone fish gain momentum.

The new study, which builds on decades of research led by Burney LeBoeuf and Dan Costa, helps bridge this gap by linking elephant seal foraging success to broader oceanographic trends. 

“This effort documented the coupling between the elephant seals’ behavior thousands of miles at sea, to their breeding success on the beach,” said Costa, a distinguished professor of ecology and evolutionary biology. 

“This could only be accomplished with a long-time series coupled with a multidisciplinary team, including oceanographers, demographers, modelers, and seal biologists.”

Measuring the ocean’s pulse

Past studies from this long-term research program revealed several groundbreaking discoveries, including the long-distance foraging migrations of elephant seals, the sheer volume of their feeding attempts, and the connection between maternal foraging success and pup health.

Now, the latest findings demonstrate that elephant seal foraging success aligns closely with broad-scale oceanographic indices that can be monitored by satellites. 

“This linkage allowed us to measure the ocean’s pulse and estimate fluctuations in fish availability five decades into the past and a few years into the future,” Beltran explained.

By providing a critical ecological baseline, this research can help inform sustainable fishing policies and assess the impacts of climate change on deep-sea fish populations. 

With growing concerns about human-driven environmental changes, long-term studies like this one are becoming increasingly essential for understanding and protecting the ocean’s vast, yet largely unexplored, ecosystems.

The study is published in the journal Science.

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