Elephant seals as helpers in researching fish populations
Studying the ecologically and economically important fish populations in the ocean’s twilight zone presents enormous challenges. However, in the North Pacific, this task now appears to be much easier, thanks to northern elephant seals, which are very familiar with this environment.
A research team from the University of California Santa Cruz (UCSC) has developed an innovative method to assess fish populations in the twilight zone—with the help of northern elephant seals. For 60 years, researchers have been studying these seals, which come ashore by the thousands each year at Año Nuevo State Park off the California coast to breed, molt, and give birth.
Using modern sensors attached to the elephant seals, the research team collected detailed data on their dives and hunting success, providing insights into the availability of prey fish across a vast area of the North Pacific, extending as far north as the Aleutian Islands.
“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,” Roxanne Beltran, assistant professor of ecology and evolutionary biology at UCSC and lead author of the study, said in a university news release. “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.”
During their seven-month migrations across the Pacific, each elephant seal travels around 10,000 kilometers and makes an average of 75,000 hunting attempts at depths between 200 and 1,000 meters—in the twilight zone, where fish populations are difficult for researchers to access.
In their new study, published on February 14 in the journal Science and featured on the cover, Roxanne Beltran and her research team demonstrate that tagging just 14 elephant seals per year would be sufficient to monitor fish availability across an ocean volume of 4.4 million cubic kilometers—a data density that would be nearly impossible to achieve using conventional methods such as research vessels or buoys.
Elephant seals are proving to be ideal sentinels of marine ecosystems, offering scientists invaluable data. While satellites provide crucial insights into temperature changes and ocean currents, they cannot directly measure fish stocks.
This is where the elephant seals come into play: with each of their numerous dives, they collect data on their hunting success—an important indicator of biological productivity in the depths for researchers.
This long-term data collection provides a unique foundation for understanding marine ecosystems and their changes. As the demand for protein-rich food continues to grow, fish stocks in the twilight zone could become a target for commercial fishing in the future—despite significant uncertainty about their actual population sizes.
The researchers emphasize that their findings are urgently needed to develop sustainable management strategies and prevent negative impacts on the ecosystem. As these fish species play a crucial role in the marine food web, a decline in their populations could have far-reaching consequences for the entire ecosystem—including species that humans directly rely on.
The researchers also discovered a strong correlation between the hunting success of elephant seals and oceanographic conditions, which can be measured via satellite. This finding enables scientists to infer fish availability not only in the present but also up to five decades into the past—and even to make predictions for the future. The study thus provides a crucial foundation for sustainable ocean management and for assessing the impacts of climate change on fish populations in the twilight zone.
Julia Hager, Polar Journal AG
Link to the study: Roxanne S. Beltran et al. ,Elephant seals as ecosystem sentinels for the northeast Pacific Ocean twilight zone.Science387,764-769(2025).DOI:10.1126/science.adp2244
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