Rhythmic clicks, grunts and roars fill the Año Nuevo Island Reserve in California, home to a large breeding colony of northern elephant seals (Mirounga angustirostris). For nearly 60 years, scientists at the University of California Santa Cruz (UCSC) have studied the seals here. Lately, they have used Fitbit-like biomonitors on the seals to understand where they migrate to each year and what they eat. In 2022, one of the students attending a 10-week-long undergraduate field course at the reserve wondered how oceanographic conditions, such as surface temperature and pressure, affect the elephant seals.

That curious question led scientists to connect oceanographic conditions in the northern Pacific Ocean to fish abundance in its deep twilight zone by measuring how successfully elephant seals foraged and reproduced. Their study, published in February in Science, analyzes this connection to estimate fish abundance in the twilight zone five decades in the past and predict it two years into the future. The study also shows how marine predators like elephant seals act as sentinels that can tell us about changes in the ocean’s inaccessible depths — if only we can learn how to decipher their reports.

The twilight, or mesopelagic, zone — the layer of ocean water that extends 200–1000 meters (656–3280 feet) deep — is an inhospitable place. “It’s completely dark, it’s extremely cold, there’s this crushing pressure,” says study co-author Allison Payne, a graduate student in the UCSC lab of lead author Roxanne Beltran. “It’s kind of like another planet.”

“We don’t know exactly how much productivity is there, and we also don’t know how things that are happening on the surface or in the upper parts of the ocean are affecting the ocean at those depths,” says co-author Conner Hale, a research technician in Beltran’s lab who participated as an undergraduate in the 2022 field course. “So that’s something that our study is able to shed more light on.”

Exploring the twilight zone is challenging: human divers can’t reach those depths, and sending subs or remotely operated vehicles (ROVs) is expensive. But as marine predators who spend about seven months every year far out at sea catching fish in the twilight zone, northern elephant seals are built for such depths. The seals from the Año Nuevo colony always return to the island reserve after their migratory trip, bringing information about what’s happening in the deep open ocean.

“The elephant seals are doing amazingly as evolutionarily informed oceanographers going out and sampling the ocean for us,” says ocean ecologist Simon Thorrold from Woods Hole Oceanographic Institute, U.S., who studies twilight-zone fish but was not involved in the new study.

During the many years studying these seals, UCSC researchers measured some seals’ weights every year before and after their foraging trips. The team behind the current study used this data as a proxy for the availability of fish in the twilight zone. They were able to correlate this to oceanographic conditions in the Pacific Ocean, such as sea surface temperature and air pressure changes brought about by climate patterns like the El Niño-Southern Oscillation. Because marine fish from the twilight zone make a daily migration to the ocean surface at night and then back into the ocean’s depths at daybreak, scientists believe ocean conditions at the surface affect them.

The study suggests changes in oceanographic conditions cascade through the food chain all the way up to apex predators such as seals. “In years with good conditions, you have lots of fish biomass, and the seals will eat about a kilogram [2.2 pounds] of fish more per day, and that leads to them having about 20% more mass in a year compared to a year with bad conditions,” says Payne.

The researchers found a very strong correlation between mass gain in seals and the Southern Oscillation Index, a measure of the air pressure gradient in the southern Pacific Ocean that indicates the intensity of the El Niño/La Niña. The correlation enabled them to estimate that there was low twilight zone fish abundance in 2021, but higher fish abundance during 2019–2020 and 2022–2024. They also looked back five decades in time and identified periods of very low fish abundance in 1984–1985 and 1994–1996, and very high abundance in 1975–1978.

The researchers then studied how oceanographic conditions affect seal reproduction in the colony using data collected over three decades about seal pups born on the island. Like most animals, well-fed seal mothers give birth to healthy pups. The study found a two-year lag between oceanographic conditions and their impact on seal reproduction, as it takes time for these effects to travel up the food chain. “In the two years before an elephant seal is born, if the conditions are good, then two years later, the elephant seal moms are four times more likely to give birth,” Payne says.

She says that pups born to well-fed mothers weighed more after they were weaned and were six times more likely to survive to one year of age. And their good fortune carried into the next generation. “The seals that are born in years with good conditions ultimately produce more than twice as many pups as the seals born in conditions that are bad.”

“The idea that these large-scale oceanographic systems … [are] also propagating deeper into those mesopelagic communities, I think that really is an important finding,” says Thorrold. “What I think was interesting was that they figured out a way of looking at the success rates of the seals foraging, and correlated [that] with some important demographic rates that they were measuring in the populations as well.”

Although the study’s findings don’t tell us about the composition of fish species in the twilight zone, how they interact with each other, or how that may change with oceanographic conditions, Thorrold says the findings definitely suggest “something interesting is happening there and we ought to be taking [it] into account.”

With increasing interest in industrial fishing in the ocean’s twilight zone and changing oceanographic conditions due to human-induced climate change, researchers say other deep-diving marine predators, such as the southern elephant seal (Mirounga leonina) and the northern fur seal (Callorhinus ursinus), could also be enlisted to keep an eye on changes in the Pacific Ocean’s depths.

“We need to use every tool that we have to figure out what’s going on in [the twilight zone] because it’s so difficult,” Payne says.

Banner image: A northern elephant seal in Año Nuevo Island Reserve. Image by Daniel Costa, permit number NMFS 23188.

Twilight zone fishing: Can we fish the ocean’s mesopelagic layer?

Citations:

Beltran, R. S., Payne, A. R., Kilpatrick, A. M., Hale, C. M., Reed, M., Hazen, E. L., … Costa, D. P. (2025). Elephant seals as ecosystem sentinels for the Northeast Pacific Ocean twilight zone. Science, 387(6735), 764-769. doi:10.1126/science.adp2244

Cai, W., Santoso, A., Collins, M., Dewitte, B., Karamperidou, C., Kug, J., … Zhong, W. (2021). Changing El Niño–Southern Oscillation in a warming climate. Nature Reviews Earth & Environment, 2(9), 628-644. doi:10.1038/s43017-021-00199-z

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