Are There Plenty of Fish in the Sea? Ask an Elephant Seal.

The deep sea contains many things: shipwrecks laden with gold, submersibles stuffed with sightseeing billionaires, and an absolutely unfathomable number of fish. If you could somehow put all the world’s fish in a giant pile—all the tuna; all the herring; all the mackerel, sardines, and anglerfish—around 90 percent, by weight, would have come from the ocean’s twilight zone.

Faced with crushing water pressure, a near-total lack of light, and vast areas to search, scientists have long struggled to keep tabs on the trove of fish hiding out in the deep. But for a recent study, scientists led by Roxanne Beltran, an expert on marine mammals at the University of California, Santa Cruz (UC Santa Cruz), strapped sensors to one of the deep sea’s most formidable predators: the northern elephant seal (Mirounga angustirostris). Tracking these hunters, the scientists show, offers a new way of monitoring the fluctuations of fish populations across swaths of otherwise obscured ocean.

Northern elephant seals have big blubbery bodies that can weigh as much as a car and reach nearly four meters (13 feet) long. But don’t let their physiques fool you, says Beltran. These animals annually migrate 10,000 kilometers (6,000 miles) round trip between North America’s west coast and open-ocean feeding grounds in the heart of the North Pacific. "They’re routinely diving down 1,000 meters (3,280 feet) below the ocean’s surface while holding their breath," says Beltran. "And the females do all of that while pregnant with next year’s pups."

Northern elephant seals spend around seven months of the year hunting nearly nonstop. During this time, each seal makes tens of thousands of dives in search of fish and squid.

For decades, scientists from UC Santa Cruz have been monitoring the weights, survival rates, and reproductive success of northern elephant seals living near California’s Año Nuevo State Park. They’ve also been attaching satellite tags to the seals to track their foraging trips. Collecting this data took three generations of scientists and hordes of undergraduate researchers, all willing to endure long days of hoisting elephant seals onto tarps so they could weigh the animals using a gigantic tripod, and otherwise struggling to keep their equipment free of seal poop.

But the real magic came when Beltran and her colleagues cross-referenced this hard-won data against records of historical ocean conditions.

"Elephant seal foraging success, survival, and reproduction all fluctuate substantially with ocean conditions," says Beltran. Looking at how elephant seals fare in different conditions, Beltran and her colleagues realized they could use the measurements of foraging success and pup growth to gauge how much these seals must be eating and, from there, estimate the otherwise invisible swings in fish populations.

Their findings suggest that populations of fish and squid in the deep sea within the northern elephant seals’ foraging range go through cycles of abundance and scarcity every three to five years. And they found that seals thrive about two years after a bout of particularly favorable oceanographic conditions. That’s "when the seals have a ton of food to eat," Beltran says.

By identifying the relationship between seals, deep-sea fish, and ocean conditions, Beltran and her team now say they can predict the size of the deep-sea fish stock up to two years out—a potential boon for fisheries scientists and conservationists.

If all of this sounds like a convoluted and complex way to count fish, it is. But no more so than how scientists already do it.

"Counting fish," John Shepherd, a fisheries management specialist at England’s University of Southampton, once said, "is like counting trees, but the trees are invisible and constantly on the move."

Current techniques to estimate fish populations include scanning the sea with sonar, conducting sample trawls to see what can be found, and auditing the catches of commercial fishers. Unfortunately, none of these methods—including Beltran’s new one—can paint a complete picture of how many fish there are, says Ray Hilborn, a fisheries scientist at the University of Washington who was not involved in the study.

"To understand the total abundance of any species we need data across their entire distribution," Hilborn says. Elephant seals, like fishing boats, are not sampling the ocean randomly to see how many fish are in each place. Instead, they seek out high concentrations of prey, and when they find one they tend to hang around the area. "So one would not expect the average fish density they encounter to be representative of the total abundance of their prey," Hilborn says.

Imperfect as it is, says Mark Morales, a postdoctoral researcher at UC Santa Cruz who was not involved with the study, the new seal-based proxy measurement is valuable.

"Fishery scientists like myself have to act a lot like detectives, relying on little bits and pieces of information here and there," Morales says. "This study offers an intriguing piece of additional information that can be used to try to estimate how many fish are in the ocean."

With climate change shifting ocean conditions and fishing fleets setting their eyes on previously unfished species, including those that live in the deep sea, Beltran and her colleagues plan to continue monitoring the health of the seals at Año Nuevo. With luck, and lots of careful observation, these animals might have even more to tell us about our changing planet.