March 14, 2025 — Northern elephant seals weigh in at several thousand pounds and quickly put on more weight when catching squid, fish, and other prey. They feed off the California coast in the so-called “twilight zone” of the ocean (200 to 1,000 meters deep) where sunlight disappears. The ocean’s twilight zone holds most of the world’s fish, but is difficult to assess on a large scale.
However, elephant seals may help. Scientists have found that just as elephant seals gain substantial weight in good times, they gain little when prey are scarce.
A new research paper published in Science recognizes northern elephant seals as an “ecosystem sentinel” that can provide fishing fleets, fisheries managers, and others with low-cost but high-value insight into how the ocean is changing and why. The finding builds on two earlier research papers published last year that help scientists identify which species respond to changes quickly enough to make good sentinels. They also looked at how to assemble a series of sentinel species to inform decisions affecting the West Coast economy and the environment.
The research supports NOAA Fisheries’ mission of tracking and forecast ocean changes that affect commercial and recreational fishing. The insight helps fisheries managers make more timely decisions and accurate decisions about fishing seasons and levels. Ocean sentinels may help gather the data more quickly and at lower cost than research ships, for instance.
The scientists, led by Roxanne Beltran at University of California at Santa Cruz, examined four decades of data on California’s burgeoning northern elephant seal population. They compared those numbers with recorded changes in the ocean and found that even small differences in how much prey mother elephant seals consumed made big differences in their body mass and survival of their pups. They found that the connection was so strong that it helped the scientists hindcast the abundance of prey in the twilight zone as far as 5 decades into the past, and predict it 2 years into the future.
“In an ideal world, we would have daily mapping of phytoplankton and zooplankton abundance throughout the entire California Current,” says Elliott Hazen of NOAA Fisheries’ Southwest Fisheries Science Center. In a 2019 paper, Hazen proposed that marine top predators make effective ecosystem sentinels. “That way, we could see how the ecosystem is responding to various changes in real time. But we don’t. So we rely on predators, like the northern elephant seal, to tell us about larger ecosystem trends. Are they fatter or are they skinnier? This tells us whether there is enough prey, which is an indicator of ecosystem health.”