February 14, 2025 — Over the past 60 years, marine biologists at UC Santa Cruz have monitored the behavior of northern elephant seals that journey to nearby Año Nuevo Natural Reserve. With the seals gathering on the beach by the thousands to breed and molt, generations of researchers have been able to amass more than 350,000 observations on over 50,000 seals.
With the help of powerful technologies and the intrepidness to get close enough to carefully tag, weigh, and observe these loud and lumbering marine mammals, the long-term research project has extensive historical and real-time data on their fitness, foraging success, at-sea behavior, and population dynamics.
Roxanne Beltran is next in line to lead the project, and her new study being published as the February 14 cover story for Science reports that seals can essentially act as “smart sensors” for monitoring fish populations in the ocean’s eerily dim “twilight zone.” This is the layer of water between 200 and 1,000 meters below sea level, where sunlight penetration all but stops, and today’s ocean monitoring tools cannot reach with ease. Ships and floating buoys only allow measurements of a tiny fraction of the ocean, while satellites can’t measure below the surface where fish occur.
Importantly, this zone holds the majority of the planet’s fish biomass. Because this is also where the seals feed, seals whose foraging success is tracked can provide a previously impossible way to measure the availability of fish populations across a vast ocean. This, Beltran said, represents a significant discovery because humans are considering harvesting these fish populations to satisfy humanity’s ever-increasing need for protein-rich foods.