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.”
West Coast Indicators of Ocean Health
NOAA Fisheries’ scientists have long turned to different species in California’s rich coastal ecosystem as indicators of ocean conditions and health. For example, the California Current Integrated Ecosystem Assessment reports on the size of krill at different points along the coast. Krill are a barometer of marine productivity because they grow larger when the tiny marine organisms they eat are plentiful.
Ecosystem sentinels take the relationship a step further. They track changes in a species not only as indicators of how the ocean is changing, but also to reconstruct past and future shifts. This can help NOAA Fisheries and state fisheries managers estimate how many fish are available to catch during the fishing season. It could provide insight for fishing fleets on how commercial species are shifting and where they can expect more profitable catches.
A 2024 paper led by Hazen recognized that the best sentinel species respond rapidly to changes in their environment and are generally easy to observe. That means they can be regularly sampled or measured. The use of sentinel species offers a low-cost means of tracking impacts on our coastal economy, which hinges on the high productivity of the California Current.
“Using ecosystem sentinels enables rapid response and adaptation to ecosystem variability and environmental change in part because they may be easier to observe and in part they may serve as leading indicators of system disruption,” Hazen’s team wrote. They described NOAA’s adoption of a similar set of indicator species along the West Coast, including some zooplankton linked to salmon survival and productivity. They have become valuable to both the fishing fleet and state, tribal, and federal fisheries managers as part of NOAA’s Integrated Ecosystem Assessment program.
While northern elephant seals breed along the California coast north of Santa Cruz, they make regular trips into the open ocean’s twilight zone to feed. Their feeding success translates into body growth, effectively gauging how much prey is available. This is similar to how fisheries surveys on NOAA research ships sample the species and biomass of fisheries at different depths.
Predators Make Good Sentinels
Another review highlighted ocean predators as good candidates to be sentinels because their survival depends on the health of the food web that provides them with prey. Generally, healthy predators reflect a healthy ecosystem.
The scientists examined 372 case studies using indicator species to identify qualities that make a good sentinel species, such as responding quickly to changes in their habitat. Researchers can further look for correlations between the species and ocean changes, identifying coming changes that might affect management plans.
“We contend that incorporating sentinel species into these plans will allow for rapid and adaptable
management in response to human change, due to sentinels' sensitivity to environmental and ecosystem change,” the scientists wrote. “For example, an ecosystem-based management plan for a Pacific herring fishery in San Francisco Bay used data from both sentinel predators of herring such as salmon, whales, murres, and sea lions, and alternative sentinel prey of those predators—such as anchovy, squid, and krill—to suggest adjustments to harvest quotas.”
EcoCast Avoids Conflicts
Scientists with NOAA Fisheries’ Southwest Fisheries Science Center also have incorporated indicator species into tools such as EcoCast that provide valuable intelligence for the fishing fleet.
EcoCast is just like a weather forecast,” the scientists offer as an analogy. “But instead of helping people make decisions about when they should wear a raincoat, it helps fishers make decisions about where to fish.”
Each day, EcoCast maps where species like leatherback turtles, California sea lions, and blue sharks are likely to be found. The locations are based on past tracking of individual animals with satellite tags that show how they may gravitate to areas with ocean conditions ideal for feeding. This near-real time information helps commercial fishing crews chart where they can pursue their target catch, such as swordfish, while avoiding areas where they are likely to also catch sea turtles or entangle sea lions.
Ecosystem sentinels could be used as a similar tool. They can tell us where and when we expect to find high concentrations of protected species—and when this might result in human-wildlife conflicts.
“Unexpected shifts in protected species can create new human-wildlife conflict when humans and protected species overlap,” says Hazen. “The more we can get ahead of those changes, the better off our ecosystems and commercial fisheries will be.”
