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Tracking an Elusive, Imperiled Seal Could Reveal Threats to the Ocean

Guadalupe fur seals began washing up on California beaches dead or emaciated in 2015, but scientists knew almost nothing about them. Now they’re using satellite tags to monitor the mammals’ movements – and find what’s killing them.

Written by Gloria Dickie Published on Read time Approx. 5 minutes
A rehabilitated Guadalupe fur seal named Silkster is released after being fitted with a satellite tag.Dana Angus

SAUSALITO, California – Wild Tangerine peers through the gap in the mesh lining of her cage, making eye contact for barely a second, then she’s gone. Her small, sleek, almost alien form – with comical ears that stick straight out of her head – cuts through the shallow pool of water that has been her home for the past four months, then she bobs back up.

Wild Tangerine is one of 76 Guadalupe fur seals that have passed through the Marine Mammal Center’s doors in Sausalito since January 2015, when the starving and stranded animals began washing up on the Pacific Coast in droves – for reasons unknown. In fact, almost nothing is known about the mammals, which are classified as threatened under the United States Endangered Species Act. Their predicament, though, has created a unique opportunity for researchers.

Three years into what the National Oceanic and Atmospheric Administration (NOAA) calls an Unusual Mortality Event, or UME, scientists are still struggling to determine what has caused more than 225 documented strandings of the animals; they have no baseline information or data on what healthy Guadalupe fur seals should be doing in the wild. The seals, like other pinnipeds, are an indicator species of ocean health. So when dozens of emaciated juveniles began washing ashore, along with hundreds of sick California sea lions, it tipped scientists off to changes in the marine environment – warming, acidification, disease – that might otherwise go unseen as climate change accelerates.

Satellite transmitter tags are being used to track rehabilitated and free-ranging Guadalupe fur seal pups and yearlings. (Jeff Harris, NOAA Fisheries, Seattle, Washington)

Tenaya Norris, a scientist at the Marine Mammal Center, decided to make the best of a bad situation. Two years ago, she began sticking satellite tags onto the backs of nearly every fur seal that was rehabilitated and released; then she and her colleagues traveled to Guadalupe Island, a protected rocky island 150 miles (250 km) off the west coast of Mexico, to begin tracking and researching wild individuals. Now she is starting to put the puzzle pieces together.

In the 1800s, Guadalupe fur seals were hunted almost to extinction for their soft pelts. They are now protected under U.S. law and their population numbers between 12,000 and 20,000. The majority are born in the breeding grounds of Guadalupe Island, though the fur seals have been spotted foraging as far north as British Columbia. Most, however, are thought to stick around the food-rich waters off the California coast.

Because the Guadalupe fur seal UME coincided with unprecedented strandings of California sea lions, the theory was advanced that the same thing – a change in the availability of key prey – was likely affecting both species. But California sea lions and the seals have different diets, and they dive in different ranges, notes Justin Viezbicke, the NOAA’s California stranding coordinator.

This map shows the post-release movements of 29 rehabilitated Guadalupe fur seals released from the Marine Mammal Center in Sausalito, California, in 2015–17. (The Marine Mammal Center)

The California sea lions’ deaths are believed to have been largely driven by a move further offshore of sardine spawning grounds, which would not affect Guadalupe fur seals much, he said. From the autopsies performed on the stranded seals, researchers found most were emaciated with parasitic or bacterial infections. “But nothing is really sticking out at this point,” said Viezbicke. “We don’t know for sure what’s causing it.”

Prior to Norris’ work, only four satellite tags had ever been deployed on Guadalupe fur seals. “That means we don’t know anything about where the species is when it’s at sea and not on land,” she said.

Scientists observed the rehabilitated fur seals making a beeline north during the El Niño event in 2015 that brought a mass of warm water – dubbed “the Blob” – to the Pacific northwest. No one knew, however, whether that was normal behavior for the species. That is why it is critical to tag non-stranded seals, too. “Why would we put out tags on stranded animals if we don’t do wild ones, too?” asked Norris, adding that such work is typically outside the scope for a UME as the baseline data already exists for other species. “But we don’t know if what they’re doing post-release is normal.”

The map shows the movements of 18 free-ranging Guadalupe fur seal pups tagged on Guadalupe Island in 2016–17. (The Marine Mammal Center)

Last year, researchers satellite-tagged five wild animals; this year, they tagged another 38. Norris has now begun organizing the transmitted locations into a map. Although some tags stopped working sooner than expected, preliminary maps show wild fur seals mostly homed-in on the productive upwelling waters further offshore of northern and southern California in 2016 and 2017, choosing not to swim further north like their rehabilitated counterparts in 2015. Next summer, Norris hopes to return to Guadalupe to deploy more tags.

For now, tracking data will be overlaid with prey and stranding data during warm-water anomalies. “The tracking can add to that story,” said Norris. “It can highlight differences between years when you see how close to shore they were and how far north they went in 2015. Usually this species is pretty far from shore, which we can tell by looking at the combined tracks since then. You start to get an idea of the impacts these warm-water events can have on a species.”

The map shows the movements of 25 juvenile and adult free-ranging Guadalupe fur seals tagged on Guadalupe Island in 2017. (The Marine Mammal Center)

And while the population of Guadalupe fur seals is still holding strong, it would not take much to have a pronounced impact on their numbers.

“Any time you have a population with a really restricted breeding range, it really only takes one catastrophic event, like a really bad El Niño, to have a really drastic effect on the population,” said Norris. The Northern fur seal, for example, was once hard hit by a previous El Niño and the population took 10 years to recover. “It’s not unusual to think that same thing would happen with this fur seal population.”

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