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The ongoing recovery of the endangered Guadalupe fur seal is one of the feel-good nature stories of North America. Thought to have been driven extinct in the 1800s because of hunting for its fur, a remnant population survived. The species now numbers as many as 40,000—mainly off Baja Mexico and California. But jubilation over the recovery is tempered by the fact that fur seals are washing ashore dead or dying.
For the first time, researchers have systematically analyzed the necropsy results of Guadalupe fur seals found on the beaches of Oregon and Washington State from 2005 to 2016. In total, 169 fur seals stranded during the 12-year period, including 139 yearlings and 20 weaned pups—evidence that younger animals foraging on their own in a big ocean are at the greatest risk.
Of the seals that stranded, 155 died. Necropsies on 93 found that the major cause of death, responsible for 44 percent of cases, was emaciation. For the others, 29 percent died of trauma, and 19 percent of infectious disease. Close to half of the necropsied seals suffered from both emaciation and infectious disease. “You don’t know which came first,” says Erin D’Agnese, who led the study while a doctoral candidate at the University of California, Davis. “Were they fighting an infection so they couldn’t forage, or was foraging not going well and they became infected?”
Infections were mostly associated with two parasites: Toxoplasma gondii, which is shed in cat feces and can be transmitted from cat litter; and Sarcocystis neurona, which is typically found in horses, but is shed by opossums. Both parasites are now common in marine mammals.
Of the deaths caused by trauma, almost half were linked to fisheries, such as entanglement with nets. About 40 percent of trauma deaths involved unspecified blunt force trauma, while bullet wounds claimed two seals, and a shark attack one.
Research focusing on Guadalupe fur seals in California previously found that strandings are more likely to occur during warm El Niño conditions, but that finding was not exactly replicated in the Washington and Oregon study. The two years with the highest number of strandings were 2012, a cooler La Niña year, and 2016, an El Niño year.
The researchers found that 90 percent of the strandings occurred from May to July, a time when ocean currents bring nutrient-rich, cold waters from the north into the northeast Pacific. The seals may have been following northward migrating prey such as squid, sardine, and anchovy when they became stranded.
Guadalupe fur seals have been documented making foraging trips well beyond the northeast Pacific, including into the waters north of Hawai‘i, Haida Gwaii off northwestern British Columbia, and even into a northern fur seal rookery in Alaska’s Pribilof Islands in the Bering Sea.
“They’re amazing—a cool species to work with,” says Seattle-based research biologist Jeff Harris, who is part of a US-Mexico team putting satellite tags on fur seals at remote Guadalupe Island, the main breeding site of this species. “They can stay out at sea for a couple of years.”
The study shows that as the seal’s population grows and it reclaims its historical range, more strandings can be expected. “They’re not up here dying from any one thing that suggests a problem,” D’Agnese says. “They’re dying from the usual causes of mortality—some natural and some human-related.”
Recent modeling suggests the seal’s population has doubled since 2010, but even that doesn’t guarantee continued success. In the past, hunting forced the species through a genetic bottleneck, meaning the seals may be less resilient in the face of unknown forces such as global warming and its effect on prey availability.
“Oceanographic change can impact quickly,” Harris confirms. “You never know what they’re going to experience out there.”