Northern Fur Seal Used to Breed on Mainland
By Jennifer McNulty, UCSC
reprinted with permission
For years, anthropologist/archaeologist Diane Gifford-Gonzalez was pestered by nagging questions about some marine mammal bones that were part of UCSC’s extensive archaeological archive. Why, she wondered, did the collection contain the remains of so many northern fur seals, a species that today is found primarily off Siberia and the Aleutian Islands in Alaska?
The northern fur seal, one of the smallest of the eared seals, accounts for fewer than 1 percent of all pinniped strandings along the Central Coast today. But the presence of its bones accounted for about one-third of all sea mammals in local archaeological sites, suggesting to Gifford-Gonzalez that things had been very different some 2,000 years ago.
Adding to the mystery was the prevalence of bones on the mainland. Today, the northern fur seal breeds only on islands. Like the California sea lion that’s so abundant around the Monterey Bay Area today, modern northern fur seals give birth and rear their young on offshore islands where they are safe from predators such as grizzly bears, coyotes, mountain lions and humans.
"The ancient fur seals in Monterey Bay spent the entire year in the waters off California." Gifford-Gonzalez knew from the bones that the distribution of marine mammals around the Monterey Bay Area had undergone dramatic changes in the past 2,000 years. What she didn’t know was why. Her quest for answers took off with the arrival on campus in 1996 of Paul Koch, an associate professor of Earth sciences. Koch is a paleontologist and geochemist who uses bone-chemical analysis to study the diet and environments of different animals. To hear Gifford-Gonzalez tell it, she had been waiting for someone with his expertise for years.
Northern fur seals are perhaps best known for their fur, which was prized by hunters in the 1800s.
Using the isotope chemistry of fragments of bones taken from the UCSC collection, Koch and his graduate student Robert Burton discovered that the northern fur seals found on the mainland in California were feeding far offshore, along the continental shelf. Furthermore, bone chemistry showed no signs that these animals had ever fed in Alaska. Unlike the northern fur seals found off the California coast today, who mostly swim down from Alaska, the ancient fur seals in Monterey Bay spent the entire year in the waters off California.
More significantly, the team ascertained the precise ages of very young animals at death, and Gifford-Gonzalez confirmed that adult females were birthing and rearing young on the mainland.
The prehistoric presence of mainland seal rookeries triggers a host of interdisciplinary questions that have implications for marine biologists studying the life histories of seals, anthropologists and archaeologists interested in the hunting practices of prehistoric Indians, conservation biologists plotting habitat and species restoration plans, and environmental scientists studying climate change.
Northern fur seals are perhaps best known for their fur, which was prized by hunters in the 1800s. At that time, the seals were breeding on San Miguel Island off the coast of Ventura and on the Farallon Islands 30 miles west of the Golden Gate Bridge, as well as on islands far to the north. Over hunting caused the disappearance of the seals from the Central Coast, and the animals only began to re-colonize San Miguel in 1968, said Gifford-Gonzalez. Now, about 5 percent of the population is born on San Miguel.
Long before the coming of the fur trade, however, something made the seals vanish from mainland beaches along the Central Coast. The population appears to have been sufficiently well established that Gifford-Gonzalez and Koch have ruled out the usual suspects: bears, coyotes, and other carnivores. More likely, they believe, would be human over predation or climate change or a combination of the two.
"Natural climate variability could impact the seals directly, or it could make things so terrible for humans on land that they would turn to ocean resources and take more than they had before," said Koch.
With new National Science Foundation funding, Gifford-Gonzalez and Koch plan to pinpoint the date of the seal’s disappearance and fill in details of the environmental record. Ninety percent of what they need is already in the UCSC archaeology archives, where animal bones, shells, and other artifacts gathered during regional construction projects are stored.
Isotope analysis of mussel shells gathered from the same archaeological sites as the bones will allow Koch to chart changes in ocean conditions, including temperature, salinity, and the intensity of upwelling, which affect food supplies. Eventually, they hope to expand their work to investigate whether the displacement of seals began here and continued north along the California coast into Oregon.
"We want to know what happened to these seals long before the historic period fur trade began," said Gifford-Gonzalez. "It looks like the present distribution of elephant seals, Steller sea lions, and California sea lions could be a result of animals moving in to fill a niche that was opened up by displacement of the northern fur seal. The next phase will try to reveal the cause, whether it was climatic change, prehistoric human predation, or perhaps cascading ecosystem effects."
Please report typos, broken links, and the like to our Webweaver
Content © 2004 - 2005. All Rights Reserved.