Today, northern fur seals are primarily native to Alaska and Siberia and are rarely seen on the Central Coast of California. But up until 1,000 years ago, these animals bred abundantly on the beaches of Monterey Bay and elsewhere in California. That discovery initially surprised researchers at the University of California, Santa Cruz, who are now trying to explain the species' abrupt disappearance from the local archeological record.
Evidence gathered by Paul Koch, associate professor of Earth sciences, Diane Gifford-Gonzalez, professor of anthropology, and former students Robert Burton and Josh Snodgrass suggests that hunting by early human populations may have been responsible for the fur seals' decline. Their latest results indicate, preliminarily, that marine climate change was not a factor. They reported their findings at the December meeting of the American Geophysical Union in San Francisco.
"The northern fur seal was more flexible and widespread in the past than current conservation literature would lead us to believe," Gifford-Gonzalez said.
If human hunting drove the fur seal out of California, this has important implications for local conservation strategies, added Koch. "The northern fur seal was a dominant animal and a very important component of the local ecosystem until 1,000 years ago, and now it's missing," he said.
For years, Gifford-Gonzalez and other archaeologists had known that fur seal bones are abundant in the archaeological record along the Central Coast, accounting for about one third of all marine mammal bones. Today, fur seals breed primarily at high latitudes, except for a small rookery on San Miguel Island in southern California, and they rarely visit the California mainland. Were ancient fur seals just Alaskan visitors that came to the California mainland in droves to feed? Or were they native to the California Coast?
To answer these questions, Gifford-Gonzalez and Snodgrass teamed up with Koch and Burton, who use the bone chemistry of both ancient and modern animals to determine their diets and ecology. Chemicals locked in ancient seal bones can pinpoint where a seal fed during its lifetime. Unique forms of carbon and nitrogen, called isotopes, are found in specific proportions at different points along the ocean and coast and serve to label local regions. The animals absorb these chemicals from their food and store the labels in their bones.
The analysis showed that fur seals in the fossil record lived in California and fed in offshore waters. They were not immigrants from Alaska. The next question was whether the fur seals were breeding on the California mainland or on islands. Today, fur seals breed exclusively on islands.
The researchers examined the distribution of the bones of baby fur seals in the archaeological record. From the abundance of pup bones at mainland excavation sites, they concluded that the seals were birthing on the mainland. There were too many bones to be explained by humans scavenging dead pups that might have floated in from an island, and the pups were too young to have reached the site by swimming.
"Animals aren't going to blow off San Miguel Island and float all the way up to Moss Landing and end up in someone's soup pot," Koch said.
Koch and Gifford-Gonzalez haven't been able to find seal bones in sites less than 1,000 years old. They reasoned that the seals were driven out either due to a natural climate change or to human hunting. To evaluate the possible role of climate change, Seth Newsome, a graduate student in Earth sciences, conducted a preliminary analysis of sea temperatures before and after 1,000 years ago.
Mussel shells in the archaeological record serve as a thermometer of past sea temperatures. Mussel shells are formed by the crystallization of calcium carbonate from water. Oxygen isotopes in the water get sorted during this crystallization process in a way that depends on the sea temperature.
So far, there's no evidence for a change in ocean temperatures at the time the seals disappeared. Almost every other model that could explain the seals' disappearance involves human predation of one sort or another, Koch said.
According to Koch, restoring fur seals to California's coast should be considered as part of conservation efforts to maintain a healthy coastal ecosystem. Conservation biologists who aim to restore local marine ecosystems are faced with the difficult challenge of choosing a baseline for their restoration efforts, he said. Should the environment be restored to its condition prior to all human interference, prior to European contact, or prior to recent destructive human activities?
Koch believes that if, in fact, the fur seals were wiped out by early humans, then the "natural" state of the local marine ecosystem is arguably one that includes abundant fur seals with rookeries on the mainland. The system involving seals had been around prior to disturbance for at least 9,000 years, perhaps for a million years, Koch said.
"It would be sort of silly to ignore what history says is stable just to create something that looks like what the European fur traders first saw," he said.
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Editor's note: Reporters may contact Paul Koch at (831) 459-5861 or pkoch@es.ucsc.edu and Diane Gifford-Gonzalez at (831) 459-2633 or dianegg@cats.ucsc.edu.
