Increased Bering Sea Ice Explains Prehistoric Fur Seal Rookeries

Records show that for more than 100 years, sea ice has retreated from its southern-most position around the Pribilof Islands by May and cleared the Bering Strait by June, allowing northern fur seals (Callorhinus ursinus) to use the Pribilofs as pupping grounds and various species of whales to migrate into Arctic waters to feed over the summer.

In the September 2007 issue of The Holocene, archaeozoologists Drs. Susan Crockford and Gay Frederick (Pacific Identifications Inc., Victoria, B.C., Canada), present evidence from an archaeological site on Unalaska Island (known as “Amaknak Bridge”) that about 3,500 to 2,500 years ago, sea ice not only extended further south than it does today but persisted longer into the summer.

No other type of evidence has documented an expansion of sea ice in the Bering Sea during this time period, which appears to explain the prehistoric distribution of at least one North Pacific marine mammal.

Although about 90% of all fur seals now give birth on the Pribilof Islands, from about 4,000 years ago until just before Europeans arrived, fur seals also maintained rookeries throughout the Aleutian Islands and along the west coast of North America from Alaska to southern California. While it seems likely that a combination of intense harvesting by native and early European hunters wiped out these southern rookeries, why they were established in the first place has been an unsolved ecological mystery.

Careful analysis of prehistoric marine mammal bones from the Amaknak Bridge site now provides a very plausible explanation.

Crockford and Frederick conclude that the expansion of sea ice had a huge impact on Northern fur seals and some whale species. They suggest that the Pribilof Islands were inaccessible from May through July of most years during the Neoglacial because of encroaching sea ice and that these condition prompted fur seals to establish breeding colonies along the Aleutian chain and down the west coast of North America.

They also conclude that the Bering Strait must have been blocked with ice virtually year round between about 4,700 BP and 2,500 yr BP, preventing migration of whales into Arctic waters. Whales that currently use the Beaufort and Chukchi Seas for summer feeding, including bowhead, gray whales and beluga, must either have established resident populations in the Bering Sea — or migrated elsewhere — for at least 2,000 years.

The inevitable repercussions of such a significant expansion of sea ice in the Bering Sea, over such a long period of time, suggests that life history patterns of many migratory marine mammals are much more flexible than they appear. In other words, species may be far more adaptable than we give them credit for.

In the face of a major climate-generated obstacle, whales accustomed to summer grazing in the Arctic simply shifted to alternative feeding grounds; fur seals found alternative birthing locations and changed their migration patterns.

Fur seals not only survived this challenging period, they thrived: evidence from archaeological sites along the west coast of North America indicate that prehistoric fur seal populations associated with southern rookeries must have been substantial.

When conditions improved after about 2,500 year ago, fur seals and whales not only re-established old migratory habits, they evidently continued to flourish because large populations of fur seals, gray whales and bowheads existed by the time European sealers and whalers hit the North Pacific in the late 18th century and began hunting with gleeful abandon.

Reference: Crockford, S. and Frederick, G. 2007. Sea ice expansion in the Bering Sea during the Neoglacial: evidence from archaeozoology. The Holocene 17 (issue 6, September): 699-706.

Pacific Identifications Inc. specializes in the identification of vertebrate animal remains for private and institution-based archaeologists and biologists. The work reported here was funded jointly by the Museum of the Aleutians, AK and the Alaska Ministry of Transport.