Pacific walrus numbers dropped by about half from 1981 to 1999, likely due to a combination of hunting and changes to the animals' icy habitat, according to new research from the U.S. Geological Survey, based on the most complete population data yet released on the difficult-to-study walruses.
The study, published online in the journal Marine Mammal Science, is the most up-to-date and comprehensive population information released about the large Arctic marine mammals that depend on dwindling amounts of floating summer sea ice to search for food, rest and raise their young.
The USGS study found that the walrus decline was steepest in the early 1980s, then moderated in the 1990s.
The study evaluated data from 1974 to 2006, including harvest records, age-structure surveys and five population surveys. It tracked calves and their fates, juvenile survival, adult walruses and other population dynamics, and is the first Pacific walrus analysis to do so in such detail, according to the study.
But it does not say how many Pacific walruses were actually around in the study period.
"We did not feel we could do a good job of estimating total population size," said Rebecca Taylor, a USGS research statistician and the lead author. "We were unwilling to make any statements about total population."
The Pacific walrus is being considered for Endangered Species Act protections, with listing considered warranted and a decision due by 2017, according to a U.S. District Court order resulting from a 2011 settlement agreement with environmentalists. Grounds for listing are climate changes in the Arctic, with reductions in summer sea ice forcing large groups of walruses to shorelines, where they endure crowded conditions and are far away from prime underwater food-foraging sites on the continental shelf. Possible oil development in the Chukchi Sea and increased ship traffic through Arctic waters are also considered possible threats to the walruses.
The U.S. Fish and Wildlife Service, the agency responsible for managing Pacific walrus populations, has cited an estimate of 129,000 walruses, based on a 2006 U.S.-Russia population count. That estimate has many caveats, however, including a very wide margin of error and that it did not account for the animals' entire range, the Fish and Wildlife Service notes.
The new USGS study provides the Fish and Wildlife Service with "some good biological baseline information" to use in a listing decision, Taylor said. However, reliable long-term walrus population data -- needed for such a decision -- has been elusive, she said.
Counting walruses is notoriously difficult, Taylor said. The animals spend much of their time foraging for food underwater, where they cannot be seen by scientists on ships or aircraft, she said.
Walruses move over a vast area and, unlike territorial animals, may not return to any particular location by habit, she said.
"They're basically the gypsies of the sea," she said. It is difficult to identify different groups of walruses, she said, and "they go with the ice, so trying to make sure you have found all of the groups is an even bigger challenge."
The dominant reason for the population decline seen in the 1980s and 1990s -- whether hunting, sea-ice loss or some other factor -- remains undetermined, Taylor said.
"That's not known," she said. "What our paper talks about is historic trends."
Also not addressed is what habitat change, including sea-ice loss, or any other factors may be doing to the Pacific walrus population currently, Taylor said.
"Just because we document a decline in 1981 to 1999, that doesn't say anything about what's going on now," she said.
Population studies continue. The USGS, Fish and Wildlife Service and Alaska Department of Fish and Game have been jointly surveying walruses in the Bering Strait, with scientists trying a new genetic-marking method to try to get more accurate counts. A joint survey was conducted last year and this year, and is planned to continue in 2015 as well, officials said.
The USGS conducts an annual walrus-tracking program, in which satellite-radio tags are attached to animals so their movements across the Chukchi Sea can be recorded.
The 2014 tracking project is underway, and radio-tag signals show some walruses hugging the Northwest Alaska coast. That program has been going on since 2008.