Pacific walrus populations in Alaska’s Bering Strait and Chukchi Sea area halved between 1981 and 1999. New research indicates that the decline may have slowed down in the years before 2000. According to Rebecca Taylor, a research statistician with the US Geological Survey, researchers relied on data about the demographic composition of the walrus population from 1976 to 2000 to make that determination.
“While our study encompassed data that went from 1975 through 2006, those age composition data went from …. there was a period in the early 80s starting with 1981 and then there was another period in the late 90s, and so it is that ’81 to ’99 time period where we have the most informative data and therefore are most confident about the population trend. “
Taylor says further study is necessary to determine the most recent status of the walrus population. She says researchers used only data for female walrus.
“Because walruses are polygamous, so you don’t need that many males to get the job done. So it’s typical with a polygamous species to track females only, so we did that with this analysis.”
She says the information USGS has indicates that in the 1980s, the female walrus population was aging, making up about 85 percent of the adults, thus producing fewer juveniles.
“So that suggests that reproduction and calf survival were probably quite low at that point, so that is what we believe initiated the population decline. “
A decade later, she says, the walrus population began to get younger, so the decline lessened as the younger walrus population grew. Taylor says her study only looked at demographic and population growth rates to quantify trends, not to find a cause. But, she says, other research indicates hunting is likely to have had a bearing on the decline.
“It was thought that the walrus population probably was depressed by hunting in the 1950s, and that regulations put in place in the 1960s probably allowed that population to increase quite a bit. And so, by sometime between the mid 70s to the early 80s it’s thought that the walrus population was probably quite high compared to what the environment could support, and that probably initiated the decline. “
An increase in hunting during the 1980s served to exacerbate the decline, she says. She says changes in sea ice conditions could also affect Pacific walrus populations. It is difficult to estimate populations of walrus, because the animals spend a good bit of their time foraging underwater.
“There is concern about whether or not harvest levels are sustainable. And there is concern about loss of sea ice, because the walrus like to follow the marginal sea ice, eat clams and other invertebrates off the ocean floor, and then come up and rest on chunks of floating ice. Because they are ice -dependent, there is concern about how changes in sea ice might affect them.”
Taylor’s research is being done to help the US Fish and Wildlife Service make a determination to list the Pacific walrus as threatened by 2017. Andrea Medeiros , a spokesperson for USFWS, says the research was prompted in response to a court order stemming from a lawsuit filed by the Center For Biological Diversity.
Taylor says it is difficult to know what is going on with the walrus population now, if we don’t know what was going on in the past. The research is the first rigourous estimate of Pacific walrus survival rates, she says.
“It’s giving us important demographic information. It is providing some baseline scientific data.”
Taylor’s study is published on-line in the journal Marine Mammal Science.