JUNEAU, Alaska — Researchers are trying to get a better handle on the size of the Pacific walrus population ahead of an expected decision by the U.S. Fish and Wildlife Service on whether the animals need special protections.

A new report from the U.S. Geological Survey estimates the population was cut roughly in half between 1981 and 1999, possibly due to overabundance of walrus and relatively high harvests in the 1980s. However, the study’s lead author, Rebecca Taylor, said other factors — such as lower availability of food, potentially because of sea ice loss — couldn’t be ruled out.

The decline was not steady and was most severe in the mid-’80s before moderating, the USGS said.

Taylor said the goal of the study, published last week in the online journal Marine Mammal Science, was to provide baseline information, including information on historic population dynamics, reproductive rates and survival rates shown by the population.

She said no one before had been able to provide rigorous survival estimates.

More recent data — from 2013 and 2014 — have been collected as part of an ongoing effort to help get a better feel for population dynamics. Taylor said the analysis will be brought more up to date.

Pacific walrus are generally found in the Bering or Chukchi seas, depending on the time of year, and use coastal haul-outs. They are notoriously difficult to count because they move around a lot, often using floating ice, and spend much of their time underwater, Taylor said.

There is one population that includes U.S. and Russia waters, she said. A 2006 survey estimated at least 129,000 animals. In 2011, the U.S. Fish and Wildlife Service found protecting the Pacific walrus as threatened or endangered was warranted, due to loss of sea ice from global warming and concerns with subsistence harvest levels. However, the agency said listing was precluded by the need to address higher-priority species.

That prompted a lawsuit, the settlement for which called for a plan to address the backlog of species. A decision on the walruses is due in 2017.

Jim MacCracken, the lead for Fish and Wildlife’s walrus program in Alaska, said the focus for determining whether listing is warranted has started to shift, with the agency looking more at population viability, such as whether the animals can adapt to changes. In the past, there was greater emphasis on threats, he said.