Grizzly bears in the region in and around Yellowstone National Park have suffered unusually high mortality rates so far this year, likely because of a dearth of natural food sources, a researcher said.

Chuck Schwartz, leader of the Interagency Grizzly Bear Study Team, said officials tallied 25 known and probable grizzly mortalities.

Twenty-two of those mortalities were human-caused, two of the deaths resulted from natural causes, and the cause of one death was undetermined.

For every bear that was reported dead, two more deaths likely went unreported, Schwartz said.

"This is not a good year for bears, as far as mortality is concerned," Schwartz told group of wildlife managers and conservationists at the annual meeting of the Yellowstone Grizzly Coordinating Committee on Wednesday.

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The Interagency Grizzly Bear Study Team is a group of researchers that has monitored grizzlies in Yellowstone since the bears were put on the endangered species list. It is funded by the U.S. Geological Survey.

Earlier this year, Yellowstone grizzlies were taken off the endangered species list, a decision that is being challenged by environmental groups.

Nine female grizzlies older than 2 years were among the reported dead: three from management actions, four from hunting incidents, and two from natural causes.

Wildlife managers say these so-called "independent age females" have the biggest impact on the grizzly populations.

In all, Schwartz said a conservative estimate for the overall grizzly population in Yellowstone is 571 bears.

This year, his team documented 50 females with cubs of the year. The average litter size was 2.16 cubs.