Researchers say human-caused grizzly-bear deaths have taken a toll in the Greater Yellowstone Ecosystem this year, despite estimates that the overall population continues to grow.

Interagency Grizzly Bear Study Team officials estimate that 39 grizzly bears have died in 2008. Seventeen of those deaths involved hunting incidents, six where the result of natural causes and four had unknown causes. The remainder of the deaths were some form of human-caused death, including management removals of problem bears.

Male grizzly bears have already surpassed a 15 percent mortality threshold that, if observed for three consecutive years, would prompt a management review by state agencies. That review could result in grizzlies being placed back under Endangered Species Act protection.

If another female grizzly is shot by a hunter, female mortalities would pass a 9 percent threshold that would trigger a similar review after two consecutive years.

Those thresholds were last surpassed in 2000.

When a person other than a wildlife manager reports a grizzly bear death, researchers count it as three toward the thresholds because roughly two-thirds of citizen-caused grizzly deaths go unreported, said study team leader and U.S. Geological Survey researcher Chuck Schwartz.

“We know that there are bears that die in the ecosystem that we don’t hear about,“ Schwartz said.

“A lot of bears have been shot in defense of life,” Schwartz said from his office in Montana. “We wanted to get the word out before a large number of hunters hit the field so we don’t end up with a bunch more dead bears on the ground. We want to emphasize to the public that you have to be careful out there. We don’t want to reconsider delisting the bear.”

Hunters are taking to the woods in large numbers for the general hunting season, and bears won’t begin hibernating for the winter for another two to four weeks.

The good news is that researchers estimate Greater Yellowstone’s grizzly population is higher than last year — 596 bears, up from 571. Schwartz and his team made the estimate based on 84 new cubs observed with 44 females. Researchers estimate that the population continues to grow about 4 percent annually.

Louisa Willcox, a wildlife advocate for the Natural Resources Defense Council, said she was shocked about this year’s bear deaths so far.

 “It’s very, very disturbing and should give us all pause,” she said.

Willcox said much of the blame belongs with the Montana, Idaho and Wyoming state wildlife agencies for not reporting the trend earlier.

“This pileup of dead bears didn’t happen overnight,” she said. “The agencies could have developed early warning systems to say this is a bad year for bears. There was none of that. Clearly, the system is broken.”

Willcox said the current system needs more transparency and wildlife managers need to respond faster to large numbers of bear deaths.

“A lot of these problems can be avoided, and the agencies haven’t responded,” she said.

Also, Willcox said this year’s deaths further validate a lawsuit that would put bears back on the endangered species list. The lawsuit was filed by the Natural Resources Defence Council, the Jackson Hole Conservation Alliance, the Sierra Club and others.

The groups argue that grizzly bears remain imperiled because of inadequate habitat for population growth and the effects of climate change on food sources such as whitebark pine seeds. The case is now before District Judge Edward Lodge in Boise, Idaho.

Schwartz urged hunters to use extra caution when hunting in grizzly country this year. He also said recent research shows that bear spray is more effective than bullets when fending off an angry grizzly.

“[Hunters] need to be smart about what they’re doing out in the woods, particularly if they kill something,” he said. “We can maintain healthy bear populations, but everyone has to participate.”