Watch out for the park bears.

Though no one has been hurt, Shenandoah National Park officials are warning visitors about aggressive bears seeking easy food in backpacks, tents and, of course, picnic baskets.

Problem bears have been a perennial problem, but park officials this fall are intensifying efforts to reduce bear and human interactions for safety reasons.

Gordon Olson, chief of resource management for the park, said, "We want to make sure we're ahead of the ball here. We've got a lot more staff involved and it's definitely helping.''

From 300 to 500 bears roam the 105-mile-long park, which is a sanctuary for wildlife because hunting is not permitted. The park has one of the densest populations of black bears documented within the U.S.

For the first time in recent memory, park officials have posted signs in problem areas such as trailheads to warn about "aggressive'' bears. Black bears are generally timid and run at the sight of humans.

One sign, posted at the Hawksbill trail just north of Big Meadows campground, warns against even camping in the area. It says "bears along this trail have harassed visitors, taken food, and damaged equipment including backpacks.''

About 30 park staff personnel and volunteers, including hosts at the park's four campgrounds, were trained this year in "aversive'' techniques with which to scare bears away. These range from yelling and using slingshots to shooting bears with bean bags or rubber bullets.

"We don't want people to think this is a dangerous place to come,'' said Karen Beck-Herzog, a spokeswoman for the park. "But a bear is a wild animal and deserves our respect. After all, we're visiting their home.''

Park officials said no one has been injured by a bear in recent memory, and the temporary bear-warning signs, tied to trees by bungee cords, are meant only to prevent problems.

So far this year, park officials have recorded 130 to 150 unfriendly or uneasy encounters with bears. Those might include a bear following a hiker or a getting spooked on an encounter with a human but then stopping and hanging around, said Olson.

The park gets about 1.1 million visitors a year.

Some problem bears are well known. One, known as the "skinny'' bear, hangs around Old Rag Mountain, one of the park's most popular hiking destinations. The creature is seeking food.

In another case this summer, a mother bear and her two cubs regularly appeared on the Skyline Drive near Big Meadows, attracting tourists with cameras and creating traffic jams.

"Problems develop through the summer,'' said Olson. "Animals become more food-conditioned around people.''

Some visitors feed bears and deer despite permanent signs at campgrounds and elsewhere that warn against giving food to wildlife.

"Sometimes we run into visitors who don't have a clue,'' said Olson. "But it's usually more productive to educate them than to cite them.''

Once bears become accustomed to human food, "it's very difficult to reverse,'' Olson said. "A bear would rather eat a Twinkie than forage.''

Finding problem bears is not easy. "Some of these animals are pretty savvy,'' said Olson. "They key in on our vehicles and uniforms,'' and slip away. "They know the next thing coming out is the slingshot.''

Contact Carlos Santos at (434) 295-9542 or csantos@timesdispatch.com.

Staff writer Frank Green contributed to this report.