The following is a news release from Yellowstone National Park.
"Grizzly bears are emerging from hibernation in the Greater Yellowstone Area, so hikers, skiers and snowshoers are advised to stay in groups of three of more, make noise on the trail and carry bear spray.
Bears begin looking for food soon after they emerge from their dens. They are attracted to elk and bison that have died during the winter. Carcasses are an important enough food source that bears will sometimes react aggressively when surprised while feeding on them.
Updated bear safety information is available on the Yellowstone bear safety Web page at http://www.nps.gov/yell/planyourvisit/bearenc.htm and in the park newspaper, which is distributed at all park entrances. The park also implements seasonal bear management areas closures to reduce encounters between bears and humans in areas where elk and bison carcasses are in high density. A listing of these closures can be found at http://www.nps.gov/yell/parkmgmt/bearclosures.htm.
Yellowstone regulations require visitors to stay 100 yards from black and grizzly bears at all times. The best defense is to stay a safe distance from bears and use binoculars, a telescope or telephoto lens to get a closer look. All visitors traveling in the park away from developed areas should stay in groups of three or more, make noise on the trail, keep an eye out for bears and carry bear spray. Bear spray has proven to be a good last line of defense, if kept handy and used according to directions when a bear is approaching within 30 to 60 feet.
While firearms are allowed in the park, the discharge of a firearm is a violation of park regulations. Even the park's law enforcement rangers who carry firearms on duty rely on bear spray, rather than their weapons, as the most effective means to deal with a bear encounter.
Visitors are also reminded to keep food, garbage, barbecue grills and other attractants stored in hard-sided vehicles or bear-proof food storage boxes. This helps keep bears from becoming conditioned to human foods, and helps keep park visitors and their property safe.
Bear sightings should be reported to the nearest visitor center or ranger station as soon as possible."