The North Circle is a remote & wild hike. Make sure you know what you are doing or consider a more civilized hike elsewhere.
Hike aware. Be prepared just-in-case you surprise a bear. Hike in groups. Make a good deal of noise. Watch for bear sign.
Conventional wisdom is that black bears are good tree climbers, Grizzlies are not. In 2005 a jogger in the Canadian Rockies decided to climb a tree rather than back away (with 2 friends) from an inquisitive Grizzly. She climbed 51ft. The Grizzly climbed 50ft high after her. Isabelle Dube fell to her death. Do not climb a tree for protection, in any situation.