Although visitors and park rangers have confirmed seeing the river otter in most of Yellowstone’s major lakes, rivers and large streams, they’re not commonly spotted. That’s likely because they spend most of the daylight hours cuddled in their dens, which they make in beaver lodges, log jams, hollow logs, bank recesses, rock recesses and even culvert pipes. The best time to see them is in the winter. Their dark coat is easy to spot against the snow and ice on the riverbank and they don't hibernate like some other Yellowstone mammals.
A relative of the weasel, the river otter has thick fur that protects it against Yellowstone’s icy winters. Their webbed feet and powerful tails help them to easily navigate through the water. Otters can close their ears and nostrils, using their whiskers to search for prey. They can swim for 2-to-3-minutes without coming up for air.
River otters eat fish, crayfish, frogs, and occasionally young muskrats. They typically seek out the slowest moving prey and then head to the riverbank or a log to eat their catch.
In the river habitat, otters are at the top of the food chain, but once on land they have a slew of natural predators. Gray wolves, coyotes, red foxes and bobcats will all eat the charismatic otter. Other animals—bald eagles, ravens, even pelicans—will try to steal fish that an otter has already caught.
Adult otters weigh 10 to 30 pounds and can grow up to four feet long, while baby otters are typically half their size.
Dawn and dusk are the best times to spot a Yellowstone river otter. Make sure to look in areas nearby a body of water. Their dark bodies are easier to spot against a white winter backdrop.
Sources: http://www.pbs.org/wnet/nature/episodes/yellowstone-otters/introduction/2103/, http://www.nps.gov/yell/planyourvisit/upload/ys6(1)part3.pdf