River otters are one of the most aquatic members of the weasel family. They can swim up to six or seven miles per hour on the surface of the water as well as underneath it, and can remain submerged for up to two minutes. Otters spend a great deal of time fishing under the ice, and obtain oxygen from open holes such as the one pictured (as well as from air bubbles under the ice). As their tracks indicate, otters come up onto the ice to eat their prey, be it fish or crayfish, their two favorite winter meals.
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