Otter vibrissae are stiff, densely innervated and mounted on a muscular pad. When a fish swims it leaves a trail of vortices in the water that persists for seconds. Otters and seals can detect and follow these trails, a capacity demonstrated in controlled experiments where blindfolded animals tracked artificial wakes and moving targets they could not see or hear.
The adaptation matches where the animals hunt. Rivers, marshes and estuaries are often turbid, and otters forage at night as well as by day. Underwater the eye also faces an optical problem, since the cornea loses most of its focusing power in water; the otter's lens can change shape strongly to compensate, but sight is still a limited resource in silty water.
River otters were removed from large parts of their range by trapping and by water pollution, and were then returned to many US states through reintroduction programmes from the 1970s onward, several of which are considered among the more successful North American carnivore restorations. Because otter distribution is now partly a product of those releases, present-day maps should not be read as a purely natural distribution.