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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

The river otter is the most playful member of the weasel family, most often seen basking on ice. © Shane Rucker
The river otter is the most playful member of the weasel family, most often seen basking on ice.

© Shane Rucker

February 2007

Spry slider

Spying a frolicking river otter is a high point on a winter walk. Look for its coded message in the snow.

Judy Nugent


On the icy edge of winter
Territorial during the short breeding season
Easy living in summer
Keys for otter conservation
Tracking otter populations

A secretive and shy animal, the river otter is not seen most of the year, but once sighted, provides an experience you won't soon forget. Known as Lontra canadensis, the river otter is the most playful member of the weasel family. It is most frequently seen in winter basking on ice near open water or running, hopping and sliding over the snow.

As with so many other species, unrestricted harvest in the early decades of European settlement depleted populations of otters, which were trapped for their warm, luxurious fur. Uncontrolled, wasteful use of land and waters damaged fish populations causing otters to disappear from many parts of their natural range. Just as conservation practices have reintroduced and bolstered game species such as turkey, waterfowl and elk, active management is slowly restoring otter populations as well. In the past 20 years, nearly 4,000 river otters live-trapped with foothold traps in Louisiana and Mississippi were released in 18 states rebuilding healthy populations today. Wisconsin and states as far south as New Mexico and as far east as New York carry out active programs to recover these natural parts of our wild heritage.

Otters are found throughout Wisconsin with greater concentration in numerous wetlands of the northern half of the state. Otters are mustelids, in the same family of weasels as mink, martens, badgers, fishers and wolverines. Adult otters are three to four feet long with tubular, furred tails that make up nearly 50 percent of their total body length. The tail alone can reach 18 inches or more in length. Adult otters average 15 to 25 pounds. Their fur is short, dense and durable, setting the standard for the fur industry, making is extremely desirable to trappers and fur buyers alike. Fur color varies from a light tan in the cheek, chin, throat and belly, to a rich, dark brown on the back. Five webbed toes, a long tail and streamlined body make otters nimble, adept and extremely efficient swimmers. You have to be a good swimmer when you catch live fish for a living! Stiff whiskers two to four inches long help them detect food in murky water. Petite diamond shaped noses offer a keen sense of smell to pick up the scent of other animals.

On the icy edge of winter

Though otters are often seen feeding or basking where an icy shelf meets flowing water, they are primarily nocturnal (active at night) or crepuscular (active at dawn or dusk). They live in abandoned beaver lodges, woodchuck burrows, muskrat dens, holes in the bank, or beneath the roots of upturned trees. Dens do not have to be near water for otters to use them.

Scavenging for food near open water or along the bank, otters can look awkward all hunched up. However, their humping lope can carry them along as fast as people can travel. Otters can run as fast as 15-18 mph. They conserve energy moving across the snow by taking a few bounds and then sliding on their tummies. From above, these tracks look a bit like Morse code, in a dot, dot, dash pattern. This telltale pattern is useful in tracking their movements.

On land, otters are both predator and prey. In winter they hunt mice, voles and red squirrels. Unfortunately, otters in turn are hunted by wolves, coyotes and bald eagles. By far, the otter's most common predator is human.

Even in winter, otters find safety staying near water. When fishing in the winter, they keep a hole or two open at all times. An opening just a few inches across is enough to give them access to the fish below.

Territorial during the short breeding season

The spring thaw and lengthening daylight provides the drive to trigger the breeding season. Otters reach sexual maturity by age two. Females can reproduce as yearlings though an average of 32 percent of them have litters compared to about 75 percent of adult females. Breeding occurs in March or April and brings increased competition among males.

Otters have a pair of scent glands located near the base of their tails. These glands produce a strong musky odor that males use to mark their territories. They also urinate on vegetation in their home range as a way of marking turf. Males can also smell when the females come into estrus. Otters communicate through touch, posture and a variety of vocalizations such as chirps, chuckles, grunts, whistles, hisses, growls, whines, coughs and screams. Fighting may occur when a female is near two adjacent males.

According to Amber Roth, currently a PhD student in Michigan and formerly a Wisconsin DNR wildlife research technician, otters, like other mustelids, can delay embryo implantation for 10-12 months depending on their health, food availability and the quality of available habitat. Active gestation is 60-63 days and females will mate again, usually within two months of having their litters. Breeding-aged females usually have one litter every year. The newborns are referred to as either pups or kits. Two to four pups are born in April or May, and are about 4 inches long and fully furred. Their eyes remain closed until they reach one month old.

The den is usually hidden from the male and is defended until the pups are old enough to depart. Males often set off from the females after mating, but may sometimes return to help rear the young.

Easy living in summer

Pups leave the dens in late spring and will stay with their mother for a year learning survival skills.

Otters are well adapted for swimming. Unlike muskrats or beavers, the otters barely makes a ripple when swimming and hardly a splash when diving. Their ears and noses have valve-like skin that closes and keeps the otters watertight. Their eyes are specially adapted for underwater vision, so much so that they are nearsighted above water. River otters can stay underwater for four minutes, dive to depths of more than 40 feet, and swim at an average speed of 7 mph.

The main food an otter is after is fish. Suckers, minnows, carp, sunfish and bass make up nearly 77 percent of their diet, though they also dine on frogs and crayfish. Otters look for food near shorelines, overhanging banks and other aquatic cover. In Wisconsin, otters prefer slower moving fish like suckers to faster moving trout, but will feed on what is available. They grab prey with their mouths and may float on their backs holding smaller fish with their forepaws. Large fish are dragged on shore to be eaten.

By fall the pups have been weaned, but still stay close to their mothers. Young otters are sometimes seen playing with live fish and will catch a fish, release it, and catch it again; or they'll toss the fish between their paws or into the air. At first blush, otters appear to be tormenting the fish, but by passing fish to other otters these "games" improve their coordination.

As family groups, adult females and pups are very sociable. In addition to mutual grooming, otter family groups float down the river together in formations called rafts. These groups stay together until the mother is ready to have the next litter, then she will push away the pups encouraging them to find unclaimed habitat. Fighting may occur as young males establish these new territories.

Keys for otter conservation

The river otter is sensitive to changes in its environment and is often found in areas far away from intense human activities. To conserve otter habitat, the national Otter Action Plan, drafted by research biologist Thomas Serfass of Maryland's Frostburg State University, calls to:

  • reduce emissions that cause acid rain and affect otters' food chains;
  • implement streambank fencing projects to keep grazing cattle farther from streamsides and nearshore aquatic habitats that otters use;
  • enhance programs to protect wetlands and limit their loss;
  • regulate mining activities nationwide that cause acid mine drainage and mitigate existing drainage that defiles streams; and
  • enhance policies and enforce laws that reduce point source and runoff water pollution.

All these policy changes would help aquatic species like otter, trout, waterfowl and even raptors that benefit from clean water, streambank stabilization and wetland protection. These are exactly the sorts of goals the Department of Natural Resources is building into the State Wildlife Grants program to bolster many species.

Tracking otter populations

From the standpoint of the Department of Natural Resources, monitoring shy creatures like river otters can be daunting. The high value of their pelts, which averaged $110 last year, only intensifies the need to accurately estimate their numbers, regulate their harvest, account for natural mortality and take steps to sustain healthy populations.

DNR biologists use several different methods to estimate populations. The first is keeping up-to-date records of the number of otters trapped, registered (mandatory) and sold. A tightly controlled trapping season limits the number of permits sold in each of three management zones. The system is matched with mandatory reporting. All otters harvested each year need to be registered with DNR conservation officers. The locations of kills, date, gender, type of trap used, and the name and address of the trapper are all recorded for each animal.

These trapping numbers are cross-referenced and double-checked against the number of pelts marketed. Periodically, DNR biologists also require trappers to turn in otter carcasses for analysis. Wildlife researchers and health experts study the teeth and reproductive organs of the carcasses to age animals, determine how many pups each female has produced, measure skull size, estimate growth rates, note geographic variations in otter populations statewide and track population trends. Carcasses are also examined to monitor for contaminants in natural food chains. For instance, mercury concentrations in trapped otters provide important clues about where this environmental contaminant is accumulating in aquatic environments and food chains. Careful monitoring helps track animal health and sustain stable statewide populations. Otter carcasses are being collected statewide this harvest season.

The second method relies on ground and aerial census work. The aerial system was pioneered starting in 2001 by Bruce Kohn, a DNR wildlife researcher now retired. He conducted a three-year study to determine how and if observing otter tracks in the snow from airplanes could effectively, accurately estimate their populations. He selected 23 30-mile transects within each of the three otter management zones. The aerial studies were conducted on bright, sunny winter days within one to three days after a fresh snowfall when the pilots and researchers could best see the Morse code like ". . _" track pattern. The transects crossed areas where one would expect to see otters, such as streams and rivers that maintain a good flow in winter where the otters would find fish and other food sources.

Amber Roth and Bruce Kohn developed a method to count otters from the air. © Paula J. Hellcamp
Amber Roth and Bruce Kohn developed a method to count otters from the air.

© Paula J. Hellcamp

At the same time, Kohn verified the aerial results by conducting traditional ground surveys. When the results were compared, the aerial surveys proved equally effective and far less expensive in gauging the population. To cover the selected transects each season takes a pilot, two observers per zone and a total of about 45 hours of flying time at an annual cost of $6,000. The Wisconsin Trappers Association saw the importance and need for this assessment for three years and offered to pay for the surveys.

Currently DNR researchers survey 69 transects statewide – 23 routes in each otter management zone. The biologists record the presence or absence of otter tracks at stream and river crossings along the 30-mile long routes. Each transect has eight or more stream or river crossings to provide adequate sample sizes and a greater chance of judging how populations are distributed. GPS locations for the western and eastern end points of each transect ensure that the same transects are surveyed each year.

In 1982, after the harvest season, otter populations were estimated at 12,600 animals, up to 15,600 by 1996, and back down to 12,500 in 2003. The harvest goals are set to maintain a minimum of 13,000 otters statewide before the season.

Analysis from the 2004 season results showed regional differences in otter reproductive rates and the sex and age composition of trapped animals. Fewer yearling female otters became pregnant in the central and southern Wisconsin zones, and the number of juvenile males harvested in the southern zone was lower than in other regions. If those trends continue, annual harvest goals will be refined to assure otter populations remain strong.

Given that each otter needs about three square miles of good quality habitat and ample supplies of fish to meet its daily needs, otters will never be as abundant as herbivores like beaver and muskrat that eat plants and can stand more crowded conditions. Sustained conservation efforts are needed to ensure a viable otter population throughout the state. If you would like to learn more about these secretive, playful animals, search "river otter" at Wisconsin Department of Natural Resources, search our EEK! site, or visit The River Otter Alliance . And when you head out for a winter woods walk, bring a camera and look for telltale signs of the otter, a Morse code like track in the snow. Seeing these creatures in the wild is a memory you'll treasure for years to come.

Outdoor writer Judy Nugent crafts feature stories from Waukegan, Ill.