At 35 to 53 inches from tip to tip, the river otter is Illinois' largest member of the weasel family. A stout tail makes up about 30 to 40 percent of its total body length. An otter uses its tail like a rudder while swimming. Adults weigh 10 to 25 pounds; males are about one third larger than females. Otters have a broad, slightly flattened head, large nosepad, stiff, bristly whiskers, small black eyes and small rounded ears. Their bodies are muscular and torpedo-shaped, allowing them to move easily through water. The legs are short and have five fully-webbed toes on each foot. The fur is dark brown or reddish brown on the back and light brown, tan or silver on the throat and belly.
Distribution & Abundance
River otters were common and found throughout Illinois during early European settlement. Unregulated harvest and habitat loss caused their numbers to decline during the mid-1800s, and sightings were rare by the early 1900s. The trapping season was closed beginning in 1929, but this didn't help much. Pollution was a major problem until the 1970s, when many laws were enacted to improve water quality in our streams and rivers.
River otters were listed as a state threatened species in 1977. Their status was downgraded to state endangered in 1989. It's likely that fewer than 100 otters existed in Illinois at this time. The largest concentration lived along the Mississippi River and its backwaters in northwestern Illinois. A smaller population occurred along the Cache River in the southern tip of the state.
The Illinois Department of Natural Resources started working on a recovery plan in the early 1990s. The chances of success seemed good because habitat and water quality had improved in many parts of the state. Also, the state's beaver population was at near-record numbers (beaver dams create excellent otter habitat) and the state was engaging in some major efforts to conserve wetlands and wooded areas along streams and rivers.
From 1994 through 1997, 346 otters were captured in Louisiana using small foothold traps and released in southeastern and central Illinois. Thanks to these efforts and expanding populations in nearby states, otters are now common and found in every county in Illinois. Their status was upgraded from state endangered to state threatened in 1999, and they were delisted in 2004.
Recovery efforts have been wildly successful. A study by Southern Illinois University estimated that the number of river otters in release areas grew to more than 8,400 in 2009 (about 1 otter per 5 square miles). This estimate does not include parts of western and southern Illinois where releases were not made because otters had colonized these areas naturally. DNR estimates that nearly 11,000 otters occurred statewide in 2009. This is expected to grow to more than 30,000 by 2014 if left unchecked.
Rivers, streams and lakes are key habitats. Those with wooded shorelines and nearby wetlands are best. Water quality isn't a major concern unless it's so bad that fish are scarce. Some types of contaminants can cause problems, but tests show that otters from Illinois have little or no accumulation of these compounds in their bodies. Beavers tend to make areas even more inviting for otters. Otters aren't able to dig their own burrows. Abandoned beaver dens are their favorite places for resting and raising young. Pools and wetlands created by beaver dams are good places for otters to search for food.
River otters are most active at night. Adult males live along large stretches of river, often up to 40 to 50 miles. Females aren't nearly as mobile. Their home ranges are only 3 to 10 miles depending on habitat quality and the time of year. A study in Missouri found that otters at one conservation area had home ranges of 7 to 48 miles long and reached an average density of one adult per five miles of shoreline. Home ranges were smaller at another area, which supported one adult per 2 1/2 miles in more diverse and higher-quality habitat.
As many as nine otters have been spotted together in Illinois. Smaller groups of three to five are more common, and usually consist of a female and her offspring. Males tend to be solitary, but sometimes join a family group or even form a bachelors club with other males.
Otters travel a lot but spend most of their time at activity centers with abundant food and cover. Examples include logjams, oxbows, pools below dams or spillways and springs or riffles that stay free of ice all winter. Otters usually follow watercourses, but cut across land if it's more convenient. Some crossings are used so often they have worn trails. Areas visited frequently or for long periods of time are fairly obvious.
Otters tend to drag large fish onto the shoreline or a logjam to dine. Scales, heads and other uneaten parts are left behind. Otter toilets (known as spraints) are marked by piles of large, coarse droppings that are dark-colored and contain fish scales and crayfish parts. Sometimes otters post territory by raking leaves and grass into a mound that they mark with droppings or a thick yellowish fluid from their anal glands.
Otters are graceful and powerful swimmers. Only the head and shoulders show as they swim along the water's surface. They might swim like this for long distances or take short, regular dives as they go. They can stay submerged for three to four minutes and swim up to a quarter mile underwater.
They have a bounding or loping gait on land. Tracks left in the snow are nearly unmistakable because of their unique "bound-slide" mode of transportation. After a few bounds forward, they slide on their bellies for 10 to 20 feet, reaching speeds of up to 15 to 18 miles per hour. While noted for their playful behavior and habit of sliding down steep riverbanks, slides are fairly rare unless an area is used heavily.
It's not unusual for otters to approach within a few feet of a boat or a person on shore because they're curious and near-sighted. Near-sightedness is an adaptation that helps them to see better while underwater.
Fish are the river otter's main prey. Food studies done in different parts of the United States show that they eat whatever species are most common and easiest to catch. Biologists found remains of sunfish, carp and shad most often in droppings collected in northwestern Illinois. Otters eat crayfish and frogs when abundant. Less common foods include salamanders, snails, clams, snakes, turtles, birds and insects.
Breeding takes place from January through April. The fertilized eggs remain dormant for 285 to 365 days, then they attach to the wall of the uterus and continue to develop. The pups are born 60 to 63 days later, arriving between January and May. Most litters have two to four young. Newborns are helpless but develop quickly. Their eyes open at about 35 days of age. Brief trips outside the den begin at 10 to 12 weeks. Females do most of the work when it comes to raising the pups, but males help occasionally once the pups leave the nest. The young otters are coaxed into the water for the first time at about 14 weeks of age. They're weaned by 4 months of age, but often remain with their mother until the following spring. Young females can breed at one year of age, but many wait until they're 2 years of age.
Conserving wetlands and wooded areas along streams and rivers are top priorities. Reducing soil erosion and preventing fertilizers and pesticides from washing into streams are important measures, even where otters aren't likely to visit. For example, soil particles washed into a stream can settle when they reach slow-moving water, covering the rock, sand, or gravel that some fish need to lay their eggs and raise young. Fewer fish means less food for otters.
Problems caused by otters have grown along with the population. Most problems stem from predation of fish in stocked ponds or aquaculture facilities. Effects of predation on natural fish communities have not been documented in Illinois, but the possibility exists given findings of studies in Missouri. Predation by otters was a major source of mortality for small alligator snapping turtles released in a stream in southern Illinois as part of an effort to restore this endangered reptile.