On this page:
- Facts About Maine River Otters
- Viewing River Otters
- Preventing Conflicts
- Trapping and Lethal Control
- Public Health Concerns
- Legal Status
- Additional Information
Figure 1: Drawing Credit - Larrison
River otters (Lontar canadensis, Fig. 1) have long, streamlined bodies, short legs, webbed toes, and long, tapered tails, all adaptations for their mostly aquatic lives. Their short, thick fur is a rich brown above, and lighter, with a silvery sheen, below. Adult male river otters average four feet in length, including the tail, and weigh 20 to 28 pounds. Female adults are somewhat smaller than males.
Although seldom seen, river otters are relatively common throughout Maine in ponds, lakes, rivers, sloughs, and streams as well as in estuaries, bays, and open waters along the coast. In colder locations, otters frequent areas that remain ice-free in winter, rapids, the outflows of lakes, and waterfalls. River otters avoid polluted waterways, but will seek out a concentrated food source upstream in urban areas.
River otters are powerful swimmers with snakelike agility; their small eyes are adapted for seeing food items in murky or dark water. (Fig. 1)
Facts about Maine River Otters
Food and Feeding Behavior
- River otters are opportunists, eating a wide variety of food items, but mostly fish. River otters usually feed on slow moving fish, such as carp, mud minnows, stickle backs, and suckers, that are from four to six inches long.
- River otters can smell concentrations of fish in upstream ponds that drain into small, slow moving streams and will follow that smell to its origin, even in urban areas.
- River otters also eat freshwater mussels, crabs, crayfish, amphibians, large aquatic beetles, birds (primarily injured or molting ducks and geese), bird eggs, fish eggs, and small mammals (muskrats, mice, young beavers).
- In late winter, water levels usually drop below ice levels in frozen rivers and lakes, leaving a layer of air that allows river otters to travel and hunt under the ice. River otters digest and metabolize food so quickly that it can take only an hour for food to pass through their intestines.
- River otters use dens for giving birth and for shelter from extreme weather. Birthing dens are lined with small sticks, shredded vegetation, and other available material.
- Den sites include hollow logs, log jams, piles of driftwood or boulders, and abandoned lodges and bank dens made by beaver.
- Dens are well hidden; those located at the water's edge will have an entry far enough below the surface to prevent it from being seen or frozen shut.
- River otters also den under boathouses, duck blinds, and other human structures up to half a mile away from water.
Reproduction and Family Structure
- River otters have what is called delayed implantation; the fertilized egg does not attach to the uterine wall for up to eight months after breeding. Thus, actual gestation ranges from 61 to 63 days.
- Two to four pups are born May through June.
- Young otters begin playing at four weeks of age and learn to swim at about seven weeks of age.
- When eight to ten weeks of age, the pups begin exploring beyond their den and are introduced to solid food.
- In late fall, the pups leave to establish their own territories. During this time, wandering youngsters are seen far from water, traveling on land between lakes, ponds, and streams. They may move from one drainage to another.
- The basic social group for river otters is a female and her offspring. Before and after breeding, male otters usually lead solitary lives.
Mortality and Longevity
- Essentially safe from predators while in water, river otters are more vulnerable when they travel on land. Predators, including coyotes, bobcats, cougars, bears, and domestic dogs, mostly take young otters.
- Humans trap river otters to obtain their pelts, control fish predation in private ponds and commercial fish hatcheries, and prevent damage to private property.
- Reduced water quality (from chemical pollution and soil erosion) as well as stream-bank habitat alteration from development pose the greatest threats to river otter populations.
Viewing River Otters
River otters are active day and night; around humans they tend to be more nocturnal. Otters spend their time feeding and at what appears to be group play. They also dry their fur, groom themselves, and mark their territory by vigorously scratching, rubbing, and rolling on the ground.
River otters are active year round and, except for females with young in a den, are constantly on the move. They tend to follow a regular circuit and cover it in one to four weeks. Males can travel 150 miles within a particular watershed and its tributaries in a year. A family may range 10 to 25 miles in a season.
To observe river otters, sit quietly on a high place such as a bridge, overhanging bank or tree, or pier, above a known feeding area, trail, or slide. Find an angle from which you can avoid surface glare. Polarized sunglasses and binoculars are useful. River otters are wary and their hearing and sense of smell are well developed. They are fairly nearsighted, however, and they may not notice you if you stay still.
Never instigate a close encounter with a river otter. Otters have been known to attack humans, and females with young are unpredictable.
Trails and Tracks
Figure 2: Drawing Credit - WDF&W
When traveling on land, river otters walk, run, or bound. Bounding is their fastest gait. When bounding, the front and hind feet are brought toward each other causing the back to arch and the tail to be lifted off the ground. Otters make trails along the edges of lakes, streams, and other waterways. Trails often lead across a small peninsula from one cove to another or alongside shallow rapids. Trails are six to seven inches wide and may lead to slides or dens. Look for tracks in soft mud, damp sand or fresh snow. (Fig. 2)
River otter tracks show five pointed toes around a small heel pad. Tracks are three to three and a half inches wide and three to four inches long. (Fig. 2)
One-foot wide slices are made in grass, dirt, sand or snow along the edge of water, frequently on islands in lakes or in openings under bushes and brambles along streams. There are often several slides in a river otter's home range. It is common to find a trail leading away from the slide.
River otters thoroughly chew their food, so their droppings contain only fine bits of fish scales, bones, crayfish parts, and fish scales, and shells. The droppings, which smell fishy and have an oily texture, are shapeless, dark, slimy, and green, red, or black. Droppings get lighter as the sun bleaches out the color.
Otters deposit their scat at latrine sites, which are prominent locations near at the water's edge, along trails, and near dens. Many otters frequent the same sites, leaving droppings as a way of communicating with each other.
River otter sounds include chirps, growls, whines, and, when alarmed, an explosive hah!
Although river otters are often blamed for preying on wild game fish, particularly trout, studies indicate that the bulk of the diet consists of non-game fish species. Occasionally -- and it is usually families containing young pups in spring -- otters cause severe problems in fish hatcheries and private ponds. Also, they sometimes den under houses, decks, and other structures near water, and the smell of their droppings and discarded food remains can be unpleasant.
To prevent conflicts or remedy existing problems:
Eliminate access to feeding sites and other areas. Because river otters have heavy bodies and aren't jumpers, a four-foot high fence constructed with three-inch mesh wire can keep them out of an enclosed area, such as where fish or aquiculture activities are concentrated (Fig. 3). Because river otters are strong, fences should be sturdy and extend six inches below the surface to prevent otters from pushing under the fence. Alternatively, include a wire apron on the animal side of the fence to prevent otters from entering from underneath (Fig. 4).
A double-wire electric fence, with wires set six and ten inches above ground will also deter river otters. Such a fence can stand alone, or supplement an existing perimeter fence. (Fig. 5) A single wire can be used around docks and houseboats.
River otters are resourceful and will thoroughly investigate fence lines to find a way to food. They are known to use abandoned animal burrows as routes under fences. Inspect fences regularly to make sure river otters have not dug or pushed their way underneath or worked their way over them.
Figure 3: Drawing Credit - Jennifer Rees
Figure 4: Drawing Credit - Jennifer Rees
Figure 5: Drawing Credit - Jennifer Rees
An existing wire fence can be modified to keep otter out of an area by adding chicken wire to the bottom three feet. Keep the bottom flush with the ground or bury the fencing six inches. (Fig. 3)
Where otters seem intent on entering from underneath the fence, include a wire apron on the animal side of the fence. (Fig. 4)
Keep river otters out of ponds and other areas with two hot wires, six and 10 inches off the ground. Make sure these are well marked to prevent accidents. (Fig. 5)
Provide fish with hiding places. Give fish safe places to hide by constructing sturdy hiding places on the bottom of ponds using cinder blocks, ceramic drain tile, wire baskets made from leftover galvanized fencing, or upside-down plastic crates held in place with heavy rocks. In larger ponds, attach a group of cut conifer trees to a heavy anchor on the bottom of the pond.
Eliminate access to convenient denting sites. Close potential entries under porches, houses, sheds, and other structures with quarter-inch mesh welded-wire (hardware cloth), boards, or other sturdy material. (Fig. 6) Aluminum flashing, or aluminum or stainless-steel hardware cloth is recommended in saltwater areas since galvanized materials quickly corrode.
Eliminate potential entries under porches, houses, sheds, and other structures with quarter-inch mesh welded-wire (hardware cloth), boards, or other sturdy material.
Eliminate noxious odors. Commercial odor-eliminators can be used to remove the smell of otter droppings and other debris under structures. Such products are available through hospital supply houses, drugstores, pet stores, and from the Internet using the keywords "Pest Control Supplies." If the smell is really bad, the beams and other areas under the structure may have to be cleaned with a bleach solution. (one and a half cups of household bleach in one gallon of water) Be very careful of fumes.
River Otters In or Under Buildings
Occasionally a river otter will find a suitable den site in or under a building. Otters normally occupy a den site for only two or three consecutive nights. However, during the mating and nesting season, females are attracted to warm, dry, dark, easily defended areas, and will remain longer if the setting remains favorable.
You may choose to let an otter occupy an area, such as under an outbuilding, if it doesn't pose a problem. Should you choose to remove the animal, you can hire a wildlife control company (call your local regional Fish and Wildlife office for a current list of contacts), or you can do it yourself.
- Seal all openings except the main entrance used by otters. Use sturdy wire mesh (quarter-inch hardware cloth or similar materials) to screen vents near ground level in houses and other structures. Tightly seal holes in foundations or under porches to prevent otters from entering.
- To determine entry points, you can use "tracking patches" of a fine layer of sand, flour, or dust placed at suspected entrances. Wadded up newspaper lightly stuffed into the entry hole also works great. Otters will push the paper out of the way when exiting.
- After dark, when the otter goes out in search of food, it will leave tracks at the den entrance. Inspect the powder or the dislodged newspaper.
- Once an otter has left the building, immediately seal the entrance with a hardware cloth "one-way door" (Fig. 7). You will not want to permanently exclude at this point because you don't know yet how many otters are inhabiting the building. The one-way door can be made from half-inch hardware cloth that is attached over the opening, and hinged at the top and left loose on the other three sides. It should be larger than the opening so that it cannot swing inward. The otter will push it open to leave, but cannot re-enter.
- After the one-way door has been installed for two or three nights, put a layer of flour on the inside and outside of the door. If all footprints are outside, and none are inside, then the otter has been shut out. If you have any doubt, smooth the dirt on both sides of the door with your hand or a tool, reapply the flour and observe. Once a couple of days have gone by with no footprints, the otter is probably gone. Another way to check is to open the door and shove a few pieces of wadded up newspaper into the entrance. If the paper stays in place for two to three nights, then the otter is gone.
- Once you are sure all otters are out, permanently seal the opening.
Important Note: Be sure all animals are out before sealing up the entrance. Pay close attention and use extra caution if trying this option during May and June when babies may be in the den.
A one-way door can be used in conjunction with a welded wire or hardware cloth barrier. (Fig. 7)
To try and drive an otter away, consider harassing the animal. Lighting up the den site with battery operated flashing lights and adding a portable radio can cause an otter to seek a more suitable habitat.
Trapping and Lethal Control
Trapping or shooting river otters should be a last resort. Lethal control can never be justified without a serious effort to first prevent problems from recurring. Removing river otters by any means is a short-term solution as other otters are likely to move in if attractive habitat is still available. (For detailed information, see "Trapping Wildlife")
Public Health Concerns
Diseases and parasites associated with river otters are rarely a risk to humans. Canine distemper, a disease that affects domestic dogs, may be found in Maine river otter populations. Have your dogs vaccinated for canine distemper to prevent them from contracting the disease.
Anyone handling a river otter should wear rubber gloves, and wash their hands well when finished.
The river otter is classified as a furbearer. A trapping license and open season are required. Because legal status, trapping restrictions, and other information about river otters can change, contact your local Inland Fisheries and Wildlife Regional Office wildlife office for updates.
If an otter is causing damage or is a nuisance, consult Maine's laws on this subject.
New England Wildlife, Habitat, Natural History, and Distribution
Written by: Richard DeGraff, and Mariko Yamasaki
University Press of New England, 2001.
(Available from: www.upne.com)
Mammals of the Pacific Northwest: A Pictorial Introduction
Written by: James R. Christensen, and Earl J. Arrison
Moscow, ID: University of Idaho Press, 1982.
Prevention and Control of Wildlife Damage
Written by: Scott E. Hygnstrom, et al.
University of Nebraska-Lincoln, Institute of Agriculture and Natural Resources, 1994.
(Available from: University of Nebraska Cooperative Extension, 202 Natural Resources Hall, Lincoln, NE 68583-0819; phone: 402-472-2188; also see Internet Sites below.)
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Written by: Ingles, L. G.
Stanford, CA: Stanford University Press, 1965.
Mammals of the Northwest: Washington, Oregon, Idaho, and British Columbia
Written by: Earl J. Arrison
Seattle: Seattle Audubon Society, 1976.
Land Mammals of Oregon
Written by: B.J. Verts, and Leslie N. Carraway
Los Angeles: University of California Press, 1998.
Adapted from: "Living with Wildlife in the Pacific Northwest"
(see Washington Department of Fish and Wildlife)
Written by: Russell Link, Wildlife Biologist, Email Russell Link, with assistance from WDFW Biologists Rich Beausoleil and Rocky Spencer
Design and layout: Peggy Ushakoff, ITT2
Illustrations: As credited
Copyright 2004 by the Washington Department of Fish and Wildlife