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General Biology, Reproduction and Behavior

The reproductive biology of river otters and all other weasels is complex because of a characteristic known as delayed implantation. Following breeding and fertilization in spring, eggs exist in a free floating state until the following winter or early spring. Once they implant, fetal growth lasts 60 to 65 days until the kits are born, usually in spring (March through May) in most areas. In the southern portion of the range the dates of birth occur earlier, mostly in January and February, implying implantation in November and December. Litters usually contain 2 to 4 kits, and the female alone cares for the young. They usually remain together as a family group through the fall and into the winter months. Sexual maturity in young is believed to occur at about 2 years of age in females, but later in males. 

River otters are chiefly nocturnal, but they frequently are active during daylight hours in undisturbed areas. Socially, the basic group is the female and her offspring. They spend much of their time feeding and at what appears to be group play, repeatedly sliding down the banks of mud or snow. They habitually use specific sites (toilets) for defecation. Their vocalizations include chirps, grunts, and loud piercing screams. They are powerful swimmers and are continuously active, alert, and quick–characteristics that give them immense aesthetic and recreational value. Their webbed feet, streamlined bodies, and long, tapered tails enable them to move through water with agility, grace and speed. Seasonally, they may travel distances of 50 to 60 miles along streams or lake shores, and their home ranges may be as large as 60 square miles. Males have been recorded to travel up to 10 miles in one night.

River otters use a variety of denning sites that seem to be selected based on availability and convenience. Hollow logs, rock crevices, nutria houses, abandoned beaver lodges and bank dens are used. They will also frequent unused or abandoned human structures or shelters. Natal dens tend to be located on small headwater branches or streams leading to major drainages or lakes. 

Damage and Damage Identification

The presence of river otters around or in a fish hatchery, aquaculture or fish culture facility is a good indication that a damage problem is imminent. Otter scats or toilets that contain scales, exoskeletons, and other body parts of the species being produced is additional evidence that damage is ongoing. Uneaten parts of fish in shallow water and along the shore is evidence that fish are being taken. Otters usually eat all of a small catfish except for the head and major spines, whereas small trout, salmon and many of the scaled fish may be totally eaten. Uneaten carcasses with large puncture holes are likely attributable to herons. River otters can occasionally cause substantial damage to concentrations of fishes in marine aquaculture facilities. Often the damage involves learned feeding behavior by one or a family of otters. 

Information is from Prevention and Control of Wildlife Damage-Cooperative Extension University of Nebraska-Great Plains Agricultural Council Wildlife Committee-United States Department of Agriculture Animal and Plant Health Inspection Service Animal Damage Control