The river otter (Lontra canadensis) is widely distributed across North America, from Florida to Alaska, and this large mustelid (weasel family) occupied every major waterway of the United States and Canada until at least the nineteenth century. By 1977, however, they occupied < 75% of their historic range due to draining of wetlands, overharvesting for fur, and pollution. Wetland conservation and restoration in the 1980s, and reintroduction programs in some states in the 1990s, successfully restored river otters to almost 90% of their historic range by 1998. In Massachusetts, river otters were never extirpated. The abundance of freshwater and coastline in the Commonwealth has provided excellent resources for the species and opportunity for dispersal or colonization across state lines.
River otters are adapted to life in or near freshwater, but they will use saltwater habitats along the Atlantic and Pacific coasts and inhabit islands such as the Elizabeth Islands, Martha’s Vineyard, and Noman’s Land. Open ocean is considered a barrier to the species, but they have recently colonized Nantucket. Tracks were photographed in 2008 by biologist Edie Rae and verified by the state furbearer biologist. Otter activity is regularly documented on Noman’s Land. Thus, river otters in this region appear to be making saltwater crossings of a few miles. Future genetic analysis of scat collected on all of the nearshore islands and the upper Cape will provide valuable information on connectivity between these populations.
Density & Home Range Size
In Prince William Sound, Alaska, coastal river otter densities were 20 – 80 otters/100 km of shoreline, and their home ranges included 20 – 40 km of shoreline. Male otters used a larger area than females in these coastal areas and overlapped with several females and other male otters. Females occupied exclusive areas with their young, but adult and sub adult helper females were also part of these family groups. Male otters can be part of large groups (9 – 30) that hunt cooperatively, particularly in marine habitats. Our wildlife cameras on Martha’s Vineyard have documented groups of 2 – 8 otters.
Coastal river otters in Alaska have smaller home ranges than otters in inland waterways because coastal areas provide an abundance of food. In southeastern Massachusetts, we might expect similar densities and ranges. As highly mobile mammals, otter may move > 10 km per day.
Otters vary in size, but typically weigh 5 – 15 kg (11 – 34 lbs) and are 89 – 137 cm (36 – 55 inches) in length. As they are most commonly seen in the water, people often underestimate their size. An otter’s tail represents 30 – 40% of its total length, and it is valuable to the animal in stabilizing the body in the water by acting as a rudder and paddle. Otters are most comfortable in the water, but are surprisingly agile on land. They stay warm in the water, and in cold climates, because their skin has more subcutaneous fat than terrestrial mammals and secretes squalene that provides water-proofing for the fur. They also have piloerector muscles in the integument of the skin, which allows them to vary the loft of their fur.
Otter teeth are adapted for crushing and cutting, and their vibrissae (whiskers) help them find food in murky water. Their hearing is excellent, and their vision is adapted for hunting underwater. They are wary animals and may detect humans or other species at considerable distances.
Reproduction & Denning
Otters mate in the spring, and copulation stimulates egg production in females. The fertilized eggs float within the uterus until they implant at a later date. They share this trait with a few other mammals. However, the river otter’s delayed implantation period of approximately 273 days is one of the longest. Implantation is triggered by a photoperiod of 10.5 hr of light and 13.5 hrs of dark in all parts of their range. Thus, implantation dates range from 15 November to 19 February (Florida to Alaska). Pups/cubs are born from January 17 to 9 May, depending on latitude, with Massachusetts otters whelping most likely in March/April. Females give birth to 1 to 6 young that are born blind, toothless, and with little fur. They are mobile with their mother by 60 days, but rely on her for food and protection for almost an entire year.
Like other mustelids, otters have a high metabolism. Because of their high metabolic rate, otters cannot store enough fat to be dormant in winter and are active year-round. They use underground dens, above the water line, and are known to take over muskrat bank dens, beaver lodges, and the burrows of other mammal species that would be near the water. They are also known to use foundations of buildings near the shore. In warm weather, otters often rest under shrubs or other cover and not inside a den. Otters use several denning or resting areas within their home range annually as they must move around the landscape to ensure they have enough food.
One study estimated an adult otter required 1 – 1.5 kg of food per day. Otters are carnivores, but are primarily piscivorous. They eat a variety of fish species, and much of their diet in southeastern Massachusetts includes fish less than 8 inches in length, such as killifish (Fundulus sp.), silversides, and small flounder. In these coastal areas, a large portion of their diet is blue crabs or lady crabs (swimming crabs). Lobsters are consumed where otters occupy rocky shorelines. In other regions, crayfish constitute a substantial portion of the diet. Amphibians (frogs and salamanders), turtles, insects, birds, and mammals (such as muskrats) comprise a smaller proportion of otter diets. Otters sometimes hunt cooperatively, and fish are consumed in direct proportion to their availability. Slower swimming species are consumed in higher proportions.
Latrines, Tracks & Sign
Otters are comfortable on land and habitually use the same “haul-out” or “latrine” areas within their home range. Otters roll in these areas to dry and clean their fur and will rest for long periods if cover is sufficient for them to feel secure. Scent mounds (mounds of grass/vegetation) are left at these sites, along with scat. Scent marking in river otters is not a territorial behavior as it is with most other carnivores, such as cats, fox, and coyotes. Scenting in otters indicates their presence rather than a territorial boundary they defend.
When moving over land, otters typically use a bounding pattern, like weasels and mink, where the back is arched and all four feet hit the ground near each other. Below is a photo of otter tracks. For more information on identifying otter tracks on Martha’s Vineyard Click here.
On snow and ice, otters typically slide while holding their feet alongside their bodies. River otters can move 24 – 29 km/hr when running or sliding overland.
Otter scat is almost always composed of fish scales or crustacean parts and pieces in southeastern Massachusetts. Fresh scat is dark in color, and the scat fades or bleaches as it ages. Here are some examples of otter scat:
A site visited regularly by otters will often have several scats: some fresh and some old. A site visited less often may only have a single scat on a mound of dirt or sand or grass.