Danny Green / North American river otter (Lontra canadensis) on back, rolling on snow bank. Yellowstone National Park, USA, January.

Danny Green / North American river otter (Lontra canadensis) female and cub walking across snow, reflected in water. Yellowstone National Park, USA, January.

Danny Green / North American river otter (Lontra canadensis) on snow covered bank, reflected in icy water. Yellowstone National Park, USA, January.

George Sanker / North American river otter (Lontra canadensis) swimming with head above water. Acadia National Park, Maine, USA. November.

George Sanker / North American river otter (Lontra canadensis) two in water with autumnal trees reflected in the water, Acadia National Park, Maine, USA, October.

George Sanker / North American River Otter (Lontra canadensis) two swimming near the surface with autumn leaves reflected in water, Acadia National Park, Maine, USA, October.

Mark Carwardine / Northern / North American river otter (Lontra canadensis) swimming, Captive, Slimbridge, Gloucestershire, UK, July

Michael Quinton / North American River Otter (Lontra canadensis) on iced over river, Yellowstone National Park, Wyoming

Karine Aigner / A rescued baby North American river otter (Lontra canadensis) on white background, Florida, USA. Captive.

Karine Aigner / A rescued baby North American river otter (Lontra canadensis) on white background, Florida, USA. Captive.

Ernie Janes / RF - American river otter (Lontra canadensis) eating fish, captive, occurs in North America. (This image may be licensed either as rights managed or royalty free.)

Ernie Janes / American river otter (Lontra canadensis) eating fish, captive, occurs in North America.

Ernie Janes / American river otter (Lontra canadensis) captive, occurs in North AMerica.

Nick Garbutt / RF - North American river otters (Lontra canadensis) (probably female with two juveniles) on the frozen river edge.Yellowstone National Park, USA. January. Digitally stitched image. (This image may be...

Nick Garbutt / RF - North American river otters (Lutra canadiensis) on the frozen river edge. Yellowstone National Park, USA. January (This image may be licensed either as rights managed or royalty free.)

Nick Garbutt / North American river otters (Lutra canadensis) probably female with two juveniles, on the frozen river edge. Upper Yellowstone River, Hayden Valley, Yellowstone, USA. January

Nick Garbutt / North American river otters (Lutra canadensis) running on the frozen river edge. Upper Yellowstone River, Hayden Valley, Yellowstone, USA. January (stitched image)

Nick Garbutt / North American river otters (Lutra canadensis) on the frozen river edge. Upper Yellowstone River, Hayden Valley, Yellowstone, USA. January

Nick Garbutt / North American river otter (Lutra canadensis) two on frozen river edge. Upper Yellowstone River, Hayden Valley, Yellowstone National Park, Wyoming, USA. January.

Nick Garbutt / North American river otter (Lutra canadiensis), two standing at edge of frozen Upper Yellowstone River, Hayden Valley, Yellowstone, USA. February.

Danny Green / North American otter (Lontra canadensis) eating a fish, Yellowstone National Park, USA, February

Roger Powell / North American river otters (Lontra canadensis) family group with one of which has just caught a crab, Lower Suwannee River, Levy County, Florida, USA April

Will Burrard-Lucas / North American river otter (Lontra canadensis), group exploring the banks of river. Grand Teton National Park, USA. May

Edwin Giesbers / RF- North American river otter (Lutra canadensis) captive, occurs in North America.

Edwin Giesbers / North American river otter (Lutra canadensis) captive, occurs in North America.

Paul Hobson / North American river otter (Lontra canadensis) playing in snow, Yellowstone National Park, Wyoming, USA, February.

Paul Hobson / North American river otter (Lontra canadensis) playing in snow, Yellowstone National Park, Wyoming, USA, February.

Paul Hobson / North American river otter (Lontra canadensis) playing in snow, Yellowstone National Park, Wyoming, USA, February.

Paul Hobson / North American river otter (Lontra canadensis) playing in snow, Yellowstone National Park, Wyoming, USA, February.

George Sanker / North American river otter (Lontra canadensis) emerging from water, Acadia National Park, Maine, USA, October.

George Sanker / North American River Otter (Lontra canadensis) on ice in a beaver pond. Acadia National Park, Maine, USA, November

George Sanker / North American River Otters (Lontra canadensis) on ice in a beaver pond. Acadia National Park, Maine, USA, November

George Sanker / North American River Otter (Lontra canadensis) looking up curiously in a beaver pond. Acadia National Park, Maine, USA, November

Charlie Summers / Northern River Otter (Lontra canadensis) resting on the banks of Gibbon River, Yellowstone National Park, Wyoming.

David Kjaer / Three North American / Canadian Otters (Lutra canadensis) lying on each other by water. Captive, UK.

Brent Stephenson / Fully grown Northern River Otter (Lutra canadensis) pup swimming along the shoreline. Walker Cove, Alaska, United States, July.

Diane McAllister / Canadian river otter (Lutra canadensis) on a log enjoying its evening meal of a Cutthroat trout, starting at the tail. Trout Lake, Yellowstone National Park, Wyoming, USA, June. Winner of the Amateur...

Diane McAllister / Canadian river otter (Lutra canadensis) on a log enjoying its evening meal of a Cutthroat trout, starting at the tail. Trout Lake, Yellowstone National Park, Wyoming, USA, June