Both species are agile swimmers and catch fish by chasing them underwater. The otter
needs to eat 20 per cent of its body weight in food every day - about 2.5kg and will be able to tackle larger prey items than mink. Otters will usually eat the fishes head first and will quite often leave the tail. The mink has a wider diet but will take smaller fish. A feeding station will contain lots of fish scales and fins especially in the winter months.
To ascertain which species is involved you should firstly look for any footprints the animal may have left in the mud around the feeding area:
Otters are much bigger than mink and have 5 toes on each foot connected by webbing (although this is not always obvious and the print marks left usually only show four toes). The overall footprint shape resembles that of a kite with a footprint approx 7cm long and 6cm across. The hind print is generally bigger and can be up to 9cm in length. The claw marks are represented as slight indentations.
Mink also have 5 toes on each foot but the prints are much smaller with fore prints approx 3.5cm long and up to 4cm wide. The hind print is up to 4.5cm long. The prints are roughly oval in shape with clearly defined claw marks
Your next piece of detective work would be to see if the animal may have left some droppings nearby:
Otter droppings (called Spraints) are black and slimy when first deposited and have a strong oily smell. Over time however they become light grey and lose their thick consistency. The droppings are made up almost exclusively of fish bones, scales and the hard shell fragments of crustaceans. Otters use droppings as scent markings and they are quite often left in exposed places (raised banks or vegetation, stones etc).
Mink droppings are generally 6-8cm long and approx 6-9mm wide. They are often twisted and pointed at one end. Close examination is likely to show a much wider range of prey remains including the fur and bone fragments of small rodents, bird feathers and also the husks and pips of fruit and berries. If the mink has been eating fish the droppings are likely to resemble thin otter spraint but with a faintly acrid smell. The droppings are often used as scent markings, although some will be inconspicuously placed.
The otter is a protected species and it is illegal to trap or harm it in anyway. The mink however is a relatively recent introduction to Great Britain, and represents a threat to some native animals, in particular the water vole. Mink are categorised as a pest species and are widely trapped.