River Surveys

Why survey for mammals by rivers?

River edges, especially around bridges, are great places to survey for mammals. The combination of fresh water, prey and habitat make rivers and river banks attractive to a wide range of animals including otters, mink, brown rat, polecat, fox, badger, deer, and of course dogs!

Mud banks under bridges cannot grow vegetation, so they stay clear to collect and reveal footprints and droppings for wildlife enthusiasts to find. As mud banks under bridges are sheltered from both sun and rain, any spraints/scats left there will stay in good condition and give off a scent for longer, which may lead some mammals like otters to select such sites for scent marking.

Otters in particular are significant as they are a key indicator of the health of the river ecosystem. In the 1950s otters were considered to be common and widespread across most of England. However, by the early 1960s, otter populations had started to crash.

Pesticides used in sheep dips and as seed dressings were the main cause of their decline. Such chemicals were particularly detrimental to animals at the top of the food chain such as birds of prey and carnivorous mammals.

These chemicals were withdrawn from use in the 1980s. Since then the health of the UK’s rivers have been making a natural, but slow, recovery, and the presence of otters has signalled the improved state of many of our rivers. In Worcestershire, otter signs are now found on all major watercourses thanks to a spread in the population during the 1990’s and 2000’s from one of a few national strongholds on the river Teme, thanks to improvements in water quality and a determined conservation effort by the Environment Agency and Worcestershire Wildlife Trust, with support from Severn Trent.

Signs

Because many mammals are largely nocturnal and/or elusive, direct observations are difficult and many surveys are based principally on the observation and recording of tracks and droppings. These can be used mainly to reveal presence, though not finding any need not denote absence of mammals. [3]

Tracks

Tracks or footprints are the imprint of the underside of a paw made in damp, soft substrates such as sand or mud. [5]

When identifying tracks, remember that size can vary depending on what the animal was doing – animals climbing a bank, leaping or running may have slipped, leaving bigger tracks. On harder ground, some of the pads or claws may not register.

Confusion is also more likely where several tracks are imposed on top of, or close to, each other. So, it’s best to base your id on clear tracks. Where this isn’t possible, think carefully about any factors that may complicate your identification. [6]

An otter’s footprint has five toes arched around the front of a large pad which has a long heel. It is usually 5 to 7 cm wide. In soft ground, claw marks and webs may be seen and print impressions can be deeper and look larger; the tail may also leave a mark. Sometimes not all five toes are visible and, depending on gait, prints can overlap, making tracks more difficult to separate from those of other mammals, particularly those of domestic dogs.

Mink tracks are considerably smaller at only 2 to 4 cm wide, and the toes are more pointed than those of the otter; mink, stoat, and otter cub tracks may be very difficult to tell apart. Badger tracks are similar in size to otter tracks, but all five toes point forwards and are in front of the heel pad. Foxes and dogs have only four toes; domestic dog prints are most commonly misidentified as otter tracks, though the four toes of the dog print (compared to the otter’s five) are a ready diagnostic distinction. [3]

Droppings

Some mammals identify and communicate with each other by scent and leave their droppings in particular locations. These may have a distinctive scent. Other mammals do not use their droppings for communication and deposit them at random.[5]

Fresh otter ‘spraints’ have a characteristic sweet-musky smell, often said to be like jasmine tea. This smell is the most reliable diagnostic feature for identifying otter droppings. As spraints age, they dry out, turn grey and crumbly, and gradually lose their scent, though old spraints can retain some of the distinctive smell for over a year. Mink droppings (or ‘scats’) smell very unpleasant.

Because otter spraints are a form of communication, they are left in prominent places where they can easily be found by other otters. Look for spraints by rivers and streams on rocks and boulders, fallen tree trunks, logs, on concrete ledges under bridges, and on otter paths especially where the animal leaves and enters the water. Spraints are often left where a side tributary joins the main river. [3]

Other signs – slides and castles

A site along a river bank used by otters to enter the water may be worn smooth to form a ‘slide’. On sandy substrates without prominent features, the otter may scrape up sand and stones into a small heap to spraint upon; this heap is sometimes called a ‘castle’. Where a bankside sprainting site is repeatedly used, the resulting increased fertility can lead to formation of a lushly growing and raised ‘spraint heap’ tussock protruding above the normal ground level. [3]