Otters are members of the Mustelidae family of carnivores, which also includes the badger, polecat, mink, ferret, pine marten, stoat and weasel. They are chocolate brown in colour with flattened heads, webbed feet and thick tapering tails. Males weigh about 9kg and average 1.1m in length, females are slightly smaller. They are largely solitary, semi-aquatic and obtain most of their food from lochs, rivers or the sea.
Ecology and Behaviour
The Scottish otter population can, broadly-speaking, be divided into two ecologically distinct, but not mutually exclusive, types: riverine/freshwater otters which are widely distributed across the country, and coastal otters which are mainly distributed along the west and north coast, the Hebrides and Shetland. Wherever they occur, they are territorial animals and this behaviour, which itself is related to the availability of essential resources, has implications on how many otters can reside along a given stretch of river or coastline.
Otters in Freshwater Habitats
In freshwater habitats, otters are largely (but not exclusively) nocturnal. They occur at very low population densities, with the average home range size of a female being around 20km of watercourse and that of a male, around 32km, although the home ranges of some male otters can be considerably larger than this. Riverine otters may travel distances of 16km or more in one night and, as home ranges frequently incorporate sections of separate river systems, movement between these can expose otters to an increased risk of mortality when crossing roads.
Otters feed largely on fish and, perhaps not surprisingly, the amount of time they spend in different parts of their home range is related to the abundance of their fish prey. Although all species of fish are taken in freshwaters, those with a high lipid content form a high proportion of their diet in Scotland, i.e. trout, salmon and eels. In rivers and streams, the first two tend to predominate in the diet, whereas in lochs, eels are more important. During periods of food shortage such as the spring, frogs and toads may become important prey, with mammals and birds also taken occasionally.
Otters in Coastal Habitats
The Scottish otter population is noteworthy in that it comprises a particularly high proportion (perhaps 50% or more) of coastal-dwelling otters which feed predominantly in the sea. Nowhere else in the British Isles are coastal habitats more important for this species than the coast and islands of western Scotland and Shetland, so much so that coastal otters are occasionally referred to as ‘sea otters’ despite the fact that they are exactly the same species as the animals which inhabit freshwaters further inland. (The true sea otter is a distinct species which occurs off the west coast of North America). Not surprisingly, some of the issues which affect coastal otters differ from those affecting the species elsewhere.
In contrast to otters in freshwater habitats, their coastal counterparts are commonly active during the day. As elsewhere, the sexes live apart for most of the time, but their home-ranges overlap, the home-range of each male overlapping that of more than one female. They generally feed on small benthic (bottom-dwelling) fish and crustaceans such as crabs, and have a strong preference for hunting in areas with dense sea weed cover in shallow, inshore rocky areas.
Home ranges in coastal habitats are typically smaller than elsewhere, allowing a higher population density to be sustained. This is possible primarily because of the high productivity of the inshore marine environment, particularly in the more favoured otter areas. In these very productive areas, otter home ranges may be as small as 4-5km of coastline.
Irrespective of marine or freshwater lifestyle, in cold water, heat loss even through the otter’s thick fur is substantial, so they must eat a lot to maintain condition. Water temperature is a very important factor determining the length of time they must hunt in order to catch sufficient food. Thus in the winter months, each otter may be obliged to fish for longer periods to compensate for the increased rate of heat loss. For a coastline to be suitable for otters, an essential component is the presence of easily accessible freshwater for bathing, so that the sea-salt can be washed from the animals’ fur and thereby maintain its insulative properties.
Although otters are less shy than was once thought and have now been recorded close to the centres of all Scotland’s cities, this is only possible if there are opportunities to hide or escape from possible danger.
Each otter will use a number of different places for shelter within its home range. The distribution and density of these structures is highly variable, so that in some areas, the distance between them may be as great as 2km, whereas elsewhere, one shelter may be present, on average, for every 150m of river or shoreline. Although, they can be difficult to find and recognise, the discovery and identification of all shelters which may be affected by a proposed development is essential, both to protect the animals themselves and to comply with the relevant legislation.
Two kinds of shelter are used: underground holts and above ground couches. Otters may dig their own holts but they very often make use of other structures ranging from enlarged rabbit holes and cavities amongst tree roots to rock piles and man made structures. They often mark these structures with spraints (faeces), although natal holts (where the young are born) can have few revealing features, probably to avoid drawing attention to them. Couches can be nest-like structures (0.3-1m in diameter) constructed from nearby vegetation but, equally, can be simply a depression in a stick pile or under a windblown tree. Couches are often in a patch of vegetation where it is the patch, rather than the couch site itself, which is important. Sometimes an otter may sleep in a different part of the patch of vegetation on different occasions or even move within the patch during the day.
In some freshwater areas, couches may be used more frequently than holts and are sometimes even used for breeding purposes. Favoured rest sites appear to be on riverine islands and in reed beds where they are present. However, the animals frequently rest in a wide variety of vegetation types. Find out more information on otter breeding sites and their conservation.
Along coasts, holts are particularly important as resting sites for otters, more so than for some inland areas. Where the substrate is sufficiently soft and peaty, holts may be excavated entirely by otters, otherwise they may adapt or enlarge existing cavities as necessary.
Other Field Signs
Although rarely seen, otters leave a number of distinctive field signs as indicators of their presence. Spraints provide the clearest evidence and are more frequently encountered than tracks, which can be difficult to decipher. Otter spraints have a high mucus content and are often formless, generally black or greenish-black in colour and may contain obvious fish bones and scales. Their most diagnostic feature is their sweet or spicy smell. Favoured sprainting sites may have been used for many years and the continual nitrogen enrichment of these spots may result in the nearby grass becoming characteristically bright green in colour. Otters also construct sign heaps on which spraints are deposited. These are most commonly formed from vegetation or soil.
Otter feeding remains may be found, usually in the form of the hard parts of crustaceans, the unpalatable bits of amphibians and the bony parts of fish. They are easy to confuse with the prey remains from other predators.
Otter tracks are about 6cm wide and each has five toes although often, on hard ground, only four show. The webs and claws are apparent only occasionally and it is worth remembering that tracks rarely look exactly like the pictures in field guides. Otter paths are 12-15cm wide and normally connect with water and holts. They are marked with spraint and in snow and ice may show evidence of slides.
Otters have a low reproductive rate with a late onset of sexual maturity and no more than a single litter of two or three cubs born per female, in any one year. Except in Shetland, where most births occur during the summer months, there is no consistent pattern of seasonality and births may occur at any time of the year. The cubs are cared for by their mother before dispersing at about a year old. This is significant because the death of the female may lead to the loss of the cubs which can be detrimental to the population as a whole. Furthermore, their low reproductive rate, coupled with a relatively short life expectancy in the wild (typically only 3-4 years), and a naturally well-dispersed low-density population structure, means that otters are highly vulnerable to localised events which can cause unusually high levels of mortality. For example, the construction of a new road which fails to incorporate special measures to enable otters to cross safely can lead to otter casualties in particular locations, thereby threatening the viability of the local population.