WILDLIFE MANAGEMENT- MANAGING FOR OTTERS
Facts about Otters
D T Rowe-Rowe

Otters are animals that usually generate much interest. They appeal to some people merely because of their attractive appearance; others are intrigued by the numerous folk-lore tales which surround them; and there are those who are strongly prejudiced against the animals, believing that they are responsible for heavy depredations on sportfish and wild waterfowl populations, or because they are sometimes responsible for killing ducks and geese.

In this technical guide facts on the biology of the two South African otter species, based on three years of field research in freshwater habitats will be presented, and the ways of dealing with problems will be suggested.
Identification

The clawless otter Aonyx capensis is the larger of the two. The combined head and body length is about 950mm and the animal weights 12 to 16 kg. The general colour is chocolate brown. The throat and neck, cheeks, and upper lip are white. The feet are not webbed, and the digits are clawless.
The spotted-necked otter Lutra maculicollis is smaller, with a combined head and body length of about 600mm and it weighs only 4 to 5 kg. Most of the spotted-necked otters recorded in KwaZulu Natal have an overall brown colour including the neck and throat. Indistinct spots can sometimes be seen on the neck. The toes are webbed and each digit possesses a claw.

When one sees an otter in the wild, all that is usually visible is the head and neck above the water, and as the fur is wet, both species tend to look very dark. To assist positive identification one should therefore look very carefully to see whether there is any white (sometimes it is more dirty-cream or buff) on the animals cheeks or mouth. If so- it is clawless. If not - spotted-necked. Another difference is that the clawless otter has many long whiskers, whereas those of the spotted-necked otter are short.

General habits
Clawless otters are active mainly during the late afternoon and early evening and occasionally at dawn or early morning. Spotted-necked otters are active during the day, with peaks in their activity during the early morning and late afternoon.

Otters are not adept at moving on land. They either walk slowly or employ an awkward seal-like trot. If a clawless otter carries an object on land, it holds the object against its chest with one forefoot and runs on three legs, whereas spotted-necked otters carry in the mouth only. When swimming the clawless otter uses its hind-legs for propulsion and the tails as a rudder, progressing by long underwater dives, or swims steadily with its head above the water all the time. The spotted-necked otter progresses by a series of dives, about 5m at a time, surfacing only momentarily to loop and dive again, creating a serpentine impression.

Shelter is taken in holes in the ground and under rocks, under roots of trees, or dense vegetation on islands or streambanks. In sandy alluvial soil the clawless otter digs its own burrows which are up to 3m long and 1m below the surface, and have one or two entrances above water level.
Otters dry and groom themselves by rubbing and rolling against various objects. Most of their rolling places are in grass, but sandbanks, earth ledges, and flat rocks are also used. At most rolling places, otters establish latrines, which, like the rolling places, are regularly used. Clawless otter faeces (scats) have a mean diameter of 25mm whereas; those of the spotted-necked otter are about 14mm in diameter. The droppings of both species usually contain crab remains and are seldom found more than 5m from the water.

Although otters travel mainly in water, they occasionally move across land, usually to get from one aquatic habitat to another. The animals appear to keep land travel to a minimum, however, and regular pathways termed ‘runs’ are formed, representing the shortest distance between water bodies. The places at which otters get in and out of water, termed ‘slides’ are worn smooth by the movement of their bodies.

Otters are usually seen singly or in twos, and sometimes in family groups of up to five, consisting of one or two adults and their young. Litters of up to three young are born during spring. At birth young clawless otters are a smokey grey colour and only acquire the adult colour later. At birth young spotted-necked otters are dark brown (almost black).
Young otters indulge in play fighting and romping, but in both species manipulative games are most common. These take the form of picking up an object, juggling with it (by clawless otters), or tossing the object into the water and attempting to retrieve it before it reaches the bottom (by spotted-necked otters). Games of this nature probably provide otters with the exercise and practice required for the capture of prey.

Food
The diet of otters in freshwater habitats was studied by collecting and examining their faeces and information on their feeding behaviour was obtained from watching animals in the wild and closely observing otters that were kept in large enclosures.

Altogether nearly 1400 clawless otter and 300 spotted-necked otter droppings were examined. Major items in the clawless otters diet were crabs 65% and frogs 23%, while minor items were fish 4%, dragonfly larvae 3% and birds 1%. The spotted necked otter’s diet consisted of crabs 37%, fish 35%, frogs 21%, dragonfly larvae 4% and birds 2%.

Seasonal variations occurred in the otter’s diets. Both species took more crabs during summer than during winter, and the incidence of fish was highest during winter. These variations are explained in terms of crabs retreating into inaccessible places during winter, and because fish are slowed down when the water is cold. The incidence of frog (mainly platannas are eaten) was highest during late autumn and winter, and is explained in terms of reduced water levels in marshes, lakes, and backwaters which were drying up, making frog capture easier.

Fishes eaten by otters were identified from scales found in the faeces, and sizes were estimated from the lengths of certain bones, while in some fish species age could be determined from scales. In trout waters, mainly small fish were eaten by both otter species: 88% and 95% of the fish remains found in clawless and spotted-necked otter faeces respectively, were from trout under 200mm long.

In the non-trout waters fishes identified from remains in otter faeces (in order of abundance) were largemouth bass, bluegill, spotted bass, minnow, eel, and scaly or yellow fish. With the exception of a few bass, all fish were under 200mm long.

A fish’s swimming ability is proportionate to its length - the longer the fish the faster it can swim. This is one of the reasons why most fish eaten by otters are small.

Food finding
It becomes much easier to understand the otters’ diets if one considers their adaptations and behavioural patterns. The clawless otter has large robust molars, no flesh cutting teeth and powerful jaws that make it possible for it to crush the shells of even the largest freshwater crabs. It has evolved dexterous forefeet, which are used more as hands than as feet. This otter hunts by sight and by feel, using its forefeet to feel under rocks and in holes, and the long whiskers help it to locate prey by sensing vibrations set up in the water when the prey moves. Almost all prey is captured by grasping it with the forefeet, then biting it.

The spotted-necked otter hunts by sight and captures all prey in its mouth. The teeth differ from those of the clawless otter in that the molars are smaller and it has flesh-cutting cheek teeth. Because of its smaller size and the smaller molars, the spotted-necked otter cannot feed on crabs of all sizes, but takes mainly small specimens. It is a faster swimmer than is the clawless otter and is therefore a better fish hunter.

Conclusions
The evolution of an animal takes place in relation to a number of natural factors. One of the important factors in the evolution of a predator is its prey - the predator evolves behavioural and physical characteristics, which enable it to capture and feed on particular prey species. The prey species in turn evolve so as to avoid excessive predation by physical protection, behavioural adaptations and reproductive rates, which exceed predation rates. Otters and their natural prey evolved a long time ago and have existed together for many millions of years. It would be contrary to the laws of nature for otters to over-utilize their prey populations as this would result in their own extinction.

In our country, the availability of otter prey has been altered somewhat by the introduction of alien sportfish and the keeping of domestic ducks and geese on farm dams. Fish (including introduced species) forms only a minor component in the diet of the clawless otter. Although fish is an important item in the diet of the spotted-necked otter, the effect on fish populations is negligible because 1) the otter is rare 2) mainly small fish are eaten 3) as trout waters in KwaZulu Natal tend to be overstocked with this introduced species, the otter, if anything, may be helping the angler.

Where many fish are kept in small ponds (such as hatcheries) otters are capable of doing considerable damage. The solution to the problem is not to try to eliminate the otters, as this could go on forever, but to keep the otters out. This is best done by enclosing the hatchery with wire netting, and having a single electric strand, about 150mm high and 200mm from the fence, around the outside. Consult your KwaZulu-Natal Nature Conservation Service District Conservation officer for the latest information on electric fencing.

Otter predation on indigenous waterfowl is negligible. At certain times of the year (usually late winter or early spring), when streams are low or dry, otters visit lakes and artificial impoundment’s mainly to capture crabs and platannas. If ducks and geese are kept on the dam, a number of these may be killed. To eliminate the otter is a very temporary measure, as another will soon take its place in the vacuum created. The best thing to do is protect the poultry in an enclosure until stream conditions improve.

Otters are rare, mainly because of the restricted niche, which they occupy. The major limiting factor is clean, unpolluted water- in heavily silted or polluted streams, crab, fish, and frog population decline to the degree that otters cannot exist. If, therefore, you have otters in your stream or dam, this is an indication of sound veld and environmental management, as the presence of otters reflects healthy aquatic conditions.

KEY POINTS
Clawless otters eat mainly crabs and frogs.
Spotted-necked otters live on crabs, fish and frogs - but the otter is rare.
Otters and their prey require clean, unpolluted water.