Otter Homepage

This site primarily concerns 'river' otters: Lutra lutra and L. canadensis, their conservation, ecology and other stuff. But just to keep you happy - a picture or two of sea otters.

NEW! The Monteray Bay (Sea) Otter Cam

Biology and Ecology of the European Otter Lutra lutra

Otters are large amphibious mammals belonging to the weasel family (Mustelidae). The nine species of otter found around the world are synonymous with aquatic habitats and are found in wetlands, rivers, lakes and coastal waters. The European otter L. lutra has an historical range from Eire to Japan, and Scaninavia to North Africa, although population density and distribution has declined greatly since the 1940s. This species can grow to 1.2m (4'), with a third of this length being tail, and weigh up to 10kg (22lb).

Otter eating an eel

Like all mustelids, otters are carnivorous. They are adaptable predators, and eat a wide range of food. Otters show a high degree of adaptation to an aquatic existance and their diet reflects this specialisation. Studies have shown that fish make up 30-90% of the diet of L. lutra, the remainder including crayfish, amphibians and water fowl. Occasionally, freshwater mussels and mammals such as rabbits, beavers and smaller mustelids (weasels, stoats and mink) are also eaten. Otters typically consume slow-moving fish, such as eels, and rarely catch salmonids such as trout and salmon. They are agile swimmers, but rarely dive for more than a minute or so.

Otters and Mink

Skull of L. canadensis (very similar to L. lutra)

Skull of Mink (Mustela vision)

(Skulls courtesy of DeLoy Roberts)

Mink (Mustela vision) are a North American species, accidentally and deliberately released into British ecosystems from fur farms. They are a partly aquatic species and are common along rivers, but lack the high degree of specialisation of otters. They are quite often mistaken with otters, but are smaller and less graceful in the water. Tracks and droppings of otters and mink are quite different, and are used in otter surveys to detect the presence of the animals.

Otter tracks and spraint

Note: 5 toes, unlike dog track

Mink tracks and scat

More Tracks

Otters reach sexual maturity at around 18 months and may live for 10 years. However, a long gestation period and small litters of 2 or 3 pups mean that the species has a low fecundity rate. Otters remain with their mother for a year before being forced out to establish their own territories. The dog otter has little to do with the pups once they are born. The territories of female (bitch) otters are around 8km (5 miles) of river bank, while male (dog) otters have territories of about 15km (10 miles) which overlap those of a number of females. Each territory has a number of secure resting sites. Temporary sites in vegetation are known as crouches while permanent dens, usually in the roots of riverside trees such as oaks are called holts. The size of a territory depends upon the size of the otter population, the availability of prey and the number of resting sites in the area.

Potential Holt Site Beneath Oak

The habitat requirements for otters still needs to be defined, but a broad range of features appear to be important. Prey is essential, and minimal cover (dense vegetation and shrubs) is acceptable if there is abundant food. Females are more sensitive to lack of cover than males, and require secure refuges to raise litters. In plentiful cover, otters will tolerate greater disturbance such as human activity. Therefore, the order of importance may be:

Food > Cover > Disturbance

The otter is a charismatic species, seen as intelligent and playful, largely thanks to the work of Henry Williamson (Tarka the Otter) and Gavin Maxwell (Ring of Bright Water). Purseglove (1988) writes;

"The fact that people will give money to save the otter, a nocturnal animal whose presence is detected even by full-time otter survey teams only by its tracks and droppings, is the best answer I know to the mean-spirited and illogical arguement: 'What's the use of saving it, if I can't see it?'"

Therefore, there was much public concern in the 1970s, when it was realised that the English otter population had 'crashed'.

The Fall and Rise of Otters in the UK

In 1962, in was reported that otter hunts were finding fewer otters. An anonymous report in 1969 - the initial analysis of hunt records - suggested that there had been a decline between 1957 and 1967. These findings prompted a more detailed examination in 1977. This further report suggested that otter populations had not been evenly or densely distributed prior to the 'crash' in the 1950s. It was proposed that many areas had already been below the carrying capacity of the local rivers due to persecution by organised hunting and / or game keepers. Other areas, such as Devon, the Somerset Levels and the Broads, with less severe pressures, had higher populations of otters.

At their height, there were only 23 otter hunts, each active for about 50 days per year and catching around 20 animals. By contrast, there were 23,000 game keepers in the UK by 1911, trapping throughout the year. Around 5 years before the 'crash' there was already a significant difference between the otter hunt success in South East and South West England, mirroring the different density of game keepers. The effect of the persecution is evidenced in the increase of most predator species during the First World War, in which most 'keepers fought.

The most startling feature of the otter decline was its rapid onset on a national scale. This suggests that a single factor was responsible, being most severe in the south and east. Between 1960 and 1974 most of the otters in the Midlands, South and South East of England were lost, with surviving populations too small to be viable, even in the former stronghold of East Anglia. The 'mean start' of this decline was 1957, with most loss occuring from 1957 to 1963. The only new factor introduced during this time and over such a large area was the use of organochlorine pesticide. It is notable that populations of avian predators such as the peregrine falcon also declined during the 1950s. The differential use of these chemicals - higher in the arable farmlands of the east and south-east, less in the pastures of the south west and west, mirror the decline in otter populations.

Strachan and Jeffries (1996) suggest that otter populations, already stressed by direct and indirect persecution from hunting and gaming interests, was not able to tolerate the introduction of pesticides.

Three UK national otter surveys have now been carried out, and the latest (1991-94) shows that the otter is making a recovery. Populations in the South West and West have consolidated and are moving eastward. Radio tracking has shown that in theory, otters could cross England in 14 days. The animals have not recolonised as a wide-spread, low density population, however, and there appears to be a 'rolling-front', moving 3.6km (2.5 miles) per year, as individuals occupy new territories.

Site Occupancy by Otters in Former National River Authority Regions of England and Wales

     Region              1977-79        1984-86       1991-94
     South West           23.6%          43.8%         67.1%
     Welsh                14.8%          29.6%         63.0%
     Northumbrian          8.3%           9.8%         26.4%
     Severn-Trent          2.3%           3.6%         20.7%
     Wessex                1.3%           0.7%         18.8%
     Yorkshire             1.8%           2.2%         10.4%
     Anglian               3.2%           1.1%          8.0%
     Southern              2.1%           2.9%          4.4%
     Thames                0.0%           0.0%          2.2%
     England & Wales       5.8%           9.0%         22.2%

Strachan and Jeffries, 1996

Green rivers are those with a natural population, red rivers have introduced otters and their progeny.

The populations in the west of England and Wales have recovered rapidly, as they retained a much larger population. The neighbouring regions, such as Wessex, have benefitted from influxes (probably of young animals) from these 'core' populations. Eastern regions have not been so fortunate, and the introduction of captive-bred otters probably saved the Anglian population from becoming extinct. Despite the promising recovery, over 75% of sites in England and Wales, and 95% of sites in the Southern Region remain empty of otters. It is estimated that it will take 70 years before otters are common again across the whole of their former range. Habitat loss; accidental fatalities on roads or in fishing gear, such as fyke nets and increasing urbanisation may all slow down the recovery. Accidental mortality is especially grave in small population, where any deaths remove a larger percentage of the remaining population. A great deal of money is being directed towards otter conservation, in the hope that these problems may be overcome. These schemes include releasing captive-bred otters (with mixed results) and habitat improvement. The latter is perhaps of greater benefit to the environment, as it is not only otters which benefit. Replanting river banks, construction of artificial holts and restocking water courses with fish are some of the measures taken. Additional, numerous Otters and Rivers Projects have been set up, covering most of England. Such projects form part of the local Biodiversity Action Plans, which seek to conserva the ecology of wetlands and rivers.