Sheila Macdonald and Chris Mason
A number of factors are involved in the decline, both in population and range, of several otter species. The most important of these are pollution (especially in the developed world), habitat destruction, and over hunting.
There is little doubt that pollution has exerted major effects on the distribution and abundance of otters, though most work linking pollution to otter declines relates to the Eurasian otter (Lutra lutra) in western Europe. A review is provided by Mason (1989).
Pollution may influence otters either indirectly or directly. Indirect effects include damage to the food supply or habitat of otters, thus lowering the carrying capacity of an affected area. Direct effects impact on the animal itself, resulting either in rapid death (acute toxicity) or in a lowered fitness (sub-lethal toxicity), reducing the animal's ability to reproduce successfully or to survive in inclement conditions.
Indirect effects are most significant when they destroy the fish stocks or other prey forming the main food supply of otters. The most widespread form of pollution is organic waste from domestic and industrial sources. Where sources are small and adequately treated, organic discharges will do little damage and, by adding nutrients to streams, may enhance fish populations to the benefit of otters. However, poorly treated effluents can wipe out fish populations for long distances downstream of the discharge. Such effects can be of particular importance where large cities are situated in the middle reaches of major rivers with organic pollution rendering long stretches of otherwise suitable habitat unusable by otters. Intensive animal husbandry (such as of pigs, cattle, or chickens) also results in problems of waste disposal, the river providing the easiest, though often illegal, solution to the problem. In contrast to the effects of cities, livestock units are often situated close to smaller streams and rivers, and so pollution from these sources can remove further refuge areas for otters.
Rural industry may also pose indirect threats to otters, particularly mining for gravel and metals. In many countries gravel is extracted directly from river beds, not only destroying natural vegetation but also sharply increasing the silt load of the water, which clogs the gills of fish. Mine water is not only frequently acid, but it is often rich in metals which are acutely toxic to fish. Mason and Macdonald (1988a) reported how the discharge from a small mine in a remote area of Wales destroyed the fish population and made an otherwise ideal river habitat unsuitable for the Eurasian otter.
Massive fish kills from pollution can deplete the food supply for otters (Photo by Martin Gaethlich).
The acidification of freshwaters is now a widespread problem in the northeast United States, Canada, and Scandinavia, and has been more recently described from upland waters in the United Kingdom. Gaseous pollutants, many of them released from power stations, are eventually returned to land in precipitation at sites remote from their origin, causing episodes of very low pH as they are washed into streams with poor buffering capacity. The effect is exacerbated in those catchments afforested with conifers and the outcome is the elimination of much of the stream fauna, including fish. Few studies linking otter distribution to acidification have been made. However, Mason and Macdonald (1987) described how in the headwaters of a Welsh river, an afforested branch, receiving episodes of acid water, were not used by the Eurasian otter, while the species was resident on an adjacent branch which drained down open hills and was not subject to low pH. More recent work in Scotland has shown a relationship between low pH and reduced otter usage in streams in a region known to be suffering from severe acidification (Mason and Macdonald 1989).
Of those compounds which cause direct effects on otters, most concern has been expressed over oil, organochlorines, and heavy metals. Oil is known to have killed coastal-dwelling Eurasian otters and sea otters (Enhydra lutris) and acts by contaminating the fur, increasing heat loss, and reducing buoyancy of the animal (Costa and Kooyman 1982). Oil may be ingested and prove toxic during grooming, causing death by hemorrhagic gastroenteropathy (Baker et al. 1981). These effects were confirmed in March 1989 when a large-scale oil spill in Prince William Sound, Alaska, killed many sea otters.
Effluents from mining operations can contaminate a stream, like this one in Greece (Photo by Martin Gaethlich).
Organochlorines and heavy metals are ingested mainly via the food supply. These pollutants are persistent, accumulating in living tissues, and they are a particular problem in freshwater because there are many sources. Rainfall will wash atmospheric pollutants and chemicals applied to land into watercourses. Some pesticides are deliberately applied to water, especially in the tropics, to control pests such as mosquitoes, black-flies, and molluscs. Many industries discharge effluents directly into rivers or indirectly via sewage works. Small amounts of persistent pollutants in effluents may become quickly concentrated in the biota. Fish take in pollutants with their food, via their gills, and through their skin. Otters obtain pollutants almost entirely through their food, but if that is contaminated, high concentrations can quickly result.
There are few direct toxicological data on otters, though the American mink (Mustela vison) has been used as a laboratory model. It is therefore necessary to make a subjective assessment of the significance of pollution loads which are recorded in otter tissues. Because of the possibility of sublethal effects, which are difficult to detect but which could markedly reduce an individual's ability to survive, the conservationist should be concerned even with comparatively low levels of pollutants within tissues. It should also be remembered that an individual may carry a suite of pollutants, which might interact to produce toxic effects that are largely unknown.
Chlorinated hydrocarbon pesticides have been used widely in agriculture, horticulture, and forestry, as well as in industries such as wool and carpet manufacture and timber preserving. Their use has now been reduced in the developed world, but large amounts are still exported to the tropics. Chlorinated hydrocarbons are highly persistent in the environment, so that even if no longer applied, their effects may last for decades. High concentrations of dieldrin, for example, have accumulated in freshwater fish in southwest England, though it has proved impossible to locate the source precisely (Hamilton 1985). Chlorinated hydrocarbons are liposoluble and so build up in high concentrations in animal tissues. They may exert their toxic effects when fat stores are mobilized in periods of stress, for example during reproduction or food shortages.
Chlorinated hydrocarbon pesticides are known to have caused massive declines in the numbers of many bird and some mammal species and high concentrations have been recorded in the Eurasian otter, the North American river otter, and the sea otter (Mason and Macdonald 1986). There is little concrete evidence that pesticides have caused a decline in otters, but because of the secretive behavior of these animals, decreases may have gone unrecorded during the peak period of pesticide usage. Few otter tissues were analyzed from this period. Chanin and Jefferies (1978) attributed the decline of the Eurasian otter in Britain, which began in the late 1950s, to the introduction of dieldrin into the environment, but they have provided no analytical data to substantiate their claim. PCB usage increased exponentially during the 1950s.
Otters no longer inhabit rivers like this one that has been cleared of bankside vegetation and channelized (Photo by Chris Mason).
Polychlorinated biphenyls (PCBs) are organochlorines, which have a wide variety of industrial uses due to their physico-chemical characteristics such as high stability, inertness, and dielectric properties. They are widely dispersed in the environment, being distributed by water and in the atmosphere. They are highly lipophilic and biologically stable. Very low concentrations in water, often below the level of detection, can be greatly amplified in living tissues, with bioconcentration in aquatic mammals being as high as 107 (Tanabe 1988).
Experiments in which mink were dosed with PCBs have shown that reproductive failure occurs when concentrations exceed 50 mg PCB/kg fat (Jensen et al. 1977). It is known that such concentrations have been exceeded in Eurasian otters from Sweden, eastern England, and the Netherlands, where numbers have declined sharply (see Fig. 1). In contrast, thriving otter populations in northern Norway and northern Scotland have generally contained low levels of PCBs (Mason et al. 1986a; Mason and Reynolds 1988). Concern has also been expressed over PCBs in the North American river otter (Henny et al. 1981). Many of the PCB analyses are recent, indicating that the compounds are still a potent threat to populations. For example, a Eurasian otter cub, born in eastern England to a mother released as part of a restocking program, was killed by a car when not yet weaned at only 11 weeks of age: it had already accumulated 62 mg of PCBs per kg fat in its liver (Jefferies and Hanson 1987). Two animals from eastern England, containing high concentrations of PCBs, exhibited pathological symptoms, such as ulcers and skin abnormalities (Keymer et al. 1988), similar to those recorded in Baltic seals, where the cause was considered to be PCB-induced adrenocortical hyperplasia, resulting in a failure of the immune system (Bergman and Olsson 1986). Clearly such observations have major implications for any proposed reintroduction program for otters.
Figure 1. Mean (and range) of PCBs (mg kg-1 in lipid) from different European populations of Eurasian otters, Lutra lutra (status of population in parentheses). 1. southern Sweden (endangered); 2. northern Sweden (severe decline); 3. coastal Norway (thriving); 4. Finland (stable); 5. Scotland (thriving); 6. Wales (expanding following decline); 7. southwest England (expanding following decline); 8. eastern England (endangered); 9. Netherlands (probably now extinct). The hatched line at 50 mg kg-1 indicates the concentration of PCBs in tissues known to cause reproductive failure in American mink (Mustela vison).
Heavy metals are widely distributed in tissues of otters (Mason and Macdonald 1986a) but, unlike organochlorines, metals occur naturally in the environment. A probable case of mercury poisoning in the North American river otter has been described from an animal living in an Ontario river contaminated with chlor-alkali wastes (Wren 1985), while elevated concentrations have also been reported from the United States and from Eurasian otters in Britain. Elevated concentrations of lead and cadmium have also been reported (Mason et al. 1986b). However, in general it seems unlikely that metals have been involved directly in widespread declines of otters, though there is the possibility of more subtle, sub-lethal effects and synergistic action between metals and organochlorines (Olsson et al. 1979).
Few otters become available for analysis of pollutants, so it may be necessary to use alternative techniques to assess potential threats to otter populations, or to provide clues to their disappearance. The analysis of potential prey is an obvious technique (see Mason 1989). Preliminary work has indicated that the analysis of pollutants in spraints (scats) provides a useful way of assessing the load of metals, radioactivity, and organochlorines in otter populations (Mason and Macdonald 1986b, 1988b; Macdonald and Mason 1988).
Otters generally require shelter in which to sleep and breed. Some, like the smooth otter (Lutra perspicillata) can excavate their own dens, but most make use of holes in river banks, caves, cavities amongst tree roots, log jams, or piles of rock and debris. Many simply take cover in dense waterside scrub, reed-beds, or marshes. Impenetrable riparian vegetation comprises an important requirement for many species. The removal of such habitat was cited as a major factor influencing the status of populations of the Neotropical otter (Lutra longicaudis), southern river otter (L. provocax) and giant otter (Pteronura brasiliensis) throughout their ranges in Latin America (Melquist 1984). Radio-tracked North American river otters avoided a reservoir with little resting and escape cover despite its suitability in other respects (Melquist and Hornocker 1983).
In Britain, den sites of the Eurasian otter are often found in eroded root systems of old riparian trees (Macdonald and Mason 1983a). Chehébar et al. (1986), working in Argentina, found the southern river otter to be restricted to lake shores covered in forest or shrubs where the animals made use of mature root systems and fallen trees. In Britain, large trees were systematically removed from river banks by water authorities in case they fell into the water and caused blockages and damage.
A degraded stream in South Africa, due to overgrazing and erosion, with no cover for otters (Photo by Dave Rowe-Rowe).
Western European rivers have been cleared of vegetation and frequently also straightened both for flood prevention and to maximize available agricultural land. Marshes have been drained. Otters disappear from areas where no suitable habitat remains but will tolerate limited alterations. Some make use of small reservoirs and canals, such as the Eurasian otter in Israel and giant otter and Neotropical otter in the Guyanas (Macdonald et al. 1986; Melquist 1984).
Deforestation for timber extraction, or for conversion to grazing lands, followed by overgrazing, is rife throughout much of South America and Asia. The riparian habitat is lost and siltation of waters reduces the otters' food supplies. Similarly, in Africa the rapidly increasing human population has resulted in unwise agricultural practices and extreme over-grazing with the same results (Rowe-Rowe 1986). Deforestation and overgrazing, common also in southern Europe, affects water flow, leading to torrents in the rainy season followed by dry river beds for the rest of the year. In Asia, clearance of mangroves and conversion of these areas to agricultural or aquacultural projects similarly reduces habitat available to otters.
The construction of dams can also threaten otter survival. In Morocco and Spain, for example, many dams have been built in the uplands where the water is retained, leaving lowland rivers dry. Irrigation water is supplied in raised channels, and artificial reservoirs are often too deep and steep-sided to provide useful otter habitat. Also, when dams are constructed in semiarid environments, where the former riparian habitat was the only vegetation in the area, the newly created lakes are generally devoid of thick vegetation on the shores, and many years can pass before soil develops and riparian habitat regenerates. In some cases, this can fragment populations and disrupt free genetic interchange between populations.
Otters are shot and trapped for their pelts, in some countries legally, but in most illegally. They are also killed because of their supposed predation of fish and crustacean stocks, for instance at fish-farms through much of Europe, and aquaculture schemes in Asia and Africa.
The North American river otter is trapped legally in 27 states in the United States, and in 11 Canadian provinces. Over the years 1985 to 1988 inclusive, 27,658, 41,222, 44,060, and 37,247 skins respectively were reported in international trade (data from Wildlife Trade Monitoring Unit). However, hunting of the North American river otter is now managed by quota and may no longer be a serious threat.
South American otters were extensively trapped for their skins in most countries until the late 1960s, when hunting was largely banned. In 1980, the net trade in the Neotropical otter was 37,443 skins. Since then, there has been a rapid decline, to 157 in 1985 (Broad 1987). Paraguay was the largest source of pelts up to the early 1980s. Some 902 skins of the threatened southern river otter were reported in international trade in 1984. There is still some residual hunting of otters in a number of countries and otter products have been reported for sale in French Guiana and Uruguay. Hunting undoubtedly has been a major cause of the decline of otters in Latin America in the past and may still be locally important, since poaching is difficult to control and the price paid for a pelt is often many times higher than a monthly wage.
Illegal hunting for pelts may also be a problem in tropical Asia, and 3,558 skins of smooth otter were exported from Bangladesh in 1980. There were also reportedly 1,032 skins of hairy-nosed otter (Lutra sumatrana) traded in 1987 (1,000 of them supposedly from China). The current status of this species is causing great concern to the Otter Specialist Group. In general, however, there is still very little information on which to assess the threats of illegal hunting to otters in Asia.
Many otters are accidentally drowned in fish traps of various types. Fyke nets (conical nets often set for eels) are responsible for many deaths of the Eurasian otter; otters also die in crustacean traps. For a review of known numbers of mortalities in Europe, see Jefferies et al. (1984). Otter deaths in fyke nets can be prevented by the use of a “stop grid” at the net entrance (Madsen 1989). Lack of recent growth of the California sea otter populations is thought to be due to entanglement in coastal set nets (Ames 1987; Van Blaricom and Estes 1988).
Otters are killed on roads by traffic. Specific numbers are only available from Europe; for example, 26 of 50 recorded Eurasian otter deaths in West Germany (Heidemann 1981).
General human disturbance is likely to be a serious problem only where the habitat is inadequate. The Eurasian, North American river, southern river, and Neotropical otters inhabit lakes used for recreation and can be found in waterways well within the boundaries of cities. Their tolerance to disturbance seems to be related to the availability of suitable escape cover. Thus, habitat destruction and human disturbance interact in their effects on otter behavior.
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