Excerpts from 'The Mink' by Nigel Dunstone NATURAL PREDATORS It has frequently been stated that the mink has no natural predators apart from man. This may be the case in some countries where there are feral populations, but even here they are likely to receive some attention from raptors. In their native lands they are preyed on by wolves, foxes, coyotes and bobcats, hawks, owls and eagles. In Russia, wolves (Canis lupus), lynx (Lynx lynx), wolverine (Gulo gulo) and even otters are their principal predators. Eagle owls may be a significant avian predator. It is doubtful whether any of these predators have a significant effect on prey populations. POPULATION DENSITY AND POPULATION DYNAMICS Population density will vary according to several factors, for example, the carrying capacity of the habitat, intraspecific aggression and territoriality, and level of predation. Other variables affect the accuracy of the population estimate depending on the method used. These include variations in trapping pressure, trapper effectiveness and even weather conditions. The highest estimates of population density come from areas with abundant and stable aquatic habitats, swamps and marshes. Estimates can be widely variable from year to year. In Montana, Mitchell (1961) obtained a mink density of one per 11.8ha (280 mink in a 33km area), but the following year the catch was only of 109 mink (one per 30.3 ha). Other estimates range from one native American mink per 18.8 ha in Wisconsin to one per 625-909 ha for feral American mink in Sweden. Differences in the population density of mink between two areas can result from various factors including availability of prey and comperition with other predators, suitable den sites and social instability through persecution. The Fur Trade People have always hunted animals for their fur, to clothe themselves or to use as a commodity to trade. Nowadays there are many more practical substitutes for fur, and it has become a pure luxury. To maintain demand, its aesthetic appeal and symbolic extravagance is backed by a massive promotional campaign. In 1987, the sale of furs was worth some estimated $US2 billion dollars world-wide. Figure 10.1 shows the annual world-wide production of mink pelts from 1976 to 1986. THE EXPLOITATION OF WILD POPULATIONS OF MINK The exploitation of fur-bearing mammals was the incentive for much of the early exploration of North America. Trappers and traders penetrated all parts of the continent, and there are frequent conflicts between representatives of the fur companies from various European nations attempting to gain control of territories with good populations of fur-bearers and key trading routes. As long ago as 1670, the Governor and Company of Adventurers of England Trading in Hudson's Bay, later incorporated into the Hudson's Bay Company, began a trade in fur which they continued until very recently. Fur-trading is thus the oldest business institution in North America. The number of animals killed world-wide for their skins is appalling. The total, excluding seals, was estimated at over 237 million in 1977-1978; a country by country breakdown is given in Table 10.1. TABLE 10.1 World summary of animals killed for fur (1977-1978). Wild animals 38 743 415 Captive-raised animals 26 999 550 Domestic animals 237 325 000 Breakdown of wild animals killed for fur by country (1977-1978) USA 18 784 261a Canada 319 103 b Latin America 3 550 000 Europe 1 478 700 USSR (exports) 1 709 075 Asia 4 391 626 Australia 2 516 861 New Zealand 2 636 209 a - 1983-1984 figures for animals trapped in the USA, although incomplete, show 11 million trapped despite higher trapping pressure to meet increased demand. This is an indication of a 'limiting out' or local extirpation of fur-hearing species to meet demand for skins. Figures from International Association of Fish and Wildlife Agencies. b - Canadian 1983-1984 figure is 4394361 (WSPA). Source.- Nilson (1980). Wild mustelids constituted a substantial proportion of the furs traded from North America in the 18th and 19th centuries. Wild-caught pelts of mink and otter were important to the economy of the northern lands. Early explorers traded directly with native Indians for furs, but later white settlers and trappers, who came to America to make quick fortunes, became the major supplier of animal pelts. However, the initial rich plunder became a lean living as the animals were over-exploited. Often trappers had to move to even more remote places when the wildlife was depleted locally. As we have already seen, for the sea mink (Mustela macrodon), once common along the coasts of Maine, New Brunswick and Nova Scotia, over-exploitation probably led to extinction. Populations of many fur-bearers, particularly the larger and slower breeding mustelids were greatly reduced by the early 1900s. The reduction in numbers of pelts of these highly prized, economically important species stimulated much ecological research on population dynamics. This eventually lead to the imposition of closed seasons. All Canadian provinces and 47 American States allow at least a limited harvest of mink. Although few States conduct population estimates, the lack of a consistent downward trend in the number of wild mink pelts harvested (Fig. 10.2) is taken to indicate that the populations are not over-exploited. The USA traps more animals for their fur than any other country, with the possible exception of USSR. In 1977-1978 some 18.8 million individuals of 23 species were captured in the USA. In the present day only a small proportion of this trade is of wild mink. US fur harvest data indicate some 300 00 - 400000 mink are trapped annually with the bulk coming from Minnesota, Wisconsin, Alaska and Iowa.. In Canada, analysis of statistics for 1985-1986 show approximately 88 000 wild mink pelts with a value of $Can 3 million, in addition to nearly 1.5 million ranched animals being marketed. The distribution and harvest density of wild mink in Canada and the USA for the 1983-1984 trapping season is shown in Fig. 10.3. Most wild mink currently trapped in the USSR are from released populations of the American mink. It is likely that the demise of the European mink is directly attributable to over-exploitation for its fur. Following World War 1, there was a boom in the fur industry in the USA, when the wearing of animal furs became a fashion rather than a necessity. High pelt prices led to the severe depletion of wild mink, marten (Martes martes), fisher ( Martes pennanti), river otter and beaver (Castor canadensis) populations. This led to the acceptance of ranch-bred fur. ORIGIN OF RANCH POPULATIONS Mink raising is a relatively new industry compared with other branches of farming. The earliest attempts to raise mink in captivity date from the early 1900s in Canada. Initially the fur trade would not contemplate using pelts from captive-bred animals as the skins of wild-caught mink had denser underfur and better resistance to fading. This led to the experimental breeding of mink to promote better fur quality. From the outset, little attention was paid to the origin of the mink. The foundation breeding stock was usually trapped in the neighbourhood of the farm. Of the 11 subspecies of American mink, the three main contributors to the genetic stock are Mustela vison vison, M.v. melampeplus and M.v. ingens. Captured animals were fed, housed and kept in good health until they produced young. Three types of ranching have been attempted: extensive range, whereby the animals are kept under conditions as close as possible to nature within large enclosures with free access to water; the colony method, whereby many mink are kept together in large houses; and thirdly, the pen system, widely used today, where each mink is housed individually within the confines of a small cage and nest box. THE BREEDING OF MUTATION MINK Despite its intial disadvantages, the ranching of mink quickly developed into a prosperous industry. There are several reasons for this success, including the large litter size that can be reared in captivity, the short time to maturity and the relative ease with which colour varieties may be produced and maintained in the breeding pen. Native American are dark brown in colour. The original intention was directed towards the production of fur equal in quality to that of wild mink. Selective breeding was imposed to promote darkness of the fur and fineness of texture. As mink were reared in increasing numbers, freak animals or mutations began to appear. These mutation mink arose by the in-breeding of small captive populations, and cross-breeding of mink subspecies from a number of different geographic areas. The Silverblu or Platinum was the first colour variety to be noted in captivity, and is the aberrant colour variety most frequently noted in feral populations in the UK. Conceivably this colour mutarion may have also arisen in wild populations of mink in the USA, but none was captured alive for breeding purposes. The first recorded appearance of a Platinum mink occurred on a Wisconsin farm in 1931 and led to the establishment of the Whittingham strain of Silverblu Platinum mink. This was the first commercially important mutation mink, and the consumers enthusiasm for it was largely responsible for the intense competition to develop new colour varieties. The colour of the fur arises from pigment granules within each shaft of guard hair. These granules vary in size, number, shape, arrangement and in the intensity of pigment they contain. These slight differences (particularly in number and size of the granules) are wholly responsible for the plethora of colour varieties that form the basis of mutation mink breeding. The science of heredity (genetics) is the study of the transmission of characteristics from parent to offspring. In reality it is not the characteristics themselves that are innherited, but the genetic disposition to react in a certain way to the environment. For the mink farmer to predict the offspring he is likely to obtain as a result of crossing different types, it is necessary to know the ancestry of the parents, their genetic constitution and at least a simplified knowledge of the principles of Mendelian inheritance. In the USA most furriers are located in New York, but the business is in decline, contracting fron 2500 companies in the 1940s to some 400 in the late 1980s. This does not necessarily signify a reduction in the market, rather, pelts are being shipped around the world to take advantage of cheap manufacturing costs and then re-imported to Europe for sale. Mink farming is a huge world-wide production industry. Well over 23 inillion pelts pass through western European auction rooms each year (see Table 10.4). Relatively few animals are now reared in the UK; in 1981 the total production was 245 000 pelts from 67 farms. In 1992 there are just 27 farms still operative. Similarly, only a small proportion of coats are sold here. In a reversal of the traditional trade, North America is now the main consumer of European pelts. Russia produces in excess of 10 million pelts annually, the bulk of which are marketed and used domestically. TABLE 10.4 World production of mink in 1987-1988. Country Million Denmark 10.20 Finland 3.80 Sweden 2.00 Norway 0.51 Holland 1.60 France 0.55 Spain 0.25 ltaly 0.38 Austria 0.05 West Germany 0.35 Belgium 0.17 Great Britain 0.26 Ireland 0.10 USSR (export only) 4.10 East Germany (export only) 0.32 Poland 0.08 Czechoslovaki 0.05 China (export only) 3.10 USA 4.70 Canada 1.50 Japan 0.68 Korea 0.05 Total 34.80 Source: Mink Factories publ. Lynx and CIWF (1988). THE WELFARE OF RANCH MINK Assessing the welfare status of any species is difficult and often involves subjective judgement of what an animal requires. This itself is often clouded by what we would feel if we were confined under similar conditions. The application of such sentiments to other animals is referred to as anthropomorphism. The fur industry would argue that production, measured by the level of breeding success, is an adequate indicator of wellbeing, whereas anti-fur groups might consider mortality and/or the incidence of abnornmal behaviour as a more appropriate estimator of welfare status. If welfare is evaluated on the basis of longevity, breeding success and reduction of stress, then there is little to commend life in the wild, since wild mink live shorter lives, raise smaller litters and experience stress from periodic shortage in resources, for example, food, mates and the aggression-fraught business of acquiring and controlling a territory. If, however, you assess welfare on the quality of life experienced, then the wild animal's free access to the basic necessities of life, the possibility of satisfying its natural inquisitiveness and instincts to swim, hunt, procure a territory, and pass on its genes to future generations, must surely be an improvement over months and in some cases years of captivity. Few guidelines have been published on the welfare of ranched \plain\f4\fs20 mink\plain\f4\fs20\i , \plain\f4\fs18 and opinions vary considerably on housing requirements and management regimes. None appear to be based on sound behavioural or physiological studies, or take any account of the ecological data concerning wild mink such as that presented in this book, even though much of this information has been published for some considerable time. However, care must be taken in extrapolating standards from the wild animal to its caged counterpart. Who is to judge if the welfare of the wild animal is good? Few attempts have been made to intensively breed and rear carnivores. Only mink, ferrets (fitch), sable and silver foxes have been successful. One of the main arguments against fur-farming is that the mink is, at best, incompletely domesticated and hence its welfare requirements are considerbly different than those of familiar domestic animals. Mink differ from such animals in two fundamental ways. Firstly, they are solitary animals, whereas most domestic stock are derived from gregarious species and remain so. Secondly, they have been in captivity for a comparatively short time, some 60-80 generations since capture from the wild. The nature of the selective breeding that has been carried out also differs. The strong selective pressure can leave little scope for the concurrent selection of those animals which behave most suitably in their new restricted environment. There seems to have been little attempt to breed for docility or lack of aggression towards conspecifics-common features of most domesticated species. The Farm Animal Welfare Council (an advisory group to the UK govemment) has listed five basic needs for all livestock kept in captivity: i) freedom from thirst, hunger and malnutrition, ii) appropriate comfort and shelter, iii) the prevention of, or rapid diagnosis and treatment of injury, disease and infection iv) freedom from fear, v) freedom to display most normal patterns of behaviour. To a certain extent, the animals best interests are also those of the farmers. We should hope that conditions (i) and (iii) are catered for if the farmer is not to lose his animals. In most instances ranch mink are protected from environmental hazards including climatic extremes, are fed and watered and given treatment for ailments. The remaining requirements are more difficult to assess. CONFINEMENT Virtually all of the mink's activity in the wild is dictated by its requirements for survival and reproduction. Labours of necessity they may be, but the dilemma of the welfare debate is how much of this activity and stimulation is also required to maintain a sense of well-being, to avoid boredom and disorientation. If physical requirements are catered for on a farm, does the animal then show a reduced need for activity? It is unlikely that there is a biological requirement to swim, but caged mink show considerable levels of locomotor activity, frequently including abnormal behaviours such as pacing arid circling. Some time ago we conducted some preliminary experiments involving supplementing the diet of a free-ranging, but radio-collared mink by providing rabbit carcasses for his consumption. This provision of an excess of food had little effect on his nightly wanderings even though he no longer needed to leave his den to forage. In one respect at least the ranch mink resembles the wild animal. Both show a low level of overall activity, perhaps 2-4 h per day. The rest of the time is spent asleep or resting. The nature of the activity shown is very different and the reasons for the inactivity of the two types of animals is probably different, energy conservation on the one hand compared to boredom on the other. The largest cages encountered on mink farms rarely exceed 30 wide x 45 high x 90cm long, and many are considerably smaller. Even this size of cage is likely to be used to house more than one individual. It is indeed remarkable that an animal that, in its native land or in feral populations, roams over kilometres of riparian, lacustrine or coastal habitat will successfully breed under such conditions of confinement. The fact that escapee animals so readily adapt to an alien countryside, and to differing species of prey and climatic conditions indicates that man's attempted domestication has had an almost negligible effect on this species. Perhaps ironically, the saving grace of fur-breeding is that most of the animals do not have to endure this confinement for long, since the maiority are killed and pelted at the age of 7-8 months. ISOLATION Given the solitary lifestyle adopted by wild mink outside of the breeding and kit-rearing seasons, it has been argued that the presence of conspecifics in close proximity will be stressful. Certainly all ranched mink appear less stressed if they are given a nest box into which they may retire. Separation of females by visually opaque screens reduces weight loss during pregnancy, indicating that the sight of another individual may be stress-inducing. It has also been noted that adjacent animals adjust their activity patterns so as not to be simultaneously active. Various experiments have been carried out to investigate the effect of visual isolation on breeding success, and these seemed to indicate that females so kept, raised larger litters (Gilbert & Bailey 1969, 1970), although more recent experiments (Hoffmeyer and Moller 1986) have been somewhat contradictory. Perhaps the intervening 20 years of 'domestication' has had an effect. The separation of the kits from their mother should be to a different part of the farm rather than to adjacent cages, thus reducing stress in this instance. Given the mink's dependence on olfactory stimulation in detecting and even recognizing conspecifics, the overwhelming aroma (!) prevalent on a mink farm could also induce stress. The same is probably true for audition. Certainly smell and hearing predominate in the mink's sensory world. BEHAVIOURAL DISORDERS The incidence of behavioural disorders is suggestive of a welfare problem. The Commission of the European Community (CEC report 1983) details four types of abnormal behaviour: (i) injurious behaviours, (ii) stereotyped behaviours, (iii) abnormal body movements, (iv) apathetic behaviour. Caged mink can exhibit all of these (Jonge et al. 1986); needless to say wild mink (feral or native) exhibit none. Particularly evident are stereotypic running up and down the cage and circling behaviour which occurs just before feeding time. The occurrence of injurious behaviours, of which the most common are tail sucking and tail biting, can be frequent. CONGENITAL DEFORMITIES Post-mortem analyses of 1249 mink kits which were either stillborn or died within 1 day of birth revealed 89 (3.8%) to be deformed. Of these, 70% suffered from dropsy, 20% from hernias (cerebral or umbilical), 7.9% from cleft palate or hare lip and 1% from cyclopia. It is impossible to get comparable information from wild populations. DISEASES On mink farms it is extremely important to minimize the incidence of diseases and parasites, particularly since even if death does not occur, the animal's loss of condition may adversely affect the fur quality. Mink are healthy vigorous animals; they rarely get sick if they have adequate food, water and clean conditions. Because of their 'dirty' habits, and the manner of feeding, ranch mink are prone to botulism unless extreme care is taken with the origin and treatment of foodstuffs. Outbreaks of botulism can wipe out an entire herd. Nutritional diseases include steatitis, caused by high levels of unsaturated fatty acids and a deficiency of vitamin E. Wet belly (urinary incontinence) is probably caused by a mineral imbalance and can ruin the pelt. Chastek's paralysis is an interesting disease, caused by the presence of an enzyme in certain species of fish (carp), which when included in the foodstuff can result in the destruction of vitamin B (thiamine). Blue colour varieties of mink, including Aleutian and Sapphire, are susceptible to a viral disease, Aleutian disease, so called because the original losses were only recognized in these varieties and the disease was thought to be hereditary. Now it is known that these colour varieties have a greater susceptibility to the disease. Females pass on the disease to the unborn kits in the uterus. This has been possibly the most economically important of the mink ailments. Nowadays ranchers test for the disease and by culling infected animals have largely bred it out of the stock. Mink are commonly innoculated against viral enteritis, botulism and distemper. \par In the 1940s and 1950s fish from the Great Lakes was a prime source of protein for local mink farms. During the middle 1960s they found that they could no longer use this cheap abundant source because it resulted in poor production and high levels of kit mortality. Analyses of Coho salmon from Lake Michigan revealed 15 ppm of poly-chlorinated biphenols (PCBs) from pesticide production. Laboratory tests showed mink to be extremely sensitive to PCBs; 30 ppm was lethal to adults, 15 ppm caused reproductive failure, and even 10 ppm depressed the growth of kits. MORTALITY IN CAPTIVITY Mink can live a long time in captivity. I kept one animal, which was used in behavioural experiments involving swimming and diving, for 8 years. Most ranch mink are pelted at 8 months, but a few breeders may be retained for two or three breeding seasons. The `natural' lifespan of farmed mink is not known. Kit mortality seems to be high, with perhaps 9-12% being born dead, or dying within the first 3 weeks of life. The mortality of the offspring of first-year breeding females was greater than for older breeders. THE ETHICS OF WEARING FUR The ethical argument against wearing fur is that it is an unnecessary luxury product, for which no animal should be trapped or farmed and killed. Some who reflect upon this subject for the first time will wonder how such cruelty can have been permitted to continue in these days of civilisation, and no doubt if men of education saw with their own eyes what takes place under their sanction, the system would have been put to an end long ago. Long ago has become long hence, and still the issue is unresolved, often clouded in prejudice and misunderstanding on both sides. The trapping of wild animals for their pelts is impossible to justify in terms of conservation or animal welfare. Over 80% of America's wild-caught fur-bearers are taken in steel-jawed leg-hold traps. For mink this represents some 300 000 annually in the USA alone. Very few trappers are 'professionals' deriving their livelihood from trapping. In the USA, some 86% of trappers are under 20 years old. Weekend trapping by townspeople is also common. Leg-hold traps suitable for mink are relatively cheap at around $U55 each, and can be easily obtained from stores or mail-order companies. The use of the leg-hold trap has been banned in some 64 countries, including Britain, because it is cruel and indiscriminate, but only a handful of North American States have even restricted its use. For every mink coat (comprising some 65 skins), it is estimated that some 180 'trash' or 'non-target' animals will also have been killed. Animals caught in leg-hold traps may often seem to suffer only minor injuries; however, X-rays show them to suffer bone fractures, torn muscles and ripped tendons. Frequently they chew off their own limbs to escape. Atkeson (1956) reported 27.6% of 209 trapped mink 'wringing-off' or 'chewing-off' their own limbs or escaping with the trap still attached to their limbs. Most trap-held animals will damage their teeth in trying to escape from the trap. Broken teeth, gum and jaw damage are common in such animals, The use of rubber-lined leg-hold traps, have been advocated by the fur-trade, but these seem to be infrequently used, and their benefits regarding the welfare of the trapped animal are probably negligible. To many people the ranching of fur is unacceptable because the animal is raised for its pelt alone. Other domestic species, farmed under equally intensive conditions, do provide subsidiary compounds. Only occasionally are additional products made available from the mink farm. Lanolin rich mink oil and a variety of compounds used in perfumery are available. The mink carcasses may be used in the production of fertilizers, but are more commonly incinerated or tipped at land-fill sites. In any case, the argument that such an industry would be justifiable if it produced items of utility as well as luxury would seem rather thin. The industry would not exist but for the fur, and the nature of additional by-products is as irrelevant to the mink farmer as it is to the mink! Another argurnent in defence of fur-farming is that the demand for natural fur is insatiable, and the humane ranching of mink would relieve the wild population of some trapping pressure. However, we must consider the extent to which supply affects demand, whereby promotion of farmed fur products ensures a buoyant market for trapped pelts also. At the moment there is very little legislation applying to the manner in which mink are housed. Training guidebooks for fur farmers emphasize production rather than welfare. In the UK, the Fur Breeders Association has produced a Code of Conduct for their members. This has now been adopted by a number of European countries. Unfortunately, not all breeders are members of the Association and not all members seem to adhere to this voluntary code. For many years there has been considerable debate over the ethics of wearing animal pelts. From a practical standpoint, synthetic furs may have some advantages over real fur garments. They are moth proof, water repellent and considerably less expensive to buy, yet aimost as warm, although possibly not as long lasting and durable as the real article. However, few mink coats are worn for their warmth and durability alone. On a world-wide scale the propaganda campaigns of the anti-fur lobby are beginning to bite. Despite this, the market for furs in some countries continues to increase in line with their growing affluence and 'westernized' ideals. As a result, global pelt production is rising. It is difficult to say when saturation point will be reached, as any drop in price due to over-production brings the product within the reach of new consumers who had never before considered buying them. Public opinion is rapidly changing at least in the UK and some other countries in Europe. In a survey of 2000 people conducted on behalf of Lynx (a UK based anti-fur campaigning group), 71% agreed (49% strongly, 22% slightly) that 'it was wrong to kill animals for their fur to make clothing'. Over 70% agreed that there should be a complete ban on the trapping and the rearing or farming of animals for their fur. INTERACTIONS WITH MAN AND OTHER ANIMALS Our children of tomorrow could curse us if they can never see a rabbit, an otter, or any other species of British mammal or waterfowl in the British countryside, which could be the mink menace is not eradicated now. Express and Echo (31 October 1979) Such was the fearsome reputation commonly held for the immigrant mink in Europe, and particularly in the UK. Although now largely discredited, enmity towards the mink is deeply ingrained, and many people would still sympathize with the sentiment expressed above. Feral populations of animals often cause severe ecological problems, but it is the economic damage resulting from their effect on man's activities that usually attracts public attention. Many of the mammal species in the UK which are considered to be economic pests are also introduced species that have become feral. The Destructive Imported Animals Act (1932) prohibits the unlicensed keeping of specific non-indigenous species and empowers the Ministry of Agriculture Fisheries and Food to destroy such animals when they are found at large. Efforts at eradicating feral populations of mammals in Britain have been successful with only three species: the muskrat (Ondatra zibethicus) which established colonies in Perthshire, the Welsh Borders and Surrey/Sussex in the 1930s; Himalayan porcupines (Hystrix brachyure) in Devon in the 1970s; and coypu (Myocastor coypus) in East Anglia in the 1980s. It is now some 60 years since American mink were first imported to Europe for farming on a commercial scale, and just over 18 years since they were first recorded breeding in the wild. Despite early doubts concerning the viability of feral populations and a Ministry of Agriculture scheme to eradicate them, there has been a dramatic colonization of a large proportion of the waterways in the UK. The pest status of the mink has been the subject of vociferous scientific debate for nearly 10 years. The landowners' view was eloquently expounded by Sir Christopher Lever in an emotive, but anthropomorphic article published in the popular scientific press (Lever 1978a) which was largely based on broad generalizations, but was widely quoted and singularly effective in provoking antagonism to the mink. The mink's corner was defended by Linn & Chanin (1978a,b) of Exeter University, authors of numerous scientific articles on the biology of this species. Until recently there has been little critical assessment of the extent of damage caused by this exotic carnivore, and much of what has been said refers to anecdotal accounts of their alleged depredations on domestic livestock, and perpetuates the various misconceptions concerning their biology. Savage disruptive alien or well-integrated immigrant, the mink is here to stay. No amount of anger or enmity will empower us to eradicate it now. So it is time to cast off emotive reactions and come to understand the new balance being struck in our riparian community. EXPLODING THE MINK MYTH One of the most notable misconceptions relates to the mink's breeding biology, reputed (incorrectly!) to compare well with that of the rabbit. The often reported 'plagues of mink' are usually the result of over-zealous trapping effort drawing in mink from adjacent territories. Mink are multiplying tremendously, and if there is not a concerted effort, they will reach epidemic proportions. What we need is concerted action. I am surprised that conservationists will not help because mink are decimating flora and fauna as much as road-side spraying. Daily Telegraph, September 1982 A plague of mink is inconceivable They are solitary, territorial animals whose intolerance of other mink will always ensure a low population density. Only at two stages in their life history do mink show any resemblance of social behaviour. Firstly, mink may consort for a limited period during the mating season. Their curiosity and wide-ranging behaviour in search of females leads to many animals being captured at this time giving the illusion of a large population. The second occasion, for another short period during July and August, occurs when the female may be seen with her litter of the year in attendance just prior to their dispersal. At this time of year, there is only a short period of darkness which she can use to provide an adequate supply of food for her rapidly growing litter. Consequently, hunting is often diurnal and the sight of three or four well-grown animals hunting a river bank or pool will undoubtably cause a river-keeper's heart to palpitate. Naive and curious, dispersing mink are easily caught, again giving the impression of 'plague' proportions. Mink regulate their own numbers according to the availabilitv of suitable habitat containing adequate prey through the mechanism of territoriality. There does not have to be a natural predator, their aggressiveness to members of their own kind is more than adequate to control the population. Newspaper reports (see Fig. 11.1) concerning mink activity show a good chronological correlation with these significant events in the mink's year which affect its visibility to man more than its level of predation. Every spring and late summer the mink biologist must steel himself to meet the wave of ill-conceived publicity regarding the status of the feral population. A major cause of the misconceptions is due, as Birks (1990) points out, to the 'scapegoat effect', whereby the media demand simplified answers to complicated ecological questions. The mink's true story does not translate well into journalese, and mostly it is not what the readership wants to read. Something or someone must take the blame, and who better than the mink. To evaluate the pest status of the mink, we need to assess firstly the extent of any significant economic loss or damage to domestic stock, the impact on native wildlife through predation or competition, and the nuisance value of mink to the general public. Secondly, so that a policy for the management of feral mink can be suggested, we need to assess the numbers, fecundity and dispersal capabilities and the potential for their control. Two additional points need to be considered when assessing pest status from dietary studies. Firstly, population studies on prey have tended to be carried out retrospectively, usually some time after the presence of mink has been determined. Secondly, most studies have averaged the dietary information across individual mink in the population, and over a long period. This may have the effect of masking depredations which occur in the short term or by particular individuals. PREDATION ON NATIVE FAUNA The results from dietary snudies have already been presented. We need only consider here those prey species which have been identified as being potentially at risk from mink, such as ground-nesting birds (ducks, moorhens, coots and seabirds), or have some commercial value (poultry, gamebirds and salmonid fishes). Mink and salmonids \par \plain\f4\fs18 Concern is frequently expressed over the effect of mink on salmonid stocks (e.g. Lever 1985), but it is unlikely they do more than check the surplus in those places and seasons of abundance. It should be pointed out that the daily intake of mink is small relative to the 1-1.5kg of fish needed to sustain an otter (Erlinge 1968). Otters, fish and fishermen seem to have co-existed happily enough in the past. By comparison, the mink is an opportunistic fisher, and not well adapted for swimming and underwater pursuit (see Chapter 4). The dietary studies in which predation on salmonids was found to be important, typically oligotrophic rivers, tend to be in areas where alternative prey were uncommon (Akande 1972, Cuthbert 1979, Chanin & Linn 1980). If the fish themselves become scarce, the mink is more likely to redirect its hunting efforts towards terrestrial prey. \par Little supportive evidence has been found for any deleterious effect on fish stocks and angling interests. Chanin & Linn (1980) examined records of commercial salmonid net fisheries in the Teign estuary (one of the first rivers to be colonized by feral mink, and subsequently subjected to considerable research effort). From 1951 to 1968 migratory sea-trout catches increased four-fold, while salmon catches changed little over the same period. A decline in the numbers of salmonids thereafter was precipitated by salmon disease (UDN, ulcerative dermal necrosis). No changes in the catch by netsmen or rod fishermen could be attributed to the increasing mink population. Linn & Chanin (1978a) rightly comment on the confusion in the popular press between killing of individuals and the depletion of populations. Fish stocks have a remarkable ability to respond to depletion because of their great fecundity. Pressure on the scale imposed by a relatively small number of mink is likely to be negligible. On the upper reaches of the oligotrophic River Teign, brown trout were too numerous for the available food supply. The mink's cropping of the resident brown trout population may be beneficial to fisheries interests by allowing the remainder to grow to a greater size. The only coarse fish species recorded in any quantity in the diet of mink has been the eel (Anguilla angiuilia) at 11-23% (Cuthbert 1979, Wise et al. 1981). The removal of eels, a predator of fish eggs and a favoured food of mink, may only serve to improve game fish stocks. Mink and waterfowl Ground-nesting birds such as waterfowl have been shown to be at risk during their breeding season and are preyed on in marsh and wetland habitats in the USA (Sargeant et al. 1973, Eberhardt & Sargeant 1977); although here the situation was complicated by loss of nesting habitat which had the effect of concentrating the mink's food. In the British Isles, waterfowl do not appear to be as important. Yet these prey groups have also been repeatedly suggested (Lever 1977, 1978a,b, 1985) as being major victims. It should be added that ornamental ducks on a small pond, especially when pinioned and kept at high density in the vicinity of streams or rivers will be, not unexpectedly, particularly vulnerable. Artificial, high-density collections of exotic wildfowl, such as those exhibited by the Wildfowl and Wetlands Trust in England can suffer badly from the depredations of mink. Inevitably waterfowl are more at risk from mink predation than any other group of prey. They are ground nesting and found precisely where the mink is likely to encounter them in its riparian wanderings. The brunt of the predation seems to fall upon moorhens, coots and ducks, and the British Trust for Ornithology has circumstantial evidence that their numbers have been suppressed on some mink-infested waters. The important fact is that the effect is local: nowhere in the UK have mink caused widespread population declines. Counts of overwintering waterfowl and wading bird species, coordinated by the Wildfowl and Wetlands Trust since the 1960s, show no signs of a national decline that can be attributed to mink predation; indeed most species of waterfowl have increased. In north-east Iceland it has been reported that the waterfowl population on Lakes Myvan and Vikingavatn had decreased from 50 000 in 1961 to 15500 in 1974. Although this was originally attributed to mink, doubt has been cast on' this aspersion in recent years. What is undoubtably true is that there are a wide range of reasons for the decline of many of our waterbird populations, including drainage of wetlands, riverbank clearance, pollution of rivers and estuaries and increased levels of disturbance resulting from increased leisure activities. Such habitats become sub-optimal, yet because of the paucity of alternative breeding sites, birds have nevertheless to use them, often breeding at higher density than is normal and so become more vulnerable to predation, not only by mink but also by otherground-based predators. Mink and sea-birds The mink's predation on colonial ground-nesting sea-birds appears to be one area where considerable damage could be caused. Some features that increase their vulnerability are the concentration of breeding activities for regional populations at a few, heavily used sites, poor defensive behaviour due to the previous lack of terrestrial predators, and the extent to which harassment can upset normal breeding behaviour resulting in reduced breeding success in addintion to the numbers of direct kills. A population of the black guillemot (Cepphus grylle) has been sharply reduced since 1976 following the arrival of mink on the Soerskar peninsula of Finland in the summer of 1974. The birds occupied an offshore island. In 1974 and the following year, damage attributed to mink was small, but in 1976, 42% of the breeding population of 371 pairs was lost. In 1977 no mink reached the island, but in 1978, 41% of the 328 nests were predated. In 1981 only 167 breeding pairs remained. Similar allegations have been made in Iceland concerning losses of breeding sea-birds including shags, common gulls, common and arctic terns, black guillemots and puffins. In northern Norway the decapitation of kittiwake chicks has also been blamed on a marauding mink. In North America the entire reproductive output of one colony of spotted sandpipers (Actitis macularia) \plain\f4\fs18 was wiped out in one incident (Oring et al. 1983). In other instances, after an initial period of destabilization, some resolution has been reached. Mink have been reported to cause considerable damage at breeding colonies of common terns (Sterna hirundo) on small islands off the western coast of Scotland, with the result that breeding has failed, and in some cases, colonies deserted. The birds' response has been to re-establish their colonies on islands further offshore where, for the moment they are immune from the mink's attack (Craik quoted in Birks 1990). Between 1972 and 1974, Gerell found eider ducks and gulls in Sweden suffering severe predation from mink, causing them to desert their nests and to abandon subsequent breeding attempts. But in the following years a balance became became established with evidence that the birds had learned to mob the predator (Gerell 1985). \par A small offshore island became an interesting feature of our study area in southern Scotland (Dunstone & Birks 1983, 1985, Ireland 1990). The 10ha island was used by a colony of herring gulls (Larus argentatus), but was also the exclusive territory of a female mink. Over the three breeding seasons we observed her activities, her food requirements and those of her kits before dispersal were met by 'predation' upon the gulls. Sea-bird carrion was plentiful on the island and was used by the mink, and a considerable quantity remained untouched by her at the end of the breeding season. In such a case, it seems that the mink's territoriality prevented heavy exploitation of the birds. Surplus killing would be unlikely by a mink habituated to colonial prey. At least with some sea-bird species, a healthy balance seems possible. Mink and mammals Large mammals are by far the most economic prey for the mink, and lagomorphs, particularly rabbits, are always taken if available. In five of the ten studies conducted on the minks' diet in the UK, the most frequently taken item was lagomorph prey. Rabbits, when common in the study area, are the most heavily exploited mammal, and often the most important food overall for their bulk contribution to the diet (Jenkins & Harper 1980, Wise 1981, Dunstone & Birks 1987). Most mammal species taken by mink are abundant, and in the case of the rabbit and the rat, are pest species themselves, and unlikely to provoke concern. There are two species which have to be viewed in a different light. In Spain, feral mink are thought to be responsible for the decline of the European desman (Galemys pyrenaicus), a small insectivore resembling a large water shrew that inhabits tumbling mountain streams in the Pyrenees. While the species undoubtedly does fall prey to mink, populations of this animal are still found in areas where mink abound. A far greater threat to the desman is posed by the flooding of the river valleys for hydro-electric schemes. Here also, widespread habitat modification has led to the two species being forced to live in close proximity. The second species for which the depredations of mink might pose a significant threat is the water vole (Arvicola terrestris). This prey approaches the optimum size for a mink and is readily accessible in the riparian niche. Populations of this semi-aquatic rodent have declined in the last decade over much of England and Wales. Female mink, at least, are small enough to enter the vole's burrows. However, only rarely have their remains been found in the scats of mink, and then at low frequency, (2-4% occurrence, Chanin & Linn 1980) even though both species live in the same habitat. There can be little doubt that the mink has played a role in the later stages of the decline of water voles, but it is likely that habitat destruction, human disturbance and river pollution, particularly from organochlorine pesticides, have played the major part. \par A full investigation of the status of the water vole is currently being undertaken in the UK. Most sites previously known to hold water voles still do, and in many mink are also present. There is evidence that the water vole may be adjusting to the presence of mink by showing a preference for small streams, brooks and ditches rather than the banks of larger rivers. Such areas would be visited less frequently by mink as they support a lower density of prey (Birks 1990). PREDATION ON DOMESTIC STOCK The occurrence of prey remains derived from domestic animals in the diet of mink is an extremely rare occurrence, even in those studies which have involved mink populations living in the vicinity of farm buildings and game-rearing pens (Dunstone & Birks 1987). Studies conducted over long periods and involving large samples of scats, show that poultry and game birds generally make up less than 1% of the diet; the highest recorded was 5.4% (e.g. Cuthbert 1979, Chanin & Linn 1980, Dunstone & Birks 1985). Initial fears about the importance of mink as an agricultural pest were exaggerated. Nevertheless, mink do take game birds; Fairley (1980) reports one female mink being shot in a pen of dead pheasants, and of feathers from game birds occurring in some mink stomachs. Similarly, Ward et al. (1986) report the occurrence of poultry feathers in 0.2% of their scat sample. Birks & Dunstone (1985) also report the occurrence of single partridge (Perdix perdix) and pheasant (Phasianus colchicus) carcasses in the dens of mink. Nevertheless the predation of mink on Galliforme prey species is often presumptive. The most disturbing reports of stock damage involve mass killings of confined animals. Many carnivores exhibit the phenomenon of surplus killing under conditions where prey are encountered at high density and cannot escape from the predator. The phenomenon is not unique to mink, but tends to receive considerable publicity when it does occur. Surplus killing incidents are unusual in the wild, but have been reported from North America (e.g. Errington 1961). Here it is interpreted as an adaptive trait, conferring an advantage upon opportunistic predators which exploit food resources of varying abundance and avaiIability. Recently, Harrison & Symes (1989) have attempted a rather different approach to assess the economic pest status of mink. During the periods of survey, detailed records have been kept by the UK Government's Ministry of Agriculture, Fisheries and Food of the reports from landowners and farmers in south-west England of the damage attributed to mink. Although not exhaustive or particularly rigorous, such an analysis can provide useful information. The surveys include some 108 reports of damage during the period 1961-1970, and another 8 during 1985 and 1986. The killing of poultry and domestic waterfowl amounted to some 60% of incidents reported (Fig. 11.2). Of these most involved small numbers, from one to five kills, but exceptionally as manv as 40 chickens were killed in a single incident (Fig. 11.3). Mink predation on poultry occurs with a higher frequency than that by foxes, badgers or other mustelids. However, it seems that incidents attributed to mink are more likely to be reported. Given that an individual laying hen is worth about £32, individual incidents are relatively trivial in terms of economic damage. Most UK poultry production is intensive, and takes place in closed buildings where, if they are adequately maintained, the birds should be safe from the depredations of mink. The recent public demand for free-range products is likely to lead to increased vulnerability to mink predation. MAFF also recorded a number of kills of young pheasants kept in release pens, some involving in excess of 100 individuals. Similar instances have been reported in America, particularly in situations involving the arificial, usually high-density, propagation of wildfowl. Predation at trout farms Given the aquatic habits of mink, the high incidence of fish in the diet of native and feral animals, and the fact that fish farms are by necessity located close to waterways, it is not surprising to find that such enterprises receive frequent visits from marauding mink. Harrison & Symes (1989) conducted a questionnaire-based survey of a sample of 75 fish farms. Of those contacted, 46% had experienced visits by mink. However, as far as the farmers were concerned herons (Ardea cinerea) posed a far more serious predator problem. The type of damage attributable to mink ranged from direct predation and damage to fish, to damage caused to underwater holding-cages as the mink attempted to gain access to the confined prey. Fish have also been reported to die from stress or panic during attempted capture by mink. Damaged fish, caused by bites and scratches, were more likely to succumb to fungal infection. The size of trout taken varies from small fingerlings to fish up to 0.5 kg. One farmer claimed to have lost 500 trout of 50-75 g in just 1 month; in the following month he trapped 40 mink! This incident occurred during August when juvenile mink would be dispersing. Such incidents should be rarer at other times of the year when the farm would lie within the territory of one animal whose food requirements will be considerably less. INTERACTION WITH NATIVE CARNIVORES Competition between mink and otters Competition occurs where two species are using a resource, such as food type or den, which is actually or potentially limiting. There is little reason to believe that feral populations of mink should not successfully co-exist with the otter. This is after all the case of native American mink with the river otter (Lutra canadensis) in the USA and Canada, and the European mink with the European otter in Russia and parts of Europe. Complete competitive exclusion is evidently not occurring. In the case of competition for food between the mink and otter, the shared prey resource will usually be fish, of which otters are the considerably superior predator in terms of hunting efficiency. The otter is a fish specialist, whereas the mink is a generalist and will take terrestrial prey if it is more readily available. If competition for fish became intense, the mink would be the first to back out. North America Observations of sympatric river otter and mink populations in North America have revealed differences in their foraging behaviour. Otters usually forage from the water for their prey, even if it is terrestrial in origin. In contrast, mink often forage for terrestrial prey while travelling along the shore. Aquatic prey are usually taken by diving into the water after sighting it from an out of water vantage point (Melquist et al. 1981). Analysis of 657 mink scats and 1902 river otter spraints from west-central Idaho showed fish were the commonest prey for both predators (mink 59%; otters 97%), but whereas mammals, birds and invertebrates were also important prey for mink, these were of negligible significance to the otter. The predominance of fish in both diets suggests a high degree of overlap, but this was belied by closer examination of the prey (Fig. 11.4). The greater body size and better adaptations for underwater hunting possessed by the otter allowed them to forage on a more extensive range of generally larger fish than did mink. For example, large-scale sculpins (Catostomus macrocheilus) of 35-45 cm were commonly taken by otters but not predated by mink. Mottled sculpins (Cottus bairdi) were thought to be less available to mink because of their bottom-dwelling habits. The greatest amount of overlap occurred in the use of cyprinid fishes, redside shiners (Richardsonius balteatus) and speckled dace (Rhinichthys osculus) \plain\f4\fs18 7-12 cm long. Kokanee (Oncorhynchus nerka), a migratory salmonid, were of considerable importance to otters (43% of fish taken) during spawning runs, but were infrequently taken by mink, at least early in the season. Later, after they had spawned, spent fish were taken by mink in shallow water or as carrion on the shore. Europe Estimates of dietary overlap of 40% between otter and feral mink have been demonstrated on a eutrophic lake and a moorland river in south-west England (Wise et al. 1981). In contrast to North American studies, otters and mink showed a lack of size selection when preying on fish. Fish constitute the bulk of the otter's diet all year round, but fish predation by mink was seasonal. Thus dietary overlap was greatest in autumn and winter, when fish were easier to catch in cold water. (Fig. 11.5). Greatest competition might be expected to occur during the summer months when fish were less available. At this rime it is generally noted that mink are preying heavily upon rabbits, if available locally. In those areas where otter diet was studied before the introduction of mink there has been no subsequent change recorded in the diet of the otter (Chanin 1996) suggesting that competition is not occurring. On a Swedish river, dietary overlap ranged from 50% in summer to 70% in winter when some foods became unavailable because of ice cover (see Fig. 11.6), Erlinge 1972). On Lake FIisbysjon there was a lower density of mink than would be expected, possibly due to interference from the large population of otters present. During winter the ice cover on lakes and rivers, which may last for 3-5 months, substantially reduces the number of aquatic areas that can be hunted by either predator. This occurs at a time when fish prey are a main dietary item for both predators. Competition for prey would appear to be strong in this case, and it is significant that the mink is the species more limited by it. In summary, dietary overlap between mink and otter in the use of fish prey will not be critical uriless the resource is limiting, as it might be in a suboptimal habitat, for example impoverished oligotrophic strearns. Even here it is likely that the mink will suffer more than the otter because it is the poorer aquatic hunter. If available, mink will then switch to terrestrial prey. In some studies competition was avoided because the different sized predators took differing prey species. This again works to the advantage of the otter, since it can take both small and large prey, while only smaller fishes can be taken by mink. Concern that mink may prey on otter cubs seems to be without foundation. Indeed, Grigor'ev & Egorov (1969) reported that the otter in Russia 'is a serious enemy' of the mink. Analysis of 880 otter spraints revealed remains of six mink. Similarly, Novikov (1956) reports the otter to 'vigorously hunt mink' in Russia. Since the two species inhabit similar aquatic habitats and are both secretive in nature, spending long periods lying up in dens, it might be expected that there would be competition between them for suitably secure sites, especially in areas where these are in short supply. In North America it has been reported that both species will use the same log jams as den sites (Melquist et al. 1981). Mink, because of their considerably smaller size, were able to live in places inaccessible to otters. Mink used a variety of structures including log jams, stream bank, brush and debris, rock crevices and beaver lodges, whereas otters only used the first two types. The use of log-jam dens by mink peaked in December when use by the otter was at its lowest, but it was not thought that this was a response to the absence of the otter. Simultaneous radio-tracking of mink and otter was achieved on 65\plain\f4\fs18\i days, mink and otter were observed to be foraging at the same time in the same log jam on 18 of those days. Although mink appeared to be using drier parts of the log jam while the otter foraged amongst logs in the main river channel, there were frequent occasions when the two species were recorded active within 5 m of each other. During 21 days of monitoring, mink were active at times when otter were resting in the same log jam or vice versa, often moving in close proximity without disturbing the other. The two dens most frequently used by otter and mink were within 3 m of one another. To what extent the resource partitioning observed in this study is due to the presence of a unique and massive log jam is not known. In Britain also, mink have been found occupying the same holt site as the otter. Indeed, on one occasion a mink hunt flushed a mink and an otter from the same stickpile on the River Creedy in Devon (Green et al. 1986). Decline of the otter That mink were blamed for the decline in otter populations in the 1960s and 1970s is not surprising. The vicious alien predator was fast spreading along our rivers at a time when the much loved otter appeared to be in equally fast decline. From the subsequent studies outlined above, there is little evidence to support the contention that mink have adversely affected the status of otters. The view is, however, well entrenched in mink mythology (Lever 1985) and frequently one reads that 'as the mink moved in, the otters gradually disappeared'. A number of causes have been considered for the decline of the otter, including mink (Chanin & Jefferies 1978). The abundance of otters, as estimated from the hunting records of otter hound packs, generally showed an increasing population up until 1956. The fact that the decline took place so quickly and simultaneously across the country, led to a feeling that it must have been brought about by a change in circumstances occurring in about 1957. Hunting itself has been suggested as the cause, but analysis of records show a remarkably constant rate of killing, up until a voluntary ban was imposed by the Masters of Otter Hounds Association when they became aware of the severity of the problem. Although deaths as a result of hunting did not cause the decline there was evidence to suggest that in areas of a large cull, the decline in the otter population was greater and undoubtedly led to a greater pressure on the otter. In many countries riparian habitat destruction, partacularly for enhanced drainage schemes, and increased disturbance brought about by an increase in leisure activities (fishing and boating) may have been involved in the decline. However, there was no evidence to suggest any difference in these activities before and after 1957. The timing of the otter's decline however, links it more closely with the introduction in 1955 of organochlorine insectides, dieldrin and aldrin, for use in sheep dip and as seed dressings. Many of the uses of these pesticides in agriculture were banned in the early to mid l960s when the devastating effects of pollutants on the aquatic food chain were realized. This insidious catastrophe was an unfortunate coincidence as far as the mink was concerned. By the time the decline of the otter had been brought to the attention of the general public, the mink was widespread and its alleged notoriety documented in the press. A convenient scapegoat was at hand. In reality, the mink was absent from many of the places where the decline of the otter was being noted. Even now, many years after the event, the view is often expressed in sporting and country magazines that the mink was a significant cause of the otters' problem. I have found that, over the years, as mink moved in, the otters gradually disappeared. Having spent most of my working life in the country and by the rivers as a gamekeeper, I actually saw the killing of a young otter by a wild mink'. Western Morning News (17 February 1981) Unlike populations of predatory birds that were similarly affected by these toxic chemicals, the otter populations have been slow to rebuild. This is possibly because there populations became too fragmented, perhaps exacerbated by hunting pressure, and movement between isolated groups was prevented because of the lack of cover caused by habitat destruction. Even when it was accepted that the mink was not to blame for the decline of the otter, doubt was then expressed as to whether their presence along a river would in any way hinder the ability of the otter population to re-establish itself. Recently, partly through a programme of captive breeding and re-introduction, populations of otters are recovering in England. They are now present on many rivers that contain mink populations. The low otter population may have facilitated the rapid spread of mink, but it is more likely that the mink will give way to the returning otter, rather than impair its recovery. Indeed, Birks (1990) reports evidence that in some areas where otters have made a significant recovery over the last 10 years, mink populations are now lower. The causal relationship has yet to be investigated; what is certain is that the mink poses no direct threat to the otter. A problem arises with the continued use of hounds to hunt mink. In view of the tendancy for mink and otter to be found on the same rivers and occasionally to use the same resting places, this will inevitably cause disturbance to the otter and could represent a significant threat in areas where they are scarce or attempting to re-colonize. Mink and other mustelids The potential interaction between American and European mink has already been discussed. In Europe, the only other carnivores with which the mink is likely to be in direct competition are the polecat (Mustela putonus) and the stoat (Mustela erminea). The staple item of diet for these predators is the rabbit (Day 1968, Walton 1968). Populations of rabbits are such that competition with mink is unlikely to be a limiting factor affecting the status of either native carnivore. Furthermore, the polecat appears to be extending its range even in areas where mink are present (Birks 1986). This may be due partly to the minks' association with waterways, on which the polecat appears to be less dependent. THE VERDICT Early worries concerning the impact of mink as a significant predator of native fauna and domestic stock, or as a competitor with native carnivores are largely without foundation. It can be seen that the relatively small size of mink, and hence their low energy requirements, coupled with their low population density, means that they have little impact overall. General conclusions from dietary studies indicate that mink are not specialist predators; they exist in a flexible balance with prey and do not appear to have threatened the viability of any one prey species. In Britain there have not been any extinctions or drastic reductions of any species of prey that can be directly and unequivocably attributed to mink. The major problem caused by mink predation concerns their effect on native bird species. While distressing incidences of killings may occur, in most cases the threat to populations is slight, unless habitat damage has already put the population in a precarious position. Here it must be remembered that mink are territorial, and the size of the territory is influenced by two factors-the habitat quality and the presence or absence of other mink. Because of this spacing mechanism, any local abundance of food or seasonal glut will only be available to the territory holders, thus allowing only a small number of animals to share the food, and preventing rnink from surrounding areas gaining access. Thus, for example, on Ross Island where there were abundant ground-nesting herring gulls available for most of their breeding season, only one female mink preyed on the colony because it lay within her territory. The colony has continued to expand despite the annual predation pressure from mink (Birks 1986). Considerable concern has been expressed about the predation by mink of domestic stock, particularly chickens, game birds and fish. Undoubtedly mink can pose a serious threat to agricultural holdings of high-density stock, for example, game-bird rearing pens, chicken farms and fish rearing ponds, as can many other predators such as the fox, otter, dog and cat. Mink are frequently blamed for the surplus killing of domestic prey, usually when encountered in confined conditions. These incidents can be guarded against by good husbandry, which will additionally provide protection against native predators. The maintenance of traps in the vicinity of pens is all that is usually required to control mink in these instances. In terms of direct losses, the economic impact across the country of mink as a predator of domestic stock is almost negligible, although the individual farmer who has just lost 50 of his best free-ranging hens down by the river might not see it that way. But there are hidden costs associated with the presence of this alien predator. The cost of control operations both in trapping equipment and time can be considerable. The unsuccessful eradication campaign of the Ministry of Agriculture in a 5-year trapping programme cost £3105 000 between 1965 and 1970. If such a campaign were even to be contemplated today it would cost many millions of pounds. The individual farmer may spend an appreciable sum of money mink-proofing his stock, but the benefits derived by also excluding other predators may be substantial. In some locations, the prevalence of mink predation may threaten the viability of certain farming operations. Such restrictions in the siting of these enterprises may also be a cost. Before the presence of feral mink, most farms kept a few free-range chickens or small flocks in deep-litter houses. Today the poultry industry is intensive with far fewer but much larger flocks kept in battery units where they are immune from attacks by predators. THE EFFICACY OF CONTROL OPERATIONS TRAPPING The cost involved in mounting 'effective' control operations can be substantial. The job is magnified by the presence of transient individuals who will rapidly replace resident territory holders that are removed. Consequently many more animals will be trapped on a given stretch of river than were initially present, and recolonization from adjacent areas is almost inevitable. In Sweden, considerable effort was expended from the 1940s to the 1960s by the Sportsmen's Association in trapping mink in an attempt to protect their hunting interests. Their annual catch is shown in Fig. 11.7. Successful attempts to trap out mink populations have only occasionally been documented. Most fail or have only a limited effectiveness until mink re-invade. One such effort was made on the River Otter in Devon in 1973 (Chanin 1981). A total of 47 mink from 27km of river were killed between April and September. It is unlikely that all mink were captured, and the area was recolonized during the next breeding season. As Chanin (1981) points out, even if mink had been eradicated on this river they would inevitably have re-invaded from adjacent rivers. The cost of establishing a 'mink-free zone' is not warranted in view of the small amount of economic damage they do. HUNTING Otter hunting with packs of hounds was banned in the UK in 1975. At least six of these packs subsequently switched to hunting mink. Other packs were formed with the specific purpose of hunting mink. By December 1984, 20 such packs had registered with the Masters of Minkhounds Association. Hunting is carried out from April to September, a period which includes the mink's breeding season. Den sites are located by the dogs as they search the riverbank, and an attempt is made to bolt the quarry which then dashes to its next refuge. There have been a considerable number of reports of damage to river banks and their vegetation caused by hunt servants in attempting to dislodge mink from their dens. Although precise figures are not available each hunt probably accounts for some 40-50 mink per season, amounting to an annual nationwide slaughter of 700-800 (Birks 1986). Only one study (Birks 1981, 1986, 1989) has addressed the effectiveness of hunting with dogs as a means of controlling mink. In an analysis of hunting records of the Cornwall and Devon Minkhounds (south-west England) from 1976 to 1980, the pack hunted on 156 days during which 84 mink were caught. Two-thirds of the mink located by the hounds successfully evaded capture. On one occasion, the minkhounds hunted through Birks' study area, at which time five or six resident mink were known to be present. Three of these residents were found by the hounds, two of which escaped into secure rocky dens, only one was killed. Although this seems a remarkably inefficient means of control, the extent of persecution may be more subtle. The recently born litter of the female were left to perish. In another case, a presumably pregnant female hunted in March failed to produce a litter that year. On mink farms interference causing stress at such an early stage in pregnancy is likely to lead to abortion. APPROPRIATE CONTROL It is doubtful if the agricultural or ecological impact of mink on domestic stock or native prey species is sufficient to warrant expenditure on wide-spread control measures. The most cost-effective way of guarding against mink attack is not by control, but by excluding the predator. Mink are small predators and can quite easily gain access to poultry enclosures and sheds through rat holes, possibly when hunting the rats themselves. Particularly valuable collections of exotic waterfowl can be protected from mink by means of a perimeter wire fence with metal baffle at the top to prevent the mink climbing in. Preventative trapping should be carried out at times of the year when it is likely to have most effect, for example, by removing pregnant females in spring and territorial residents in autumn after dispersal movements have largely finished. The mink must be accepted as endemic across much of northern Europe, its further spread into ecologically or agriculturally sensitive areas should be prevented. In 1977 permission was sought, but refused, to establish a mink farm on the island of Westray, Orkney, Scotland. This island supports one of the largest colonies of breeding sea-birds in the British Isles, amounting to some quarter of a million birds of 19 species. A small endemic rodent, the Orkney vole (Microtus arvalis orcadensis) would also have been vulnerable. The Mink (Keeping) Order 1987 prohibits the keeping of mink on any offshore island of Great Britain other than those where they are farmed or already established in the wild. LEGISLATION PERTAINING TO MINK Made under the Destructive Imported Animals Act 1932, the Mink (Keeping) Order 1987 prohibits the keeping of mink within Great Britain except under licence, and requires occupiers to notify the presence of (unlicenced) mink on their land. The keeping of mink is prohibited on any offshore island of Great Britain other than the Isle of Wight, the Isles of Arran (and nearby Holy Island), Harris and Lewis and its associated islands north of a line dividing Harris from the islands of Shillay, Pabbay, Killegray, Langay and Gilsay. The keeping of mink in the Caithness and Sutherland districts of the Highland region of Scotland is also forbidden. Licence fees and keeping conditions are specified in the Keeping Regulations and penalties are proscribed under the Act. Licence fees, £3115, in 1992, are kept under review and have to reflect the administration and inspection costs involved. In addition, the Wildllfe and Countryside Act 1951 makes it an offence to release mink or to allow them to escape into the wild. Importation, except under licence, is prohibited by the Rabies (Importation of Dogs, Cats and Other Mammals) Order 1974, as amended. GENERAL OVERVIEW The popular view is of an out-of-control bloodthirsty predator which rampages along our riverbanks and wreaks havoc on a grand scale, leading to major losses of fish stocks and waterside wildlife. The alternative view is that the mink is occupying a previously vacant niche, with the result that its permanent impact upon our fauna will be relatively slight. Failure to tease the myth from the reality could have dire consequences. Where the mink's innocence is falsely protested in response to an ecological problem, then possibly avoidable damage may occur through inaction or through tackling the wrong cause. Conversely, where the mink is falsely blamed, the genuine causes of wildlife problems rnay be ignored with serious consequences. In some instances 'the cure may be worse than the disease', as traps set to control mink may ensnare other wildlife.