Population studies on feral American mink Mustela vison in Ireland

Populations of feral American mink Mustela vison were investigated in several study areas in Ireland, principally by capture-recapture techniques. The populations were found to be essentially self-regulating, with stress, weight loss and mortality evident during the winter and spring. The highest mean density recorded, in terms of animals per unit length of river or lake shore, was 1.4 individuals km⁻¹ with a peak density of 2.3 individuals km⁻¹, but population densities were considerably lower in most areas. Trappability and population structure, in age, sex and residency classes, varied between habitats, but population turnover was high, leading to complete replacement of individuals in two populations monitored for two years. The availability of the preferred prey, crayfish Austropotamobius pallipes appeared to be a major factor determining population size and preferred breeding areas. Control methods are either unnecessary or are likely to fail in most situations, and may be counterproductive.     

Sexual Dimorphism in the Skull of the Feral American Mink (Mustela vison Schreber)

Sexual dimorphism in the skull of the feral American mink is discussed, based on 15 measurements of 53 adults from southern Norway. The differences in means are highly significant. Variation is found to be greatest in males. Sexual dimorphism is thought to be an adaptation for different reproductive roles of the sexes in a polygynous mating system with lack of male parental care.     

Modelling the benefits of American Mink Mustela vison management options for terns in west Scotland

American Mink Mustela vison is a semi-aquatic predator that has invaded the west coast of Scotland and many of its associated islands. We developed a GIS model of their potential range based on their dispersal abilities and habitat use, which revealed that most islands in west Scotland are accessible to Mink, and that these host a large proportion of the region's Common Sterna hirundo and Arctic Terns S. paradisaea . Mink are predators on tern eggs and chicks, and statistical modelling of long-term productivity data demonstrated that unprotected sites within their range have an average productivity of 0.33 chicks per pair, whereas that at sites where Mink were trapped was 253% higher. We assessed the benefits of current Mink control projects for terns in the Western Isles and the remainder of west Scotland using a population modelling approach. This showed that both projects delivered considerable benefits for Common Terns, because a large proportion of their numbers were within the area of the control programmes and in sites that would be accessible to Mink if no control were in operation. For Arctic Terns, the benefits were less clear, as a larger proportion of their numbers were outside the control areas, and many of these were in sites isolated from, or unsuitable for, Mink. We discuss the implications of these findings for future strategic planning of Mink management in west Scotland.     

Comparative Enzymologic Study of Catalytic Properties of Blood Serum Cholinesterase of the American Mink Mustela vison

Comparative enzymologic study of catalytic properties of cholinesterase (ChE) in blood serum of the American mink Mustela vison Schr. has revealed several peculiarities of this enzyme. First, using the method of substrate–inhibitor analysis, homogeneity of the ChE preparation has been established, i.e. only one ChE has been found in mink serum. Second, the rate of acetylcholine hydrolysis was higher than of thiocholine substrates, among which propionylthiocholine was hydrolyzed at the highest rate. Third, propionylthiocholine had the highest V/K _M value that reflects to a degree affinity of the substrate to enzyme. Fourth, the phenomenon of substrate inhibition, which is not inherent for mammalian serum cholinesterases, is revealed and kinetically analyzed. Fifth, study of inhibitory specificity has not revealed differences of the mink serum ChE from other serum ChE.     

Metrical and Non-metrical Skull Variations in Norwegian Wild Mink (Mustela vison Schreber)

Variations in fifteen metrical measurements and six non-metrical traits in 131 mink skulls were investigated. The skulls were subdivided geographically into three samples: Eastern Norway, Western Norway and Trøndelag. Separation into juveniles and adults was in accordance with the criterion that all sutures are closed in adults. This seemed to work well. No age or sex variations were found in the non-metrical traits. Relatively little geographic variation found in either the metrical measurements or the non-metrical traits indicated little genetic variation. Gene-flow is regarded as one of the reasons for this.     

A comparison of the feeding biology of Mink Mustela vison and otter Lutra lutra

The feeding habits and prey selectivity of Mink Mustela vison and otters Lutra lutra were compared in two localities in Devon: a eutrophic lake and a moorland river, in which both species occurred and had access to the same prey populations. The effects of prey availability on the predators' diets were assessed by comparing prey consumed, as revealed by scat analysis, with estimates of prey abundance and size range. Otters specialized in fish at all times of year but showed seasonal variation in species taken. Selection for slow-moving fish and seasonal changes in behaviour of some fish species were the probable causes of this variation. Otters diversified more into non-fish food in summer, when fish availability was reduced. The main alternative prey in the lacustrine habitat was waterfowl, but in the riverine habitat, rabbits. Mink were more generalized carnivores, taking a variety of fish, waterside and terrestrial prey in all seasons. These three prey categories were taken to an almost equal extent in the lake but terrestrial prey dominated in the riverine habitat. Fish were taken most frequently in winter and birds and mammals in summer. Neither predator showed selection in respect of prey size. In each area, about one third of the otter and Mink diets was common to both species. Fish was the principal group of the shared component, and dietary overlap in respect of them was greatest in autumn and winter. In view of the dietary preferences of each predator, the existence of alternative prey items and limited degree of dietary overlap, it is considered unlikely that the two species competed for food to any extent. Other factors must therefore be responsible for the spread of feral Mink and the decline in otter populations in many parts of Britain.     

The first linkage map of the American mink (Mustela vison)

Described herein, the first microsatellite linkage map for the American mink consists of 85 microsatellite markers resolved into 17 linkage groups. The map was constructed using 92 F(1) progeny from five sire families created by crossing mink with different colour types. The linkage groups ranged from 0 to 137 cM. These linkage groups were assigned to 12 of the 14 mink autosomes using a somatic cell hybrid panel. The total map covered 690 sex-averaged Kosambi units with an average marker spacing of 8 cM. This map will facilitate further genetic mapping of monogenic characters and QTL.     

Comparative Enzymologic Study of Catalytic Properties of Brain Cholinesterase of the Mink Mustela vison

Study of preparations of different parts of brain of the American mink Mustela vison Bris. has revealed the highest activity of acetylcholinesterase (AChE) in the caudate nucleus. Using the method of substrate-inhibitor analysis, homogeneity of this AChE preparation has been shown. Acetylthiocholine was hydrolyzed with the highest rate to exceed even acetylcholine, its advantage being also revealed in a _c/K _M value that reflects to a degree the affinity of the substrate to the enzyme. The study of inhibitory specificity showed no differences of AChE in the human brain and in erythrocytes.     

The Pineal Gland of the Mink, Mustela vison: Light-, Fluorescence- and Electron Microscopical Studies

The pineal gland of normal and experimental female mink has been studied by light-, fluorescence- and electron microscopy. The general structure of the mink pineal is described. Two main cell types are recognized. One, termed pinealocyte, predominates in number. Though slight morphological differences (e.g. electron density of the cytoplasm and content of organelles) were observed, this study indicates that the pineal of mink only contains one single population of pinealocytes. The other, termed glial cell, inserted between the pinealocytes, is characterized by the presence of elongated processes, containing microfilaments. Different treatments (ovariectomy and LH—RH administration) and different endocrine states during the year induced morphological changes in the pinealocytes. A rich network of nerve fibres containing electron-dense granules (40–50 nm) is observed. Microspectrofluorometrically these fibres exhibit the spectral characteristics of cateholamines. All the pinealocytes show a yellow fluorescence. This cellular fluorophor shows the same microspectrofluorometric characteristics as does the fluorophor of serotonin. Occasionally, synaptic ribbons are observed in the perikaryon and the processes of the pinealocytes. A large number of cellular junctions between pinealocytes and endothelial cells is present. Their presumed function(s) are discussed. There is evidence of a blood-brain barrier within the mink pineal gland.     

Mapping of 53 loci in American mink (Mustela vison)

Fifty-three genes were mapped in the American mink genome using polymerase chain reaction (PCR)-based analysis of a Chinese hamster-American mink somatic cell hybrid panel. Heterologous primers designed for cat gene mapping were used in this study. Forty-nine of these loci were localized into expected chromosome regions according to Zoo-FISH data, whereas four loci--ALPL, CDC20, ERF-2, and Fc(Mv)23617--were mapped out of expected conserved regions. PCR products amplified with primers corresponding to these four markers were partly sequenced and verified using BLAST. The results showed the homology to be more than 90% between mink and human or cat counterparts. At present, the gene map of American mink has expanded to 127 loci.     

Sex Identification in the Mink (Mustela vison Schreber) by Metrical Measurements of the Skull

The dental length and length of the first molar in the upper jaw of mink are characteristics used to estimate a discriminant function for sex identification of Norwegian populations. Reliability of the method is theoretically 93.2%.     

The impact of American mink Mustela vison and European mink Mustela lutreola on water voles Arvicola terrestris in Belarus

In the U.K. the impact of introduced American mink Mustela vison , on water voles Arvicola terrestris , may be exacerbated by habitat loss and fragmentation. Pristine wetlands in Belarus, which American mink invaded in the early 1990s, provide a three-pronged opportunity to test this hypothesis. First, we examine the evidence that, even in the unmanaged wetlands of our Belarussian study site, American mink have reduced water vole populations. Second, we ask whether habitat size, type and isolation mitigate the impact of American mink predation. Thirdly, we explore whether water voles are at greater risk of predation from American than European mink because of their patterns of habitat use. Following the invasion of American mink, water voles were most abundant in small, still-water sites, far from river banks, while American mink were most active in large, running-water sites. Small mammal remains were found in a higher percentage of American than European mink scats, and of these, more were water vole in American mink scats. The occurrence of water voles in scats of both mink species declined after the American mink invaded and established. Our results provide at least circumstantial evidence that American mink limit water vole populations even in unmanaged wetland eco-systems, and that they have a greater impact than their European congener at least partly because they make greater use of isolated marshes. Although by no means providing complete protection, the configuration and dispersion of available habitat mitigated the impact of American mink on water voles. This raises the possibility that habitat restoration, especially through the establishment of isolated enclaves, could help reduce the effect of American mink in the U.K. These observations are of broader interest in the context of assessing the effect of multiple pressures on vulnerable species.     

Cytodifferentiation in the Pars Distalis of the Pituitary Gland in the Fetal and Juvenile Mink,Mustela vison

Hypophysial tissue of mink (Mustela vison) embryos, newborn young and kits aged 1 day to 27 weeks was examined by light and electron microscopy. Among the embryos five different ontogenetic stages were distinguished. It appears that the primordium of the pituitary gland differentiates in early embryonic life. Several types of glandular cells containing variable amounts of granular material were recognizable by the time of birth. The role of the granular cells and the origin of the lumina in the pars distalis are speculated upon (in the Discussion).     

Fine Structure of the Pars Distalis of the Pituitary Gland in the Female Mink,Mustela vison

Pituitary glands of intact and experimental adult females of mink, Mustela vison, were examined by electron microscopy. Conventional methods involving removal of endocrine glands (ovaries and adrenals), administration of radioactive isotope, ¹³¹I, blocking agents (thiouracil and metopirone) and hormones (thyroxine, hydrocortisone, thyrotropin and luteinizing hormone releasing hormones) were employed. Five categories of granular cells were distinguished both by their ultrastructural characteristics and qualitative changes throughout the year and following different treatments. The cell types are described and their functions discussed. From conventional electron microscopical studies it proved difficult to draw any satisfactory conclusions about the gonadotropic cells. Further investigation by means of immunocytochemistry and radioimmunoassay techniques is required to determine, whether the presumptive gonadotropic cell type produces both FSH and ICSH or only one of these hormones. Morphologically two types of agranular cells were identified. Their morphological inter-relationship and function are discussed briefly.     

Multivariate variation in feral American mink (Mustela vison) from Southern Norway

Multiple group principal component analysis was used to investigate the morphological relationships between feral American mink from three areas of Southern Norway based on 15 skull dimensions. The first principal axis calculated from the variance-covariance matrix of log-transformed data is usually interpreted as a growth axis. In the present analysis, the scores of the first principal axis of males were found to be uncorrelated with estimated age for specimens younger than seven months old, and was interpreted as a growth-free size axis. However, the second component of males was found to reflect mainly growth in the postorbital area. The first component of females was thought to reflect a mixing of growth-out and growth-in size variation. One of the reasons for separating size and size-free variation in the present work was to test whether growth-dependent size influenced the assessment of geographic variation within each sex. The scores on the components reflecting size variation (growthout and growth-in) were, however, not found to deviate significantly between the areas. Discriminant analyses run on the component scores, both with size variation included and excluded, gave the same result. In mean, the mink from Western Norway deviated significantly from the mink from Trondelag and Eastern Norway, while the mink from the two latter areas did not deviate morphometrically. The deviation found was in shape rather than in size of both sexes.     

Immunocytological analysis of meiotic recombination in the American mink (Mustela vison)

Using immunolocalization of MLH1, a mismatch repair protein that marks crossover sites along synaptonemal complexes, we estimated the total length of the genetic map, the recombination rate and crossover distribution in the American mink ( Mustela vison ). We prepared spreads from 130 spermatocytes of five male minks and mapped 3320 MLH1 foci along 1820 bivalents. The total recombination length of the male mink genome, based on the mean number of MLH1 foci for all chromosomes, was 1327 cM. The overall recombination rate was estimated to be 0.48 cM/Mb. In all bivalents, we observed prominent peaks of MLH1 foci near the distal ends and a paucity of them near the centromeres. This indicates that genes located at proximal regions of the chromosomes should display much tighter genetic linkage than physically equidistant markers located near the telomeres.