Methods for the identification of farm escapees in feral mink (Neovison vison) populations

Abstract

In Denmark, American mink (Neovison vison) have been bred for their fur since the mid-1920s. Mink escaping from farms may supply the feral population. Often, it is of biological and management interest to separate the population of feral mink (i.e. mink caught in the wild) in two groups: 1) mink born on farms i.e., escapees, and 2) mink born in the wild. In this study, two methods were used for separating feral mink into the two groups: a) Comparison of body length of farmed mink and feral mink, and b) Presence of a biomarker (tetracycline: an oral antibiotic used on mink farms). A total of 367 wild caught mink (from the mainland of Denmark and the island of Bornholm), and 147 mink from farms, collected during the period 2014–2018, were used for the analysis of body length. For the testing of tetracycline (TC) as a biomarker, 78 mink from farms where there was knowledge about TC treatment (with or without) were examined for fluorescent markings in the canine teeth. Results from both univariate analyses and Gaussian mixture model analysis demonstrated clear divisions between the mean body length (mean ± S.E., range) of farmed males (52.1 cm ± 0.4, 48–68) and farmed females (mean 44.0 ± 0.2, 40–50), and between farmed mink and wild caught mink. Mixture analysis identified two groups within each sex of the wild caught mink, one assigned to farmed mink (born in captivity) and another group of smaller mink suspected of being born in the wild. On Bornholm, the mean (±SD, range) length of males born in the wild was 43.7cm (± 0.3, 36–57) and for females 37.5cm (± 0.3, 32–45). The mean length (±SD, range) of males born in the wild in the mainland of Denmark was 42.5cm (± 2.3, 36–46) and for females 36.1cm (± 1.0, 34–37). Among the feral mink from mainland Denmark, 28.4% of males and 21.6% of females were identified as escapees, while 0% of the males and 1% of the females were identified as escapees among the wild caught mink on Bornholm. Eight percent of mink from farms using tetracycline were false negatives, while no false positives were found among mink from farms not using TC. TC fluorescence was found in five of 217 mink caught in the wild equivalent to 22% escapees in mainland Denmark. No TC markings were found in mink caught in the wild on Bornholm. In conclusion, both methods a) the body length of mink, and b) fluorescent biomarkers in canine teeth are considered as useful tools to identifing mink that have escaped from farms.

Age and Reproduction of Wild-Born and Escaped Mink (<em>Neovison vison</em>) Caught in Danish Nature

“…During 2018, 247 mink were submitted to the National Veterinary Institute of Denmark. Of these mink, 208 could be separated belonging to either of the groups wild-born or captive-born; 96 mink considered born in nature and 112 mink to be captive-born, which is in accordance to [7]. Six mink of 84 (7%) with known color in the wild-born group were recessive colored, while 43 of 93 (46%) of the captive-born group with known color were recessive colored.…”

“…In a previous investigation of mink caught in the wild in Denmark, comparison of body length of captive-born mink and wild-born mink revealed a substantial number of mink escapes from Danish mink farms [7]. Hence, newly escaped farmed mink may bias studies of wild-born mink reproduction and demography, and thereby the question of a self-sustainable population.…”

“…In a previous study of Danish mink both univariate analyses and Gaussian mixture model analysis demonstrated clear divisions between the body length of farmed mink and wild-born mink. Mixture analysis identified two groups within each sex of the wild caught mink, one assigned to farmed mink (born in captivity) and another group of smaller mink suspected of being born in the wild [7]. Hence, mink sampled for the present study were separated in two groups: Mink born in nature (wild-born) and mink born on farms (captive-born) according to [7].…”

“…Mink are known to escape from Danish farms and mink can be found in the wild in most parts of Denmark due to farm escapes [6]. In previous Danish studies, 80% mink in the years 1998-2000 and approximately 30% mink caught in nature during the winters of 2014 -2018 were found to be mink recently escaped from farms, leading to a question whether a truly feral mink population exists in Denmark [7][8][9][10].…”

Cardiopulmonary nematodes of wild carnivores from Denmark: Do they serve as reservoir hosts for infections in domestic animals?

“…Mink were originally imported to Denmark for fur production and kept in captivity. Today, mink can be found in the wild all over Denmark due to escapees from farms ( Pagh et al, 2019 ), but the population size is unknown. Eurasian otters, beech martens ( Martes foina ) and polecats ( Mustela putorius ) are believed to be less common.…”