EN
Reproduction of the American mink Mustela vison Schreber, 1777 population introduced to Belarus was studied in 1983 — 1992. Material consisted of 52 male genitalia, carcasses of 45 pregnant females, and observations of 19 litters with blind cubs and 31 litters with larger cubs, The study was conducted in two areas in northern Belarus: Rossony, where the decrease of mink numbers due to excessive trapping was recorded, and Gorodok, where the expansion of American mink in the presence of the native European mink Mustela lutreola was observed. Mating period of American mink lasted from mid February to mid April. Young were born from April till mid June. Litter size was on average 4.2 (blind cubs) and 3.3 in older cubs (> 2 months old). Reproduction intensity on Drissa river and its tributaries was related to density. On average, 3.3 embryos per pregnanat female were found in 1-yr females and 4.3 in > 2-yr old females at the density of 13.3 mink/10 km of watercourse. In the low density population (2.8 mink/10 km due to overexploitation by hunters) reproduction intensity increased to an average of 4.2 embryos in 1-yr females and 6.0 in £ 2-yr females. During population decrease, percentage of young increased and the sex ratio became biased towards females. In an expanding population of American mink that had newly colonised Lovat river, the reproduction was very intense (7.3 - 7.6 embryos/female). The results show a great reproductive plasticity of American mink that obviously helped this species to successfully colonise new ranges after introduction.