Population biology of stoats Mustela erminea and weasels Mustela nivalis on game estates in Great Britain.

Abstract

1. British gamekeepers commonly trap and shoot stoats and weasels in order to increase the abundance of game. We provide details of the population biology of 822 stoats and 458 weasels collected on 25 game estates and use simple population models to assess the effects of culling.
2. Seventy-one per cent of stoats and 94% of weasels were trapped, while 26% of stoats and 5% of weasels were shot. While trapped samples exhibited typically male-biased sex ratios, the sex ratio of shot stoats was even. Eight of 305 female stoats and six of 77 female weasels were visibly pregnant, with mean litters of 9.0 and 6.2 embryos, respectively. Median ages at death were 11.6 and 8.0 months for male and female stoats, respectively, and 9.3 and 9.2 months for male and female weasels. Male and female stoats, but not male and female weasels, had significantly different rates of survival.
3. Model weasel populations continued to increase (λ = 1.35) despite culling as a result of high productivity when sufficient food was available. Model stoat populations declined slightly (λ = 0.95), probably as a result of concerted culling effort when young stoats were dependent on maternal survival. This suggests that persistence of culled stoat populations may depend on immigration.
4. To reduce stoat populations without affecting the survival of dependent juveniles, culling effort could be focused on trapping females in late winter and shooting females in early spring, where landscape and climate permit. For control of weasel populations, trapping effort should be, and in practice often is, focused on late spring, following a period of high natural mortality.
5. High rates of immigration mean that culling by gamekeepers will not ordinarily lead to any long-term decline in actual stoat and weasel populations. We suggest that measures taken to enhance immigration will improve the long-term status of stoats and weasels in regions where their conservation is desirable, and whilst this persists the impact of culling will be short-lived and local.