1. Following a long history of eradication campaigns against predators, wolverine Gulo gulo distribution and numbers became critically low in Norway in the 1960s. Consequently, wolverines were protected by law during the 1970s and 1980s. This led to an increase in wolverine numbers and re-establishment in some formerly occupied habitats, resulting in increased depredation on domestic reindeer Rangifer tarandus and sheep Ovis aries. Wolverine control and licensed hunting have been used increasingly as tools to reduce the depredation since protection was introduced. However, the factors influencing the level of this depredation have not been studied. We therefore examined losses of domestic sheep grazing unattended on upland summer ranges in the Snøhetta plateau of south central Norway during 1979–94, in relation to wolverine population numbers, reproduction and control measures. This area was recently re-occupied by wolverines and reproduction has been recorded regularly from the beginning of the study period.
2. The number of ewes and lambs recorded in organized grazing areas increased more than fivefold during the period, and losses increased proportionally with sheep numbers. The differences in losses among municipalities and grazing co-operatives were probably related to sheep breeds and local variation in wolverine density.
3. The heavy Dala sheep breed was most at risk, whereas losses from the lighter Norwegian short-tailed and Norwegian fur-bearing breeds were lower than expected.
4. Killing of wolverines led to fewer lambs being lost in the same year that the wolverine killing took place, but the effect quickly declined, implying a rapid re-establishment of wolverines in the local area.
5. Lambs were more vulnerable to wolverine predation than ewes, and losses were higher in wolverine cub-rearing areas than in the area as a whole.
6. We conclude that: (i) control programmes for wolverines, as practised today, will have little long-term effect in reducing sheep losses, unless wolverines are eradicated or severely reduced in numbers; (ii) the introduction of more agile sheep breeds will probably reduce losses; (iii) in areas where wolverine control to protect sheep is a threat towards wolverine conservation, the present sheep husbandry system of grazing unguarded sheep in the mountains should be replaced by alternative forms that provide the sheep with more protection. This is particularly important in wolverine denning areas.