Mortality of domestic sheep in free-ranging flocks in southeastern Norway

Lamb mortality during three summer grazing seasons (1988 to 1990) among 1,399 lambs in three flocks of free-ranging domestic sheep in southeastern Norway was modeled using logistic regression. Ewe mortality during the same three seasons among the 295 ewes in one of the three flocks was also modeled. During the period, mortality among the 1,399 lambs and 295 ewes was 7.2% and 12.5%, respectively. Other work has shown predation by brown bears to be the single most important mortality factor in all three flocks, especially among ewes. Year, herd, sex, age of dam, and spring growth rate (i.e., from birth to release onto summer range) were statistically significant (P < or = .05) in explaining mortality among lambs. Lamb mortality was higher in 1989 and 1990, higher in HerdA (the most isolated herd that grazed adjacent to the Swedish border), higher among lambs of yearlings, among male lambs, and among lambs with low spring growth rate. Ewe mortality increased with age (P = .07). Adjustment of age for weight weakened the statistical association between age and mortality, however (P = .16). The negative association between lamb mortality and age of dam and the positive association between ewe mortality and ewe age may both be related to the quality of maternal care provided by ewes of different ages. Subsequently, attempts to compensate for losses, especially those due to predation by bears, lead to a younger herd that may increase the herd's overall vulnerability.