Weather, disease, and bighorn lamb survival during 23 years in CanyonlandsNational Park

The Island-in-The-Sky desert bighorn herd (Ovis canadensis nelsoni), the la rgest in Canyonlands National Park, Utah, is used extensively by Utah Divis ion of Wildlife Resources to re-establish herds in former habitat throughou t the state. This study was undertaken because park managers became concern ed about delayed herd recovery following 4 years of transplant removals and suspended trapping operations. This concern led to my study of effects of weather on yearly lamb:ewe ratios for the park's 23-year database. I used w eather variables regulating the annual growing season, soil-moisture regime , and body condition of ewes to develop multiple-regression models. Regress ion models, developed from 10 and 16 years of data, explained 91% and 61% o f the variation in lamb:ewe ratios, respectively; I estimated lamb:ewe rati os for remaining years of data and compared them with observed values. Maxi mum ambient temperatures in May of the same year lambs were born and precip itation from January through March 2 years before lambs were born (thought to relate to body condition of ewes) were variables correlated most strongl y with lamb:ewe ratios. Peak of lambing occurs in April and May. May is the pivotal month, which, on average, has maximum temperatures that promote ne gative soil-moisture storage and a decrease in forage nutrients. Disease (c ontagious ecthyma) was first noticed in 1986 and apparently reduced lamb su rvival through 1992. I used data from disease years to estimate lamb surviv al, but not for model development.