Additive and compensatory predation mortality by cougar on elk

We studied the impact of predation by cougar ( Puma concolor) on elk ( Cervus elaphus) over a 17 year time span in two watersheds in western Washington. Elk numbers ranged widely in each watershed, from a low of 130 increasing to ≈600 in the Green River and from 600 increasing to ≈1,400 in the White River. We studied adult cow elk survival (n=244) and calf survival (n=548) in both watersheds using radio-marked animals. We manipulated cougar numbers by conducting targeted removals over 7 years and estimated cougar density prior, during, and after the removals. Cougar predation on elk calves was strongly additive (R²=0.80). Cougar predation on cow elk was partially additive in the Green River where other factors had less influence on adult elk survival. Survival of adult cow elk in the White River was less affected by cougar predation than other factors in part due to greater public access. Based on age and femur marrow condition we estimated that 25% of all radio-marked adult elk mortalities due to cougar were compensatory. We have also investigated predation rate by cougar using GPS-marked cougar (n=12). We estimated that 15% of adult elk >= 2 years old that were killed by cougar were likely compensatory and that 75% were likely additive. Removing cougars allowed elk in both watersheds to recover to, or exceed, herd objectives for both watersheds, and restore elk hunting opportunity. Our data strongly indicates that cougar were limiting elk, and that cougar removals allowed elk to escape the predator pit they were in. But simply removing cougar in other elk-cougar systems may not restore elk depending on the relative influence of other factors affecting mortality, amount of additive predation mortality, and suite of interacting predators and prey.