Differential survival in the presence of spatially structured ptarmigan harvest suggests additive mortality

The effects of hunting on wildlife populations vary dramatically, depending on the timing and magnitude of harvest, and population-specific states and vital rates. We examined the hypothesis that spatially and seasonally concentrated harvest decreases annual survival probabilities of willow ptarmigan (Lagopus lagopus). We estimated survival of radio-marked willow ptarmigan at 2 categories of sites: those where ptarmigan were easily accessible and heavily hunted and those that were remote and received little or no hunting pressure in Alaska, USA. We predicted that seasonal survival estimates during the willow ptarmigan hunting season would be lower in access corridors than at remote sites and that this would result in lower annual survival unless subsequent seasonal compensatory mortality occurred. Consistent with our prediction, annual survival was higher at remote sites (adult males: 0.50, 95% credible interval [CrI] = 0.42-0.57; adult females: 0.36, 95% CrI = 0.26-0.46; juveniles: 0.39, 95% CrI = 0.29-0.50) than at accessible sites (adult males: 0.36, 95% CrI = 0.26-0.46; adult females: 0.23, 95% CrI = 0.12-0.32; juveniles: 0.25, 95% CrI = 0.13-0.37) for all demographic groups. Concentrated harvest occurred in accessible sites during the hunting season (Aug-Mar). During the post-breeding season (Aug-Nov), when willow ptarmigan were near their breeding sites and the hunting season was open, survival was higher for those from remote sites than for those from accessible sites when accounting for demographic group (adult male, adult female, juvenile). In contrast, during winter (Dec-Mar), when willow ptarmigan had moved away from breeding territories and were no longer segregated into remote and accessible sites, survival estimates did not differ between those from remote breeding territories and those from accessible breeding territories. This pattern of differential survival indicates an association between concentrated hunting at accessible sites and reduced survival, and it suggests an additive component of harvest mortality. In addition, the timing of these survival patterns relative to seasonal movements suggests that early season harvest (prior to seasonal movements away from breeding territories) may have a greater impact on local breeders in accessible areas than later harvest.