Is hunting mortality additive or compensatory to natural mortality? Effects of experimental harvest on the survival and cause-specific mortality of willow ptarmigan

Abstract

Summary1. The effects of harvest on the annual and seasonal survival of willow ptarmigan Lagopus lagopus L. were tested in a large-scale harvest experiment. Management units were randomly assigned to one of three experimental treatments: 0%, 15% or 30% harvest. Seasonal quotas were based on the experimental treatment and estimates of bird density before the hunting season. Survival rates and hazard functions for radio-marked ptarmigan were then estimated under the competing risks of harvest and natural mortality. 2. The partially compensatory mortality hypothesis was supported: annual survival of ptarmigan was 0AE54 ± 0AE08 SE under 0% harvest, 0AE47 ± 0AE06 under 15% harvest, and was reduced to 0AE30 ± 0AE05 under 30% harvest. Harvest mortality increased linearly from 0AE08 ± 0AE05, 0AE27 ± 0AE05 and 0AE42 ± 0AE06 from 0% to 30% harvest, whereas natural mortality was 0AE38 ± 0AE08, 0AE25 ± 0AE05 and 0AE28 ± 0AE06 under the same treatments. 3. Realized risk of harvest mortality was 0AE08-0AE12 points higher than our set harvest treatments of 0-30% because birds were exposed to risk if they moved out of protected areas. The superadditive hypothesis was supported because birds in the 30% harvest treatment had higher natural mortality during winter after the hunting season. 4. Natural mortality was mainly because of raptor predation, with two seasonal peaks in fall and spring. Natural and harvest mortality coincided during early autumn with little potential for compensation during winter months. Peak risk of harvest mortality was 5· higher than natural mortality. Low natural mortality during winter suggests that most late season harvest would be additive mortality. 5. Environmental correlates of natural mortality of ptarmigan included seasonal changes in snow cover, onset of juvenile dispersal, and periods of territorial activity. Natural mortality of ptarmigan was highest during autumn movements and nesting by gyrfalcons Falco rusticolus L. Mortality was low when gyrfalcons had departed for coastal wintering sites, and during summer when ptarmigan were attending nests and broods. 6. Our experimental results have important implications for harvest management of upland gamebirds. Seasonal quotas based on proportional harvest were effective and should be set at £15% of August populations for regional management plans. Under threshold harvest of a reproductive surplus, 15% harvest would be sustainable at productivity rates ‡2AE5 young per pair. Impacts of winter harvest could be minimized by closing the hunting season in early November or by reducing late season quotas.

Demographic transition among hunters: a temporal analysis of hunter recruitment dedication and motives in Denmark

“…Unrestricted harvesting within the seasonal limits has historically been regarded as an activity that only takes out a surplus of the populations (Pedersen et al 2004), despite the fact that scientists provided new knowledge about the eff ects of hunting as early as in the 1990s (Kastdalen 1992, Smith and Willebrand 1999, Willebrand and Hornell 2001. Th e studies show quite clearly that harvesting may add to the natural mortality in willow ptarmigan in contrast to the old view of harvesting from a surplus (Sandercock et al 2011). Th is view is still present among hunters and landowners, but even among these groups there is increasing recognition that hunting of willow ptarmigan has the potential to be unsustainable.…”

“…Harvest of willow ptarmigan Lagopus lagopus and rock ptarmigan Lagopus mutus , both species hereafter referred to as ptarmigan, have declined since the late 1990s from 550 000 ptarmigan in the 1999/2000 hunting season, to 170 000 birds shot during the 2011/2012 season. Th e infl uence of harvest on population levels is unclear, but negative eff ects have been documented for harvest rates from 15% or more (Sandercock et al 2011). A recent study in Fennoscandia (Lehikoinen et al 2014) has shown that many ground nesting birds, including ptarmigans but also not hunted species, have declined in numbers during the same period, indicating that other factors than harvest alone aff ects the ecosystem.…”

How Does Africa's Most Hunted Bat Vary Across the Continent? Population Traits of the Straw-Coloured Fruit Bat (Eidolon helvum) and Its Interactions with Humans

“…These data provide further understanding of this species' ecology, demography and response to hunting, valuable for a wide range of studies. For example, as explored further elsewhere , factors such as a species' life history strategy, its population status, individual animal movements, the seasonal timing of the harvest, and heterogeneity in individual's susceptibility to harvest determine a population's response to harvesting pressure (Sandercock et al, 2011 …”

Grouse Hunting Regulations and Hunter Typologies in Norway

“…Key-words: bag limit, game, harvest, willingness-to-pay, ptarmigan Declining grouse populations is a pressing management issue worldwide (Storch, 2007). The underlying causes are complex, and while no study has yet identified one single factor that explains the declines at larger spatial scales, the most prominent threat seems to be habitat loss, degradation and fragmentation (e.g., Webb, Boarman, & Rotenberry, 2004;Marzluff & Neatherlin, 2006): changes that are long-lasting and not easily reversed.Additionally, we see growing evidence that game-bird hunting may be more additive to other causes of mortality than previously thought (Smith & Willebrand, 1999;Pedersen et al, 2004;Pöysä et al, 2004;Sandercock, Nilsen, Brøseth, & Pedersen, 2011;Connelly, Hagen, & Schroeder, 2011; but see also Sedinger, White, Espinosa, Partee, & Braun, 2010).It thus seems clear that harvest regulations are inevitable for mitigating grouse population declines. Management agencies then face a partly conflicting quest; they must achieve the ecological goal without overly restricting hunting opportunities and thereby jeopardize hunter satisfaction.…”

“…Additionally, we see growing evidence that game-bird hunting may be more additive to other causes of mortality than previously thought (Smith & Willebrand, 1999;Pedersen et al, 2004;Pöysä et al, 2004;Sandercock, Nilsen, Brøseth, & Pedersen, 2011;Connelly, Hagen, & Schroeder, 2011; but see also Sedinger, White, Espinosa, Partee, & Braun, 2010).…”