Consequences of past and present harvest management in a declining flyway population of common eidersSomateria mollissima

Abstract

Harvested species population dynamics are shaped by the relative contribution of natural and harvest mortality. Natural mortality is usually not under management control, so managers must continuously adjust harvest rates to prevent overexploitation. Ideally, this requires regular assessment of the contribution of harvest to total mortality and how this affects population dynamics.To assess the impact of hunting mortality on the dynamics of the rapidly declining Baltic/Wadden Sea population of common eiders Somateria mollissima, we first estimated vital rates of ten study colonies over the period 1970–2015. By means of a multi‐event capture–recovery model, we then used the cause of death of recovered individuals to estimate proportions of adult females that died due to hunting or other causes. Finally, we adopted a stochastic matrix population modeling approach based on simulations to investigate the effect of past and present harvest regulations on changes in flyway population size and composition.Results showed that even the complete ban on shooting females implemented in 2014 in Denmark, where most hunting takes place, was not enough to stop the population decline given current levels of natural female mortality. Despite continued hunting of males, our predictions suggest that the proportion of females will continue to decline unless natural mortality of the females is reduced.Although levels of natural mortality must decrease to halt the decline of this population, we advocate that the current hunting ban on females is maintained while further investigations of factors causing increased levels of natural mortality among females are undertaken. Synthesis and applications. At the flyway scale, continuous and accurate estimates of vital rates and the relative contribution of harvest versus other mortality causes are increasingly important as the population effect of adjusting harvest rates is most effectively evaluated within a model‐based adaptive management framework.

Drivers of Spatiotemporal Variation in Survival in a Flyway Population: A Multi-Colony Study

“…We used data from the Danish Wadden Sea to test for an effect of blue mussel body condition on survival at Vlieland, whereas for the other colonies we used annual averages over several widely distributed stations located within the wintering area. Furthermore, we tested the effect of hunting pressure estimated as a killrate h , i.e., the probability of dying due to hunting within a year (Tjørnløv et al, 2019). Moreover, we identified 1984Moreover, we identified , 1985Moreover, we identified , 1996Moreover, we identified , 2001Moreover, we identified , 2003Moreover, we identified , 2008Moreover, we identified , and 2009 as years with strikes of avian cholera, and tested how outbreaks of epidemic disease in a given colony affected survival of the females nesting there.…”

“…Finally, spring counts of eiders migrating into the Baltic Sea suggests that the flyway population sex ratio has shifted toward an increasing male bias (Lehikoinen et al, 2008;Berg, 2014;Ramula et al, 2018), which can mainly be attributed to sex-specific survival of prime reproductive-age individuals (≥5-year-old) during the breeding season (Ramula et al, 2018). An evaluation of hunting at the flyway scale suggests that in recent decades hunting has been sustainable and wellregulated (Tjørnløv et al, 2019), ultimately resulting in a ban on hunting females in Denmark (where most hunting takes place) from 2014 onward (Christensen and Hounisen, 2014).…”

Vital rate estimates for the common eiderSomateria mollissima, a data‐rich exemplar of the seaduck tribe

“…First, due to the relative ease of trapping nesting female common eiders (Waltho & Coulson, 2015), most survival estimates relate to adult females rather than males or juveniles. Since males are rarely the limiting sex, most seaduck MPMs are female‐only, although having additional estimates of male survival could help to distinguish sex‐specific mortality (Flint, 2015; Allen et al., 2019; Tjørnløv et al., 2019). Second, only a small minority of studies provided enough information to categorize population trend; while we appreciate that much data‐gathering relates to finer‐scale questions, we suspect that a broad‐scale categorization would be possible in many cases, facilitating greater contextualization of reported demographic parameters.…”

Long‐term trends in the survival rates of adult female Common EiderSomateria mollissimaat three colonies in Iceland

“…2006) and since 2014 only males have been legal quarry (Tjørnløv et al . 2019). Control of predators such as Arctic Foxes Vulpes lagopus and gulls Larus spp.…”

“…Because of the economic and cultural importance of the Common Eider down collected at breeding sites (Doughty 1979, Jónsson 2001, Petersen 2005, Common Eiders have been legally protected from human hunting and persecution in Iceland since 1787, unlike some populations in North America and Europe that are exposed to sport and subsistence hunting (Stephensen & Sigurðsson 1855). Gilliland et al (2009) estimated that 55 000-70 000 Common Eiders were shot annually in Greenland during 1993-2000, and Desholm et al (2002 estimated that approximately 115 000 were shot annually in Fenno-Scandinavia, although the hunting season for females was shortened in Denmark in 2004/2005(Bregnballe et al 2006 and since 2014 only males have been legal quarry (Tjørnløv et al 2019). Control of predators such as Arctic Foxes Vulpes lagopus and gulls Larus spp.…”