Résumé
1. 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. 2. 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 modelling approach based on simulations to investigate the
effect of past and present harvest regulations on changes in flyway
population size and composition. 3. 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. 4. 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. 5. 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.