Adaptive harvest management for the Svalbard population of pink‐footed geese: 2019 progress summary
This report describes an Adaptive Harvest Management (AHM) program designed to maintain the Svalbard population of Pink-footed Geese (Anser brachyrhynchus) near their target level (60,000) by providing sustainable harvests in Norway and Denmark. Specifically, this report provides recent monitoring and assessment results and their implications for the 2019 hunting season.
In this report we provide results for both the set of nine discrete population models used since 2013 and for a recently developed Integrated Population Model (IPM). Updating model weights (set of nine models) and model parameters (IPM) use the most recent monitoring information available (i.e. through spring 2019). We also compare optimal harvest strategies based on the two modeling approaches. Of growing concern, however, has been the observation that the predictive ability of the original population models has declined over time. Going forward, we suggest that use of the IPM is a superior modeling approach for setting hunting seasons for
Population sizes in May as estimated by the IPM are in general agreement with counts and capture-markrecapture estimates but are more precise. Estimates of survival from natural causes were relatively constant over time with a mean of 0.93 and estimates of adult kill rate ranged from 0.04 to 0.13. Estimates of countryspecific kill rates suggest that most of the overall increase in kill rate in recent years is attributed to increasing harvest pressure in Denmark. With respect to productivity, the IPM produced estimates of preseason age ratio that were variable over time, but on average suggested that young constituted about 18% of the population just
prior to the hunting season. Estimates of the preseason age ratio correlated well with the number of days above freezing in May in Svalbard. Our results suggest that population size has stabilized because of declining survival rates that have accompanied an increase in kill rates. Adoption of the IPM this year would result in a harvest quota for the 2019 hunting season of 22,000, based on a May population estimate of 76,500 (95% CI: 66,800 – 86,600) and 8 days above freezing in Svalbard. Using the agreed upon allocation of the total allowable
harvest, the quotas are 6,600 for Norway and 15,400 for Denmark. Assuming the total quota is met, the IPM predicts a population size in May 2020 of 64,900 (95% CI: 50,300 – 83,700).
We note that the updated set of original models used since 2013 suggests an allowable harvest of 40,000 in 2019 based on a November 2018 count of 91,900 geese (comprised of 12,900 young and 79,000 adults). We are skeptical of this large quota, however. Using all available demographic data suggests that the November 2018 population size was lower than that indicated by the count and that the winter population experienced more natural mortality than expected. These insights demonstrate the inherent risk of basing a harvest quota
on a single population count, irrespective of other demographic data.
|Adaptive harvest management for the Svalbard population of pink‐footed geese: 2019 progress summary
|Fred Johnson, Henning Heldbjerg, Kevin K. Clausen, Jesper Madsen
|Other Government Series
|USGS Publications Warehouse
|Wetland and Aquatic Research Center