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No. 40: Adaptive Harvest Management for the Svalbard Population of Pink-Footed Geese. 2014 Progress Summary

Johnson, F.A., Madsen, J. & Jensen,G.H. 2014. Adaptive Harvest Management for the Svalbard Population of Pink-Footed Geese. 2014 Progress Summary. Aarhus University, DCE – Danish Centre for Environment and Energy, 22 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 40. dce2.au.dk/pub/TR40.pdf

Summary

This document describes progress to date on the development of an adaptive harvest-management strategy for maintaining the Svalbard population of pink-footed geese (Anser brachyrhynchus) near their agreed target level (60 thousand) by providing for sustainable harvests in Norway and Denmark.  Specifically, this report provides an assessment of the most recent monitoring information and its implications for the harvest management strategy.

The development of a passively adaptive harvest management strategy requires specification of four elements: (a) a set of alternative population models, describing the effects of harvest and other relevant environmental factors; (b) a set of probabilities describing the relative credibility of the alternative models, which are updated each year based on a comparison of model predictions and monitoring information; (c) a set of alternative harvest quotas, from which a 3-year quota is chosen; and (d) an objective function, by which alternative harvest strategies can be evaluated and an optimal strategy chosen. 

By combining varying hypotheses about survival and reproduction, a suite of nine models have been developed that represent a wide range of possibilities concerning the extent to which demographic rates are density dependent or independent, and the extent to which spring temperatures are important.  Five of the models incorporate density-dependent mechanisms that would maintain the population near a carrying capacity (i.e., in the absence of harvest) of 65k – 129k depending on the specific model.  The remaining four models are density independent and predict an exponentially growing population even with moderate levels of harvest.

The most current set of monitoring information was used to update model weights for the period 1991 – 2013.  Current model weights suggest little or no evidence for density-dependent survival and reproduction.  These results suggest that the pink-footed goose population may have recently experienced a release from density-dependent mechanisms, corresponding to the period of most rapid growth in population size.  There was equivocal evidence for the effect of May temperature days (number of days with temperatures above freezing: TempDays) on survival and on reproduction.

During the summer of 2013 we computed an optimal harvest strategy for the 3-year period 2013 – 2015. The strategy suggested that the appropriate annual harvest quota is 15 thousand. The 1-year harvest strategy calculated to determine whether an emergency closure of the hunting season is required this year suggested an allowable harvest of 25.0 thousand; thus, a hunting-season closure is not warranted.  If the harvest quota of 15 thousand were met in the coming hunting season, the next population count would be expected to be 71.0 thousand.  If only the most recent 4-year mean harvest were realized (11.3 thousand), a population size of 74.8 thousand would be expected.  Simulations suggest that it will take approximately seven years at current harvest levels to reduce population size to the goal of 60 thousand.  However, it is possible that the extension of the forthcoming hunting season in Denmark could result in a total harvest approaching 15 thousand; in this case, simulations suggest it would only take about three years to reach the goal.