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Nr. 64: Adaptive Harvest Management for the Svalbard Population of Pink-footed Geese. 2015 Progress Summary

Johnson, F.A. & Madsen, J. 2015. Adaptive Harvest Management for the Svalbard Population of Pink-footed Geese. 2015 Progress Summary. Aarhus University, DCE – Danish Centre for Environment and Energy, 24 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 64. dce2.au.dk/pub/TR64.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 (60,000) 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, and the extent to which spring temperatures influence survival and reproduction.  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 – 2014.  Current model weights suggest little 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 is equivocal evidence for the effect of May temperature days (number of days with temperatures above freezing: TempDays) on survival and on reproduction.

The optimal harvest strategy for the 2013–2015 hunting seasons prescribed a harvest quota of 15,000 per year.  The harvest in the 2014 hunting season was 14,991, compared to 11,081 in 2013, mostly due to an increase in harvest in Denmark during January 2015.  The percentage of young in the fall of 2014 was 10.3%, which is lower than average.  The observed population size of 59,000 in May 2015 was much lower than expected.  For the 2015 hunting season, observed population size and temperature days suggest that an emergency closure should be considered.  In the event a harvest of 15,000 is maintained, predicted population size in May 2016 is 51,700 (95% CL: 41,600-64,300), based on observed TempDays = 9 in May 2015 and the most recent model weights.  On the other hand, if the season were closed this year, we would expect a population size of 66,700 (95% CL: 53,600-82,900) in May 2016.  A total harvest of 6,700 would be expected to result in a 2016 population size at goal (i.e., 60,000).