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No. 86: Adaptive Harvest Management for the Svalbard Population of Pink-Footed Geese. 2016 Progress Summary

Johnson, F.A. & Madsen, J. 2016. Adaptive Harvest Management for the Svalbard Population of Pink-Footed Geese. 2016 Progress Summary. Aarhus University, DCE – Danish Centre for Environment and Energy, 24 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 86 http://dce2.au.dk/pub/TR86.pdf

 

Summary

This document describes progress to date on the implementation of an adaptive harvest-management strategy designed to maintain the Svalbard population of pink-footed geese (Anser brachyrhynchus) near their 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 2016 harvest management strategy.

The development of an adaptive harvest management (AHM) strategy requires specification of four elements: (a) a set of alternative population models, which bound the uncertainty about effects of harvest and other relevant environmental factors; (b) a set of probabilities (or weights) 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 to choose; and (d) a management 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 were developed.  Those models 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 65,000 – 129,000 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 – 2015.  Current model weights suggest little evidence for density-dependent survival and reproduction.  These results suggest that the pink-footed goose population may have 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 in Svalbard (number of days with temperatures above freezing) on survival and reproduction.

During the initial years of AHM (2013-2015), the optimal harvest strategy prescribed a harvest quota of 15,000 per year, to be shared by Norway and Denmark.  The total harvest during the 2013-2015 hunting seasons was similar, but more variable (mean = 11,944, sd = 1,798), than during the preceding three years (mean = 11,380, sd = 588).  Population size was similar, but less variable, during the three years of AHM (mean = 74,823, sd = 1,165) compared to the preceding years (mean = 76,867, sd = 6,859).  Recent population counts confirm the suspicion that the count in May 2015 of 59,000 was biased low, and adjustments have been made.  The percentage of young in autumn during the three-year period of AHM was lower and less variable (mean = 0.120, sd = 0.018) than during the preceding three years (mean = 0.171, sd = 0.064).

Beginning with the 2016 hunting season, harvest quotas will be prescribed on an annual basis rather than every three years because of the potential to better meet population management objectives.  The optimal harvest strategy remains “knife-edged,” however, meaning that small changes in resource status can precipitate large changes in the annual harvest quota.  This is likely to be of concern to hunters, and the International Working Group is actively investigating ways in which large swings in harvest quotas might be dampened.  Based on updated model weights, the recent observations of population size (74,800), the proportion of the population comprised of one-year-old birds (0.138), and temperature days in Svalbard (20), the optimal harvest quota for the 2016 hunting season is 25,000.  The large increase in quota compared to that during first three years of AHM reflects stakeholders’ desire to reduce population size to the goal of 60,000, recognizing that population size remains relatively high and above-average production is expected in 2016 due to a warm spring.  The annual harvest quota is expected to average about 8,700 (sd = 9,600) over the long term.  We stress again, however, that high annual variability in the annual quota can be expected unless the management objective is modified to dampen it and/or the pink-footed goose population exhibits more density dependence.