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No. 28: Adaptive Harvest Management for the Svalbard Population of Pink-Footed Geese

Johnson, F.A. & Madsen, J. 2013. Adaptive Harvest Management for the Svalbard Population of Pink-Footed Geese. Assessment for the 2013-2015 hunting seasons. Aarhus University, DCE – Danish Centre for Environment and Energy, 20 pp. Technical Report from DCE – Danish Centre for Environment and Energy No. 28 dce2.au.dk/pub/TR28.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 optimal harvest quota for the 2013-2015 hunting seasons and describes a process for evaluating whether emergency hunting closures would be needed during that period.

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 1991 – 2012 period. Current model weights suggest no evidence for density-dependent survival and only slightly more evidence for the three models incorporating density-dependent 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.

Based on the most recent model weights, an optimal harvest strategy was computed for the 3-year period 2013-2015. The strategy suggests that the appropriate annual harvest quota for the 2013-2015 period is 15 thousand; hence there is no need to take emergency measures to close the upcoming hunting season. For comparison, the estimated harvest in 2012 was 11 thousand. If the harvest quota of 15 thousand were met, the autumn 2013 population count is expected to be 76 thousand. If only the most recent 3-year mean harvest were realized (11.5 thousand), an autumn population size of 80 thousand is expected. Thus, it may be that harvest is approaching the magnitude needed to stabilize the population.