Sustainable harvesting strategies of Willow Ptarmigan in a fluctuating environment

Stochasticities in population dynamics as well as uncertainties in parameters often make it difficult to obtain reliable predictions of future population fluctuations. Here we use a long-term data set to model the fluctuations of a Willow Ptarmigan (Lagopus lagopus) population in Central Sweden, We use this model to examine how different hat-vest strategies affect mean annual yield, and how these results are influenced by uncertainties in parameter estimates. Restricted proportional harvest (each hunter is allowed to shoot only a limited number of grouse per day) gave slightly higher mean annual yields than proportional threshold harvesting (harvesting only a fixed proportion of the difference between the estimated population size and the threshold when this difference is positive). However, variance in annual yield was reduced by restricted proportional harvesting because periods with low population size became shorter. Uncertainties in population parameters did not affect which strategy was optimal although those uncertainties strongly influenced the expected yield and the uncertainties in the hunting statistics. Inaccuracies in population projections are therefore important to estimate and to model when developing sustained harvest strategies for fluctuating populations.