Optimal harvest of age‐structured populations of moose Alces alces in a fluctuating environment
2001
Sæther, Bernt‐Erik | Engen, Steinar | Solberg, Erling J.
We analysed sex‐ and age‐specific harvesting strategies of moose using an age‐structured population model that includes density dependence as well as environmental stochasticity. In order to find the strategy that maximises the mean annual yield we simulated the process over a large number of years. The mean annual yield is a function of the three parameters (number of harvested individuals of calves, adult (≥ ½ years old) bulls and adult females) that are involved in the definitions of the strategies. We compare, by numerical maximisation of a function in several variables, two harvest strategies: proportional harvesting, i.e. removal of a certain proportion of individuals in a given age‐and sex‐class, and threshold harvesting, i.e. all individuals of a given sex‐ and age‐class are harvested when the size of this subpopulation exceeds a certain threshold. In general, proportional harvest gives a smaller mean annual yield than threshold harvesting. The variance in the annual yield is, however, larger for threshold than for proportional harvesting. These differences between the two harvest strategies increase when the annual survival of calves is low, when there is high environmental stochasticity and when there is strong density regulation operating on survival. For both harvest strategies, the optimal harvest strategy involves high harvest of calves and adult bulls, whereas adult females should hardly be harvested.
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