Sex differences and data quality as determinants of income from hunting red deer Cervus elaphus
Milner-Gulland EJ., Coulson T., Clutton-Brock TH.
When hunting species in which the sexes differ substantially in value, sex-selective harvesting can increase income dramatically. In some hunted species, for example the red deer Cervus elaphus in Scotland, there are also marked ecological differences between the sexes. In red deer, stag mortality and dispersal rates are substantially higher when hind densities are high. Hence, there is a trade-off between having enough hinds to produce valuable stags, but keeping densities low enough to minimise losses from dispersal and stag mortality. We develop a model, parameterised for red deer on Rum, to explore these trade-offs. This stochastic, age and sex-structured model includes two neighbouring estates with differing harvesting policies. Due to stag dispersal, estates with low hunting levels act as sources of stags for neighbouring estates that harvest more heavily. The optimal harvesting strategy depends on the actions of neighbours, but keeps hinds below 50% of carrying capacity and imposes heavy hunting pressure on stags. Scottish deer estates aim to harvest fewer stags and more hinds than the model suggests as optimal, which could lead to substantially reduced incomes. We explore the reasons for this mismatch between predicted optimal behaviour and actual harvesting strategies by incorporating realistic levels of uncertainty, bias and infrequent population counts into our model. We show that the estates' harvesting strategies lead to approximately optimal hind harvesting, because hind numbers are generally underestimated in counts, whereas the uncertainty surrounding population sizes leads to a lower than optimal stag harvest. The most effective method of improving incomes is to increase count frequency. This modelling approach is broadly applicable, both for the management of hunted species under uncertainty and for spatially explicit conservation policies such as no-take areas.