Harvest Optimization in Populations with Age Distributions

A harvester wishing to crop a population is faced with the choice of which age groups to remove. This paper considers how the choice should be made in a population of females with seasonal births, when the harvest is to be sustained indefinitely. The numbers which can be removed from any age class depend on their expectation of future progeny, as measured by a "harvest cost" function (related to Fisher's [1958] reproductive value). The smaller the harvest cost in any age class, the greater the proportion which can be harvested. Thus the smallest harvest is always obtained from a partial crop of the age class with greatest harvest cost. Under certain restricted conditions, the greatest harvest comes from a partial crop of the age class with smallest harvest cost, but it is more usual for the greatest harvest to require a crop of two age classes. The use of the harvest cost function is illustrated with data from a population of ringed seals.