SEASONAL DENSITY-DEPENDENCE, TIMING OF MORTALITY, AND SUSTAINABLE HARVESTING

Birth-pulse populations are often characterized with discrete-time mod els, that use a single function to relate the post-breeding population size to the post-breeding size of the previous year. Recently, models of seasonal density dependence have been constructed that emphasize i nteractions during shorter time periods also. Here, we study two very simple forms of density-dependent mortality, that lead to Ricker and B everton-Holt type population dynamics when viewed over the whole year. We explore the consequences of harvest timing to equilibrium populati on sizes under such density dependence. Whether or not individual mort ality compensates for the harvested quota, the timing of harvesting ha s a strong impact on the sustainability of a harvesting quota. Further , we show that careless discretization of a continuous mortality schem e may seriously underestimate the reduction in population size caused by hunting and overestimate the sustainable yield. We also introduce t he concept of the demographic value of an individual, which reflects t he expected contribution to population size over time in the presence of density dependence. Finally, we discuss the possibility of calculat ing demographic values as means of optimizing harvest strategies. Here , a Pareto optimal harvest strategy will minimize the loss of demograp hic value from the population for a given yield. (C) 1998 Elsevier Sci ence B.V. All rights reserved.