The dynamics in harvested populations
Abstract: We are interested in the effect of harvesting on population fluctuations and for that purpose we develop and analyze an age-structured model where recruitment is a stochastic process and the adult segment of the population is harvested. When a constant annual harvest is taken the coefficient of variation of the adult population increases for most parameter values due to the age truncation effect, i.e. an increased variability in a juvenescent population due to the removal of older individuals. However, if a constant proportion of the adults is harvested the age truncation effect is sometimes counteracted by a stabilizing dynamic effect of harvesting. To study the effect of a variable harvest we also modelled harvesting as a stochastic processes. We find that harvest noise always has a magnifying effect on the fluctuations of the exploited population. We use cod survey data between 1988 and 2010 and auxiliary environmental data to estimate the parameters in a single species state space model, which is used to predict future development of the eastern Baltic cod (Gadus morhua) under different fishing mortality and assumptions about future abiotic conditions. We have demonstrated that exploitation is far more important than abiotic conditions in driving the temporal fluctuations of the Eastern Baltic cod. We analyzed a stage-structured stochastic environment model of the vendace (Coregonus albula) population in the Bothnian Bay with winter water temperature and winter salinity as environmental variables. We calculated the elasticity of the vendace stochastic growth rate (λs) with respect to the mean and standard deviation in vital rates, salinity and water temperature. We found that the mean value of salinity had the largest influence on λs. The high elasticity of λs with respect to the mean juvenile survival for a fished population indicates that one should avoid harvest the youngest individuals.
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