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UBC Theses and Dissertations

Optimal stock size and harvest rate in multistage life history models : Pacific salmon Oncorhynchus spp. Moussalli, Elie Ibrahim


A simple life history model that analyzes the relationships between habitat capacity, survival rates, optimal escapement, and harvest rate for Pacific salmon (Oncorhynchus spp.), is implicit in nearly all considerations of salmon management. This model consists of density dependent events at spawning and rearing, and density independent prespawning mortality, egg-to-fry survival and smolt-to-adult survival. The analysis of this model yields simple relationships between key parameters and optimal escapement, and optimal harvest rate. Interventions with the salmonid life cycle such as flow regulation, habitat improvement or degradation have simple consequences on optimal escapement and harvest rate. Similarly, detectable long term trends in the ocean survival rate will affect the optimal escapement and harvest rate and thus may be incorporated in management decisions. If there is a density dependent limit later in the life history, an increase in the survival rate will result in a lower optimum escapement and a higher optimum harvest rate. If there is no density dependent limit later in the .life history an increase in survival will result in a higher optimum escapement and a higher optimum harvest rate. I also show that management by escapement requires information on habitat capacity and stock productivity; whereas management by harvest rate requires only the latter. This model is generalized to a multistage form to represent any harvestable population consisting of successive Beverton and Holt type density dependent stock-recruitment stages. This multistage stock-recruitment model is expressed recursively in terms of the cumulative parameters up to and including any arbitrary stage. The optimal spawning stock size (escapement) and harvest rate are, analyzed in terms of the parameters of any intermediate stage. The results of this extension confirm those of the salmonid model and generalize them.

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