MODELING THE PROXIMATE BASIS OF SALMONID LIFE-HISTORY VARIATION, WITHAPPLICATION TO ATLANTIC SALMON, SALMO-SALAR L

The great diversity of life-history patterns in the salmonids has stim ulated many theoretical studies. However, virtually all studies are ba sed on ultimate considerations, in which predictions are made by compa ring the expected reproductive success of different developmental or l ife-history pathways and choosing the one (or ones) with the highest f itness. Such models are post hoc because they attribute fitness to ind ividuals at the completion of the particular phase of the life cycle a nd do not attempt to characterize the mechanisms that animals use to a chieve the life-history pattern. We describe a model, based on proxima te considerations, for salmonid life histories, focused on Atlantic sa lmon Salmo salar L. The model involves identification of the times at which developmental conversions are initiated or inhibited and the con nection between physiological states and the thresholds for such conve rsions. Developmental paths are based on the comparison of the current physiological status of the fish (and its change of state) with a gen etic threshold. The state of the fish and rate of change of state are determined by environmental opportunity, but the threshold is genetic. This approach therefore immediately generates a genotype-environment interaction. We use expected reproductive success to determine the fit ness of individuals with different genetically determined thresholds. Instead of finding an optimal life history, our theory generates fitne ss surfaces for different life histories, so that variation is inheren t in this approach. We describe and explain the structure of the model and present evidence on which this structure is based, thus providing a framework within which one can understand how ecology relates to th e physiological mechanisms leading to the developmental changes of sme lt metamorphosis and maturation.