One of the great challenges of biology is to understand pattern and variation simultaneously. In the salmonids, this challenge arises in the context of the major life-history events of migration from fresh water to the sea and returning from sea-water to fresh water. We have developed
life-history models that combine proximate (physiological mechanism) and ultimate (natural selection) considerations and that allow us to understand both the pattern and the variation in Atlantic and coho salmon and steelhead trout. The conceptual framework can be implemented by stochastic
dynamic programming and leads to generalizations about top-down and bottom-up control of life histories, the evolution of diadromy, implications for the management of fisheries, the recovery of salmonid populations, and effective aquaculture. The salmonids are one of the best examples of the
principle that nature is indeed complicated and variable but much of that variability can be understood.
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