Anadromous fish require specific types of habitat for each stage of their life cycle. Anadromous life cycles are viewed as a series of habitat needs: pre-spawner estuary holding, pre-spawner upstream migration, pre-spawner in-river holding, spawning/incubation, juvenile winter rearing, juvenile summer rearing, downstream migration, and estuary rearing.

Abundance and distribution of anadromous salmonids is influenced by many biotic and abiotic factors. Biotic influences include interactions with predators, inter-species competition, intra-species competition, parasites, and disease organisms. Abiotic factors include water temperature, water nutrient content, and stream substrate composition. These factors, operating in combination, create both density-dependent and density-independent effects on fish populations.

Density-independent effects on populations are primarily driven by habitat quality. For example, density-independent effects increase as habitat quality deteriorates such as through decreasing size of sediment particles. Density-independent effects on the total production of a life history pattern can be shown as a multiplicative relationship (Table 30). Conversely, density-dependent processes are influenced by habitat quantity. Density-dependent mortality increases as larger populations occupy the same or smaller area (Lestelle et al. 1996).

Life history patterns of anadromous salmonid stocks have evolved to exploit the unique temporal and spatial variation in each river ecosystem. Timing of life history stages is adapted to patterns of stream flow, variations in predation risk, and other factors. In all river systems, relative success of the various life history patterns varies each year. In any given year, the relative success of a particular anadromous life history pattern depends on the abundance and distribution of suitable habitat for each life stage and the ability "to migrate at the appropriate time between links in the habitat chain" (Lichatowich et al. 1995). Furthermore, when rates of input of water, sediment, and wood are altered by natural events or human activities, the entire stream ecosystem is modified which influences reproductive success of each life history pattern. Thus life history patterns that do not presently contribute greatly to anadromous salmonid production may become important as habitat changes over years and decades.

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