| Summary: | The distributions and abundances of trout and salmon are strongly influenced by their habitat. The habitat includes both abiotic and biotic factors, which interact in complex webs. Habitat probably has strongest effects during population bottlenecks, when the standing stock approaches the carrying capacity of the environment. Various approaches to modelling interactions between habitat and population density and mean weight have been explored, but further work is needed in this area of investigation. The importance of depth, current, substrate, cover, and to a lesser extent, temperature and oxygen availability to the various stages of the life cycles of salmon and trout are briefly reviewed. By drawing on published data, it is possible to define broad ranges of acceptable conditions for the life stages of each species. However, it is not possible to partition this variation into between-population differences, within-population preferences, within-population tolerances, and effects of interactions between habitat variables. To pursue this important issue further, a structured approach using experimentation both in the field and in suitable laboratory systems is recommended. There is abundant evidence that habitat requirements of salmon and trout overlap. Trout tend to out-compete salmon except often in areas of particularly fast flows and, perhaps, remote from the river bank. The habitat requirements of year classes of salmon and trout overlap and therefore, there is scope for interactions between them depending on the spatial arrangement of habitats and the occurrence of bottlenecks. It is particularly important to understand where the bottlenecks to production lie and to focus on these in the first instance. Otherwise, there is a risk of manipulating habitat that is already in excess, or increasing numbers of a population that will subsequently be constrained, e.g., by over-wintering habitat. For this reason, it is prudent to accept that although manipulations of habitat may appear to be beneficial when ...
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