<em>Abstract.</em>—Stream carrying capacity for anadromous salmonids that rear to the smolting stage in freshwater can be predicted from a sequence of cause-response functions that describe fish preferences for macro-habitat features. The channel unit (e.g., pool, glide, riffle) is a useful stratum for quantifying rearing capacity for salmonids, and is a hydrologically meaningful unit for predicting the response of stream morphology to watershed processes. Thus, channel units are the natural link between habitat-forming processes and habitat requirements of salmonids. Maximum densities of juvenile salmonids that can be supported in a channel unit are related to availability of preferred habitat features including velocity, depth, cover, and substrate. Within channel unit types, maximum densities of salmonid parr will shift predictably as availability of cover from wood and boulders increases. Within stream reaches, additional variation in maximum rearing densities can be accounted for by light penetration and nutrient load. As salmonids grow, their habitat preferences change and the preferred habitat associated with their increasing size becomes less and less available. Further, territory size of salmonids increases exponentially with fish length, such that the demand for territory to support surviving members of a cohort increases at least through their first year of life. Changing habitat preferences and space demands, juxtaposed against shrinking habitat availability with the onset of summer low flows often results in a bottleneck to rearing capacity for age >1 salmonids in wadable streams. Habitat measurements in Oregon streams indicate that depths preferred by steelhead (anadromous rainbow trout) <em>Oncorhynchus mykiss </em>become scarce as parr exceed 15 cm in length, which coincides with the approximate threshold length for steelhead smolts. We present a generalized framework, called the Unit Characteristic Method, for accumulating effects of these habitat factors at the channel unit and reach-level scales to estimate carrying capacity for rearing salmonids in a basin. Our subsequent chapter in this book presents a demonstration of how this method can be applied to predicting salmonid production in streams.
Associations among Fish Length, Dam Passage History, and Survival to Adulthood in Two At-Risk Species of Pacific Salmon