Prediction of Stream Carrying Capacity for Steelhead: the Unit Characteristic Method

—We describe and demonstrate the Unit Characteristic Method (UCM) as a means by which measurements of habitat from typical stream surveys can be used to estimate the capacity of a stream to rear juvenile steelhead Oncorhynchus mykiss. Channel unit features of importance include surface area by unit type, depth, substrate, and cover. The influence of a stream’s primary productivity is represented in the method through measures of alkalinity and turbidity. We tested the fit of model predictions to juvenile steelhead production observed in seven watersheds ranging in size from 26 to 1,420 km. Model predictions of capacity were significantly correlated to observed maximum production of juvenile steelhead (P < 0.005, R = 0.88), as was watershed area (P < 0.005, R = 0.88). The UCM predictions revealed that parr capacity was unevenly distributed in the watersheds, and that habitat quality (smolt capacity/m) differed between reaches among all watersheds by up to 15-fold across seven basins surveyed, and ranged more than 10-fold between reaches within four of seven test watersheds. Thus, the UCM can be used to discriminate stream reaches and features that either warrant habitat restoration or conservation. Key factors driving high or low habitat quality differed between reaches, and included pool area, riffle depth, boulder substrate, alkalinity, fine sediment, and turbidity. The UCM provides a framework for understanding the habitat features that determine the production potential of a basin, for identifying factors that limit production, and for predicting potential fish benefits from differing habitat management strategies.