Biophysical factors driving the distribution and abundance of redband/steelhead trout (Oncorhynchus mykiss gairdneri) in the South Fork John Day River Basin, Oregon, USA

Abstract

The overall goal of this study was to identify multiple scales of habitat use and habitat electivity by redband/steelhead trout and define the limiting factors affecting the distribution patterns of this species during summer flows. The main objective in chapter 2 was to identify the most important habitat associations that control the distribution patterns of threatened populations of redband/steelhead trout in the South Fork John Day River Basin. I found that the factors influencing the distribution patterns of trout in the basin were dependent on the thermal context and spatial extent of analysis. The inclusion of water temperature alone at large spatial scales explained most of the distribution of trout at the basin scale and that channel morphology was a very important factor at small spatial scales. The main objective in chapter 3 was to identify geophysical factors influencing the distribution pattern of threatened populations of redband/steelhead trout in the basin at multiple spatial scales. I found that the trout distribution patterns in the area could be described as a clustered distribution, with a strong selection for upstream-cold water reaches and small, well oxygenated, running water pools. My study reveals that the most influential factor at large spatial scales was water temperature and smaller scale channel morphology. Chapter 4 examines whether longitudinal-summer-stream temperature profiles in semi arid-environment streams can be used to index carrying capacities of threatened populations of redband/steelhead trout. My results show that, in the South Fork John Day Basin, stream temperature can be used as indicator of trout carrying capacity. The distribution redband/steelhead trout in summer is largely determined by the- physiologically influenced-preference that individuals have for habitats within specific temperature ranges.