Distribution, habitat use, and growth of juvenile Chinook salmon in the Metolius River Basin, Oregon

Abstract

Chinook salmon (Oncorhynchus tshawytscha) have been absent from their historic spawning and rearing grounds in the Metolius River Basin in central Oregon since 1968, when fish passage was terminated at the Pelton Round Butte Hydroelectric Project on the Deschutes River. Plans have been developed to reestablish passage of anadromous fish through the Project. However, only anecdotal evidence exists on the historic distribution of spring Chinook juveniles in the Basin. A recent approach to characterizing habitat quality for anadromous fishes in the Basin was the development of HabRate (Burke et al. In Press), which presented a relative quality rating of habitat based upon published fish-habitat relationships at the stream reach spatial scale. The present study was initiated to test the predictions of HabRate for summer rearing juvenile Chinook salmon in the Metolius Basin. Chinook salmon fry were released in the winters of 2002 and 2003, and their densities and sizes were quantified via snorkeling and fish collection in six unique study reaches in the upper Metolius River Basin. Each of these stream reaches varied in terms of temperature, habitat availability, invertebrate drift availability, and fish community composition. My observations were not consistent with the qualitative predictions of HabRate. Moreover, habitat utilization was not consistent among study reaches. Similar to other qualitative habitat rating models (e.g. Habitat Suitability Indices (Raleigh et al. 1986) and Instream Flow Incremental Methodology (Bovee 1982)), HabRate's predictions rely solely on physical habitat characteristics, with the assumption that habitat will be used consistently among stream reaches (i.e. a pool in one reach is of equal importance as a pool in another reach). My results suggest that the unique ecological setting of each study reach provides the context for understanding the patterns of growth, habitat use, and diurnal activity of juvenile Chinook salmon. The inclusion of ecological components, such as food availability, the bioenergetic constraints of temperature, and the risk of predation can make these models more biologically realistic. Growth of juvenile Chinook salmon among study reaches had a curvilinear relationship to water temperature, and was also positively related to the drift density of invertebrate biomass. In three collection seasons (fall 2002, spring 2003 and fall 2003) 41 to 69% of the variations in fork lengths were explained by a multiple regression model including temperature and invertebrate drift. Based on these findings, I present a conceptual growth capacity model based on the tenets of bioenergetics as a basis for understanding the relative quality of the habitat among stream reaches for juvenile Chinook salmon. Fish community composition can help to explain observed patterns in habitat utilization and diel activity patterns. In the study reaches that had a greater presence of adult trout (potential predators), observations of juvenile Chinook salmon in mid-channel habitat were infrequent to non-existent during the day and abundances were higher in all habitat types at night. In the study reaches with colder water temperatures, observed juvenile Chinook salmon densities were higher at night. I suggest that habitat selection and diurnal activity patterns in some study reaches are reflective of strategies taken by the fish to minimize risks of predation.