Identification and Characterization of Juvenile Spring Chinook Salmon Overwinter Rearing Habitat in Upper Grande Ronde Valley

Management and restoration of spring Chinook salmon require knowledge of life history attributes, including overwinter rearing habitat within natal and rearing streams and spatiotemporal occupancy. Early migrant (i.e., fish overwintering downstream of natal areas) survival to Lower Granite Dam is typically lower for Catherine Creek juvenile spring Chinook salmon compared to other such populations in the Grande Ronde subbasin. Our objective was to identify and characterize overwintering reaches and microhabitat use within Catherine Creek for naturally produced juvenile early migrant Chinook salmon. Radiotelemetry techniques were selected to yield high resolution occupancy data and microhabitat suitability. Fish were collected using a rotary screw trap and implanted with a coded radio-tag from mid-October to early-December during 2009 and 2010. Fish weighing ≥ 8.5g were selected for tagging to restrict tag burden to ≤3%. Effort was made to relocate each fish once a week, and microhabitat use data was collected for 30 randomly selected fish per week. Mortality of radio-tagged juvenile Chinook salmon appeared relatively low (12%) during 2009. During 2009, early migrants occupied a reach of Catherine Creek residing between Union, OR and the mouth of Mill Creek for overwinter rearing (i.e., ~ 30 km), while distribution during 2010 extended outside the study area (i.e., >90 km). Median weekly linear range was high during fall migration however, decreased toward zero (i.e., no movement) during winter both years. During both 2009 and 2010, a considerable increase in movement occurred during late-winter and coincided with elevated water temperatures. During both years, linear regression revealed no significant relationship between total linear range and size. A gradient shift occurs within lower Catherine Creek near the mouth of Pyles Creek, where Catherine Creek transitions from complex riffle, run, pool habitat to homogenized deep run habitat. Catherine Creek early migrant juvenile spring Chinook salmon selected microhabitat non-randomly during both 2009 and 2010. Juvenile spring Chinook salmon preferred deep water and slow currents near cover and the bank throughout their distribution; however, coarse substrates were optimal within the high gradient reach, while silt was most suitable in low gradient reaches. Multiple regression analyses (PCA) indicate that Catherine Creek early migrant juvenile Chinook salmon occupy slow pools near cover in high gradient reaches, while slow runs with cover are occupied in low gradient reaches. Our results increase knowledge of Catherine Creek juvenile spring Chinook salmon overwinter rearing behavior, seasonal microhabitat suitability, and suggest critical management issues for this severely depressed population.