Urban and agricultural development has resulted in drastically modified riverine corridors that are often considered to be detrimental to the recovery of anadromous salmonid populations. Although mitigation features (e.g., large wood and shallow-water areas) are frequently incorporated in flood control infrastructure to offset the impacts of streambank stabilization, little is known regarding their effectiveness and the habitat characteristics associated with enhanced nearshore rearing conditions in large rivers. We evaluated two measures of habitat use by emigrating juvenile Chinook Salmon Oncorhynchus tshawytscha relative to different shoreline types (rock revetment [riprap], mitigated, and natural) and analyzed associations between environmental characteristics and habitat occupancy using a large number of presence/absence samples in the lower Sacramento River, California. We found both measures of habitat use to be significantly higher at natural shorelines and those including mitigation features than at shorelines consisting predominantly of rock revetment. A predictive logistic regression model suggested that the density of woody material and inundated terrestrial vegetation, depth, and substrate type significantly affected habitat occupancy. Despite a moderate predictive capability (62% of correctly classified records in a leave-one-out simulation), the model was useful in identifying habitat characteristics associated with significantly increased habitat use in this large, low-gradient river, most notably the presence of instream cover (wood or vegetation), gently sloping streambanks, fine substrate, and variable nearshore current velocity. Conversely, habitat occupancy by juvenile Chinook Salmon diminished with large, rocky substrate and increased depth, characteristics favored by introduced predatory Smallmouth Bass Micropterus dolomieu. This study illustrates the value of incorporating mitigation features and identifies characteristics that enhance habitat use by emigrating juvenile Chinook Salmon.Urban and agricultural development along large, salmon-bearing streams of the Pacific Northwest has resulted in the need for and construction of extensive flood control features, such as levees and weirs, to protect valuable agricultural lands, public infrastructure, and private property. As a consequence, nearshore riverine habitat, which is
Fish behavior in response to levee repairs at River Mile 85.6 of the Sacramento River was monitored using 60 VR2W 180-kHz and 45 VR2W 69-kHz receivers to create a large-scale, two-dimensional (2-D) positioning system. Research teams released 250 late-fall Chinook salmon smolts from Coleman National Fish Hatchery into the river near Knights Landing, California. Smolts were implanted with VEMCO V6 180-kHz acoustic tags. Other research teams released late-fall Chinook, steelhead trout, smallmouth bass, striped bass, green sturgeon, and white sturgeon tagged with 69.0-kHz equipment. A stretch of river that was approximately 750 × 90 m in size was monitored from 13 January 2011 until 5 March 2011. More than 7,000 2-D positions were calculated for 215 late-fall Chinook salmon released near the study site. The majority (88.8%) of smolts never came within 20 m of the levee repair site, and their tracks occurred in the channel or away from the levee repair site with individuals rapidly moving through the study site. The average transit time through the study site was 14 minutes 04 seconds. No significant difference was found in the migratory rates of smolts released at different locations (p = 0.084). There was a strong diel component to the movement of juvenile Chinook salmon smolts at the study site (p < 0.01). A significant difference was observed in the survival rate of the smolts when they were grouped by size (<130 mm = small; >130 mm = large), but not by release date. The levee repair site did not appear to have a positive influence on smolt recruitment. Many of the smolts that approached the repair site subsequently moved away from the area, and smolts detected within the levee repair area were not observed to have significantly longer transit times through the site than fish that were observed in the channel or on the opposite bank (p = 0.54). A small number of individuals exhibited a searching or foraging behavior, and several of these individuals had a higher degree of variation in their bearings.
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