Juvenile Chinook Salmon Summer Microhabitat Availability, Use, and Selection in a Central Idaho Wilderness Stream

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Abstract We measured summer microhabitat use, availability, and selection by age‐0 Chinook salmon Oncorhynchus tshawytscha in the Big Creek drainage, Idaho. Age‐0 fish selected for low‐velocity (0‐25 cm/s), moderate‐depth (40‐80 cm) habitats that were located within 80 cm of cover. Pools (52%) and runs (38.5%) were the most commonly used habitat types, while pebbles (33.7%) and sand (23%) were the most often used substrates. Cover type use was predominated by woody debris (54.8%) and rock outcrops (23.7%). Run (38.5%) and riffle (32.9%) were the most available habitats in Big Creek, while pebble (38.4%) and cobble (28.2%) were the most available substrates. Mean water velocity (47 cm/s) availability and distance to cover (108 cm) availability were greater than those selected by age‐0 Chinook salmon, while mean total water depth (30 cm) availability was lower than that selected by the fish. Linear regression was used to show that an increase in juvenile Chinook salmon total length was significantly (P < 0.05) related to increased total water depth (r2 = 0.68), focal water depth (r2 = 0.73), and focal water velocity (r2 = 0.49) use. The relationship of habitat use and fish total lengths indicate that even within a short temporal period, juvenile Chinook salmon will select for different habitats as they grow. Upper and lower Big Creek microhabitat availability characteristics differed significantly (P < 0.05). Upper Big Creek had more fish per unit of preferred rearing habitat than lower Big Creek, which suggests that either summer microhabitat availability or redd density partially explain the density differences observed in Big Creek. Microhabitat use and availability data were useful for identifying habitat selection of age‐0 Chinook salmon in Big Creek. The data from this study can be used for future identification, quantification, and restoration of suitable Chinook salmon rearing habitat in other Pacific Northwest streams.

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