We investigated the spawning biology of California golden trout Oncorhynchus mykiss aguabonita, an endemic subspecies of rainbow trout, in the Golden Trout Wilderness, California. We investigated the influence of stream temperature on the seasonal and daily timing of spawning, measured characteristics of completed redds, and quantified microhabitat use and preferences by spawning females. We also quantified size at sexual maturity, degree of sexual size dimorphism, and the operational sex ratio during the spawning season. Golden trout spawning began after peak stream discharge in mid-May, when maximum daily water temperatures consistently exceeded 15 degrees C and continued for approximately 3 weeks. Stream temperatures also influenced the daily timing of spawning activity, with highest activity in the afternoon when stream temperatures approached the daily maximum. Among sexually mature fish (>95 mm), males were significantly larger than females, and ripe males were 3-9 times more abundant than ripe females. Females spawning early in the season were larger and constructed their redds in larger substrates than later-spawning females. Redds were characterized by very small substrates, and females showed distinct preferences for particular substrate sizes, water depths, and water velocities. Females selected spawning sites characterized by mean substrate sizes of 4-12 mm, water depths of 5-20 cm, and water velocities of 30-70 cm/s. Compared to spawning microhabitat preferences of other salmonids, female golden trout used smaller substrates, shallower water depths, and dug shallower nests. These differences may be the result of the unusually small size of golden trout in our study population, relative to salmonids used in previous studies of spawning microhabitat.
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