Increments of a fish otolith are commonly used to estimate age and somatic growth; yet the accuracy of such estimates first requires an understanding of the periodicity with which increments are formed. We conducted a rearing experiment to evaluate daily formation of increments in otoliths from spring Chinook salmon (Oncorhynchus tshawytscha), an anadromous fish from the Yakima River, Washington. Specifically, we compared the known number of post-emergence days that fish were alive to the number of otolith increments formed after an emergence check. Our results indicated daily formation of otolith increments, thus corroborating previous studies and supporting the use of otolith increments to estimate age and somatic growth of individual Chinook salmon. Given the positive relationship between body size and survival to adulthood, continued use of otolith microstructure to quantify age and growth will help identify factors critical for the recovery of listed Chinook salmon populations.