A Method for Preparing Chinook Salmon Otoliths for Age Determination, and Evidence of Its Validity

Abstract

Otoliths from North American chinook salmon Oncorhynchus tshawytscha are often opaque and difficult to age, Of the several methods used to prepare chinook salmon otoliths for age determination, I found that storage in buffered isotonic saline and glycerine for 3 months made it possible to assign ages to otoliths that were unreadable by other methods. Known-age fish were used to validate the method. Known ages and otolith ages agreed for 80.2–93.9% of age-0.1 to age-0.5 fish. (In this system of age notation, the first digit indicates the number of annuli formed in freshwater and the second digit indicates the number formed in salt water.) Both under- and overaging errors were made, but no consistent bias could be demonstrated. Ages determined from scales and otoliths from unknown-age fish agreed for 77.8–83.0% of age-1.2 to age-2.4 fish. The otolith method provided virtually the same freshwater age as the scale method, but the scale method tended to underestimate saltwater ages because of scale resorption. The frequency of normal (non-crystallized) and aberrant (crystallized) otoliths varied among populations, and wild populations tended to have fewer aberrant otoliths than hatchery populations. Unlike regenerated scales, aberrant otoliths can be used successfully to estimate fish age, Otoliths provide a reliable method for estimating the age of spawning chinook salmon.