Use of Otolith Microstructure to Discriminate Stocks of Juvenile Central Valley, California, Fall‐Run Chinook Salmon

Abstract

AbstractWe examined the utility of otolith microstructure analysis as a means of discriminating among stocks and between rearing types of juvenile fall‐run Chinook salmon Oncorhynchus tshawytscha in California's Central Valley. Otolith samples were collected from 11 geographically discrete stocks of fall‐run Chinook salmon of river and hatchery rearing types over 2 years. Six measured otolith attributes and three statistics derived from these measurements served as dependent variables in a multifactor analysis of variance of paired river and hatchery stocks. Morphometrics that demonstrated no significant interaction between year and stock or rearing type were used as predictors in discriminant function analyses; 32% of samples were correctly classified by stock and 80% were correctly classified by rearing type. Measures of otolith size at fry emergence were the best predictors for distinguishing among both stocks and rearing types with discriminant function analysis. We hypothesize that observed differences in otolith size at emergence among stocks and rearing types reflect differential responses in development of Chinook salmon embryos reared under different temperature regimes. Although classification success among stocks was rather limited, results for correctly classifying fall‐run Chinook salmon by rearing type were encouraging. We suggest that observed differences in otolith microstructure between rearing types be augmented with thermal marking in Central Valley hatcheries to provide a fully reliable tool for determining the proportionate contribution of natural and hatchery production to spawner populations. This information alone would greatly advance stock management of Central Valley fall‐run Chinook salmon.