Use of otolith microstructure to estimate growth rates of juvenile Chinook salmon from a central valley, California stock

We compared two approaches to back-calculation with otolith microstructure to develop a method for accurately estimating growth rates of juvenile fall-run Chinook salmon Oncorhynchus tshawytscha in California's Central Valley. Total otolith width was a strong determinant of fork length (FL) in linear regressions used to determine the y-intercept in the fish size-otolith size relationship in two study groups of Chinook salmon. The Fraser-Lee back-calculation model estimated FL at first feeding in both study groups that did not differ significantly from lengths of first-feeding Chinook salmon in a reference group. In comparison, the biological-intercept method produced back-calculated lengths that were significantly greater in one study group than lengths of first-feeding Chinook salmon in the reference group. Chinook salmon emergence dates, estimated from counts of daily growth increments beyond the first-feeding check, corresponded with observed emergence periods in the river and hatchery populations from which the study groups were sampled. Size-at-age relationships were well described by a power function in both study groups, where mean FL over time approached an apparent asymptote at approximately 80 mm after 90 d posternergence. Growth rate estimates, using back-calculated size from the Fraser-Lee model, averaged 0.50 mm/d in one study group and 0.43 mm/d in the other study group. These estimates fell within the range of previous growth rate estimates for juvenile Chinook salmon in Central Valley riverine, floodplain, and delta environments and were about 2.5 times higher on average than an estimate for the San Francisco Estuary and about 2.3 times lower on average than estimates from the Strait of Georgia. We discuss the utility of otolith microstructure to not only estimate growth rates, but also to reconstruct emergence-date distributions in cohorts of emigrating juvenile Chinook salmon for stock identification purposes.