98 Transactions of the American Fisheries Society 133:98–120, 2004  Copyright by the American Fisheries Society 2004 Assessment of High Rates of Precocious Male Maturation in a Spring Chinook Salmon Supplementation Hatchery Program DONALD A. LARSEN* AND BRIAN R. BECKMAN Integrative Fish Biology Program, Northwest Fisheries Science Center, National Marine Fisheries Service, 2725 Montlake Boulevard East, Seattle, Washington 98112, USA KATHLEEN A. COOPER School of Aquatic and Fisheries Science, University of Washington, Seattle, Washington 98195, USA DAN BARRETT Cle Elum Supplementation and Research Facility, Yakama Nation, 800 Spring Chinook Way, Cle Elum, Washington 98922, USA MARK JOHNSTON Yakama Nation, Nelson Springs Research Center, 771 Pence Road, Yakima, Washington, 98902 USA PENNY SWANSON Integrative Fish Biology Program, Northwest Fisheries Science Center, National Marine Fisheries Service, 2725 Montlake Boulevard East, Seattle, Washington 98112, USA WALTON W. DICKHOFF Integrative Fish Biology Program, Northwest Fisheries Science Center, National Marine Fisheries Service, 2725 Montlake Boulevard East, Seattle, Washington 98112, USA; and School of Aquatic and Fisheries Science, University of Washington, Seattle, Washington 98195, USA Abstract.—The Yakima River Spring Chinook Salmon Supplementation Project in Washington State is one of the most ambitious efforts to enhance a natural salmon population currently under way in the United States. Over the past 5 years we have conducted research to characterize the developmental physiology of natural and hatchery-reared wild progeny spring Chinook salmon Oncorhynchus tshawytscha in the Yakima River basin. Fish were sampled at the main hatchery in Cle Elum, at remote acclimation sites, and, during smolt migration, at downstream dams. Through- out these studies, we characterized the maturational state of all fish using combinations of visual and histological analyses of testes, computation of gonadosomatic indices, and measurement of plasma 11-ketotestosterone (11-KT). We established that a plasma 11-KT threshold of 0.8 ng/mL can be used to designate male fish as either immature or precociously maturing approximately 8 months prior to final maturation (1–2 months prior to release as ‘‘smolts’’). Our analyses revealed that 37–49% of the hatchery-reared males from this program undergo precocious maturation at 2 years of age and that a portion of these fish appear to residualize in the upper Yakima River basin throughout the summer. An unnaturally high incidence of precocious male maturation may result in the loss of returning anadromous adults, the skewing of female : male sex ratios, and ecological and genetic impacts on wild populations and other native species. As precocious male maturation is significantly influenced by the growth rate at specific times of year, in future studies we will alter maturation rates through seasonal growth rate manipulations. Throughout the Pacific Northwest of the United States, supplementation strategies for Pacific salm- on Oncorhynchus spp. are being employed by hatcheries in an attempt to rebuild natural spawn- * Corresponding author: don.larsen@noaa.gov Received February 11, 2003; accepted June 18, 2003 ing stocks and augment fisheries. Successful sup- plementation relies on improving the postrelease survival of hatchery salmon while reducing the potential negative genetic and ecological impacts on wild fish. Although the concept varies in prac- tice, a supplementation hatchery may be defined as a rearing facility for breeding and propagating a stock of fish with genetic resources equivalent