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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. Throughout 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.

Two gonadotropins, GTH I and GTH II, were isolated from pituitaries of spawning coho salmon (Oncorhynchus kisutch) using sequential extractions with ammonium acetate (pH 9.0) and 40% ethanol, precipitation with 80% ethanol, gel filtration chromatography (Sephadex G-100), anion-exchange chromatography (Mono-Q Sepharose), and gel filtration chromatography (Sephadex G-75). Coho salmon GTH I and GTH II stimulated steroidogenesis in vitro in a similar dose-dependent manner when incubated with either ovaries or testes of prepubertal coho salmon. An in vivo bioassay using coho salmon parr demonstrated that coho salmon GTH I and GTH II did not contain thyrotropic activity. Molecular weights were estimated by gel filtration chromatography to be 43,000 and 39,000 for GTH I and GTH II, respectively. Analysis of coho salmon GTH I and GTH II on reversed-phase high-performance liquid chromatography (rpHPLC) revealed that they consist of alpha and beta subunits with N-terminal amino acid residues of Tyr, Gly (alpha, beta of GTH I) and Tyr,Ser (alpha, beta of GTH II). Coho salmon GTH I-beta and GTH II-beta differed from each other in amino acid composition, N-terminal amino acids (Gly vs. Ser), and molecular weights in SDS-PAGE (19,000 vs. 20,000) and had a high degree of chemical similarity to chum salmon GTH I-beta and GTH II-beta, respectively. Specific rabbit antisera to the beta subunits of coho salmon GTH I and GTH II were generated. The observation of two GTHs with distinctly different chemical characteristics in coho salmon is similar to what has previously been found in chum salmon.

Previous studies conducted at the Cle Elum Spring Chinook Salmon Supplementation Hatchery in Washington State demonstrated that 37-49% of the male Chinook salmon Oncorhynchus tshawytscha released from this facility in its first years of operation precociously matured at age 2 rather than the more typical age 4. We examined the effects of altering seasonal growth rate on the incidence of age-2 male maturation in an experimental subset of that population and compared their physiological development (size, growth rate, condition factor, whole-body lipid, gill Na þ ,K þ -ATPase activity, and plasma insulin-like growth factor-I [IGF-I]) with that of both hatchery (production) and wild fish. Altering summer and autumn rations resulted in four growth trajectories with the following size and precocious male maturation rates: the high summerÀhigh autumn growth trajectory produced fish averaging 25 g and 69% precocious maturation; the high summerÀlow autumn trajectory yielded fish that averaged 18 g and exhibited 58% precocious maturation; the low summerÀhigh autumn trajectory produced 18-g fish with 51% precocious maturation; and the low summerÀlow autumn trajectory yielded fish averaging 16 g and 42% precocious maturation. Production fish averaged 22 g and exhibited a 53% precocious maturation rate. The high summer growth treatments and production fish were largest among all groups and had higher plasma IGF-I, adiposity levels, and precocious male maturation rates than did the low summer growth treatments. Wild fish were significantly smaller and leaner and had much lower plasma IGF-I levels than all other groups. Gill Na þ ,K þ -ATPase activity was not different between groups, suggesting that there was no differential effect on smoltification. Growth modulation reduced the precocious male maturation rate by 39% among experimental treatments and by 21% between production fish and the lowÀlow treatment. However, the maturation rate and adiposity of hatchery fish differed markedly from those of wild fish, suggesting that more dramatic alterations of rearing regime may be required to further reduce the prevalence of this phenotype in cultured fish.

The independent effects of size and fatness, 1 year prior to maturity, on male sexual development of chinook salmon (Oncorhynchus tshawytscha) were tested. Beginning in March 1995, ration size and dietary lipid were independently manipulated to produce four groups of spring chinook salmon differing in size and fatness. Size, growth rate, adiposity, liver triacylglycerol and glycogen contents, and plasma insulin and insulin-like growth factor I (IGF-I) levels were monitored to follow the metabolic states of fish in the different treatment groups and to observe whether plasma levels of these growth mediators predict sexual development. Differences in size and fatness were well established by the first autumn (September 1995), 1 year prior to sexual maturity, and common ration size groups were pooled for further rearing. Subsequently in winter-spring (February-March), 6 months prior to sexual maturity, there were no within-tank differences in size or fatness. Nevertheless, the effects of size and fatness, from 1 year earlier, on incidence of sexual maturity were significant. Overall, size appeared to have the primary effect, but for smaller fish, an effect of fat content was indicated. Plasma insulin levels, and in limited cases, IGF-I levels, were correlated with growth rate and size, but were not accurate indicators of sexual development.

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