GROWTH AND HORMONAL CHANGES ASSOCIATED WITH PRECOCIOUS SEXUAL-MATURATION IN MALE CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA (WALBAUM))

One-year-old chinook salmon (Oncorhynchus tshawytscha (Walbaum)), rear ed in seawater, develop a bimodal weight-frequency distribution prior to the precocious sexual maturation of some of the males (jacks). Alth ough jacks are larger than immature fish, they are still too small to be of commercial value. To investigate the differential size between j acks and immature fish, chinook salmon were tagged and reared in seawa ter for 5 months prior to sexual maturation (March-August). During thi s time, immature males, females, and jacks were sampled monthly for we t weight and plasma concentrations of triiodo-L-thyronine (T3), testos terone, and growth hormone (GH). Fish were also sampled in the precedi ng December as controls. The weight-frequency distribution of the samp led fish became significantly bimodal in May, with the upper modal gro up consisting mostly of jacks. Jacks had the highest relative growth ( RG) rates from April-June. Jacks had significantly elevated plasma T3 concentration in March only, and had significantly elevated testostero ne levels throughout the experiment. There were no significant differe nces in plasma GH concentrations between jacks and immature fish; howe ver, in jacks, plasma GH concentrations were positively correlated wit h RG during April-May, and plasma testosterone levels were positively correlated with RG for June-August. Although a number of hormonal diff erences between jacks and immature fish coincided with the differentia l growth rates, no measured hormone concentration could explain the ob served differences in growth and size.