The effect of whole body lipid on early sexual maturation of 1+age male chinook salmon (Oncorhynchus tshawytscha)

Early sexual maturation of male chinook salmon (maturation 1 to 4 years pri or to females in the same age class) results in reduced effectiveness of st ock enhancement programs and a financial loss to the salmon farming industr y, previous studies in Atlantic salmon have shown that the age of maturity in males is affected by growth and/or body energy stores, but the relative roles of these two factors are not well understood. Therefore, an experimen t was designed to determine when spermatogenesis was initiated, to characte rize the endocrine changes during the onset of puberty in male salmon, and to determine if the level of whole-body lipid affects the incidence of earl y male maturation in a wild stock (Yakima River) of 1 + spring chinook salm on. Fry were fed a commercial diet from February until August and were then divided into groups of 320 fish (mean weight, 5.6 g) and fed one of five e xperimental diets (two replicate groups/diet) containing 4%, 9%, 14%, 18% o r 22% lipid and 82%, 77%, 73%, 69%, or 65% protein for 13 months. Fish were reared on natural photoperiod and ambient temperature (6 degrees C to 16 d egrees C), and pair-fed to a level based on the tank with the lowest feed c onsumption. Fish were weighed monthly and sampled to determine body composi tion, pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels, plasma insulin-like growth factor I(IGF-I) levels, and stage o f gonadal development. Throughout the experimental period the mean fish weight was similar among t reatment groups. However, from December through the end of the experiment i n the following September, maturing males were significantly larger than no nmaturing fish. Initial lipid levels in 0-age experimental fish were near 6 %, which is similar to wild fish of the same stock and age captured in the Yakima River during August. Fish fed diets containing more than 4% lipid in creased in whole-body lipid content during the first 2 months of feeding an d then maintained at relatively constant levels during the course of the ex periment. Whole-body lipid levels for the dietary treatment groups averaged 5.6%, 7.1%, 8.2%, 9.4%, and 9.6% from October through the following Septem ber. Based on histological examination of the testes of experimental fish, type B spermatogonia and primary spermatocytes were first observed in some of th e yearling males during November. These were designated maturing males. Pit uitary FSH levels were significantly higher in maturing than nonmaturing ma les at this time and for the remainder of the study. Pituitary FSH levels i ncreased as spermatogenesis proceeded in maturing fish, whereas pituitary L PI levels increased in maturing 1 + males only during July and August, when testes were in late stages of spermatogenesis and in September during sper miation. Plasma IGF-I levels were significantly higher in maturing males th an nonmaturing fish from December through the end of experiment. Since matu ring males were significantly larger than nonmaturing fish of both sexes fr om December through September, the difference in IGF-I levels could be due to differences in growth or due to maturation. The percentage of maturing males was significantly influenced by whole-body lipid, increasing from 34% in fish fed the 4% lipid diet to 45% in fish fe d the 22% lipid diet. These data suggest that whole-body lipid levels influ enced the incidence of maturation of male spring chinook salmon. In additio n, both endocrine and histological indicators suggest that maturation was i nitiated in males approximately a full year prior to the time the fish will spawn. (C) 2000 Elsevier Science B.V. All rights reserved.