Effects of growth rate/body size and a low lipid diet on the incidence of early sexual maturation in juvenile male spring Chinook salmon (Oncorhynchus tshawytscha)

Two experiments were conducted sequentially to examine the roles of growth rate (size) and body fat on the incidence of early sexual maturation in male spring Chinook salmon (Onchorynchus tshawytcha). In both experiments two replicate groups of fish for each treatment were reared on experimental diets for 17 months after first feeding (February). Fish were sampled approximately monthly to monitor growth and determine whole body lipid level, gender and the state of sexual maturation. In the first experiment fish were pair-fed either a low (7%) lipid diet at one of six ration levels (satiation, or 88%, 76%, 64%, 52%, 40% of satiation) or a commercial feed (22% lipid) at the 64% level. The incidence of 1+ age male maturation in July ranged from 66.2% to 92.8% in fish with mean body weights ranging from 51 to 110 g during the previous December, which is within the period of initiation of maturation. Maturation rates in fish fed the low lipid experimental feed and the higher lipid commercial feed at the same ration level (64%) were similar suggesting that dietary (body) lipid level had no effect on maturation in relatively fast growing juvenile Chinook salmon. In the second experiment, fish were fed a commercial feed (22% lipid) using a regimen designed to produce fish of 10, 15, 20, 25, 30, and 110 g in December. The incidence of 1+ age male maturation the following July ranged from 12% to 51.0% for fish that had mean body weights ranging from 10 to 108 g in December. The relationship between December fish weight and maturation in both experiments was modeled with a quadratic equation. A mean threshold body size for initiation of maturation of 7.9 g was obtained for this stock of Chinook salmon by meta-analysis using data from both experiments and from a previous experiment conducted at the same facility under similar rearing conditions. The results of this study support previous conclusions that growth rate or size is the major factor influencing onset of puberty in males. Development of rearing strategies that produce healthy small nonmaturing 1+ age male Chinook salmon will require a better understanding of the relative contributions and interactions of multiple factors on the bioenergetics of this species.