Sexual growth dimorphism in sea bass Dicentrarchus labrax

The growth of 100 sea bass feminized by estradiol treatment and 200 individually tagged controls was followed from the age of 10 months [27±1.7 g, mean±confidence interval (95%)] to 45 months (1160±46 g) in order to analyze the evolution of sexual dimorphism as a function of the age and sexual maturity. One hundred percent females were found in the treated group ( n=94) versus 55% in the control nontreated group ( n=171). Males remained significantly smaller than both females and feminized fish from the beginning of the experiment. The resulting weight advantage [(female mean weight−male mean weight)/male mean weight, expressed as a percentage] for females was much more pronounced during the early stages of development (67% at 10 months of age), decreased in the second year of life and then became stabilized around 25%. Females and feminized fish retained the same growth, suggesting that growth is related to phenotypic sex in the sea bass. The growth rate was influenced by sex in the fourth year of life, period during which the females grew faster in terms of both weight and length. Compared to the males, they had a relatively higher ratio of digestive tract to body weight (+26%), but lower ratios of visceral fat to body weight (−49%) and muscle lipid content (−16%) at slaughtering, at which time they had finished their second vitellogenic cycle (45 months).