Reproduction and physiology
PROBASS addressed the problem of very high percentage of males obtained in European sea bass (Dicentrarchus labrax) aquaculture. Males mature reproductively earlier, have slower growth rates due partly to usage of energy for testicular growth, and have 20-40% less body weight at harvest time. The objectives of PROBASS were to gain knowledge on the hormonal, biochemical and molecular regulation of sex differentiation in sea bass, and develop a methodology by which sex ratios can be manipulated to produce more females. Environmental factors influence sex differentiation and sex ratios in many fish, including the sea bass, therefore PROBASS will evaluate the effects of temperature and rearing density on the hormonal, biochemical and molecular parameters regulating sex differentiation, and develop an environmental feminization protocol for the industry. Use of female-dominant populations will increase production efficient and competitiveness of the industry, since females fish grow earlier and produce more muscle mass.
The first part of PROBASS was the development of the required methodologies to study the process of sex differentiation. These methodologies include expression of sex differentiation genes (SOX, SF 1 and DAX-1); expression and tissue levels of GnRHs, GtHs, GH and IGF I and II; steroidogenic enzyme gene expression (P450c17, P450arom and P450c11b); tissue levels of sex steroids (T, 11-KT and E2); and androgen and estrogen receptor expression and binding. The second part included the production of male- and female-dominant stocks by selective grading. From these stocks, morphometric data and tissue samples were collected for an ontogenic analysis of the brain-pituitary-gonad axis during sex differentiation, in order to identify sexually dimorphic profiles among the above studied parameters. The third part examined the effects of manipulations of rearing temperature and stocking density during early development on sex differentiation and resulting sex ratios, with the objective of identifying a temperature/density regime, which produces female-dominant stocks. Samples were again collected to evaluate the influence of temperature/density on sex differentiation at the level of the brain, pituitary and gonad. The fourth part was a trial carried out in grow-out facilities, to determine if fry reared under the developed temperature/density manipulation regime produce female-dominant stocks in industrial production facilities. PROBASS aimed at clarifying essential biological functions in European sea bass, which can benefit aquaculture production. It has resulted in the development of a new system of production using temperature manipulations to produce more females. Since females grow more than males, culture of female-dominant stocks has improvew aquatic production and increased competitiveness of the aquaculture industry. This new environmental method has eliminated the use of steroid hormones to control sex ratios, thus reducing the impact of aquaculture on the environment.