Evolution of sex ratio and egg production of gilthead seabream (Sparus aurata) over the course of five reproductive seasons

Abstract

Spawning performance -relative fecundity and fertilization success- was studied in two hatchery-reared broodstocks of gilthead seabream (Sparus aurata) held under relatively constant well-water temperature (18–20 °C) and simulated natural photoperiod, for five consecutive spawning seasons, between 3 and 7 years of age. The spawning season lasted between 4 and 6 months each year, and the total number of eggs produced annually ranged between 1 480 000 and 3 100 000 eggs/kg female body weight, being the highest during the first and second spawning season. No difference was observed in monthly relative fecundity between years -although high variation existed within years, whereas fertilization success was the highest during the second and third reproductive season, and dropped significantly during the last year of the study. The male percentage of the broodstocks was 35% during the first spawning season of the females, and thereafter it decreased further and remained stable at around 15%–20% for the second and third spawning season. Substitution of older females with smaller males to readjust male percentage to 50% in the fourth spawning season, was followed by sex change of larger males to females and a drop of the male percentage to 18% in the following spawning season. The present study demonstrated the long spawning season of gilthead seabream -especially under constant water temperature, the high fecundity and fertilization success, and the stabilization of male percentage to ∼20% after the first spawning season or when a broodstock is modified to increase male percentage. These results are useful to the aquaculture industry, demonstrating that there is no need to add males to gilthead seabream broodstocks over the years, as the sex ratio is stabilized and egg production and fertilization success remain high with a relatively low male:female sex ratio.