The lsraeli Joumal of Aquaculture - Bamidgeh 51(3), 1999, 91-105


Sergei Gorshkov, Galina Gorshkova, Wayne Knibb and Hlllel Gordin

Genetics Department. Israeli Oceanographic and Limnological Research, National Center for Mariculture, P.O. Box 1212, Elat 88112, Israel

(Received 29.1.99, Accepted 16.6.99)


Israel's cultured sea bass stocks were imported from France in 1976 and 1982. The sex ratio and growth performance of 14 groups of offspring of sea bass raised at the National Center for Mariculture (NCM) in Eilat during the last six years were examined in this study. The overall sex ratio lo r the 14 groups favored males: 78.6% vs 21.4%. However, the sex ratio varied widely (from 98.1% to 44.2% for males) among different cohorts and progeny produced at different times. At the same time, There were no deviations in sex ratio within a cohort or progeny during ontogenesis; i.e. the proportion of females to males among different age groups remained stable, indicating that sex in Dicentrarchus labrax, once determined, is irreversible.
Sexual dimorphism in length and weight was observed even before sexual maturity. Starting at age 10-12 months, the size-weight divergence between sexes was evident for all cohorts and progeny studied. Despite considerable variation in body size and weight among the different cohorts, females tended to be longer and significantly heavier than males. Effects of maturation on growth seem to be minor under our conditions as no significant weight differences were found between mature precocious males and immature fish.
Histological examinations of gonads showed that, at age six months, 17% of males had some oogonia cells (intratesticutar oocytes) imbedded In testicular tissue. The proportion of such males declined to 0 at age 10 months. Starting at this age, the histological patterns of gonads, typical for males and females, were well established.
Experiments on hormonal sex inversion showed that the labile period for gonadal masclinization and feminization appears to occur sometime prior to day 115 post-hatching. Under our rearing conditions, about four months may be considered the age of morphological sex differentiation. Overall, while our data did not identify particular genetic and/or environmental reasons for the deviations in sex ratio, they do illustrate possible plasticity of sex determination in sea bass.