Energy reserves, rather than body size, are associated with ovarian development in two-year-old female European sea bass (Dicentrarchus labrax L.).

In fish, early puberty can lead to divergent growth trajectories and increased size dispersion within populations. In European sea bass, this pattern is evident by the end of the second year, with females of greater body weight generally tending to reach puberty earlier, although this association is not always consistent. This study investigates the factors influencing the relationship between growth and reproductive maturation in this teleost fish. Four female groups were defined based on body weight (small, S; large, L) and gonadal development (immature, I; advanced, A): SI, SA, LI, and LA. Results showed that larger fish size did not always correspond with a more advanced gonadal stage (late vitellogenesis). Furthermore, females with a higher condition factor during the spring-summer period preceeding putative gonadal development (PGD; autumn-winter months), exhibited elevated plasma levels of follicle stimulating hormone and insulin-like growth factor 1 (Igf-1). These females showed a higher gonadosomatic index (GSI), as well as elevated plasma levels of 17β-estradiol and vitellogenin during the PGD period. This suggests that body condition before PGD may be critical for fish to subsequently meet reproductive demands, as females likely with insufficient energy reserves were unable to sustain the costs associated with maturation later on (i.e., SI, LI vs. SA, LA). Levels of igf expression were linked to ovarian development and were positively correlated with intraovarian lipid gene expression. Of note, SA and LA females displayed increased ovarian expression of igf-2 and lipoprotein lipase (lpl) in comparison to that of SI and LI females. The ovarian igf-2 correlated with GSI and ovarian expression of lpl, suggesting that igf-2 might potentially promote lipid uptake. These findings indicate that body size alone does not predict ovarian development in European sea bass, whereas lipid accumulation appears critical for triggering gonadal development, emphasizing the role of energy status in the relationship between growth and reproduction in this species.