Long photoperiod on sea cages delays timing of first spermiation and enhances growth in male European sea bass (Dicentrarchus labrax)

Two large groups of fingerling (3.2 g) male European sea bass (around 75,000 individuals each) reared in net sea cages for a period of two years were exposed to two different photoperiods starting in June: either ambient photoperiod (37º 24′ N) (NP) or a constant long photoperiod (LP; 18 h light: 6 h dark, 18 L: 6D). Long days had limited effect on somatic growth at the time of incipient gonad development (IGD, 1st annual cycle; November–March), while they were effective at the time of full gonad development (FGD, 2nd annual cycle). Moreover, the annual patterns of morphological indices, i.e., Fulton's condition factor (CF), visceral fat index (VFI) and particularly gonadosomatic index (GSI), were delayed by one or two months as the result of the long days compared to the control group (NP). LP did not prevent the onset of puberty but delayed spermiation by one month, and modified accordingly the phase and/or amplitude of the annual rhythms of pituitary gonadotropin releasing hormone (sbGnRH) content and plasma levels of luteinising hormone (LH) and sex steroids, particularly that of 11-ketotestosterone (11-KT). In general, high levels of sbGnRH (≥ 4 ng/pit) were associated with elevated plasma LH (1–3 ng/ml at IGD and ≥ 8 ng/ml at FGD), and the sex steroid peaks usually preceded LH peak in both treatments (1–2 months earlier). Finally, full spermiation and the maximum GSI values coincided with the LH peak, highlighting the close relationship that exists between LH plasma levels and gonadal development.