Long-term exposure to continuous light inhibits precocity in juvenile male European sea bass ( Dicentrarchus labrax, L.). I. Morphological aspects

Abstract

The effect of long-term exposure to a continuous light photoperiod was investigated in juvenile male sea bass ( Dicentrarchus labrax L.) during the first annual cycle. Four-month-old fish were exposed to a natural photoperiod (40°N) (NP) and 24 h continuous light (LL), for 12 consecutive months, under natural temperature conditions (13.3–25.1 °C). Long term exposure to LL regime was highly effective for the inhibition of the gonadal development and consequently the prevention of precocious males. The NP group showed a steady increase in gonadosomatic index (GSI) values from December to a peak in January (1%), while the LL group had a similar pattern but it did not surpass the 0.2% GSI at the peak level. Similarly, the LL group exhibited mostly immature stages of germ cell development from December to March contrasting with the NP group, which presented more advanced stages of maturation at these times. The NP exhibited a high incidence of precocious fish (21.9%, 15% and 2.6% in February, March and April, respectively), while the LL regime dramatically reduced these percentages to 0%, 3.32% and 2.25%, in February, March and April, respectively. The exposure of LL provoked a significant reduction of the visceral fat (VFI) and hepatosomatic index (HSI) throughout the reproductive period, suggesting an accelerated consumption of the fat reserves of the fish with respect to the control group. However, the carcass index, which indicates the eviscerated weight of the fish, increased significantly ( P<0.05) in the LL group compared with the NP group. Finally, the exposure to LL induced a significant ( P<0.05) loss of weight and length with respect to the NP group.