Long-term effect of photoperiod manipulation on growth, maturation and flesh quality in turbot

•We investigate the effect of continuous light during the production cycle of turbot.•Long-term rearing on continuous light reduced growth.•Short-term exposure to continuous light stimulated growth.•Lower male plasma 11-ketotestosteron levels were seen in continuous light groups.

A group of juvenile turbot (mean (SE), initial weight 22 g (1.5)) were reared in indoor and covered outdoor tanks under ambient temperature conditions for 46 months until harvest (mean final weight, 2.7 kg). The photoperiod treatment was divided into 5 phases where the fish in each phase were exposed to either 16 h of light and 8 h of darkness (LD16:8, extended light, E) or continuous light (C). Thus, the following five photoperiod combinations were tested: a) LD16:8 (Control) group (EEEEE), b) Group 2A (ECEEE), c) Group 2B (EECEE), d) Group 2C (EEECE), and e) Continuous light (CCCCC). Long term rearing on continuous light reduced growth, whereas short term exposure to continuous light stimulated growth as the final mean weights of Groups 2C and 2B were 16 and 11% higher than those of the Control and the Continuous light groups. Significantly higher male plasma 11-ketotestosterone levels were seen in the Control group in July 2011 (5.67 ng mL− 1) and June 2012 (2.56 ng mL− 1) than in the other experimental groups, indicating delayed maturity in groups reared under continuous light. Estradiol 17-β levels were low in all groups throughout the experiment indicating low or no female maturation. Photoperiod regime had only minor effect on flesh quality traits of the fish. Based on current results, we recommend that turbot is farmed at extended light (i.e. LD16:8) in combination with continuous light during spring and summer (i.e. Group 2C in this study) during the second production year.