Effects of photoperiod on juvenile turbot (Psetta maxima): Physiological status and growth.

Turbot juveniles (32 g of artificial photoperiods : four constant photoperi od regimes (8L:16D, 12L:12D, 16L:8D, 24L:0D) and two changing regimes, one increasing from 12 to 16 h light per day, and the other decreasing from 12 to 8 h light per day. All other environmental conditions were maintained co nstant :temperature : 17 +/- 0.5 degrees C, salinity : 34.5 ppt, light inte nsity : 2 W.m(-2), O-2 over 6 mg.l(-1). The effects of these photoperiods o n growth, feed efficiency, nitrogenous excretion, physiological status (blo od plasma osmolarity, chloride, sodium, potassium and thyroid hormones cont ents) were studied. All over the experiment, fish looked healthy and no mortality was observed. No significant differences in growth response to photoperiod were observed . Specific growth rate (1.7-1.9), apparent food conversion ratio (0.70-0.75 ), protein efficiency ratio (2.5-2.7) and protein utilization coefficient ( 36-39 %) were not photoperiod-dependent. There were no changes in fish body composition in terms of proteins, lipids, ash and water contents. No signi ficant differences in blood plasma osmolarity, chloride, sodium and potassi um concentrations, related to photoperiods, were observed. T-4 concentratio ns were similar for all photoperiods tested, they ranged from 2 to 4 ng.ml( -1) while T-3 levels significantly decreased at the end of experiment in tu rbot maintained under the shortest day-lengths (8 hours light and decreasin g photophase). A major post-prandial increase in total ammonia nitrogen exc retion was observed under photophases 8 h, 12 h and 16 h. Under permanent l ighting, this peak was significantly lower. Urea nitrogen excretion showed a nocturnal peak related to photoperiod. In fish submitted to permanent lig hting, the same urea pattern was observed. In the Atlantic strain of turbot tested, growth and feed efficiency were no t influenced by photoperiod at least within 60 days.