Effect of Photoperiod on Growth of Trout (Oncorhynchus mykiss) in Cold Ambient Sea Water

The effects of four photoperiod regimes on growth performance of the rain- bow trout (Oncorhynchus mykiss W.) in cold ambient seawater conditions were compared in indoor seawater tanks (salinity 18 g/l; 6-8°C) in Janu- ary-February. The photoperiods were the natural photoperiod (10 h light: 14 h dark), 20 h light:4 h dark, 16 h light:8 h dark, and 0 h light:24 h dark. Fish were fed a diet consisting of 47% protein, 20% fat, 11% ash, and 8.5% moisture. At the end of eight weeks, the relative growth rate and feed intake were significantly higher in fish exposed to 20L/4D than in other photore- gimes. Total protein intake (g), protein retention (%), and N content in fish (%) significantly improved with the increase of light hours, indicating that optimal growth in the study conditions is obtained in extended light regimes (20L/4D or 16L/8D)