Photoperiod regulation of growth of juvenile Atlantic halibut (Hippoglossus hippoglossus L.)

Juvenile Atlantic halibut (Hippoglossus hippoglossus L., initial weight (SD) 31.1 g (+/-5.8 g)) were reared for 147 days at a constant temperature of 11 degrees C and subjected to four different light regimes from February 10: continuous light (LD 24:0), simulated natural photoperiod of Bergen (60 degrees 25'N, LDN), constant 8 h light:16 h dark (LD 8:16) and LD 8:16 switched to continuous light on May 4 (LD 8:16-24:0). Fish exposed to LD 24:0 had a significantly higher specific growth rate (1.05% day(-1)) than those on LDN (0.98% day(-1)). Fish exposed to LD 8:16 had the poorest growth (0.80% day(-1)). The group on LD 8:16-24:0 (overall growth rate 0.94% day(-1)) grew poorly at first but subsequently increased growth rate after having been exposed to continuous light. This response was delayed by at least 21 days, suggesting that the fish needed some time to adapt to the change in photoperiod. Sensitivity to continuous light became less pronounced under summer photoperiods (above approximately 18 h daylength), which suggested that continuous light could be used to improve growth in juvenile halibut when the natural daylength is less than approximately 18 h. Overall mortality was 11.9%, with significantly higher mortality being observed in the LDN (17.4%) and LD 8:16-24:0 (17.1%) groups than in the LD 24:0 (6.2%) and LD 8:16 (6.8%) groups. Size-dependent mortality was seen in LD 8:16-24:0 in all periods from May 4 to July 6, with mortality being higher amongst small fish. (C) 2000 Elsevier Science B.V. All rights reserved.