Interaction of temperature and photoperiod on growth of Atlantic halibut Hippoglossus hippoglossus L.

The combined effect of temperature and photoperiod on the growth of juvenil e Atlantic halibut Hippoglossus hippoglossus (initial weight 11.6 g) was in vestigated under different temperature (6 degrees C and 12 degrees C) and p hotoperiod (continuous light and simulated natural photoperiod) combination s by studying the effects on fish weight, specific growth rate (SGR), feed consumption (F%), feed conversion (FCE) and oxygen consumption and ammonia excretion under routine and fasting conditions. SGR was greater at the high er temperature, and growth was faster under continuous light than under nat ural photoperiod, irrespective of temperature, SGRs being 0.83% and 0.98%, and 1.61% and 1.71% at 6 degrees C and 12 degrees C respectively. The weigh t data revealed a significant interaction between temperature and light, an d provided evidence of a stronger growth-enhancing effect of continuous lig ht at the lower temperature. F% increased with temperature but was independ ent of photoperiod, whereas FCE was higher among fish exposed to continuous light, irrespective of temperature. Independently of photoperiod, oxygen c onsumption and ammonia excretion increased with temperature, but the differ ences in these rates in relation to photoperiod became more pronounced at t he lower temperature, where continuous light gave the highest rates. Higher oxygen consumption during darkness suggested that light may suppress activ ity in halibut, It was proposed that reduced activity and anabolic effects of photoperiod contribute to explain the increased growth and growth effici ency in fish subjected to continuous light.