No abstract

To investigate the interactive e¡ects of temperature and photoperiod on the growth performance, feeding parameters and muscle growth dynamics in juvenile Atlantic halibut (Hippoglossus hippoglossus L.), a total of 1212 juvenile halibut, including 383 tagged ¢sh (mean initial weight of tagged individuals: 17.6 AE 0.3 g SE), were reared under a simulated natural light regime for Bergen (60125 0 N) or continuous light at 9,12 and15 1C from 3 December 2007 until11 March 2008. The mean weight and growth rate were signi¢cantly higher at 12 and 15 1C than at 9 1C. In addition, signi¢cantly higher mean weight and growth rate were observed in halibut reared under continuous light at a low temperature, indicating an interactive e¡ect of temperature and photoperiod on growth performance. No e¡ect of temperature or photoperiod was found with respect to feed conversion e⁄ciency, whereas a higher feed consumption at increasing temperature and a higher overall daily feeding rate at continuous light at a low temperature were observed. Indications of continuous light having a stronger e¡ect at low temperatures on muscle growth dynamics were found. A di¡erence in the size class distribution of ¢bre diameter was found between photoperiod treatments at 9 1C, suggesting that continuous light resulted in elevated hypertrophic growth at low temperature. This may suggest that the increased growth rate found at continuous light at 9 1C may be a result of hypertrophic growth in juvenile halibut.

To investigate the possible direct effect of a stepwise reduction in temperature with increasing size on growth, feeding parameters and muscle growth patterns of juvenile Atlantic halibut (Hippoglossus hippoglossus L.), 804 juvenile halibut (mean initial weight individuals: 14.2 g ± 0.2 SEM) were reared at constant 9, 12 and 15°C or shifted (T-step, i.e. 15-12°C after 36 days) for 99 days. Despite indications of lower optimal temperature for growth with increasing size, equal end weights were obtained between the constant 12°C, constant 15°C and T-step groups. Best overall growth was observed for the group kept at constant 12°C. The limited effect of the T-step group may relate to the size at movement (too big), the temperatures investigated (close to optimum) and the time and size interval investigated (too narrow). Differences in growth were reflected more by alterations in feed intake (C T and F%) than by differences in feed conversion efficiencies (FCE). Differences were found with respect to the density of muscle cells, whereas no differences were found between the average muscle cell diameters. The mean diameter of muscle cells tended to increase only slightly with increasing fish weight, while the mean density of muscle cells tended to decrease. Using an optimum temperature of 12°C, an indication of a possible increased rate of hyperplasia in relation to higher growth was seen.