Growth, feed utilization and growth heterogeneity in juvenile turbot Scophthalmus maximus (Rafinesque) under different photoperiod regimes

Juvenile turbot (45 g. SE= 1.3) were reared under three photoperiods, 08L:16D, 12L:12D and 20L:04D at slightly elevated ambient temperature for Ireland. Over the 297-day experimental period, the overall growth rate of the 12L:12D (0.82% d^-1) treatment was higher than for both 08L:16D (0.80% d^-1) and 20L:04D (0.77% d^-1). Overall relative feed intake (FI = % consumption*day^-1) was higher for the 20L:04D (FI = 0.81% d^-1, SE = 0.06) treatment than for the 08L:16D (0.63% d^-1, 0.04) and 12L:12D (0.64% d^-1 0.04) treatments, whereas feed conversion efficiency (FCE=weight gain* consumption^-1) was lower in the 20L:04D (FCE=0.67, SE=0.08) group when compared with the 08L:16D (0.88, 0.06) and 12L:12D (0.88, 0.06) treatments. Present results show that the long-term extended fixed photoperiod may act as an irritant, inducing stress, suppressing growth and reducing feed utilization. It is hypothesized that the progression of size-dependent hierarchies over time can be divided into two distinct phases herein referred to as 'hierarchy resolution' and 'hierarchy stabilization' phases (or phases 1 and 2) characterized by increasing and decreasing growth heterogeneity respectively. Growth heterogeneity is measured as coefficient of variation of weight and rank correlation of initial weight of a phase and corresponding growth rate.