Effects of Extended Daylength on Autumn Growth Dynamics of Juvenile Atlantic Salmon, Salmo salar

Abstract

The growth and growth dynamics of juvenile Atlantic salmon (Salmo salar) exposed from mid-August to mid-December to either a normal light:dark cycle (LDN) or a constant 16-h light:8-h dark (LD 16:8) photoperiod were compared. Exposure to the LD 16:8 resulted in 38.1% of the fish entering the upper modal group (UMG) of the length–frequency distribution, while only 15.4% entered the UMG under LDN. The length at which fish entered the UMG (critical fork length) did not differ between photoperiods. Fish in the small (5.8–6.7 cm) and middle (6.8–7.7 cm) size-ranges under LD 16:8 had greater growth rates than fish in the same size-ranges under LDN. Fish in the large (8.3–9.2 cm) size-range showed no significant variation in growth rates between photoperiods. At the end of the experiment, mean thyroxine (T4) levels were significantly (P < 0.05) lower in fish from the LD 16:8 photoperiod (1.69 ng/mL) in comparison with LDN fish (3.96 ng/mL). Triiodothyronine (T3) levels and the T3/T4 ratio were elevated under LD 16:8.