Effects of pre-smolt photoperiod regimes on post-smolt growth rates of different genetic groups of Atlantic salmon ( Salmo salar)

Abstract

The aim was to study relationships between photoperiod in freshwater and growth rates prior to and following seawater exposures for fish from different stocks. Individually tagged 1+juveniles originating from four different stocks of Atlantic salmon were reared under the constant photoperiods LD10:14, LD12:12, LD14:10 and LD16:8 and under simulated natural increasing photoperiods from early spring starting from LD11:13 (SNP-11; fish from LD10:14 and LD12:12) or from LD15:9 (SNP-15; fish from LD14:10 and LD16:8). The groups were exposed to running seawater during May and June under LD22:2 and transferred to natural light regime in late July to monitor the subsequent growth rates in seawater for the next 6 months. The genetic origin of the fish was the factor of most influence for variation in the proportion and growth rates of the upper modal group fish in freshwater. The Dale stock showed the best growth performance, followed by the Vosso, Hatchery and the Lone stock. During the first 3–6 weeks in seawater, there was an evident and common drop in growth rates for all the stocks. The largest part of the variation in growth could be attributed to pre-smolt photoperiods, as the drop in growth rates was less pronounced for the fish previously held under increasing photoperiod in freshwater. During the next 5 months in seawater, the significance of stock was partly re-established. The Dale, Vosso and Hatchery stocks reached comparable sizes, significantly larger than the Lone stock. In addition, the initially slow growing groups under constant photoperiod in freshwater grew at the highest rate resulting in no differences in final sizes between treatment groups, possibly implying that their transfer to long daylength in seawater stimulated growth and/or completed the smoltification process. Relationships between photoperiod in freshwater, smoltification and growth rate before and after seawater exposure are discussed.