Post-smolt growth and maturation of out-of-season 0+Atlantic salmon (Salmo salar) reared under different photoperiods

Out-of-season smelts are currently transferred to the sea in the autumn under a decreasing photoperiod. The following experiment was undertaken to determine the effect of photoperiod on the seawater performance of out-of-season smolts. The growth and maturation of out-of-season 0 + post-smelts were monitored under different photoperiod regimes following seawater transfer. Prior to seawater transfer all fish were subjected to the same photoperiod which advanced the development of hypoosmoregulatory ability and smelt coloration. 600 smelts were transferred to 6 seawater tanks on the 11th December to form 6 groups. From the 11th December to the 11th July, three duplicate groups were subjected to a simulated natural photoperiod (SNP), a 9 h daylength (LD 9:15) or constant light (LL, i.e., a continuation of the freshwater photoperiod). From the 11th July until the termination of the experiment on the 4th December one group from each of treatments LD 9:15 and LL was subjected to SNP (groups LD 9:15/SNP and LL/SNP). The three treatments exhibited significantly (P < 0.01) different growth; fish in treatment LL and LD 9:15 exhibited the highest and lowest weights respectively. Percentage maturation was highest in group LL (24.6%) compared to the other groups (range 0-9.3%). Mortality was similar in all groups. Although these experiments were conducted in a pumped-ashore seawater system the results demonstrate the potential for photoperiod manipulation of fish in cages for improving the seawater performance of out-of-season smelts. (C) 1999 Elsevier Science B.V. All rights reserved.