Growth, osmoregulation and sexual maturation of underyearling Atlantic salmon smolt Salmo salar L. exposed to different intensities of continuous light in sea cages

Atlantic salmon smolts are usually transferred to sea water during early sp ring, but photoperiod treatment can be used to produce underyearling (0+) s molts for transfer to sea water in late autumn, 7-8 months earlier than usu al. This study investigated the effect of exposure to additional continuous light (LL) of different intensities on 0+ smolts after transfer to sea wat er, 0+ smolts transferred to sea cages in mid-October were exposed to natur al light (NL) or LL of one of three intensities (low, medium or high) until January, whereupon they experienced NL until harvest in December, Fish in all groups displayed a normal ability to hypoosmoregulate, assessed by moni toring plasma chloride concentrations after transfer to sea water, No consi stent differences in length, weight or condition were found between LL grou ps, The LL groups had greater growth in length than the NL group during the time of LL exposure, followed by an increase in weight when exposed to NT, , This resulted in fish of larger size than the NL group. By August, the fi sh in the NL group had caught up with the LL groups in respect of length, w eight and condition factors. This suggests that the LL treatment led to inc reased winter growth and phase-shifted a seasonal pattern of growth. The in cidence of sexual maturation was low (<1%), with no differences between gro ups.