More Than One Way To Smoltify A Salmon? Effects Of Dietary And Light Treatment On Smolt Development And Seawater Growth Performance In Atlantic Salmon

Post-smolt mortality and stunted growth is a problem in Norwegian salmon farming that can be partly traced back to sub-optimal smolt quality in connection with ongoing changes in smolt production practices. In the present study, we compared smolt development and post-smolt performance in seawater (SW) of Atlantic salmon subjected to 1) traditional light treatment (short photoperiod (SP) long photoperiod (LL)), 2) a dietary treatment alone (LL-LL + diet), 3) a combination of light and diet (SP-LL + diet) treatment and 4) no stimulation (LL-LL control) during the freshwater (FW) phase. The transition diet consisted of feed supplemented with a salt mixture and the amino acid tryptophan. Effects of light and dietary treatment were tested on two different size classes (40 and 130 g) of parr. Response patterns to treatments were independent of initial size and findings discussed below apply to both. Fish in the continuous light groups (LL-LL and LL-LL + diet) had the highest mass gain during the FW phase while light treated fish had a reduction in condition factor indicative of a true smolting. Hypoosmoregulatory ability in May was increased by diet, but not by light. However, a strong and treatment independent decrease in gill NKA alpha 1a gene expression indicated a complete shift from a FWto a SW type gill Na+, K+-ATPase. Moreover, plasma Cl- concentration and osmolality after 7 days in SW in May were well within the range expected for fully SW adapted fish in all treatment groups. Despite good hypoosmoregulatory abilities in all fish, there were strong treatment dependent effects on their growth during the two month SW residence. SW growth was negatively correlated with growth and change in K during the last part of the FW phase. A markedly higher specific growth rate and appetite in the SP-LL group than in the LL-LL group was likely a result of the light stimulated endocrine changes that occurred during smolting. Dietary treatment seemed to compensate for the lack of light treatment since feed intake and growth in LL-LL + diet fish was comparable to that in the SP-LL treated smolts. Feed intake and growth in the SP-LL + diet groups was highest, giving support for a notion that light and diet act through separate and additive mechanisms.