A 2X2 FACTORIAL STUDY ASSESSING PHOTOPERIOD AND FEEDING RATE ON THE OVERALL QUALITY OF ATLANTIC SALMON Salmo salar POST-SMOLTS RAISED IN LAND-BASED FRESHWATER SYSTEMS

Producing larger Atlantic salmon Salmo salar smolts (up to 1 kg in size) in freshwater systems is a novel approach for land-based salmon production. In this scenario, there is a need to establish optimum environmental conditions to maximize smolt quality, through optimum induction of smoltification, reduction in early maturation, and minimization of losses to disease upon transfer to sea cages and thereafter. Photoperiod has been shown to have a strong influence on both smoltification and the initiation of sexual maturation; however, determining the optimum photoperiod regime, in terms of osmoregulatory and reproductive physiological outcomes, during pre-growout freshwater rearing, has not been adequately assessed. Feed restriction during pre-growout has also been shown to reduce early maturation in post-smolts. We investigated two photoperiod regimes: i) LD24:0 (i.e. 24 hours light, zero hours dark), and ii) LDN (simulated natural photoperiod, Bergen latitude) applied to Atlantic salmon post-smolts raised in freshwater systems at 13^oC up to 1,000g in mean weight. All fish were reared on LD24:0 from first feeding up to 40g, at which point the fish received a 6-week LD12:12 artificial winter while simultaneously being split into full ration (FR) and reduced (60%) ration (RR) groups. Following the 6-week winter, fish were exposed to 8 weeks of LD24:0 to induce smoltification, after which both the FR and RR groups were further split into either LD24:0 or LDN groups and raised under these conditions with a target final weight of 1,000g in the best performing group. Gill samples were collected at the beginning of the 6-week artificial winter, for gene expression (NKAα1a and 1b, NKCC, DIO2a) and ATPase activity; gill samples for the same testing were further collected at i) the return to LD24:0, ii) midway through the LD24:0 smoltification window, iii) just prior to splitting the fish into photoperiod treatments, and iv) at 300g. Brain and pituitary were sampled during these latter sampling points, in order to assess brain DIO2b mRNA activity and pituitary gene expression, as well as head kidney to assess immunocompetence through microarray transcriptome analysis of annotated immune response pathways. Plasma was also collected during the three sampling points in the LD24:0 smoltification window, to assess chloride/osmolality, 11-ketotestosterone, cortisol, and thyroxin. Additionally, fish performance (length, weight) was assessed at regular monthly intervals, as well as condition factor, fin condition, and cataracts. At the time of abstract submission, the study is ongoing; the best performing fish are approximately 500g, and many tissue analyses data are still forthcoming. Presently, both photoperiod and feeding regime are independently statistically associated with growth performance, with best performing fish in the FR groups and poor condition factor exhibited in the RR groups. Full results will be presented at Aquaculture America 2018.