Long term effects of smolt and post-smolt production strategy on mortality, growth, sexual maturation and melanized focal changes in farmed Atlantic salmon (Salmo salar L.)

Diet-based smolt production and land-based post-smolt rearing are emerging strategies in Atlantic salmon aquaculture. However, their impact on fish performance and welfare over a full production cycle is poorly understood. To address this, we exposed 0+ Atlantic salmon parr (~50 g) initially kept on continuous light to two different photoperiods at 12 °C: either a ‘square wave’ regime with 6 weeks of short day (12 h light / 12 h dark) followed by a return to continuous light (hereafter LD-LL ), or continuous light throughout (hereafter LL ). When LD-LL returned to continuous light, half of the LD-LL tanks and half of the LL tanks were provided a salt ( S ) enriched diet (5.0 % NaCl, 0.5 % CaCl 2 ) in the expectation that it will aid the seawater transition, establishing four different freshwater (FW) regimes: LD-LL , LL , LD-LLS and LLS . FW rearing ended 4 weeks after initiation of the different dietary regimes (October). Then, fish from each FW regime (~100–130 g) were equally distributed between three different post-smolt scenarios in a common garden design: In tanks at 12 °C under LL with 15 ppt or 34 ppt, or directly into sea-cages. This created 12 groups (×4 FW regimes and ×3 post-smolt scenarios). Five months later (March), the post-smolts in tanks on 15 and 34 ppt were transferred into sea-cages (at ~1 kg). The experiment ended 7 months later (September, at about 3.5 kg). Response parameters were weight and length, mortality, and the prevalence of sexual maturation, vertebral deformities, and melanized focal changes (MFC) in the fillet at harvest. FW diet or photoperiod had no effect on mortality, but mortality was higher in those fish that had been moved directly to sea-cages versus those that remained on land for longer. FW diet did not affect growth, but the size at transfer to sea-cage was greater in LL (~130 g) than LD-LL smolts (~100 g). Post-smolt rearing on land in 34 ppt stimulated growth to harvest relative to 15 ppt, however, otherwise there was no effect of FW photoperiod or post-smolt scenario on final weights. Maturity levels were higher following the two land-based regimes (male: 53–68 %, female: 6–20 %) compared to those moved directly to sea-cages as smolts (male: 10–25 %, female: 0 %). LD-LL caused a general increase in male maturation compared to LL, while the S diet increased male maturation in fish directly transferred to sea-cages. MFC prevalence was higher in fish reared on land in 34 ppt (~9.5 %) relative to those reared on 15 ppt (~3.1 %), and was also higher in immature (~6.7 %) fish compared to maturing fish (~3.3 %). All groups had an equal and relatively high level of vertebral deformities (≥1 def. Vertebra ~44 %, >10 def. Vertebrae ~23 % and > 20 def. Vertebrae ~9 %), mostly ‘cross stitch vertebrae’. Harvest yield (total biomass produced excluding mature fish) was higher (~1.8 fold) in groups transferred directly to sea-cages as smolts versus those that remained on-land during early post-smolt rearing. The study suggests that post-smolt rearing on land to 1 kg can jeopardize fish welfare by stimulating a high level of maturation and give reduced harvest yield when compared to transferring smolts directly to a sea-cage. • No effect of salt-enriched diet or smoltification photoperiod on harvest weight. • Salt-enriched diet stimulated grilse maturation in sea cage post smolts. • More grilse maturation in on-land than sea cage post smolts. • More muscular melanin deposits in 34 ppt than 15 ppt on-land post smolts. • More muscular melanin deposits in immature than mature fish. • Higher harvest yield in sea cage than on-land post smolts.