Freshwater history exerts a larger effect than the feeding regime at sea on the occurrence of rainbow trout sexual maturation in seawater cages

Sexual maturation of rainbow trout during grow-out at sea poses economic risks for aquaculture due to reduced growth, feed intake, and downgraded harvest quality. This study evaluated the effects of freshwater rearing systems and feed rationing at sea on maturation in a commercial production site in Western Norway. Two batches of trout reared in Recirculating Aquaculture Systems (RAS) or Flow-Through (FT) systems were transferred to sea cages in March 2024 (mean sizes 450 g and 179 g, respectively). All fish received full rations for the first three months at sea, after which half of the cages from each system were subjected to feed restriction (one fasting day every three days; ~70% ration) until harvest. Fish reared in RAS showed a higher incidence of sexual maturation than FT fish irrespective of the feed regime (12% vs. 0%). This difference was associated with more intensive freshwater conditions, particularly elevated temperatures during the last period of the freshwater phase, and larger size at sea transfer. Gonadotropic upregulation in RAS trout was evident by June, prior to visible maturation, suggesting activation of the reproductive axis triggered by increasing photoperiod and temperature following spring transfer. Maturation occurred in both sexes, unlike in Atlantic salmon where is male-predominant. Feed restriction did not reduce maturation rates but negatively affected growth, feed efficiency, and harvest quality. Thus, ration reduction is not an effective strategy against early maturation and incurs additional costs. These results highlight a trade-off in freshwater rearing strategies: while intensive RAS conditions promote faster growth, they may increase the risk of early maturation and associated welfare issues at sea.