Stage- and Temperature-Dependent Responses of Baltic Salmon (Salmo salar L.) Juveniles to Probiotic Supplementation During Rearing in Flow-Through Hatchery

This study evaluated the effects of a commercially produced, ready-to-use probiotic supplement, dominated by lactic acid bacteria, on the growth performance and selected health indicators of salmon (Salmo salar L.) fry, parr, and presmolts reared for restocking purposes. The results indicated a stage-specific response to probiotic application. In salmon fry, short-term immersion baths were associated with reduced juvenile mortality and improved tolerance to fluctuations in temperature and dissolved oxygen. In parr, immersion treatments were linked to improved growth performance, condition factor, and a lower incidence of gill-cover necrosis compared to the Control group. In presmolts, short-term dietary supplementation with probiotics was associated with increased growth rates relative to Controls. Probiotic application was associated with changes in bacterial isolates and, in some groups, lower antimicrobial resistance (AMR) indices, particularly after 14 days of immersion treatment. Probiotic supplementation was associated with improved growth and increased tolerance to environmental stressors, indicating its potential to support fish welfare and sustainable aquaculture.