Effects of Temperature and Salinity on Parr-Smolt Transformation and Seawater Performance in Juvenile Atlantic salmon (Salmo salar L.): Flow Through Systems versus Recirculating Aquaculture Systems

Abstract

To ensure a successful acclimatization of juvenile Atlantic salmon to open net-pens in seawater (SW), it is crucial to maintain optimal conditions throughout their various developmental stages in land-based rearing facilities. Recently, discussions have emerged about potential differences between traditional Flow Through Systems (FTS) and Recirculating Aquaculture Systems (RAS) concerning fish welfare and performance in SW. This study explores the influence of temperature and salinity on the parr-smolt transformation (PST) and subsequent seawater (SW) performance in juvenile Atlantic salmon (Salmo salar L.) reared in FTS and RAS. The effect of various temperature regimes (8 °C, 12 °C, 14 °C, and 8 °C to 12 °C) during the freshwater (FW) phase, as well as two different salinity regimes after completion of PST: 1) direct transfer to 32 ppt for approximately 21 days and 2) a gradual increase to 32 ppt over the same periodwere investigated. These factors were evaluated in terms on their impact on smolt quality, SW tolerance, and overall performance in SW environments. The findings revealed that higher rearing temperatures (12 °C and 14 °C) significantly enhanced growth, smolt index, and kidney Na+, K+ ATPase (NKA) enzyme activity. Notably, smolts raised in RAS displayed greater variability in NKA enzyme activity compared to the more stable development observed in RAS. The study also noted a delayed response in kidney NKA enzyme activity compared to the gills, which indicated that renal adaptation to osmoregulatory demands in salmon lags behind gill responses. Additionally, gradual salinity increases prior to seawater open net-pen transfer improved welfare outcomes and reduced mortality rates, particularly in RAS, where such adjustments appear important for mitigating stress and enhancing survival. These findings highlight critical differences between FTS and RAS, underscoring the need for tailored management strategies in different rearing systems to optimize smolt resilience and performance in SW. Optimizing both temperature and water quality management in land-based rearing systems, particularly in RAS, is crucial for enhancing smolt quality and survival rates during the early stages of SW adaptation, thereby reducing mortality and improving production efficiency in commercial salmon farming.