Impact of Temperature Reduction from 14 °C to 12 °C in RASs on Atlantic Salmon: Increased Mineral Accumulation in RASs and Enhanced Growth Post-Transfer to Seawater

Robust, healthy, and fast-growing smolt is of high importance for fish farmers as a way of reducing the mortality and production time of Atlantic salmon (Salmo salar) in open sea pens. Lowering the water temperature in flow-through systems (FTSs) compared to recirculating aquaculture systems (RASs) has shown promising results for the growth and health of fish post-transfer to sea; unfortunately, limited information is available on the same parameters in replicated RAS setups. Hence, the current study aimed to compare the performance of Atlantic salmon reared at 12 and 14 °C over a 9-week RAS period and a subsequent 10-week post-transfer period, while also investigating the accumulation pattern of minerals in RASs. The results showed a 100% survival and comparable condition factors and cardiosomatic index (CSI) across both temperatures. During the RAS period, the thermal growth coefficient (TGC) was higher at 12 °C, but body weight gain and feed consumption were lower. No differences in mineral retention or fecal stability were observed. However, the production water accumulated more dissolved phosphorus (DP) and total iron (Fe) at 12 °C. Post-transfer, the TGC remained higher for fish with a rearing history of 12 °C. This fish also had higher body weight gain and feed consumption while exhibiting a lower hepatosomatic index (HSI) and viscerosomatic index (VSI), indicating improved muscle growth. Overall, lower water temperature reduced growth and increased DP and Fe in RASs. However, it improved post-transfer weight gain of muscle tissue, highlighting its benefits for seawater performance.