Jannicke Vigen

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.

Metabolic responses to different dietary levels of vitamin D ₃ and available phosphorus (avP) was investigated in cultured Atlantic salmon ( Salmo salar ). The study was carried out in triplicate with a 2‐level full factorial design with a center point and three additional points within the design space. Over a 17‐ week period at the initial weight of 453 ± 9 g, salmon reared indoor on land, were fed diets containing different levels of vitamin D ₃ and avP. In summary,growth performance, measured as specific growth rate (SGR), improved when either vitamin D ₃ or avP was increased independently, but declined when both were elevated excessively. Bone mineralization was maintained at intermediate vitamin D ₃ inclusion levels, though high vitamin D ₃ reduced bone ash. A diet containing 0.63 mg/kg vitamin D ₃ and 0.70% avP supported growth and mineral retention. A significant reduction in fecal soluble phosphorus when vitamin D ₃ was increased in the diet, suggests that management of vitamin D ₃ can contribute to improved waste control and reduced environmental load for phosphorus. Finally, the study highlighted that today’s commercial dietary inclusion of vitamin D ₃ can result in lower vitamin D ₃ accumulation in muscle of land‐based farmed Atlantic salmon compared to wild Atlantic salmon.

A four-week trial was conducted to compare the effects of a conventional flow-through system diet (FTS) and an experimental RAS diet (ERAS) on fish performance, water quality and general system implication in a replicated recirculation aquaculture system (RAS). Six identical RAS, each with a total system water volume of 1500 L and cylindrical rearing tanks of 1000 L were stocked with Atlantic salmon (Salmo salar) post-smolts with an average weight of 199.7 ± 28 g, to an average stocking density of 30 kg/m³ and reaching approximately 49 kg/m³ at the trial end. The ERAS diet were composed to inhabit typical RAS feed characteristics compared with the FTS diet, such as a higher fecal stability and reduced protein levels (−12%), but at the same time increased fat content (+8%) to secure similar gross energy levels (22–23 MJ kg⁻¹) between the two diets. Water quality parameters were measured individually. The total accumulation of minerals and metals was analyzed in water from different parts of the system at the start and end of the trial period for both diets. No differences in growth, condition factor, feed conversion rate (FCR), or survival of fish fed the two dietary adaptations were observed. The system using the ERAS diet showed significantly higher pH (+1.2%) and alkalinity (+17%) and lower total ammonia nitrogen (TAN) (−18%) and NO₂⁻ (−46%) compared with the FTS diet. The count of drum filter activations was also significantly lower (−13%) with the ERAS diet. Compared with the FTS diet, the ERAS diet had a lower probability (−4%) of generating particles smaller than 50 μm, and that the RAS was also more effective in removing particles from the drum filter, prompting a lower daily activation of the filter of 22.1 ± 3.0 counts compared with 25.5 ± 3.5 for the FTS diet. Mineral analysis showed a significantly lower accumulation of total phosphorus (TP) (−90%) and dissolved phosphorus (DP) (−92%) in the RAS units using the ERAS diet compared with those using the FTS diet. Compared with a traditional flow-through diet, these results highlight the benefits of using an RAS-adapted diet that matches the energy requirement of flow-through diets regarding water quality, system performance, satisfactory growth, and condition.

Growth and histological parameters were evaluated in Atlantic salmon (74 g) that were fed alternative phospholipid (PL) sources in freshwater (FW) up to 158 g and were transferred to a common seawater (SW) tank with crowding stress after being fed the same commercial diet up to 787 g. There were six test diets in the FW phase: three diets with different doses of krill meal (4%, 8%, and 12%), a diet with soy lecithin, a diet with marine PL (from fishmeal), and a control diet. The fish were fed a common commercial feed in the SW phase. The 12% KM diet was compared against the 2.7% fluid soy lecithin and 4.2% marine PL diets, which were formulated to provide the same level of added 1.3% PL in the diet similar to base diets with 10% fishmeal in the FW period. A trend for increased weight gain with high variability was associated with an increased KM dose in the FW period but not during the whole trial, whereas the 2.7% soy lecithin diet tended to decrease growth during the whole trial. A trend for decreased hepatosomatic index (HSI) was associated with an increased KM dose during transfer but not during the whole trial. The soy lecithin and marine PL diets showed similar HSI in relation to the control diet during the whole trial. No major differences were observed in liver histology between the control, 12% KM, soy lecithin, and marine PL diets during transfer. However, a minor positive trend in gill health (lamella inflammation and hyperplasia histology scores) was associated with the 12% KM and control diets versus the soy lecithin and marine PL diets during transfer.