If osmotic stress and reduced seawater tolerance are predisposing factors for infectious pancreatic necrosis (IPN) outbreaks in farmed Atlantic salmon, increased survival by enhancing access to energy would be expected. The aim of the present study was, therefore, to increase energy access in 1-year old Atlantic salmon after sea transfer by increasing the level of dietary fat, by exchanging some of the dietary oil with more easily oxidized medium chain triacylglycerols, or by dietary supplementation of potentially energy enhancing additives such as clofibrate and tetradecylthioacetic acid (TTA). A natural outbreak of IPN occurred 8 weeks after sea transfer, and a significant dietary effect explaining 76% of the variation in mortality was observed. Relative percentage survival for the fish fed TTA in sea water was 70% when compared with the unsupplemented control, reducing mortality from 7.8 to 2.3%. Muscle fat content and plasma chloride were related to IPN mortality, suggesting that reduced hypoosmoregulatory capacity might be a predisposing factor to the onset of an IPN outbreak. Based on the observation of a threefold increase in white muscle mitochondrial fatty acid oxidizing activity by TTA, it is suggested that TTA has resulted in a re-allocation of dietary fatty acids from storage to energy producing oxidation.

We have previously documented increased survival by feeding tetradecylthioacetic acid (TTA) during a natural outbreak of infectious pancreatic necrosis in post-smolt S1 Atlantic salmon. The aim of the present study was to test the effects of dietary TTA in S0 smolt at a location where fish often experience natural outbreaks of heart and skeletal muscle inflammation (HSMI) during their first spring at sea. The experimental groups were fed a diet supplemented with 0.25% TTA for a 6-week period prior to a natural outbreak of HSMI in May 2007. Relative percent survival for the groups fed TTA was 45% compared with control diets, reducing mortality from 4.7% to 2.5%. Expression of genes related to lipid oxidation was higher in cardiac ventricles from salmon fed TTA compared with controls. In addition, salmon fed TTA had periodically reduced levels of plasma urea, and increased cardiosomatic index and growth. Reduced mortality and increased growth after administration of TTA may be related to a combination of anti-inflammatory effects, and an altered metabolic balance with better protein conservation because of increased lipid degradation.

We have investigated the feed intake and growth in autumn‐transferred Atlantic salmon (S0) during their first spring in the sea, a period of low performance in commercial production. We have compared the results with those obtained from spring‐transferred smolt (S1), in order to determine whether this reduction in performance is accompanied by changes in nutrient retention, levels of muscle fat, energy status or condition factor (CF). The practical importance of the results obtained in the small‐scale experiments was evaluated by studies performed at two commercial farms, both using S0 salmon. The feeding rate, rate of growth and degree of feed utilization were low during the first spring in sea, for both S0 and S1 smolt. In both commercial farms, the apparent feed intake in S0 was reduced by approximately 50% in the spring. This low‐performing period coincided with reduced fat and energy retention, low levels of muscle fat and poor CF. Fat retention was reduced from 44.8% (March–May) to 15.4% (May–June) in S0, whereas protein retention did not change, indicating that the energy demand was high during the first spring in sea.

In the present study, the possible e¡ect of dietary treatment on early sexual maturation in post-smolt Atlantic salmon, without any negative e¡ect regarding growth, was investigated. The experiment was performed using 4400 individually marked (Pit tag) 11 salmon, fed either a control diet or a diet supplemented with 0.5% tetradecylthioacetic acid (TTA) in duplicates for 3,6 or 12 weeks after sea transfer. Compared with the control, dietary supplementation of TTA resulted in a threefold reduction in incidence of sexual mature males (0.6% vs. 1.8%). A curve-linear relationship between relative reduction in maturation and weeks of feeding TTA was found, indicating that the e¡ect is most marked as a result of the ¢rst weeks of feeding and then levelling o¡. No negative dietary impact on growth was observed. As the level of fat in the muscle was reduced by dietary TTA, it seems that post-smolt supplemented dietary TTA do not accumulate high enough energy stores to start the maturation process, whereas the energy-enhancing e¡ect of TTA due to increased fatty acid oxidation capacity may maintain the growth potential. Compared with immature salmon, sexually maturing ¢sh revealed increased spring growth before the onset of maturation.