Farmed salmon feeds have changed from purely marine-based diets with high levels of EPA and DHA in the 1990s to the current 70 % plant-based diets with low levels of these fatty acids (FA). The aim of this study was to establish the impacts of low dietary EPA and DHA levels on performance and tissue integrity of Atlantic salmon (Salmo salar). Atlantic salmon (50 g) in seawater were fed fourteen experimental diets, containing five levels (0, 0·5, 1·0, 1·5 and 2·0 %) of EPA, DHA or a 1:1 EPA + DHA plus control close to a commercial diet, to a final weight of 400 g. Lack of EPA and DHA did not influence mortality, but the n-3-deficient group exhibited moderately slower growth than those fed levels above 0·5 %. The heart and brain conserved EPA and DHA levels better than skeletal muscle, liver, skin and intestine. Decreased EPA and DHA favoured deposition of pro-inflammatory 20 : 4n-6 and 20 : 3n-6 FA in membrane phospholipids in all tissues. When DHA was excluded from diets, 18 : 3n-3 and EPA were to a large extent converted to DHA. Liver, skeletal and cardiac muscle morphology was normal in all groups, with the exception of cytoplasm packed with large or foamy vacuoles and sometimes swollen enterocytes of intestine in both deficient and EPA groups. DHA supplementation supported normal intestinal structure, and 2·0 % EPA + DHA alleviated deficiency symptoms. Thus, EPA and DHA dietary requirements cannot be based exclusively on growth; tissue integrity and fish health also need to be considered.Key words: Aquafeed: DHA: EPA: Essential fatty acids: Fat: Phospholipids Continued growth of the Atlantic salmon (Salmo salar) farming industry depends on the availability of sustainable feed ingredients in the world market. For optimal use of ingredients with limited availability, information regarding nutritional requirements is of utmost importance. Fatty acid (FA) composition of salmon diets has changed considerably over the last several decades. Although 90 % of traditional Norwegian salmon diets were composed of marine ingredients in the 1990s, current diets only contain approximately 30 % marine ingredients (1)

Salmon slaughtered by standard routines (control) or stressed by confinement for 10 min before stunning and then stored at 0.4 or 3.3ЊC for 9 days were compared. Handling stress led to lower muscle phosphocreatine (pϽ0.001), adenosine-5'-triphosphate (pϽ0.05) and shorter pre-rigor period. Storage temperature affected external quality index, white muscle pH and K-value (degradation products of ATP). Stress produced a softer fillet (pϽ0.001). A lower breaking strength (pϽ0.01) was found in fish stored at 0.4ЊC. Sensory tests distinguished the control/ stress groups within the 0.4ЊC chilling regime and the 0.4ЊC/3.3ЊC chilling groups within the control regime. Stress caused a lower score for texture (pϽ0.05) both at 0.4 and 3.3ЊC and for odor at 3.3ЊC in a descriptive sensory test. No detectable effects of stress or storage temperature were found on flavor or color.

A 12-week feeding trial was conducted to investigate the interactive e¡ects between water temperature and diets supplemented with di¡erent blends of ¢sh oil, rapeseed oil and crude palm oil (CPO) on the apparent nutrient and fatty acid digestibility in Atlantic salmon. Two isolipidic extruded diets with added ¢sh oil ¢xed at 50% and CPO supplemented at10% or 25% of total added oil, at the expense of rapeseed oil, were formulated and fed to groups of Atlantic salmon (about 3.4 kg) maintained in £oating cages. There were no signi¢cant e¡ects (P40.05) of diet on growth, feed utilization e⁄ciency, muscle total lipid or pigment concentrations. Fatty acid compositions of muscle and liver lipids were mostly not signi¢cantly di¡erent in salmon fed the two experimental diets but showed elevated concentrations of 18:1n-9 and18:2n-6 compared with initial values. Decreasing water temperatures (11^6 1C) did not signi¢cantly affect protein, lipid or energy apparent digestibilities of the diets with di¡erent oil blends. However, dry matter digestibility decreased signi¢cantly in ¢sh fed the diet with CPO at 25% of added oil. Increasing dietary CPO levels and decreasing water temperature signi¢cantly reduced the apparent digestibility (AD) of saturated fatty acids. The AD of the saturates decreased with increasing chain length within each temperature regimen irrespective of CPO level fed to the ¢sh. The AD of monoenes and polyunsaturated fatty acids was not a¡ected by dietary CPO levels or water temperature. No signi¢cant interaction between diet and water temperature e¡ects was detected on the AD of all nutrients and fatty acids. The results of this study showed that the inclusion of CPO up to about 10% (wt/wt) in Atlantic salmon feeds resulted in negligible di¡erences in nutrient and fatty acid digestibility that did not a¡ect growth performance of ¢sh at the range of water temperatures generally encountered in the grow-out phase.

The present study aimed to determine the minimum requirements of the essential n-3 fatty acids EPA and DHA in Atlantic salmon (Salmo salar) that can secure their health under challenging conditions in sea cages. Individually tagged Atlantic salmon were fed 2, 10 and 17 g/kg of EPA + DHA from 400 g until slaughter size (about 3·5 kg). The experimental fish reared in sea cages were subjected to the challenging conditions typically experienced under commercial production. Salmon receiving the lowest EPA + DHA levels showed lower growth rates in the earlier life stages, but no significant difference in final weights at slaughter. The fatty acid composition of various tissues and organs had remarkably changed. The decreased EPA + DHA in the different tissue membrane phospholipids were typically replaced by pro-inflammatory n-6 fatty acids, most markedly in the skin. The EPA + DHA levels were maintained at a higher level in the liver and erythrocytes than in the muscle, intestine and skin. After delousing at high water temperatures, the mortality rates were 63, 52 and 16 % in the salmon fed 2, 10 and 17 g/kg EPA + DHA. Low EPA + DHA levels also increased the liver, intestinal and visceral fat amount, reduced intervertebral space and caused mid-intestinal hyper-vacuolisation. Thus, 10 g/kg EPA + DHA in the Atlantic salmon diet, a level previously regarded as sufficient, was found to be too low to maintain fish health under demanding environmental conditions in sea cages.

Initial levels of white muscle high-energy phosphates, IMP, ATP:IMPratio, adenylate energy charge and pH were used as indicators of handling stress when Atlantic salmon were slaughtered. Ante-mortem handling included fish that were either anesthetized (baseline), quickly netted and killed by a blow to the head (unstressed) or chased to exhaustion in the holding tank (stressed). During subsequent storage in ice, freshness was evaluated in terms of IMP, HxR, Hx and K-values. HPLC showed initial differences in distribution of metabolites induced by struggling gradually fell during storage. K-values were different only for 2 days post mortem, but the effect of handling stress was discernible as higher mean K-values and different IMP and HxR contents for up to 7 days post mortem.

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Rainbow trout with initial body weight 1144 g were fed two diets with high (feed A) or low (feed B) water stability for 6 weeks. During the last 2 weeks either stable or fluctuating oxygen saturation, salinity and temperature was introduced. High water stability of the feed was associated with harder pellets, less dust formation and less broken pellets compared to feed with low water stability. During the first 4 weeks, feed intake was 23% higher in trout fed diet B than in those fed diet A. Fluctuating environment resulted in a severe drop in feed intake for both dietary groups. At termination of the trial, stomachs of trout fed feed B contained mashed pellets and free water and oil. Stomachs of trout fed feed A contained more intact pellets and little fluid. The apparent digestibility of protein, starch, dry matter and energy was highest in feed A. The apparent digestibility of P and Zn was highest at stable environment, and the absorption of P was highest from feed A. In conclusion, physical quality affected the nutritional value of the feeds, and differences in nutritional value at stable or fluctuating environment appeared to be related to feed intake.