Feed intake, growth rate and body composition of juvenile Baltic salmon exposed to different constant temperatures

Feed intake, specific growth rate and changes in body composition of age 1+ and 2+ Baltic salmon, Salmo salar L, were studied for fish held under constant temperature conditions. The 1+ fish (60 g) were reared for 6 weeks at 11, 15, 17, 19 or 23 °C and 2+ fish (250 g) were held at 15 °C. Feed intake of 1+ salmon increased from 176 kJ kg-1 day-1 at 11 °C to 275 kJ kg-1 day-1 at 17 °C and decreased to 229 kJ kg-1 day-1 at 23 °C. Specific growth rate increased from 1.18% day-1 at 11 °C to 1.59% day-1 at 15 °C and decreased to 0.56% day-1 at 23 °C. Optimum temperatures for feed intake and growth were estimated at 17.8 °C and 15.6 °C, respectively, and estimated upper thermal limits for feeding and growth were 29.0 °C and 24.1 °C, respectively. Models for feed intake and growth rate in relation to temperature and fish size are presented. The utilization efficiency of ingested energy decreased from 57% at 11 °C to 22% at 23 °C. For all groups of 1+ fish, most (approximately 86%) of the weight gain consisted of water. Lipid deposition accounted for about 52% of the body energy gain irrespective of rearing temperature.     

Feed intake, growth and feed utilization of offspring from wild and selected Atlantic salmon (Salmo salar)

The aim was to assess whether selection for increased growth rate in Atlantic salmon (Salmo salar) is associated with increased feed intake and/or better feed utilization. Growth responses of offspring from selected and wild lines of Atlantic salmon (initial weight 814±14 g and 533±12 g, respectively) were tested in a 14-week experiment. Selected and wild salmon increased body weight by 79 and 39%, respectively, during the experiment. Relative feed intake (DFI), thermal growth coefficient (TGC) and feed efficiency ratio (FER) were significantly higher in the selected (DFI: 0.67% BW d⁻¹, TGC: 2.96×10⁻³ and FER: 1.16), than in the wild, line (DFI: 0.48% BW d⁻¹, TGC: 1.39×10⁻³ and FER: 0.93). FER was positively correlated with mean growth rate (r=0.90, n=6, P<0.05), and differences between the lines indicated a 4.6% increase in FER per generation of selection. Fish of the selected line had a significantly lower intake of protein and energy per kilogram gain, so the higher growth rate of the selected line was the result of both greater feed consumption and more efficient feed utilization for growth. This implies that selection for increased growth rate in Atlantic salmon may improve both of these traits.     

Feed intake of Atlantic salmon parr Salmo salar L. in relation to temperature and feed composition

The design and testing of a feed intake monitoring system based on feed waste collection is described. The system was used in a study of feed intake and growth of Atlantic salmon Salmo salar L. parr in relation to temperature (2 °C or 8 °C) and feed composition (21% fat, 50% protein and 22.5 MJ kg^?1 or 34% fat, 40% protein and 24.8 MJ kg^?1). Fish had lower feed intake and slower growth at the lower temperature, but temperature‐corrected growth (TGC) was better in fish held at the lower temperature. There was an increase in TGC over time at low temperature, probably as a result of long‐term thermal acclimation. Feed conversion for an increase in body size from 19 to 38 g was better for the fish held at low temperature. Feed consumption of low‐fat feed was higher than that of high‐fat feed, and growth of the fish fed the low‐fat feed was better at the higher temperature. These data are consistent with the ideas that the fish compensated for differences in feed energy densities to maintain energy and nutrient intakes, and that lipostatic factors may be operating to regulate feed intake and growth.     

Lipostatic regulation of feed intake in Atlantic salmon Salmo salar L. defending adiposity at the expense of growth?

Production of Atlantic salmon Salmo salar L. utilizes feeds with high fat concentrations to give low feed:gain. However, increased dietary fat content inevitably leads to increased body fat deposition, and salmon with very high fat concentrations in the muscle (fillet) are regarded as being of inferior quality. Adiposity is thought to participate in the regulation of feed intake by means of negative feedback control. Thus, elevated adiposity is predicted to result in lower feed consumption, and thereby possibly impair growth. We tested the hypothesis that high body fat content in salmon would lead to reduced feed intake and growth. Salmon (740 g) were preconditioned with high‐ or low‐fat feed (38.8 and 27.8% dietary fat content) for 10 weeks to establish differences in body fat storage (build‐up phase). Thereafter, fat and lean fish (19.4% and 16.7% body fat content) were fed the high‐ and low‐fat feeds for an additional 7 weeks (Phase Two). During Phase Two, the fat fish consumed 30% less feed than lean fish, which resulted in corresponding differences in growth. The differences in adiposity seen at the end of the build‐up phase were still evident at trial end. Groups of fish, which were preconditioned with the same feed during the build‐up phase, had similar feed consumption and growth to each other during Phase Two, indicating that body fat was playing an important role in the regulation of feed intake.     

In vitro protein digestion, and growth of Atlantic salmon with different trypsin isozymes

Growth and in vitro digestion was examined in three iso-trypsin phenotypes (patterns 1, 2 and 2) of Atlantic salmon, Salmo salar. Fish grew equally well during the 128 day experiment irrespective of phenotype. At day 64 and 91, weight-specific growth rate of 2 phenotype animals were significantly lower (p < 0.05). At the end of the experiment, significant differences (p < 0.05) were noted between groups with respect to size. An in vitro method was used to examine protein digestion by the different trypsin phenotypes. Three fish meals (FM I > FM II > FM III) of high, medium and low quality, as defined by chemical analyses, were used. Phenotype 1 was able to differentiate between three fish meals ranking FM I > FM II > FM III. In contrast, phenotypes 2 and 2 were not able to separate between meals. In vitro digestion efficiency was dependent upon the three salmon types, ranking phenotype 2 > 1 = 2.     

Variation of juvenile Atlantic salmon (Salmo salar) body composition along sedimentary links*

Johnston P, Bergeron NE. Variation of juvenile Atlantic salmon (Salmo salar) body composition along sedimentary links.
Ecology of Freshwater Fish 2010: 19: 187–196. © 2010 John Wiley & Sons A/S Abstract – The objective of this study was to determine the proximate body composition of juvenile Atlantic salmon along the sequence of aquatic habitat types created by longitudinal changes in the riverbed substrate of two rivers (i.e., sedimentary links units). Interesting trends in the body composition were observed but our initial hypothesis, that fish of the upstream sections have higher energy content, was nevertheless not verified. No common longitudinal pattern was detected in the body composition (water, lipid, energy density) along the studied rivers. Trends in the body constituents were different between age‐classes and rivers, whereas they were highly variable within‐ and among‐samples. There was however a common trend in the pattern of variations, with the coefficient of variations increasing in the downstream direction for almost all constituents, age‐classes and rivers. Potential abiotic and biotic factors that might have contributed to these observations are discussed.     

Is there lipostatic regulation of feed intake in Atlantic salmon Salmo salar L.?

High‐energy feeds, with increased fat concentrations, are often used in salmon farming to improve feed:gain ratio. However, fish are thought to regulate ingestion to meet their energy and nutrient intake requirements. Further, feeds with excessive fat content will lead to increased adiposity, which is thought to exert a negative feedback on feed intake via lipostatic regulation mechanisms. A test of the lipostatic model of feed intake regulation was carried out on juvenile Atlantic salmon Salmo salar L. (c. 165 g) in which body fat content (5.6% and 9.4% body fat) had been manipulated by feeding feeds with different fat concentrations. Thereafter, the fish were offered the high‐ and low‐fat feeds (15.6% and 26.5% dietary fat content) simultaneously, and our hypothesis was that feed preference and intake would reflect the state of fat storage. Thus, we predicted that, when given a choice of feeds, the lean fish would eat more, and that the body fat status of the groups would converge over time. The results indicated a general preference for the leaner feed irrespective of adiposity level, but leaner fish consumed more feed, grew faster and deposited more body fat than their fatter counterparts. Over time, body compositions converged among treatments, and differences in feed intake ablated. These findings seem to provide supportive evidence for a lipostatic regulation of feed intake in fish.     

The influence of feeding regime on growth and slaughter traits of cage-reared Atlantic salmon

The effects of different feeding methods on growth and slaughter traits of Atlantic salmon, Salmo salar, have been examined. Swimming activity associated with feeding was greatest amongst fish fed using automatic feeders, intermediate amongst those fed predetermined rations by hand, and least in groups of fish fed to satiation twice per day. Fish fed to satiation ate more and grew faster than did those provided with rations predicted to support high rates of growth, but the fish fed to satiation had higher feed:gain ratios than those given predetermined rations as two meals per day. The muscle tissues of the fish fed to satiation had higher lipid (%) and lower moisture (%) contents than did those of the fish fed the predetermined rations, but differences in muscle composition could be accounted for by differences in the size of the fish. Lipid was positively, and moisture negatively, correlated with fish size, whereas per cent protein and dress-out losses were little influenced by fish size and feeding regime. The fish fed to satiation were slightly heavier for a given length than those in the other groups. There was, however, no evidence that the largest fish had the highest condition factor, nor could condition factor be used to predict per cent lipid in the muscle tissue. Size variation increased in all groups of fish, and this was, in part, due to the cessation of growth of maturing individuals. Size variation was, however, most marked in the groups fed using automatic feeders, suggesting that interindividual competition for food was greatest amongst these fish.     

Ultrastructural pathology of Baltic salmon, Salmo salar L., yolk sac fry with the M74 syndrome

The ultrastructural pathology in liver, brain, skeletal and cardiac muscle of Baltic salmon yolk sac fry with the M74 syndrome is described. In the clinical stage of disease, the main pathological findings in the liver were a depletion of glycogen, condensation of nuclear chromatin, hydropic degeneration of mitochondria and a dilation of the bile canaliculi. In the terminal stage, additional findings were lipid accumulation and myelin whorls in the cytoplasm. The rough endoplasmic reticulum (RER) was degranulated and vesiculated and in some individuals, it formed concentric membranous whorls. Mitochondria showed several additional lesions, such as matrix densities, pleomorphism and cristae abnormalities. Skeletal myocytes were degenerated, and intracellular lipid accumulation was seen in the myocardium. In the brain, an increased frequency of cells exhibiting pyknosis or karryorhexis was recorded. The cytoplasm of these cells formed an amorphous mass of moderate density. The evaluation of brain and skeletal muscle was complicated by sporadic occurrence of pathological findings in the reference material, i.e. clinically healthy Baltic salmon yolk sac fry. As these yolk sac fry are suspected to have a subclinical thiamine deficiency, reference material in future studies should include salmon yolk sac fry from Atlantic populations or originating from reared broodstock.     

饲喂频率对黑鲷生长及体生化成分的影响

选取同一种饵料设置4个饲喂频率水平(分别为1次/d、2次/d、4次/d、6次/d),采用静水连续充气养殖系统,在盐度为26.0~27.0、温度为26.8~24.5℃的条件下对黑鲷成鱼和鱼种进行为期40d的生长实验。结果表明:(1)摄食率不受饲喂频率的显著影响,饲喂频率影响黑鲷的生长是由饲料转化率的改变引起的。(2)黑鲷成鱼和鱼种的水分含量随饲喂频率的增高而增加,黑鲷成鱼脂肪含量随饲喂频率的增高而下降,黑鲷鱼种脂肪含量随饲喂频率的增高而增加。(3)在本试验条件下,适宜饲喂频率随着黑鲷鱼体重的增加而提高,黑鲷鱼种的适宜饲喂频率为2次/d,黑鲷成鱼的适宜饲喂频率为4次/d。     

Atlantic salmon, Salmo salar, utilizes wax ester-rich oil from Calanus finmarchicus effectively

Against a background of decreasing availability of fish oils for use in aquaculture, the present study was undertaken to examine whether a wax ester-rich oil derived from the calanoid copepod Calanus finmarchicus, could be used effectively by Atlantic salmon when supplied in their diet. Individually tagged Atlantic salmon of initial weight around 500 g were divided into replicate tanks of two dietary groups and fed either a fish oil supplemented diet, or an experimental diet coated with Calanus oil. Wax esters accounted for 37.5% of the lipids in the Calanus oil diet but were absent from the fish oil diet in which triacylglycerols (TAG) were the major lipid class. Over the feeding period (140 days) the salmon fed fish oil displayed a greater increase in length, but there was no significant difference between the two groups in weight gained. The specific growth rates (0.75) and the feed conversion ratio of fish fed the two diets were similar throughout the study. No differences were observed in the apparent digestibility coefficients (ADC) of fish fed Calanus oil or fish oil. The ADC of fatty acids decreased with chain length and increased with unsaturation. Long-chain alcohol utilization showed a similar tendency although there was a notable difference in that saturated long-chain alcohols were utilized better than the comparable fatty acid homologue. In fecal lipid of fish fed Calanus oil, the content of 16:0 alcohol decreased in both the free long-chain alcohol and wax ester fractions, while the corresponding fatty acid increased in the feces of both dietary groups of fish. In contrast, the proportion of the 22:1n−11 alcohol increased in both fecal wax esters and free long-chain alcohol fractions whereas 22:1n−11 fatty acid displayed no accumulation. The observed patterns of fatty acid and long-chain alcohol compositions in fecal lipid compared to those of the initial dietary lipid are consistent with the digestive lipases of salmon preferentially hydrolyzing esters containing polyunsaturated fatty acid (PUFA) moieties. The wax esters of Calanus oil contained substantial amounts of the n−3 PUFA, 20:5n−3 and 22:6n−3, that were effectively deposited in muscle and liver tissues. No major differences were seen in either lipid content/lipid classes or in gross fatty acid composition of these tissues between the two dietary groups. It is concluded that that Atlantic salmon in seawater can effectively utilize diets in which a major lipid component is derived from zooplankton rich in wax ester without any detrimental change in growth or body lipid composition. This finding gives support to the use of lipid from zooplankton from high latitudes as an alternative or as a supplement to fish oil and a provider of long-chain n−3 PUFA in diets for use in salmon aquaculture.     

Feeding and growth of exercised and unexercised juvenile Atlantic salmon in freshwater,and performance after transfer to seawater

Juvenile Atlantic salmon (Salmo salar), selected from the upper modal group, were first held for 122 days (from December to May) in freshwater, and then for 49 days in seawater (34% salinity). In freshwater, the fish were exposed to either standing water or water currents corresponding to a swimming speed of 1 Bl s^–1. Natural light conditions prevailed throughout the whole experiment.In the freshwater phase, food intake, growth rate and condition factor of the fish increased with increasing daylength. Condition factors, however, decreased slightly during the last month of the freshwater period, suggesting that the fish underwent parr-smolt transformation. The growth rate and food intake of the fish subjected to standing water were approximately 15% lower than those of fish exposed to water currents.Transient suppression of appetite and growth, and an increase in plasma concentration of Cl^–, were seen after exposure of the fish to seawater. Within 28 days, all these parameters had been restored to pre-transfer levels. Previous exercise did not appear to have influenced the hypoosmoregulatory capacity of the fish. Growth rate and food intake observed during the seawater phase were similar, irrespective of the treatment experienced by the fish during freshwater rearing.     

Growth and digestibility effects of soya lecithin and choline chloride on juvenile Atlantic salmon

Atlantic salmon, Salmo salar L., fingerlings with a mean weight of 1.5 g were fed one of four casein-gelatine-based purified diets supplemented with soya lecithin (LC) and choline chloride (CH) for 84 days. The diets were supplemented with either: 0 g kg⁻¹ CH and 0 g kg⁻¹ LC, 0 g kg⁻¹ CH and 30 g kg⁻¹ LC, or 5 g kg⁻¹ CH and 0 g kg⁻¹ LC, or 5 g kg⁻¹ CH and 30 g kg⁻¹ LC. The same diets were also fed to 100-g salmon to assess the effects of LC and CH supplementation on digestibility. Fingerlings fed the diet with neither LC nor CH (0 g kg⁻¹ LC and 0 g kg⁻¹ CH) grew at a significantly slower rate than fish fed the supplemented diets. There were significant effects on growth of supplementation of both LC and CH. The results indicate that the choline requirement of Atlantic salmon fingerlings is satisfied by 4 g kg⁻¹ inclusion in a gelatine-casein-based diet, and that dietary soya lecithin can fully replace choline chloride. The digestibility study with the larger fish indicated a beneficial effect of lecithin on the digestibility of both protein and energy.     

饲料蛋白水平对方格星虫稚虫生长和体组成的影响

以初始体重为39.20±0.24mg的方格星虫稚虫为实验对象,分别饲喂蛋白水平为25.21%、29.87%、35.03%、40.67%、45.47%、50.12%、55.29%、60.38%、64.85%的9种等能微颗粒饲料56d,研究饲料蛋白水平对方格星虫稚虫生长和体组成的影响。结果表明,饲料蛋白水平对方格星虫稚虫的生长性能有显著影响(P<0.05)。随着饲料中蛋白水平的提高,稚星虫的增重率和特定生长率都呈先增后降的趋势,当饲料蛋白水平为45.47%时,稚星虫增重率和特定生长率均达到最大值,二次曲线回归分析确定当增重率达到极值时,饲料蛋白水平为46.79%;对稚星虫体组成成分的分析表明,饲料蛋白水平对方格星虫稚虫体组成中脂肪和蛋白质的含量影响显著(P<0.05),稚星虫体蛋白含量随着饲料蛋白水平的上升有显著升高的趋势,64.85%组稚星虫体蛋白含量最高,显著高于其他各蛋白水平组(P<0.05)。体脂肪含量随着饲料蛋白水平的上升有降低的趋势,64.85%组稚星虫的体脂肪含量最低,显著低于25.21%、29.87%、35.03%和40.67%组(P<0.05)。饲料蛋白水平对稚星虫体灰分含量没有显著影响(P>0.05)。以增重率为评价指标时,方格星虫稚虫对饲料中蛋白的适宜需求量为46.79%。     

摄食促进物质对许氏平鲉摄食和生长的影响 不同地理群体星突江鲽养殖子一代群体遗传多样性分析

以初始体重为49．83±．17g的许氏平鲇Sebastesschlegeli为实验对象,进行为期30d的摄食生长实验。共配制6种等氮等能饲料,其中5种饲料以鱼粉和豆粕为蛋白源（豆粕蛋白替代30％的鱼粉蛋白）配制出基础饲料,分别在基础饲料中不添加摄食促进物质和添加0．3％的商品诱食剂、二甲基-β-丙酸噻亭（DMPT）、甜菜碱和柠檬酸配制成5种豆粕替代饲料,其余1种为鱼粉不替代饲料作为对照组,研究摄食促进物质对许氏平鲇摄食和生长的影响。结果表明,当豆粕取代饲料中添加甜菜碱和柠檬酸时,许氏平铀的特定生长率、饲料效率、蛋白质效率以及肝脏、肠胰蛋白酶活力都显著高于不添加摄食促进物质组（P〈0．05）;当不添加摄食促进物质时,摄食率显著低于鱼粉不替代组（P〈O．05）;而添加DMPT、甜菜碱和柠檬酸时,摄食率与鱼粉不替代组比较没有显著差异（P〉0．05）;肝体指数都明显地低于鱼粉不替代组。结果表明,豆粕替代30％的鱼粉蛋白时,添加0．3％的甜菜碱或柠檬酸能够显著提高许氏平鲇幼鱼对饲料的摄食率和生长率。     

Fat distribution in Atlantic salmon Salmo salar L. in relation to body size and feeding regime

Tissue (fillet, viscera and carcass) distributions of fat were examined in Atlantic salmon, Salmo salar L. (≈740→≈1400→≈2000 g) to test the hypothesis that the fillet becomes increasingly important as a fat depot when fish increase in size. The salmon were fed for 11 weeks on either a high‐fat feed (H: ≈39% fat) or a low‐fat feed (L: ≈28% fat), and half of the fish were then subjected to a dietary switch to create four feed treatments (HH, HL, LL and LH). Fillet fat concentration increased with the passage of time, and the fillet also represented an increasing percentage of the body mass (≈48.5→≈55→≈58.5%) as the fish increased in size. As a consequence, the fillet became increasingly important as a fat depot, containing ≈30% of the body fat in the small fish at the start of the experiment, and ≈50% in the fish sampled at the end of the trial. The proportion of fat localised in the viscera was little influenced by either fish size or feeding treatment, and was within the range of 19–25%, whereas the carcass held a decreasing percentage of the body fat stores as fish size increased. There was a highly significant negative correlation between the percentages of body fat found within the carcass (C) and fillet (F): F = 73.589–0.9285C (R² = 0.973; n = 13). Although the fillet became more important as a fat depot as fish increased in size, the percentage of the body fat reserves found within this tissue appeared to plateau at 50–55%.     

Effects of dietary moisture content of extruded diets on physical feed quality and nutritional response in Atlantic salmon (Salmo salar)

Feeds in modern aquaculture must be of high physical quality to withstand mechanical impacts. However, physical quality of feed has been shown to affect nutritional responses. In this study, different physical feed qualities were created using different drying time. Effects on physical feed quality, feed intake, nutrient digestibility and growth in Atlantic salmon were investigated. Different drying times of four commercial‐like diets resulted in dry matter contents of 95.9%, 94.1%, 92.3% and 90.8%. A fifth diet containing 70% dry matter was made by soaking pellets. Triplicate groups of Atlantic salmon (961 ± 12.1 g ind^?1, mean ± SEM) were first fed ad libitum and thereafter a restricted ration. The diets differed significantly in bulk density, DORIS durability, hardness, water stability after 60 and 120 min, but not in water stability after 240 min. Feed intake and growth were not affected by physical quality of the dry diets, whereas soaking had a positive effect on feed intake. Restricted compared with ad libitum feeding increased nutrient digestibility, but digestibility was not affected by varying drying time among diets. However, soaking resulted in slightly reduced digestibility of starch. Most methods used to analyse physical quality did not correlate with feed intake and growth; only long‐term water stability affected feed intake.     

Cholesterol and short-chain fatty acids in diets for Atlantic salmon Salmo salar (L.): effects on growth, organ indices, macronutrient digestibility, and fatty acid composition

The influence of dietary cholesterol (CHOL) and short-chain fatty acids (SCFA; sodium salts of acetic, propionic and butyric acid, 5:5:2 w/w/w) on growth, organ indices, macronutrient digestibility, and fatty acid composition of Atlantic salmon Salmo salar was investigated. Salmon (initial average weight 0.7 kg) held in seawater (7°C) for 175 days were fed one of six diets: 1, without CHOL/SCFA supplement; 2, with 0.5% SCFA; 3, with 2.0% SCFA; 4, with 1.0% CHOL; 5, with 1.0% CHOL and 0.5% SCFA; 6, with 1.0% CHOL and 2.0% SCFA.
Neither SCFA nor CHOL supplements had any significant effects on specific growth rate (SGR), mortality, apparent digestibility coefficients (ADC) of macronutrients, total lipid content. Hepatosomatic index (HSI) was slightly increased in salmon fed the CHOL supplement ( P  < 0.05). Hepatic CHOL concentration, but not the hepatic CHOL pool, was significantly increased ( P  < 0.001) by dietary CHOL supplementation.
The fatty acid compositions of fillet and gut tissues were not influenced by dietary treatment, while significant effects of CHOL supplements were observed in faeces and liver. Less saturated fatty acids and more mono- and poly-unsaturated fatty acids were excreted with faeces in salmon fed CHOL supplements. Salmon fed CHOL supplements significantly reduced the relative concentration of hepatic palmitic acid (C16 : 0), arachidonic acid (C20 : 4 n-6) and docosahexaenoic acid (C22 : 6 n-3), while the contents of oleic acid (C18 : 1 n-9) and eicosenoic acid (C20 : 1 n-9) were significantly increased. SCFA did not influence the observed effects of dietary CHOL.
The present study shows that dietary CHOL supplements profoundly altered excretion and liver metabolism of individual fatty acids in salmon. The impact of this alteration on physiological performance has not been elucidated.     

The onset of downstream movement of juvenile Atlantic salmon, Salmo salar L., in a chalk stream

The downstream movements of wild Atlantic salmon, Salmo salar L., from their established feeding territories in the River Itchen, Hampshire, UK, were logged continuously over an 11-month period using a passive integrated transponder (PIT) antenna system. The time of these movements was then related to a number of monitored and calculated environmental parameters. Initial downstream movement of smolts in April was correlated with the onset of darkness, at which time salmon moved from their established feeding territories alone. No relationship was found between the number of smolts migrating and river flow or maximum daily water temperature. The timing of downstream movement of parr between October and March was random with regard to sunset and time of maximum daily water temperature, suggesting the environmental cue that initiates movement may be different outside the spring smolt period.     

Influence of a dietary shift on temporal changes in fat deposition and fatty acid composition of Atlantic salmon post-smolt during the early phase of seawater rearing

The proportion of body fat in farmed fish correlates with the concentration of fat in the feed, and the fatty acid composition of the storage fat usually reflects that of the lipids in the feed. We examined the time course of changes in fatty acid compositions of fillet, viscera and carcass of Atlantic salmon post‐smolt over 14 weeks after transfer from fresh water to seawater. The fish had been fed either high‐(34%) or low‐ (22%) fat feeds based upon either fish or vegetable oils during freshwater rearing. Changes in tissue fat concentrations and fatty acid compositions were studied to assess the extent to which lipid turnover and fatty acid metabolism might contribute to temporal changes in fatty acid profiles. When given a 41% protein, 31% fat, fish oil‐based feed, the tissue fatty acid profiles of salmon fed vegetable oil‐based feeds in fresh water gradually came to resemble those of fish fed the fish oil‐based feed throughout freshwater and seawater rearing. The changes in tissue fatty acid compositions were greatest during the second half of the study, corresponding to the time at which growth rates of the fish were highest (SGRs weeks 0–6, 0.3–0.6% day^?1; weeks 0–14 SGRs > 1% day^?1). As the fish increased in size and body fat increased, their tissue fatty acid compositions seemed to be influenced more by deposition of fatty acids obtained from the feed than by lipid turnover and fatty acid metabolism.