Gerd Marit Berge

Effect of dietary fat level on weight gain, digestibility, and fillet composition of Atlantic halibut

Abstract Two experiments were carried out to obtain information on the effect of dietary fat levels on juvenile (6–12 g) and larger (0.6–1.5 kg) Atlantic halibut ( Hippoglossus hippoglossus ). The juvenile halibut were fed dry diets containing 12% fat and 54% protein and 21% fat and 49% protein, respectively. The larger fish were fed moist diets formulated to be isonitrogenous (19–21% protein on a wet-weight basis), to contain equal amounts of carbohydrate (9–10%) and increasing amounts of fat (8, 12, 16 or 20%, respectively). Dietary fat level did not result in significant differences in weight gain for any of the two size classes of halibut. No significant effects of the various diets were seen for feed efficiency, chemical composition of fillets and livers or digestibility of dietary dry matter, fat and protein for the larger fish. Large individual variation was observed in all recorded data.

Fish protein hydrolyzate in starter diets for Atlantic salmon (Salmo salar) fry

Triplicate groups of Atlantic salmon fry with an initial weight of 0.15 g were fed four experimental diets for 20 weeks. The diets were: a commercial steam pelleted salmon starter (Diet 1); a Norse-LT 94® fish-meal based control diet (Diet 2); 5% of amino acid nitrogen in Diet 2 replaced by amino acid nitrogen from fish protein hydrolyzate (Diet 3); 8% of amino acid nitrogen in Diet 2 replaced by amino acid nitrogen from fish protein hydrolyzate (Diet 4). The growth rate and the final weight of fish fed Diets 3 and 4 were higher (P < 0.05) than that of fish fed Diet 2 at 20 weeks. Diet 1 was at the same level as Diets 3 and 4. The results indicated a positive effect on growth of juvenile salmon by replacing 5 or 8% of the amino acid nitrogen in a LT fish-meal based diet with amino acid nitrogen from fish protein hydrolyzate. The were no differences in mortality, condition factor, liver to body weight or gut to body weight ratios which could be related to the diets.

Determination of digestibility of commercial salmon feeds

The aim of the study was to develop an applicable method for determination of the digestibility of nutrients in commercial feeds. Leaching of markers and dry matter (DM) were compared between a commercial extruded feed containing La 2 O 3 , the same commercial feed coated with oil and other additional markers, and the commercial feed that had been ground and re-pelleted with water, marker and a crude alginate binder. Two markers, Y 2 O 3 and Cr 2 O 3 , were added in both the coated and the moist feeds. The optimal inclusion level of binder and water to minimize the leaching of DM from the moist pellet was determined in a preliminary study. The lowest leaching of DM was achieved with the inclusion of 2% alginate and 35% water. Added markers tended to be less homogeneously included in the coated feed than in the moist feed, and the markers leached relatively more than the DM from the coated feed. Apparent digestibility coefficients (ADCs) of DM, fat, nitrogen (N), energy, organic matter (OM), and phosphorus (P) in the commercial dry feed and the moist feed were compared in two experiments with Atlantic salmon. Differences between the ADCs in the two experiments were observed for N and P. The ADC estimates of the moist feed were slightly lower than those of the dry feed for all nutrients. Similar ADC estimates for each nutrient were obtained for all markers, except for Y 2 O 3 which produced a slightly higher ADC for N than did La 2 O 3 . By using the moist feed method recommended from this study, a difference of about 1.4% units in ADC of energy and of 0.8% units for N between the original and the modified test feed was demonstrated. This means the error of ADC estimates is low compared to using fixed digestibility coefficients for each of protein, fat and carbohydrate, a method that has been employed for declaration of commercial fish feed. Further development of the method is discussed.

Soya Saponins Induce Enteritis in Atlantic Salmon (Salmo salar L.).

Soybean meal-induced enteritis (SBMIE) is a well-described condition in the distal intestine of salmonids, and saponins have been implicated as the causal agent. However, the question remains whether saponins alone cause SBMIE. Moreover, the dose-response relationship has not been described. In a 10 week feeding trial with Atlantic salmon, a highly purified (95%) soya saponin preparation was supplemented (0, 2, 4, 6, or 10 g/kg) to two basal diets, one containing fishmeal as the major protein source (FM) and the other 25% lupin meal (LP). Saponins caused dose-dependent increases in the severity of inflammation independent of the basal diet, with concomitant alterations in digestive functions and immunological marker expression. Thus, saponins induced inflammation whether the diet contained other legume components or not. However, responses were often the same or stronger in fish fed the corresponding saponin-supplemented LP diets despite lower saponin exposure, suggesting potentiation by other legume component(s).

Atlantic salmon (Salmo salar) muscle precursor cells differentiate into osteoblasts in vitro: Polyunsaturated fatty acids and hyperthermia influence gene expression and differentiation

The formation and mineralisation of bone are two critical processes in fast-growing Atlantic salmon (Salmo salar). The mechanisms of these processes, however, have not been described in detail. Thus, in vitro systems that allow the study of factors that influence bone formation in farmed Atlantic salmon are highly warranted. We describe here a method by which unspecialized primary cells from salmon white muscle can differentiate to osteoblasts in vitro. We have subsequently used the differentiated cells as a model system to study the effects of two factors that influence bone formation in Atlantic salmon under commercial farming conditions, namely polyunsaturated fatty acids, PUFAs, and temperature. Muscle precursor cells changed their morphology from triangular or spindle-shaped cells to polygonal or cubical cells after 3 weeks in osteogenic medium. In addition, gene expression studies showed that marker genes for osteoblastogenesis; alp, col1a1, osteocalcin, bmp2 and bmp4 increased after 3 weeks of incubation in osteogenic media showing that these cells have differentiated to osteoblasts at this stage. Adding CLA or DHA to the osteoblast media resulted in a reduced PGE(2) production and increased expression of osteocalcin. Further, temperature studies showed that differentiating osteoblasts are highly sensitive to increased incubation temperature at early stages of differentiation. Our studies show that unspecialized precursor cells isolated from salmon muscle tissue can be caused to differentiate to osteoblasts in vitro. Furthermore, this model system appears to be suitable for the study of osteoblast biology in vitro.

Requirements of n-3 very long-chain PUFA in Atlantic salmon (Salmo salar L): effects of different dietary levels of EPA and DHA on fish performance and tissue composition and integrity.

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.

Medium-Chain and Long-Chain Fatty Acids Have Different Postabsorptive Fates in Atlantic Salmon

An increasingly larger proportion of the oils used in diets for farmed fish are plant derived and rapeseed oil is most commonly used. Despite high dietary lipid levels and a marked change in lipid composition, the transport and metabolic fate of absorbed fatty acids is not fully understood in teleost fish. The main purpose of this study was to trace the postabsorptive metabolic fate of 2 fatty acids of different chain length: oleic acid [(3)H-18:1(n-9)], constituting 70% of fatty acids in rapeseed oil, and the medium-chain decanoic acid [(14)C-10:0], which does not require carrier molecules for membrane passage. The fatty acids and their metabolites were traced in portal and peripheral blood, liver, heart, skeletal muscle, and visceral adipose tissue at time intervals from 3 to 48 h after feeding. The portal vein was the primary transport route for both 10:0 and 18:1(n-9) from the intestine to the liver the first 6 h after feed intake. From 12 to 48 h, the peripheral route became increasingly more important. The study also indicates a possible direct transport route of fatty acids from the intestine to the surrounding viscera. Our data demonstrate that whereas 18:1(n-9) is primarily deposited as TG in skeletal muscle and visceral adipose tissue, 10:0 is used by the heart and skeletal muscle as a source for rapid energy production.

Low levels of very-long-chain n-3 PUFA in Atlantic salmon (Salmo salar) diet reduce fish robustness under challenging conditions in sea cages

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.

Regulation of the Omega-3 Fatty Acid Biosynthetic Pathway in Atlantic Salmon Hepatocytes

Limited availability of the n-3 fatty acids EPA and DHA have led to an interest in better understanding of the n-3 biosynthetic pathway and its regulation. The biosynthesis of alpha-linolenic acid to EPA and DHA involves several complex reaction steps including desaturation-, elongation- and peroxisomal beta-oxidation enzymes. The aims of the present experiments were to gain more knowledge on how this biosynthesis is regulated over time by different doses and fatty acid combinations. Hepatocytes isolated from salmon were incubated with various levels and combinations of oleic acid, EPA and DHA. Oleic acid led to a higher expression of the Δ6 fatty acid desaturase (fad) genes Δ6fad_a, Δ6fad_b, Δ6fad_c and the elongase genes elovl2 compared with cells cultured in medium enriched with DHA. Further, the study showed rhythmic variations in expression over time. Levels were reached where a further increase in specific fatty acids given to the cells not stimulated the conversion further. The gene expression of Δ6fad_a_and Δ6fad_b responded similar to fatty acid treatment, suggesting a co-regulation of these genes, whereas Δ5fad and Δ6fad_c showed a different regulation pattern. EPA and DHA induced different gene expression patterns, especially of Δ6fad_a. Addition of radiolabelled alpha-linolenic acid to the hepatocytes confirmed a higher degree of elongation and desaturation in cells treated with oleic acid compared to cells treated with DHA. This study suggests a complex regulation of the conversion process of n-3 fatty acids. Several factors, such as that the various gene copies are differently regulated, the gene expression show rhythmic variations and gene expression only affected to a certain level, determines when you get the maximum conversion of the beneficial n-3 fatty acids.