Abstract
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 17g/kg of EPA+DHA from 400g until slaughter size (about 3·5kg). 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 17g/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, 10g/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.
Highlights
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
Membrane PL are the source of VLC-PUFA substrates required for the synthesis of eicosanoids, arachidonic acid (ARA; 20 : 4n-6)
No differences were found in the final body weight (P = 0·65), total length (P = 0·71), hepatosomatic index (P = 0·19) or cardiosomatic index (P = 0·10) (Table 3) among the three dietary groups at the final sampling in sea cages
Summary
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. Variable results concerning the health impact of increased levels of n-6 fatty acids and reduced n-3 fatty acids in salmonids have been reported, ranging from no apparent adverse effects[4,5] to impaired macrophage function[6], lower leucocyte production[7], or decreased fish resistance to infection[8] This indicates that understanding how low dietary EPA and DHA levels influence the composition, function and health of specific tissues is essential. They found no effect on mortality when the dietary EPA and DHA level was reduced to 1·4 % of fatty acids (4 g/kg feed) This trial was conducted under controlled experimental conditions, which do not reflect the fluctuating environmental conditions that salmon experience in commercial aquaculture and where an accumulated loss through the entire production cycle generally ranges from 15 to 20 %(14). Determining the lower dietary requirement of Atlantic salmon for EPA and DHA, and whether environmental conditions and stress influence this requirement, is necessary
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