Introduction
The red flesh color is an important quality trait for farmed Atlantic salmon. T he color is due to the salmon’s ability to deposit the carotenoid astaxanthin in the muscle. The retention of astaxanthin in the muscle is not very efficient, normally less than 10% of the ingested astaxanthin is retained in the muscle . There are indications that the fillet color has decreas ed in recent years . Reduced inclusion of marine ingredients in the modern salmon diet (Ytrestøyl et al., 2015, Aas et al., 2019) has led to reduced levels of several nutrients (phospholipids, vitamin A and cholesterol) that may affect flesh pigmentation in Atlantic salmon, both during absorption in the gut and through interference with metabolic conversion of astaxanthin to vitamin A . In addition to its provitamin A function (Schiedt et al., 1985), astaxanthin is a powerful antioxidant that reacts with free radicals and singlet oxygen (Naguib et al., 2000, Dose et al., 2016) . Increased oxidative stress has been suggested as a possible cause of decreased flesh pigmentation in salmon, but experimental evidence supporting this hypothesis is limited. In this project, the effect of dietary retinol and astaxanthin concentrations in combination with stress on astaxanthin deposition and metabolism were tested in Atlantic salmon.
Materials and methods
Diets with two concentrations of astaxanthin (30 and 60 mg/kg) combined with three levels of vitamin A (6500, 35 000, 100 000 IU/kg), six diets in total, were fed to A. salmon for 16 weeks (start weight 190g: final weight:1000g). The trial was done in tanks with flow through seawater and feed intake were measured for calculation of astaxanthin retention in the muscle. After the 16- week feeding trial, four of the diets (high/low astaxanthin and retinol) were fed for a period of 5 weeks and split in a group that were stressed and a control group without stress. The stress was induced by lowering the water level and oxygen concentration in the tanks three times/week. Samples of muscle, liver and intestine were sampled for analysis of astaxanthin and breakdown products by HPLC and NMR. Visual color was assessed by a Minolta Chroma Meter and SalmoFanTM. Hepatocytes and enterocytes were isolated and incubated with 14C-labelled astaxanthin to study the metabolism of astaxanthin to retinol in vitro. Samples of liver, muscle and intestine were taken for analysis of gene expression by microarray.
Results
Growth rate was not significantly affected by dietary astaxanthin or retinol concentration. The flesh color measured by Minolta redness (a*-value) and Salmofan scores was lower at the highest dietary vitamin A concentration. This was confirmed by analysis of astaxanthin concentration in NQC by NIR and HPLC. A decreasing astaxanthin concentration with increasing vitamin A in the diet was also found in liver and plasma, but not in intestine. The retention of astaxanthin in the muscle was highest in the salmon fed the diet with 30 ppm astaxanthin and medium vitamin A concentration. Analysis of samples from the stress trial and in vitro trial as well as gene expression are currently on-going and will be presented.
References
Aas, T.S., Ytrestøyl, T. & Åsgård, T. (2019) Utilization of feed resources in the production of Atlantic salmon (Salmo salar) in Norway: An update for 2016. Aquaculture Reports, 15, 100216.
Dose, J., Matsugo, S., Yokokawa, H., Koshida, Y., Okazaki, S., Seidel, U., Eggersdorfer, M., Rimbach, G., Esatbeyoglu, T. Free Radical Scavenging and Cellular Antioxidant Properties of Astaxanthin. Int. J. Mol. Sci. 2016, 17, 103.
Naguib, Y.M., 2000. Antioxidant activities of astaxanthin and related carotenoids. J. Agric. Food Chem. 48, 1150-1154
Schiedt , K., Leuenberger, F.J., Vecchi , M., Glinz , E. 1985. Absorption, retention and metabolic transformation of carotenoids in rainbow trout, salmon and chicken, Pure and Applied Chemistry 57, 685-692.
Ytrestøyl, T., Aas, T.S. & Åsgård, T. (2015) Utilisation of feed resources in production of Atlantic salmon (Salmo salar) in Norway. Aquaculture, 448, 365-374.