MICROALGAE AND ORGANIC MINERALS AFFECT LIPID RETENTION EFFICIENCY AND FILLET QUALITY IN ATLANTIC SALMON (Salmo salar L.)  

Introduction

Microalgae are recognized as prominent future sustainable sources of long chain n-3 polyunsaturated fatty acids (LC n-3 PUFA) rich oils as food grade fisheries providing fish oil and fish meal may have already reached their limit of sustainability (Heal G & Schlenker 2009). The salmon feed industry alone requires more than 50% of the global fish oil production (ca. one million tons annually) (Naylor et al., 2009). In the present study spray dried microalgae biomass (Schizochytrium sp.) was used to replace fish oil as source of LC n-3 PUFA in medium and low fish meal diets for Atlantic salmon supplemented with either inorganic or organic minerals.

Materials and methods

Atlantic salmon (mean weight 400 g) were fed 5 extruded diets containing 48.5 g kg^-1 EPA+DHA (in dietary oil) and balanced crude protein, crude lipid, digestible energy, total saturated fatty acids, and n-3/n-6 fatty acid ratio using different oil blends and plant protein mixes including pure spray dried Schizochytrium sp. biomass (Alltech Inc, USA). The diets were supplemented with either organic minerals (Zn, Cu, Se, Mn and Fe; Alltech Inc, USA) or standard inorganic minerals. Results on pellet technical quality characteristics and salmon smolt on-growing performance, fatty acid retention efficiency, fillet technical quality and whole body mineral content will be presented.

Results

No differences on growth, feed efficiency and whole body mineral content were observed. The saturated fatty acids and DHA+EPA levels in salmon fillets were similar in all treatments. Retention efficiency of monounsaturated fatty acids and EPA+DHA in salmon body significantly improved in the fish fed diets containing increasing levels microalgae (Fig.1).

The extrusion conditions were not optimised for the different raw material mixes; still, pellet technical quality was high in all feeds.

Improved fillet quality in terms of less gaping was observed with increasing dietary inclusion level of Schizochytrium sp. and even more pronouncedly the use of dietary organic minerals. The dietary inclusion of organic minerals altered salmon's lipid transport pattern by improving the digestibility and the levels in the fillet of several dietary fatty acids, such as DHA and monounsaturated fatty acids, whereas no such differences were detected in the whole body. Improved dress out percentage, reduced liver fat levels and gaping occurrence were observed with the experimental diets compared to the fish oil control.

No effects on blood chemistry or whole body mineralization were seen by dietary inclusion of either microalgae or different mineral types.

Discussion and conclusions

The microalgae biomass included up to 5% in extruded feeds for salmon could successfully replace fish oil as source of LC n-3 PUFA, as previously seen in Kousoulaki et al. (accepted). Increasing levels of microalgae in the diet lead to improved EPA+DHA retention efficiency, increased fish dress-out percentage and reduced liver lipid levels. These effects promote salmon farming sustainability as well as improved fish welfare. The observed positive effects of dietary microalgae and organic minerals on salmon fillet technical quality are interesting to the processing industry as fillet gaping is the reason for the downgrading in the secondary processing of salmon (Mitchi 2001). Possible explanations for the observed biological effects may involve improved skeletal muscle aerobic metabolism (Larsson et al., 2012) due to differential function of organic minerals and microalgae components. Se, Zn and Co are co-enzymes in important functions, among other, for muscle and connective tissue maintenance. Moreover, some minerals exert antioxidant effect protecting essential compounds that are prone to oxidative stress, such as HUFAs. Previously we found increased f-actin production in the intestinal tissue of fish fed increased levels of microalgae (Kousoulaki et al., accepted). F-actin is present in the intestinal cell tight junctions, and should it also be produced at higher levels in the muscle tissues it may be another complementary explanation to the currently observed improved fillet texture in terms of gaping in the fish fed the Schizochytrium sp. supplemented diets.

References

Heal, G., Schlenker, W., 2009. Economics: Sustainable fisheries. Nature 455, 1044-1045.

Kousoulaki, K., Østbye, T-K.K., Krasnov, A., Torgersen, J.S., Mørkøre, T., and Sweetman, J. Microalgae feed for future omega-3 rich farmed fish: Fish Metabolism, Health and Fillet nutritional quality. Journal of Nutritional Science (accepted).

Larsson T, Krasnov A, Lerfall J, Taksdal T, Pedersen M, Mørkøre, T., 2012. Fillet quality and gene transcriptome profiling of heart tissue of Atlantic salmon with pancreas disease (PD). Aquaculture 330-333, 82-91.

Michie, I., 2001. Causes of downgrading in the salmon industry. S.C. Kestin, P. Warris (Eds.), Farmed Fish Quality, Blackwell Science, Oxford. Pp. 129-136.

Naylor, R.L., Hardy, R.W., Bureau, D.P., Chiu, A ., Elliott, M., Farrell, A.P., Forster, I., Gatlin, D.M., Goldburg, R.J., Hua, K., Nichols, P.D., 2009. P. Nat. Acad. Sci. USA 106, 15103-15110.