FATTY ACID DEPOSITION IN FILLETS OF POST-SMOLT ATLANTIC SALMON Salmo salar FED DIETS SUPPLEMENTED WITH DHA-RICH OIL EXTRACTED FROM Schizochytrium spp.

The increasing demand for natural sources of omega-3 fatty acids combined with the pressure to improve sustainability of aquaculture operations are conducive to developing innovative solutions for the salmon producers.  The objective of this study was to assess the effect of a novel oil (DHA Natur^TM) extracted from the microalgae Schizochytrium spp. on weight gain, feed intake, feed conversion, proximate (dry matter, crude protein, crude lipid, ash) and fatty acid composition, and polyunsaturated fatty acid retention efficiency of Atlantic salmon (Salmo salar).  Salmon (initial body weight: 552.0±13.7 g) were fed one Control diet (Diet A: 10% South American fish oil + 18% canola oil + 0% DHA Natur^TM) and four diets containing graded levels of DHA Natur^TM (Diet B: 0.677%; Diet C: 1.354%; Diet D: 2.031%; Diet E: 2.708%) for 112 days.  The dietary content of DHA+EPA was 2.62 % (as-is) across diets.  Dietary fish oil to vegetable oil ratios varied from 0.36 (Diet A) to 0.20 (Diet E).  The vegetable oils used in the feed formulas were canola oil and flaxseed oil (degummed).  Each diet was randomly allocated to three 800-liter circular tanks at 25 salmon per tank and maintained in saltwater (25 ppt) at 13.9±0.6°C in a recirculating aquaculture system.

Growth, feed intake, feed conversion (FCR; feed:gain) and proximate composition of skinless fillets were not significantly affected by dietary treatments (P>0.05).  Although not significant, fillet DHA (% as-is in fillet) showed upward trends in all experimental diets, with those containing higher levels of DHA Natur^TM inclusion presenting with an overall greater rate of total fillet DHA content after 112 days.  When fillet DHA content (% as-is) was plotted against time (days), the positive regression slopes indicated diets with higher dietary DHA Natur^TM inclusion generally had higher rates of DHA deposition: Diet A (Control): 0.0008 %/day, Diet B: 0.0011%/day, Diet C: 00012%/day, Diet D: 0.0010%/day, Diet E: 0.0016%/day.  Fillet DHA deposition rate (mg/°C*day) did not differ significantly between experimental diets (P=0.9401) and ranged from 1.01 (Diet A) - 1.15 (Diet D).  However, there was a significant negative linear relationship between EPA deposition rate (mg/°C*day) and DHA Natur^TM inclusion (P=0.0046) as a result of the low content of this fatty acid in Schizochytrium spp.  EPA retention efficiency did not differ significantly between treatments, which suggested the potential EPA requirement was met at all inclusion levels of DHA Natur^TM.

In conclusion, the oil extracted from Schizochytrium spp. (DHA Natur^TM) was a nutritious and beneficial source of available DHA for post-smolt Atlantic salmon.  The results in this study indicated DHA from this novel ingredient was as available and efficient as that from high quality South American fish oil.