Apparent digestibility of proximate nutrients, energy and fatty acids in nutritionally-balanced diets with partial or complete replacement of dietary fish oil with microbial oil from a novel Schizochytrium sp. (T18) by juvenile Atlantic salmon (Salmo salar L.)

Oils derived from thraustochytrid microorganisms, particularly Schizochytrium sp., are rapidly gaining attention as sustainable feedstocks to replace conventional edible oils for many industrial applications. A novel strain of Schizochytrium sp. (T18) has shown rapid growth under optimized heterotrophic cultivation, efficiently using inexpensive carbon and nitrogen feedstocks and capable of producing a high docosahexaenoic acid (DHA)-rich oil. This is the first study directed towards its nutritional evaluation as a potential feed resource for salmonid aquaculture. The study examined the effects of partial or complete replacement of dietary marine fish oil with Schizochytrium sp. (T18) microbial oil on apparent digestibility (AD) of dietary proximate nutrients, energy and fatty acids (FAs) when fed to juvenile (32 g) Atlantic salmon (Salmo salar L.). Four isonitrogenous (50% crude protein), isolipidic (20% lipid) and isocaloric (19 MJ/kg digestible energy) practical-ingredient experimental diets were formulated to replace 0, 33, 66 and 100% of dietary fish oil with extracted Schizochytrium sp. (T18) oil. The oil used contained 50% polyunsaturated FAs (PUFA); of which 82% (e.g., 42% of total FAs) was comprised of the dietary n-3 long-chain PUFA (n-3 LC-PUFA) DHA and contained extremely low levels of harmful contaminants (e.g., heavy metals, lipid oxidation products, pathogens). Partial or complete replacement of dietary fish oil with Schizochytrium sp. (T18) oil had no significant (P >= .064) effects on AD of dietary dry matter (76-77%), protein (94%), lipid (89-90%) or energy (83-84%). Similarly, AD was unaffected (P >= .647) for dietary PUFA (97%) and n-3 PUFA (98%) with only minor differences (P <= .046) detected for monounsaturated FAs (MUFA; 91-92%) and n-6 PUFA (95-96%). Alternatively, AD of dietary saturated FAs (SFA) and DHA were significantly (P < .001) improved (70-76% and 95-98%, respectively) with increasing replacement of fish oil with Schizochytrium sp. (T18) oil. Results suggest that conventional fish oil can be completely replaced in juvenile farmed salmon feeds by Schizochytrium sp. (T18) oil with no negative effects on digestibility of dietary proximate nutrients, energy or fatty acids and, in fact, promoting a significant linear dose-response increase in AD of dietary SFA and a statistically significant increase in dietary DHA digestibility.