Reevaluating polyunsaturated fatty acid essentiality in Rainbow Trout

The National Research Council (NRC) reports that the essential fatty acid requirements of Rainbow TroutOncorhynchus mykisscan be satisfied by 0.7-1.0% 18:3(n-3) or 0.4-0.5% n-3 long-chain (LC) polyunsaturated fatty acids (PUFAs; defined byNRCas 20:5[n-3] + 22:6[n-3]) in the diet. These requirements were defined roughly 50 years ago and do not consider the importance of n-6PUFAs in the diet. Therefore, we assessed survival, growth performance, and tissue fatty acid composition of juvenile Rainbow Trout (mean initial weight +/- SE = 24.6 +/- 0.1 g; 10 fish/tank, 4 tanks/diet) that were fed experimental diets (53% protein; 13% lipid) containing fish oil, fully hydrogenated soybean oil, or fully hydrogenated soybean oil with five different combinations of fatty acid ethyl esters (18:2[n-6], 18:3[n-3], 20:4[n-6], 20:5[n-3], and 22:6[n-3]) to illuminate the relative essentiality of n-3 and n-6PUFAs. Final individual weight (78.2-132 g), weight gain (216-433%), feed conversion ratio (0.93-1.42), specific growth rate (2.05-2.98% body weight/d), and hepatosomatic index (1.4-2.1) were significantly affected by dietary treatment whereby Rainbow Trout that received the fish oil-based diet outperformed all other treatments. Growth of fish that were given different combinations of fatty acid ethyl esters was not statistically different. Fatty acid tissue composition generally reflected dietary treatment, with the greatest profile changes observed in intraperitoneal fat, fillet, and liver and the smallest changes occurring in brain and eye tissues. Results largely validate previous reports indicating that Rainbow Trout are physiologically able to synthesizeLC-PUFAs from C18PUFAs and therefore do not necessarily requireLC-PUFA-rich feeds. However, numeric increases in the growth of Rainbow Trout suggest that this species achieves an energetic advantage if offered n-3 and n-6LC-PUFAs in the diet.