Significance of docosahexaenoic acid for rainbow trout (Oncorhynchus mykiss) larvae

N‐3 fatty acids are essential for rainbow trout (Oncorhynchus mykiss). Our investigations specify the fatty acid requirements of rainbow trout larvae. One group of larvae was fed with Artemia ssp. rich in linolenic acid (18:3 n‐3) and octadecatetraenoic acid (18:4 n‐3), while the other group received a commercial fish feed containing high levels of long‐chain polyunsaturated fatty acids (PUFA), mainly docosahexaenoic acid (22:6 n‐3). After two and four weeks of feeding, the growth and fatty acid pattern of the larvae were determined. The fatty acid composition of the diet is reflected in the triglyceride composition of the fish, but there is no sign of conversion of 18:3 n‐3 and 18:4 n‐3 into long‐chain polyunsaturated fatty acids. This is caused by limited chain elongation rather than by low enzyme activity of desaturase. In the triglycerides of the larvae, high levels of 18:4 n‐3 were determined. This fatty acid was not transformed into the corresponding long‐chain polyunsaturated fatty acids. Deficiency of 22:6 n‐3 resulted in reduced larval growth. Therefore, it can be stated that docosahexaenoic acid is essential for rainbow trout larvae.