Rainbow trout prefer diets rich in omega-3 long chain polyunsaturated fatty acids DHA and EPA

The control of feed intake in fish in aquaculture requires the development of new techniques to improve diet composition, feed conversion efficiency and growth. The aim must be sustainability and an effective use of resources. The effect of replacing traditional aqua-feed ingredients (fishmeal and fish oil) by a 100% plant-based diet is known to drastically decrease fish performance (survival and growth). The present study examined the feed preference of rainbow trout Oncorhynchus mykiss for three diets containing distinct levels of omega-3 long chain polyunsaturated fatty acids (omega-3 LCPUFA): eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (0% for low, 5% for medium and 20% for high, total fatty acid content). Feed preference values for each group (low v. medium omega-3 diets, medium v. high omega-3 diets and low v. high omega-3 diets) were observed using two selffeeders positioned at opposite sides of the tank. The hypothesis was that the decrease of fish growth and survival rate of fish fed with 100% plant-based diet could be explained by the absence of omega-3 LCPUFA relating to decrease of food intake. This could explain the tasting role of omega-3 LCPUFA in the feeding behavior of rainbow trout (which reflects the motivation to consume feed). The results showed that rainbow trout could discriminate between the diets containing different level of omega-3 LCPUFA even if unable to differentiate between level of 5% (no preference observed in low v. medium omega-3 diets). Overall they had a preference for diet high in omega-3 LCPUFA: 59.5% preference for high omega-3 diet in high v. low omega-3 diets, and 75.6% preference for high omega-3 diet in medium v. high omega-3 diets respectively. This preference was repeated after 21 days and for a further 21 days when the feeds were exchanged between the two self-feeders in each tank: 63.3% preference for high omega-3 diet in high v. low omega-3 diets, and 69,5% preference for high omega-3 diet in medium v. high omega-3 diets respectively. The tests also indicated a difference in the extent of food waste of each of the three diets revealed by uneaten pellets after feed demands. During two periods of test, high omega-3 diet was the most appreciated, the least wasted and the most eaten (all choice groups) whereas the most uneaten feed remained the least appreciated diet in three choices diets (low omega-3 diet in low v. medium omega-3 diets, medium in medium v. high omega-3 diets and low in low v. high omega-3). In conclusion, this study highlighted the influence of omega-3 LCPUFA in the feeding behavior of juvenile rainbow trout, levels of omega-3 LCPUFA drove dietary choices in the fish.