The study by scientists at the French National Institute for Agricultural Research (INRA) was carried out in the knowledge that the effect of replacing traditional aqua-feed ingredients (fishmeal and fish oil) by a 100% plant-based diet drastically decreases fish performance (survival and growth).
The researchers were working on the hypothesis that feed with low or zero levels of the omega-3 fatty acids EPA and DHA was less attractive for fish. The fish therefore ate less of it and consequently fared less well.
The study, published in the journal Physiology & Behavior, examined the feed preference of rainbow trout (Oncorhynchus mykiss) for three diets containing different levels of the omega-3 long chain polyunsaturated fatty acids (ω-3 LCPUFA) EPA and DHA - 0% for low, 5% for medium and 20% for high, total fatty acid content.
Feed preference values for each group (low versus medium ω-3 diets, medium versus high ω-3 diets and low versus high ω-3 diets) were observed using two self-feeders positioned at opposite sides of the tank.
The results showed that rainbow trout could discriminate between the diets containing different levels of ω-3 LCPUFA even if unable to differentiate between levels of 5% (no preference was observed in low v. medium ω-3 diets).
Overall, they had a preference for feed high in ω-3 LCPUFA: a 59.5% preference for high ω-3 diet in high v. low ω-3 diets, and 75.6% preference for high ω-3 diet in medium v. high ω-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: a 63.3% preference for high ω-3 diet in high v. low ω-3 diets, and 69,5% preference for high ω-3 diet in medium v. high ω-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 ω-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 ω-3 diet in low v. medium ω-3 diets, medium in medium v. high ω-3 diets and low in low v. high ω-3).
In conclusion, this study highlighted the influence of ω-3 LCPUFA in the feeding behaviour of juvenile rainbow trout, levels of ω-3 LCPUFA being shown to drive dietary choices in the fish.
Read the full study, Rainbow trout prefer diets rich in omega-3 long chain polyunsaturated fatty acids DHA and EPA, here.