Realised genetic gains on growth, survival, feed conversion ratio and quality traits after ten generations of multi-trait selection in rainbow trout Oncorhynchus mykiss, fed a standard diet or a "future" fish-free and soy-free diet

There is limited scientific evidence on the real impact of selective breeding in aquaculture on the medium term, while the composition of aquafeeds is rapidly evolving towards plant-based raw materials. We compared a rainbow trout line selected in freshwater for fillet production (improved growth, carcass yield and fillet fat) for ten generations (G10) with an unselected control line from the same base population (G0). We crossed G10 and G0 neomales to the same G10 females, thus creating a Selected and a Control group expected to diverge by half the true difference between G0 and G10. Those were grown to 1.6 kg, and two feeds were compared across the two lines from 264 to 374 days post-hatching. One was a commercial standard, the second was a "future" feed devoid of fishmeal, fish oil and soy-based products, with microalgae as a source of docosahexaenoic acid (DHA). After doubling the difference between the Selected and the Control to estimate the true performance of G0, we saw that G10 was improved relative to G0 for body weight (+61%), feed conversion ratio (-17 to-20%), fillet fat (+28-53%) and carcass yield (+4.2%), but not for fillet yield. Survival was not affected by selection. Both feeds had a similar performance in terms of growth, but the future feed showed a higher FCR, probably due to a feed intake measurement issue. Fish had a good EPA+DHA content (>1.2 g/100 g wet weight) with both feeds, partly linked to endogenous synthesis of these fatty acids. There was little if any genotype by feed interaction. This study shows that selective breeding can produce fast growing, feed efficient and thus provide opportunity for more sustainable fish culture. We showed that highly nutritious fish can be produced with good growth performance without using any fish meal, fish oil or soy-based product.