Individual feed efficiency in fishes: direct measurement methods and indirect predictors to develop selective breeding programs in two major aquaculture species: European sea bass Dicentrarchus labrax and Nile tilapia Oreochromis niloticus

n finfish aquaculture, feed is expensive and has environmental impacts. Improving feed efficiency (FE) to produce the same amount of fish with less feed is a major objective for sustainable aquaculture. This can be achieved by selective breeding but this requires methods for individual phenotyping, and measuring individual feed intake (FI) is technically challenging for fishes. The two best methods, to date, are either to rear fish individually or to tag them externally so that they can be visually identified while reared in small groups.I investigated some important issues related to estimation of individual FE, on two major aquaculture species, European sea bass Dicentrarchus labrax and Nile tilapia Oreochromis niloticus. I demonstrated that estimating individual FE at juvenile stage in Nile tilapia was predictive of FE over the whole production cycle. Then, I compared the two phenotyping methods in Nile tilapia, to discover that they did not provide equivalent estimations. Finally, I also found that the most efficient fish at restricted feeding were not the most efficient at satiety, in both species.Both phenotyping methods remain, however, tedious. I therefore investigated potential indirect selection criteria that are easier to measure. In European sea bass, there was no correlation of individual FE with individual metabolic rate (as oxygen consumption). Furthermore, I addressed methodological issues for implementation of stable isotope analyses with non-lethal sampling of fish scales as an indirect selection criterion.To conclude, selecting fish at juvenile stage seems reliable and will cut selection costs. Phenotyping method for individual FE towards selective breeding depends, however, upon the species. In particular, species collective behaviour must be considered when choosing the most suitable method. Furthermore, fish should be phenotyped at holding conditions and feeding levels that are as close as possible to commercial practices. Metabolic rate might be useful as an indirect criterion if coupled with measures of ammonia excretion or body composition. Scale stable isotope analyses are technically feasible and require further investigation