Résumé
Improving fish feed efficiency is of major interest to reduce feed use in aquaculture and improve its sustainability through reduced costs and environmental impacts. Fish efficiency is defined as the ability of fish to convert feed intake into body weight gain over a given time period. The main challenge to improve feed efficiency in breeding programs is our capacity to accurately measure feed intake at the individual level on a large number of fish. In previous studies, individual feed efficiency has been evaluated on fish reared in isolation. Each fish was given a known amount of feed, and two hours after feeding, uneaten pellets were manually collected and counted to determine the exact feed intake. However, this method is really tedious and difficult to use in aquaculture production outside laboratory conditions. Another drawback of this method is that fish are isolated in individual aquaria with no interactions with congeners. The measurement of natural abundance of carbon and nitrogen stable isotopes (δ13C and δ15N) of organisms could reflect feed efficiency as the difference between isotopic values of an organism and its feed is related to protein retention. This method would have the advantages of being rapid, low cost and a non-destructive for fish. To test the correlation between individual fish efficiency and stable isotope values, 178 individuals of seabass were raised in individual aquariums during six weeks. The quantity of food ingested by each fish was recorded every day. Every two weeks, seabass were individually weighed. Feed conversion ratio (FCR) of each individual was then calculated as the ratio between the quantity of food ingested and body weight gain. In parallel to the weight measurements every two weeks, few scales and part of caudal fin were sampled from each fish. Following the six initial weeks in individual aquaria, fish were reared for an additional 9 weeks in a collective tank. At the end of this growing period, scales, fin, muscle and blood were sampled on each fish. δ13C and δ15N values of scales, fin, muscle and blood at each sampling periods were measured by EA-IRMS.Our results show a very strong negative correlation between individual FCR and body weight gain (R = -0.95, p < 0.001). δ15N values of muscle sampled after 15 weeks of experimentation showed a negative correlation with individual FCR (R = -0.2, p < 0.05). At the all times of sampling, δ13C and δ15N values of scales, fin and blood were not correlated to individual feed efficiency.In conclusion, carbon or nitrogen stable isotopes cannot be used as a reliable proxy to evaluate feed efficiency of the seabass D. labrax. This study does not corroborate previous studies on rainbow trout (Oncorhynchus mykiss) showing that δ13C of muscle and δ15N of liver were potential good biomarkers to evaluate individual feed efficiency in fish (Dvergedal et al., 2023). Further studies are necessary to better understand the metabolic processes driving isotopic incorporation and such differences between fish species.