The specific growth rate (SGR) of a cohort of 2000 tagged juvenile European sea bass was measured in a common tank, during two sequential cycles comprising three-weeks feed deprivation followed by three-weeks ad libitum re-feeding. After correction for initial size at age as fork length, there was a direct correlation between negative SGR (rate of mass loss) during feed deprivation and positive SGR (rate of compensatory growth) during re-feeding (Spearman rank correlation R=0.388, P=0.000002). Following a period of rearing under standard culture conditions, individuals representing 'high growth' phenotypes (GP) and 'high tolerance of feed deprivation' phenotypes (DP) were selected from either end of the SGR spectrum. Static and swimming respirometry could not demonstrate lower routine or standard metabolic rate in DP to account for greater tolerance of feed deprivation. Increased rates of compensatory growth in GP were not linked to greater maximum metabolic rate, aerobic metabolic scope or maximum cardiac performance than DP. When fed a standard ration, however, GP completed the specific dynamic action (SDA) response significantly faster than DP. Therefore, higher growth rate in GP was linked to greater capacity to process food. There was no difference in SDA coefficient, an indicator of energetic efficiency. The results indicate that individual variation in growth rate in sea bass reflects, in part, a trade-off against tolerance of food deprivation. The two phenotypes represented the opposing ends of a spectrum. The GP aims to exploit available resources and grow as rapidly as possible but at a cost of physiological and/or behavioural attributes, which lead to increased energy dissipation when food is not available. An opposing strategy, exemplified by DP, is less 'boom and bust', with a lower physiological capacity to exploit resources but which is less costly to sustain during periods of food deprivation.

-Little is known about the genetic basis of residual feed intake (RFI) variation in fish, since this trait is highly sensitive to environmental influences, and feed intake of individuals is difficult to measure accurately. The purpose of this work was (i) to assess the genetic variability of RFI estimated by an X-ray technique and (ii) to develop predictive criteria for RFI. Two predictive criteria were tested: loss of body weight during feed deprivation and compensatory growth during re-feeding. Ten heterozygous rainbow trout clones were used. Individual intake and body weight were measured three times at threeweek intervals. Then, individual body weight was recorded after two cycles of a three-week feed deprivation followed by a three-week re-feeding. The ratio of the genetic variance to the phenotypic variance was found high to moderate for growth, feed intake, and RFI (VG/ VP = 0.63 ± 0.11, 0.29 ± 0.11, 0.29 ± 0.09, respectively). The index that integrates performances achieved during deprivation and re-feeding periods explained 59% of RFI variations. These results provide a basis for further studies on the origin of RFI differences and show that indirect criteria are good candidates for future selective breeding programs. rainbow trout / clone / residual feed intake / indirect criteria / selection

Feed efficiency is a major goal for aquaculture sustainability, and selecting fish to genetically enhance this trait would be highly valuable. However, no selective breeding program specifically targeted to feed efficiency exists for farmed fish, mostly because of the difficulty of measuring individual feed intake. However, a negative phenotypic correlation between feed efficiency and weight loss at fasting has been previously demonstrated in sea bass submitted to feed deprivation (FD). We mated sea bass parents selected for their high (FD+) or low (FD-) weight loss at fasting to produce FD+ and FD- progeny, which were reared in a single tank to avoid common environmental effects. At 8 months of age, 1200 of those fish were submitted to three alternating periods of fasting (3 weeks) and re-feeding (3 weeks). Individuals were weighed at the end of each feeding and fasting period. Their line of origin was identified by genotyping of 12 microsatellite markers, resulting in 1130 unambiguously assigned fish (484 FD-, 686 FD+). FD- offspring lost significantly less weight than FD+ offspring in this feed deprivation trial. After that the feed efficiency of eight groups of 50 FD+ fish and eight groups of 50 FD- fish was evaluated in four successive 20-day periods. At the end of the fourth period, 10 fish per tank were sacrificed to evaluate their carcass yield. The FD- fish had a better overall growth and were fatter, and FD+ fish had a better carcass yield. A better feed efficiency was expected for the FD- fish, but differences between the two groups for this trait, measured either with feed efficiency ratio or with residual feed intake, were not consistently significant Although the two lines were clearly divergent for several traits, demonstration of feed efficiency differences between the FD+ and the FD- lines was not consistently observed in sea bass. A second generation of selection may allow further divergence in the lines and reveal differences in feed efficiency. (C) 2013 Elsevier B.V. All rights reserved

No commercial breeding programs have yet started to improve feed utilization efficiency in fish, mainly because of the difficulty in accurately measuring individual feed intake in fish reared in groups. Our general goal is to propose indirect criteria to be used for breeding of commercial lines. In a previous study undertaken with rainbow trout clones, a genetic correlation was detected between residual feed intake (RFI), and body weight variation during successive periods of feed deprivation (FD) and refeeding (RF). To assess the pertinence of such indirect criteria for future breeding programs, we set up a large experiment using sea bass (Dicentrachus labrax), a recently domesticated species with broad genetic diversity. The objectives of the present study were to analyze the relationship of FD and RF with RFI, and to assess the consistency of such indirect criteria over time and their correlation with carcass quality. Fish originating from a full factorial design combining eight dams and 41 sires, were raised in the same tank. At 306 days post fertilization 2000 fish were individually tagged and their body weight recorded over a growth period of three weeks, followed by two successive periods of three weeks of feed deprivation and three weeks of ad libitum re-feeding. Fish performances were then classified, FD−, FD+, RF− and RF+ for fish exhibiting loss (FD) or gain (RF) of weight relatively lower (−) and higher (+) than the population mean. Fish were sorted into four groups (FD−/RF−, FD+/RF+, FD−/RF+, FD+/RF). Each of the 4 groups was split between three replicated tanks of 50 fish. Body weight gain and feed intake were measured for each replicate every three weeks over a six months period to estimate residual feed intake. The fish then underwent a period of three weeks of feed deprivation followed by a period of three weeks of ad libitum re-feeding. At the end of each period, individual weight gain variations were recorded, as well as muscle fat using ultrasonic measurement.The relationship with RFI variations was high for FD (P = 0.04, n = 12), and close to significance for a criterion merging FD and RF performances (P = 0.06, n = 12). FD but not RF was negatively correlated with muscle fat (P < 0.05, n = 600). In conclusion, selecting fish losing less weight during a three week feed deprivation period should lead to improve RFI, but also to increase muscle fatness. Using a combination of FD and RF as indirect criteria would result in slower progress in RFI, but would not impact carcass quality traits. The next step for inferring the potential genetic gain that can be expected is to assess the heritability of such criteria.