Résumé : The objective of this study was to characterize the body composition, nutrient utilization, energy storage sites and major economic traits of trout selected at pan-size for or against body weight corrected muscle fat content (fat line, FL and lean line, LL respectively). The study focused on the...The objective of this study was to characterize the body composition, nutrient utilization, energy storage sites and major economic traits of trout selected at pan-size for or against body weight corrected muscle fat content (fat line, FL and lean line, LL respectively). The study focused on the effect of selection in fish size larger than the size where selection has been applied, and the possible effect of diet composition on the differences between lines. FL and LL trout were fed two diets differing for energy content and protein/lipid ratio during 62-days. The feeding trial (day 370 to day 432) started with 327g fish. Dietary protein and lipid contents were 58.7% and 8.3% dry matter (DM) respectively for diet D8 and 47.7% and 26.7% DM for diet D27. Growth, feed intake, feed utilization and traits related to body shape and composition were recorded. In both lines diet D27 improved growth rate, feed efficiency, protein retention and fat gain (P < 0.05), and was associated with a higher viscero-somatic index, fillet fat content and lower trimmed fillet yield than diet D8. At the end of the trial, LL fish were slightly heavier than FL ones (P = 0.02, no line x diet interaction). The main differences between lines were observed in lipid gain and retention. Values of lipid retention (% of intake) were higher in FL fish (P = 0.04) regardless of diet, and suggested a greater ability of FL fish in de novo lipid synthesis. Fat deposition (% of weight gain) was greater in FL fish during the trial and occurred in fillet (P = 0.04), on internal side of the belly flap (P = 0.04), as well as in other sites of the body, as indicated by higher overall body lipid content at day 432 (P = 0.02, no line x diet interaction). Nevertheless, carcass and fillet yields were not different between lines regardless of diet. In conclusion, selection for body weight corrected muscle fat content modified the ability of the fish to utilize nutrients and to store more or less fat in the different body sites. The differences were expressed in a large range of dietary protein/fat ratios. Line x diet interactions were recorded for a very limited number of traits, indicating that the combined use of genetic and nutritional tools should be efficient to manage carcass quality (growth, body shape, processing yields) and fat deposition (quantity and body location) in trout, with no need to adapt diet formulation according to lines. (C) 2007 Elsevier B.V. All rights reserved.
Résumé : A two-way selection for muscle lipid content was performed in rainbow trout. One-year-old pan-size individuals were mass selected using Distell Fish Fatmeter values corrected for absolute weight (fat index, FI), this being a non-destructive measurement of the muscle lipid content in live fish....A two-way selection for muscle lipid content was performed in rainbow trout. One-year-old pan-size individuals were mass selected using Distell Fish Fatmeter values corrected for absolute weight (fat index, FI), this being a non-destructive measurement of the muscle lipid content in live fish. Direct and correlated responses at pan-size (around 260 g) were estimated after two generations of selection. Absolute Fat values and FI values in the upward line (fat line, FL) and downward line (lean line, LL) differed from the control line and from each other. The mean realized heritability achieved for FI was 0.25. The difference in Fat values between FL and LL resulted in a significant difference in muscle lipid content (29.6% dry matter content in FL, vs. 25.6% in LL, P=0.003). The differences in Fat and FI values between FL and LL were sustained until maturation at 2 years old. At the age of 1 year, the two lines did not differ significantly in terms of weight, length or most body shape traits (height, width or cross-section shape). However, FL fish had a better condition factor than LL (1.44 vs. 1.38), and a greater belly thickness (11% relative increase), the control line displaying intermediate values. The abdominal internal fat coat was thicker in FL, and represented a larger proportion of the entire abdominal thickness than in LL. The relative visceral weight (in % of total body weight), carcass and fillet yield were not modified by selection. The frequency of precocious males was significantly higher in FL (16.5%, vs. 11% and 10.1% in control and LL fish, respectively), but there was no difference between the reproductive traits of 2-year-old LL and FL females. These results indicate that breeding based on Fatmeter values was efficient in selecting the fillet lipid content in rainbow trout, with limited correlated responses at the size where selection was applied. In particular, no adverse effects were recorded of a reduction in muscle lipid content on carcass or fillet yield under the experimental conditions applied during this study.