We previously showed that individual feed efficiency in the aquariums had a significant genetic basis in sea bass (genomic h² = 0.47). However, further exploration of the physiological factors influencing individual variations in feed efficiency, especially regarding lipid distribution and composition, is crucial. It is commonly assumed that leaner animals are expected to exhibit greater efficiency . Surprisingly , our study revealed that sea bass with higher global fat content exhibit greater efficiency, although further exploration of this relationship was not pursued. Therefore, the objective of the present study was to delve deeper into the relationship between individual efficiency and lipid distribution and composition in European sea bass.
First, 458 sea bass from the commercial breeding program of EMG-Ichtus were phenotyped in aquariums to estimate their feed efficiency as the residual body weight gain (rBWG). All fish were genotyped to estimate genetic parameters . Based on estimated breeding values , two categories were created : rBWG + gathered the 120 fish with the highest EBVs while rBWG - gathered the 120 fish with the lowest EBVs. These 24 0 fish were reared in 6 groups of 40 fish (each group containing fish from the same EBV category) until 748g. Then, we measured several harvest traits: harvest weight , muscle fat content, fillet weight, head weight, viscera weight and headless carcass weight. Fatty acid composition was determined using gas chromatography on samples extracted from the right fillets. The left fillets underwent MRI analysis () to estimate the repartition of lipids in the filet (Figure 1).
rBWG was heritable (0.22 ± 0.08) . T here were no difference in fill et yield and headless carcass yield between rBWG+ and rBWG- fish suggesting no adverse effects of improved feed efficiency on these traits. However , rBWG + fish exhibited significantly greater viscera-to-body ratio and lower head-to-body ratio compared rBWG- fish. MRI analysis indicated that rBWG+ fish were fatter (13.5% fat ) than rBWG- fish (11.6% fat ). These findings confirm a link between higher feed efficiency and increased fat content in sea bass, a unique observation in current literature. Elevated levels of palmitic and palmitoleic acid in more efficient fish suggest a higher tendency to convert protein into lipids. One hypothesis suggests that these efficient fish exhibit higher protein retention, but surplus protein may be converted into lipids in the absence of structural growth, as indicated by higher viscera -to-body ratio in more efficient individuals. However, all fillet areas were equally rich in lipids for both groups. Finally , there were promising results regarding EPA and DHA content, which were richer in more efficient fish. Notably, EPA and DHA also displayed heritability (0.17 and 0.07 respectively) suggesting the potential for selective breeding to enhance favorable fatty acid composition.