A study was undertaken with marketable size turbot to evaluate the effects of dietary fat levels on chemical composition, lipogenesis and flesh quality. Four experimental diets containing graded levels of fish oil in order to obtain 10%, 15%, 20% and 25% of crude fat were fed to triplicate groups of turbot (initial body weight of 660 g) for 12 weeks in full strength seawater at temperature of 17°C. Nutrient digestibility was not influenced by dietary fat levels. The best growth performance was observed in fish fed 10% and 15% dietary fat. High dietary lipid levels led to higher fat deposition in whole fish, although lipid level in muscle remained low (1.1% in dorsal muscle and 1.7% in ventral muscle irrespective of diet). Significant subcutaneous fat accumulation was detected in turbot. No protein sparing effect by lipid was observed in turbot fed high dietary fat. Hepatic lipogenic enzymes (glucose-6-phosphate dehydrogenase, G6PD; malic enzyme, ME and acetyl CoA carboxylase) did not show any clear change in activity in response to dietary fat content. With regards to quality parameters, there were no differences in gutted and fillet yields among treatments. Sensory analyses of dorsal fillets indicated only a difference in exudation (corresponding to loss of water) and whiteness within treatments in accordance with instrumental colour analyses and on ventral fillets, only a difference of sweet flavour was observed. No differences in hardness were detected by either instrumental texture analysis or sensory analysis. In conclusion, although high dietary lipid levels affected growth and whole body composition of turbot adversely they induced very few alterations in flesh quality.

The fatty acid (FA) content of fish is generally said to reflect fatty acid composition of the diet. In fact, incorporation of FA into tissues is modulated by various metabolic factors, and final composition will depend upon the initial FA content, cumulative intake of dietary fatty acids, growth rate and duration. Analysis of time course of changes in FA composition should be easier with animals having different initial FA profiles, which are subsequently fed a diet with the same FA composition. Data from two studies, one with brown trout and another with turbot were used. Fish were first fed with diets containing one of three different oils (soybean oil (SO), linseed oil (LO) and fish oil (FO)), and subsequently fed the same fish oil-based diet (washout period). If we suppose a model fish having the same initial composition as those fed vegetable oil and which incorporate fatty acid in the same way as the control fish always fed fish oil, we may compute a model of dilution of initial fatty acid content with increasing growth and absolute fat deposition. Experimental data can be compared with a reference fatty acid profile given by this model for the same fatty acid increase. Application of the model to experimental data shows that while muscle neutral lipid (NL) FAs roughly follow this dilution model, those of muscle polar lipids (PL) undergo much faster changes than model values based on increase of total polar lipid quantities. Among observed differences between the model and experimental values, DHA is of particular interest as this fatty acid displays lower change rates (significant in turbot neutral lipids) than expected in contrast to other fatty acids.

The aim of the study was to investigate the replacement of fish oil by vegetable oils and the effects of a washout with a return to fish oil on growth performances and lipid metabolism. Three experimental fish meal based, isonitrogenous (crude protein content: 57.5%) and isolipidic (crude lipid content: 16.5%) diets, were formulated containing either 9% of added fish oil (FO), soybean oil (SO) or linseed oil (LO). Each diet was distributed to triplicate groups of 25 marketable size turbot (initial body weight of 579 g) grown in seawater at a water temperature of 17°C. Fish were fed once a day to visual satiety. At the end of the growth trial which lasted 13 weeks, all groups of turbot were fed FO diet for 8 weeks. The growth of turbot was high, but the incorporation of vegetable oils in the diets resulted in a slight decrease in growth as compared to those fed the fish oil based diet. Feed and protein efficiency and whole body composition were not affected by dietary lipid sources. Total lipid content was low in the muscle of turbot (below 2%), ventral muscle being fatter than dorsal muscle. Liver and muscle fatty acid (FA) composition reflected dietary FA composition. Liver and muscle of fish fed SO diet were rich in 18:2n-6 whereas those of fish fed LO diet were rich in 18:3n-3. Liver and muscle of fish fed SO and LO diets had lower levels of 20:5n-3 and 22:6n-3 in comparison to those of fish fed FO diet. In turbot, hepatic lipogenic enzyme activities were low and not influenced by dietary lipid source. At the end of the second period, after transfer to FO based diets, muscle FA composition of fish fed previously SO and LO diets was still different to those of fish fed the FO diet. The values of 18:2n-6 and 18:3n-3 respectively were lower than the values found at the end of the growth period but higher than those of fish fed the FO diet. An increase of FA levels, characteristic of fish oil, was observed in the liver and muscle of fish previously fed vegetable oils. Data obtained show that replacement of fish oil by vegetable oils is possible without any significant impact on growth performance of turbot, that dietary lipids are an effective vector to influence the nutritional quality of finished product and that a duration of 8 weeks is not sufficient to bring the FA profile of turbot of this size back to that of fish fed fish oil over the whole period.

European sea bass juveniles (14.4±0.1 g mean weight) were fed diets containing different levels of fish oil then of n-3 highly unsaturated fatty acids (n-3 HUFA) for 12 weeks. The fish performance as well as fatty acid (FA) composition of neutral and polar lipids from whole body after 7 and 12 weeks feeding were studied. The requirements of juvenile sea bass for n-3 highly unsaturated fatty acids (n-3 HUFA) were studied by feeding fish diets containing six different levels of n-3 HUFA ranging from 0.2% to 1.9% of the diet, with approximately the same DHA/EPA ratio (1.5:1). The growth rate at the end of the trial showed significant differences. Fish fed low dietary n-3 HUFA (0.2% DM of the diet) showed significantly lower growth than the diet 3 (0.7%), then no further improvement (P>0.05) of growth performance was seen by elevating the n-3 HUFA level in the diet up to 1.9% (diet 6). No difference in feed efficiency, protein efficiency ratio or protein retention was observed among treatments, nor in protein and total lipid content. However, the n-3 HUFA levels in diets highly influenced fish fatty acid composition in neutral lipid, while polar lipid composition was less affected. Comparison of polar lipid content after 7 or 12 weeks indicated that DHA remained stable at the requirement level, while arachidonic acid decreased with time. Results of this experiment suggest that the requirement for growth of n-3 HUFA of juvenile sea bass of 14 g weight is at least 0.7% of the dry diet.

The effects of essential fatty acid deficiency and temperature on the fatty acid profiles of polar lipids (PL) and neutral lipids (NL) from various tissues (muscle, liver, gills, eyes and brain) of European sea bass juveniles were compared in a two factorial design. Fish (60 g) were held for 84 days at 22 or 29 °C (upper limit for growth) and fed either at a lower or a higher level than n − 3 HUFA minimal requirement for growth (0.4% and 2.2% n − 3 HUFA dry matter for diets LD and HD, respectively). Essential fatty acid deficiency had a major influence on fatty acids in NL fraction of all tissues, a more moderate influence on PL of muscle, liver and gills, while a low and very low diet influence was observed on eyes and brain PL fatty acid content, respectively. DHA and EPA content in brain PL as well as DHA in eyes PL were not affected by diet. DHA contents were similar in gill PL of 22-HD, 29-HD and 22-LD but was reduced in 29-LD fish. Most of brain PL fatty acids displayed a significant effect of temperature (at 29 °C; 18 : 0, 18 : 1n − 9 contents were higher, and 20 : 5n − 3, 22 : 6n − 3, 20 : 4n − 6 contents were lower than at 22 °C). Temperature had more influence on PL than on NL fatty acid content, except in liver. A lipogenic activity seemed to occur, both 16 : 0 and 18 : 0 were high particularly in liver NL and dependent on temperature (higher at 29 than at 22 °C). An enhanced 18 : 3n − 6 content in fish fed on the deficient HUFA diet indicated a desaturation activity, mainly in liver NL and gill PL, with higher arachidonic acid content in PL of gills than other tissues. Muscle fatty acid profiles in NL and PL were more similar to those of whole body than other tissues, however total lipid content then PL : NL ratio differed. This study shown that beside known characteristics of each tissue in term of PL fatty acid content, each tissue have also characteristics in term of response to temperature and dietary deficiency influence. Among then, neural tissue displayed the highest capacity to regulate DHA content in PL, preserving functionality despite HUFA deficiency.

The purpose of this study was to check if it is possible to influence sea bass juvenile metabolism by a conditioning of larvae from day 6 post hatching to day 45 to a low or a high HUFA compound diet (LH, 0.8% EPA + DHA and HH, 2.2% EPA + DHA) when reared at 16 or 22 °C. Following a 3-month intermediate period (at 19 °C using a commercial diet), the adaptability of the 4 initial larval groups to a HUFA experimental deprived diet (0.5% EPA + DHA) were tested at 19 °C in a 60 day-experiment (d-151-211). The four experimental duplicated conditions were ex-LH16 and ex-HH16 for the 2 groups previously reared at 16 °C (initial weight, 7.3 ± 0.5 g) and ex-LH22 and ex-HH22 for the 2 groups previously reared at 22 °C (initial weight, 11.1 ± 0.5 g). Survival was maximal and there was a 1.6-2 fold increase in mass during the experiment. Growth was similar in the 4 experimental groups: NS difference in growth curve slopes (P = 0.7). At the end of the experiment (d-211), whole body fat levels were in the same range in all groups (13-15% WW). The fatty acid (FA) composition in polar lipids (PL) and total lipids (TL) were significantly affected by initial weight related to larvae conditioning, which can be mainly attributed to a dilution effect (impact of initial FA content on final FA content versus relative mass increase during the course of the experiment). Conversely to this trend, DHA content in PL was higher in the ex-LH groups than in the ex-HH groups whatever thermal conditioning of larvae was. This indicated that ex-LH groups had a better capacity to adapt to a deficient HUFA diet than ex-HH fish. The relative expression of the delta-6 desaturase (Δ6D) was significantly higher in ex-LH than in ex-HH groups (P < 0.001) between d-151 and d-181, which suggested that Δ6D transcription in ex-LH groups was positively modulated by the HUFA-deprived diet. This stimulation of the first step of the desaturation/elongation pathway could allow synthesizing FA needed to compensate low dietary HUFA supply. This study shows for the first time that it seems possible to influence juvenile fish metabolism by a nutritional conditioning during the larval stage.

Four replicated groups of sea bass (Dicentrarchus labrax) larvae were fed diets containing an extra-high level of highly unsaturated fatty acids (HUFA) (XH; 3·7 % EPA þ DHA), a high level of HUFA (HH; 1·7 %), a low level of HUFA (LH; 0·7 %) or an extra-low level of HUFA (XLH; 0·5 %) from day 6 to day 45 (experiment 1; XH1, HH1, LH1, XLH1). After a subsequent 1-month period feeding a commercial diet (2·7 % EPA þ DHA), the capacity of the four initial groups to adapt to an n-3 HUFA-restricted diet (0·3 % EPA þ DHA; R-groups: XH2 R , HH2 R , LH2 R , XLH2 R ) was tested for 35 d. Larval dietary treatments had no effect on larval and juvenile survival rates. The wet weight of day 45 larvae was higher in XH1 and HH1 (P,0·001), but the R-juvenile mass gains were similar in all treatments. D-6-desaturase (D6D) mRNA level was higher in LH1 and XLH1 at day 45 (P,0·001), and higher in LH2 R and XLH2 R , with a significant increase at day 118.Concomitantly, PPARa and PPARb mRNA levels were higher in XLH1 at day 45, and PPARb and g mRNA levels were higher in XLH2 R at day 118, suggesting possible involvement of PPAR in stimulation of D6D expression, when drastic dietary larval conditioning occurred. The low DHA content in the polar lipids (PL) of LH1 and XLH1 revealed an n-3-HUFA deficiency in these groups. Larval conditioning did not affect DHA content in the PL of R-juveniles. The present study showed (i) a persistent D6D mRNA enhancement in juveniles pre-conditioned with an n-3 HUFA-deficient larval diet, over the 1-month intermediate period, and (ii) brought new findings suggesting the involvement of PPAR in the D6D mRNA level stimulation. However, such nutritional conditioning had no significant effect on juvenile growth and lipid composition.

No abstract