Feed discrimination and selection in self-fed European sea bass Dicentrarchus labrax L.

Abstract This study was conducted to evaluate feed discrimination and preference of European sea bass Dicentrarchus labrax L., taking into account the effects of feed location, previous feeding experience and light conditions. Fish (63 g) were held in tanks and exposed to a 12L:12D photoperiod and ambient temperature. The following feeding practices were applied in triplicate: three‐choice feeding (self‐ feeding with feeds formulated for carnivorous, herbivorous and omnivorous fish rotated on a weekly basis) and monofeeding (self‐feeding with one of these feeds). After 9 weeks (phase I), fish previously held in monofeeding, were subjected to three‐choice feeding for 2 weeks (phase II). In phase I, fish discriminated between feeds after their rotational displacements and preferred the feed formulated for carnivorous species. This also supported the best growth. Fish used in phase II preferred the same feed. There was no preference for any location within tanks. Feeding was generally diurnal, although in some tests with the feeds for herbivores and omnivores, diurnal and nocturnal feed demands were similar and fewer in number than with the feed formulated for carnivores.     

Muscle cellularity in relation to somatic growth in the European sea bass Dicentrarchus labrax (L.)

Sea bass, Dicentrarchus labrax (L.), juveniles (1.32 g) grew at a rate of 4.2% per day, when fish were fed ad libitum and reared at optimal thermal conditions (ambient sea water temperature, 20-26°C). At 13°C, feeding and spontaneous activity were severely restricted and somatic growth was reduced to 0.6% per day. Over a period of 6 weeks, both muscle fibre hyperplasia and nuclear division were higher in the ambient-temperature group compared with the fish reared at 13°C. Despite the differences in growth rate and spontaneous activity, muscle fibre hyperplasia was paralleled by nuclear division in the lateral axial muscle in both temperature groups and the number of nuclei per myofibre did not differ significantly between the two temperature groups. It is concluded that at optimal thermal and feeding conditions, somatic growth of sea bass juveniles is mediated through an increase in the number of nuclei and muscle fibres.     

Evaluation of zinc supplementation in European sea bass (Dicentrarchus labrax) juvenile diets

This study aimed to investigate the effect of increased zinc dietary levels on the growth performance, feed utilization, immune status and induced wound healing in European sea bass (Dicentrarchus labrax). Fish weighing 10 g were fed five diets containing organic zinc at 30, 70, 110 and 150 mg kg^?1 diet and one inorganic zinc source (zinc oxide) at 150 mg kg^?1 diet for a period of 12 weeks. No significant (P>0.05) differences were found in the growth performance parameters (final weight, specific growth rate), immunological indices tested (respiratory burst activity in whole blood) and the wound‐healing process. Zinc accumulation in the skin, vertebrae and liver increased significantly (P<0.01) with increased dietary zinc levels but not in muscle. The highest zinc concentrations were obtained in skin tissue, followed by vertebrae, liver and muscle tissue. Using the concentration of zinc in skin as a response criterion, broken‐line analysis showed that the supplementation of 148 mg organic Zn kg^?1 diet seemed to be the optimum dietary zinc supplementation level for sea bass juveniles.     

Phosphorus requirement of European sea bass (Dicentrarchus labrax L.) juveniles

A growth trial was conducted to estimate the phosphorus requirement of European sea bass juveniles. Six experimental isonitrogenous and isoenergetic semi‐purified diets (casein based) were formulated to contain 0.48%, 0.65%, 0.77%, 0.86%, 1.05% and 1.25% phosphorus (diets D1, D2, D3, D4, D5 and D6 respectively). Dicalcium phosphate was used as dietary phosphorus source. Twelve groups of 25 fish of 10 g initial body weight were allocated to 55‐L tanks in a thermoregulated water recirculating system. Each experimental diet was assigned to duplicate groups of these fish. The trial lasted for 10 weeks and fish were fed two times a day, 6 days a week, to apparent visual satiation. At the end of the trial, final weight of fish fed diet D1 was significantly lower than that of the other groups, except of fish fed diet D4. Mortality of fish fed diet D1 was significantly higher than that of fish fed the other diets, except for diet D6. Feed efficiency and protein efficiency ratios were significantly lower with diet D1 than with the other diets. Nitrogen retention (% nitrogen intake) of fish fed diet D1 was significantly lower than in the other groups, except that of fish fed diet D4. Energy retention (% energy intake) was not significantly different among groups. At the end of the trial there were no differences in whole body composition among groups. Whole body phosphorus content averaged 0.72% (on a fresh weight basis) and was not significantly affected by dietary phosphorus content. Phosphorus retention averaged 6.1 g kg^?1 weight gain and was not significantly different among groups. Phosphorus retention (% phosphorus intake) was significantly higher in fish fed diets D2 and D3 than in fish fed higher dietary phosphorus levels. Expressed per unit body weight per day, phosphorus retention was not significantly different among groups fed diets D2 to D6, while phosphorus losses linearly increased with dietary phosphorus intake. Results of this trial indicate that the phosphorus requirement of sea bass juveniles was satisfied with a diet containing 0.65% of phosphorus.     

Methionine- and cystine-supplemented diets for juvenile sea bass (Dicentrarchus labrax)

Growth studies indicated that methionine-deficient diets can be supplemented with the free sulphur amino acids, methionine and cystine. The best growth rate was obtained with the diets for which the supplemented methionine:cystine ratio was 20:80. No significant effect on body composition of the methionine:cystine ratio could be detected. The serum free methionine increased with the supplemented methionine content in the diet. The serum free cystine did not exhibit such a relationship. The best growth rate and food conversion were obtained with the diet containing 0.91% methionine, 1.29% cystine and a methionine:cystine ratio of 36:64 on a sulphur molar basis.     

Enhancement of growth and feed utilization of the European sea bass (Dicentrarchus labrax) fed supplementary dietary salt in freshwater

The effects of increased dietary sodium chloride supplement on growth, feed efficiency (FE), feed dispensed rate and fillet composition and morphological indices of European sea bass (initial weight of 4.9 g) were investigated in freshwater using five different levels of supplemental salt (0 (control diet, with no salt supplemented), 1%, 3%, 5% and 9% of diet). Over the experimental period of 50 days, the final weight of dietary salt (DS) groups was found to be approximately 19% higher than the control group. The final weight, specific growth rate and FE were greatest at the moderate level (1–5%) of salt supplementation. Optimum level of DS inclusion was calculated as 3%. Feed efficiency was significantly enhanced with increasing salt level up to 5%. A negative relationship between daily feed intake and FE (r²=0.66) was observed. The average muscle ratio (MR) in groups fed 1–5% supplemented salt was around 51.4% whereas fish receiving control and 9% salty diets had 5.3% lower MR (48.7%) (P<0.05). However, no significant effects of salt inclusion were found on viscera or hepatosomatic indices. Muscle ash and dry matter content slightly rose with increasing DS in the diet, but with no statistical difference (P>0.05). Similarly, no significant changes were observed in either muscle protein or lipid content among the groups (P>0.05).     

Effects of different dietary oils in sea bass (Dicentrarchus labrax) nutrition

Five diets having the same proximate composition but containingdifferent types of supplemental oils, singly or in combination, were used forgrowing sea bass from 95 g to about 200 g in smallseacages. The oils tested were olive oil, soybean oil and fish oil. The dietsformulated contained EPA and DHA levels ranging from 0.88 to 1.35% of the diet.Growth parameters and fish body composition were not significantly affected bythe type of oil used. The same was generally apparent for liver andhematological characteristics. The content of phospholipids in EPA and DHA washighest in the livers of fish fed diets supplemented with fish oil. A positivecorrelation was found between dietary and liver n-6 PUFA. Histological sectionsindicated extended pathological symptoms (intensive liver degeneration andhemorrhages, changes in the gill structure) in the fish receiving dietssupplemented only with plant oils. These symptoms existed but to a smallerfrequency and degree in livers of fish fed diets supplemented with plant andfish oil, while were not apparent in those fed the fish oil diet.     

Effects of different dietary oils in sea bass (Dicentrarchus labrax) nutrition

Five diets having the same proximate composition but containingdifferent types of supplemental oils, singly or in combination, were used forgrowing sea bass from 95 g to about 200 g in smallseacages. The oils tested were olive oil, soybean oil and fish oil. The dietsformulated contained EPA and DHA levels ranging from 0.88 to 1.35% of the diet.Growth parameters and fish body composition were not significantly affected bythe type of oil used. The same was generally apparent for liver andhematological characteristics. The content of phospholipids in EPA and DHA washighest in the livers of fish fed diets supplemented with fish oil. A positivecorrelation was found between dietary and liver n-6 PUFA. Histological sectionsindicated extended pathological symptoms (intensive liver degeneration andhemorrhages, changes in the gill structure) in the fish receiving dietssupplemented only with plant oils. These symptoms existed but to a smallerfrequency and degree in livers of fish fed diets supplemented with plant andfish oil, while were not apparent in those fed the fish oil diet.     

Saponins and phytosterols in diets for European sea bass (Dicentrarchus labrax) juveniles: effects on growth,intestinal morphology and physiology

This study aimed to elucidate the effects of two soy antinutrients, saponins and phytosterols, on growth, intestinal health and function of European sea bass juveniles. Seven fishmeal‐based diets were formulated: a control diet without antinutrients and six experimental diets containing low or high levels of soyasaponins (SapL and SapH, respectively), low or high levels of phytosterols (PhytL and PhytH, respectively) and low or high levels of soyasaponins + phytosterols (SapPhytL and SapPhytH, respectively). A feeding trial was conducted for 59 days after which blood was collected for plasmatic cholesterol quantification and intestinal samples were collected for histology, digestive enzymes activity and gene expression analysis. Histology was also performed at day 15. PhytL resulted in high distal intestine (DI) relative weight and decreased plasma cholesterol, while PhytH caused inflammatory changes in the DI. SapH depressed maltase and alkaline phosphatase activity in DI and the histological evaluation indicated some inflammatory changes. The SapPhytH resulted in decreased maltase activity in the DI and indications of inflammatory changes that were supported by the results on gene expression profiles. The antinutrients tested did not compromise growth, however, caused some gastrointestinal disturbance that may affect fish in the long term.     

A shortcut method for sexing juvenile European sea bass, Dicentrarchus labrax L.

The objective of this study was to develop a rapid and effective method of sexing juvenile sea bass with minimum labour and material. To this end, the gonad squash mount technique was applied along with macroscopic techniques for sexing a large number of experimental fish at the age of 215–275 days post fertilization (p.f.). At this age, 90% of the 3894 fish could be unambiguously sexed by macroscopic examination of their gonads, whereas the remaining proportion was identified using squash mount preparations (10%). The accuracy of the observations was measured up to classical histology procedures. Undifferentiated fish accounted for 0.1% of the total population only. The application of the squash mount technique holds out the prospect of considerable improvement in the efficiency and rapidity of current sexing techniques in sea bass.     

Characterization of mevalonate metabolism in the sea bass (Dicentrarchus labrax L) liver

The activities of mevalonate kinase, mevalonate 5-phosphate kinase and mevalonate 5-pyrophosphate decarboxylase, were examined in sea bass (Dicentrarchus labrax L) liver. The activities of the three enzymes were studiedin vitro in relation to the influence of protein content, time of incubation, pH, temperature, mevalonate, ATP and Mg⁺⁺ concentration. Protein content in the assay medium affected the three enzymes differently. Mevalonate kinase, mevalonate 5-phosphate kinase, and mevalonate 5-pyrophosphate decarboxylase activities were linear up to 0.2, 0.4 and 0.8 mg protein, respectively. With respect to the time course studies, the enzymes also behaved differently. Mevalonate kinase activity increased over forty minutes, reaching a plateau thereafter, while mevalonate 5-phosphate kinase and decarboxylase increased over the entire assay period. All the three enzymes showed a maximum in activity at pH 7.5. The effect of reaction temperature showed that phosphorylation increased to maximum around 35°C for mevalonate kinase and 30°C for mevalonate 5-phosphate kinase while decarboxylation rates remained constant well until 30°C temperature decreasing afterwards. The enzymes behaved differently as a function of mevalonate concentration. Mevalonate 5-phosphate formed was maximal when the initial mevalonate concentration was 272 M, whereas mevalonate 5-pyrophosphate and CO₂ were formed maximally at mevalonate concentrations of 136 M and 68M, respectively. Optimal ATP concentration in the medium was 3 mM for decarboxylase and 6 mM for kinases, and Mg⁺⁺ requirements varied from 4 mM for decarboxylase to 6 mM for kinases.     

Compensatory growth response of European sea bass (Dicentrarchus labrax L.) under cycled starvation and restricted feeding rate

The compensatory growth response of the European sea bass (Dicentrarchus labrax) that faced cycling starvation and restricted ration was assessed. Juveniles (10.5 g) were stocked into 15 tanks at a density of 25 fish per tank. Five different feeding regimes were tested on triplicate groups of fish: C_Satiation: control fed for 60 days without deprivation, C_Restricted: 25% restricted feeding, S‐R: 1 day starvation then 4 days C_Restricted feeding, R‐F: 1 day C_Restricted feeding then 4 days C_Satiation feeding, and, finally, S‐F: 1 day starvation then 4 days C_Satiation feeding. The specific growth rate of fish in the C_Satiation (2.5 ± 0.06% day^?1), S‐F (2.5 ± 0.11% day^?1) and R‐F (2.4 ± 0.18% day^?1) were significantly higher than that of C_Restricted (2.2 ± 0.05% day^?1) or S‐R (2.0 ± 0.01% day^?1). Fish in S‐F group were able to achieve catching up with the C_Satiation. There was no significant difference in feed conversion rates but R‐F and S‐F consumed approximately 34% more feed than C_Satiation following the first re‐feeding day. Although, the highest lipid content was observed in C_Satiation (14.4%), S‐R (33.3% dry matter) had the highest water content compared with the C_Satiation and C_Restricted (37.8% and 36.9% dry matter respectively). In conclusion, it may be concluded that sea bass has rapid response to cycling starvation/re‐feeding and that a 25% restricted feeding ratio is insufficient to invoke a compensatory growth response in sea bass.     

Estrogen binding sites in the ovary of the European sea bass (Dicentrarchus labrax L.)§

The binding characteristics of the European sea bass estrogen receptor (ER) were determined in ovarian cytosolic and nuclear extracts. Results were consistent with the presence of a single class of high affinity and low capacity ER, the first to be characterized in teleost ovary. This will contribute to our understanding of changes in ER affinity and abundance during sex differentiation in this economically important species.     

The effect of dietary ribonucleic acid incorporation in performance of European sea bass (Dicentrarchus labrax) juveniles

The aim of this study was to evaluate the effect of dietary ribonucleic acid (RNA) on the performance of European sea bass juveniles. Two diets containing fish meal as the only nitrogen source were formulated with 39% or 50% of crude protein (LP and HP diet, respectively). Two other diets were formulated similarly to LP diet, but included yeast-RNA extract at 6.2% and 12.4% levels (LP6 and LP12 diets, respectively) by replacing dextrin. Total nitrogen content of the LP12 diet was equivalent to that of the HP diet. The four experimental diets were isoenergetic (22 kJ GE g⁻¹ DM). Triplicate groups of 25 fish (IBW=13 g) were fed the test diets for 10 weeks. The reduction of dietary protein level did not significantly affect growth rate but significantly increased feed intake and decreased feed efficiency. However, nitrogen retention (%N intake) was significantly improved with the decrease in dietary protein level. Dietary RNA incorporation did not affect feed intake or feed efficiency, although it reduced growth rate of fish fed the highest dietary RNA level. Nitrogen retention (%N intake) significantly decreased with the dietary supplementation of RNA. Dry matter, nitrogen and energy digestibilities were not significantly affected by dietary treatments. Final whole-body composition was also unaffected by dietary treatments, except for ash content which was significantly higher in the LP12 group. Hepatosomatic index was significantly increased by the dietary incorporation of RNA. The dietary supplementation of RNA had no protein sparing effect and significantly reduced energy retention. It may be concluded that dietary RNA supplementation had no nutritive value for European sea bass.     

Peptide molecular weight distribution of soluble protein fraction affects growth performance and quality in European sea bass (Dicentrarchus labrax) larvae

Dicentrarchus labrax were fed from 10 to 40 days posthatching with six microdiets differing in the inclusion level (60 g kg^‐1 and 120 g kg^‐1) and type of protein hydrolysate (PH; yeast, YPH; pig blood, PBPH; pig red blood cells, PRBCPH). A microdiet containing 120 g kg^‐1 fish PH (FPH) was used as a control. PH differed in their amino acid (AA) profile and molecular weight distribution and therefore the tested microdiets too. The estimated content in FAA and di‐ and tripeptides in the FPH microdiet was 2 g kg^‐1 and 44 g kg^‐1, respectively. FAA estimated levels in YPH and PBPH microdiets were 26 g kg^‐1 and 53 g kg^‐1, whereas levels of di‐ and tripeptides were 30 g kg^‐1 and 60 g kg^‐1, respectively. The estimated levels of FAA in PRBCPH microdiets were 8 g kg^‐1 and 17 g kg^‐1, whereas estimated levels of di‐ and tripeptides were 11 g kg^‐1 and 22 g kg^‐1, respectively. Results revealed that FPH may be replaced by alternative PH from yeast and pig blood products, as fish fed those diets performed, in terms of growth, survival, digestive function and incidence of skeletal deformities, as well as those fed the FPH microdiet. Using YPH, PBPH and PRBCPH, the inclusion level of PH in microdiets might be reduced to a half with respect to current practices using FPH.     

Effects of conventional and organic feed on the mineral composition of cultured European sea bass (Dicentrarchus labrax)

European sea bass (Dicentrarchus labrax) is a widely consumed marine fish in Mediterranean areas, and different farming techniques are applied for fish culturing to satisfy the growing demand for seafood. The aim of this study was to investigate the effects of conventional and organic feed on the quality of cultured European sea bass (Dicentrarchus labrax) collected during the growth period. The concentrations of ash, moisture, essential macro‐elements (Ca, K, Mg, Na and P), micro‐elements (Co, Cr, Cu, Fe, Mn, Ni and Zn) and toxic elements (As, Cd, and Pb) were determined in feeds and in fillets of cultured fish. The results were compared to those obtained from wild sea bass. Results showed that the differences between organic diet‐fed and conventional diet‐fed sea basses varied in relation to the specific element measured and the growth period. The former showed higher concentration of Fe, Mg and Cr, and lower Na content. The amount of P, Na, Fe, Cu and Cr in wild sea bass was significantly (P ≤ 0.05) higher than that found in farmed fish. The levels of toxic elements in cultured sea bass were always within the allowed limit for fishery products. Wild samples had significantly (P ≤ 0.05) higher content of arsenic and lead than farmed sea bass. Cultured fish represent a valuable dietary source of essential macro‐ and micro‐elements. Controlled rearing systems and feedings were related to a decrease in the presence of some toxic metals in cultured fish compared with wild fish.