Incidence and prevalence of Diplectanum aequans and its influence on the fitness of juvenile sea bass (Dicentrarchus labrax) in the Black Sea

High rates of unexplained mortalities (up to 70%) are anticipated in the cultured juvenile sea bass (Dicentrarchus labrax) in farms in the eastern Black Sea at least for the last 8 years. Diplectanum aequans, often blamed for the observed mortalities, is the only prevalent parasite impacting the health of sea bass in the brackish water of the Black Sea. To investigate the seasonal fluctuation of D. aequans prevalence, mean intensities and the potential effects of D. aequans on the fitness of sea bass, eight cages from one farm (Farm A) and four cages at another distinct site (Farm B) were surveyed monthly from May to October of 2008. Six hundred and sixty‐one juvenile sea bass from Farm A and 236 from Farm B were individually examined for the presence and intensity of the parasite. The prevalence of D. aequans in cultured sea bass (96.3 ± 11.11; Mean ± SD) ranged from 66.7% to 100% in June, the beginning of 6‐month growing season. The mean intensity was 6.08 ± 2.19, significantly higher than that (2.74 ± 0.87; P < 0.05) in December (P < 0.05), the end of the growing season. Fish with low fitness had significantly higher number of parasite than the fish with higher fitness (P < 0.005). Although D. aequans negatively influenced host's condition factor, the impact was not enough to lead mortalities in the cultured sea bass in the Black Sea.     

Muscle activity as a key indicator of welfare in farmed European sea bass (Dicentrarchus labrax L. 1758)

Groups of adult sea bass were reared at either low (10 kg m^?3) or high (50 kg m^?3) stocking densities respectively for 84 and 116 days. To monitor the red muscle activity, about 20 fish from both densities were surgically implanted with EMG (Electromyograms) radio transmitters, after EMG calibration during exhaustive swimming exercise (U_crit test). Blood samples and morphometric measurements were also taken. EMG showed that the muscle activity of fish reared at 50 kg m^?3 was on average twofold higher than fish kept at lower density. Cortisol was significantly more elevated at higher density and haemoglobin, haematocrit and RBCC (red blood cells count) showed the same trend, while lysozyme decreased. Patterns for glucose and lactate were less clear. The results showed that the contemporary use of functional (EMG) and physiological (haematological and biochemical) profiles could give a more comprehensive view of the fish status validating the diagnosis of fish stress induced by culture practices.     

Deposition of conjugated linoleic acid in market size sea bass (Dicentrarchus labrax) and its effects on performance,composition and fillet sensory and texture attributes

The effect conjugated linoleic acid (CLA) accumulation in fish fillet on sensory and texture attributes has been evaluated. Fish were fed diets with graded levels of CLA, which were administered for different durations prior to harvest. Fish were sampled for fillet proximate composition and fatty acid profile for sensory and texture parameters at the end of the trial. Growth performance, feed conversion ratio and muscle proximate composition were not affected by dietary treatments. Muscle fatty acid profile was not affected by dietary treatments despite the increase in saturated fatty acids in fish fed diet with 10 g kg⁻¹ of CLA for 16 weeks before harvest. Besides, the deposition levels of CLA isomers increased gradually depending on the CLA treatment, reaching its maximum level in fish fed 10 g kg⁻¹ CLA diet for 20 weeks before harvest. Fish fed 10 g kg⁻¹ were juicier (P < 0.05) compared with control group, whereas fish fed 5 g kg⁻¹ CLA diet were less (P < 0.05) chewy than fish fed control diet regarding sensory attributes. Texture parameters were not significantly affected by dietary CLA. These results suggest that market size European sea bass could successfully incorporate CLA isomers with some minor positive effects on sensory analyses.     

Effects of dietary Bergamot (Citrus bergamia) peel oil on growth,haematology and immune response of European sea bass (Dicentrarchus labrax) juveniles

The aim of the present study was to evaluate the effects of essential oil extracted from bergamot (Citrus bergamia) peel on growth performance and health status of European sea bass juveniles (5.10 + 0.05 g). Test diets supplemented with 0.0%, 0.5%, 1.0% and 2.0% bergamot peel oil (BPO₀, BPO_0.5, BPO₁ and BPO₂) were fed to experimental fish for 60 days. At the end of the experiment, the best growth performance was obtained in the BPO_0.5 and BPO₁ groups (p < .05). No significances were found for haematological parameters including red blood cell count, haematocrit and haemoglobin concentration at the end of the feeding experiment. There were significant differences between the total protein, cholesterol, alkaline phosphatase and glutamic oxaloacetic transaminase (p < .05) between the BPO0.5 and BPO1 supplemented groups and the control group with no BPO addition. The addition of BPO showed significant differences in immune parameters such as lysozyme activity, total myeloperoxidase content, superoxide dismutase and glutathione peroxidase activity between BPO₀ and BPO supplemented groups (p < .05). It might be concluded that dietary addition of 0.5%‐1% BPO could increase the growth performance and feed utilization with improved health status in sea bass juveniles.     

Sea bass Dicentrarchus labrax (L.) bacterial infection and confinement stress acts on F‐type lectin (DlFBL) serum modulation

The F‐lectin, a fucose‐binding protein found from invertebrates to ectothermic vertebrates, is the last lectin family to be discovered. Here, we describe effects of two different types of stressors, bacterial infection and confinement stress, on the modulation of European sea bass Dicentrarchus labrax (L.) F‐lectin (DlFBL), a well‐characterized serum opsonin, using a specific antibody. The infection of the Vibrio alginolyticus bacterial strain increased the total haemagglutinating activity during the 16‐day testing period. The DlFBL value showed an upward regulation on the first, second and last days and underwent a slight downward regulation 4 days post‐challenge. In contrast, the effect of confinement and density stress showed a decrease in the plasma concentration of lectin, ranging from 50% to 60% compared with the control. The modulation of DlFBL is in line with the hypothesis that humoral lectins could be involved and recruited in the initial recognition step of the inflammation, which leads to agglutination, and the activation of mechanisms responsible for killing of the pathogens.     

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 propolis enriched diet on growth performance and plasma biochemical parameters of juvenile European sea bass (Dicentrarchus labrax L.) under acute low‐temperature stress

To study the effect of propolis in crude form (CPP), prepared without any chemical refinement (CPP) on Dicentrarchus labrax under low‐temperature stress, sea bass juveniles were randomly divided into three groups: a control group fed with basal diet and two treatment groups fed with basal diet supplemented with 1.25 and 2.5 g kg⁻¹ of propolis. At the end of a 10‐week feeding trial, sea bass were exposed to low‐temperature stress at 12 °C for 24 h. The growth performance, RNA/DNA ratio and changes in serum biochemical parameters were investigated. Dietary intake of propolis stimulated the specific growth rate (SGR), feed conversion efficiency (FCE), RNA/DNA ratio and alkaline phosphatase (ALP) enzyme activity, while decreasing plasma triglycerides and aspartate aminotransferase (AST) activity. Supplement of 2.5 g kg⁻¹ CPP in diet significantly increased the mean SGR and FCE up to 9% and 13.4%, respectively, in comparison with the control group. Low‐temperature stress elevated serum triglycerides, glucose and cortisol levels in all groups; however, glucose and cortisol reached significantly lower end values in group receiving highest amount of propolis in diet. This study suggests that ingestion of basal diet supplemented with 2.5 g kg⁻¹ of propolis could prevent adverse effects of low‐temperature stress and promote the growth of sea bass.     

Growth parameters,innate immune response and resistance to Listonella (Vibrio) anguillarum of Dicentrarchus labrax fed carvacrol supplemented diets

The research was aimed to assess the effect of dietary carvacrol (0.025% and 0.05%) on sea bass (Dicentrarchus labrax) growth, immune response and resistance to Listonella anguillarum. Fish (69.2 ± 0.22 g) were fed the experimental diets for 9 weeks. Dietary carvacrol did not negatively affect fish survival, growth performance, feed intake and feed conversion ratio (P > 0.05) nor carcass yield and viscerosomatic, hepatosomatic and mesenteric fat index (P > 0.05). Serum and head kidney leucocytes were collected after 1, 4 and 8 weeks of feeding. Carvacrol significantly reduced serum proteins, immunoglobulins and lysozyme activity (P < 0.01) and moderately increased phagocytosis and pinocytosis of head kidney macrophages. The release of reactive oxygen species by leucocytes was reduced in carvacrol‐fed fish, even if significantly (P < 0.05) only in those fed 0.05% carvacrol for 1 week. Dietary carvacrol did not significantly affect the aspecific immune response, although a potential antioxidant activity might be speculated. Moreover, feeding carvacrol provided an appreciable resistance to a challenge with L. anguillarum, when a bacterial dose lower than the Lethal Dose₅₀ was used. Cumulative mortality in fish fed 0.025% carvacrol was significantly lower than that of untreated controls (75% Relative Per cent Survival).     

New perspective to control of tenacibaculosis in sea bass Dicentrarchus labrax L

This work was carried out to investigate the effects of injection of Tenacibaculum maritimum formalin‐killed cells (FKC), extracellular products (ECPs) and crude lipopolysaccharide (LPS) as well as 1% feed supplements of oil extracts of Echinacea purpurea and Origanum vulgare on sea bass immunity improvement to the favour of T. maritimum experimental infection control after 4 weeks of the experiment. Tenacibaculum maritimum isolated from naturally infected sea bass showed brown to yellowish‐brown lesions (sores) on gills, skin and/or fins and identified by different biochemical methods and polymerase chain reaction technique. Immune parameters namely, total protein, globulin and lysozyme activity, as well as the relative level of protection were improved by T. maritimum (FKC), (LPS), (ECPs), O. vulgare and E. purpurea, respectively compared with control. Histopathological examination of T. maritimum naturally infected sea bass indicated many pathological changes in gill, skin and musculatures. Present study could be concluded that application of T. maritimum (FKC), (LPS), (ECPs), O. vulgare or E. purpurea improved sea bass immunity to the favour of disease resistance against T. maritimum. Further investigations on the combination between the previous control methods and the vaccine application methods will be needed.     

Comparative pathogenicity study of ten different betanodavirus strains in experimentally infected European sea bass,Dicentrarchus labrax (L.)

Viral encephalopathy and retinopathy (VER), otherwise known as viral nervous necrosis (VNN), is a severe pathological condition caused by RNA viruses belonging to the Nodaviridae family, genus Betanodavirus. The disease, described in more than 50 fish species worldwide, is considered as the most serious viral threat affecting marine farmed species in the Mediterranean region, thus representing one of the bottlenecks for further development of the aquaculture industry. To date, four different genotypes have been identified, namely red‐spotted grouper nervous necrosis virus (RGNNV), striped jack nervous necrosis virus (SJNNV), tiger puffer nervous necrosis virus and barfin flounder nervous necrosis virus, with the RGNNV genotype appearing as the most widespread in the Mediterranean region, although SJNNV‐type strains and reassortant viruses have also been reported. The existence of these genetically different strains could be the reason for the differences in mortality observed in the field. However, very little experimental data are available on the pathogenicity of these viruses in farmed fish. Therefore, in this study, the pathogenicity of 10 isolates has been assessed with an in vivo trial. The investigation was conducted using the European sea bass, the first target fish species for the disease in the Mediterranean basin. Naive fish were challenged by immersion and clinical signs and mortality were recorded for 68 days; furthermore, samples collected at selected time points were analysed to evaluate the development of the infection. Finally, survivors were weighed to estimate the growth reduction. The statistically supported results obtained in this study demonstrated different pathogenicity patterns, underlined the potential risk represented by different strains in the transmission of the infection to highly susceptible species and highlighted the indirect damage caused by a clinical outbreak of VER/VNN.     

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.     

Appraisal of a high protein distiller’s dried grain (DDG) in diets for European sea bass,Dicentrarchus labrax fingerlings on growth performance,haematological status and related gut histology

High protein distiller's dried grains (HP‐DDG) are co‐products of ethanol production that uses prefractionation technology. An 8‐week growth trial was conducted to investigate the effect of partial replacement of soybean meal (SBM) by three levels of HP‐DDG (30%, 40% and 50%) on growth performance, physiological parameters and histological changes of the intestine of European sea bass, Dicentrarchus labrax. The results indicated that an increased dietary level of HP‐DDG of more than 30% significantly increases growth performance and improved the FCR of sea bass. In addition, replacement of SBM by HP‐DDG enhanced feed intake efficiency and the health status of fish. Haematology and serum biochemistry (haemoglobin (Hb), red blood cells (RBCs), white blood cells (WBCs), packed cell volume (PCV %) and humeral immune parameters including total protein, albumin, globulin, cholesterol, lysozyme activity and total antioxidant capacity significantly increased with increase HP‐DDG inclusion levels. The findings of this study indicated that HP‐DDG is a good complementary protein source for inclusion aquaculture diets and levels above 30% as a replacement of SBM did not compromise growth performance and physiological parameters of sea bass while enhancing some important indices of health status in this species.     

Total nitrogen,carbon and phosphorus digestibility in gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) fed with conventional and organic commercial feeds: implications for particulate waste production

Organic fish farming is postulated as a more environmentally friendly activity than conventional activities. However, environmental sustainability as regards waste output has not been addressed. In this respect, the most important environmental problems associated with fish farming are those related with the discharge of particulate wastes. In this study, total nitrogen, carbon and phosphorus digestibility was estimated in a wide size range of gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) fed a commercially available conventional diet and an organic feed, as a previous step to assessing particulate waste output. Neither fish size nor feed nutrient content was seen to influence nutrient digestibility in either species. The organic diet provided higher carbon but lower nitrogen and phosphorus digestibility than conventional feed in both species. Simulations under real ongrowing conditions resulted in lower values for nitrogen and higher values for carbon particulate wastes when conventional feed was supplied to both species. Particulate phosphorus wastes were 2.6‐ to 3.4‐fold higher for fish fed the organic diet. Restrictions concerning NO the raw materials used for organic diet formulation and differences in nutrient bioavailability between the diets seemed to be the underlying reason for the differences found.     

Antibody response in sea bass (Dicentrarchus labrax L.) in relation to water temperature and oxygenation

The influence of water temperature and water oxygenation on the specific antibody response was evaluated by indirect ELISA in sea bass (Dicentrarchus labrax L.) immunized against human‐γ‐ globulins emulsified in Freund's complete adjuvant. A higher antibody response was observed in fish reared at 24 and 30 °C, than at 12 and 18 °C. For the fish group reared at 24 °C, the immunological observation was carried out until 300 days after immunization. At the end of this period, fish kept at 24 °C still showed immunological competence, although the antibody response began to decrease significantly from 120 days after the immunization. Fish reared under mild hyperoxia and normoxia conditions had higher antibody responses than fish reared under mild hypoxia conditions. Moreover, at the end of the experimental period of 56 days the antibody response of fish reared under hyperoxia conditions was significantly higher than the immune response detected in fish reared under normoxia and hypoxia conditions.     

Fertilization strategies for Sea Bass Dicentrarchus labrax (Linnaeus, 1758): effects of pre‐incubation and duration of egg receptivity in seawater

Studying gamete biology can provide important information about a species fertilization strategy as well as their reproductive ecology. Currently, there is a lack of knowledge about how long sea bass Dicentrarchus labrax eggs can remain viable after being activated in seawater. The objectives of this study were to understand the effects of pre‐incubation of fresh and overripe sea bass eggs in seawater and to determine the duration of egg receptivity. Pooled eggs (fresh and overripe) from four females were pre‐incubated in seawater for 0 min (control), 0.5 min, 1 min, 3 min, 10 min and 30 min and then fertilized by pooled sperm from four males. The fresh eggs had a higher fertilization success than overripe eggs. Our results revealed a significant effect of pre‐incubation time for both the fresh (P < 0.01) and overripe eggs (P < 0.01). Fertilization success of eggs significantly declined for both these treatments after 3 min of pre‐incubation, which clearly indicates that sea bass eggs are able to be fertilized by sperm for up to 3 min after release into seawater. This study has particular importance for understanding fertilization strategies, reproductive potential, as well as reproductive ecology of sea bass.     

Effects of different carotenoid supplementation sources with or without sodium taurocholate on growth,feed utilization,carotenoid content and antioxidant status in fry of the European seabass,Dicentrarchus labrax

A total of 210 European seabass fry with an average body weight of 0.51 ± 0.03 g were divided into seven experimental treatments (three replicates per treatment). The seven experimental diets (45.64% CP) were formulated to contain synthetic astaxanthin (SA; 0.1 g kg^?1), marigold flower meal (MFM; 0.2 g kg^?1) or crab waste meal (CM; 10 g kg^?1), with or without sodium taurocholate (ST; 0.4 g kg^?1); the control diet contained no supplementation. The experiment was conducted for 8 weeks. The results show that seabass fry fed the diets containing different sources of carotenoids, especially with ST, demonstrated higher growth efficiency, survival (%), feed efficiency, protein and ether extract contents of fish body. The best results were found with Diet_MFM+ST. Whole‐body carotenoid levels increased significantly with all supplementation treatments; the highest level was observed with SA, followed by fish fed Diet_MFM and Diet_CM, especially with ST co‐supplementation. Moreover, thiobarbituric acid‐reactive substances were significantly reduced in fish fed the diets containing different sources of carotenoids compared to the control diet. The total antioxidant status significantly increased in fish fed Diet_SA and Diet_CM alone or with ST compared to the control group. In conclusion, these results reveal that Diet_MFM+ST is the most suitable treatment for improving growth, feed efficiency, and the protein and ether extract contents of seabass fry compared to other sources of carotenoids. The best oxidant/antioxidant balance and carotenoid content were achieved using SA, CM and MFM with ST supplementation.