Labrax Fry Research Articles

Use of Natural Zeolites as a Detoxifier of Heavy Metals in Water and the Flesh of Reared European Seabass Dicentrarchus labrax

Abstract A study was conducted to investigate the effects of natural zeolites as a water clarifier on the heavy metal removal efficiency from the underground saltwater used for rearing Dicentrarchus labrax fry. Five concentrations of zeolites were tested: 0 (Z0), 2.5‰ (Z2.5), 5‰ (Z5), 7.5‰ (Z7.5) and 10‰ (Z10). Fry with an initial body weight of 1.53±0.018 g/fish were stocked in 15 aquaria at a density of 10 fry/aquarium. The fish were fed a commercial diet (42% protein and 12.34% lipid) twice daily (09:30 and 14:00) at 5% of their body weight per day for 42 days. Growth, feed utilization, survival and heavy metal removal efficiency were evaluated. The growth performance and feed utilization indices gradually improved with increasing zeolite concentration, with the most significant (P≤0.05) values detected at Z10. The survival rate decreased significantly at Z10 compared with the control (Z0). Increasing the zeolite concentration significantly (P≤0.05) improved the removal efficiency of heavy metals in the rearing water with adsorption selectivity of Pb˃Cd˃Fe˃Cu˃Zn. Furthermore, an increase in the detoxification rate of heavy metals in fish flesh with increasing zeolite level was detected with the removal selectivity of Fe˃Cu˃Zn˃Pb˃Cd. In conclusion, it can be stated that natural zeolites can be used effectively to reduce heavy metals in polluted waters and subsequently in fish flesh in addition to improving fish performance.

Amyloodiniosis in cultured Dicentrarchus labrax: parasitological and molecular diagnosis, and an improved treatment protocol

Amyloodinium ocellatum, the causative agent of amyloodiniosis (marine velvet, velvet disease), affects marine and brackish fish in various warm and temperate habitats. We recorded disease outbreaks with high morbidity and mortality rates in marine-cultured European seabass Dicentrarchus labrax fry at 2 locations in northwest Egypt. The sudden outbreak, high morbidity and mortality rates, and skin lesions with a velvety appearance in affected fish all indicated A. ocellatum infection. This was further confirmed by microscopic findings of the parasitic stage (trophonts) in skin and gill smears. While ecological factors including water temperature and salinity were all amenable to parasite establishment and propagation, mortality rates differed between the 2 farms, with rates of mortality well correlated with prevalence and intensity of A. ocellatum infections. Characterization by PCR targeting rDNA gene fragments and subsequent DNA sequencing and phylogenetic analysis further confirmed the molecular identity of the A. ocellatum isolate, which was genetically similar to isolates from other geographical locations. Finally, an improved treatment method using dual hyposalination and copper sulfate exposure to increase the efficiency and decrease the toxicity of copper sulfate was tested. The gradual reduction in water salinity coupled with copper sulfate treatment was more efficient at controlling the disease than only applying copper sulfate. To our knowledge, this is the first parasitological and molecular characterization of A. ocellatum in marine cultures in Egypt. The high molecular identity and close phylogenetic relationship further confirmed the monophyletic nature of A. ocellatum isolates.

Isolation of Lacinutrix venerupis strains associated with disease outbreaks in sea bream Sparus aurata and European sea bass Dicentrarchus labrax.

Four Gram-negative bacterial isolates were recovered from 2 disease outbreaks that occurred in 2013 affecting European sea bass Dicentrarchus labrax fry and sea bream Sparus aurata adults. Main symptoms were erratic swimming, eroded fins and, in the sea bream outbreak, haemorrhages on the body surface; bacteria were always recovered from internal organs, almost in pure culture. On the basis of phenotypic characterization and 16S rRNA gene sequence analysis, the isolates were identified as Lacinutrix venerupis, a bacterium not previously reported as a fish pathogen. The highest 16S rDNA sequence similarities were recorded with the type strain of this species (99.9-100% similarity), while other species showed similarities below 97%, the closest relative being L. mariniflava (96.3% similarity). Phenotypic characterization showed some discrepancies with the L. venerupis type strain (mainly in BIOLOG GN profile); however, DNA-DNA hybridization assays with L. venerupis and L. mariniflava type strains confirmed that these isolates belong to the former species (levels of DNA relatedness were 98-100% and 38-50%, respectively). Finally, a virulence evaluation of the isolates using Senegalese sole Solea senegalensis fry was also performed; significant mortalities (80-100% mortality within 4 d) were recorded after intraperitoneal injection, but only with high doses of bacteria (107colony forming units fish-1). Further studies will be necessary to determine the importance of this species as a fish pathogen.

Pterocladia (Rhodophyta) and Ulva (Chlorophyta) as feed supplements for European seabass, Dicentrarchus labrax L., fry

The aim of this study was to evaluate the suitability of two marine macroalgae, Ulva lactuca (Chlorophyta) and Pterocladia capillacea (Rhodophyta), meals as a supplement to enhance the nutritional value of formulated feeds for European seabass Dicentrarchus labrax fry. Seven isoproteic (50 % crude protein), isocaloric (500 Kcal/100 g gross energy) diets containing four levels (0 or control, 5, 10, and 15 %) of either Ulva meal (UM) or Pterocladia meal (PM) were tested. Each diet was fed to triplicate groups of D. labrax fry (initial body weight, 0.23 ± 0.02 g for Ulva- and 0.14 ± 0.01 g for Pterocladia-fed fish), to apparent visual satiety for 8 weeks. The results indicated that feeding seabass at 5 % UM or PM level (U5 and P5 diets) produced the best growth, feed utilization, nutrient retention, and survival rates among all the dietary groups. Feeding fish with a 5 % PM-added diet has also improved stress response after a 5-min air exposure test, prior to the termination of the feeding trial. These findings suggest that both Pterocladia and Ulva meals could be potentially used as an additional feed component (at 5 %) for enhancement of seabass D. labrax fry performance, nutrients composition, and stress resistance especially when subjected to transportation from hatchery to weaning ponds/tanks.

Effect of dietary taurine supplementation on growth performance and feed selection of sea bass Dicentrarchus labrax fry fed with demand-feeders

: Two series of experiments were conducted for the evaluation of the effect of dietary taurine supplementation on survival, growth performance and feed preference of sea bass Dicentrarchus labrax fry when fed with demand-feeders a fish/soybean meal-based diet. In the first experiment, fish were imposed one of four diets supplemented with taurine (0, 0.1, 0.2 or 0.3% taurine on a dry weight basis), while in the second they had simultaneous free access to the four diets supplemented with taurine. When not allowed to choose, fish exhibited better specific growth rate of 3.2 and 3.2% per day with diets supplemented with 0.2 and 0.3% taurine than those of 2.3 and 2.2% per day of 0 and 0.1% taurine, respectively. When fish were free to access the diets, they selected more actively the 0.2 and 0.3% taurine-supplemented diets than the 0 and 0.1% taurine. As survival was not significantly different among trials, the results indicate that sea bass fry require a 0.2% taurine in the diet for better growth when fish meal and soybean meal are the primary sources of protein. The positive correlation between growth performance and nutrient preference is discussed as a method for improvement of fry food formulation.

A photographic larval fish counting technique: comparison with other methods, statistical appraisal of the procedure and practical use

The problems involved in counting small or fragile live aquatic organisms have not yet been entirely overcome at hatcheries. We describe a counting method based on photographing a whole population and show it to be four to ten times more precise than the usual weighing or hand counting methods. A simple photographic system was designed to fit the requirements of the method. For large populations, formulae are given to calculate population and sample sizes according to the precision required and using stratified random sampling of fractions of each photograph. No statistically significant differences were observed when single populations were counted by several operators. The mean human counting error was about 1%. Actual counts of Dicentrarchus labrax fry (mean weight 30 mg) are used to illustrate the statistical developments. From one sample representing 17.5% of the total population, three inexpert operators were able to quantify a population of about 120000 fry, with ± 1.7% precision, in 3 h.