Juvenile Flounder Research Articles

Otolith microchemistry identifies shallow, intertidal-dominated estuaries as important nurseries for sand flounder in New Zealand

Estuaries provide critical habitat, food, and refuge for juvenile fish. Elemental concentrations in otoliths can retrospectively identify estuarine nursery habitats that contribute disproportionately to adult populations, providing valuable information for fisheries and coastal ecosystems. The present study aimed to (1) compare elemental signatures of juvenile (age 0+) sand flounder Rhombosolea plebeia otoliths collected from 9 estuaries in east Otago, New Zealand, and (2) assess the application of otolith microchemistry for identifying the nursery habitats contributing to adult (age 1+) sand flounder from the coastal shelf population. Otoliths from juvenile flounder were collected in November 2020, and adults of the same cohort were collected in the winter/spring of 2022. Laser ablation inductively coupled plasma mass spectrometry quantified 12 element concentrations within the post-settlement region of juvenile and adult otoliths using depth-profiling and ablation transects, respectively. Significant differences in post-settlement elemental concentrations were detected among estuaries. Juvenile and adult sand flounder were classified to their nursery origin using canonical analysis of principal coordinates. Grouping estuaries by the New Zealand Estuary Trophic Index classification scheme improved the overall juvenile classification accuracy from 56.7% (51/90) to 82.2% (74/90). Adult sand flounder (n = 78) were classified to have originated from shallow intertidal dominated estuaries (62%), followed by Otago Harbour (32%) and shallow, short residence time river and tidal river with adjoining lagoon estuaries (6%). The results highlight how otolith microchemistry can provide information on the use of critical estuarine nursery habitats for a commercially important species.

Analysis of the Skin and Brain Transcriptome of Normally Pigmented and Pseudo-Albino Southern Flounder (Paralichthys lethostigma) Juveniles to Study the Molecular Mechanisms of Hypopigmentation and Its Implications for Species Survival in the Natural Environment.

Southern flounder skin pigmentation is a critical phenotypic characteristic for this species' survival in the natural environment. Normal pigmentation allows rapid changes of color for concealment to capture prey and UV light protection. In contrast, highly visible hypopigmented pseudo-albinos exhibit a compromised immune system and are vulnerable to predation, sensitive to UV exposure, and likely have poor survival in the wild. Skin and brain tissue samples from normally pigmented and hypopigmented individuals were analyzed with next-generation RNA sequencing. A total of 1,589,613 transcripts were used to identify 952,825 genes to assemble a de novo transcriptome, with 99.43% of genes mapped to the assembly. Differential gene expression and gene enrichment analysis of contrasting tissues and phenotypes revealed that pseudo-albino individuals appeared more susceptible to environmental stress, UV light exposure, hypoxia, and osmotic stress. The pseudo-albinos' restricted immune response showed upregulated genes linked to cancer development, signaling and response, skin tissue formation, regeneration, and healing. The data indicate that a modified skin collagen structure likely affects melanocyte differentiation and distribution, generating the pseudo-albino phenotype. In addition, the comparison of the brain transcriptome revealed changes in myelination and melanocyte stem cell activity, which may indicate modified brain function, reduced melanocyte migration, and impaired vision.

Effects of constant and variable salinity regimes on the occurrence, feeding, and growth of two juvenile flatfishes (Pseudopleuronectes yokohamae and Platichthys bicoloratus)

Extreme environmental conditions, such as high and low salinity, are stressors for marine fish growth. Juveniles of marbled flounder Pseudopleuronectes yokohamae and stone flounder Platichthys bicoloratus use estuaries as nursery grounds. Our field surveys showed that the occurrence of juvenile stone flounder seemed to be unrelated to salinity, whereas that of juvenile marbled flounder was positively correlated with salinity. To examine the effects of salinity on feeding and growth of these species, we conducted a laboratory experiment in which juveniles were reared individually under constant and variable salinity regimes for seven days. Cultured marbled flounder of two sizes (23–29 and 48–58 mm standard length) were reared under constant salinities of 5, 15, and 30 and fluctuating salinities of 5–30 and 15–30 or 5–15. Wild stone flounder (23–37 mm standard length) were reared under constant salinities of 1, 5, and 30 and fluctuating salinities of 5–30 and 1–30. Commercial pellets were fed to fish twice daily until satiation. The feeding and growth of marbled and stone flounders were considerably low at constant salinities of 5 and 1, respectively. Meanwhile, suppression of feeding and growth was not observed in fish with salinities varying from 5 to 30 or 1 to 30, respectively. The results indicated that extremely low salinity inhibits feeding and growth of juveniles only with continuous exposure. Estuaries with fluctuating salinities are potential nursery grounds for both marbled flounder and stone flounder, although marbled flounder tend to avoid low-salinity environments in the field. This study showed that extremely low-salinity environments, which reduce juvenile growth when consistently exposed, would not affect distribution or growth if they were temporary around low tide.

Relevance of flounder caging and proteomics to explore the impact of a major industrial accident caused by fire on the Seine estuarine water quality

On September 26th 2019, a major fire occurred in the Lubrizol factory located near the Seine estuary, in Rouen-France. Juvenile flounders were captured in the Canche estuary (a reference system) and caged one month in the Canche and in the Seine downstream the accident site. No significant increases of PAHs, PCBs and PFAS was detected in Seine vs Canche sediments after the accident, but a significant increase of dioxins and furans was observed in water and sewage sludge in the Rouen wastewater treatment plant. The proteomics approach highlighted a dysregulation of proteins associated with cholesterol synthesis and lipid metabolism, in fish caged in the Seine. The overall results suggested that the fire produced air borne dioxins and furans that got deposited on soil and subsequently entered in the Seine estuarine waters via runoff; thus contaminating fish preys and caged flounders in the Seine estuary.

Morphometric and meristic discrimination within juvenile flounder species in New Zealand indicate genetic and environmental disparities

ABSTRACT Morphometrics and meristics are measurable and countable traits, respectively, that are under simultaneous control by genetic and/or environmental factors. Resolving morphometric and meristic spatial variation can be a useful technique for identifying population structure and connectivity. The present study investigated morphometric and meristic character variation within three flounder species, sand flounder (Rhombosolea plebeia), greenback flounder (R. tapirina), and yellowbelly flounder (R. leporina) in New Zealand, predominantly in the South Island. Four morphometric measurements and two meristic characters were obtained from 857 juvenile flounder, standardised, and analysed. Consistent with a previous genetic analysis, the differentiation of morphometric and meristic characters supported the idea that sand flounder populations north and south of 42o S were reproductively isolated, and although statistically non-significant, there was an indication that yellowbelly flounder collected from Lake Ellesmere and Orowaiti Lagoon may have been reproductively isolated. Only juvenile sand flounder and greenback flounder were collected in the tidal reaches of estuaries throughout east Otago and the populations are considered undifferentiated based on intra-species morphometric and meristic characters. Our results demonstrated that the influence of environmental variation and adaptive physiology of juvenile flounder likely contributed to the morphometric and meristic variation of individuals among estuaries and years.

Actual State of Population and Harvesting of the Baltic Shrimp Palaemon adspersus (Decapoda, Palaemonidae) in the North-Western Section of the Black Sea

Data on the sex and size-mass composition of the Baltic shrimp <i>Palaemon adspersus</i> Rathke, 1837, population were obtained basing on the analysis of samples from 35 fyke net catches in the north-western section of the Black Sea over 2017-2020. The Baltic shrimp in the catches was represented by the specimens 20-65 mm long, and the main portion (52%) was formed by the specimens 40-50 mm long. According to LBB model, over 2017-2020 the relative fishing mortality level increased from 0.7 to 1.2. At the same time, in 2020 relation of the actual biomass to the biomass ensuring the maximum sustainable yield <i>(B/B</i><sub>msy</sub>) was equal to 1.0, indicating the optimal intensity of the Baltic shrimp harvesting in the Ukrainian waters. According to the model calculations, the minimal allowed length for harvesting should be 35 mm. Analysis of the fyke nets catches revealed significant number of undersized specimens of the commercial and protected fishes in the bycatch, including juveniles of gobies, turbot and flounder. Actual harvesting intensity do not result in <i>P. adspersus</i> overfishing in Ukrainian waters. In order to mitigate the negative impact of shrimp harvesting on the non-target species, it is necessary to regulate it by limitation of the fishing gears number and optimization of their designs. It was revealed that the fyke nets number in the Ukrainian waters should be limited to about 800 units.

European flounder foraging movements in an estuarine nursery seascape inferred from otolith microchemistry and stable isotopes

Despite the importance of estuarine nurseries in the regulation of many fish stocks, temporal and spatial movements and habitat use patterns of juvenile fish remain poorly understood. Overall, combining several movement metrics allowed us to characterize dispersal patterns of juvenile flounder, Platichthys flesus, along an estuarine seascape. Specifically, we investigated otolith microchemistry signatures (Sr:Ca and Ba:Ca ratios) and stable isotope ratios (δ13C and δ15N) in muscles of these juveniles, during three consecutive years to assess inter-annual fluctuations in their home range and isotopic niches. The morphological condition and lipid content of individuals were lower in years of high as compared to low dispersal along the estuarine gradient. We discuss these results in relation to the ecosystem productivity and intra- and inter-specific competition level, which in turn affects movements and foraging behaviors of juvenile flounders.

PCO2-driven seawater acidification affects aqueous-phase copper toxicity in juvenile flounder Paralichthys olivaceus: Metal accumulation, antioxidant defenses and detoxification in livers

Ocean acidification potentially influences the biotoxicity of metals and the antioxidant defense systems of marine organisms. This study investigated how pCO2-driven seawater acidification (SA) affected aqueous-phase copper (Cu) toxicity in the juvenile flounder Paralichthys olivaceus from the perspective of hepatic oxidative stress and damage to better understand the mechanisms underlying the biological effects produced by the two stressors. Fish were exposed to aqueous-phase Cu at relevant ambient and polluted concentrations (0, 5, 10, 50, 100 and 200 μg L−1) at different pH levels (no SA: pH 8.10; moderate SA: pH 7.70, pCO2 ∼1353.89 μatm; extreme SA: pH 7.30, pCO2 ∼3471.27 μatm) for 28 days. A battery of biomarkers in the livers was examined to investigate their roles in antioxidant defense and detoxification in response to coexposure. Hepatic Cu accumulation (30.22–184.90 mg kg−1) was positively correlated with Cu concentrations. The biomarkers responded adaptively to different redox states following SA and Cu exposure. In unacidified seawater, increases in Cu concentrations significantly induced hepatic lipid peroxidation (LPO, by up to 27.03 %), although compensatory responses in antioxidant defenses and detoxification were activated. Moderate SA helped maintain hepatic redox homeostasis and alleviated LPO through different defense strategies, depending on Cu concentrations. Under extreme SA, antioxidant-based defenses were activated to cope with oxidative stress at ambient-low Cu concentrations but failed to defend against Cu toxicity at polluted Cu levels, and LPO (by up to 63.90 %) was significantly induced. Additionally, thiols (GSH and MT) responded actively to cope with Cu toxicity under SA. SOD, CAT, EROD, and GST were also sensitively involved in defending against hepatic oxidative stress during coexposure. These findings highlight the notable interactive effects of SA and Cu and provide a basis for understanding antioxidant-based defenses in marine fish confronting environmental challenges.

Bioaccumulation of trace metal elements and biomarker responses in caged juvenile flounder at a polluted site: Effects of fish density and time exposure

This study investigates the effect of fish density and exposure duration on trace metal elements (TME) bioaccumulation and several biomarkers response. Juvenile flounders were caged at low, medium and high densities and exposed during 15 or 30 days in the Seine estuary. The concentrations of the TME measured in the muscle of the caged fish were all in agreement with their bioavailability percentage in the sediments. Higher concentrations of TME were found in flounders' muscle exposed for 15 days compared with those caged for 30 days. For the same exposure time, the density of fish had no effect on the accumulation of the TME in the flounders' muscle. Biomarkers responses varied according to density and duration of exposure. Special care should be taken in their interpretation. We underline that for an optimal assessment of TME pollution in the field, 15 days with low densities of fish per cage are sufficient.

Metabolomic and transcriptomic analyses reveal response mechanisms of juvenile flounder (Paralichthys olivaceus) to sublethal methylmercury

Although methylmercury (MeHg) has been recognized as a typical heavy metal posing huge damages to various life processes of fish, the response mechanisms of marine fish at early life stages (ELSs) to MeHg is still poorly understood. In this study, non-targeted liquid chromatography-mass spectrometry (LC-MS) based metabolomic and transcriptomic approaches were used to explore response mechanisms of juvenile flounder (Paralichthys olivaceus) to long-term sublethal MeHg exposure (0 and 1.0 μg L-1; 30 d). After exposure, growth parameters of flounder were significantly decreased. Metabolomic and transcriptomic analyses of liver tissue showed obvious difference about biological pathways and identified biomarkers (around 2502 genes and 16 secondary metabolites). Those significantly differentially expressed genes (DEGs) and their enriched pathways were mainly related to immune response, oxidative stress, lipids metabolism, glycometabolism, amino acid and nucleotide metabolism and regulation of protein processes, while those identified secondary metabolites were mainly enriched in tryptophan metabolism, biosynthesis of unsaturated fatty acids, linoleic acid metabolism and glutathione metabolism. Additionally, multi-omic method was used to explore response mechanisms of key pathways under MeHg stress. In this regard, only 57 DEGs and 6 secondary metabolites were significantly enriched in 7 pathways to constitute an integrated regulatory network, including glutathione metabolism, thyroid hormone synthesis, linoleic acid metabolism, biosynthesis of unsaturated fatty acids, tryptophan metabolism pathway, serotonergic synapse and African trypanosomiasis. Above all, we could speculate that antioxidative function, lipids metabolism, nervous system and amino acid metabolism were the more sensitive targets in response to MeHg stress, which were conductive to deeply understand the response mechanisms of fish at ELSs under MeHg exposure. Those identified biomarkers could also be widely used for toxicological studies of pollutants and ecological risks monitoring.

Expression of insulin‐like growth factor genes in olive flounder, Paralichthys olivaceus, fed a diet with partial replacement of dietary fish meal

AbstractThis study analyzed the expression of growth‐related genes of olive flounder, Paralichthys olivaceus, fed a low‐fish meal (FM) diet to investigate the replacement of the FM diet in the field. The expression of growth hormone (GH)/insulin‐like growth factor (IGF) axis genes in the brain/pituitary/liver and plasma hormone concentrations were measured. A basal experimental diet was formulated using 63% FM and 12% soybean meal as the primary protein sources, and then two other diets were made by replacing 20% and 30% of the FM with soy protein concentrate, tankage meal, and poultry by‐products meal. Each diet was fed to duplicate groups of juvenile flounder (150 ± 3.0 g) twice a day. After 20 weeks, the plasma growth‐related hormone concentrations were similar between the control and FM20 groups. Moreover, GH/IGF axis gene mRNA expression in the brain/pituitary/liver was similar between the control and FM20 groups. Immunoblotting of muscle and liver showed the same pattern. This study showed the utility of replacing 20% of the FM diet and supports the possibility of field application.

Seasonal Dynamics of Faunal Diversity and Population Ecology in an Estuarine Seagrass Bed

Biodiversity is important for communities to be resilient to a changing world, but patterns of diversity fluctuate naturally over time. Understanding these shifts — and the species driving community dynamics — is crucial for informing future ecological research and conservation management plans. We investigated the impacts of seasonality, small-scale changes in seagrass cover, and small-scale spatial location on the epifaunal communities occupying a temperate seagrass bed in the South Island of New Zealand. By sampling epifaunal communities using a fine-mesh push net two to three times per season for 1 year, and using a combination of multivariate and hierarchical diversity analyses, we discovered that season, seagrass cover, and the location within the bay, and their interactions, explained 88.5% of the variation in community composition. Community composition and abundances, but not numbers, of species changed over seasons. The most common taxa were commercially important Caridean shrimp and juvenile flounder (Rhombosolea spp.), and both decreased in abundance in summer (shrimp: 1.40/m2 in winter to 0.80/m2 in summer; flounder: 0.15/m2 in winter to 0.01/m2 in summer). Other commercially important species were captured as juveniles, including blue cod (Parapercis colias), kahawai (Arripis trutta), and whitebait (Galaxias spp.). The only adult fish captured in the study were two pipefish species (Stigmatopora nigra and Leptonotus elevatus), which had distinctly seasonal breeding patterns, with reproductively active adults most likely to be found in the spring and fall. Our study highlights the importance of estimating biodiversity parameters based on sampling throughout the year, as some species will be overlooked. We demonstrate that the temperate estuarine seagrass-affiliated animal communities differ in response to season and fine-scale local environments, causing fluctuations in biodiversity throughout the year.

A new strategy in rearing of European flounder: using live Enchytraeus albidus to enhance juvenile growth

This study reports the use of the annelid worm Enchytraeus albidus as a live feed for reared juvenile European flounder, Platichthys flesus. We investigated growth, lipid composition and mortality in small scale (hundreds of fish) and large scale (thousands of fish) rearing. Live Enchytraeus had a relatively low content of the essential omega-3 fatty acid, docosahexaenoic acid (DHA), but similar levels of other essential fatty acids and amino acids as the normally used commercial dry feed. Despite low DHA levels in live worms, juvenile flounder fed Enchytraeus grew twice as fast as fish fed commercial dry feed. DHA levels in membrane phospholipids of juvenile flounder became depleted when fed Enchytraeus, but this had no apparent negative consequences for development or health of the fish. Our study suggests that the use of Enchytraeus as a live feed for European flounder is feasible at a large scale.

Changes in functioning of the largest coastal North Sea flatfish nursery, the Wadden Sea, over the past half century

The international Wadden Sea is an important flatfish nursery. Information from the Dutch Wadden Sea indicates that the flatfish nursery function of the area has been affected during the last decades. Increased seawater temperature has affected settling, habitat suitability for and growth performance of the various flatfish species. Settling of plaice, flounder and to a lesser extent sole larvae occurs earlier nowadays. In the 1960s, 0-, I-, II- and III-group plaice were present, but since 2000, II-group has disappeared and densities of I-group have decreased. For juvenile flounder, II-group almost disappeared, and for dab, a decline in densities of all age groups was observed from the 1990s onwards. Summer temperatures exceed the optimum for the cold-water species (plaice, flounder and dab) with increasing frequency, level and duration. Only for 0-group sole, the period with optimal growth conditions has become longer and has resulted in increased growth. Mortality rates in 0-group plaice have increased, coinciding with an increase in water temperatures and an increase in the abundance of predators. The decrease in density of juvenile plaice and dab in the Wadden Sea has not affected recruitment to North Sea stocks, suggesting that other areas have taken over part of the nursery function. The predicted increase in seawater temperature in the next decades will continue to improve the conditions for sole. The temperature tolerance of plaice and dab and to a lesser extent flounder will further reduce their scope for growth and may ultimately result in their disappearance from the Wadden Sea.

Mealworm, Tenebrio molitor, as a feed ingredient for juvenile olive flounder, Paralichthys olivaceus

An 8-week feeding trial was designed to evaluate the potential of mealworm (Tenebrio molitor) as a locally available unconventional source of protein in a practical diet for olive flounder (Paralichthys olivaceus). Three hundred juveniles (initial weight, 33.5 ± 0.01 g) were randomly distributed into five dietary groups in triplicate (20 fish per tank), and each group was hand-fed one of the experimental diets containing a graded level of a mealworm meal (MW) replacing 0, 20, 40, 60, and 80 % of fish meal (FM) protein (designated as MW0, MW20, MW40, MW60, and MW80). The specific growth rates of juvenile flounders revealed a decreasing trend with increasing dietary MW levels. However, feed utilization efficiency tended to improve with the increasing percentage of dietary FM replacement up to 40 % but gradually decreased in higher replacement levels. The results displayed a linear and quadratic negative effect of dietary MW inclusion level on nutrient digestibility. A significant negative linear trend was detected between dietary MW content and plasma total cholesterol. Except for lipid content, which displayed a significant decreasing trend as the dietary MW content increased, no other changes were evident in the fillet proximate composition. Significant positive linear and quadratic trends were observed between fillet C18:1n-9 (oleic acid; OA) and C18:2n-6 (linoleic acid; LA) levels and dietary MW content, where the OA and LA values increased in a dose-dependent manner. The fillet Σn3 / Σn6 ratio and atherogenic index demonstrated significant decreasing trends with dietary inclusion of MW while the fillet thrombogenic index and hypocholesterolaemic/hypercholesterolaemic fatty acid ratio exhibited significant positive trends with increasing values. Myeloperoxidase activity declined with replacement of up to 40 % FM protein but thereafter gradually increased at higher replacement levels. Glutathione peroxidase activity generally was enhanced with increasing dietary MW inclusion. Dietary MW inclusion had no significant impact on the bacterial community of the juvenile flounder’s hindgut. Although replacing up to 80 % of FM with MW may promote an immunostimulatory effect, in order to avoid unfavorable effects on feed efficiency and nutritional value of the fillet lipid fraction no more than 40 % of FM is recommended to be replaced by a MW in diets for olive flounder juveniles. Our findings could promote the utilization of MW in olive flounder feed, which may not only reduce dependence on high priced and finite feed ingredients, but also enhance farmed fish health status.

Nutritional evaluation of cricket, Gryllus bimaculatus, meal as fish meal substitute for olive flounder, Paralichthys olivaceus, juveniles

AbstractAn 8‐week feeding trial was designed to evaluate the potential of cricket meal, Gryllus bimaculatus, as a locally available unconventional source of protein in a practical diet for olive flounder, Paralichthys olivaceus. Three hundred juvenile fish (Initial weight, 33.5 ± 0.01 g) were randomly distributed into five dietary groups in triplicates (20 fish per tank) and each group was hand‐fed one of the experimental diets containing graded level of a cricket meal (CR) replacing 0, 20, 40, 60, and 80% of fish meal (FM) protein (designated as CR0, CR20, CR40, CR60, and CR80). Although replacing more than 40% of dietary FM protein resulted in lower growth rates and feed utilization efficiency, juvenile flounders fed the highest level of dietary CR (CR80) still performed as well as control group (CR0). Replacing more than 20% of FM with CR markedly reduced apparent nutrient digestibility in fish. Plasma total cholesterol concentration showed decreasing trend with increasing inclusion levels of CR in the diet. Whole body and fillet lipid content decreased with increasing levels of dietary CR. There was a remarkable enhancement in the levels of C18 fatty acids and reduction in the levels of n − 3 highly unsaturated fatty acids in the fillets of fish fed with the CR diets, which became more prominent with greater levels of dietary FM replacement. Antioxidant enzyme activities tended to increase with increasing dietary CR levels but gradually decreased when the replacement level exceeded 60%. Overall, the efficiency of CR as a promising substitute for FM in the flounder diets has been confirmed not only in relation to growth rates, but also in terms of immunopotency.

Taurine synthesis via the cysteic acid pathway: effect of dietary cysteic acid on growth, body taurine content, and gene expression of taurine-synthesizing enzymes, growth hormone, and insulin-like growth factor 1 in Japanese flounder Paralichthys olivaceus

Here, we investigated the effect of dietary cysteic acid on the growth performance, sulfur amino acid content, and gene expression levels of taurine-synthesizing enzymes, growth hormone (GH), and insulin-like growth factor 1 (IGF-1) in Japanese flounder Paralichthys olivaceus. Juvenile flounder (0.9 g) were fed one of four diets for 30 days: with 0.25, 0.5, or 1.0% cysteic acid (C0.25, C0.5, C1.0) supplementation and without supplementation (control). Fish in the C0.25 and C0.5 groups showed significantly better growth than those in the control group (P 0.05), the expression level of IGF-1 in C1.0 fish was significantly higher than that in controls (P < 0.05). Our results suggest that Japanese flounder can synthesize taurine from cysteic acid, that dietary supplementation with up to 0.5% cysteic acid promotes fish growth, and that dietary cysteic acid can affect the GH-IGF axis in Japanese flounder. These findings thus highlight the importance of the cysteic acid pathway for taurine synthesis and growth in this species.

Brown seaweed ( Sargassum fulvellum ) inclusion in diets with fishmeal partially replaced with soy protein concentrate for Japanese flounder ( Paralichthys olivaceus ) juveniles

Brown seaweed ( <i>Sargassum fulvellum</i> ) inclusion in diets with fishmeal partially replaced with soy protein concentrate for Japanese flounder ( <i>Paralichthys olivaceus</i> ) juveniles

Assessment of the efficacy of using taurine supplements to improve growth and feed utilization of juvenile starry flounder (Platichthys stellatus) given diets based on soy-protein.

A feeding trial was conducted to assess the feasibility of supplementing taurine in soy-based diets for juvenile starry flounder Platichthys stellatus. The basal diet (Crude protein 66.5%, crude lipid 8.5%) was supplemented with 0 (control), 0.5%, 1.0%, 1.5%, 2.0% and 2.5% taurine to formulate six test diets. Each diet was fed to 40 juvenile fish (22.25 g) in triplicate tanks (120 L) attached to a sea water circulation-system. Fish were fed twice daily by hand to apparent satiation during the 56-d trial. At the end of the trial, fish were counted and weighed for the analyses of growth performance, diet utilization and survival after a 24-h fast. Blood, intestines and muscles were collected for the analyses of serum oxidation resistance, digestive enzymes and body compostion. Livers were collected from the remaining fish at 4 h post-feeding for metabolic enzymes analyses. The results showed that fish fed diets supplemented with 1.0–2.5% taurine grew from 22.25–22.26 g to 47.88–50.40 g with higher average weight gain (25.62–28.12 vs 23.07 g ), specific growth rate (1.37–1.46 vs 1.27%/d ), feed intake (1.04–1.06 vs 1.00%/d), protein efficiency (2.50–2.61 vs 2.44) and lower feed conversion rate (0.84–0.83 vs 0.89) than the control treatment. Diets supplemented with 1.5–2.5% taurine significantly elevated the activities of pepsin (2.47–2.55 vs 2.22, U mg−1 prot), trypsin of distal intestine(14.55–15.24 vs 11.94, U mg−1 prot), hepatic glucokinase (126.62–129.42 vs 105.56, U mg−1 prot) and fatty acid synthetase (125.56-136.89 vs 108.45, U mg−1 prot). All diets supplemented with taurine increased the activities of lipase (32.23–36.67 vs 29.53, U g−1 prot) and trypsin (35.85–37.89 vs 33.54, U mg−1 prot) of proximal intestine, hepatic aspartate transaminase (736.990–832.38 vs 699.24, U mg−1 prot), alanine aminotransferase (477.40–551.86 vs 373.97, U mg−1 prot) and glycogen synthase (2.16–2.59 vs 1.97, U mg−1 prot), as well as serum superoxide dismutase (4.33–4.59 vs 4.07, U mg−1 prot ) and glutathione peroxidase (42.23–50.25 vs 39.17, mol mg−1 prot). Therefore, taurine supplementation benefits juvenile starry flounder growth, digestion, nutrients metabolism and oxidation resistance. The optimal taurine requirement for starry flounder is 1.75%, and the recommended supplementation level is at least 1.6% for maximizing growth of fish fed a low-fishmeal diet (13.6%).

Evaluation of the three different sources of dietary starch in an extruded feed for juvenile olive flounder, Paralichthys olivaceus

The overall performance and well-being of juvenile olive flounder were evaluated in response to three isonitrogenous and isolipidic extruded feed pellets containing either wheat flour (WF), potato (PS) or corn starch (CS), as a dietary carbohydrate source. Following the 2-week acclimation period, triplicate groups of fish (16.6 ± 0.01 g) were assigned to each experimental diet and fed to apparent satiation for 8 weeks. The results of the extruded feed pellet's physical quality tests revealed that dietary inclusion of PS could significantly enhance the water stability of the extruded feed pellets. In addition, PS and CS diets had significantly higher bulk density than WF diet. Feeding juvenile flounders with these experimental diets, however, did not induce any significant changes in their overall performance. Significantly higher serum lysozyme activity was recorded in fish fed the PS and CS diets than those fed the WF diet. Fish fed the WF and CS diets had significantly higher superoxide dismutase activity than PS group. Expression of the immune-related genes, including SOD2, EPX-L, TNF-L, IL-6, and TIPRL, was notably up-regulated in liver of fish fed the PS and CS diets compared to those fed the WF diet. Interestingly, however, an inverse expression pattern was found for the same genes in the spleen where lower expression levels were detected in the PS and CS groups than the WF group. Overall, our findings revealed that while dietary utilization of PS produced an extruded feed pellet with markedly higher physical quality than either WF or CS, neither of these starch sources were able to significantly impact the flounder's performance. Further studies are needed to elucidate the potential long-term consequences of the invoked immune response in the PS and CS groups.