The physiological performance and immune responses of juvenile Amur sturgeon (Acipenser schrenckii) to stocking density and hypoxia stress

Network-based integration of omics, physiological and environmental data in real-world Elbe estuarine Zander

Coastal and estuarine environments are under endogenic and exogenic pressures jeopardizing survival and diversity of inhabiting biota. Information of possible synergistic effects of multiple (a)biotic stressors and holobiont interaction are largely missing in estuaries like the Elbe but are of importance to estimate unforeseen effects on animals' physiology. Here, we seek to leverage host-transcriptional RNA-seq and gill mucus microbial 16S rRNA metabarcoding data coupled with physiological and abiotic measurements in a network analysis approach to decipher the impact of multiple stressors on the health of juvenile Sander lucioperca along one of the largest European estuaries. We find mesohaline areas characterized by gill tissue specific transcriptional responses matching osmosensing and tissue remodeling. Liver transcriptomes instead emphasized that zander from highly turbid areas were undergoing starvation which was supported by compromised body condition. Potential pathogenic bacteria, including Shewanella, Acinetobacter, Aeromonas and Chryseobacterium, dominated the gill microbiome along the freshwater transition and oxygen minimum zone. Their occurrence coincided with a strong adaptive and innate transcriptional immune response in host gill and enhanced energy demand in liver tissue supporting their potential pathogenicity. Taken together, we show physiological responses of a fish species and its microbiome to abiotic factors whose impact is expected to increase with consequences of climate change. We further present a method for the close-meshed detection of the main stressors and bacterial species with disease potential in a highly productive ecosystem.

Effects of different stocking densities on growth, nutritional quality, stress and antioxidant response in Labeo rohita; cultured in in-pond raceway system

A 171-day long experimental trial was undertaken to study intricate physiological response of rohu (Labeo rohita) under stress caused by high stocking density in In-pond raceways system (IPRS). Fingerlings of rohu (initial body weight: 250 ± 1.20 g) were cultured at three different stocking densities; low density (LD) (2.27 kg/m3), medium density (MD) (3.79 kg/m3) and high density (HD) (5.30 kg/m3) in raceways of IPRS production system. Each treatment was in triplicate. Fish growth exhibited a decline in HD treatment statistically as its average weight gain/fish/day was 4.21 g as compared to MD (4.82 g) and LD (4.74 g). Nutritional profile of rohu indicated by the content of crude protein, fatty acids, and profile of amino acids was up to the set dietary benchmarks. Survival rate of fish in all the treatment groups was greater than 99%. The elevated cortisol levels observed in the HD treatment in contrast to the other treatments suggested the presence of stress. The levels of superoxide dismutase, catalase and glutathione peroxidase were also higher in HD as compared to other treatments. However, there were no difference in the level of MDA between the three treatments. Activity of amylase, protease was significantly different in treatment whereas the difference in lipase activity was found to be insignificant. It can be concluded that medium stocking density i.e. 3.79 kg/m3 outperformed the high density (5.30 kg/m3) in different aspects of this study. Nevertheless, additional research is imperative to ascertain whether any intermediate stocking density between medium (3.79 kg/m3) and high (5.30 kg/m3) such as 4 kg/m3, 4.5 kg/m3, or 5 kg/m3, could potentially serve as suitable options for rohu. It is also suggested that brood stock of rohu should be genetically improved to obtain stress resilient fingerlings which will perform better at high stocking density at large scale production level.

Investigating the optimum stocking density of tilapia (Oreochromis niloticus) for intensive production focused to in-pond raceway system

The primary objective of this trial was to study the effects of stress caused by stocking density in tilapia (Oreochromis niloticus) cultured in the in-pond raceway system (IPRS). Fingerlings (Initial body weight = 30.00 ± 1.20 g) were reared at different stocking densities i.e. low stocking density (n = 13,000; 1.77 kg/m3), medium stocking density (MSD) (n = 17,000; 2.32 kg/m3), and high stocking density (HSD) (n = 21,000; 2.86 kg/m3), all confined within the raceways of the IPRS. Each group was studied in triplicates. The observed growth revealed a regression in the HSD treatment, evident in its reduced weight gain per fish per day, in contrast to other treatments. Survival rate across all treatments was above 99%. Notably, the HSD treatment exhibited an elevated level of cortisol; however, this intensified crowding stress did not significantly undermine the nutritional value of the fish in HSD and other experimental treatments. Furthermore, an elevation in the levels of superoxide dismutase, catalase, and glutathione peroxidase was noted within the HSD treatment in comparison to the other treatments to reduce the damage caused by reactive oxygen species. As the trial proceeded, functions of digestive enzymes like amylase, protease, and lipase diminished consistently across all treatments. This could possibly signify a deliberate redirection of energy resources toward stress alleviation rather than the usual digestive processes. In summation, it can be reasonably deduced that a stocking density of approximately 2.32 kg/m3 (MSD) emerges as the optimal threshold for effectively cultivating tilapia within an intensive aquaculture framework.

Probiotic performance of B. subtilis MS. 45 improves aquaculture of rainbow trout Oncorhynchus mykiss during acute hypoxia stress

The aim of this study was to produce mutant strains of Bacillus subtilis with high probiotic performance for use in the aquaculture of rainbow trout Oncorhynchus mykiss. The main strain of B. subtilis (MS) was irradiated with gamma rays (5.3 KGy). Subsequently, the B. subtilis mutant strain no. 45 (MS. 45) was selected for bacterial growth performance, resistance to acidic conditions, resistance to bile salts and antibacterial activity against Aeromonas hydrophila and Pseudomonas fluorescens. After 60 days, the rainbow trout (70.25 ± 3.89 g) fed with MS. 45 and MS were exposed to hypoxia stress (dissolved oxygen = 2 ppm). Subsequently, immune indices (lysozyme, bacterial activity and complement activity), hematological indices [hematocrit, hemoglobin, WBC, RBC, mean corpuscular volume (MCV)] and antioxidant factors (T-AOC, SOD and MDA)) were analyzed after and before hypoxia exposure. The expression of immunological genes (IFN-γ, TNF-α, IL-1β, IL-8) in the intestine and the expression of hypoxia-related genes (HIF-1α, HIF-2α, FIH1) in the liver were compared between the different groups under hypoxia and normoxia conditions. Growth, immunological and antioxidant indices improved in group MS. 45 compared to the other groups. Stress indices and associated immunologic and hypoxia expressions under hypoxia and normoxia conditions improved in MS. 45 compared to the other groups. This resulted in improved growth, immunity and stress responses in fish fed with the microbial supplement of MS. 45 (P < 0.05) under hypoxia and normoxia conditions, (P < 0.05), resulting in a significant improvement in trout aquaculture.

Effect of temperature fluctuation on the physiological stress response of hybrid pearl gentian grouper during waterless keeping alive

Temperature fluctuations are inevitable and have an important impact on the survival of fish during transportation. Therefore, the effect of temperature fluctuation (15 ± 1 °C, 15 ± 2 °C, 15 ± 3 °C) on the muscle quality, physiological, and immune function of hybrid pearl gentian grouper before waterless keeping alive, during keeping alive (0 h, 3 h, 6 h, 9 h, 12 h), and after revival for 12 h was investigated. The plasma glucose concentration of grouper gradually decreased to 0.645 ± 0.007 mg/mL, 0.657 ± 0.006 mg/mL, and 0.677 ± 0.004 mg/mL after keeping alive for 12 h under different temperature fluctuations of 15 ± 1 °C, 15 ± 2 °C, and 15 ± 3 °C, respectively. The cortisol concentration and lysozyme activity of pearl gentian grouper significantly increased (P < 0.05) during the keeping alive period. The results suggested that fish bodies would produce acute stress response, strengthen immune defense ability, and quickly consume a lot of energy to adapt to the low-temperature anhydrous environment. In all treatment groups, the activities of plasma alanine transaminase (ALT) and aspartate aminotransferase (AST) and the content of creatinine gradually increased with the prolongation of the survival time. The hardness and springiness of muscle decreased from 5965.99 ± 20.15 and 0.90 ± 0.00 to 3490.69 ± 27.59 and 0.42 ± 0.01, respectively. In the meanwhile, the change of glycogen and lactic acid content was opposite, indicating that temperature fluctuation harmed the liver, kidney function, and muscle quality. In the later stage of keeping alive, the superoxide dismutase (SOD) and catalase (CAT) activities decreased, especially in the temperature fluctuation group of ±3 °C (125.99 ± 5.48 U/mgprot, 44.21 ± 0.63 U/mgprot), leading to an imbalance of fish immunity. In summary, higher temperature fluctuation would influence the physiological function and immune defense ability and decrease the quality of pearl gentian grouper.

Transcriptome Analysis of Brain and Skin Reveals Immune Responses to Acute Hypoxia and Reoxygenation in Pseudobagrus ussuriensis

Pseudobagrus ussuriensis is an unscaled fish that is more susceptible to skin damage than scaled fish. To investigate the impacts of hypoxia and reoxygenation on skin and brain immunity, juvenile P. ussuriensis were subjected to hypoxia conditions (DO: 0.8 ± 0.05 mg/L) for durations of 0, 3, 6, and 12 h, followed by 12 h of reoxygenation (DO > 6 mg/L). Histological analysis showed a significant increase in the number of skin mucosal cells after 12 h of hypoxia and a significant decrease after 12 h of reoxygenation when compared to the control group. As the duration of hypoxia increased, an increase in antioxidant (SOD, CAT, GSH, MDA) and immune (cortisol, LZM) physiological parameters of the skin and brain appeared. The results of transcriptomic studies showed that the number of differential genes was greater in skin than in brain. Most of the immune pathways in both tissues under hypoxia conditions were all nonspecific immunity (TNF, IL-17, chemokines), while both tissues maintained their homeostasis through active energy supply and cell cycle regulation. Meanwhile, both physiological parameters and RNA transcriptome results showed that 12 h of reoxygenation could not completely eliminate the negative effects of 12 h of hypoxia. This study offers new insights into the immune responses of P. ussuriensis skin and brain during acute hypoxia and reoxygenation.

Multivariate characterization of biochemical and physiological attributes umpires Pangasianodon hypophthalmus for a welfare-based open-water cage culture

The striped catfish, Pangasianodon hypophthalmus, dominates in the Indian inland freshwater cage culture because of its rapid growth and adaptive feeding habits; however, suitable stocking density based on growth without compromising the health of fish is important. Furthermore, fish growth and survival are inversely proportional to stocking density. Many farmers face size differentiation and poor survival at higher stocking densities. To address the aforementioned practical issue, the current study was carried out to study the influence of different stocking densities on the growth performance of P. hypophthalmus in cages. The fingerlings of P. hypophthalmus (10.63 ± 0.27 g) were stocked in triplicate in five different stocking densities (20, 30, 40, 50, and 60 m-3), and the fishes were given commercial feed for 240 days. The outcome demonstrated that the fish stocking densities and their growth attributes were inversely related. Stocking density in the range of 20 to 40 m-3 produced the highest final weight, relative growth rate, and specific growth rate. The feed conversion ratio was significantly lower in 20, 30, and 40 m-3 than the higher densities of 50 and 60 m-3. The level of serum biochemical markers such as serum glutamate oxaloacetate transaminase (SGOT) and serum glutamate pyruvate transaminase (SGPT) and glucose and cortisol levels elevated significantly in higher stocking densities. The muscle quality changed as crude fat and muscle pH decreased at 50 and 60 m-3, leading to a drop in drip loss and frozen leakage rate. The vital water quality parameters were found within a suitable range. According to the outcomes of principal component analysis (PCA), elevated levels of SGOT, SGPT, glucose, and cortisol were detrimental to fish growth. The stocking density of 30 m-3 had the highest benefit-cost ratio (B:C) and return on investment (RI), followed by 20 and 40 m-3. Higher economic return was achieved at lower densities (30-40 m-3). This study suggests that rearing P. hypophthalmus at stocking densities of about 30-40 m-3 could be considered as an optimum for the inland freshwater cage culture which gives the best performance in terms of growth and production in tropical reservoirs of India. The best-suited stocking density is also taken into consideration based on the characterization of multivariate biochemical and physiological attributes.

Dietary flaxseed (Linum usitatissimum) oil supplementation affects growth, oxidative stress, immune response, and diseases resistance in rainbow trout (Oncorhynchusmykiss)

This paper describes the effects of flaxseed (Linum usitatissimum) oil (FSO) as a feed additive on growth performance, oxidative stress, immunity, and disease resistance in rainbow trout (Oncorhynchus mykiss). Four-hundred-and-twenty rainbow trout individuals (mean weight: 25.66 ± 1.33 g) were fed with different doses of FSO (0.5, 1, and 1.5%) ad libitum two times a day for 9 weeks. At the end of the feeding, growth performance was evaluated and the fish were challenged with two different bacteria (Yersinia ruckeri and Aeromonas hydrophila). At the end of the 3rd, 6th, and 9th weeks, blood and tissue samples were collected from 9 fish per treatment to evaluate innate immune response, cytokine gene expression levels, antioxidant enzyme activities and lipid peroxidation levels, and digestive enzyme activities. Determination of haematological parameters and histological examination was also carried out to evaluate the general health status of the fish. Results showed that the final weight and specific growth rate of fish supplemented with FSO increased significantly (p < 0.05). FSO-supplemented fish showed higher resistance to Y. ruckeri infection than the control group (p < 0.05). However, survival rates of all groups in A. hydrophila challenge test were similar (p > 0.05). Among the investigated innate immune response parameters, the potential killing activity of phagocytes, myeloperoxidase activity, and lysozyme activity increased in the FSO-supplemented groups (p < 0.05). Almost all cytokine gene expression levels in the experimental groups up-regulated especially after 9 weeks of feeding in the head kidney and intestine (p < 0.05). Similarly, superoxide dismutase and catalase activities were found to be significantly higher in the FSO group than in the control (p < 0.05) whereas, the lipid peroxidation levels drastically declined as a result of the FSO supplementation (p < 0.05). These results suggest that FSO can improve growth, enhance immune response, and lower oxidative damage in rainbow trout when supplemented at the rates of 0.5-1.5% for 9 weeks.

Cytochrome P450 superfamily in spotted sea bass: Genome-wide identification and expression profiles under trichlorfon and environmental stresses

Cytochrome P450s (CYPs), as one of the most diverse enzyme superfamilies in nature, play critical functions in antioxidant reactions against endogenous and exogenous compounds. In this study, we performed genome-wide characterization of CYP superfamily members and analyzed their expression patterns under several abiotic stresses in spotted sea bass, which is known as an economically important fish species in the Chinese aquaculture industry. A total of 55 CYP genes were identified and divided into 17 families within 10 clans. The analysis of phylogeny, gene structure, and syntenic relationships provided evidence for the evolution of CYP genes and confirmed their annotation and orthology. The expression of CYP genes was examined in the liver during trichlorfon stress using quantitative real-time PCR. The results showed that 20 tested CYP genes displayed significant mRNA expression changes, indicating that they may play crucial roles in the metabolism of trichlorfon and can be potential biomarkers for trichlorfon pollution. Moreover, by screening transcriptomic databases, 10, 3 and 19 CYP genes exhibited differential expression patterns in response to hypoxia, alkalinity and heat stress, respectively. Taken together, this study provided insights into the regulation of CYP genes by toxicological and environmental stresses, laid basis for extensive functional studies of the CYP superfamily in spotted sea bass and other teleost species.

Acute low-dose phosphate disrupts glycerophospholipid metabolism and induces stress in juvenile turbot (Scophthalmus maximus)

Phosphate, as the main nutrient factor of lake eutrophication brought by pollutants discharged from agriculture and industry, is always considered to be a low-toxicity substance to aquatic animals. But the toxicity mechanism is unclear, and published information is limited. In this study, a 96 h acute stress experiment was conducted on juvenile turbot (Scophthalmus maximus) with 0, 10, and 60 mg/L phosphate solutions. Metabonomic analysis revealed that low-dose phosphate (10 mg/L) disrupted glycerophospholipid, purine, and glycolipid metabolism, as well as the tricarboxylic acid (TCA) cycle in juveniles, even at 96 h of stress, which may lead to cell structure damage and signal recognition disorder between cells. Upregulated key genes in the main glycerophospholipid metabolic pathways, which matched the results of the metabolomic study, were detected. Furthermore, low-dose phosphate (10 mg/L) induced oxidative stress and immunotoxicity in fish, resulting in the raising of relevant genes expression such as cat and sod in liver and kidney. In addition, all phosphate-treated groups had induced lesions on gill tissue, as evidenced by pathological observations. In this study on toxic effects on and mechanism of phosphate in aquatic animals using metabolomics, gene expression, and histopathology, we confirm that acute low-dose phosphate could disrupt glycerophospholipid metabolism and induce stress in juvenile turbot. This can provide advice on the amount of phosphate accumulation for marine fish farming and on protecting species diversity and marine ecosystem from the point of view of phosphate toxicity to marine animals.

Blood biomarkers as diagnostic tools: An overview of climate-driven stress responses in fish

Global climate change due to anthropogenic activities affects the dynamics of aquatic communities by altering the adaptive capacities of their inhabitants. Analysis of blood provides valuable insights in the form of a comprehensive representation of the physiological and functional status of fish under various environmental and treatment conditions. This review synthesizes currently available information about blood biomarkers used in climate change induced stress responses in fish. Alterations in informative blood-based indicators are used to monitor the physiological fitness of individual fishes or entire populations. Specific characteristics of fish blood, such as serum and plasma metabolites, cell composition, cellular abnormalities, cellular and antioxidant enzymes necessitate adapted protocols, as well as careful attention to experimental designs and meticulous interpretation of patterns of data. Moreover, the sampling technique, transportation, type of culture system, acclimation procedure, and water quality must all be considered for valid interpretation of hemato-biochemical parameters. Besides, blood collection, handling, and storage time of blood samples can all have significant impacts on the results of a hematological analysis, so it is optimal to perform hemato-biochemical evaluations immediately after blood collection because long-term storage can alter the results of the analyses, at least in part as a result of storage-related degenerative changes that may occur. However, the scarcity of high-throughput sophisticated approaches makes fish blood examination studies promising for climate-driven stress responses in fish.

An accidental discovery of mannan-oligosaccharide's protection effect against air exposure and its potential mechanism in hybrid grouper (Epinephelus lanceolatus ♂ × Epinephelus fuscoguttatus ♀)

The original objective was to explore the potential benefiting effects of three prebiotics in hybrid grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀). Therefore, three experimental diets (basal diet + 1% fructooligosaccharide, Diet F; basal diet + 1% inulin, Diet I; basal diet + 0.3% mannan-oligosaccharide, Diet M) and one basal diet (Diet C) were prepared and a feeding trial was conducted. However, at the end of the fourth week into the feeding experiment, a water-leaking accident occurred and fishes of all groups went through an unexpected air exposure event. Surprisingly, different prebiotic-supplemented groups showed significantly different air exposure tolerance: the mortality of M group was significantly lower (P ≤ 0.05) than all the other groups. Examination of antioxidant, non-specific immunity, and stress parameters revealed that comparing to control group, M group showed significantly increased catalase (CAT), acid phosphatase (ACP), and alkaline phosphatase (AKP) activities, decreased superoxide dismutase (SOD) activity, and similar cortisol level (P ≤ 0.05). Real-time PCR experiment revealed that M group significantly increased the expression of CAT, glutathione peroxidase (GPx), and manganese superoxide dismutase (MnSOD) genes in head kidney (P ≤ 0.05). Overall, M exhibited the best anti-air exposure/antioxidative stress effects among the three prebiotics and could be considered a promising feed additive to relieve air exposure/oxidative stress in hybrid grouper culture.

Stocking density mediated stress modulates growth attributes in cage reared Labeo rohita (Hamilton) using multifarious biomarker approach

The present study was conducted for 240 days to evaluate the effects of stocking density based on growth attributes, digestive enzymes, muscular composition, biochemical and physiological responses of Labeo rohita fingerlings in tropical inland open water cages. L. rohita (30.35 ± 1.08 g) were randomly distributed into three treatments, namely low stocking density, LSD (10 m^-3), medium stocking density, MSD (20 m^-3) and high stocking density, HSD (30 m^-3) in triplicates. Fish were fed twice daily with CIFRI CAGEGROW® floating feed (crude protein-28%, crude fat-4%). Fish growth and feed efficiency were higher (p < 0.05) in LSD, however, MSD registered a higher yield. Amylase and protease activity reduced whereas lipase activity increased with increasing stocking density. Muscle crude protein and crude fat formed an inverse correlation. The fillet quality deteriorated at higher stocking densities based on Muscle pH, drip loss and frozen leakage rate. The stress biomarkers level (glucose, cortisol, superoxide dismutase and catalase) increased in serum under crowding conditions. Glutamate oxaloacetate transaminase and glutamate pyruvate transaminase in serum were significantly increased in HSD. Serum protein levels decreased with the increase in stocking densities. Body ionic imbalance (Na⁺, Cl^- and K⁺) was observed under crowding stress. Based on growth attributes and multiple biomarker responses, L. rohita @ 10 m^-3 was found to be the optimum density for inland open water cage culture.

Sex Determination and Differentiation in Teleost: Roles of Genetics, Environment, and Brain

The fish reproductive system is a complex biological system. Nonetheless, reproductive organ development is conserved, which starts with sex determination and then sex differentiation. The sex of a teleost is determined and differentiated from bipotential primordium by genetics, environmental factors, or both. These two processes are species-specific. There are several prominent genes and environmental factors involved during sex determination and differentiation. At the cellular level, most of the sex-determining genes suppress the female pathway. For environmental factors, there are temperature, density, hypoxia, pH, and social interaction. Once the sexual fate is determined, sex differentiation takes over the gonadal developmental process. Environmental factors involve activation and suppression of various male and female pathways depending on the sexual fate. Alongside these factors, the role of the brain during sex determination and differentiation remains elusive. Nonetheless, GnRH III knockout has promoted a male sex-biased population, which shows brain involvement during sex determination. During sex differentiation, LH and FSH might not affect the gonadal differentiation, but are required for regulating sex differentiation. This review discusses the role of prominent genes, environmental factors, and the brain in sex determination and differentiation across a few teleost species.

Transcriptomic analysis to elucidate the effects of high stocking density on grass carp (Ctenopharyngodon idella)

Background

Grass carp (Ctenopharyngodon idella) is one of the most widely cultivated fishes in China. High stocking density can reportedly affect fish growth and immunity. Herein we performed PacBio long-read single-molecule real-time (SMRT) sequencing and Illumina RNA sequencing to evaluate the effects of high stocking density on grass carp transcriptome.

Results

SMRT sequencing led to the identification of 33,773 genes (14,946 known and 18,827 new genes). From the structure analysis, 8,009 genes were detected with alternative splicing events, 10,219 genes showed alternative polyadenylation sites and 15,521 long noncoding RNAs. Further, 1,235, 962, and 213 differentially expressed genes (DEGs) were identified in the intestine, muscle, and brain tissues, respectively. We performed functional enrichment analyses of DEGs, and they were identified to be significantly enriched in nutrient metabolism and immune function. The expression levels of several genes encoding apolipoproteins and activities of enzymes involved in carbohydrate enzymolysis were found to be upregulated in the high stocking density group, indicating that lipid metabolism and carbohydrate decomposition were accelerated. Besides, four isoforms of grass carp major histocompatibility complex class II antigen alpha and beta chains in the aforementioned three tissue was showed at least a 4-fold decrease.

Conclusions

The results suggesting that fish farmed at high stocking densities face issues associated with the metabolism and immune system. To conclude, our results emphasize the importance of maintaining reasonable density in grass carp aquaculture.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07924-4.

Nannochloropsis oculata feed additive alleviates mercuric chloride-induced toxicity in Nile tilapia (Oreochromis niloticus)

Using microalgae to alleviate the adverse effects of aquaculture pollutants, including metals, has recently gained much attention. In this context, bioaccumulation, hematological indices, oxidative and antioxidant responses, and histopathological alterations were investigated in Nile tilapia (Oreochromis niloticus) fed with either a control diet or diets containing Nannochloropsis oculata (N. oculata) after exposure to mercuric chloride in order to evaluate the role of this microalgae in protecting against mercury-induced toxicity. Fish exposed to HgCl2 at a dose of ¼ LC50 (0.3 mg/L) (Hg group) for 7-21 days exhibited a significant increase in total mercury concentration with a bioaccumulation pattern of liver>gills>muscle, and a significant decrease in all blood indices except mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), monocyte counts, and neutrophil counts. Malondialdehyde (MDA) levels were significantly increased in the Hg group at all time points relative to the control. Glutathione peroxidase (GPx) activity was significantly increased at days 14 and 21, while catalase (CAT) and GPx activities increased and decreased, respectively, at day 7 compared to the control. Additionally, lysozyme activity and immunoglobulin M (IgM) were significantly decreased in the Hg-exposed group. Severe histopathological alterations were evident in the liver, kidneys, and gills. However, supplementation with N. oculata at a low (5%, 50 g/kg feed) or high (10%, 100 g/kg feed) dose stabilized all parameters and reduced the severity of the histopathological alterations with the high N. oculata diet showing more prominent effects. These results suggest that feeding N. oculata protects Nile tilapia against mercuric chloride-induced toxicity.

Effects of Stocking Density on Growth Performance and Stress Responses of Bester and Bester ♀ × Beluga ♂ Juveniles in Recirculating Aquaculture Systems

Simple Summary

The disappearance of many wild sturgeon populations from natural waters and the high demand for sturgeon products (meat and caviar) have led to increased aquacultural production of sturgeons. The development of recirculating aquaculture systems requires the optimization of rearing technologies so that high stocking densities have a minimal impact on fish health. Due to the ease with which sturgeons hybridize, the optimization of sturgeon aquaculture technologies involves the identification of hybrid lines with high technological performance and high stress resistance. One of the best known hybrids is that between the beluga and the sterlet (bester). The present study aims to evaluate the hybrid obtained by crossing the bester (female) with the beluga (male) as a new candidate for the recirculating aquaculture systems. The results show better performance and better density adaptation of the bester × beluga hybrid compared with the bester hybrid. However, at higher stocking densities, a negative impact on growth performance and physiological response was observed.

Abstract

The study aimed to compare the growth performance and physiological responses of bester (B) and backcrossed bester ♀ × beluga ♂ (BB) in response to crowding stress under different stocking densities, as well as to establish a threshold stocking density for rearing BB in a recirculating aquaculture system (RAS) without welfare impairment. For this purpose, in the first trial (T1), B (181.15 ± 21.21 g) and BB fingerlings (181.98 ± 28.65 g) were reared in two stocking densities of 2 kg/m² and 4 kg/m² in fiberglass tanks (1 m³) for 6 weeks. In a parallel trial (T2), the BB hybrids (335.24 ± 39.30 g) were kept in four initial stocking densities, ranging from 5 kg/m² to 12 kg/m². The results of T1 revealed better growth indices (i.e., final mean weight, weight gain, specific growth rate) at lower stocking densities for both hybrids; however, in terms of growth performance, the BB hybrid showed better results when compared with the B hybrid. BB hybrids registered significantly (p < 0.05) lower serum cortisol and MDA and higher lysozyme than B hybrids, showing higher tolerance to crowding stress. Nevertheless, at higher densities, selected serum parameters (i.e., hematological indices, cortisol, glucose, protein, malondialdehyde, lysozyme) and growth performance indices used to evaluate the hybrids indicate that high stocking density could affect the growth and welfare of BB hybrids, and that the selected serum parameters could be used as good indicators for chronic stress caused by overcrowding conditions.

Stocking density alters growth performance, serum biochemistry, digestive enzymes, immune response, and muscle quality of largemouth bass (Micropterus salmoides) in in-pond raceway system

The effects of stocking density on growth performance, serum biochemistry, digestive enzymes, immune response, and muscle quality of largemouth bass (Micropterus salmoides) reared in nine in-pond raceway systems (IPRS, 22.0 m × 5.0 m × 2.0 m) were studied. M. salmoides with initial an body weight of 8.25 ± 0.51 g and body length of 6.99 ± 0.44 cm were reared at an initial stocking density of 90.91 ind./m³ (low stocking density, LSD), 113.63 ind./m³ (middle stocking density, MSD), and 136.36 ind./m³ (high stocking density, HSD) with triplication. After 300 days of culture, MSD recorded the highest final body weight, weight gain, specific growth rate, and yield, but the food conversion ratio in MSD was the lowest. The viscerosomatic index in LSD was significantly higher than other groups. The fish serum reared at HSD showed significantly lower total protein, higher total cholesterol, triglyceride, total bilirubin, glucose content, alanine transaminase, and aspartate transaminase activity. Significantly lower intestinal amylase, lipase, trypsin activities, hepatic superoxide dismutase (SOD) and catalase (CAT) activities, and higher malondialdehyde content were detected in HSD compared to others. The content of crude lipid, saturated fatty acid decreased, and total essential amino acid, delicious amino acid, and polyunsaturated fatty acid increased in muscle with stocking density increase. No significant difference was observed in muscle texture. Profitability analysis indicated the benefit-to-cost ratio varied between 1.10 and 1.68, of which MSD was significantly higher than others. The optimal stocking density for M. salmoides should be 113.63 ind./m³ in an IPRS farm.

Exposure to hypoxia causes stress erythropoiesis and downregulates immune response genes in spleen of mice

BACKGROUND

The spleen is the largest secondary lymphoid organ and the main site where stress erythropoiesis occurs. It is known that hypoxia triggers the expansion of erythroid progenitors; however, its effects on splenic gene expression are still unclear. Here, we examined splenic global gene expression patterns by time-series RNA-seq after exposing mice to hypoxia for 0, 1, 3, 5, 7 and 13 days.

RESULTS

Morphological analysis showed that on the 3rd day there was a significant increase in the spleen index and in the proliferation of erythroid progenitors. RNA-sequencing analysis revealed that the overall expression of genes decreased with increased hypoxic exposure. Compared with the control group, 1380, 3430, 4396, 3026, and 1636 genes were differentially expressed on days 1, 3, 5, 7 and 13, respectively. Clustering analysis of the intersection of differentially expressed genes pointed to 739 genes, 628 of which were upregulated, and GO analysis revealed a significant enrichment for cell proliferation. Enriched GO terms of downregulated genes were associated with immune cell activation. Expression of Gata1, Tal1 and Klf1 was significantly altered during stress erythropoiesis. Furthermore, expression of genes involved in the immune response was inhibited, and NK cells decreased.

CONCLUSIONS

The spleen of mice conquer hypoxia exposure in two ways. Stress erythropoiesis regulated by three transcription factors and genes in immune response were downregulated. These findings expand our knowledge of splenic transcriptional changes during hypoxia.

Dietary exposure to methyl mercury chloride induces alterations in hematology, biochemical parameters, and mRNA expression of antioxidant enzymes and metallothionein in Nile tilapia

Methyl mercury chloride "MMC" (CH₃ClHg) is an ubiquitous environmental toxicant that causes a variety of adverse effects. In the present study, we investigated the effects of sub-chronic toxicity of MMC on Nile tilapia (Oreochromis niloticus) through the evaluation of growth performance and hematological, biochemical, and oxidative stress biomarkers. From 150 healthy fish, five equally sized treatment groups were created: a control (CT) group fed with a basal diet and four MMC treatment groups exposed to 0.5, 1, 1.5, and 2 mg of MMC per kg of basal diet for 60 days. MMC exposure significantly reduced the growth performance and survival of O. niloticus and decreased red blood cell count and hemoglobin concentration. Treated fish exhibited normocytic normochromic anemia in addition to leucopenia, lymphopenia, granulocytopenia, and monocytopenia. Moreover, MMC exposure significantly affected liver function, including a reduction in the total protein levels while increasing cholesterol and triglyceride levels. It also markedly increased the production of stress biomarkers such as glucose and cortisol levels. Furthermore, MMC significantly elevated the levels of hepatic enzymes, induced tissue damage, and caused inflammation, as indicated by the upregulation of mRNA expression of hepatic metallothionein. Finally, MMC exposure induced oxidative stress by altering the antioxidant status of the liver and downregulating the mRNA expression of superoxide dismutase, glutathione peroxidase, and glutathione S-reductase. In conclusion, MMC toxicity induced hematological and biochemical alterations, leading to an enhanced state of oxidative stress in O. niloticus.

Combined effects of enzymes and probiotics on hemato-biochemical parameters and immunological responses of juvenile Siberian sturgeon (Acipenser baerii)

A 10-week feeding trial was run to investigate the separate and simultaneous effects of exogenous enzymes (Enz), probiotics (Pro), and Pro-Enz mixtures on the hematology indices, serum biochemical parameters, and innate-immunity status of juvenile Siberian sturgeon. The fish (138.06 ± 3.64 g) were randomly dispersed into 12 tanks (20 individuals per tank) and fed with Enz (Phytase, protease, and xylanase), Pro (Pediococcus pentosaceus and Lactococcus lactis), and Pro-Enz cocktail. At the end of the feeding bioassay, the highest values of red blood cell count, hemoglobin concentration, hematocrit level, and lymphocyte percentage followed by the lowest neutrophil percentage were obtained in Pro-Enz treatment (P < 0.05). Despite a significantly lower level of alkaline phosphatase in the fish fed with Pro supplemented diet (P < 0.05), no significant difference was found in the serum level of alanine aminotransferase and aspartate aminotransferase among the experimental groups (P > 0.05). Total protein content was significantly upregulated in serum and skin mucus samples from those fed with supplemented diets compared to the control group (P < 0.05). In both serum and skin mucus samples, higher immune responses in terms of lysozyme activity, immunoglobulin M, total protein was seen in Pro-Enz treatment compared to the control group followed by the serum complement components (P < 0.05). The results indicate that the combinational supplementation of Siberian sturgeon diet with the exogenous enzymes and probiotics modulates the physiometabolic responses and innate immune system to a higher grade than their individual supplementation.

Physiological and metabolic responses in juvenile Colossoma macropomum exposed to hypoxia

This study aimed to evaluate hematological, biochemical, and gasometric parameters of tambaqui juveniles (Colossoma macropomum) exposed to hypoxia and subsequent recovery. Six animals were subjected to normoxia (basal) treatment with dissolved oxygen (DO) 6.27 ± 0.42 mg L^-1. Water flow and aeration were reduced for 3 days (hypoxia), during which DO was 0.92 ± 0.37 mg L^-1. Water flow and aeration were then reestablished with DO remaining similar to basal. The treatments were as follows: normoxia (basal); 24 h after initiating hypoxia (24H); 72 h after initiating hypoxia (72H); 24 h after reestablishing normoxia (24R); 48 h after reestablishing normoxia (48R); and 96 after reestablishing normoxia (96R). The highest glucose level was recorded at 24H (P < 0.05); the highest lactate level was at 72R; and the highest blood pH was at 24H and 72H (P < 0.05). The highest concentration of PvCO₂ was at 24H (P < 0.05), while at 96R it was equivalent to basal (P > 0.05). The variable PvO₂ was only higher than basal at 24R (P < 0.05). Juvenile C. macropomum managed to reestablish the main stress indicators (glucose and lactate) at 96R, while the other indicators varied during the study, with homeostatic physiology being reestablished during the recovery period.

Environmental hypoxia causes growth retardation, osteoclast differentiation and calcium dyshomeostasis in juvenile rainbow trout (Oncorhynchus mykiss)

Hypoxia generally refers to a dissolved oxygen (DO) level that is less than 2-3 mg/L. With ongoing global warming and environment pollution, environmental or geological studies showed hypoxia frequently occurs in global aquatic systems including ocean, river, estuaries and coasts. A preliminary study was performed to evaluate hypoxia tolerant of rainbow trout (Oncorhynchus mykiss) with parameters of mortality, behavior, endocrine and metabolite, identifying three DO levels including normoxia (Ctrl, 7.0 mg/L), non-lethal hypoxia (NH, 4.5 mg/L) and lethal hypoxia (LH, 3.0 mg/L). Furthermore, trout was treated by Ctrl, NH and LH for six hours to mimic the acute hypoxia in wild and/or farming conditions. A significantly higher mortality was observed in LH group. Trout of NH and LH showed stressful responses with unnormal swimming, increased serum cortisol and up-regulated gill hif1α transcription. Despite trout of NH and LH increased the oxygen delivery abilities by increasing the serum hemoglobin levels, the anerobic metabolism were inevitably observed with increased lactate. This study also showed a prolonged influence of NH and LH on growth after 30-days' recovery. Based on RNA-Seq data, different expression genes (DEGs) associated with stress, apoptosis, antioxidant, chaperone, growth, calcium and vitamin D metabolism were identified. Enrichment analysis showed DEGs were clustered in osteoclast differentiation, apoptosis and intracellular signaling transduction pathways. Results further showed NH and LH significantly decreased bone calcium content and disrupted the growth hormone-insulin-like growth factor (GH-IGF) axis. Our study might contribute to a better understanding of the effects of hypoxia on rainbow trout.

Transcriptome and physiology analysis identify key metabolic changes in the liver of the large yellow croaker (Larimichthys crocea) in response to acute hypoxia

The large yellow croaker (Larimichthys crocea) is one of the most important marine economic fish in the southeast coast of China. However, hypoxia stress become a major obstacle to the benign development of L. crocea industry. To understand the energy metabolism mechanism adapted to hypoxia, we analyzed the transcriptome and physiology of L. crocea liver in response to hypoxia stress for different durations. We obtained 243,756,080 clean reads, of which 83.38% were successfully mapped to the reference genome of L. crocea. The heat map analysis showed that genes encoding enzymes involved in glycolysis/gluconeogenesis were significantly upregulated at various time points. Moreover, genes encoding enzymes related to the citrate cycle, oxidative phosphorylation, and amino acid metabolism were significantly downregulated at 6 and 24 h, but upregulated at 48 and 96 h. The change of liver in physiology processes, including respiratory metabolism, and activities of the carbohydrate metabolism enzymes showed a similar trend. The results revealed that the respiratory metabolism of L. crocea was mainly anaerobic within 24 h of hypoxia stress, and aerobic metabolism was dominant after 24 h. Carbohydrate metabolism plays a crucial role in energy supply and amino acid metabolism is an important supporting character to cope with acute hypoxia stress. There was no significant change in lipid utilization under short-term acute stress. This study increases our understanding of the energy metabolism mechanism of the hypoxia response in fish and provides a useful resource for L. crocea genetics and breeding.

Evaluation of the effects of different stocking densities on growth and stress responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus in recirculating aquaculture systems

This study was aimed at evaluating the physiological and metabolic responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus to stocking density. Hybrid grouper juveniles (mean ± SE = 25.43 ± 2.36 g live mass) were stocked for 22 weeks in a recirculating aquaculture system (RAS) under four different densities: low stocking density (LD; 1.03 kg m^-3 ), medium stocking density (MD; 2.06 kg m^-3 ), high stocking density (HD; 3.09 kg m^-3 ) and extra-high stocking density (EHD; 4.11 kg m^-3 ). Biometric variables were recorded and plasma, liver, intestine and stomach samples were taken for biochemical analysis at the end of the experimental period. Final stocking densities were 6.27, 16.04, 23.77 and 28.32 kg m^-3 , respectively, with significant differences in growth performance. Our results showed that the best growth rates and feed utilisation occurred in the MD group. Higher plasma cortisol and glucose levels and lower triglyceride levels reflected the stress responses in the EHD group. Moreover, the activity of aspartate and alanine transaminases was elevated in the HD and EHD groups due to enhanced gluconeogenesis. The activity of the digestive enzyme pepsin significantly increased in the MD group. We found that 2.06-3.09 kg m^-3 is the most suitable starting density for culturing juvenile hybrid grouper in recirculating aquaculture systems.

Effects of stocking density on the growth performance, bacterial load and antioxidant response systems of noble scallop Chlamys nobilis

Stocking density is a crucial factor in shellfish aquaculture that affects overall growth performance and health status. Present study analyzes the effects of stocking densities on growth, survival and hemolymph immune status of noble scallop Chlamys nobilis. The scallops with the same size were separately placed in the lantern cages (10 layers per cage) using high stocking density (500 scallops per cage) and low stocking density (100 scallops per cage) and cultivated in the same location for 60 days. The results indicated that the scallops cultivated at high stocking density had significantly higher mortality and slower growth than those cultivated at low stocking density. Moreover, the hemolymph of scallops cultivated at high density showed significantly higher bacterial load, higher reactive oxygen species (ROS), higher expression level of Nrf2 and lower expression level of Keap1, as well as lower antibacterial ability of Vibrio parahemolyticus than that of scallops cultivated at low density. The present results demonstrated that long-term overcrowding is detrimental for the scallops, which can not only lead to high mortality and slow growth, but also cause more vulnerable to pathogenic bacteria. Therefore, we speculated that high stocking density culture practice of scallops in China might be the root of infectious bacteria outbreaks.

Fish response to hypoxia stress: growth, physiological, and immunological biomarkers

Water quality encompasses the water physical, biological, and chemical parameters. It generally affects the fish growth and welfare. Thus, the success of a commercial aquaculture project depends on supplying the optimum water quality for prompt fish growth at the minimum cost of resources. Although the aquaculture environment is a complicated system, depending on various water quality variables, only less of them have a critical role. One of these vital parameters is dissolved oxygen (DO) level, which requires continuous oversight in aquaculture systems. In addition, the processes of natural stream refinement require suitable DO levels in order to extend for aerobic life forms. The depletion of DO concentration (called hypoxia) in pond water causes great stress on fish where DO levels that remain below 1-2 mg/L for a few hours can adversely affect fish growth resulting in fish death. Furthermore, hypoxia has substantial effects on fish physiological and immune responses, making them more susceptible to diseases. Therefore, to avoid disease outbreak in modern aquaculture production systems where fish are intensified and more crowded, increasing attention should be taken into account on DO levels.

Effects of stocking density on growth, stress, and immune responses of juvenile Chinese sturgeon (Acipenser sinensis) in a recirculating aquaculture system

The present study was to investigate the effects of stocking density on the welfare of juvenile Chinese sturgeon (Acipenser sinensis) cultured in a recirculating aquaculture system (RAS) for three months. Fish (average weight 760.86 g) were reared in triplicate under three densities: low density (4.80 kg m^-2), medium density (8.99 kg m^-2), and high density (12.68 kg m^-2). The results showed that the 12.68 kg m^-2 treatment significantly suppressed fish growth, gene expression of glutathione-s-transferase (GST), and enhanced serum adrenocorticotropic hormone (ACTH), cortisol, glucose, lactate levels, and the heat shock protein 70 (HSP70) mRNA level (P < 0.05). Additionally, the 12.68 kg m^-2 treatment significantly decreased serum glutathione peroxidase and superoxide dismutase activities, and increased the formation of malondialdehyde (P < 0.05). Serum immunoglobulin M (IgM), lysozyme, alkaline phosphatase, and acid phosphatase activities showed a significant decline in this group (P < 0.05). Transcriptions of immune-related genes, including lysozyme, hepcidin antimicrobial peptide 1 (HAMP1), tumor necrosis factor-ɑ (TNF-ɑ), interleukin 1β, and interleukin 8 significantly reduced in the 12.68 kg m^-2 group (P < 0.05). In the somatotropic axis, both the serum values and gene expressions of growth hormone (GH) and insulin-like growth factor-I (IGF-I) resulted in a down-regulation in the 12.68 kg m^-2 group (P < 0.05). Overall, these results suggest that high stocking density could negatively influence the growth, stress, and immune responses of juvenile Chinese sturgeon. Consequently, the appropriate stocking density recommended for culturing juvenile Chinese sturgeon in an RAS is between 4.80 kg m^-2 and 8.99 kg m^-2.

Molecular characterization and expression regulation of the factor-inhibiting HIF-1 (FIH-1) gene under hypoxic stress in bighead carp (Aristichthys nobilis)

Factor-inhibiting HIF-1 (FIH-1) is an asparagine hydroxylase that interacts with hypoxia-inducible factor 1α (HIF-1α) to regulate transcriptional activity of HIF-1. Few studies of fish FIH-1 have been reported to date. In this study, the cDNA of FIH-1 gene was cloned and characterized for bighead carp, Aristichthys nobilis (AnFIH-1). The AnFIH-1 cDNA is 2065 bp in length, encoding a protein of 357 amino acid (aa) residues, which contains a JmjC homology region of the jumonji transcription factors. AnFIH-1 shares high identities with other vertebrate FIH-1 (79.1-96.4%), especially in the JmjC homology region, suggesting its conserved function. During the embryonic stages of A. nobilis, AnFIH-1 had significantly high expression levels in unfertilized egg and blastula. In healthy tissues, its predominant mRNA expression was detected in muscle. The mRNA levels of AnFIH-1 were significantly upregulated in the liver, gill, hypothalamus, and spleen after hypoxic treatment, and then decreased to pretreatment levels after 6-h re-oxygenation. However, in the muscle, continual increasing of mRNA expression was observed after hypoxic shock and re-oxygenation. These results indicate that FIH-1 may play an important role in physiological regulation for adapting to hypoxia stress in A. nobilis.

Evaluation of the effects of different stocking densities on the sediment microbial community of juvenile hybrid grouper (♀Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus) in recirculating aquaculture systems

Aquatic microorganisms are an important part of aquatic ecosystems because they are involved in nutrient cycling and water quality, eventually influencing fish productivity. However, at present, reports on the effect of stocking density on microorganisms in sediment samples in recirculating aquaculture systems (RAS) are relatively rare. In this study, the changes in the microbial community in an RAS were investigated under different stocking densities of juvenile hybrid grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatus). Total DNA was extracted from the sediment samples, the 16S rDNA gene was amplified, and the bacterial community was analysed by Illumina high-throughput sequencing. We identified 741 OTUs from a total of 409,031 reads. Based on the analysis of bacterial composition, richness, diversity, bacterial 16S and rDNA gene abundance; sediment sample comparisons; and the existence of specific bacterial taxa within four densities, we concluded that the dominant phyla in all samples were similar and included: Proteobacteria, Bacteroidetes, Nitrospirae, Planctomycetes, Verrucomicrobia, and Chloroflexi. However, their relative distributions differed at different fish densities. Linear discriminant analysis further indicated that the stocking treatment influenced the sediment bacterial community. This study indicates that under RAS aquaculture, mode density is a factor regulating the microbial community, which provides insights into microbe management in RAS culture.

Influences of Chlorella vulgaris dietary supplementation on growth performance, hematology, immune response and disease resistance in Oreochromis niloticus exposed to sub-lethal concentrations of penoxsulam herbicide

Little is known regarding the impact of penoxsulam, a fluorinated benzenesulfonamid rice herbicide, on Oreochromis niloticus (O. niloticus). Therefore, the current study was undertaken to highlight the effects of penoxsulam exposure on O. niloticus and to evaluate the advantages of Chlorella vulgaris (CV) dietary supplementation against the induced effects. The 96-h lethal concentration 50 (LC₅₀) penoxsulam value for O. niloticus was estimated at 8.948 mg/L by probit analysis in a static bioassay experiment. Next, 360 healthy fish were randomly allocated into 6 treatment groups. The T1 group served as the negative control and was fed a basal diet. The T2 group served as the positive control and was fed a basal diet supplemented with 10% CV. The fish in the T3 and T4 groups were exposed to 1/10 the 96-h LC₅₀ of penoxsulam (0.8948 mg/L) and were fed the basal diet alone or the basal diet supplemented with 10% CV, respectively. The fish in the T5 and T6 groups were exposed to 1/5 the 96-h LC₅₀ of penoxsulam (1.7896 mg/L) and fed the basal diet alone or the basal diet supplemented with 10% CV, respectively. Sub-acute penoxsulam exposure significantly altered hematological indices, as well as compromised the fish's immune defense mechanisms, including the phagocytic percentage, phagocytic index, nitric oxide production, immunoglobulin M levels and lysozyme, anti-trypsin and bactericidal activities subsequently decreasing O. niloticus's resistance to the Aeromonus sobria challenge and increasing disease symptoms and the mortality rate. Furthermore, sub-chronic penoxsulam exposure markedly altered growth performance, oxidant/antioxidant status and liver status and down-regulated the expression of interleukin-1β (IL-1β) and tumor necrosis-α (TNF-α). Interestingly, incorporating 10% CV into the diet protects fish against sub-acute penoxsulam-induced immunotoxicity via improvement of immune responses that increases the resistance against bacterial infection. Further, it improved the growth performance, oxidant/antioxidant status, liver status and markedly up-regulated immune-related gene expression, IL-1β and TNF-α, in the spleens of fish sub-chronically exposed to penoxsulam. These outcomes showed that dietary CV supplementation can protect the commercially valuable freshwater fish O. niloticus against penoxsulam toxicity and may be a potential feed supplement for Nile tilapia in aquaculture.

Acute hypoxic stress: Effect on blood parameters, antioxidant enzymes, and expression of HIF-1alpha and GLUT-1 genes in largemouth bass (Micropterus salmoides)

Dissolved oxygen (DO) plays a crucial role in survival, growth, and normal physiological functions of aquatic organisms. Nevertheless, the mechanisms involved in hypoxic stress and adaptation have not been fully elucidated in Largemouth bass (Micropterus salmoides). To reveal the effect of acute hypoxia on Largemouth bass, we simulated acute hypoxia (DO: 1.2 ± 0.2 mg/L) in the laboratory and analyzed physiological parameters (RBCs, Hb, SOD, CAT, NA⁺/K⁺-ATPase, GPx, and MDA) and gene expression (HIF-1alpha and GLUT-1) in Largemouth bass exposed to various durations of acute hypoxia (0, 1, 2, 4, 8, 12, and 24 h). Our results indicated that acute hypoxic exposure significantly increased RBCs but decreased Hb. In addition, antioxidant enzyme activity was enhanced significantly in the liver and muscles at the initial stage of acute hypoxic exposure, but decreased significantly in gills during the entire process of hypoxic exposure. Furthermore, the expression levels of HIF-1alpha and GLUT-1 mRNA were significantly up-regulated in Largemouth bass under acute hypoxic exposure. In conclusion, our study provides a valuable basis for further elucidation of hypoxic adaptation and facilitates husbandry for an economically valuable species.

Effect of crowding stress on the immune response in turbot (Scophthalmus maximus) vaccinated with attenuated Edwardsiella tarda

The aim of this study was to evaluate the immune responses in turbot, Scophthalmus maximus, treated with 1 × 10⁷ cfu/ml attenuated Edwardsiella tarda (0.1 ml/fish) under low density (LD; ∼5.25-5.13 kg/m², initial to final density), medium density (MD; ∼10.41-13.95 kg/m²), and high density (HD; ∼20.53-30.77 kg/m²) conditions for 8 weeks. The results showed that there was a peak value in the percentage of sIg+ (surface immunoglobulin-positive) cells in blood leucocytes (BL), spleen leucocytes (SL), and pronephros leucocytes (PL) during the sixth week in the HD, which was delayed by week compared with the other groups. The specific immunoglobulin M (IgM) antibody levels increased from the first week in all groups and reached a peak in the fifth week in the LD and MD groups, but in the sixth week in the HD group. The serum cortisol levels were greater in the HD group compared with the other groups in the last 3 or 4 weeks. These results show that stocking turbot at a LD obtained the most effective immunization, and thus we conclude that crowding stress may reduce the ability to deal with immune challenge.

Effects of Acute Hypoxia and Reoxygenation on Physiological and Immune Responses and Redox Balance of Wuchang Bream (Megalobrama amblycephala Yih, 1955)

To study Megalobrama amblycephala adaption to water hypoxia, the changes in physiological levels, innate immune responses, redox balance of M.amblycephala during hypoxia were investigated in the present study. When M. amblycephala were exposed to different dissolved oxygen (DO) including control (DO: 5.5 mg/L) and acute hypoxia (DO: 3.5 and 1.0 mg/L, respectively), hemoglobin (Hb), methemoglobin (MetHb), glucose, Na⁺, succinatedehydrogenase (SDH), lactate, interferon alpha (IFNα), and lysozyme (LYZ), except hepatic glycogen and albumin gradually increased with the decrease of DO level. When M. amblycephala were exposed to different hypoxia time including 0.5 and 6 h (DO: 3.5 mg/L), and then reoxygenation for 24 h after 6 h hypoxia, Hb, MetHb, glucose, lactate, and IFNα, except Na⁺, SDH, hepatic glycogen, albumin, and LYZ increased with the extension of hypoxia time, while the above investigated indexes (except albumin, IFNα, and LYZ) decreased after reoxygenation. On the other hand, the liver SOD, CAT, hydrogen peroxide (H₂O₂), and total ROS were all remained at lower levels under hypoxia stress. Finally, Hif-1α protein in the liver, spleen, and gill were increased with the decrease of oxygen concentration and prolongation of hypoxia time. Interestingly, one Hsp70 isoforms mediated by internal ribozyme entry site (IRES) named junior Hsp70 was only detected in liver, spleen and gill. Taken together, these results suggest that hypoxia affects M. amblycephala physiology and reduces liver oxidative stress. Hypoxia-reoxygenation stimulates M. amblycephala immune parameter expressions, while Hsp70 response to hypoxia is tissue-specific.

Molecular characterization of mudskipper (Boleophthalmus pectinirostris) hypoxia-inducible factor-1α (HIF-1α) and analysis of its function in monocytes/macrophages

Hypoxia-inducible factor-1α (HIF-1α) plays a critical role in immune and inflammatory responses and is important in controlling a variety of processes in monocytes and macrophages. However, very little information is available about the functions of HIF-1α in fish monocytes/macrophages (MO/MФ). In this study, the cDNA sequence of the mudskipper (Boleophthalmus pectinirostris) HIF-1α gene (BpHIF-1α) was determined. Sequence comparison and phylogenetic tree analysis showed that BpHIF-1α is clustered in the fish HIF-1α tree. Constitutive expression of BpHIF-1α mRNA was detected by real-time quantitative PCR in all tested tissues, and the expression was found to be dramatically increased in the skin, liver, spleen, and kidney after Edwardsiella tarda infection. In addition, hypoxia and infection induced the expression of the BpHIF-1α transcript and protein in MO/MФ, respectively. Hypoxia caused an increase in phagocytic and bactericidal capacity of mudskipper MO/MФ in a BpHIF-1α-dependent manner. BpHIF-1α induced an anti-inflammatory status in MO/MФ upon E. tarda infection and hypoxia. Therefore, BpHIF-1α may play a predominant role in the modulation of mudskipper MO/MФ function in the innate immune system.

Effects of stocking density on antioxidant status, metabolism and immune response in juvenile turbot (Scophthalmus maximus)

This study was designed to evaluate the physiological and immune responses of juvenile turbot to stocking density. Turbot (average weight 185.4g) were reared for 120days in a land based recirculating aquaculture system (RAS) under three stocking densities: low density (LD, ~9.3-26.1kg/m², initial to final density), medium density (MD, ~13.6-38.2kg/m²) and high density (HD, ~19.1-52.3kg/m²). Fish were sampled at days 0, 40, 80 and 120 to obtain growth parameters and liver tissues. No significant difference was detected in growth, biochemical parameters and gene expression among the three densities until at the final sampling (day 120). At the end of this trial, fish reared in HD group showed lower specific growth rate (SGR) and mean weight than those reared in LD and MD groups. Similarly, oxidative stress and metabolism analyses represented that antioxidants (superoxide dismutase (SOD), catalase (CAT), glutathione (GSH)) and metabolic enzymes (glycerol-3-phosphate dehydrogenase (G3PDH) and glucose-6-phosphate dehydrogenase (G6PDH)) clearly reduced in the liver of turbot reared in HD group. The gene expression data showed that glutathione S-transferase (GST), cytochrome P450 1A (CYP1A), heat shock protein 70 (HSP 70) and metallothionein (MT) mRNA levels were significantly up-regulated, and lysozyme (LYS) and hepcidin (HAMP) mRNA levels were significantly down-regulated in HD group on day 120. Overall, our results indicate that overly high stocking density might block the activities of metabolic and antioxidant enzymes, and cause physiological stress and immunosuppression in turbot.

Stress and immune responses in skin of turbot (Scophthalmus maximus) under different stocking densities

Fish skin and its mucus provide the first line of defense against chemical, physical and biological stressors, but little is known about the role of skin and its mucus in immune response to crowding stress. In the present study, we investigated the stress and immune responses in skin of turbot (Scophthalmus maximus) under different stocking densities. Turbot (average weight 185.4 g) were reared for 120 days under three densities: low density (LD), medium density (MD), and high density (HD). After 120 days, fish were weighed and sampled to obtain blood, mucus and skin tissues which were used for analyses of biochemical parameters and genes expression. The results showed HD treatment significantly suppressed growth and enhanced plasma cortisol and glucose levels (P < 0.05). In mucus, the activities of lysozyme (LZM), alkaline phosphatase (ALP) and esterase in HD treatment were lower than LD and MD treatments (P < 0.05) In skin, HD treatment resulted in up-regulation in malondialdehyde (MDA) formation and heat shock protein 70 (HSP 70) mRNA level, and down-regulation in activity of superoxide dismutase (SOD) and the transcriptions of glutathione-s-transferase (GST), interleukin-1β (IL-1β), tumor necrosis factor -α (TNF-α), insulin-like growth factor- (IGF-) and LZM (P < 0.05). Overall, the data suggested that overly high stocking density was a stressor which caused an immunosuppression in skin of turbot. Moreover, this information would help to understand the skin immunity and their relation with stress and disease in fish.

Cinnamon (Cinnamomum sp.) inclusion in diets for Nile tilapia submitted to acute hypoxic stress

The aim of this study was to evaluate the possible effects of diets supplemented with probiotics and different cinnamon levels (powder and essential oil) on immunological parameters of Nile tilapia after being subjected to acute stress by hypoxia. Three hundred and thirty juvenile male tilapia fish (66.08 ± 2.79 g) were distributed in 30 tanks of 100 L capacity (11/cage) with a water recirculation system. The animals were fed for 71 days with diets containing extruded cinnamon powder at different levels (0.5, 1, 1.5, 2%), cinnamon essential oil (0.05, 0.1, 0.15; 0.2%) and probiotics (0.4%), all in triplicate. At the end of the experiment, the fish (200.36 ± 19.88 g) of the different groups were subjected to stress by hypoxia. Hypoxia was achieved by capturing the animals with a net, keeping them out of the water for three minutes, and then sampling the blood 30 min after the procedure to determine the levels of cortisol, glucose, haematocrit, lysozyme, bactericidal index, total protein, and its fractions. The animals kept blood homeostasis after hypoxic stress. Diet supplementation with 0.5% cinnamon powder improved the fish immune response, since it resulted in an increase of 0.5% in γ-globulin level. Administration of 0.15% cinnamon essential oil resulted in an increase of α1 and α2-globulins, which may be reflected in increased lipid content of the carcass and the hepatosomatic index. More studies are necessary to better understand the effects of these additives for fish immunity.

Oxygen stress: impact on innate immune system, antioxidant defence system and expression of HIF-1α and ATPase 6 genes in Catla catla

Catla catla catla (2.28 ± 0.1 g) were exposed to six different levels of dissolved oxygen: 1 (DO-1), 3 (DO-3), 5 (DO-5), 7 (DO-7), 9 (DO-9) and 11 (DO-11) mg/L. DO-5 served as control. In DO-1 and DO-3, the number of red blood cells (RBC), lysozyme, respiratory burst activity and nitric oxide synthase were significantly (p < 0.05) lower compared to the control one. In DO-7 and DO-9, RBC and lysozyme were significantly (p < 0.05) higher compared to the control one. Thiobarbituric acid reactive substances was significantly (p < 0.05) higher in catla exposed at low (1 and 3 mg/L) and high (9 and 11 mg/L) dissolved oxygen compared to others. In muscles and hepatopancreas, reduced glutathione was significantly (p < 0.05) higher in DO-5 and DO-7 and in gills of DO-5 compared to others after 1 h. In muscles, glutathione S-transferase (GST) was significantly (p < 0.05) lower in DO-5 and DO-7 compared to others. In hepatopancreas, GST and glutathione peroxidise (GPx) were significantly (p < 0.05) higher in DO-1 and DO-3 compared to others. In gills, GPx was higher in DO-9 and DO-11 after 48 h. In brain, hypoxia-inducible factor (HIF)-1α mRNA level was induced in DO-1 and DO-3 compared to others after 1 h of exposure. In gills and hepatopancreas, HIF-1α mRNA level was significantly (p < 0.05) higher in DO-1 compared to others after 1 h. The ATPase 6 mRNA level was significantly (p < 0.05) higher in brain and hepatopancreas of DO-1 after 1 h and in gills and hepatopancreas of DO-3 and DO-9, respectively, after 48 h compared to others.

Feeding rates affect stress and non-specific immune responses of juvenile blunt snout bream Megalobrama amblycephala subjected to hypoxia

This study aimed to investigate the effects of feeding rates on stress response, innate immunity and hypoxia resistance of juvenile blunt snout bream Megalobrama amblycephala. Fish were randomly assigned to one of six feeding rates (2, 3, 4, 5, 6 and 7% of body weight/day) for 60 days. Then, fish were subjected to hypoxic conditions and haemato-immunological parameters were analyzed pre- and post-challenge. Low feed ration resulted in decreased liver superoxide dismutase and catalase activities and reduced glutathione contents. Inadequate feeding also adversely affected the immune functions of fish, as was characterized by the relatively low haemato-immunological parameters (including alternative complement (ACH50), myeloperoxidase (MPO), plasma protein profiles and transferring) and high hypoxia-induced mortality. High feed ration did not lead to the improvement in antioxidant capability, immune responses and survival. In addition, plasma cortisol, glucose and transferrin levels as well as lysozyme activities all increased significantly after hypoxia challenge, whereas the opposite was true for plasma ACH50 and MPO activities as well as protein profiles in terms of hypoxia challenge. An interaction between feeding rate and hypoxia was also observed in plasma cortisol, glucose and protein profiles. In conclusion, a feeding rate of 4-5% of body weight/day is optimal to boost the innate immunity of juvenile blunt snout bream. Low ration resulted in decreased antioxidant capability, compromised immune functions and reduced hypoxia resistance, while over feeding did not benefit the health status.

Influence of supplemented diet with Pediococcus acidilactici on non-specific immunity and stress indicators in green terror (Aequidens rivulatus) during hypoxia

Green terror fish were fed for two months with three types of feed including: control diet (C), diet containing 2% fish oil (O) and diet containing fish oil supplemented with 0.1% Pediococcus acidilactici bacteria (PA). At the end of the feeding period, 50 fish of different groups (n = 3) with an average weight of 4.28 g were transferred to 9 tank. Hypoxia tests were set out by sampling at three times including before hypoxia (BH) initiation, hypoxia stress spot (H) and starting mortality (SM). In fish submitted to the diet containing P. acidilactici bacteria, the immune indices of lysozyme activity (4.08, 4.19 and 4.85 μg/ml)], complement activity (2.65, 2.77 and 2.1 U/ml) and total immunoglobulin (10.05, 10.25 and 9.9 μg/ml) improved in all stages of sampling (BH, H and SM), respectively (p < 0.05). The positive effects of the bacteria application also extends for the stress indicators including: cortisol (0.175, 0.3 and 0.335 μM/ml), glucose (0.9, 1.25 and 0.6 μg/ml) and lactate (2.7, 3 and 3.35) μg/ml and plasma electrolytes consisting of Na(+) (178.5, 175.43 and 175.8 mmol/l) and Cl(-)(123.85, 119.30 and 118.43 mmol/l) in all sampling stages (BH, H and SM), respectively (p < 0.05). Put it all together, P. acidilactici, acting as a probiotic, helps reducing stress symptoms in green terror fish.

Effect of stocking density and different diets on growth of Percula Clownfish, Amphiprion percula (Lacepede, 1802)

The aim of this study was to evaluate the influence of stocking density (0.5, 1, 2 and 3 fishL⁻¹) and commercial marine fish diets (diet A, B, C and D) over four months on specific growth rate, condition factor, percentage without anomalous pigmentation (partial or total lack of white bands -miss-band) and survival of juvenile Amphiprion percula.

Results showed that at 0.5 fishL⁻¹ densities induced the best survival (100%) and also the maximum percentage of fish without miss-band (58.33 +/−4.417%). The maximum SGR was obtained for the 0.5 fishL⁻¹ (0.459 ± 0.023% cm/day). However, the best condition factor (2.53 +/− 0.27) was achieved for 2 fishL⁻¹ densities. There were no significant differences in survival (68.9 to 84.5%), fish without miss-bands (18.03 to 26.92%) and condition factor (1.92 to 2.1) among diets during the experimental period. On the other hand, diet C (with 41% crude protein) supported the best SGR (0.485 ± 0.001% cmday⁻¹).

The results suggested that stocking density are critical and more relevant when compared with the different diet tested, namely on specific growth rate, condition factor, the miss-band and survival of juvenile percula clownfish.

This study has particular significance with regards to anemonefishes husbandry in terms of survival and production efficiency.

Two HSPs gene from juvenile Amur sturgeon (Acipenser schrenckii): cloning, characterization and expression pattern to crowding and hypoxia stress

In this study, the cDNA sequences of HSP70 and HSP90 were isolated from the special chondr-ganoid scale, Amur sturgeon, for the first time. Homology analysis indicated that amino acid sequences of HSP70 and HSP90 shared high identity with other species (82.68-99.07 and 90.19-98.07%, respectively). The tissue expression analysis showed that the asHSP70 and asHSP90 mRNA were ubiquitously expressed in all the examined tissues under unstressed condition. The expression pattern of HSP70 and HSP90 under chronic (crowding) and acute (hypoxia) stress was examined by q-PCR in liver, spleen and kidney. Results showed that stocking density could significantly influence the expression of HSP70 at day 20 and/or day 40. In contrast to stocking density, levels of HSP70 transcripts indicated a remarkable increase in all examined tissues after hypoxia stress. HSP90 levels in liver and spleen increased significantly in high stocking density. By comparison, significant increase of asHSP90 in kidney was only found in high stocking density at day 40. Similar to HSP70, the levels of HSP90 transcripts showed significant increases after hypoxia stress except the transcript of liver in H2 group 6 h after hypoxia. The assessment of asHSP70 and asHSP90 mRNA levels under crowding and hypoxia stresses indicated that asHSP70 and asHSP90 gene might be good indicators of stressful situations for Amur sturgeon. Taking serum globulin and electrolytes account, we suggest that crowding and hypoxia stress can result in considerable stress for Amur sturgeon.