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Background Rainbow trout ( Oncorhynchus mykiss ) is becoming popular with the increased demand for fish protein. However, the limited resources and expense of fish meal and oil have become restrictive factors for the development of the rainbow trout related industry. To solve this problem, plant-derived proteins and vegetable oils have been developed as alternative resources. The present study focuses on evaluating the effects of two experimental diets, FMR (fish meal replaced with plant-derived protein) and FOR (fish oil replaced with rapeseed oil), through the alteration of the gut microbiota in triploid rainbow trout. The commercial diet was used in the control group (FOM). Results Amplicon sequencing of the 16S and 18S rRNA genes was used to assess the changes in gut bacteria and fungi. Our analysis suggested that the α-diversity of both bacteria and fungi decreased significantly in the FMR and FOR groups, and β-diversity was distinct between FOM/FMR and FOM/FOR based on principal coordinate analysis (PCoA). The abundance of the Planctomycetota phylum increased significantly in the FMR group, while that of Firmicutes and Bacteroidetes decreased. We also found that the fungal phylum Ascomycota was significantly increased in the FMR and FOR groups. At the genus level, we found that the abundance of Citrobacter was the lowest and that of pathogenic Schlesneria , Brevundimonas , and Mycoplasma was highest in the FMR and FOR groups. Meanwhile, the pathogenic fungal genera Verticillium and Aspergillus were highest in the FMR and FOR groups. Furthermore, canonical correspondence analysis (CCA) and network analysis suggested that the relatively low-abundance genera, including the beneficial bacteria Methylobacterium , Enterococcus , Clostridium, Exiguobacterium , Sphingomonas and Bacteroides and the fungi Papiliotrema , Preussia , and Stachybotrys, were positively correlated with plant protein or rapeseed oil. There were more modules that had the above beneficial genera as the hub nodes in the FMR and FOR groups. Conclusions Our study suggested that the FMR and FOR diets could affect the gut microbiome in rainbow trout, which might offset the effects of the dominant and pathogenic microbial genera. This could be the underlying mechanism of explaining why no significant difference was observed in body weight between the different groups.

Salmonids can accumulate lipids in their fillets, creating marbled features with alternate red (Muscle Fibers, MF) and white stripes (Myosepta, MS). To investigate the regulation of this important quality trait, diets with low and high lipid levels were fed to the fish and found that high lipid diet significantly elevated muscular lipid deposition in MS but not in MF. Then, a whole-transcriptome analysis was performed and results showed that the mRNA expression of ACSL1 and GADD45A was downregulated by the lncRNAs MSTRG.19477.1 and XR_005039693.1, resulting in consistent lipid contents in the MF from both groups. The lncRNAs MSTRG.21618.1, XR_005034756.1, XR_002473790.2, XR_002472790.2, and MSTRG43906.1 increased lipid deposition in MS30 by upregulating the mRNA expression of ELOVL2, DGAT2, LCAT, etc. In conclusion, the present study revealed that selective muscular lipid deposition and several lncRNAs may play key roles in regulating the marbling features of rainbow trout.

Background Transfer operations are one of the stress sources that cause mortality in fish. This study investigated the effects of different dissolved oxygen concentrations on oxidative stress and several biochemical parameters during the transfer of adult rainbow trout ( Oncorhynchus mykiss ). The aim was to determine the optimum dissolved oxygen concentration to minimize stress and potential tissue damage during transfer. Result The study analyzed GSH-Px, SOD, and LDH enzyme activities; MDA, Cp, TOS, TAS, OSI, cortisol, glucose, urea, uric acid, creatinine, Ca, Mg, and MDA levels; as well as Cp, TOS, TAS, and OSI values in serum and muscle tissue. Findings indicated that transfer and preparation for transfer induced oxidative damage in fish. Comparing different dissolved oxygen levels, it was observed that hypoxic and hyperoxic conditions increased ROS levels, suppressed the antioxidant mechanism, and caused oxidative stress. Conclusions Among the tested conditions, transfer under normoxic conditions with a dissolved oxygen concentration of 10 mg/L was the most effective in minimizing oxidative stress and tissue damage. This suggests that maintaining adequate oxygen levels during transfer plays a crucial role in reducing physiological stress in rainbow trout.

Background: Natural products are key sources of anti-inflammatory agents, yet the potential of fish visceral extracts remains largely unexplored. This study evaluated the anti-inflammatory activity of a spleen extract from rainbow trout (Oncorhynchus mykiss). Methods: A crude spleen extract and its four solvent fractions were tested in LPS-stimulated RAW264.7 macrophages. Nitric oxide production and expression of iNOS, COX-2, and cytokines were assessed by qRT-PCR and Western blotting. The most active fraction, OSB (n-butanol layer), was further analyzed for its effects on NF-κB signaling, macrophage polarization, and ROS generation. Results: The crude spleen extract significantly reduced NO production and downregulated iNOS and COX-2 expression at both the transcriptional and translational levels. Among the four fractions, the OSB fraction exhibited the most potent and consistent anti-inflammatory effects. OSB markedly suppressed LPS-induced expression of iNOS, COX-2, and pro-inflammatory cytokines, while enhancing anti-inflammatory cytokines. Mechanistic analyses demonstrated that OSB inhibited NF-κB activation by preventing the nuclear translocation of the p65 subunit. Additionally, OSB attenuated LPS-induced ROS production and reduced the expression of M1 macrophage markers, indicating inhibition of M1 polarization. Conclusions: The OSB fraction from rainbow trout spleen exhibits potent anti-inflammatory activity by modulating the NF-κB pathway and suppressing M1 macrophage polarization, suggesting its potential as a natural therapeutic agent.

The present study was performed to investigate the effects of dietary yeast β-1,3/1,6-D-glucans after 15-, 30-, and 45-day feeding periods on the development of oxidative stress, antioxidant defenses, and lysosomal functioning in gills and hepatic tissue of rainbow trout, Oncorhynchus mykiss (Walbaum). Many manipulations in fish farming also cause stress reactions in fishes that strongly impact their immune systems. [...]the risk of diseases and the vulnerability of fishes to various infections in fish farms is also increased by stress-induced weakening of the immune system (Nguyen et al. 2019). Various studies in a wide range of fish species have revealed the effect of ß-glucans on growth, antibody production, immune-related gene expression, survival, resistance, and protection against pathogens, and their role as an adjuvant (Meena et al. 2013, Petit and Wiegertjes 2016, Petit et al. 2019, Song et al. 2020). ß-Glucans are a heterogeneous group of glucose polymers found, inter alia, in the cell walls of plants, bacteria, fungi, and protozoa. ß-Glucans have a basic chain, and in some cases, side chains, which comprise ß(l,3), ß(l,4), and/or ß(l,6)-bound glucose groups, depending on the origin of the glucan (Douxfils et al. 2017, Felicioli et al. 2020). Many hormone effects are realized through these organelles as is the regulation of metabolic pathways (DeMartino and Goldberg 1978). [...]studying the relationship between immunological response, redox reactions, and the functioning of lysosomes in fishes is an important approach in the study of biochemical mechanisms underlying dietary treatment with ß-glucans.

A twelve-week feeding experiment was undertaken to explore the impact of substituting dietary fish meal (FM) and fish oil (FO) with complex protein (CP) and canola oil (CO) in the diet of triploid rainbow trout on the quality of their fillets. The control diet (F100) contained FM (60%) and FO (18.6%) as the main protein and lipid sources. Based on this, 50% and 100% of FM and FO were substituted by CP and CO and they were named as F50 and F0, respectively. The results showed that there were no significant differences in the specific growth rates, condition factors, gutted yields, fillet yields and yellowness values as the substitution levels increased (p > 0.05). The F50 treatment obtained the highest values of fillet springiness and chewiness, improved the umami and bitter taste of the fillets by increasing the contents of inosine-5′-monophosphate and histidine, and increased lipid, protein, C18: 1n-9 and C18: 2n-6 contents (p < 0.05). The F0 treatment obtained the highest values of fillet hardness and pH, attenuated the sweet taste of the fillets by decreasing the content of glycine, and decreased the contents of EPA and DHA (p < 0.05). Both F50 and F0 treatments could increase the redness value, decrease the lightness and hue values of fillets, and increase the odor intensity, resulting in the typical fillet odors of green, fatty, orange and fishy (p < 0.05). In general, 50% and 100% of FM and FO substitution did not affect the growth of trout, but it did affect quality. Compared to the F100 treatment, the fillet quality of the F0 treatment was similar to the F50 treatment and could improve the appearance and odor intensity of the fillets. However, the difference was that the F50 treatment increased the springiness, umami, bitterness and lipid nutritional value of the fillets, but the F0 treatment increased the hardness, decreased the sweetness, and decreased the lipid, EPA and DHA contents of the fillets.

In the context of future aquaculture intensification, a longitudinal ten-year evaluation of the current traditional rainbow trout production in Ireland was performed. Publically available and independent data obtained from local authorities were gathered and analysed. Inlet and outlet concentrations of parameters such as BOD 5 , ammonium, nitrite, dissolved oxygen and pH for four consecutive flow-through fish farms covering the four seasons over a ten-year period (2005–2015) were analysed. The objectives of the study were (i) to characterize the impact of each fish farm on water quality in function of their respective production and identify any seasonal variability, (ii) to quantify the cumulative impact of the four farms on the river quality and to check if the self-purification capacity of the river was enough to allow the river to reach back its background levels for the analysed parameters, (iii) to build a baseline study for Ireland in order to extrapolate as a dataset for expected climate change and production intensification. For most of the parameter analysed, no significant impact of the fish farming activity on water quality/river quality was observed. These results, the first ones generated in Ireland so far, will have to be completed by a survey on biodiversity and ecotoxicology and compared after production intensification and the likely future introduction of water treatment systems on the different sites.

Background: In Europe, rainbow trout is one of the main fresh water fish farmed in a constantly developing environment that requires innovative studies to improve farm management, fish welfare and environmental sustainability. The aim of this paper is to investigate the trend of water quality parameters over 10 years, after a feeding strategy change from pellet to extruded feed. Methods: The study was conducted on a farm in central Italy, based on parallel raceways. The cycle started from young rainbow trout (90 ± 2 g) that were grown until they reached market size. A water sample of 500 cm3 was collected monthly from 2009 to 2019 from the lagoon basin in order to investigate the trends of the total suspended solids (TSS), biochemical oxygen demand (BOD5), chemical oxygen demand (COD), nitrites (NO2-N), nitrates (NO3-N), total ammonia nitrogen (TAN), total phosphorus (TP) and pH. Results: All of the studied parameters (TSS, BOD5, COD, NO2-N, NO3-N, TAN and TP) showed a significant improvement from 2009 to 2019. The pH parameter did not display notable variation during the studied period. The feed conversion ratio (FCR) was also investigated and exhibited a significant improvement from 1.4 to 1.1. Conclusion: Based on the decrease of all the investigated parameters, it is possible to say that extrusion is currently an excellent processing feed technique in aquaculture with a good level of respect for the environment.

In contrast to astrocytes in mammals, fish astrocytes promote axon regeneration after brain injury and actively participate in the regeneration process. Neurolin, a regeneration-associated, Zn8-labeled protein, is involved in the repair of damaged optic nerve in goldfish. At 1 week after unilateral eye injury, the expression of neurolin in the optic nerve and chiasm, and the expression of Pax6 that influences nervous system development in various brain regions in the rainbow trout (Oncorhynchus mykiss) were detected. Immunohistochemical staining revealed that the number of Zn8+ cells in the optic nerve head and intraorbital segment was obviously increased, and the increase in Zn8+ cells was also observed in the proximal and distal parts of injured optic nerve. This suggests that Zn8+ astrocytes participate in optic nerve regeneration. ELISA results revealed that Pax6 protein increased obviously at 1 week post-injury. Immunohistochemical staining revealed the appearance of Pax6+ neurogenic niches and a larger number of neural precursor cells, which are mainly from Pax6+ radial glia cells, in the nuclei of the diencephalon and optic tectum of rainbow trout (Oncorhynchus mykiss). Taken together, unilateral eye injury can cause optic nerve reaction, and the formation of neurogenic niches is likely a compensation phenomenon during the repair process of optic nerve injury in rainbow trout (Oncorhynchus mykiss).