Background: Supplementing aquafeeds with emulsifiers can enhance lipid utilization, yet the physiological effects of lysolecithin, derived from enzymatic lecithin conversion, remain under-explored. Objectives: This study examined the effects of lysolecithin supplementation on hepatopancreatic transcriptome and gut microbiota in largemouth bass (Micropterus salmoides) fed stearin-based high-lipid diets. Methods: Two diets were formulated: a control containing 130 g kg⁻¹ stearin fish oil (SO), and in the experimental diet (SL), 3.1 g kg⁻¹ rapeseed oil was replaced with 3.1 g kg⁻¹ lysolecithin oil. Each diet was fed to three replicate groups for 56 days. Hepatopancreas and distal intestine were sampled for transcriptome profiling, and gut microbiota were characterized at 28 and 56 days. Results: Lysolecithin supplementation resulted in 424 differentially expressed genes compared with the control (322 up- and 102 downregulated). KEGG enrichment indicated major effects on lipid metabolic processes, notably activation of the PI3K-AKT signaling pathway, enhanced adipocyte lipolysis, and modulation of adipocytokine signaling, suggesting improved insulin sensitivity and lipid mobilization. Histological analysis showed mild distal intestinal inflammation in the SO group. Gut microbiota composition shifted over time; lysolecithin increased the relative abundance of Cetobacterium and reduced potential opportunistic taxa compared with the control. Conclusions: Overall, dietary inclusion of lysolecithin improved lipid utilization in largemouth bass, likely by enhancing lipid metabolism and promoting beneficial gut microbial profiles. These findings support lysolecithin as a promising feed additive for optimizing high-lipid aquafeeds.

While the detrimental effects of high temperature stress on fish growth and disease resistance have been widely reported, its impact on muscle quality has received limited attention. In this study, largemouth bass Micropterus salmoides with an initial body weight of 45.73 g were subjected to a 60-day growth trial (~25 °C), followed by a 5-day acute warming phase and a subsequent 30-day chronic high temperature exposure (32 °C). Through integrated analyses of morphological parameters, texture characteristics, TUNEL assay, gene expression analysis, and metabolomics in muscle, the effects of high temperature stress on the meat quality of largemouth bass were systematically examined. The results showed that high temperature stress significantly upregulated key genes in the ubiquitin-proteasome pathway (trim13, foxo1α) and key genes in the autophagy-lysosome pathways (lc3α, lc3β, bcl2l1, ctsl2), induced apoptosis in muscle cells, and led to significant reductions in myofiber diameter and density. In terms of textural properties, high temperature stress significantly decreased parameters such as springiness, adhesiveness, and cohesiveness, as well as water holding capacity. Metabolomic analysis further revealed that high temperature induced remodeling of energy metabolism and significant reprogramming of purine and amino acid metabolic pathways, resulting in decreased levels of key flavor compounds, including IMP, GMP, flavor amino acids (glutamic acid, alanine, methionine, arginine, proline), and peptides (glu-glu-lys and glu-cys-gly), thereby adversely affecting muscle flavor quality. The findings of this study provide a theoretical basis for understanding the impact of thermal stress on the eating quality of farmed fish.

The replacement of fishmeal (FM) in aquafeeds for carnivorous fish remains challenging due to reduced palatability and adverse effects on liver health and intestinal microbiota. Marine by-products-based additives containing fish protein hydrolysates and seaweed polysaccharides have shown potential to overcome these limitations. This study evaluated the effects of graded supplementation of Haiweisu (HWS), a multi-marine by-product formulated with squid viscera hydrolysate, small-molecule components from fish protein hydrolysate, seaweed polysaccharides, and seaweed residue as a carrier, in a FM-free diet for juvenile largemouth bass. Four isonitrogenous and isolipidic diets were prepared: a FM-free control diet (CON) and three diets supplemented with 10, 20, or 30 g/kg HWS (designated S10, S20, and S30, respectively). Each diet was fed to triplicate groups of fish (29.26 ± 2.61 g) for 56 days. Results showed that HWS supplementation linearly increased final body weight, weight gain rate, and feed intake, while significantly reducing the feed conversion ratio (p < 0.05). All HWS-supplemented groups exhibited markedly lower hepatic lipid accumulation and plasma total cholesterol levels compared with the CON group, accompanied by alleviated hepatocellular steatosis and inflammatory infiltration as revealed by Oil Red O and H&E staining. Moreover, HWS significantly enhanced intestinal microbiota alpha diversity (Ace, Chao, Sobs, and Shannon indices), decreased the relative abundance of the dominant genus Mesomycoplasma, and enriched potentially beneficial genera including Methylobacterium, Delftia, and Sphingomonas (p < 0.05). In conclusion, dietary HWS supplementation effectively improved growth performance, alleviated hepatic steatosis and inflammation, and beneficially reshaped the intestinal microbiota in juvenile largemouth bass fed a FM-free diet. These findings support HWS as a promising functional additive for sustainable FM-free aquafeeds in carnivorous fish species.

Metabolic disorders induced by high‐starch diets are the primary cause limiting the use of carbohydrates in aquafeeds. This study aimed to investigate the intervention effects of zinc on metabolic disorders induced by high‐starch diets in Micropterus salmoides . Five diets were formulated, including a control diet (115 g kg ⁻¹ starch) and three high‐starch diets (163 g kg ⁻¹ starch) supplemented with 0 (HS‐LZn), 20 (HS‐MZn) and 150 (HS‐HZn) mg kg ⁻¹ Zn. After 8 weeks of feeding, intraperitoneal glucose (Glu) tolerance tests (IPGTTs) and intraperitoneal insulin tolerance tests (IPITTs) were conducted to evaluate insulin sensitivity. High‐starch diets inhibited feed intake but improved feed utilization. Diet HS‐HZn significantly increased Zn content in the hepatopancreas. Zn retention decreased significantly as the dietary Zn supplementation level increased from 0 to 150 mg/kg. High‐starch diets significantly improved the retentions of protein and energy. High‐starch diets exerted more pronounced effects on glycogen metabolism and glycolysis. Diets HS‐LZn and HS‐MZn induced severe hepatic glycogen accumulation, concomitant with significant decreases in activities of hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK), glycogen synthase (GS) kinase‐3β (GSK‐3β), and acid alpha‐glucosidase (GAA). Dietary 150 mg/kg Zn supplementation significantly decreased hepatic glycogen and increased activities of HK, PFK, citrate synthase (CS), and GAA. High‐starch diets did not induce severe lipid metabolic disturbances or obvious fat deposition. The insulin tolerance test revealed that the area under the curve (AUC) and incremental AUC (iAUC) for Glu in the HS‐LZn group were significantly higher than the control and HS‐HZn groups. Meanwhile, phosphorylated AKT (p‐AKT) level and immunofluorescence intensity were significantly decreased in the HS‐LZn group, while zinc improved AKT phosphorylation. Untargeted metabolomics analysis demonstrated that high‐starch diets without zinc supplementation inhibited multiple amino acid metabolic pathways.

Hydropower regulation can alter riverine habitats and potentially influence fish diet, gut microbiota, and nutritional characteristics. In this study, upstream and downstream populations of Hemibagrus guttatus (H. guttatus) were investigated in a dam-regulated reach of the Hongshui River associated with the Datengxia Water Conservancy Project using 12S rRNA and COI metabarcoding, 16S rRNA gene sequencing, and muscle nutritional analyses. Fish from the two habitats exhibited distinct dietary compositions, accompanied by significant differences in gut microbial diversity, community structure, and predicted microbial functions. Upstream fish harbored higher microbial diversity and greater abundances of Firmicutes-associated taxa, including several putatively beneficial bacteria, whereas downstream fish showed enrichment of Proteobacteria-associated taxa and functions related to environmental adaptation. Muscle composition also differed between habitats, with upstream fish containing higher moisture and crude lipid contents and downstream fish exhibiting higher crude protein content. These habitat-associated differences suggest that natural dietary resources and intestinal microbiota are closely linked to host nutritional characteristics. These findings highlight the importance of considering upstream–downstream habitat origin when evaluating the trophic, microbial, and nutritional characteristics of H. guttatus and provide a biological reference for the management and conservation of fish populations in dam-regulated rivers.