Assessment on the effects of different lipid sources in juvenile largemouth bass (Micropterus salmoides) diets.

Simple SummaryNowadays, owing to its limited availability and high cost, fish meal is no longer an affordable protein source in fish feed. Therefore, it is necessary to find new sustainable protein sources to replace fishmeal in the diet of fish or in aquafeeds. Recently, yellow mealworm (Tenebrio molitor) has been widely used as a protein source in the formulated feed of broilers, pigs, shrimp and fish, due to its high protein content, excellent amino acid profile and abundant functional substances. In this study, we found that yellow mealworm meal has high digestibility in largemouth bass (Micropterus salmoides) and an appropriate level (less than 19.52%) in the diet can promote growth and improve liver health in largemouth bass. Therefore, we assume that utilization of yellow mealworm in the diet of largemouth bass as a protein source is feasible.This study investigated the effects of yellow mealworm meal (TM) on growth performance, hepatic health and digestibility in juvenile largemouth bass (Micropterus salmoides). The fish were fed with the basic feed and the test feed (70% basic feed and 30% raw materials) containing Cr2O3, and feces were collected for digestibility determination. The fish were fed with five isonitrogenous (47% crude protein) and isolipidic (13% crude lipid) diets, in which fishmeal (FM) was replaced with 0% (TM0), 12% (TM12), 24% (TM24), 36% (TM36) and 48% (TM48) TM. The fish were reared in cylindrical plastic tanks in a recirculating aquaculture system for 11 weeks. The apparent digestibility coefficients (ADC), of dry matter, crude protein and crude lipid, in largemouth bass of TM were 74.66%, 91.03% and 90.91%, respectively. The ADC of total amino acid (TAA) of TM in largemouth bass was 92.89%, and the ADC of essential amino acid (EAA) in TM in largemouth bass was 93.86%. The final body weight (FBW), weight gain rate (WGR) and specific growth rate (SGR) in the TM24 group were significantly higher than those in other groups. Similarly, the highest mRNA expression levels of hepatic protein metabolism genes (pi3k, mtor, 4ebp2 and got) and antioxidant enzyme (glutathione peroxidase, Gpx; catalase, Cat) activities were observed in the TM24 group. Moreover, the expression levels of anti-inflammatory factors (il-10 and tgf) in liver were up-regulated and the expression levels of pro-inflammatory factors (il-8 and il-1β) in liver were down-regulated. Quadratic regression model analysis, based on weight gain rate (WGR) against dietary TM level, indicated that the optimum level of dietary TM replacing FM in largemouth bass diet was 19.52%. Appropriate replacement levels (less than 36%) of FM by TM in the diets can enhance the antioxidant capacity and immunity of largemouth bass. However, high levels of FM substitution with TM (more than 48%) in the feeds can damage the liver health and inhibit the growth of largemouth bass. Notably, largemouth bass has high ADC and high utilization of TM, which indicates that it is feasible to use TM as feed protein source for largemouth bass.

Use of fermented tea residues as a feed additive and effects on growth performance, body composition, intestinal enzyme activities, and inflammatory biomarkers in juvenile largemouth bass (Micropterus salmoides)

Effects of different lipid sources on growth performance, fatty acids composition in tissue and expression of genes related to lipid metabolism in largemouth bass (Micropterus salmoides)

The present study investigated the effects of Astragalus membranaceus extract (AME) on growth performance, immune response, and energy metabolism of juvenile largemouth bass (Micropterus salmoides). Seven diets containing 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and 0.6% AME (Con, AME0.1, AME0.2, AME0.3, AME0.4, AME0.5, and AME0.6 groups) were formulated and fed to M. salmoides for 8 weeks. Final body weight (FBW), feed intake (FI), weight gain (WG), and specific growth rate (SGR) were all significantly higher in AME0.4 group than in Con group (P < 0.05). Feed conversion rate (FCR) was significantly improved in AME0.5 group compared with Con group (P < 0.05). Whole-body crude protein contents were significantly increased in AME0.2 group (P < 0.05). Whole-body crude lipid contents were significantly lower in AME0.2 and AME0.3 groups, while muscle lipid was upregulated by dietary AME (P < 0.05). Hepatic malondialdehyde (MDA) contents were significantly lowered in AME0.3 and AME0.4 groups, and catalase (CAT) activities were significantly increased in AME0.1 and AME0.2 groups (P < 0.05). Plasma aspartate aminotransferase (AST) level was significantly lowered in AME0.5, and AME0.6 groups, and alanine aminotransferase (ALT) level was lowered in AME0.5 groups (P < 0.05). Plasma triglyceride was declined in AME0.6 group, and glucose was decreased by 0.3%-0.5% AME (P < 0.05). Significantly higher hepatocyte diameter, lamina propria width, and submucosal layer thickness were recorded in AME0.6 groups, while the longest villi height was obtained in AME0.2 and AME0.3 groups (P < 0.05). The mRNA expression levels of insulin-like growth factor 1 (igf1) revealed the growth-promoting effect of AME. The anti-inflammatory and antiapoptotic effects of AME were demonstrated by transcription levels of interleukin 8 (il-8), tumor necrosis factor-alpha (tnf-a), caspase, B-cell lymphoma-xl (Bcl-xl), bcl-2 associated x (Bax), and bcl-2-associated death protein (Bad). The transcription levels of lipid metabolism and gluconeogenesis related genes, including acetyl-CoA carboxylase alpha (acc1), fatty acid synthase (fasn), fatty acid binding protein 1 (fabp1), phosphoenolpyruvate carboxykinase 2 (pepck2), and glucose-6-phosphatase catalytic subunit 1a (g6pc), were reduced by AME treatment, while the levels of glycolysis-related genes, including glucokinase (gck) and pyruvate kinase (pk), were the highest in AME0.2 and AME0.3 groups (P < 0.05). According to polynomial regression analysis of SGR, WG, FCR, whole-body crude lipid, MDA, and ALT, the optimal AME supplementation level was estimated to be 0.320%-0.429% of the diet. These results provided insights into the roles of AME in regulating immunity and metabolism, which highly indicated its potential as immunostimulants and metabolic regulators in diverse aquatic animals.

The aim of this study was to investigate the effects of sodium butyrate (SB) supplementation on growth performance, antioxidant enzyme activities, inflammatory factors, and hypoxic stress in largemouth bass (Micropterus salmoides). Diets were supplemented with different doses of SB at 0 (SB0), 0.5 (SB1), 1.0 (SB2) and 2.0 (SB3) g/kg. The hypoxic stress experiment was performed after 56 days of culture. The results showed that compared with the SB0 group, the final body weight, weight gain rate and protein deposition rate of the SB3 group were significantly increased (P<0.05), while FCR was significantly decreased (P<0.05). The contents of dry matter, crude lipids, and ash in the SB2 group were significantly higher than those in the SB0 group (P<0.05). The urea level was significantly decreased (P<0.05), and the glucose content was significantly increased (P<0.05) in the SB supplement group. Compared with the SB0 group, the SB2 group had significant reductions in the levels of serum triglyceride, cholesterol, elevated-density lipoprotein cholesterol, and low-density lipoprotein (P<0.05), and significant reductions in the levels of liver alkaline phosphatase and malondialdehyde (P<0.05). The total antioxidant capacity of the SB1 group was higher than that of other groups (P<0.05). Compared with the SB0 group, the mRNA expression of TLR22, MyD88, TGF-β1, IL-1β and IL-8 in the SB2 group significantly decreased (P<0.05). The cumulative mortality rate was significantly decreased in the SB2 and SB3 groups in comparison with that in the SB0 group after three hours of hypoxic stress (P<0.05). In a 56-day feeding trial, SB enhanced largemouth bass growthby increasing antioxidant enzyme activity and inhibiting TLR22-MyD88 signaling, therefore increasing cumulative mortality from hypoxic stress in largemouth bass.

This study evaluated the optimal dietary copper (Cu) levels and their effects on growth performance, body composition, and antioxidant capacity in juvenile largemouth bass (Micropterus salmoides). A total of 360 fish (initial average weight (1.67 ± 0.01 g) and initial average length (2.5 ± 0.2 cm)) were randomly assigned to 18 tanks, each containing 20 fish and six dietary Cu concentrations: 2.13 (control), 3.00, 3.66, 4.58, 4.64, and 5.72 mg/kg. The results indicated that fish receiving 3.00 mg/kg of Cu exhibited the best final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR), with a significantly reduced feed conversion ratio (FCR). While body composition (moisture, protein, lipid, and ash) remained consistent across groups, plasma total protein (TP) levels increased with Cu supplementation. Elevated triglycerides (TG) and albumin (ALB) were noted at 4.64 and 5.72 mg/kg, respectively, while glucose (GLU) levels decreased with an increase in dietary Cu. Antioxidant capacity, assessed via hepatic glutathione (GSH) and the activities of catalase (CAT), and showed significant improvements at 3.00 and 3.66 mg/kg Cu, while superoxide dismutase (SOD) showed the highest activity at a dietary Cu level of 5.72 mg/kg. Additionally, the expressions of tgf-β and tnf-α genes were significantly upregulated at a dietary Cu level of 5.72 mg/kg, while il-8 and il-10 genes were upregulated at dietary 3.66 mg/kg. The expression of nrf2 was significantly upregulated in response to a dietary Cu level of 3.66 mg/kg compared to the control group, and the expression of the keap1 gene was significantly upregulated in the fish fed with 5.72 mg/kg of dietary Cu. The results indicated that appropriate dietary supplementation could promote the growth performance and improve the antioxidant status the immunity of largemouth bass, and the optimal Cu requirement for FCR and SGR were approximately 3.10 mg/kg and 3.00 mg/kg, respectively.

This study evaluated the effects of dietary soybean lecithin (SBL) on the growth, haematological indices, immunities, antioxidant capabilities, and inflammatory and intestinal barrier functions because little information of dietary SBL could be obtained in juvenile largemouth bass (Micropterus salmoides). The fish were fed identical diets except for SBL added at 0, 2, 4 and 8%. It was found that 4 and 8% SBL significantly increased fish weight gain and daily growth rate (p < 0.05), while 4% SBL was optimal for enhancing RBC, HGB, PLT, MCV, MCH, WBC and MON in blood, and ALB and ALP in serum (p < 0.05). SBL (4%) also significantly elevated the antioxidant enzymes activities of T-SOD, CAT, GR, GPx, GST and T-AOC and GSH contents; increased mRNA transcription levels of Nrf2, Cu/Zn-SOD, CAT, GR, GST3 and GPx3; and decreased MDA contents. Keap1a and Keap1b levels were markedly down-regulated (p < 0.05). SBL (4%) significantly enhanced levels of the immune factors (ACP, LZM and C3) and the mRNA expression levels of innate immune-related genes (C3, C4, CFD, HEPC and MHC-I) compared with the control groups (0%) (p < 0.05). SBL (4%) significantly increased IgM and T-NOS in the intestine (p < 0.05) and significantly decreased levels of TNF-α, IL-8, IL-1β and IFN-γ and increased TGF-β1 at both transcription and protein levels in the liver and intestine (p < 0.05). The mRNA expression levels of MAPK13, MAPK14 and NF-κB P65 were significantly decreased in the intestine in the 4% SBL groups (p < 0.05). Histological sections also demonstrated that 4% SBL protected intestinal morphological structures compared with controls. This included increased intestinal villus height and muscular thickness (p < 0.05). Furthermore, the mRNA expression levels of the intestinal epithelial cell tight junction proteins (TJs) (ZO-1, claudin-3, claudin-4, claudin-5, claudin-23 and claudin-34) and mucin-5AC were significantly up-regulated in the 4% SBL groups compared with the controls (p < 0.05). In conclusion, these results suggested that 4% dietary SBL could not only improve growth, haematological indices, antioxidant capabilities, immune responses and intestinal functions, but also alleviate inflammatory responses, thereby providing reference information for the feed formulations in cultured largemouth bass.

Dietary valine affects growth performance, intestinal immune and antioxidant capacity in juvenile largemouth bass (Micropterus salmoides)

Previous studies have shown that the critical swimming speed (Ucnt) of largemouth bass, Micropterus salmoides Lacépède, is significantly influenced by a number of factors including body mass (Beamish, 1970), training (Farlinger and Beamish, 1978), water temperature (Hocutt, 1973; Kolok, 1991) and photoperiod (Kolok, 1991). These studies share a common approach in that they were primarily concerned with comparing mean values of Ucrit of fish from different treatment groups, and individual variation within the groups was treated as statistical noise. While this approach is valid, it may overlook a significant source of performance variation - the variation found among individuals.Recent research (for a review, see Bennett, 1987) into the locomotor performance of amphibians and reptiles has suggested that individual variation is substantial and repeatable (i.e. performance variation of an individual forced to perform multiple times will be minor compared to the variation found when a number of individuals are forced to perform once). This research has been conducted on animals at a constant temperature, and also before and after an acute temperature change. No study has been done to determine whether performance repeatability is maintained following a chronic change in temperature. This may be particularly important with respect to fishes because many fish undergo physiological and morphological changes when subjected to chronic changes in water temperature. In the current study, variations in the Ucrit of individual bass were quantified, and performance repeatability was established at 11 and 22°C and after an acute and chronic decrease in water temperature from 20 to 10°C.Juvenile largemouth bass were obtained from the Wray Fish Hatchery, Colorado Division of Wildlife, during November and August. The fish used during November (N=7) varied from 7.8 to 9.8cm (mean 9.0cm) in fork length and from 6.2 to 12.2 g (mean 9.0 g) in body mass. The fish used during August (N=21) varied from 8.7 to 11.0 cm (mean 9.4 cm) in fork length and from 8.2 to 17.2 g (mean 11.4 g) in body mass. Correlations between Ucrit (when expressed in terms of body lengths per second, BLs−1) and fork length and body mass were not significant for the fish in either season.The bass were collected from holding ponds, then transported to Boulder where they were held at field water temperatures and photoperiods (November 11°C, 12h:12h light:dark; August 20-22°C, 14h:10h light:dark). All fish were fed commercial fish pellets ad libitum once per week, a feeding regime at which the fish maintained their body mass but did not grow. The fish were fasted for a minimum of 48 h before they were challenged with a bout of swimming.Ucnt was determined using the swim chamber previously described by Kolok (1991). Water continually circulated from a 2001 reservoir into a 2501 trough holding the submersed swim chamber. Water in the trough was chilled to the desired temperature using a chiller unit partially immersed in the reservoir. A 0.5 horsepower (373 W) water pump was used to generate water flow within the swim chamber. Water flows within the chamber were measured using an on-line pitottype flowmeter and the water flow was straightened using two flow straighteners located 20 and 40cm from the pump discharge region. The fish swam in a 7.7cm inner diameter 55 cm long clear acrylic tube attached downstream from the flow straighteners. Black plastic shaded the middle 35 cm portion of the swim chamber and fish generally remained in the shaded area until they were close to fatigue.The on-line flowmeter was calibrated using a Marsh-McBirney electronic flow meter and direct volume displacement. Water flow patterns within the chamber were analyzed using a 20 MHz pulsed Doppler system and a Doppler probe consisting of a 20 MHz ultrasonic crystal mounted in an 8 mm diameter plastic tube. Water flow was determined at 13 positions within the chamber at water velocities of 20 and 50 cm s−1, and was found to be essentially rectilinear at both velocities.Ucrit values were generated using a protocol modified from that of Kolok (1991). Individuals were placed into the swim chamber and given 1.5 h to habituate to a water velocity of 15cms−1. The 1.5 h habituation period was much shorter than the 24h period recommended by Beamish (1978). However, Kolok (1991) found that there were no significant differences between the performances of largemouth bass habituated to the chamber for 1.5 or 24 h at either 5 or 20°C. At the end of the habituation period, the fish were subjected to a swimming challenge in which water velocity was increased by 5 cm s−1 every 20 min until fatigue. The fish used in this study were approximately 10cm in fork length; therefore, the velocity increment used was approximately 0.5 BL s−1. A fish was assumed to be fatigued when it could no longer be encouraged, by gentle prodding, to swim off the flow straighteners at the rear of the chamber.For the repeatability tests at a constant temperature, seven fish during November (11°C) and nine fish during August (22°C) were challenged with two bouts of swimming. The fish were given at least 4 days post-capture to habituate to the laboratory environment before their first swimming challenge. Immediately following recovery from the first bout of swimming, each fish was weighed, measured for fork length and uniquely freeze-branded. The fish were then given 4 days to recover from the first swimming challenge (the fish were offered food on the second day after the challenge) before the performance challenge was repeated.The mean Ucrit of the fish swimming at 22°C was 3.81 BLs−1 during the first trial and 3.84 BLs−1 during the second. The mean Ucrit of the fish swimming at 11°C was 3.02 BLs−1 during the first trial and 3.01 BLs−1 during the second trial. These Ucrit values are consistent with previous findings on nonbranded juvenile large-mouth bass at similar temperatures; Farlinger and Beamish (1978) obtained a Ucrit of 3.41 BLs−1 at 25°C, while Kolok (1991) found it to be 3.60 BLs−1 at 22°C and 2.91 BLs−1 at 10°C.Variation in the individual values of Ucnt of fish at 11 and 22°C were substantial and repeatable. Ucrit values of individual fish from trial 1 varied from 2.56 to 3.44BLs−1 at 11°C, while at 22°C they varied from 3.17 to 4.03 BLs−1 (Fig. 1). This degree of variation was substantial enough that the best performers at 11°C had higher Ucnt values than the worst performer at 22°C. Correlations between the rank order performance of the fish in trial 1 and 2 were significant at 11°C (Spearman rank correlation coefficient, N=7, p=0.86, P=0.036) and 22°C (N=9, p=0.77, P=0.030), suggesting repeatability of performance. These results are consistent with the results from other ectothermic vertebrates, specifically lizards, snakes and toads (Bennett, 1987).For the repeatability tests in which water temperature varied, 12 fish were challenged with a bout of swimming at 20°C. Immediately following recovery from the first bout of swimming, each fish was weighed, measured for fork length and uniquely freeze-branded. Four days after the last fish had swum, the water temperature in the holding tanks was decreased to 10°C over 2 days. This rate of change was chosen because preliminary data suggested that a more rapid transfer would lead to significant mortality.The fish were then challenged with two bouts of swimming at 10°C, the first bout starting 48 h after the temperature drop and the second bout approximately 4 weeks after the drop in temperature. Because there was only one swim chamber available, there was a 48 h variation in the exposure time to the cold water between the first and last fish challenged with a bout of swimming. Variation in the amount of time the fish were subjected to cold water, however, was not significantly correlated with swimming performance in either the acute (N=12, r2=0.015, P=0.70) or chronic (N=12, r2 = 0.0002, P=0.96) challenge. At the end of the experiment, the fish were weighed and measured for fork length a second time. Neither body mass nor fork length changed significantly (t-test, P=0.7 and 0.9, respectively) during the 4 week acclimation period.There were significant differences (Kruskal Wallis, P<0.05) in the swimming performance of the fish acclimated to 20°C, 10°C (acute) and 10°C (chronic). The mean Ucrit of the fish decreased significantly (nonparametric multiple comparison, Zar, 1984) from 3.77 BL s−1 to 2.43 BL s−1 when they were exposed to a drop in water temperature from 20 to 10°C over 48 h. After 4 weeks at 10°C, the mean i/crit of these fish increased from 2.43 to 2.63 BLs−1, but this increase was not statistically significant. The performance of individual largemouth bass within the three treatment conditions was quite variable, with the difference between the best and worst performer being 0.97BLs−1 at 20°C, 1.48 BLs−1 at 10°C (acute) and 0.88BLs−1 at 10°C (chronic) (Fig. 2). When these performances were ranked, there was a significant correlation between the rank order performances of the fish in the three groups (Kendall correlation of concordance, N =12, W=0.75, P<0.025).Significant correlations between the rank order performance of a number of individuals before and after an acute temperature change have been shown for a number of lizard species (Bennett, 1980; Huey and Hertz, 1984), for yearling rainbow trout (J. E. Keen and A. P. Farrell, personal communication) and now for juvenile largemouth bass. These results suggest that in fish, as well as in other ectothermic vertebrates, individuals do not specialize at being good performers at either cold or warm temperatures. It appears, however, that the best performers remain so regardless of the temperature at which they perform.Fish acclimated to cold water frequently have elevated swimming performances compared to fish acutely exposed to the same water temperature (Roots and Prosser, 1962; Griffiths and Alderdice, 1972). Prior to the current study on largemouth bass, it was unknown whether an acclimatory response to cold water would significantly alter the rank order performance of the acclimated fish. This was not the case for the largemouth bass acclimated to 10°C for 4 weeks. While the rank order of one or two largemouth bass changed dramatically after the acclimation period, the significant correlation among the rank order performances of the fish at 20°C, 10°C (acute) and 10°C (chronic) was maintained (Fig. 2). This result suggests that the effect of acclimation to cold water was consistent from individual to individual, and that the best-performing fish would remain so even after acclimation to a different water temperature.The results of this study suggest that variation in the swimming performance of individual juvenile largemouth bass is substantial and repeatable for fish tested twice at one temperature, tested at different temperatures, or tested after a 4 week acclimation to a different temperature. These results strongly suggest that individual variation in Ucrit is more than statistical noise and that it is a source of variation that can be exploited when designing future experiments.All of the largemouth bass used in this study were graciously provided by the Colorado Division of Wildlife's Wray Hatchery.

A 56-day feeding experiment was conducted to determine the dietary lysine requirement of juvenile largemouth bass (Micropterus salmoides) and investigate the effects of dietary lysine on growth, whole-body composition, and hepatic gene expression related to glycolipid metabolism via the AKT/FoxO1 pathway. The juveniles ( 17.34 ± 0.02 g) were fed six graded lysine levels (2.11% (control), 2.56%, 2.92%, 3.33%, 3.68%, and 4.09%, dry diet). The results showed that the 3.33% dietary lysine level significantly increased the final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) and improved the feed conversion ratio (FCR) compared with the control group. The whole-body composition was not significantly affected by dietary lysine levels, while lowest hepatic lipid contents were found in the 2.92% and 3.33% dietary lysine groups. Regarding glycolipid metabolism, compared with the control group, 3.33% dietary lysine improved the protein kinase B (AKT) and inhibited the forkhead box O1 (FoxO1), thus upregulated the pyruvate kinase (PK) mRNA levels to enhance glycolysis. Furthermore, sterol-regulatory element binding protein-1c (SREBP1c) and peroxisome proliferator-activated receptor-γ (PPAR-γ) were downregulated by 3.33% dietary lysine, which caused the downregulation of lipid synthesis-related genes acetyl-CoA carboxylase-1 (ACC) and stearyl-CoA desaturase (SCD) mRNA. In addition, 3.33% dietary lysine promoted the expression of the lipolysis-related genes peroxisome proliferator-activated receptor-α (PPAR-α) and carnitine palmitoyl transferase-1 (CPT1). According to the quadratic regression analysis based on the FCR and SGR values, the optimal dietary lysine levels were estimated to be 3.03% and 3.07% of the diet (6.39% and 6.48% of dietary protein), respectively.

Intestinal flora and immunity response to different viscous diets in juvenile largemouth bass, Micropterus salmoides

The study aimed to evaluate the effects of fermented tea residue (FT) on growth performance, intestinal morphology, liver antioxidant capacity and Aeromonas hydrophila infection in juvenile Largemouth bass. A total of 240 fish were randomly distributed in 12 tanks with 20 fish per tank (4 treatments with 3 replications) and fed with diets FT at the rate of 0 (control), 2, 4 and 6%. The weight gain rate (WGR), specific growth rate (SGR) and intestinal villi height (VH) of juvenile largemouth bass were significantly higher than those of the control group after feeding FT (P&amp;lt; 0.05); meanwhile, the liver superoxide dismutase (SOD), glutathione peroxidase (GSH-PX) and catalase (CAT) activities of juvenile largemouth bass were significantly higher and the malondialdehyde (MDA) levels were significantly lower than those of the control group after feeding FT (P&amp;lt; 0.05). Mortality occurred in all groups of largemouth bass after the injection of A.hydrophila, but feeding FT reduced the cumulative mortality compared with the control group (P&amp;lt; 0.05). In juvenile largemouth bass infected with A.hydrophila, the relative mRNA expression of the intestinal anti-inflammatory factors IL-10 and TGF-α was significantly higher and that of the pro-inflammatory factors IL-1, IL-15, IL-8, and TNF-α was significantly lower (P&amp;lt; 0.05). In summary, it can be seen that a 2% FT addition can improve the liver antioxidant capacity of juvenile largemouth bass, enhance the resistance to A.hydrophila and increase the growth of largemouth bass.

Aquaculture plays a crucial role in China’s agricultural sector, with improved growth performance and feed efficiency in cultured species representing key industry challenges. Among nutritional strategies, feed attractants have received increasing attention for their ability to stimulate feeding behavior and enhance feed utilization. This study hypothesized that dietary supplementation with a formulated feed attractant would enhance feeding activity, improve physiological condition, and modulate antioxidant and immune responses in juvenile largemouth bass (Micropterus salmoides). A total of 270 fish (initial weight: 12.5 ± 2.5 g) were randomly assigned to three groups: CON (basal diet), YXX0.05 (basal diet + 0.05% attractant), and YXX0.1 (basal diet + 0.10% attractant). After 56 days of feeding, no significant differences were observed in final body weight (FBW) or whole-body composition (p &gt; 0.05). However, feed intake (FI) increased by 5.1%, and the condition factor (CF) improved significantly by 7.6% (p &lt; 0.05) in the YXX0.05 group, while the viscerosomatic index (VSI) was reduced by 3.6% (p &lt; 0.05), suggesting enhanced feeding motivation and improved body compactness. In terms of physiological responses, compared to the control group, the YXX0.05 group exhibited a significant increase in liver total antioxidant capacity (T-AOC) (p &lt; 0.05), a significant decrease in malondialdehyde (MDA) content (p &lt; 0.05), an increase in serum lysozyme (LZM) activity (p &lt; 0.05), and a significant decrease in aspartate aminotransferase (AST) activity (p &lt; 0.05), reflecting enhanced immune status and potential liver protection. In conclusion, although growth performance metrics such as FBW and specific growth rate (SGR) remained unchanged, the feed attractant at 0.05% inclusion significantly improved feed intake, body condition, and physiological health markers. These results suggest the attractant has practical value in improving fish welfare and nutrient utilization efficiency, providing a functional dietary strategy for sustainable largemouth bass farming.

Largemouth bass virus (LMBV) is an iridovirus that was isolated from wild adult largemouth bass Micropterus salmoides in the southeastern United States in 1994. Although originally isolated from moribund wild fish, its virulence to juvenile largemouth bass is uncertain. To help clarify this point, two LMBV titrations were made in juvenile largemouth bass. Titers of LMBV in fathead minnow cells were 104.8 and 105.8 tissue culture infectious doses—50% cytopathic endpoint (TCID50) per milliliter, respectively. Tenfold serial dilutions of LMBV employed in each cell culture titration, injected intraperitoneally (0.1 mL/fish) into largemouth bass produced calculated lethal dose—50% mortality endpoints (LD50s) of 282 (102.45) and 288 (102.46) infectious doses in two consecutive infectivity trials. Virus yield of assayed infected fish averaged 108.5 TCID50/g and 107.7 TCID50/g in viscera of moribund and dead fish in the two trials and 106.5 TCID50/g in surviving exposed fish 14 d after infection. In a second experiment, largemouth bass had 100% mortality 5 d after injection while virus immersed fish had a significantly (P ≤ 0.005) lower mortality of 17% at 14 d. Similarly treated juvenile striped bass Morone saxatilis suffered 63% mortality after injection and significantly (P ≤ 0.005) lower mortality of 10% after immersion. In a third study of 25 d, 100% of injected largemouth bass died by 5 d after injection, and all of them were virus-positive. Injected striped bass had a significantly (P ≤ 0.005) lower mortality of 24%; all three fish were virus-positive initially, two fish were virus-positive at 18 d, and none were positive at 25 d. Juvenile largemouth bass were highly susceptible to LMBV injection and striped bass were moderately susceptible, but both species were only mildly susceptible when exposed by immersion.

First-year overwinter survival is a frequent bottleneck to the recruitment of Largemouth Bass Micropterus salmoides. Early ontogeny to piscivory provides increased overwinter survival through increased growth and the accumulation of lipids. This ontogeny is thought to be slowed by dense and complex vegetative habitats, but this hypothesis has not been directly tested. To address this question, we conducted enclosure experiments for 5 weeks during the typical time that juvenile Largemouth Bass would transition to piscivory (i.e., midsummer). Thirty-two enclosures were constructed across two 0.10-ha ponds and given one of four stem densities of simulated vegetation (0, 50, 250, and 500 stems/m2). Three juvenile Largemouth Bass and 30 juvenile Bluegills Lepomis macrochirus were added to each enclosure. All fish were sampled twice per week. Largemouth Bass growth was measured on all sample dates, and stomach samples were collected to determine diets on one date each week. Bluegill sizes and densities were manipulated to maintain the number of fish at 30 fish that were 25–35% of Largemouth Bass TL after each sampling event. Largemouth Bass stomach contents (percent by weight) were initially dominated by insects, and the bass transitioned to mostly fish prey by the end of the experiment. The use of fish prey, as measured by the presence of surviving Bluegills in the enclosures, significantly increased at the beginning of the second week at all stem densities, but fewer fish prey were eaten by bass in the 250-stems/m2 treatment than in all other treatments throughout the experiment. Largemouth Bass also grew less in the 250-stems/m2 treatment. We concluded that vegetation density does affect the foraging rate of piscivorous juvenile Largemouth Bass, but not necessarily the timing of the ontogeny to piscivory. However, differing results between this experiment and other previously published studies suggests vegetation may have an interactive effect with available prey types.