Therapeutic and prophylactic immunization against Streptococcus iniae infection in hybrid striped bass (Morone chrysops×Morone saxatilis)

Vaccination strategies have traditionally been used as preventative or prophylactic measures against disease (prophylactic immunization) in uninfected fish. Alternatively, therapeutic or remedial measures, such as antibiotic administration, are commonly employed to treat disease in infected fish. Vaccination as a therapeutic measure (therapeutic immunization), however, has not been adequately explored in sub‐clinically infected fish. Therapeutic and prophylactic immunization with three Streptococcus iniae vaccines, formalin‐killed whole S. iniae cells (FKC vaccine), concentrated S. iniae extracellular products (greater than 2 kDa) (ECP vaccine) and a combination of killed cells and extracellular products (FKC+ECP vaccine), were tested in hybrid striped bass, Morone chrysops×Morone saxatilis, previously naturally infected with S. iniae. Fish (mean weight 10.0 g) were injected intraperitoneally (IP) or intramuscularly (IM) with one of each of the vaccines, tryptic soy broth (TSB‐control) or non‐injected (non‐injected control) to evaluate therapeutic effects (Trial 1). Survivors of the natural infection and ECP and FKC+ECP vaccine immunization and another lot of non‐injected control fish were immersion challenged with 1.47 × 10⁶ CFU of S. iniae mL^?1 at 44 days post‐immunization to evaluate vaccine efficacy (Trial 2). Hybrid striped bass (1.0 g) were also IM injected with S. iniae ECP vaccine at an aquaculture facility and immersion challenged with 1.47 × 10⁶ CFU of S. iniae mL^?1 12 weeks post‐immunization (Trial 3). The ECP and FKC+ECP vaccines, regardless of injection route, significantly (P<0.001) increased survival in asymptomatic, sub‐clinically infected fish thereby providing therapeutic merit. Hybrid bass immunized IP or IM had mean per cent survival values ranging from 78 to 96 at 44 days post‐immunization (Trial 1) and 69–97 post challenge (Trial 2). Survival of fish injected with TSB or immunized with FKC vaccine was significantly lowered and ranged from 12 to 13 by IP injection and 40 to 50 by IM injection and thus, the FKC vaccine had no therapeutic effect. The survival of hybrid striped bass IM immunized with S. iniae ECP vaccine in field Trial 3 was 91 and the RPS was 83. These results demonstrate that therapeutic immunization using S. iniae ECP and FKC+ECP vaccines can control a natural S. iniae infection. Furthermore, S. iniae ECP or FKC+ECP vaccines can also be used prophylacticly to protect hybrid striped bass against subsequent pathogen challenge.     

Dose-dependent influences of dietary β-1,3-glucan on innate immunity and disease resistance of hybrid striped bass Morone chrysops×Morone saxatilis

To investigate potential use of dietary β-1,3-glucan for health management of hybrid striped bass, juvenile fish were fed diets supplemented with yeast glucan (MacroGuard^®) at 0.05%, 0.1% or 0.2% of diet for 4 weeks, followed by immune response assays and a bath challenge with Streptococcus iniae . Dietary glucan significantly ( P <0.05) enhanced neutrophil oxidative radical production, and fish fed 0.1% glucan had a significant ( P <0.05) reduction in mortality (10%) after bacterial challenge compared with fish fed the control diet (46.7%). However, accumulative mortality of fish fed 0.2% glucan was not significantly different from that of fish fed the control diet. To further elucidate this observation, macrophages from sub-adult hybrid striped bass were isolated and cultured in L-15 medium with 10% foetal calf serum and penicillin/streptomycin supplemented with 0.2, 0.5, 1, 2, 5, 20 and 100 μg soluble glucan (MacroGuard^®) mL⁻¹ for 24 and 48 h. Intracellular superoxide anion production was significantly ( P <0.001) increased by 0.5 μg glucan mL⁻¹, but significantly ( P <0.001) suppressed by doses >5 μg glucan mL⁻¹. It is concluded that dietary yeast glucan has potential for use in diet formulations of hybrid striped bass to limit the adverse effects of S. iniae , but dosage should be an important consideration in administration.     

Selective breeding for the hybrid striped bass (Morone chrysops, Rafinesque ×M. saxatilis, Walbaum) industry: status and perspectives

The hybrid striped bass (HSB) farming industry has considerable potential for growth into domestic retail markets, but expansion of this industry is limited by high production costs that dictate high prices for HSB. It is widely recognized within the industry that selective breeding of an improved HSB will be required to increase production efficiency and reduce market prices. A National Program of Genetic Improvement and Selective Breeding for the HSB Industry has been initiated and some progress has been made toward domestication of the parent species of the hybrid. However, uncertainty remains as to which breeding procedures will most rapidly yield sustainable genetic gains in key production traits. This paper consolidates and reviews general information on the biology of temperate basses (genus Morone) relevant to selective breeding of improved HSB. The topics covered include control of reproduction, geographic distribution of stocks and population genetic variation. This is followed by a brief review of the current application of selective breeding techniques, including those based on molecular markers. Finally, we discuss potential avenues for genetic improvement of HSB in a selective breeding program.     

Dietary vitamin E requirement of hybrid striped bass (Morone chrysops female × M. saxatilis male)

This study was conducted to determine the dietary vitamin E requirement of juvenile hybrid striped bass ( Morone chrysops female ×  Morone saxatilis male). Semi-purified diets supplemented with 0.2 mg Se kg⁻¹ from Na₂SeO₃ and either 0 (basal), 10, 20, 40, 60, or 80 mg vitamin E kg⁻¹ as  DL -α-tocopheryl acetate were fed to hybrid striped bass initially averaging 1.8 ± 0.1 g (mean ± SD) for 12 weeks. Fish fed the basal diet, which contained 5.8 mg α-tocopherol kg⁻¹ dry weight, were darker in colour and had reduced weight gain, as well as generally reduced haematocrit values compared with fish fed diets supplemented with vitamin E. In addition, fish fed diets containing less than 20 mg supplemental vitamin E kg⁻¹ had significantly ( P  < 0.05) reduced weight gain and feed efficiency compared with those fed diets supplemented with vitamin E at 20–80 mg kg⁻¹. Dietary supplementation of vitamin E caused incremental increases in the concentration of α-tocopherol in both plasma and liver tissues. However, hybrid striped bass fed graded levels of vitamin E did not exhibit a dose response in terms of ascorbic acid-stimulated lipid peroxidation of hepatic microsomes. Regression analysis of weight gain data using the broken-line model indicated a minimum vitamin E requirement ( ±  SE) of 28 ( ±  3) mg kg⁻¹ dry diet. Based on these data, the dietary vitamin E requirement of hybrid striped bass appears to be similar to that determined for other fish species.     

Quantitative dietary lysine requirement of juvenile striped bass Morone saxatilis

Two feeding trials of 8 and 10 weeks each were conducted to quantify the dietary lysine requirement of juvenile striped bass, Morone saxatilis. Diets in both experiments contained approximately 420 g crude protein kg^–1 and 13.4 MJ digestible energy (DE) kg^?1. L ‐Lysine‐HCl was added to the basal diet to yield five and six treatments in the two experiments. Diets in the first experiment were determined to contain 9.2, 14.1, 14.6, 19.9 and 21.0 g available lysine kg^?1 on a dry‐matter basis. Diets in the second experiment were determined to contain 14.8, 18.1, 21.3, 24.5, 27.6 and 30.9 g available lysine kg^?1 on a dry‐matter basis. Weight gain, specific growth rate (SGR), feed conversion ratio (FCR), and apparent nitrogen utilization (ANU) were significantly (P < 0.05) improved by increasing dietary lysine concentrations to approximately 20 g kg^?1 of diet. Least‐squares regression analysis of weight gain and SGR in the first experiment indicated a minimum dietary lysine requirement of 20.1 ± 2 g kg^?1 dry diet. Least‐squares regression analysis of the same criteria measured in the second experiment yielded the following estimates of dietary lysine requirements (g kg^?1 dry diet): 19.8 ± 2.3 for weight gain, 21.7 ± 1.5 for SGR, 23.7 ± 3.5 for FCR and 18.6 ± 1.3 for ANU. From these results the minimum recommended dietary lysine requirement for optimal growth of juvenile striped bass is approximately 21 g kg^?1 dry diet which equates to 49 g kg^?1 dietary protein or 1.57 mg kJ^?1 DE. Although higher than that reported for hybrid striped bass, this requirement level is similar to those reported for many other fish species.     

Selenium nutrition of hybrid striped bass (Morone chrysops × M. saxatilis) bioavailability, toxicity and interaction with vitamin E

Two concurrent 12-week feeding trials were conducted to evaluate the bioavailability of inorganic sodium selenite and organic seleno-DL-methionine and to investigate the potential interaction between selenium and vitamin E in juvenile hybrid striped bass. In experiment 1, purified diets utilizing casein, gelatin and an amino acid premix as protein sources with a basal selenium concentration of 0.11 mg Se   kg⁻¹ were supplemented with either Na₂SeO₃ to provide selenium concentrations of 1.19, 2.00, 5.17 and 21.23 mg Se kg⁻¹ or with seleno-DL-methionine to provide 0.90, 1.26 and 2.55 mg Se kg⁻¹ and fed to juvenile hybrid striped bass in aquaria. A second experiment evaluated potential interactions by feeding these purified diets with or without supplemental vitamin E or sodium selenite, singularly or in combination. No overt selenium deficiency signs were exhibited by fish in either of the experiments; however, signs of selenium toxicity including retarded weight gain (WG), reduced feed intake and feed efficiency ratio (FER) as well as increased mortality, were observed in fish fed the diet containing more than 20 mg Se kg⁻¹. Whole-body selenium and whole-body selenium retention were linearly influenced by sodium selenite and selenomethionine. However, there was no significant effect of dietary selenium, vitamin E or their interaction on WG, FER and survival. Slope-ratio analysis showed that bioavailability of seleno-DL-methionine as a selenium source for juvenile hybrid striped bass was significantly ( P  < 0.01) higher (3.3-fold) than sodium selenite.     

Hypoxia affects performance traits and body composition of juvenile hybrid striped bass (Morone chrysops × M. saxatilis)

Performance traits and body composition of juvenile hybrid striped bass (Morone chrysops × M. saxatilis) in response to hypoxia were evaluated in replicate tanks maintained at constant dissolved oxygen concentrations that averaged 23.0 ± 2.3%, 39.7 ± 3.0% and 105.5 ± 9.5% dissolved oxygen saturation. Fish were fed a commercially formulated feed daily to apparent satiation. Total feed intake and fish growth and yield increased linearly in response to increased dissolved oxygen concentration. Nutrient utilization was reduced significantly only at the greatest level of hypoxia. With the exception of whole body protein content, whole body compositional indices and nutrient retention efficiencies were linearly related to dissolved oxygen concentration. Results demonstrate that as hypoxia becomes more severe, juvenile hybrid striped bass feed intake is reduced, which affects growth and nutrient retention.     

Differentiation of striped bass, Morone saxatilis (Walbaum), from a hybrid: effectiveness of training

Abstract The ability of consumers and fishermen to distinguish-between wild striped bass, Morone saxatilis (Walbaum), and its hybrids may be important to marketing farm-raised fish and managing wild stocks. The ability of randomly selected persons to distinguish between wild and hybrid specimens was examined and it was found that a significant improvement in recognition occurred with only minimal training. Average correct response of 70% without training rose to nearly 90% after training. Moreover, the respondents' sex and age were important factors in determining successful identification.     

Dietary cholesterol and lecithin have limited effects on growth and body composition of hybrid striped bass (Morone chrysops × M. saxatilis)

The effects of cholesterol and lecithin on growth and body composition of juvenile hybrid striped bass ( Morone chrysops  ×  M. saxatilis ) were investigated by feeding juvenile hybrids (initial weight 5.0 g) diets containing cholesterol at either 0 or 1% and lecithin at either 0, 2, 4, or 6% in a 2 × 4 factorial design. Each of the eight diets was fed to fish in triplicate 38-L aquaria maintained as a brackish water recirculating system for 8 weeks. Weight gain, feed efficiency, muscle ratio and hepatosomatic index were not significantly ( P  > 0.05) affected by dietary supplementation of cholesterol or lecithin. Supplementation of the diet with lecithin at 4 and 6% significantly ( P  < 0.05) decreased intraperitoneal fat accumulation regardless of dietary cholesterol level. Neither muscle nor liver lipid levels were significantly altered by dietary supplementation of cholesterol or lecithin although both liver and plasma lipid classes were affected. Dietary cholesterol decreased concentrations of liver and plasma free fatty acids and liver phospholipids while increasing concentrations of liver triglycerides and plasma phospholipids. Dietary lecithin did not consistently affect plasma and liver lipid classes although changes in phospholipid levels approaching significance ( P =0.0502 and P =0.0513, respectively) were observed. Thus it is concluded that dietary supplementation with cholesterol or lecithin had no substantial beneficial effects on growth or body composition of juvenile hybrid striped bass.     

Hepatic glucose utilization and lipogenesis of hybrid striped bass (Morone chrysops × Morone saxatilis) in response to dietary carbohydrate level and complexity

The influence of carbohydrate level and complexity on in vitro hepatic glucose utilization and lipogenesis were determined in hybrid striped bass, Morone chrysops ♀ × Morone saxatilis ♂. Six isocaloric, isonitrogenous diets containing glucose, maltose, or dextrin at two different levels (200 or 400 g kg^?1 diet) were fed to adult fish for 15 weeks. Liver explants were obtained at near‐maximum postprandial glycaemic response and incubated with radioactive labelled substrates. Glycogen synthesis from [¹⁴C]glucose was not different among treatments and was less than ¹⁴CO₂ formation. ¹⁴CO₂ production increased as a function of carbohydrate level but was unrelated to carbohydrate complexity. There was no detectable conversion of [¹⁴C]glucose to lactate for any treatment. Rates of de novo lipogenesis from [1‐¹⁴C]acetate were high in comparison to [U‐¹⁴C]glucose or [9,10‐³H]palmitate incorporation into liver lipids and differed in response to carbohydrate level and complexity. [9,10‐³H]palmitate esterification was an order of magnitude less than glycogen and CO₂ production but 4–10 times greater than [¹⁴C]glucose incorporation into liver lipids. Palmitate incorporation did not differ among treatments. Incorporation of [¹⁴C]glucose into liver lipids was higher in fish fed diets containing 400 g kg^?1 carbohydrate. These data support the idea that glucose is not a major oxidative substrate in hybrid striped bass and indicate that the level of soluble carbohydrate should be limited to 200 g kg^?1 diet or less for hybrid striped bass.     

Dietary tryptophan requirement of hybrid striped bass (Morone chrysops × M. saxatilis)

Requirements for six of the 10 indispensable amino acids (IAA) have not been quantified for hybrid striped bass. In this study, we estimate the requirement for l ‐tryptophan by nonlinear regression analysis of several growth indicators. Fifteen isocaloric, isonitrogenous diets were formulated to contain 1.0, 1.3, 1.6, 1.9, 2.2, 2.5, 2.8, 3.1, 3.4, 3.7, 4.0, 5.0, 6.0, 10.0 or 14 g l ‐tryptophan kg^?1 diet and fed to reciprocal cross hybrid striped bass for 7 weeks. After 5 weeks, survival of fish receiving the basal diet was 40% while surviving fish fed this diet were in poor health. Survival of fish receiving 1.3 g Trp kg^?1 diet declined to 70% by termination of the trial. Survival in other treatments was 100%. Fish weight gain was 100% or greater for fish receiving 1.6 g Trp kg^?1 diet or more. Hepatosomatic index, muscle ratio and intraperitoneal fat (IPF) ratio also responded to dietary tryptophan concentration. IPF was inversely related to dietary tryptophan concentration. The dietary tryptophan requirement was estimated to be between 2.1 and 2.5 g kg^?1 diet (6–7 g kg^?1 protein), depending on response variable, using four‐ and five‐parameter saturation kinetics models. These findings will increase the precision of diets formulated for hybrid striped bass.     

Dietary threonine requirement of juvenile hybrid striped bass (Morone chrysops♀×M. saxatilis♂)

Two growth studies were conducted to determine the dietary threonine requirement of reciprocal cross hybrid striped (sunshine) bass. Semipurified diets were prepared with crystalline amino acids and lyophilized fish muscle to supply 350 g crude protein kg⁻¹ diet. The basal diet contained 4.9 g threonine kg⁻¹ from fish muscle, and test diets were supplemented with graded levels of L-threonine. In the first experiment, fish initially averaging ≊ 9.8 g each were fed diets containing threonine levels of 4.9, 7.5, 10.0, 12.5, 15.0 and 17.5 g kg⁻¹ dry diet for 7 weeks. Weight gain, feed efficiency and protein efficiency ratio (PER) were significantly ( P < 0.01) influenced by dietary threonine level. Based on weight-gain responses, a threonine requirement (± SE) of 8.4 (± 0.8) g kg⁻¹ dry diet was determined, and dietary threonine levels of 10.0 g kg⁻¹ diet or greater resulted in the highest levels of free threonine in plasma.
Based on the results of the first experiment, a second feeding trial was conducted with diets containing threonine levels of 4.9, 6.5, 8.0, 9.5, 11.0 and 12.5 g kg⁻¹ dry diet. Fish initially averaging ≊ 3.0 g each were fed each diet for 8 weeks. Weight gain, feed efficiency and PER values of fish were markedly improved, with increases in dietary threonine up to 8.0 g kg⁻¹ dry diet. Regression analysis of weight gain, feed efficiency and PER data using the broken-line model resulted in threonine requirement estimates of 9.7, 8.5 and 8.6 g kg⁻¹ dry diet, respectively. Based on these data, the threonine requirement of juvenile sunshine bass was determined to be ≊ 9.0 g kg⁻¹ dry diet or 26 g kg⁻¹ of dietary protein.     

Amino acid availability from animal, blended, and plant feedstuffs for hybrid striped bass (Morone chrysops × M. saxatilis)

The refinement of hybrid striped bass feed formulations has been progressing rapidly. There is still, however, limited information available regarding nutrient digestibility and amino acid availability from common feedstuffs. A pair of experiments was conducted to determine the protein digestibility and amino acid availability to sunshine bass from an assortment of commercially available animal protein feedstuffs, blended animal products and plant protein feedstuffs in extruded diets. The feedstuffs tested were blood meal, poultry by‐product meal, fish solubles, Pro‐Pak 60, Pro‐Pak 65, ProCon 65RDB, and 60FMC for the animal protein feedstuffs and brewer's yeast, canola meal, peanut meal, and sunflower meal for the plant protein feedstuffs. Test diets consisted of a mixture of nutritionally complete reference diet and test ingredient. Triplicate tanks of fish were fed their respective diets for 7 days prior to collection of faeces by stripping. Apparent digestibility coefficients of protein (ADC‐CP) in the animal products ranged from 47% for 60FMC to a high of 70% for fish solubles. ADC‐CPs for animal products were not different across products. Blood meal, poultry by‐product meal, Pro‐Pak 60, Pro‐Pak 65, and ProCon had intermediate ADC‐CPs of 63, 55, 63, 57 and 52%, respectively. ADC‐CP in plant feedstuffs ranged from 43% for canola meal to 80% for peanut meal. Brewer's yeast, canola meal, and sunflower meal had intermediate ADC‐CPs at 54, 43 and 69%, respectively. Apparent amino acid availability coefficients were variable across animal products and did not necessarily correlate to the ADC‐CPs for a given feedstuff. Isoleucine availability was low in blood meal at 38% compared with 59% or better for the remaining amino acids. Lysine, tyrosine and phenylalanine availability from fish solubles was low at 31, 35 and 44%, respectively. Amino acid availability from Pro‐Pak 60 was consistently higher across all amino acids for the animal products and blends tested. Of the plant products tested, peanut meal was the best performing feedstuff relative to amino acid availability.     

The effect of phytase on apparent digestibility of four practical plant feedstuffs fed to striped bass, Morone saxatilis

This study was conducted to determine the effect of phytase on apparent dry matter (DM), crude protein and phosphorus (P) digestibility of four plant feedstuffs (isolated soya protein, ISP; soyabean meal, SBM; corn gluten meal, CGM and wheat middlings, WM) fed to striped bass Morone saxatilis . One reference diet was formulated based solely on animal feed ingredients. Four test diets were prepared by mixing 60–75% of the reference diet with 40–25% of the test ingredient in order to reach similar levels of total P (0.67–0.73%) and to have a minimum of 35% protein (36.1–53.0%). Chromic oxide was added at 0.5% of the diet and used as an indigestible marker. A phytase solution was sprayed postpelleting on the test diets at a concentration of 1000 phytase units kg⁻¹ (PU kg⁻¹) dry diet. The test diets (with or without phytase supplementation) and the reference diet (nine treatments total) were fed to 3-year-old striped bass and were replicated in time ( n =3) during the 4-week period. Digestibility of DM was significantly lower ( P  < 0.001) for SBM and WM when compared with CGM and ISP and was not influenced by phytase supplementation. Crude protein digestibility was also not affected by phytase supplementation and was significantly lower ( P  < 0.09) for SBM when compared with ISP. Phosphorus digestibility was improved by approximately 23% with the addition of phytase for all four feedstuffs tested ( P  < 0.001). Furthermore, WM had significantly lower ( P  < 0.0001) P digestibility when compared with the other feedstuffs, irrespective of phytase supplementation.     

Genetic effects on carcass-quality traits in hybrid striped bass (Morone chrysops♀×Morone saxatilis♂)

A 10 × 10 factorial mating design was used to examine the genetic effects on various carcass‐quality traits in hybrid striped bass (Morone chrysops♀×Morone saxatilis♂). A total of 448 offspring were raised in a ‘common‐garden’ environment and carcass‐quality traits were assessed at 389 days post fertilization; parentage of each fish was inferred from genotypes at 5–10 nuclear‐encoded microsatellites. There was a significant effect of dam and sire on body weight and fillet weight and a significant effect of dam on viscera weight and condition factor. When carcass‐quality traits were standardized to body weight, there was a significant effect of dam, sire and dam × sire interaction on viscera percentage (proportion of viscera weight to body weight). Phenotypic correlations between body weight and non‐standardized carcass‐quality traits were high (r_p=0.80–0.92), whereas phenotypic correlations between body weight and standardized carcass‐quality traits were low (r_p=0.13–0.19). Genetic correlations between body weight and fillet weight (dams and sires), and body weight and viscera weight (dams only) were significant and positive, while the genetic correlations between body weight and viscera percentage (dams and sires) were significant and negative. Significant general combining ability values for favourable carcass‐quality traits in the same breeder (dam or sire) occurred only in one or two of 20 (dams and sires) used in this study, suggesting that simultaneous improvement in multiple traits in hybrid striped bass may require evaluation of large numbers of candidate breeders. The effect of sex on body weight and all non‐standardized carcass‐quality traits was significant and appeared to be due to faster growth in females. These results indicate that selection for faster growth of hybrid striped bass would lead to a correlated increase in fillet weight and viscera weight but not necessarily to an increase in the proportion of fillet and waste product generated. The significant dam, sire and dam × sire interaction effects on viscera percentage and the negative genetic correlations between body weight and viscera percentage indicate that selection for reduced proportion of viscera relative to body weight might be feasible.     

Lysine supplementation of commercial fishmeal‐free diet in hybrid striped bass Morone chrysops x M. saxatilis affects expression of growth‐related genes

Our recent results in hybrid striped bass (HSB) concluded that ideal protein theory accurately predicts first‐limiting amino acids in commercial diet formulations if accurate amino acid availability data are used and that appropriate levels of supplemental lysine are needed to improve fish performance from fishmeal‐free diets. Our goal in this study was to elucidate how dietary lysine supplementation of a commercial fishmeal‐free diet influences the expression of two genes, myostatin and myogenin, controlling myogenesis in differentially growing groups of HSB. Real‐time RT‐PCR results in HSB suggest that the levels of lysine added to the diet (17.8, 35.1, 51.0 g kg^?1 of diet) has an impact on myogenin relative to the basal unsupplemented diet, but no effect on myostatin. Moreover, our data further suggest that the amount of dietary lysine supplementation influenced the ratio of myostatin/myogenin expression in HSB and that this pattern mimicked that of most of the growth, composition of growth and nutrient retention data from our previous study and may therefore be a useful marker for selecting fish for improved growth performance.     

Effect of triiodothyronine on the growth and survival of larval striped bass (Morone saxatilis)

This study was carried out to test the effect of triiodothyronine (T₃) on the growth and survival of larval striped bass (Morone saxatilis). Growth and survival of striped bass held in 5 ppt seawater and treated with various doses of T₃ were measured beginning at 5 and 16 days after hatching. Body content of T₃ was measured by radioimmunoassay. T₃ dissolved in the 5 ppt seawater was taken up by larval striped bass in a dose-dependent manner, and affected the growth and survival of the fish. At 5 days after hatching, T₃ at 100 ng ml^–1 and 50 ng ml^–1 retarded the growth of larval striped bass and caused a lower survival rate than T₃ at 25 ng ml^–1 or the control treatment. At 16 days after hatching, T₃ at 100 ng ml^–1 retarded the growth of larval fish and caused a higher mortality. T₃ at 10 ng ml^–1 and 1 ng ml^–1 did not show any effect on either survival or growth. Body content of T₃ returns to control levels within days following end of treatment. The results indicate that exogenous T₃ can be detrimental to the growth and survival of larval striped bass.     

Refined understanding of sulphur amino acid nutrition in hybrid striped bass, Morone chrysops♀×M. saxatilis♂

Previous studies have indicated that the level of total sulphur amino acids (TSAA) (methionine+cystine) is most limiting in practical diet formulations for hybrid striped bass (HSB), especially if animal feedstuffs are replaced with plant feedstuffs. Reduction in costly animal feedstuffs such as fish meal, while maintaining adequate dietary levels of TSAA, may enhance the cost effectiveness of production. Therefore, this study investigated three different aspects of sulphur amino acid nutrition of HSB including: (1) the efficacy of crystalline methionine hydroxy analogue (MHA) and liquid MHA (Alimet?) relative to l‐ methionine in meeting the requirement for TSAA; (2) the cystine‐sparing value for methionine; and (3) the influence of various sulphur amino acid supplements on ammonia excretion. During three separate feeding trials (2, 6 and 10 weeks in duration), juvenile HSB were fed various diets including a basal diet deficient in TSAA (0.33% or 0.51% of diet), and experimental diets supplemented on an equal‐sulphur basis with different levels of either l‐ methionine, Alimet? or crystalline MHA. Diets containing TSAA at 1% of diet and different ratios of cystine to methionine (60:40, 55:45, 50:50 and 45:55) were also fed to re‐evaluate the sparing effects of cystine on methionine. In trial 1, over the course of 10 weeks, Alimet? was 73% as effective in promoting weight gain as l ‐methionine at the same concentration while MHA was 83% as effective. After 6 weeks in trial 2, fish fed Alimet? at 1.25% of diet displayed similar growth performance as those fed TSAA at 1.0% of diet, while weight gain of fish fed Alimet? at 1% was only 58% of that displayed by fish fed TSAA at 1.0%. No significant differences in weight gain, feed utilization or survival were observed among fish fed diets containing various ratios of cystine to methionine, although the diet with 60:40 cystine to methionine had the lowest numerical responses. Inclusion of MHA or Alimet? did not affect TAN excretion of HSB. These findings will aid in refining diet formulations for HSB to ensure adequate sulphur amino acid nutrition.