Combination effects of dietary EPA and DHA plus alpha‐tocopherol: effects on performance and physiological status of Caspian brown trout (Salmo trutta caspius) fry

The effects of dietary n‐3 highly unsaturated fatty acids [eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA)] with α‐tocopherol on growth, non‐specific immune response and oxidative status were investigated in Caspian brown trout, Salmo trutta caspius, fry. Six experimental diets containing three different dietary levels of n‐3 HUFAs (low: 1 + 0.5% of total fatty acids, DHA+ EPA, medium: 2 + 1%, DHA + EPA, high: 4 + 2%, DHA + EPA) with two different levels of α‐tocopherol (low: 300 and high: 1000 mg kg^?1 diet) were prepared and named: LL, LH, ML, MH, HL and HH (HUFA/α‐tocopherol) groups, respectively. Diets were fed to triplicate groups of 60 fry with an initial weight of 600 ± 25 mg for 10 weeks. Results showed that increase in dietary DHA and EPA up to high level improved fry growth in terms of the body weight and specific growth rate, particularly when dietary α‐tocopherol levels were high, suggesting a higher antioxidant protection value when these fatty acids are high. At all dietary n‐3 HUFA levels, increase in α‐tocopherol from low to high level enhanced the alternative complement (ACH50) activity. Fry fed diets medium and high n‐3 HUFA displaying significantly higher lysozyme activity (P < 0.05). Moreover, fish fed medium or high levels of n‐3 HUFA had significantly lower prostaglandin E2 (PGE2) than those fed low n‐3 HUFA (P < 0.05). Significant differences in antioxidant enzymes (catalase, glutathione peroxidase, glutathione S‐transferase, glutathione reductase and superoxide dismutase) activity were also observed between groups, with higher activity in high levels of n‐3 HUFA (P < 0.05). Results of this study suggest that the effect of dietary n‐3 HUFA on examined non‐specific immunity parameters are not uniform; however, these impacts are closely related to the α‐tocopherol supplement and their interaction. In conclusion, increased dietary levels of n‐3 HUFA and α‐tocopherol would enhance growth performance and welfare of this species.     

Efficacy of 17 α‐methyl testosterone and letrozole on sex reversal of protogynous grouper,Epinephelus tauvina (Forskal, 1775) and spawning performance of sex‐reversed males

This study aimed to test the efficacy of 17 α‐methyl testosterone (17 α‐MT) alone and in combination with letrozole, an aromatase inhibitor, for the induction of sex reversal in protogynous greasy grouper, Epinephelus tauvina. Further, the long‐lasting effects of these treatments and spawning performance of sex‐reversed males were also investigated. Greasy grouper with oocytes in the perinucleolus stage were implanted with 5 mg 17 α‐MT kg^?1 body weight (T₁), 5 mg 17 α‐MT and 0.2 mg letrozole kg^?1 body weight (T₂) and 5 mg 17 α‐MT with 0.4 mg letrozole kg^?1 body weight (T₃) and no androgens/enzyme inhibitor implanted (C). The 17 α‐MT alone and in combination of letrozole‐induced sex reversal in greasy grouper, whereas untreated control fish (C) showed normal ovarian development. However, T₂ and T₃ group showed 100% sex reversal and completion of spermatogenesis up to functional male phase in 2 and 3 months, respectively, whereas T₁ group resulted in only 66.67% functional male with motile spermatozoa after 4 months. Sex‐reversed males successfully fertilized the eggs during induced spawning. There were significant differences on fertilization and hatching rates between T₂ group (79.00 ± 4.36%; 77.67 ± 2.87%, respectively) and T₁ group (57.67 ± 3.17%; 63.87 ± 2.91%, respectively). The result suggested that 17 α‐MT (5.0 mg kg^?1 BW) in combination with letrozole (0.2 mg kg^?1 BW) has the potential to produce 100% sex‐reversed male in short period in greasy grouper, which might greatly help in seed production of greasy grouper.     

The effects of different proportions of the 17β‐estradiol and 17α‐methyltestosterone on growth,sex reversal and skin colouration of the electric blue hap (Sciaenochromis ahli Trewavas, 1935)

This study was performed to investigate the effects of 17β‐estradiol (ES) and 17α‐methyltestosterone (MT) on growth, development, survival, sex ratio and colour change in the electric blue hap (Sciaenochromis ahli Trewavas, 1935). The hormones were not supplemented to the control feed, while six other feeds were prepared by adding 20, 40 and 60 mg kg^?1 17β‐ES or 20, 40 and 60 mg kg^?1 17α‐MT to each, resulting in seven different feed treatments. Average live weight of the fish supplemented with these diets was 0.42 ± 0.04 g. At the end of the study, the highest weight gain was observed in fish fed 60 mg kg^?1 17α‐MT group (2.62 ± 0.11 g) and the difference with the groups fed with 17β‐ES was found to be significant. All fish fed 17α‐MT were male, while the rates of feminization in fish fed 17β‐ES at 20, 40, 60 mg kg^?1 were 91.11%, 88.88% and 93.33% respectively. Survival rates were respectively determined as 80%, 95.56%, 84.44%, 93.33%, 77.78%, 84.44% and 84.44% for the control, 20, 40, 60 mg kg^?1 17β‐ES and 20, 40, 60 mg kg^?1 17α‐MT treatments. The best colouration was achieved in the 17α‐MT groups (P < 0.05). The L* values varied between 32.98 ± 4.44 and 61.35 ± 2.19, a* values between ?7.06 ± 0.22 and ?3.42 ± 0.11, and b* values between ?7.74 ± 0.10 and 11.65 ± 0.03, while Chroma (C*) and Hue (H°_ab) angle values varied between 7.54 ± 0.22 and 13.60 ± 0.01 and between 119.76 ± 0.05 and 239.73 ± 4.86. In conclusion, the 17α‐MT feeding was found to have a greater effect on the growth, feed conversion ratio, masculunization and pigmentation of the electric blue haps than the 17β‐ES treatment.     

Effects of total fish oil replacement to vegetable oils at two dietary lipid levels on the growth,body composition,haemato‐immunological and serum biochemical parameters in caspian brown trout (Salmo trutta caspius Kessler, 1877)

This research aimed to evaluate the effects of two dietary fat levels [low fat (LF) (10%), high fat (HF) (20%)] and sources [fish oil (FO), vegetable oil (VO)] on the growth and some physiological parameters of Caspian brown trout fingerlings for 60 days. Tuna oil or blends of canola and soybean oils (85:15) were added to diets to design four feeds namely LFFO, HFFO, LFVO and HFVO according to the fat levels and sources. The fish fed the LFFO diet had lower weight gain than the other fish (P<0.05). The total n‐6 fatty acids increased in fish fed diets with the blends of VO, while the total n‐3 fatty acids decreased in these fish (P<0.05). Serum lysozyme activity was higher in fish fed the HFVO diet than the other fish (P<0.05). Serum glucose, total cholesterol, triglyceride and very low‐density lipoprotein were lower in fish fed LFFO than the other fish (P<0.05). The present study demonstrates that in terms of fish growth, VOs can be used as an alternate source of dietary fat, whereas fish health and nutritional value are improved with the LFFO diet. According to these results, a partial substitution of FO by VO in high‐level fat diets is suggested for long‐term feeding of Caspian brown trout.     

Synchronization of ovulation in cultured northern whitefish (Coregonus peled,Gmelin 1788) using [D‐Arg6Pro9Net]‐sGnRH analogue and its effect on egg quality

Female northern whitefish were treated with salmon [D‐Arg⁶Pro⁹Net]‐sGnRHa emulsified in Freund′s incomplete adjuvant as a sustained release treatment (sGnRHa‐FIA) or with [D‐Arg⁶Pro⁹Net]‐sGnRHa dissolved in physiological saline as acute release treatment (sGnRHa‐PS). Females in four experimental groups (A, B, C, D) and one control group (E) were injected intraperitoneally as follows: A and B – sGnRHa‐FIA at doses of 50 and 25 μg kg^?1, respectively; C – double injection (DI) of sGnRHa‐PS at 25 μg kg^?1 administered 3 days apart; D – single injection (SI) of sGnRHa‐PS at 25 μg kg^?1; and E – control group receiving a 1:1 mixture of FIA and physiological saline. All treatments induced and synchronized ovulation. Mean time to ovulation was significantly reduced (P < 0.05) in GnRHa‐treated groups compared with control. Hormone treatments did not affect the relative fecundity. Slight differences were found in ovarian fluid pH between group A and D (P < 0.05). Except for group C, egg size was significantly reduced (P < 0.01) in hormonally synchronized groups compared with the control. Survival to the eyed stage of eggs from females in groups A, B and C was significantly lower (P < 0.01) than in groups D and E. Per cent hatched alevins was lower (P < 0.01) in groups A, B and C than in groups D and E. We conclude that synchronization of ovulation in northern whitefish can be induced by a single acute injection of [D‐Arg⁶Pro⁹NEt]‐sGnRHa at 25 μg kg^?1 BW if given near the natural spawning time determined by water temperature below 2°C.     

Characterization of β‐catenin 1 during the gonad development in the common carp (Cyprinus carpio)

β‐catenin gene is a pivotal gene for gonad development and maintenance of ovarian function in mammals. However, little is known about its expression and function in gonad development of fish. In this study, a complete cDNA (3342 bp) sequence of β‐catenin 1 was cloned from the common carp, Cyprinus carpio, by RACE PCR, which encodes a 780‐amino‐acid protein. Quantitative real‐time PCR demonstrated that β‐catenin 1 mRNA expressions were high in the testis and ovary tissue and the expression increased as the testes developed and the early stage ovaries developed. Western blot results revealed a single immunoreactive band with an estimated molecular weight of 90 kDa in testes. Immunohistochemistry analysis revealed that the β‐catenin 1 protein was concentrated mainly in the cytoplasm of early development stage of oocyte cells and in the cytomembrane of developing and mature sperm cells. 17β‐Ethinylestradiol injecting intraperitoneally into the fish decreased the relative β‐catenin 1 mRNA expression level except 1 μg/g 72 hr and 5 μg/g 48 hr of treatments in the ovary by real‐time PCR. These results suggest, for the first time, that β‐catenin 1 is an essential protein in gonad development and might be involved in ovarian early development of C. carpio.     

Comparative effect of Freund's adjuvant and Aloe vera L. gel on the expression of TNF‐α and IL‐1β in vaccinated Cyprinus carpio L. with Aeromonas hydrophila C. bacterin

The comparative effects of Freund's and Aloe vera gel as adjuvants on the expression of IL‐1β and TNF‐α genes were studied in vaccinated common carp (Cyprinus carpio) with Aeromonas hydrophila bacterin. Fishes were intraperitoneally immunized with A. hydrophila bacterin in combination with Aloe vera gel or Freund's and also without any adjuvant. At day 28 after immunization, all groups were challenged by lethal dose of A. hydrophila (10⁷ cells/fish). Changes in the expression of IL‐1β and TNF‐α genes were evaluated in anterior kidney before challenge and 12, 24, 72 and 7 days postchallenge using quantitative real‐time PCR. Higher expression levels of both genes were observed in all vaccinated groups compared with non‐immunized group. Fishes which received Aloe vera gel showed higher expression of IL‐1β and TNF‐α in relation to animals which vaccinated with or without Freund's adjuvant. We concluded that Aloe vera gel in compared with Freund's adjuvant had a more stimulatory effect on the expression of immune‐related genes in vaccinated common carp and it can use as a novel adjuvant in aquaculture.     

Effects of estradiol‐17β on survival,growth performance,gonadal structure and sex ratio of the tiger puffer,Takifugu rubripes (Temminck & Schlegel, 1850), fingerlings

Systematic research was performed to assess the survival, growth performance, gonadal structure and sex ratio during and after sex reversal induced by estradiol‐17β in tiger puffers. The fish of 20 days after hatching (dah) were immersed in different doses (0.2, 2, 20, 100 ppb) for 60 days. There were significant differences in survival rates among groups (p < .05). With the increase in the estradiol‐17β dose, the survival rates of tiger puffers decreased, revealing a linear regression relationship with log₁₀ (estradiol + 1; p < .05). The growth tendency of fish in the low‐dose group was similar to those in the control group over the same period. The growth in the 20 ppb dose group was significantly slower than in the low‐dose and control groups in fish of same age (p < .05), but was significantly faster than in the 100 ppb dose group (p < .05) after 60 days. Intersex fish were categorized into intersex I, II and III types, according to the feminization degree from low to high. Phenotypic intersex puffer fish were found in the 2, 20 and 100 ppb dose groups. Intersex ratios in 2, 20 and 100 ppb were 10%, 43.3% and 46.7%, respectively, the ratios except the ones in 2 ppb group being significantly higher than those in the control group (p < .05). Moreover, 100.0% male induction for intersex II and III types occurred in 20 and 100 ppb groups. Together, these results indicated that estradiol‐17β at a dose of 20 ppb was most effective for pseudo‐female production.     

Effect of dietary essential fatty acids on level of oestradiol‐17β and vitellogenin,reproductive performance and larval quality of striped catfish (Pangasianodon hypophthalmus) in out‐of‐spawning season

A study was conducted to determine the effect of essential fatty acid in striped catfish (Pangasianodon hypophthalmus) broodstock diets on a level of oestradiol‐17β and vitellogenin, reproductive performance and larval quality in out‐of‐spawning season. Five female and five male broodstock were stocked into a net cage (3 × 5 × 1.5 m³) and fed with pelleted diets containing 1.5% fish oil and different amounts of corn oil (CO) in diet formulation, that is 0%, 1%, 2% and 3% respectively. Gonadal development, oestradiol‐17ß and vitellogenin concentrations were examined every 2 weeks. Oestradiol‐17ß and vitellogenin concentrations showed an increase during the maturation process. The significantly highest percentage of female mature, fecundity, hatching rate, larval production and larval survival rate were obtained from fish fed with 2% corn oil in the diet. This result revealed that the administration of 2% corn oil in the diet as a source of n‐6 (linoleic acid) and 1.5% fish oil as a source of n‐3 (linolenic acid) can improve the reproductive performance of striped catfish broodstock in out‐of‐spawning season.     

Effects of α‐ketoglutarate on the growth performance,amino acid metabolism and related gene expression of mirror carp (Cyprinus carpio)

A 70‐day feeding experiment was conducted to assess the effects of α‐ketoglutarate (AKG) supplementation in different protein sources of diets on the growth performance, nitrogen metabolism and related gene expression of mirror carp. Two kinds of protein diets (soybean meal protein source and fish meal and soybean meal compound protein source) and two levels of AKG (0 g kg^?1 and 15 g kg^?1, respectively) were utilized. Each experimental diet was fed three times a day with five replicates of 20 fish per tank with an average individual weight of 217.93 ± 0.78 g. The results indicated that AKG significantly improved the protein efficiency ratio and decreased the feed conversion rate (FCR) (p < .05). Soybean protein significantly decreased the weight gain rate and increased the FCR (p < .05). In addition, diets with AKG significantly improved the concentration of glutamine and glutamic acid (Glu) in the foregut and hindgut (p < .05), as well as the glutamine synthetase (GS) activity in the foregut (p < .05). Soybean protein significantly decreased the concentration of Glu and GS activity in the hindgut (p < .05). AKG significantly improved the contents of threonine, alanine, proline and essential amino acid in muscle (p < .05). Dietary supplementation with AKG significantly improved GS gene expression in the intestine and reduced Rhesus glycoprotein (Rhag and Rhbg) gene expression in the gill (p < .05). From the results, we know that a diet with AKG is beneficial for the growth and amino acid metabolism of carp.     

Influence of dietary linoleic acid (18:2n‐6) and α‐linolenic acid (18:3n‐3) ratio on fatty acid composition of different tissues in freshwater fish Songpu mirror carp,Cyprinus Carpio

Six isonitrogenous and isoenergetic purified diets were formulated to feed Songpu mirror carp for 60 days. The control diet (CD) was only supplemented with soybean oil. The other five experimental diets contained soybean oil, linseed oil and lard oil blended at various inclusion levels to attain different linoleic acid (LA)/α‐linolenic acid (LNA) ratios (0.53, 1.04, 2.09, 3.95, 6.82) with a constant total C₁₈ polyunsaturated fatty acids (PUFA; LA+LNA, 2% dry weight) content. The fatty acid (FA) profiles of hepatopancreas, dorsal muscle, intestine, intraperitoneal fat (IPF), spleen and kidney reflected those of the diets, but with some differences. The spleen showed the lowest correlation with diet compared with other tissues, followed by the hepatopancreas (P < 0.05). The intestine and IPF showed relatively higher correlation. On the other hand, the control group had the lowest tissue‐diet correlation, followed by the LA/LNA0.53 group (P < 0.05), whereas the LA/LNA2.09 showed the highest. The LA/LNA ratios in the tissues were up‐regulated in the LA/LNA0.53, 1.04 groups and down‐regulated in the LA/LNA3.95, 6.82 groups. This was due to when LA (or LNA) was highly added in diet, the decrease in this FA was huge in tissue. The contents of saturated fatty acids and monounsaturated fatty acids increased in the control group, but seemed not influenced by dietary LA/LNA ratios. These results demonstrated that the FA deposition was tissue‐specific, and also influenced by the dietary FA composition in the experimental fish. Finally, we suggest that 2.09 is the optimal LA/LNA ratio (2% C₁₈ PUFA) of Songpu mirror carp for fillet FA composition.     

Dietary effects of poly‐β‐hydroxybutyrate on the growth performance,digestive enzyme activity,body composition,mineral uptake and bacterial challenge of rainbow trout fry (Oncorhynchus mykiss)

This study investigated the effects of diet supplementation with poly‐β‐hydroxybutyrate (PHB) on growth performance, digestive enzyme activity, lipid metabolism, mineral uptake and bacterial challenge of the rainbow trout fry (initial weight: 111.3 ± 13.7 mg) during a 6‐week experimental period. In the experimental set‐up, the commercial diet of rainbow trout fry was replaced with 0.5%, 1% and 2% PHB. The results of our work showed that the replacing of diets with PHB in experimental treatments decreases the weight of rainbow trout fry during the first 2 weeks before significantly increasing final weight at the end of the 6‐week period. PHB also improved digestive enzyme activity in experimental treatments. The highest total protease, pepsin activity and pancreatic enzyme secretion were observed with the 0.5% PHB treatment. A higher concentration of Na and K was observed in the whole body of the fry fed on 1% and 2% PHB‐supplemented diets. Enhanced survival rates occurred in all groups of fry after bath exposure to Yersinia ruckeri compared to rates in those fed the control diet. Our results suggest that the diet supplemented with PHB may improve growth performance, digestive enzyme activity and the functioning of the immune system. These positive effects could be considered for new applications in aquaculture.     

Biomarkers of bone development and oxidative stress in gilthead sea bream larvae fed microdiets with several levels of polar lipids and α‐tocopherol

Although dietary marine phospholipids are able to improve culture performance of marine fish larvae in a further extend than soybean lecithin, both types of phospholipids (PL) markedly increase oxidative risk. The inclusion of a fat‐soluble antioxidant such as the vitamin E α‐tocopherol could allow a better control of oxidative stress. The objective of this study was to determine the combined effect of graded levels of α‐tocopherol with different levels and sources of krill phospholipids (KPL) and soybean lecithin (SBL) on growth, survival, resistance to stress, oxidative status, bone metabolism‐related genes expression and biochemical composition of sea bream larvae. Sea bream larvae were completely weaned at 16 dph and fed for 30 days seven microdiets with three different levels of PL (0, 40 and 80 g kg^?1 diet) and two of α‐tocopherol 1500 and 3000 mg kg^?1 diet. Sea bream larvae fed diets without PL supplementation showed the lowest survival, growth and stress resistance, whereas increase in PL, particularly KPL, markedly promoted larval survival and growth. However, feeding SBL markedly increased TBARs and GPX gene expression increasing the peroxidation risk in the larvae. Besides, KPL inclusion improved incorporation of n‐3 HUFA and, particularly, EPA into larval tissues, these fatty acids being positively correlated with the expression of BMP‐4, RUNX 2, ALP, OC and OP genes and to bone mineralization for a given larval size class. The increase in dietary α‐tocopherol tends to improve growth in relation to the n‐3 HUFA levels in the diet, denoting the protective role of this vitamin against oxidation. Indeed, dietary α‐tocopherol decreased the oxidative stress in the larvae as denoted by the reduction in larval TBARs contents and gene expression of SOD and CAT, but not GPX. Thus, increase in dietary α‐tocopherol effectively prevented the formation of free radicals from HUFA, particularly EPA, but did not affect the incidence of bone anomalies or the expression of genes related to osteogenetic processes.     

Dietary α‐linolenic acid affects lipid metabolism and tissue fatty acid profile and induces apoptosis in intraperitoneal adipose tissue of juvenile grass carp (Ctenopharyngodon idella)

A 56‐day feeding trial was conducted to elucidate the effects and mechanism action of dietary α‐linolenic acid (ALA, 18:3n‐3) on lipid accumulation and fatty acid profile of muscle, hepatopancreas and intraperitoneal fat (IPF) in juvenile grass carp using three isonitrogenous and isoenergetic semi‐purified diets containing 0.0% (control group), 1.0% and 2.0% ALA, respectively. The lowest intraperitoneal fat (IPF) ratio was found in 2.0% group. In the muscle, hepatopancreas and IPF, docosahexaenoic acid (DHA, 22:6n‐3) and eicosapentaenoic acid (EPA, 20:5n‐3) contents increased with the increase in dietary ALA. In the IPF, caspase 3, caspase 8 and caspase 9 showed the highest activities in 2.0% group, while the value of Bcl‐2/Bax (B‐cell leukaemia 2/Bcl‐2‐associated X protein) reached the lowest. Meanwhile, swelling of the IPF mitochondria was observed in 2.0% group. The gene expressions of fatty acid desaturase (FAD) and fatty acid elongase (ELO) in the hepatopancreas and muscle showed significantly higher levels in the treatment groups, whereas an opposite trend was existed in the IPF. Fatty acid synthase (FAS), sterol regulatory element binding protein‐1c (SREBP‐1c) in the IPF and hepatopancreas reached the lowest in 2.0% group. Overall, dietary ALA could promote n‐3 highly unsaturated fatty acids (HUFAs) synthesis and suppress the accumulation of lipid by decreasing the expression of related genes and promoting the apoptosis in IPF.     

Dietary β‐glucan modulate haematological parameters,cytokines and gene expression in TLR and ERK pathways of rainbow trout (Oncorhynchus mykiss) during infection by Aeromonas salmonicida

To explore the role of β‐glucan (0, 0.05%, 0.1% and 0.2%) in resisting bacteria, rainbow trout (Oncorhynchus mykiss) was challenged with Aeromonas salmonicida after 42‐day β‐glucan administration, and then sampled at 0, 2, 4 and 6 days post infection (dpi). The data showed that 0.2% β‐glucan reduced the accumulated mortality rates compared with the ICG (infected control group) (p < .05). The white blood cells, red blood cells and haemoglobin were higher in the 0.2% β‐glucan group (BG) than the ICG (p < .05). 0.1% and 0.2% β‐glucan elevated serum total antioxidative capability and glutathione activity but alleviated the increase of serum alkaline phosphatase activity and glucose concentration (p < .05) during infection. Serum TNF‐α, IL‐1β, IL‐8 and IgM in three BGs elevated remarkably on 6 dpi compared with the ICG (p < .05). Expression of tnf, il1b and cxcl8 of the head kidney in the 0.1% and 0.2% BGs was higher than the ICG on 4 dpi while ighm expression in the 0.2% BG was higher than in the ICG on 2 and 6 dpi (p < .05). 0.1% and 0.2% β‐glucan increased the expression of tlr5m, tlr5s, tmek and myd88 in the spleen after infection (p < .05). Similarly, 0.2% β‐glucan up‐regulated the expression of tmek, myd88, oncmyk‐dab and c3 in head kidney (p < .05). Overall, 0.2% dietary β‐glucan effectively decreased accumulated mortality rate by modulating the biochemistry process, cytokines, and activating TLR and ERK signalling pathways during A. salmonicida infection.     

Effect of oxidized fish oil and α‐tocopherol on growth,antioxidation status,serum immune enzyme activity and resistance to Aeromonas hydrophila challenge of Chinese mitten crab Eriocheir sinensis

The impact of dietary α‐tocopherol on juvenile Chinese mitten crab Eriocheir sinensis was experimentally evaluated in a 10‐week study. Crab were fed with nine diets including three levels of α‐tocopherol (0, 100 and 300 mg kg^?1 diet) and three levels of fish oil oxidation (fresh, moderate and high) in triplicates. Fresh and moderate oil oxidization enhanced weight gain, but moderate and high oil oxidization lowered survival and feed efficiency. The 100‐mg α‐tocopherol kg^?1 diet resulted in higher hepatopancreas MDA than other α‐tocopherol diets. High oil oxidization led to the lowest serum superoxide dismutase (SOD) and glutathione peroxidase (GPH‐PX). The serum SOD and GPH‐PX activities in crab fed 100 mg α‐tocopherol were higher than in those fed other α‐tocopherol diets. The diet without α‐tocopherol addition lowered lysozyme and phenoloxidase (PO) activities compared to other α‐tocopherol diets. Fresh fish oil diet increased PO activity compared to oxidized oils. High oil oxidization caused significantly more mortality than fresh or moderate oxidization after 7‐d postchallenge with Aeromonas hydrophila. Supplementation with α‐tocopherol significantly enhanced resistance to bacterial infection. This study indicates that α‐tocopherol can protect lipid from peroxidation and enhance disease resistance.