Cobia Research Articles

Population genetics of cobia (Rachycentron canadum) in the Gulf of Thailand and Andaman Sea: fisheries management implications

Population genetics has been recognized as a key component of policy development for fisheries and conservation management and aquaculture development. This study aims to evaluate the genetic diversity and population structure of native cobia (Rachycentron canadum) in the Gulf of Thailand and Andaman Sea, establishing the existing population distributions and contributing information to aid in the development of policy, prior to extensive aquaculture development. Microsatellite analysis of natural cobia populations in these two ocean basins shows similar levels of gene diversity at 0.844 and 0.837, respectively. All populations and almost all microsatellite loci studied show significant heterozygote deficiency. Genetic differentiation between local populations is low and mostly not significant (R ST = −0.0109 to 0.0066). The population shows no marked structure over the long geographic barrier of the Thai–Malay peninsula, either when analyzed using pairwise genetic differences or evaluated without predefined populations using STRUCTURE. Additionally, a Mantel test shows no evidence of isolation by distance between the population samples. The significant heterozygote deficiency at most of the loci studied could be explained by the possibility of null alleles. Alternatively, given the behavior of forming small spawning aggregations, seasonal migration, and hitchhiking on large marine animals, the population genetics could be complex. The population of cobia at each location in Thai waters may be inbred, as a result of breeding between relatives, which would reduce heterozygosity relative to Hardy–Weinberg frequencies, while some of these populations could be making long distance migrations followed by admixture between resident and transient groups. This migration would cause population homogeneity in allele frequencies on a larger geographic scale. The results suggest that fisheries management for this species should be considered at both national and international levels, and until the possibility of local adaptation is fully investigated, policy development should apply the precautionary principle to ensure the preservation of genetic diversity and the sustainability of local and regional fisheries.

Quality and safety of fish curry processed by sous vide cook chilled and hot filled technology process during refrigerated storage

Fish curry, a traditional Indian dish was prepared from farmed fish Cobia (Rachycentron canadum), packaged by two different cook-chill processes namely, sous vide cook chilled and hot filled technology and held at 2 °C. Biochemical composition revealed that fish curry contained 5% protein and 6% fat. Omega-3 fatty acids, eicosapentaenoic acid (EPA) retained 55.44% while docosahexaenoic acid (DHA) retained 29% during cook-chilling process. The major fatty acids in fish curry were C18:2, C12:0, C16:0 and C18:1. Shelf-life of sous vide cook chilled and hot filled technology processed fish curry were 8 and 12 weeks, respectively. Total bacterial counts were detected after 4 weeks and 12 weeks in sous vide cook chilled and hot filled technology processes, respectively. Total staphylococci were detected in sous vide cook chilled and hot filled technology processed cobia fish curry after 4 and 12 weeks, respectively. Total bacilli, anaerobic sulfite reducing clostridia, Salmonella, and lactic acid bacteria were absent. Hot filled technology process was more efficient and could be applied for chilled fish curry preservation for 12 weeks without any safety problems.

Effects of shrimp waste meal on growth performance and chitinase activity in juvenile cobia (Rachycentron canadum)

The effects of replacing fish meal (FM) with shrimp waste meal (SWM) in diets fed to juvenile cobia (Rachycentron canadum) were investigated through a 6-week trial. SWM was added to replace FM at fractions of 0%, 10%, 20% and 25% of the diet. The result showed the survival rates of the fish were higher than 86.7%. The weight gain and feed conversion rate showed an increasing trend as the SWM proportion in diet increased from 0% to 25%, but the protein efficiency ratio showed a decreasing trend in efficiency. In addition, the hepatosomatic index increased significantly when the SWM percentage was 20% and 25%. There were no significant differences in the protein and ash concentration of the muscle among all dietary groups. However, the muscle lipid content was low when fish fed in diets with high SWM level. Chitinase activity was extremely high in the pyloric caeca tissue of cobia, and increased specific activities of chitinase were only found in the foregut of cobia fed diets containing 10% SWM for a 6-week period. A challenge test showed that SWM could not enhance cobia resistance to Photobacterium damselae ssp. piscicida infection. Our results suggest that the administration of a 10% SWM diet could potentially reduce the use of FM in the diet of cobia.

Effect of dietary vitamin C on the growth performance and innate immunity of juvenile cobia (Rachycentron canadum)

This study was conducted to evaluate the effects of dietary vitamin C on growth performance, hematologic parameters and innate immune responses in juvenile cobia, Rachycentron canadum. Seven practical diets were formulated to contain 0.0 (as the basal diet), 13.6, 27.2, 54.4, 96.6, 193.4 and 386.5 mg ascorbic acid equivalent kg−1 diet. Each diet was fed to triplicate groups of juvenile cobia with initial body weight of 5.5 g in 500-L cylindrical fiberglass tank. The results of 8 weeks feeding trial showed that typical vitamin C-deficient signs such as spinal deformation and body nigrescence were observed in the fish fed the basal diet. Fish fed the basal diet had significantly lower weight gain, specific growth rate (SGR), protein efficiency ratio (PER) and feed efficiency (FE) than those fed the diets supplemented with vitamin C, but no significant differences were observed among diets supplemented with vitamin C. However, survival rate was significantly affected by the dietary vitamin C levels, fish fed the basal diet had lower survival rate than those fed the diets supplemented with vitamin C. The ascorbic acid concentration in liver was correlated positively with the dietary vitamin C levels, however, the thiobarbituric acid reactive substances (TBARS) concentrations in liver was not significantly affected by the dietary vitamin C levels, although, fish fed the basal diet had the highest TBARS values among all treatments. The activities of serum lysozyme, superoxide dismutase (SOD), alkaline phophatase (AKP) and total immunoglobulin (Ig) were significantly influenced by the dietary vitamin C levels, fish fed the basal diet had lower lysozyme, SOD, AKP and total Ig than those fed diets supplemented with vitamin C. The serum glucose and triglyceride concentrations were significantly affected by the dietary vitamin C levels. Fish fed the basal diet had lower red blood cell and hemoglobin values than those fed the vitamin C supplemented diets. The challenge experiment with Vibrio harveyi showed that lower cumulative survival was in fish fed the unsupplemented diet, the cumulative survival were significantly increased with increase of the dietary ascorbic acid levels from 13.6 to 96.6 mg kg−1, while the cumulative survival reached plateau when dietary ascorbic acid levels increased from 96.6 to 386.5 mg kg−1. These results indicated that dietary vitamin C did significantly influence on growth performance and immune response of juvenile cobia.

Effect of previous ascorbic acid treatment on the fatty acid profile of cobia (<i>Rachycentron canadum</i>) fillets during frozen storage

The present research focuses on the nutritional value of the lipid retention associated with frozen cobia ( Rachycentron canadum ). The effect of a previous soaking in an aqueous ascorbic acid (AA) solution on the fatty acid profile of fish fillets during a further frozen storage period (–18 °C) was investigated. Two different AA concentrations were tested (0.25% and 0.50%) and compared to control conditions. As a result of the frozen storage period (up to 6 months), marked decreases were found in the contents of fatty acid groups such as monounsaturated, polyunsaturated and n-3 polyunsaturated, as well as in the n-3/n-6 ratio. However, a preserving effect on such fatty acid parameters could be observed resulting from the previous AA treatment, which was greater when applying the 0.50% AA concentration. Assessment of the polyene index indicated an increased lipid oxidation development during the frozen storage time; this increase was partially inhibited by the previous AA soaking.

An NMR-based metabolomic assessment of cultured cobia health in response to dietary manipulation

Commercial aquaculture feeds rely heavily on fishmeal and fish oil, which can be expensive and ecologically unsustainable. To evaluate the efficacy of reduced fishmeal diets for outgrowth, a dietary study was conducted on the finfish cobia, Rachycentron canadum. NMR-based metabolomic techniques were used to assess the effect of decreasing dietary fishmeal on the health of the cobia. Filtered serum 1H NMR spectra analysed by principal components analysis (PCA) showed cobia fed reduced fishmeal diets were metabolically different than cobia on control diets. In particular, tyrosine and betaine increased in cobia fed reduced fishmeal diets while glucose decreased, suggesting that these cobia were not receiving the necessary nutritional components required for energy and growth. The formulated control diet contributed to enriched growth and significantly elevated lactate levels suggesting enhanced gut microflora metabolism in response to dietary components. The results show that NMR-based metabolomic analysis is a useful tool in aquaculture studies.

Chemical and Electrical Approaches to Sedation of Cobia: Induction, Recovery, and Physiological Responses to Sedation

AbstractTo support the growing interest in marine fisheries research in areas such as biotelemetry, tagging, and tracking, we assessed the ability of various sedatives to facilitate this research in juvenile cobias Rachycentron canadum (∼300 g), namely, tricaine methanesulfonate (MS‐222; 150 mg/L), carbon dioxide (CO2; ∼750 mg/L), eugenol (60 mg/L), benzocaine (150 mg/L), and pulsed‐DC electrosedation (100 V, 30 Hz, 25% duty cycle, 5‐s exposure). Induction times (CO2 [z] > benzocaine [y] > eugenol [y] > MS‐222 [y] > electrosedation [x]), recovery of equilibrium (CO2 [z] > eugenol [z] > MS‐222 [y] > benzocaine [y] > electrosedation [x]), and responsiveness to tactile stimulus (eugenol [z] > MS‐222 [y] > benzocaine [y] > CO2 [xy] > electrosedation [x]) differed significantly among the sedative treatments (treatments with the same letters are not significantly different). Total handling time from initial sedative exposure to recovery differed among the sedatives as well (CO2 [z] > eugenol [y] > benzocaine [x] > MS‐222 [x] > electrosedation [w]), with cumulative means ± SEs of 5.9 ± 0.2 min for CO2, 4.1 ± 0.2 for eugenol, 2.7 ± 0.2 min for benzocaine and MS‐222, and 1.0 ± 0.2 min for electrosedation. Physiological responses differed significantly over time, with transient increases in plasma cortisol, glucose, osmolality, and lactate that were resolved within 6 h. The overall magnitude of the physiological responses differed among sedatives, depending on the response variable; however, in each case, CO2 elicited the greatest response. Although variations in induction and recovery times were observed, it is likely that these differences can be reasonably accommodated within the context of typical research by adjusting the sedative treatments or allowing for longer induction and recovery times as needed.Received November 18, 2011; accepted August 17, 2012

Textural and biochemical properties of cobia (Rachycentron canadum) sashimi tenderised with the ultrasonic water bath

The present study investigated the tenderisation effects ultrasound processing (UT) on farmed cobia sashimi. Age-treated cobia trunk muscles (AT) were used as the control. The pH, total volatile base nitrogen, trimethylamine nitrogen, thiobarbituric acid reactive substances, ATP catabolism components, K1 value, and texture were evaluated. The texture of AT sashimi reached the optimal firmness range with 8.53N at day 7. However, AT samples could not be served raw after day 7 because of their poor freshness indexes, including a TVBN value of 18.53g/100g, a TMAN value of 3.25mg/100g, and a TBARS value 0.983MDAmg/100g. Moreover, the K1 value of AT sashimi was 20.21% at day 5. UT was employed to efficiently tenderise cobia sashimi with an initial firmness of 9.70–7.82N after 90min of treatment. The results of this study indicate that UT accelerates the biochemical reaction rate, as evidenced by the increases in the TVBN, TMAN, and TBARS contents; however, these values were very low. The results of this study could provide basic information for the development of a novel ultrasonic tenderisation technique in raw seafood designed for restaurants and consumers.

First record of Neobenedenia melleni (Monogenea: Capsalidae) in sea-farmed cobia (Rachycentron canadum) in Brazil

Neobenedenia melleni (MacCallum, 1927) (Monogenea) is a widespread pathogen in marine teleost cultures all over the world. The present paper reports this parasite species in farmed cobia (Rachycentron canadum) in Brazil, for the first time. Some comments on preventive actions for avoiding the disease are made.

DHA is essential, EPA appears largely expendable, in meeting the n − 3 long-chain polyunsaturated fatty acid requirements of juvenile cobia Rachycentron canadum

Cobia may require both eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids to meet dietary n − 3 long-chain polyunsaturated fatty acid (LC-PUFA) demand. Growth performance is reduced when LC-PUFA-rich fish oil is replaced with soybean oil, a LC-PUFA deficient lipid, in cobia diets, but individual requirements for EPA and DHA have not been quantitatively determined. We assessed the growth performance and tissue fatty acid composition of juvenile cobia fed a fish oil-based positive control diet (FISH), a soy oil-based negative control diet (SOY), or experimental diets based on soy oil supplemented with EPA, DHA, or both at 50% or 100% of the concentrations typically observed in fish oil: (SOY + 50% EPA, SOY + 100% EPA, SOY + 50% DHA, SOY + 100% DHA, SOY + 50% BOTH, SOY + 100% BOTH). Growth performance of fish fed the SOY was impaired relative to those fed the FISH diet. Supplementing the soybean oil-based diet with DHA, alone or in combination with EPA, restored performance. EPA supplementation had no effect on growth performance. Fatty acid composition of cobia fillet, liver, brain, and eye tissues was significantly affected by diet, but tissue profile change among fish fed any of the soy oil-based diets was less overt in the brain compared to the periphery. Diets supplemented with DHA and/or EPA resulted in increased tissue levels of these fatty acids, however, the maximal levels were observed in the FISH treatment. Our data suggest the dietary n − 3 LC-PUFA requirement of juvenile cobia can be largely satisfied by DHA, and that EPA, if required, is required only in trace amounts. Soybean oil supplemented with DHA is an effective alternative to fish oil in juvenile cobia diets.

Identification of a cobia ( Rachycentron canadum) CC chemokine gene and its involvement in the inflammatory response

The chemokines regulate immune cell migration under inflammatory and physiological conditions. We investigated a CC chemokine gene ( RcCC1) from cobia ( Rachycentron canadum). The full-length RcCC1 cDNA is comprised 673 nucleotides and encodes a four-cysteine arrangement 99-amino-acid protein typical of known CC chemokines. The genomic DNA of RcCC1 consists of three exons and two introns. Phylogenetic analysis showed that RcCC1 was closest to the MIP group of CC chemokines. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed RcCC1 was constitutively expressed in all tissues examined, with relative strong expression in gill, blood, kidney, spleen, and head kidney. The RcCC1 transcripts in the head kidney, spleen, and liver were quickly up-regulated after stimulation with formalin-inactivated Vibrio carchariae (bacterial vaccine) or polyriboinosinic polyribocytidylic acid (poly I:C). These results indicate RcCC1 not only plays a role in homeostasis, but also may be involved in inflammatory responses to bacterial and viral infection.

Evaluation of poultry by-product meal in commercial diets for juvenile cobia ( Rachycentron canadum)

A 10-week feeding trial was conducted to evaluate the potential use of poultry by-product meal (PBM) as a partial replacement of fish meal protein in the commercial diets for juvenile cobia. Five isonitrogenous (approximately 45%) and isolipidic (about 11%) diets were formulated to contain graded levels of PBM, and fish meal protein was replaced with a pet food-grade PBM at 15, 30, 45, 60% level (PBM15, PBM30, PBM45, PBM60, respectively) without lysine and methionine supplementation. The reference diet (PBM0) contained fish meal and soybean meal as protein sources. Each diet was fed to groups of 20 juvenile cobia initially averaging approximately 5.8 g in triplicate 500-l tanks twice daily to apparent satiation. The results showed that growth performance and survival for fish fed PBM-supplemented diets were not significantly lower compared to fish fed the reference diet ( P > 0.05). However, protein efficiency ratio and feed efficiency ratio were significantly affected by the replacement level of fish meal protein with PBM, fish fed the PBM30 and PBM45 diets had higher PER and FER than fish fed the reference diet and the other diets. The condition factor, viscerosomatic index and intraperitoneal fat ratio were not significantly affected by the dietary treatments, however, fish fed the PBM45 diet had a higher hepatosomatic index than fish fed the other diets. There were no significant differences in moisture, crude lipid, ash, calcium and phosphorus content in whole body among all treatments, but the fish fed the reference diet had higher crude protein in whole body than fish fed the PBM-supplemented diets. There were no differences in liver superoxide dismutase, catalase, glutathione S-transferase and glutathione peroxidases activities among fish fed the experimental diets. Hematocrit, hemoglobin, red blood cell and total immunoglobulin were not significantly affected by the replacement level of fish meal protein with PBM. With the exception of plasma glucose content, there were no significant differences in plasma triglyceride, cholesterol and total protein concentration in fish fed the experimental diets. The results of this study indicated that good quality terrestrial PBM can successfully replace fish meal in the commercial diets for cobia, and the optimal level of fish meal replacement with PBM was determined by quadratic regression analysis to be 30.75% on the basis of maximum protein efficiency ratio.

Acute exposure of juvenile cobia Rachycentron canadum to nitrate induces gill, esophageal and brain damage

Cobia Rachycentron canadum is a fast growing fish with world-wide potential for aquaculture, and has been considered for rearing in recirculating aquaculture systems (RAS). Nitrate is considered the least toxic nitrogenous product in the ammonia nitrification process, but as it may accumulate in RAS, toxic levels can be reached. The objective of this study was to evaluate the acute toxicity and the histopathological effects of nitrate on juvenile cobia. Juveniles (6.87 ± 0.36 g; 11.8 ± 0.19 cm) were acutely exposed to six concentrations of nitrate (500–3000 ppm NO 3 −-N) plus a control during 96 h. At the end of this period of exposure, juvenile cobia were sampled for histopathological evaluation. The estimated LC 50 of nitrate to juvenile cobia was equal to 2407 and 1829 mg/L NO 3 −-N at 24 and 96 h, respectively. Cobia exposed to sub-lethal nitrate concentrations showed histopathologic alterations in the gills, esophagus and brain. The gills revealed epithelial hyperplasia with complete lamellar fusion, telangiectasia, and lamellar shorting induced by necrosis, and the esophagus presented hyperplasia of epithelium and mucus cells. In the brain, glial cells proliferation, satellitosis (microglial cells surrounding neurons with swollen and prenecrotic neurons), and Virchow-Robin spaces (enlarged perivascular spaces, EPVS) were observed. The results of the present study indicate that juvenile cobia have a high tolerance to acute exposure of nitrate. However, assorted histopathological responses were observed for cobia at sub-lethal nitrate concentrations. Therefore, further studies are needed to estimate safe chronic nitrate levels for juvenile cobia culture.

Environmentally conditioned, year-round volitional spawning of cobia (Rachycentron canadum) in broodstock maturation systems

Year-round control of the spawning cycle of cobia (Rachycentron canadum) has been established by using water temperature manipulation. To compare the effectiveness of using this method to induce volitional spawning in cobia, two 80 m3 recirculating aquaculture systems (RAS) were used. Temperatures in one of the maturation tanks (‘Mat 1’) were maintained between 27 and 29°C for 12 months of the 15.5-month study period. Temperatures in the second maturation tank (‘Mat 2’) were allowed to fluctuate naturally throughout the year and ranged from 20 to 32°C. A total of 101 spawning events occurred in the tanks between the spring of 2008 and the summer of 2009 (3 April 2008 to 17 June 2009). Of the 38 total spawning events in Mat 1, 17 of them (44.7% of all Mat 1 spawning events) occurred during the off-season (fall and winter). The egg viability rates did not differ significantly (P > 0.05) between on- and off-season spawns in Mat 1. Conversely, cobia broodstock exposed to natural water temperatures (no environmental manipulation) in Mat 2 followed the natural pattern of warm water (>26°C) dependence, limiting egg production to spring and summer seasons. This method of water temperature manipulation allows for effective control of the cobia reproductive cycle without compromising egg viability.

Acute toxicity of ammonia on juvenile cobia (Rachycentron canadum, Linnaeus, 1766) according to the salinity

Juvenile cobia (Rachycentron canadum) (total length 15.16 ± 0.92 cm and weight 19.26 ± 4.5 g) were exposed to different concentrations of ammonia–N (unionized plus ionized ammonia as nitrogen), using the static renewal method at different salinity levels of 5, 20, and 35‰ at pH 8.1 and 25°C. The 24, 48, 72, 96 h LC50 values of ammonia–N for R. canadum juveniles were 60.28, 48.57, 37.42, 22.73 mg l−1 at 35‰; 51.25, 43.63, 28.17, 19.05 mg l−1 at 20‰; and 39.48, 25.31, 19.50, 8.13 mg l−1 at 5‰, respectively. The 24, 48, 72, 96 h LC50 values of NH3–N (unionized ammonia as nitrogen) were 1.81, 1.46, 1.12, and 0.68 mg l−1 at 35‰; 1.75, 1.49, 0.96, and 0.65 mg l−1 at 20‰; and 1.52, 0.97, 0.71, and 0.31 mg l−1 at 5‰, respectively. As the salinity decreased from 35 to 5‰, susceptibility of ammonia–N increased by 34.5, 47.88, 50.56, and 64.23% after 24, 48, 72, and 96 h exposure, respectively. Furthermore, we found that exposure of fish to ammonia–N caused an increase in oxygen consumption of 129.1, 157.5, and 192% and a decrease in the ammonia excretion level of 53.4, 38.2, and 23.3% with respect to the control.

Substituição do óleo de peixe por óleo de soja em dietas para beijupirá (Rachycentron canadum)

A substituição do óleo de peixe em dietas para beijupirá (Rachycentron canadum) foi avaliada usando-se cinco proporções de óleo de peixe e soja. Duzentos juvenis (±12g) foram distribuídos aleatoriamente em 20 tanques e alimentados até a saciedade aparente durante 42 dias. À medida que se aumentou o teor de óleo de peixe nas dietas, houve aumento proporcional no desempenho. Os juvenis alimentados com dietas que continham 75 e 100% de óleo de peixe apresentaram ganhos de peso mais altos que os das dietas com 0 e 25% de óleo de peixe, enquanto os alimentados com a dieta com 50% de óleo de peixe apresentaram crescimento intermediário. Não foram observadas diferenças nas concentrações de proteína e lipídio na carcaça dos peixes, mas os teores de umidade e cinzas diferiram entre os tratamentos. A inclusão de teores relativamente altos de ingredientes de origem vegetal diminuiu o consumo alimentar, o que afetou negativamente o crescimento. Os resultados indicam a possibilidade de substituição de até 50% do óleo de peixe por óleo de soja em dietas para juvenis do beijupirá.

Analysis of the genetic diversity of the lymphocystis virus and its evolutionary relationship with its hosts

Lymphocystis disease virus (LCDV) is the causative agent of lymphocystis disease. In this study, the mcp gene of LCDV and the cyt b gene of the host fish were selected as molecular markers, and the phylogenetic relationships between LCDV and its host were analyzed. The 25 LCDV isolates examined in this study were attributed to seven LCDV genotypes: genotype I (LCDV-1), genotype II (LCDV-cn, etc.), genotype III (LCDV-rf), genotype IV (LCDV-rc and LCDV-sb), genotype V (LCDV-cb), genotype VI (LCDV-tl), and genotype VII (LCDV-sa). Genotype VII is a new genotype. LCDV1 was found to have differentiated first, followed by LCDV-rf; then LCDV-tl; LCDV-cb; and then LCDV-sa; and by LCDV-rc and LCDV-sb; and finally by LCDV-cn, LCDV-C, and LCDV-jf. From the host evolutionary perspective, Rachycentron canadum was found to have differentiated first, followed by Trichogaster leeri, Chanda baculis, and Sebastes schlegeli, Lateolabrax sp., Sparus aurata, Platichthys flesus, and Paralichthys olivaceus. Comparison of the phylogenies of the host fish species and LCDVs revealed no significant evidence of cospeciation between LCDVs and their host fish. In-depth studies of the genetic variation in LCDVs can enhance our understanding of the mechanism of LCDV infection, which may provide important insights into the prevention and treatment of lymphocystis disease.

Effects of dietary chitosan and Bacillus subtilis on the growth performance, non-specific immunity and disease resistance of cobia, Rachycentron canadum

The present study was performed to investigate the effects of various levels of dietary Bacillus subtilis and chitosan on the growth performance, non-specific immunity and protection against Vibrio harveyi infection in cobia, Rachycentron canadum. Fish were fed with the control diet and six different experimental diets containing three graded levels of B. subtilis at 2 × 10 10 CFU g −1 (0.0, 1.0, 2.0 g kg −1 diet) for each of two levels of chitosan (3.0 and 6.0 g kg −1 diet). The results of 8 weeks feeding trial showed that the survival rate ranged from 81.3% to 84.0% with no significant difference ( P > 0.05). The SGR (%) in the fish fed with dietary treatments was significantly higher than that of the control fish except diet 6 group with 2.0 g kg −1 B. subtilis and 3.0 g kg −1 chitosan. The serum lysozyme activities were significantly higher in 6.0 g kg −1 chitosan groups and no significant differences were observed among B. subtilis levels. The serum ACP activities were significantly higher in 3.0 g kg −1 chitosan groups at 0.0 and 1.0 g kg −1 B. subtilis levels; at low chitosan level, the cobia fed diets with 1.0 g kg −1 B. subtilis had significantly higher serum ACP activity, but at high chitosan level, the cobia fed diets with 2.0 g kg −1 B. subtilis had significantly higher serum ACP activity. The phagocytosis and respiratory burst activity in the fish fed with dietary treatments was significantly higher than that of the control fish except diet 3 group with 6.0 g kg −1 chitosan. Moreover, fish fed the diet containing 2.0 g kg −1 B. subtilis and 6.0 g kg −1 chitosan had significantly higher post-challenge survival on the 7th day following V. harveyi infection and post-challenge survival showed clearly the synergistic effect of chitosan and B. subtilis. Based on these results, the combination of 1.0 g kg −1 B. subtilis and 6.0 g kg −1 chitosan is optimal for the growth, innate immunity and disease resistance of cobia with an 8-week oral administration.

Purifi cation of a Toxic Metalloprotease Produced by the Pathogenic Photobacterium damselae subsp. piscicida Isolated from Cobia (Rachycentron canadum)

The aim of the present study was to purify and characterize a toxic protease secreted by the pathogenic Photobacterium damselae subsp. piscicida strain CP1 originally isolated from diseased cobia (Rachycentron canadum). The toxin isolated by anion exchange chromatography, was a metalloprotease, inhibited by L-cysteine, ethylenediaminetetraacetic acid (EDTA), ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA), 1,10-phenanthroline, N-tosyl-L-phenylalanine-chloromethyl ketone (TPCK), and N-alpha-p-tosyl-L-lysine-chloromethyl ketone (TLCK), and showed maximal activity at pH 6.0-8.0 and an apparent molecular mass of about 34.3 kDa. The toxin was also completely inhibited by HgCl2, and partially by sodium dodecyl sulfate (SDS) and CuCl2. The extracellular products and the partially purified protease were lethal to cobia with LD50 values of 1.26 and 6.8 microg protein/g body weight, respectively. The addition of EDTA completely inhibited the lethal toxicity of the purified protease, indicating that this metalloprotease was a lethal toxin produced by the bacterium.

Effects of dietary probiotic on the growth performance, non-specific immunity and disease resistance of cobia, Rachycentron canadum

The present study was performed to investigate the effects of a commercially available probiotic product (compound probiotic) containing Bacillus subtilis 7.0 × 109 CFU g−1, Bacillus licheniformis 3.0 × 109 CFU g−1, Lactobacillus spp. 5.0 × 108 CFU g−1 and Arthrobacter spp. 1.0 × 108 CFU g−1 on the growth performance, non-specific immunity and protection against Vibrio harveyi infection in cobia (Rachycentron canadum). Fish were fed diets containing six graded levels of compound probiotic (0.0, 1.0, 2.0, 3.0, 4.0 and 5.0 g kg−1) for 8 weeks. The results showed that the survival rate ranged from 81.1% to 84.4% with no significant difference among dietary treatments (P > 0.05) after feeding experiment. Dietary compound probiotic significantly increased the specific growth rate (SGR), serum lysozyme, alternative complement pathway (ACP) activity, phagocytosis percentage (PP) and respiratory burst activity of head-kidney macrophages of cobia. Moreover, feeding of supplemented diets containing compound probiotic resulted in significantly lower mortality against the pathogens Vibrio harveyi compared with the control group. To elevate the growth and immune resistance ability of cobia, an optimal dose of dietary compound probiotic administration determined by second-order polynomial regression analysis was 3.3 g kg−1, on the basis of the SGR and mortality after challenge with V. harveyi.