Cobia Research Articles

Biochemical composition and activities of digestive and antioxidant enzymes during the egg and yolk‐sac larval development of the cobia ( Rachycentron canadum )

Cobia Rachycentron canadum is an economically important fish and is cultured throughout southern China and other part of the world. However, the knowledge of metabolism and nutritional requirements of cobia embryos remains limited. This study investigated changes of the biochemical composition, as well as the activities of related digestive and antioxidant enzymes during the development of both eggs and yolk-sac larvae of the cobia (Rachycentron canadum). The results showed that egg mass at the fertilized stage increased gradually to the blastula stage and showed a significant increase at the segmentation stage and a significant decrease at the larval period. The main components of embryos and larvae were carbohydrates, lipids and proteins. Protein and lipid contents decreased at the blastula stage, increased significantly at the segmentation stage and then decreased thereafter. Carbohydrate contents decreased significantly during the development. Enzymatic activities showed a progressive increase from the embryo to the yolk-sac larval stage. The activities of pepsin and trypsin increased significantly during the development. Amylase activity remained low before the blastula stage and increased rapidly thereafter. Lipase showed an ‘increase–stable–increase’ tendency. Changes of digestive enzyme activities indicated the dynamics of the biochemical composition. Activities of glutathione peroxidase, catalase and superoxide dismutase significantly increased during embryonic and yolk-sac stages, and malondialdehyde contents followed the same trend. The results of the current research provide vital information to enhance yolk exploitation with a specific focus on organogenesis. Moreover, considerable knowledge on the embryonic development of the cobia is presented.

A scalable, satellite-transmitted data product for monitoring high-activity events in mobile aquatic animals

A growing number of studies are using accelerometers to examine activity level patterns in aquatic animals. However, given the amount of data generated from accelerometers, most of these studies use loggers that archive acceleration data, thus requiring physical recovery of the loggers or acoustic transmission from within a receiver array to obtain the data. These limitations have restricted the duration of tracking (ranging from hours to days) and/or type of species studied (e.g., relatively sessile species or those returning to predictable areas). To address these logistical challenges, we present and test a satellite-transmitted metric for the remote monitoring of changes in activity, measured via a pop-off satellite archival tag (PSAT) with an integrated accelerometer. Along with depth, temperature, and irradiance for geolocation, the PSAT transmits activity data as a time-series (ATS) with a user-programmable resolution. ATS is a count of high-activity events, relative to overall activity/mobility during a summary period. An algorithm is used to identify the high-activity events from accelerometer data and reports the data as a count per time-series interval. Summary statistics describing the data used to identify high-activity events accompany the activity time-series. In this study, we first tested the ATS activity metric through simulating PSAT output from accelerometer data logger archives, comparing ATS to vectorial dynamic body acceleration. Next, we deployed PSATs with ATS under captive conditions with cobia (Rachycentron canadum). Lastly, we deployed seven pop-off satellite archival tags (PSATs) able to collect and transmit ATS in the wild on adult sandbar sharks (Carcharhinus plumbeus). In the captive trials, we identified both resting and non-resting behavior for species and used logistic regression to compare ATS values with observed activity levels. In captive cobia, ATS was a significant predictor of observed activity levels. For 30-day wild deployments on sandbar sharks, satellites received 57.4–73.2% of the transmitted activity data. Of these ATS datapoints, between 21.9 and 41.2% of records had a concurrent set of temperature, depth, and light measurements. These results suggest that ATS is a practical metric for remotely monitoring and transmitting relative high-activity data in large-bodied aquatic species with variable activity levels, under changing environmental conditions, and across broad spatiotemporal scales.

Enzyme-Assisted Aqueous Extraction of Cobia Liver Oil and Protein Hydrolysates with Antioxidant Activity

Cobia, Rachycentron canadum, is a medium-size marine fish with emerging global potential for offshore aquaculture. The processing waste, cobia liver, is a raw material rich in polyunsaturated fatty acid oils. In this study, an environmentally friendly green process, aqueous extraction (AE), was used to extract the cobia liver oil. The effect of cooking time and substrate water ratio on the oil extractability was investigated herein. The cooking time of 15 min, and substrate water ratio of 1:2 obtained the highest extraction efficiency. However, the oil extractability was only 18.8%. Thus, enzyme-assisted aqueous extraction (EAAE) was used to increase oil extractability and recovery of protein hydrolysates. The commercial proteases—including alcalase, papain, trypsin, and pepsin—were employed in pretreated cobia liver in order to increase oil release during AE. The EAAE results showed that maximum oil extractability was 38% by papain pretreatment. EAAE greatly improved the extraction efficiency; the oil extractability was double than that of AE (18.8%). The fatty acid profiles revealed that ω-3 polyunsaturated fatty acid contents of extracted oil obtained from AE and EAAE were 21.3% and 19.5%, respectively. Besides, the cobia liver hydrolysates obtained from EAAE by alcalase, papain, pepsin, and trypsin pretreatment showed scavenge DPPH radical activity with EC50 values of 0.92, 1.03, 0.83, and 0.53 mg, respectively. After in vitro simulated gastrointestinal digestion, the protein hydrolysates exhibited scavenge DPPH radical activity with EC50 values of 1.15, 1.55, 0.98, and 0.76 mg for alcalase, papain, pepsin, and trypsin, respectively. The study showed that the EAAE process can be used for extracting fish oil from fish waste while simultaneously obtaining the protein hydrolysates with antioxidant activity.

Estimating Shifts in Phenology and Habitat Use of Cobia in Chesapeake Bay Under Climate Change

Cobia (Rachycentron canadum) is a large coastal pelagic fish species that represents an important fishery in many coastal Atlantic states of the U.S. They are heavily fished in Virginia when they migrate into Chesapeake Bay during the summer to spawn and feed. These coastal habitats have been subjected to warming and increased hypoxia which in turn could impact the timing of migration and the habitat suitability of Chesapeake Bay. With conditions expected to worsen, we project current and future habitat suitability of Chesapeake Bay for cobia and predict changes in their arrival and departure times as conditions shift. To do this we developed a depth integrated habitat model from archival tagging and physiology data from cobia that used Chesapeake Bay, and applied the model to contemporary and future temperature and oxygen output from a coupled hydrodynamic-biogeochemical model of Chesapeake Bay. We found that arrival occurs earlier and departure time occurs later when temperatures are warmer and that by mid- and end-of-century cobia may spend on average up to 30 and 65 more days, respectively, in Chesapeake Bay. By mid-century we do not expect habitat suitability to change substantially for cobia, but by end-of-century we project it will significantly decline and shift closer to the mouth of Chesapeake Bay. Our study provides evidence that cobia will have the capacity to withstand near term impacts of climate change, but that their migration phenology varies from year to year with changing temperatures. These findings emphasize the need to incorporate the relationship between fishes and their environment into how fisheries are managed. This information can also help guide managers when deciding the timing and allocation of a fishery.

Cloning and expression analysis of hypoxia-related gene HO in cobia

The heme oxygenase gene plays a role in many biological processes and is an important protective factor in hypoxic stress. Research on the HO gene of fishes has attracted increasing attention, but there is yet little information about the cobia (Rachycentron canadum) heme oxygenase gene. In this study, the full-length cDNA of the cobia heme oxygenase gene (heme oxygenase, HO) was cloned using rapid amplification of cDNA ends technology. The full-length cDNA was 1639 bp and the open reading frame (ORF) was 840 bp encoding 279 amino acids, with 5′-UTR 518 bp and 3′-UTR 281 bp. The results of real-time quantitative PCR showed that cobia HO was expressed in all the tissues tested, with the highest expression in gills. The mRNA expression of gill HO increased after hypoxia stress. There was no significant change in the expression of HO mRNA in the liver after hypoxia stress. The amino acid sequences of HO in diverse fish and mammals have varying levels of homology. Our results indicate that the biological functions of HO may be similar in fish and mammals, suggesting an important protective factor in response to hypoxic stress.

Effects of salinity on growth characteristics and osmoregulation of juvenile cobia, Rachycentron canadum (Linnaeus 1766), reared in potassium‐amended inland saline groundwater

AbstractThe suitability of inland saline groundwater as a medium to culture juvenile cobia, Rachycentron canadum, was assessed. In the first experiment, juvenile cobia stocked in raw (unamended) saline groundwater at salinities of 5, 10, and 15 g/L exhibited complete mortality after 108, 176, and 195 hr, respectively. The second experiment evaluated the rearing of juvenile cobia (mean weight ~9.23 ± 0.12 g) in potassium (K+)‐amended saline groundwater (100% K+ fortified) and reconstituted seawater at salinities of 5, 10, and 15 g/L to assess growth and osmoregulation in distinct culture media. Following 60 days of culture, all fish survived the experimental period. Final mean bodyweight of cobia reared in K+‐amended saline groundwater (103.2–115.8 g) and seawater (111.2–113.8 g) of different salinities did not vary significantly (p > .05). No differences (p > .05) were observed in specific growth rate, weight gain (%), and feed conversion ratio between treatment groups. Serum osmolality increased with salinity and was significantly higher (p < .05) for fish in K+‐amended saline groundwater (353–361 mOsmol/Kg) than in reconstituted seawater (319–332 mOsmol/Kg), although differences were not observed between salinities by water type. Cobia stocked in saline groundwater of different salinities were osmoregulating normally, and the higher values observed may be because of variations in ionic composition and other interfering ions in saline groundwater. Trial results suggest that juvenile cobia can achieve optimal growth in K+‐amended saline groundwater of low and intermediate salinities.

Effects of three methionine sources in diets on temporal B0AT1 and ASCT2 mRNA expression in different intestinal segments of cobia, Rachycentron canadum

AbstractA 4‐week feeding trial was conducted using five groups of hybrid grouper (61.15 ± 0.15 g) to explore the potential effects of three methionine (Met) sources. Five isonitrogenous and isolipidic diets were formulated that included a fishmeal (FM) diet; nonsupplemented (NS) diet; or NS diets with the addition of the L‐methionine (L‐Met), DL‐methionine (DL‐Met), or coated Met (Co‐Met) to obtain the same Met level as the FM diet. Fish were randomly distributed into sea cages (30 fish per cage). Weight gain and specific growth rate in the DL‐Met group and FM group were significantly higher than those in all other groups (p < .05). In the proximal and distal intestines, of the 20 gene and time combinations (10 per gene), there were 14 combinations (70%) in which there were no significant differences in gene expression levels between the FM and DL‐Met groups (p > .05). The main reason for the same growth effect between the DL‐Met and FM groups may be attributed to the synchronized absorption at most time points after feeding, which had similar expression patterns of B0AT1 and ASCT2 of the proximal and distal intestines between two groups.

Influence of season and feeding intensity on the fatty acid composition of wild cobia (Rachycentron canadum, Linnaeus, 1766) in the Dungun coast, Malaysia

Influence of season and feeding intensity on the fatty acid composition of wild cobia (Rachycentron canadum, Linnaeus, 1766) in the Dungun coast, Malaysia

Stomach content and seasonal variation in diet composition of cobia, Rachycentron canadum in the Dungun coast, Malaysia

Stomach content and seasonal variation in diet composition of cobia, Rachycentron canadum in the Dungun coast, Malaysia

Functional characterization, tissue distribution and nutritional regulation of the Elovl4 gene in golden pompano, Trachinotus ovatus (Linnaeus, 1758)

The elongases of very long-chain fatty acids (Elovls) are involved in the rate-limiting of the carbon chain elongation reaction in fatty acid (FA) biosynthesis in vertebrates. One member of the Elovls family, Elovl4, has been regarded as a critical enzyme involved in the biosynthesis pathway of polyunsaturated fatty acids (PUFAs). To explore the role of Elovl4 in PUFA synthesis in Trachinotus ovatus, the cDNA of the Elovl4b gene is cloned from T. ovatus (ToElovl4b). The ORF of ToElovl4b was 918 bp and encoded 305 amino acid (aa) protein sequences. Sequence alignment showed that the deduced amino acids contained significant structural features of the Elovl4 family, such as a histidine box motif (HXXHH), multiple transmembrane domains and an endoplasmic reticulum (ER) retention signal. Moreover, phylogenetic analysis revealed that ToElovl4b was highly conserved with that of Rachycentron canadum Elovl4b. Moreover, heterologous expression in yeast demonstrated that ToElovl4b could efficiently elongate 18:2n-6, 18:3n-6 and 20:5n-3 FAs up to 20:2n-6, 20:3n-6 and 22:5n-3, respectively. Furthermore, the tissue expression profile indicated that mRNA expression of ToElovl4b was higher in the gonads and brain than in other tissues. Additionally, nutritional regulation suggested the highest mRNA levels of ToElovl4b in liver and brain were under feeding with 1:1 FO-SO (fish oil, FO; soybean oil, SO) and 1:1 FO-CO (corn oil, CO)), respectively. These new insights were useful for understanding the molecular basis and regulation of LC-PUFA biosynthesis in fish.

Evaluating a Suitable Aquaculture Site Selection Model for Cobia (Rachycentron canadum) during Extreme Events in the Inner Bay of the Penghu Islands, Taiwan

Despite numerous studies on the effect of a cold weather disaster on fisheries in 2008, no operational systems have been developed to monitor the threat of such an event to mariculturists in the Penghu Islands (PHI) region of Taiwan. The present study employed a suitable aquaculture site selection map of the inner bay of the PHI to reduce aquaculture losses and mortality rates of cobia (Rachycentron canadum) during extreme events. Daily marine environmental data, including sea surface temperature (SST), chlorophyll-a concentration (chl-a), and wind speed in the winter, were collected. An extreme event was defined as a period of over 11 days in a month of strong winds (>6 m/s). Four parameters in the PHI inner bay, including SST, cold-water intrusion days, chl-a, and offshore distance to the PHI coastline, were used to evaluate suitable aquaculture sites for cobia culture. The results indicated that La Niña events could not be used as a factor to detect cold-water intrusion events and select suitable aquaculture sites in the PHI. The evaluated suitable aquaculture site selection map, obtained using an arithmetic mean model and a geometric mean model, revealed that the avoidance sites during extreme events were concentrated in the northern and northwestern PHI. Suitable areas were concentrated in the southeastern areas. We further suggested that commercial cobia aquaculture operations in the PHI inner bay could be moved to the suitable sites in southeastern PHI during extreme events.

Isolation and characterization of pathology in case of massive mortality by Photobacterium damselae subsp. piscicida in Rachycentron canadum

This study aimed to investigate outbreak with high mortality in cultured juvenile cobia occurred in Southeast Brazil in 2011. Fish displayed retarded growth rates, lethargy, fin ulceration, skin depigmentation, corneal opacity, and physical deformities. Internally, livers were increased in volume and pale in different degrees. Firm whitish nodules were disseminated in the liver, kidney and spleen. A moderate number of parasites identified as Neobenedenia melleni were recovered from the body surface. Microscopically, severe hepatic steatosis and extensive granulomatous lesion were identified in all fish sampled. Microbiological analysis of moribund fish revealed the presence in pure culture of a Gram-negative bacterium identified as Photobacterium damselae subsp. piscicida using biochemical and molecular characteristics. Analysis of the partial 16S rRNA sequences confirmed the results demonstrating high identity (98%). The isolates were sensitive to chloramphenicol and enrofloxacin and resistant to ciprofloxacin, florfenicol, doxycycline hydrochloride, norfloxacin, oxytetracycline, and tetracycline. Chronic pasteurellosis was considered as the main problem in the farm, while hepatic steatosis and parasitic infestation may have contributed to the development of the process.

Effects of three medicinal herbs Bidens pilosa, Lonicera japonica, and Cyathula officinalis on growth and non-specific immune responses of cobia (Rachycentron canadum)

This study investigates the effects of three medicinal herbal extracts, namely Bidens pilosa (BPE), Lonicera japonica (LJE), and Cyathula officinalis (COE), on nonspecific immune parameters of cobia (Rachycentron canadum) in vitro and in vivo. During in vitro tests, BPE treatment increased reactive oxygen species (ROS) production in a dose-dependent manner in primary head kidney leukocytes. Similarly, ROS production rates were enhanced by LJE (50 and 100 mg/ml) and COE (100 mg/ml). This suggests that these three herbal extracts possess immunostimulating properties. We then conducted two feeding trials to examine the effects of these three herbal extracts on growth and innate immune parameters of cobia, and sought an optimal dietary supplementation proportion required for activating the non-specific immune responses. In the first trial, we supplemented the diet with 1, 5, or 10% of the individual extracts. After a ten-week feeding trial, no negative impacts on weight gain, feed conversion rate, and survival rate were observed in fish offered experimental diets. Further, ROS production, phagocytic capacity of the head kidney leukocytes, and serum lysozyme activity were enhanced by differing degrees in fish fed the herbal extracts compared to fish in the control group. A similar albumin/globulin ratio was seen between each experimental group and the control group regardless of the type and dose of herbal extract used, indicating these medicinal herbal extracts are safe for cobia. We then performed a 30-day feeding trial with lower extract concentrations (1, 3, and 5% of the diet) to identify dose responses in cobia at various time points so that we could establish a cost-effective manner of administering the three extracts for cobia. All BPE fed fish had higher ROS production compared to the control group, while phagocytosis rate and index were simultaneously raised in only the BPE30 group (3% BPE). Immune parameters such as ROS production, phagocytic rate, and serum lysozyme activity were triggered when fish received 30 g LJE per kg of feed. However, ROS production only increased in the LJE10 group (1% LJE) on day 30 and was not enhanced in the LJE50 group (5% LJE). Additionally, although the phagocytic rate and phagocytic index were induced in the LJE50 group, serum lysozyme activity was not elevated in this group (LJE50) at any time point examined. ROS production was greatly improved in all COE fed groups, but only the COE30 group (3% COE) showed prolonged enhanced phagocytic rate over the 30-day feeding trial.

Movements, habitat utilization, and post-release survival of cobia (Rachycentron canadum) that summer in Virginia waters assessed using pop-up satellite archival tags

BackgroundCobia (Rachycentron canadum) is a cosmopolitan marine fish that inhabits tropical, sub-tropical, and temperate marine and estuarine waters and supports a major recreational fishery along the U.S. Atlantic and Gulf of Mexico coasts. Recent changes in U.S. cobia management have sparked controversy and highlighted limitations in our understanding of the species’ biology. This study utilized pop-up satellite archival tags (PSATs) to assess the movements, habitat utilization, and post-release survival of cobia that summer in Virginia waters.ResultsPSATs were deployed on 36 cobia caught in Virginia state waters using standard recreational techniques in August 2016 and August–September 2017. All fish larger than 37-in total length were tagged, and several of these were hooked deeply. No mortalities were inferred for the 20 cobia whose PSATs reported and remained attached for at least 10 days. Premature release of tags was an issue, and only five PSATs remained attached for the full 180-day deployment period. Some fish undertook long seasonal movements, with one individual entering Florida waters well beyond the current stock demarcation boundary. Several fish overwintered in waters offshore of North Carolina near the continental shelf break. Cobia demonstrated a strong affinity for waters ≥ 20 °C, even in the coldest months. They displayed distinct seasonal differences in habitat utilization, spending high proportions of their time near the surface during the summer months and extended periods at depths ≥ 20 m in the winter months.ConclusionsCobia are hardy fish with low post-release mortality when handled respectfully. Their migratory patterns are clearly temperature driven, and seasonal changes in habitat utilization means varied vulnerability to fishing gears. Further understanding of cobia migratory patterns, particularly in conjunction with spawning activity, is needed to best manage this species.

CHEMICAL COMPOSITION AND AMINO ACID PROFILE OF FRESH AND STEAMED COBIA (Rachycentron canadum L.)

Cobia ( Rachycentron canadum ) is economically important fish with good prospects because of fast growing and easily cultured. Cobia is usually processed by many methods including steaming. The purposes of this research were to determine the effect of steaming process on proximate and amino acids composition. The chemical composition of fresh and steamed cobia’s meat were determined by proximate analysis. Amino acid compositions of fresh and steamed cobia’s meat were measured using HPLC (high performance liquid chromatography). Steaming process reduced 11.31% of water, 1.06% of ash, 26.04% of fat, and 9.11% of protein. Fresh and steamed cobia’s meat contained 17 amino acids consisting of 9 essential amino acids and 8 nonessential amino acids. The highest essential amino acid in fresh cobia’s meat was arginine (2,262 mg/100 g) and the highest nonessential amino acid was glutamic acid (3,894 mg/100 g). Steaming process reduced amino acids generally. The highest essential amino acid in steamed cobia’s meat was leucine (1,379 mg/100 g) and the highest nonessential amino acid was glutamic acid (2,370 mg/100 g). Taurine content of fresh cobia’s meat was 120.84 mg/100 g and changed into 94.33 mg/100 g after steaming process.

Effects of rearing density on growth, physiological responses, and flesh quality in juvenile cobia (Rachycentron canadum)

AbstractRearing density is recognized as a critical consideration in fish husbandry: whereas higher densities may increase productivity and reduce production costs, they also represent a potential source of chronic stress and may negatively affect growth performance and product quality. Accordingly, we investigated the effects of rearing density on growth, physiological responses, and flesh quality of cobia (Rachycentron canadum). Fish (46.32 ± 0.22 g, mean ± SEM) were reared for 42 days in a low salinity (17.6 g/L) recirculation system according to treatments designated as low density (5 fish/tank, 3.30 ± 0.02 kg/m3), medium density (10 fish/tank, 6.67 ± 0.03 kg/m3), or high density (20 fish/tank, 13.15 ± 0.16 kg/m3). Weight gain and specific growth rate were significantly higher among fish reared at low density; however, feed intake, feed conversion ratio, hepatosomatic index, condition factor, and dress‐out were equivalent among treatments. Plasma cortisol, lactate, and osmolality levels were not affected by rearing density, and values were within basal ranges reported for juvenile cobia. Fillet drip‐loss did not vary among treatments, but rigor mortis developed more rapidly among fish reared at high density. Rearing juvenile cobia at high densities may reduce growth performance and may influence the timing of rigor development.

RNA-seq analysis reveals divergent adaptive response to hyper- and hypo-salinity in cobia, Rachycentron canadum.

Salinity is an important abiotic stress that affects metabolic and physiological activities, breed, development, and growth of marine fish. Studies have shown that cobia (Rachycentron canadum), a euryhaline marine teleost fish, possesses the ability of rapid and effective hyper/hypo iono- and osmoregulation. However, genomic studies on this species are lacking and it has not been studied at the transcriptome level to identify the genes responsible for salinity regulation, which affects the understanding of the fundamental mechanism underlying adaptation to fluctuations in salinity. To describe the molecular response of cobia to different salinity levels, we used RNA-seq analysis to identify genes and biological processes involved in response to salinity changes. In the present study, 395,080,114 clean reads were generated and then assembled into 65,318 unigenes with an N50 size of 2758bp. There were 20,671 significantly differentially expressed genes (DEGs) including 8805 genes adapted to hypo-salinity and 11,866 genes adapted to hyper-salinity. These DEGs were highly represented in steroid biosynthesis, unsaturated fatty acid metabolism, glutathione metabolism, energy metabolism, osmoregulation, and immune response. The candidate genes identified in cobia provide valuable information for studying the molecular mechanism of salinity adaptation in marine fish. Furthermore, the transcriptomic sequencing data acts not only as an important resource for the identification of novel genes but also for further investigations regarding cobia biology.

Contemporary and future distributions of cobia, Rachycentron canadum

AbstractAimClimate change has influenced the distribution and phenology of marine species, globally. However, knowledge of the impacts of climate change is lacking for many species that support valuable recreational fisheries. Cobia (Rachycentron canadum) are the target of an important recreational fishery along the U.S. east coast that is currently the subject of a management controversy regarding allocation and stock structure. Further, the current and probable future distributions of this migratory species are unclear, further complicating decision‐making. The objectives of this study are to better define the contemporary distribution of cobia along the U.S. east coast and to project potential shifts in distribution and phenology under future climate change scenarios.LocationChesapeake Bay and the U.S. east coast.MethodsWe developed a depth‐integrated habitat suitability model using archival tagging data from cobia that were caught and tagged in Chesapeake Bay during summer months and coupled those data with high‐resolution ocean models to project the contemporary and future distributions of cobia along U.S. east coast.ResultsDuring the winter months, suitable cobia habitat currently occurs in offshore waters off North Carolina and further south, whereas during the summer months, suitable habitat occurs in waters from Florida to southern New England. In warmer years, the availability of suitable habitat increases in northern latitudes. Under continued climate change over the next 40–80 years, suitable habitat is projected to shift northward and decrease over the shelf.Main conclusionsHabitat distributions suggest cobia overwinter offshore and could inhabit waters further north during warmer months, into state jurisdictions that do not have strict management regulations for cobia. When waters are warmer, distributions are projected to shift poleward and seasonal migrations may begin earlier. These results can inform resource allocation discussions between fishery managers and resource users.

Substituting fishmeal with lupin Lupinus angustifolius kernel meal in the diets of cobia Rachycentron canadum: Effects on growth performance, nutrient utilization, haemato-physiological response, and intestinal health

The purpose of the study was to investigate the impacts of dietary narrow-leafed lupin Lupinus angustifolius kernel meal (LKM) on growth performance, feed utilization, digestibility, haematology, as well as liver and intestinal health of juvenile cobia Rachycentron canadum. Five isonitrogenous (crude protein around 460.0 g/kg) diets were formulated to replace 0, 200, 400, 600 and 800 g/kg of fishmeal protein with the protein from LKM designated as LKM0, LKM20, LKM40, LKM60 and LKM80, respectively. Triplicate groups of cobia were fed the respective diets for a period of 56 days. At the end of the feeding trial, growth indices expressed as final body weight (FBW) and specific growth rate (SGR) decreased following the linear and quadratic models with increasing levels of LKM. Feed intake, protein retention and energy retention followed the same trend, whereas, feed conversion ratio (FCR) and hepatosomatic index (HSI) followed a reverse trend with FBW and SGR. A significant decrease in the apparent digestibility coefficient of protein, lipid and dry matter was observed in cobia when more than 400 g/kg protein from fishmeal was substituted by LKM protein. Haematological indices such as red blood cell and haemoglobin declined significantly in fish fed with LKM60 and LKM80, while a significant increase was observed in aspartate transaminase and alanine aminotransferase. Histopathological alterations including higher lipid deposition, necrosis and vacuolization were found in the liver of fish fed higher (>600 g/kg) fishmeal protein replacement levels. Similarly, intestinal structure was affected by higher replacement level of fishmeal protein (>600 g/kg) as fold height and goblet cells decreased significantly with increasing levels of LKM. The results indicate that 200 g/kg fishmeal protein can be replaced by protein from LKM and substitution beyond that level adversely affected the growth performance of cobia.

Genotypic diversity, and molecular and pathogenic characterization of Photobacterium damselae subsp. piscicida isolated from different fish species in Taiwan.

Photobacteriosis, caused by Photobacterium damselae subsp. piscicida (Phdp), is a serious disease in marine fish species worldwide. To date, the epidemiological characterization of this pathogen in Taiwan remains limited. In this study, we collected 39 Phdp isolates obtained from different farmed fish for phenotypic and genotypic analysis. Phenotype bioassays using API-20E and API-20NE systems showed that the Phdp is a homogeneous group. However, genotyping using the pulsed-field gel electrophoresis (PFGE) technique revealed genetic variability among Phdp isolates when 13 and 11 different PFGE band patterns were obtained with SmaI and NotI as restriction enzymes, respectively. Phylogenetic analysis using 16S rDNA and the Fur gene clustered Taiwanese isolates and other species of P. damselae in the same clade. In contrast, the ToxR phylogenetic tree, a powerful discriminatory marker, separated the two subspecies. Furthermore, the virulence-associated genes, AIP56, P55, PDP_0080, Sod and Irp1, were detected from all isolates. Virulence testing with nine representative isolates in cobia (Rachycentron canadum) and Asian sea bass (Lates calcarifer) showed that some were highly pathogenic with 80%-100% mortality rates. This study provides epidemiological data of Phdp infections in farmed fish in Taiwan, which is necessary to develop comprehensive prevention and control strategies for the disease.