Effects of Loma morhua (Microsporidia) infection on the cardiorespiratory performance of Atlantic cod Gadus morhua (L).

The microsporidian Loma morhua infects Atlantic cod (Gadus morhua) in the wild and in culture and results in the formation of xenomas within the gill filaments, heart and spleen. Given the importance of the two former organs to metabolic capacity and thermal tolerance, the cardiorespiratory performance of cod with a naturally acquired infection of Loma was measured during an acute temperature increase (2 °C h^?1) from 10 °C to the fish's critical thermal maximum (CT_Max). In addition, oxygen consumption and swimming performance were measured during two successive critical swimming speed (U_crit) tests at 10 °C. While Loma infection had a negative impact on cod cardiac function at warm temperatures, and on metabolic capacity in both the CT_Max and U_crit tests (i.e. a reduction of 30–40%), it appears that the Atlantic cod can largely compensate for these Loma‐induced cardiorespiratory limitations. For example, (i) CT_Max (21.0 ± 0.3 °C) and U_crit (~1.75 BL s^?1) were very comparable to those reported in previous studies using uninfected fish from the same founder population; and (ii) our data suggest that tissue oxygen extraction, and potentially the capacity for anaerobic metabolism, is enhanced in fish infected with this microsporidian.     

Effect of acclimation temperature on the acute stress response in juvenile Atlantic cod, Gadus morhua L., and haddock, Melanogrammus aeglefinus L.

Juvenile Atlantic cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) were subjected to 30 s air exposure stressors following acclimation to 4, 10 and 14 and 4, 8 and 14°C respectively. Both species responded to the stressor with increases in plasma cortisol at all temperatures tested. At 14°C cortisol levels peaked within 1 h post‐stressor, and returned to pre‐stressor levels within 24 h. In contrast, at 4°C, peak cortisol levels were not attained until 6 h post‐stressor in haddock and remained elevated beyond 24 h in both species. The rate of plasma glucose accumulation was greater at higher temperatures in both species and no increase was seen at 4°C. Lysozyme activity in cod, in response to the stressor, was lower than the values reported for some other species and increased slightly at 14°C. The results show that cod and haddock acclimated to different temperatures respond to common, acute stressors in a manner similar to other teleosts.     

Influence of incubation temperature on body movements of Atlantic cod (Gadus morhua L.) embryos and on size at hatch

Body movements of cod (Gadus morhua L.) embryos reared from fertilization to hatch at 5.4°C were observed at various stages of development and at six experimental temperatures ranging from 0–10°C. Frequency of cod embryo body movements increased from zero at 42 degree‐days post fertilization to maximal at 73–82 degree‐days (1 or 2 days prior to hatch). Embryos were most active at 2°C (mean of 5.5 movements per 10 min), with activity declining to less than 1/10 min at 8–10°C. Lengths of hatched cod larvae reared at a series of constant temperatures (from 4–10°C) from fertilization to hatch were greater at lower incubation temperatures. Incubation temperatures of 2–4°C were found to be optimal for incubation of cod eggs.     

Yersiniosis in Atlantic cod,Gadus morhua (L.), characterization of the infective strain and host reactions

A disease outbreak in farmed Atlantic cod caused by Yersinia ruckeri is reported. Mortality started following vaccination of cod reared in two tanks (A and B). The accumulated mortality reached 1.9% in A and 4.8% in B in the following 30 days when treatment with oxytetracycline was applied. Biochemical and molecular analysis of Y. ruckeri isolates from the cod and other fish species from fresh and marine waters in Iceland revealed a high salinity‐tolerant subgroup of Y. ruckeri serotype O1. Infected fish showed clinical signs comparable with those of Y. ruckeri ‐infected salmonids, with the exception of granuloma formations in infected cod tissues, which is a known response of cod to bacterial infections. Immunohistological examination showed Y. ruckeri antigens in the core of granulomas and the involvement of immune parameters that indicates a strong association between complement and lysozyme killing of bacteria. Experimental infection of cod with a cod isolate induced disease, and the calculated LD₅₀ was 1.7 × 10⁴ CFU per fish. The results suggest that yersiniosis can be spread between populations of freshwater and marine fish. Treatment of infected cod with antibiotic did not eliminate the infection, which can be explained by the immune response of cod producing prolonged granulomatous infection.     

Heritability of economically important traits in the Atlantic cod Gadus morhua L

During the development of breeding programme for Atlantic cod Gadus morhua L., in Iceland, genetic parameters were estimated for 1402 individuals, which were assigned with DNA profiling to 140 dams and 70 sires. The cod was reared in cages on the eastern and western coasts of Iceland from 2004 to 2005. At the average body weight of 1.8 kg, the estimated heritability (h² ± SE) for body weight, gutted weight and the condition factor (CF) were 0.31 ± 0.06, 0.34 ± 0.04 and 0.24 ± 0.06 respectively. Genetic correlation (r_G) in body weight between the two rearing locations was estimated as 0.95, which reflects a low G × E interaction. The estimated heritability for hepatosomatic index (HSI) and fillet yields was 0.061 ± 0.04 and 0.04 ± 0.04 respectively. The HSI and fillet yields were highly genetically correlated with body weight or 0.67 and 0.82 respectively. The genetic correlation between the CF and body weight was estimated as 0.31. There appears to be substantial amount of additive genetic variation for body weight suggesting that selection is likely to be successful. Low heritability for fillet yields and the HSI indicates less promise of genetic improvement. Assigning of parentage to individuals with DNA profiling was 80% successful.     

Atypical Aeromonas salmonicida infection in naturally and experimentally infected cod, Gadus morhua L.

Cod, Gadus morhua L., of wild origin, were reared at different temperatures for 12 months. During this period, moribund and newly dead fish were examined and samples collected for bacteriology and histopathology. Atypical Aeromonas salmonicida was isolated from 10 individuals reared at or above 7 °C. The isolates were homogeneous with respect to biochemical and antibiogram characters and similar to the ssp. achromogenes National Collection of Industrial and Marine Bacteria, UK, type strain 1110 and reference strains that have been isolated from salmonids and haddock in Iceland. Histopathological analysis of the naturally infected cod showed typical ulceration associated with atypical A. salmonicida infection and also widespread granulomatous formations. One‐year‐old cod of farmed origin, kept at 9 °C, received intraperitoneal or intramuscular injection with different doses of atypical A. salmonicida, isolated from the above wild cod. Mortalities were monitored for 28 days and the LD₅₀ calculated. The route of bacterial injection influenced the mortality rate and LD₅₀ value and affected, to some extent, the pathological changes observed and humoral immune parameters. Pathological changes, including haemorrhage, early stages of granuloma formation and necrotic changes, were seen in several organs. Infection appeared to induce non‐specific antibody activity against trinitrophenyl (TNP)‐haptenated protein and may have activated the complement system. Specific antibody response against atypical A. salmonicida was not detected.     

Tracking escaped Atlantic cod (Gadus morhua L.) aggregated around Norwegian farms: Considerations for management strategies

To examine site fidelity and potential for ecosystem impacts, 20 escapee cod (Gadus morhua L.) from fjord‐based cod farms were tagged with acoustic transmitters, released in the vicinity of the same farm, and their subsequent behaviour and survival monitored using a fixed array of acoustic receivers. Most individuals demonstrated high fidelity to their farm where tagged, and this increased during the polar night. Only five individuals (25%) were occasionally detected at another farm (4 km distance), and only three cod (15%) migrated out of the fjord during the 6‐month study period. Vertical movements were consistently greater in the hours around noon than at night, a behaviour inconsistent with that observed for wild fish. A high mortality rate (40%) was observed during the first 4 months post‐release, and actual mortality may have been even higher, limiting the potential for negative ecosystem effects from cod escapees. Findings from this study are used to discuss the development of recapture strategies for minimising potential impacts from cod escapees to ensure sustainable farming activity.     

Inter‐individual variation in the dominant intestinal microbiota of reared Atlantic cod (Gadus morhua L.) larvae

The diversity of the microbiota in 15 Atlantic cod (Gadus morhua L.) larvae from three different hatcheries with variable food type and water treatment was characterized with culture dependent and independent approaches. The culture‐dependent approach was phenotypic characterization of bacterial isolates based on 16 tests, and the culture‐independent approach was terminal restriction fragment length polymorphism (T‐RFLP) of PCR amplified 16S rRNA. Both approaches revealed large inter‐individual differences in the microbiota of cod larvae. Data from real time PCR indicate substantially lower variation in bacterial number larva^?1 both within and between hatcheries than the data on colony forming units, resulting in an estimated four orders of magnitude variation in culturability between larvae. Considerably higher richness and diversity were observed with T‐RFLP than with phenotypic characterization, and no correlation between the two approaches was found for diversity indexes calculated at the individual level. Dissimilarity in the composition of the microbiota between individuals from the same rearing environment was comparable to the dissimilarities between hatcheries. The large inter‐individual variation is important to consider when designing experiments, as pooling of individuals will completely mask this variability. We conclude that colonization of individual cod larvae by bacteria seems to be a stochastic and not a deterministic process, which is not well understood.     

Optimum temperature and food-limited growth of larval Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank

We estimated recent growth of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae collected on the southern flank of Georges Bank in May 1992–94 from the ratio of RNA to DNA (R/D) and water temperature. Growth of both species increased with water temperature to about 7°C and then decreased. The highest growth rates were observed in May 1993 at water temperatures around 7°C. These data confirm an earlier observation of comparable temperature optima for growth of Atlantic cod and haddock larvae in the north‐west Atlantic. Comparisons of field growth rates and temperature optima with data for larvae cultured at high temperatures and prey densities in the laboratory suggest that growth may have been food‐limited at higher temperatures on Georges Bank. Given that 7°C is the long‐term mean water temperature on the southern flank in May and that climate models predict a possible 2–4°C rise in water temperatures for the western North Atlantic, our findings point to a possible adverse effect of global warming on Atlantic cod and haddock.     

Pre-anaesthetic metomidate sedation delays the stress response after caudal artery cannulation in Atlantic cod (Gadus morhua)

Recovery from caudal artery cannulation with and without pre-anaesthesia metomidate sedation was assessed in Atlantic cod (Gadus morhua). The levels of plasma cortisol, glucose, electrolytes and acid–base parameters were compared between sedated and unsedated cod and to those in uncannulated individuals, where the samples were obtained by sacrificial sampling (reference level). Metomidate sedation delayed the stress response, causing sedated cod plasma cortisol to return to the reference level more slowly [day 4 post surgery (PS)] than in unsedated cod (day 2 PS). Plasma glucose was elevated in both sedated and unsedated cod up to and including day 5 PS. Plasma K⁺ was lower and pH was higher in cannulated cod than in the reference from 24 h PS until the end of experimentation, indicating a stress effect of sacrificial sampling on plasma K⁺ and pH that was likely caused by an acute stress response. Metomidate sedation delayed the stress response following CA cannulation and should therefore not be used as a pre-anaesthetic sedation in Atlantic cod. The caudal artery cannulation can be a useful tool in obtaining repeated blood samples from Atlantic cod given an adequate recovery time, which was determined to be 6 days irrespective of pre-anaesthesia sedation status.     

Enzymes from the gut bacteria of Atlantic cod,Gadus morhua and their influence on intestinal enzyme activity

Gut‐associated bacteria of fish are known to produce enzymes which aid in digestion. The presence and activities of these bacteria in cold‐water fishes like Atlantic cod are less known. Therefore, we have characterized the activities of extracellular enzymes of GP21 (Pseudomonas sp.) and GP12 (Psychrobacter sp.), two bacteria isolated from the gastrointestinal tract of Atlantic cod. Additionally, we examined if these bacteria when delivered through feeds could influence the activity of selected intestinal enzymes. GP21 was able to produce amylase, chitinase, cellulase and protease, whereas GP12 could produce only chitinase and protease. These enzymes were produced extracellularly and they were found to be catalytically active at acidic conditions (pH 2–5) and at temperatures ranging from 15 to 30 °C. Orally delivered bacteria could possibly influence the activity of intestinal enzymes after 40 days, rather than after 20 days of feeding. Thus GP21 and GP12, the potential probiotic organisms, could support digestion in Atlantic cod.     

Electron microscopical study of a new genus and new species of microsporida in the gills of Atlantic cod Gadus morhua L.*

Abstract. A microsporidium, forming xenoparasitic complexes (xencmas) of the cell-hypertrophy tumour type, was found in the gills of the Atlantic cod Gadus morhua L. and studied with the electron microscope. It seemed to be similar to Nosema branchiate Nemeczek, 1911, in the haddock Melanogrammus aeglefinus (L.) except in size of spore. The xenoma (cyst) was basically like the Glugea cyst and the parasite had some other characters in common with Glugea; it was apansporoblastic, unikaryotic, disporoblastic and underwent partial development in parasitophorous vacuoles. It differed from Glugea in lacking plasmodial stages, producing usually 1 or 2 spores (rather than 16) in a vacuole, showing no distinct tendency for different stages to occur in different zones of the cyst and having tubules in the parasitophorous vacuoles. The name Loma morhua n. g., n. sp. (type) was proposed and the genus assigned to the family Glugeidae Thélohan, 1892. The parasite of haddock gill was transferred to the new genus, becoming L. branchialis (Nemeczek, 1911) n. comb.     

Optimising of culture conditions and stimulation of head kidney macrophages from Atlantic cod, Gadus morhua L.

The cultivation of Atlantic cod, Gadus morhua L., head kidney macrophages was optimal at 4–6 °C. The viability of cells in culture was examined by the trypan blue exclusion test 2, 5 and 7 days after seeding. Simultaneously, the number of adherent cells at the various times was checked. The morphology of the cells was also evaluated during the cultivation period. Enhanced respiratory burst activities were demonstrated in macrophages after stimulation with known immunostimulants like lipopolysaccharide, tuftsin and an acid peptide fraction previously shown to stimulate Atlantic salmon leucocytes efficiently. Generally, the Atlantic cod macrophages seemed to respond to a lesser extent compared with macrophages obtained from both mammalian and other fish species.     

Technological solutions and operational measures to prevent escapes of Atlantic cod (Gadus morhua) from sea cages

Escapes of cod (Gadus morhua) from sea cages represent an economic problem for farmers and a potential environmental problem. We estimate that 0–6% of cod held in sea‐cage farms in Norway were reported to have escaped each year from 2000 to 2005, which is a high proportion compared with salmon. We interviewed employees at 19 coastal sea‐cage cod farms in Norway to investigate both how and why cultured cod escape and to document cage handling and management strategies that were effective in minimizing escapes. Based on the interviews, we describe five working hypotheses that may explain why a greater proportion of cod than salmon escape: (1) cod are more willing to escape than salmon; (2) cod bite the net cage and create wear and tear; (3) net cages have insufficient technical standards for cod culture; (4) cod are placed in sea cages at considerably smaller sizes than salmon; and (5) cod are more popular feed for predators. Preliminary testing of the hypothesis that cod bite netting and create holes was done by placing pre‐damaged net panels with cut twines and control panels inside sea cages. Holes in the pre‐damaged net panels increased in size over a period of 3 months. The type of damage indicated that biting of netting twines was the likely cause.     

Evaluation of a novel pentaplex microsatellite marker system for paternity studies in Atlantic cod (Gadus morhua L.)

Five Atlantic cod (Gadus morhua L.) microsatellites (Gmo8, Gmo19, Gmo35, Gmo37 and Tch11) were selected for parentage assignment. All markers were co‐amplified using dye‐labelled primers and sized by automated sample processing in a 3130xl Genetic Analyser. The genotyping of samples was reproducible and alleles from all loci could be genotyped confidently (precision and accuracy above 99.7 and 99.1% respectively). The number of alleles, observed allelic and genotypic frequencies were calculated. A population database was established for fish originating from randomly selected wild‐caught Norwegian coastal‐ and North‐East Arctic cod. Combined paternity exclusion and the probability of match were 99.97 and 1.5 × 10^?9 respectively. The multiplex assay system was used successfully for parental assignment of 2336 juveniles from different mass spawning breeding tanks. Of all offspring, 91.2% were assigned to a single parental pair. In total, 523 different matings were recorded, and 69 of 70 dams and all of the 30 sires contributed to the offspring.     

Growth and gut morphology of diploid and triploid juvenile Atlantic cod (Gadus morhua)

The objective of this paper was to compare the growth and gut morphology of juvenile diploid and triploid Atlantic cod (Gadus morhua) reared under similar conditions. Individually tagged 36‐week‐old diploid (mean weight 49.3 ± 13.8 g) and triploid (mean weight 43.6 ± 11.2) juvenile cod were measured at intervals during a 29‐week growth trial. Data for weight, length, condition factor (K), hepato‐somatic index (HSI), gonado‐somatic index (GSI), Relative Gut Length (RGL) and pyloric caeca number were collected and results were analysed in relation to ploidy status, gender and family contribution. At the end of the experiment, only one family (M2xF3) had many representatives with a relatively even distribution of sexes and ploidies. Diploid females were significantly heavier and had higher K than triploid females in the M2xF3 family (body weight 371.2 ± 120.2 vs. 298.4 ± 100.7 g; K 1.1 ± 0.1 vs. 0.93 ± 0.1), but no differences were found between diploid and triploid males. In the other families (pooled data), no differences in body weight were found between the ploidy groups. In general, triploids had a shorter intestine (RGL) and fewer pyloric caeca than their diploid siblings regardless of gender suggesting possible impairments in nutrient utilization and growth.     

Growth,survival and feed efficiency for post-metamorphosed Atlantic cod (Gadus morhua) reared at different temperatures

Two growth trials were completed on post-metamorphosed Atlantic cod (Gadus morhua) for a period of four weeks to determine the optimal temperature for best growth and feed efficiency. The same experiment was repeated twice under similar conditions to determine the effect of four temperature regimes (10, 12, 14 and 16 °C) on randomly selected juvenile cod with an initial weight ranging from 0.34 to 0.51 g. Post-metamorphosed cod grown at 14 and 16 °C were significantly larger at the end of the experiments than the fish grown at 10 or 12 °C, with specific growth rates following a similar trend (p < 0.05). Fish held at 16 °C utilized feed less efficiently than those held at the lower temperatures in experiment II and the highest feed efficiency was observed in fish held at 10 °C (p < 0.05). The results of the feed efficiency and the maximum growth per degree (dG / dT_max) in experiment II suggest that the feed efficiency was likely maximized at a lower temperature than those used in this study. By using data collected from sub-samples of fish in experiment II, maximum growth (G_max) was estimated at 14.5 °C using a growth temperature model. Cannibalism was a problem in the first experiment but was greatly reduced in experiment II by feeding a larger size feed pellet (2 mm).     

Carnobacterium maltaromaticum vs. Vibrio (Listonella) anguillarum in the midgut of Atlantic cod (Gadus morhua L.): an ex vivo study

The present investigation addresses whether the midgut (MG) of Atlantic cod (Gadus morhua L.) is an infection route for Vibrio (Listonella) anguillarum serotype 02 β and if Carnobacterium maltaromaticum, a probiotic bacterium, can out‐compete the pathogen and modulate the autochthonous MG microbiota. This was investigated by using an ex vivo method the intestinal sac, utilized previously in studies on Atlantic cod and Atlantic salmon (Salmo salar L.). Exposure of the MG to V. (L.) anguillarum did not reveal any cell damage indicating that the MG does not appear to be an infection route for V. (L.) anguillarum in healthy Atlantic cod. This finding together with previous observations on Atlantic cod and Atlantic salmon indicate that intestine as an infection route might vary between these two species. When the MG was exposed to C. maltaromaticum, no cell damage or cellular disruptions were observed. As budding from the apices of microvilli was observed in all treatments exposed to bacteria, we suggest that budding might be involved in the primary barrier against bacterial infection. However, to clarify this hypothesis, further studies are needed. Exposure of the MG to the probiotics and pathogenic bacteria indicated that C. maltaromaticum, to some extent, is able to out‐compete V. (L.) anguillarum but the topic merits further investigation. Analysis of the MG microbiota after sterile saline solution and bacterial exposure indicates that bacteria related to Staphylococcus sciuri belong to the autochthonous gut microbiota in Atlantic cod.     

Muscle growth of triploid Atlantic cod juveniles (Gadus morhua)

This study has investigated the muscle growth of diploid and triploid Atlantic cod (Gadus morhua) juveniles raised in replicate tanks over a period of 29 weeks and analysed at three sampling points (February, June and September). Data for weight, length, condition factor (K), muscle fibre growth and myogenic progenitor cells (MPCs) number were collected and results were analysed in relation to body growth and ploidy status. Diploids were significantly heavier than triploids throughout the trial (~10–20%) and had K in June and September samplings. Over the whole period, the rate of muscle fibres' recruitment was 318 fibres per day and 252 fibres per day for diploid and triploid cod respectively. The larger body weight of diploids resulted in a total number of fast fibre number of 114 979 compared to 91 086 in triploids. The average diameter of the 2.5% of the smallest fibres (2.5th percentile) was higher in diploids than triploids at the start of the trial, with a reversed picture for the average of the upper 2.5% (97.5th percentile) at the end of the trial. The probability density function of the estimated muscle fibre diameters showed similar fibre size distribution between size‐matched diploids and triploids at all sample points. The peak fibre diameter was approximately 25 μm in February and increased to approximately 50 μm in June and September, irrespectively of ploidy. Pax 7 were used as molecular markers for MPCs. A positive correlation between Pax 7⁺ cells and total body length was observed only among triploid fish at the onset of the experiment.     

Dietary Trace Metal Supplements Promote Blood Vessel Melanosis in Fillets of Juvenile Farmed Atlantic Cod,Gadus morhua L.

Chronic over‐exposure to diet‐borne copper has been implicated in the development of black stripes caused by melanin deposits (melanosis) around the blood vessels in fillets of farmed Atlantic cod, Gadus morhua L. To test this hypothesis a 6‐mo pilot study using feeds containing controlled amounts of copper has been performed. Cumulative data from each feed group showed that 56% of cod fed a diet containing 10 mg/kg of added copper had black stripe while 58% of cod on a diet containing 5 mg/kg of added copper were affected. Cod on a diet with no added copper had 33% of individuals positive for black stripe, and in a final group on a diet containing no supplementary trace metals and minerals 16% of individuals were affected. Weight gain, increase in length, hepatosomatic index, and condition factor were unaffected by the changes made to mineral supplements. Melanosis has been observed in 24% wild cod of market size (n = 30) whilst 85% of market‐sized farmed cod examined have had black stripe (n = 403).