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.     

The effect of dietary chitin on the autochthonous gut bacteria of Atlantic cod ( Gadus morhua L.)

In the present study the impact on autochthonous (adherent) bacteria in proximal intestine (PI) and distal intestine (DI) of Atlantic cod ( Gadus morhua L.) was evaluated following feeding of a control diet and a diet supplemented with 5% chitin. The autochthonous gut bacteria were investigated using denaturing gradient gel electrophoresis (DGGE). Analysis of the microbiota associated with PI and DI of Atlantic cod indicate that dietary chitin modulate the intestinal bacterial community. For example, band 25 ( Escherichia coli–like), band 14 ( Anaerorhabdus furcosa–like) and band 29 (uncultured bacterium–like) in PI were depressed by dietary chitin (P < 0.05). The number of bands (23.7 ± 5.4) in DI of fish fed chitin was marginally higher than the control fish (16.7 ± 2.1) (P = 0.065), and the relative abundances of band 6 (swine faecal bacterium–like) were marginally stimulated by dietary chitin (P = 0.095). Furthermore, the present study reports several novel sequences not previously reported in the gastrointestinal tract of Atlantic cod. Whether the dietary effect of chitin on gut bacterial community has any positive effect of fish health merits further investigations.     

Strong phenotypic and genetic correlation between size and first maturity in Atlantic cod Gadus morhua L. reared in commercial conditions

Three‐year classes of Atlantic cod Gadus morhua were studied throughout their rearing in the eastern coast of Iceland from 2004 to 2011. The growth and status of maturity were recorded during the rearing. For one of the year classes, genetic parameters for body weight and maturity status were estimated from 757 individuals, which were the offspring of 40 dams and 20 sires. The estimate for heritability of body weight was h² = 0.34 at the average weight of 630 g, and heritability for proportion of maturity was h² = 0.17 given the same weight. The relationship between body weight and the proportion of mature individuals at first winter revealed a strong genetic correlation of r_G = 0.90. The phenotypic relationship between body weight and proportion of maturity was estimated with a Bayesian logistic regression as P(yi = 1(mature)) = Φ(β0 + β1_weight + β2_sex). The best fit yielded β0 = ?2.9320 with a 95% interval between ?3.2807 and ?2.5394, β1_weight = 0.0041 with a 95% interval ranging from 0.0035 to 0.0046 and β1_sex = ?0.0201 with a 95% interval from ?0.2003 to 0.1445. The gender had no notable effect. This strong phenotypic and genetic correlation in body weight and maturation suggests that an increased growth rate will consequently lead to a higher proportion of mature individuals in the population. As a consequence, genetic manipulations to simultaneously increase growth and delay maturation may present a challenge.     

The settlement and reproductive success of Lepeophtheirus salmonis (Krøyer 1837; Copepoda: Caligidae) on atypical hosts

The reproductive success of Lepeophtheirus salmonis settled on host and non‐host fish has been compared. Triplicate single species tanks of Atlantic salmon, marine three‐spined sticklebacks, saithe and Atlantic cod were exposed to 10 adult female L. salmonis per tank (n=30 lice per species). Adult female L. salmonis settlement and egg string production occurred only on salmon and cod, with no egg production occurring on saithe and three‐spined sticklebacks. The number of eggs in egg strings, hatching success of eggs and the survival of all larval stages to the copepodid stage were severely affected by the species of fish on which female L. salmonis had settled. L. salmonis settled on cod produced significantly fewer eggs, lower hatching rates and lower survival rates of larvae than females on Atlantic salmon. The production of egg strings by L. salmonis females infecting cod, which successfully hatch and moult through to the infective copepodid stage, albeit in small numbers, is discussed in terms of the implications to aquaculture and salmon and cod farming scenarios.     

The growth of larval cod and haddock in the Irish Sea: a model with temperature,prey size and turbulence forcing

We applied a physiological individual‐based model for the foraging and growth of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae, using observed temperature and prey fields data from the Irish Sea, collected during the 2006 spawning season. We used the model to estimate larval growth and survival and explore the different productivities of the cod and haddock stocks encountered in the Irish Sea. The larvae of both species showed similar responses to changes in environmental conditions (temperature, wind, prey availability, daylight hours) and better survival was predicted in the western Irish Sea, covering the spawning ground for haddock and about half of that for cod. Larval growth was predicted to be mostly prey‐limited, but exploration of stock recruitment data suggests that other factors are important to ensure successful recruitment. We suggest that the presence of a cyclonic gyre in the western Irish Sea, influencing the retention and/or dispersal of larvae from their spawning grounds, and the increasing abundance of clupeids adding predatory pressure on the eggs and larvae; both may play a key role. These two processes deserve more attention if we want to understand the mechanisms behind the recruitment of cod and haddock in the Irish Sea. For the ecosystem‐based management approach, there is a need to achieve a greater understanding of the interactions between species on the scale a fish stock is managed, and to work toward integrated fisheries management in particular when considering the effects of advection from spawning grounds and prey–predator reversal on the recovery of depleted stocks.     

A follow‐up study of selected biomarkers of health in cod Gadus morhua L. collected from the southern Baltic off the Polish coast

Selected biomarkers of health were examined in 50 post‐spawning cod Gadus morhua collected in November 2015 from the southern Baltic. The biomarkers included condition factor (CF), macroscopic lesions, histopathology of spleen, liver and gonads, and morphometry of follicular atresia and hepatic and splenic melanomacrophage cells (MMC). All fish appeared in good body condition. One fish had a dermal ulcer, and in seven, macroscopic nematodes were noted within body cavity. Microscopic lesions in the liver included biliary myxozoanosis, microsporidial and necrocentric granulomas, parasitic hepatitis, multifocal necrosis, foci of cellular alterations, spongiosis, peliosis and cytoplasmic fibrillar inclusions. The spleen and gonads had microsporidial and/or necrocentric granulomas. Some of the biomarkers showed differences as compared to spawning cod collected in May from the same location in 2012, most importantly values an order of magnitude lower for splenic MMC in post‐spawning fish. In post‐spawning fish, there were statistically significant correlations between MMC, CF, follicular atresia, parasitic hepatitis and microsporidiosis. This is the first comparison of biomarkers of health in post‐spawning and spawning Baltic cod. Future studies need to examine the relationships of biomarkers to levels of pollutants in the environment and in tissues of cod.     

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.     

Effect of cultivated copepods (Acartia tonsa) in first‐feeding of Atlantic cod (Gadus morhua) and ballan wrasse (Labrus bergylta) larvae

The aim of this study was to compare the nutritional composition and effects of short periods with cultivated copepod nauplii versus rotifers in first‐feeding. Atlantic cod (Gadus morhua) and ballan wrasse (Labrus bergylta) larvae were given four different dietary regimes in the earliest start‐feeding period. One group was fed the copepod Acartia tonsa nauplii (Cop), a second fed enriched rotifers (RotMG), a third fed unenriched rotifers (RotChl) and a fourth copepods for the seven first days of feeding and enriched rotifers the rest of the period (Cop7). Cod larvae were fed Artemia sp. between 20 and 40 dph (days posthatching), and ballan wrasse between 36 and 40 dph, with weaning to a formulated diet thereafter. In addition to assessing growth and survival, response to handling stress was measured. This study showed that even short periods of feeding with cultivated copepod nauplii (7 days) had positive long‐term effects on the growth and viability of the fish larvae. At the end of both studies (60 days posthatching), fish larvae fed copepods showed higher survival, better growth and viability than larvae fed rotifers. This underlines the importance of early larval nutrition.     

Influence of episodic upwelling on capelin, Mallotusvillosus, and Atlantic cod, Gadusmorhua, catches in Newfoundland coastal waters

Wind-induced upwelling in the north-west Atlantic has been hypothesized to influence catch rates of fish by gear fishing at fixed locations. Passive movement to shallow depths during upwelling has been proposed to increase encounter rates of fish with net leaders set at the coast. However, supporting evidence is not conclusive, possibly because fish respond to strong events rather than all events. We investigated whether catch rates of capelin, Mallotusvillosus, and Atlantic cod, Gadusmorhua, were related to the strength of upwelling, as measured by the rates of water temperature change, or to stage of upwelling. Capelin trap catches were positively related to increases in water temperature, representing the relaxation phase. This result was due primarily to increases in catch after strong (> 4°C change) rather than after typical (< 4°C change) upwelling events. Cod trap catches were not related to upwelling strength but did increase 1–2 days after typical events. The data suggest that upwelling increases capelin movement, while the return of warmer surface water after an upwelling event increases cod movement.     

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.     

Experimental mycobacteriosis in Atlantic cod, Gadus morhua L

Piscine mycobacteriosis causes losses in a number of fish species both in the wild and in aquaculture worldwide. Mycobacterium salmoniphilum infections have on several occasions been reported in farmed Atlantic salmon, Salmo salar L. The present study tested and confirmed the susceptibility of Atlantic cod, Gadus morhua L., an important yet relatively novel aquaculture species, to infection with M. salmoniphilum. Atlantic cod injected intraperitoneally with a suspension of this bacterium were maintained together with cohabitant (COH) fish in a flow-through marine water system at 10-11 °C. The fish were supervised daily and samples taken at 2, 7, 14, 23, 34 and 53 weeks post-infection and examined pathologically, bacteriologically and using molecular biology. Injected mycobacteria were re-isolated in high concentrations from both injected and COH fish groups. Death attributable to mycobacterial infection was observed in both injected (47%) and COH (28%) fish groups. Extensive development of granuloma in visceral organs, mainly the mesenteries, spleen, kidney and liver (lesser extent) and at later stages of the infection in heart tissues and gills, was observed in both injected and COH fish. Granulomas underwent a temporal progression of distinct morphological stages, culminating in well-circumscribed lesions surrounded by normal or healing tissue. Acid-fast bacilli were detected in both granulomas and non-granulomatous tissues. This study confirms that Atlantic cod is highly susceptible to M. salmoniphilum infection and that this bacterial species may be a threat to cod both in the wild and in the aquaculture.     

Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century

Two approaches were used to qualify observed variability in Greenland cod (Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self‐sustaining cod stock off Greenland with high SSB that produced a series of above‐average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’.     

Diet of blue cod,Parapercis colias,living on undisturbed biogenic reefs and on seabed modified by oyster dredging in Foveaux Strait,New Zealand

• 1. Little has been done to assess the potential impact of habitat modification by bottom fishing gear on the feeding habits of demersal fishes. An analysis is presented of the diet of blue cod in Foveaux Strait, southern New Zealand, based on the gut content of fish taken in winter 1999 from two sites where each site consisted of both undisturbed biogenic reefs and reefs modified by oyster dredging.
• 2. Of the 420 guts collected, 13% were empty. The overall mean wet weight of gut content was <4 g. No significant habitat or site effects were detected for the proportion of empty guts or the amount of food consumed.
• 3. A pattern was detected that blue cod on dredged habitats generally fed on more crustaceans than those on undistributed habitats. Blue cod from undisturbed habitat also displayed a more diverse diet than those taken from dredged habitat. These results suggest that long‐term disturbance of seabed habitat by the oyster fishery in Foveaux Strait has caused changes to the diet of blue cod. The findings also suggest that actions should be taken to protect the biogenic reefs from further damage if the blue cod fishery and related resources are to be effectively managed.
• 4. Changes in prey diversity with increasing fish size were also found, with prey diversity (Shannon–Wiener index) increasing from 0.83 to 1.35 over a range of fish size from <25 cm to larger than 34 cm. A total of 52 prey taxa were identified in the diet of blue cod. Crustaceans were the main component, followed by mollusca and polychaeta. Fish, echinodermata and ‘other’ were less important in the diet.

Copyright © 2002 John Wiley & Sons, Ltd.     

Activation of intestinal epithelial cells in Atlantic cod,Gadus morhua,induced by algal derivatives

This study investigated the influences of two algal derivatives, alginic acid and fucoidan (FUC) on the activities of the intestinal epithelial cells (IEC) of Atlantic cod. The cells were isolated from three regions of the intestines, and primary cultures were incubated with the derivatives followed by temporal analysis of cellular activities. Epithelial cell survival in the fore and hind intestine was not significantly affected by the algal derivatives at both time points (3 and 24 h postincubation, hpi). Cellular survival in the mid‐intestine decreased significantly at 3 hpi. Stimulation of myeloperoxidase (MPO) activity of IEC induced by the algal derivatives was observed to be region dependent. Myeloperoxidase activity in the cells from fore‐ and mid‐intestines was not significantly affected by the two algal derivatives. Both algal derivatives significantly stimulated the activity at 3 hpi in the hind intestinal cells, and FUC‐exposed cells remained significantly elevated until 24 hpi. The results showed that algal derivatives could be applied as potential immunostimulants in cod with stimulatory action of MPO activity in the mucosal immune cells of the hind intestine.     

A comparison of reciprocating flow versus constant flow in an integrated, gravel bed, aquaponic test system

Murray cod, ^{Maccullochella peelii peelii}, and Green oak lettuce, Lactuca sativa, were used to test for differences between two aquaponic flood regimes; reciprocal flow (hydroponic bed was periodically flooded) and constant flow (hydroponic bed was constantly flooded), in a freshwater aquaponic test system, where plant nutrients were supplied from fish wastes, while plants stripped nutrients from the wastewater before it was returned to the fish. The Murray cod had FCRs and biomass gains that were statistically identical in both systems. Lettuce yields were good and a significantly greater amount of both biomass and yield occurred in the constant flow treatment. Constant flow treatments exhibited greater pH buffering capacity, required fewer bicarbonate (buffer) additions to control pH and maintained lower conductivity levels than reciprocal flow controls. Water consumption in the two systems was statistically identical. Overall, results suggest that a constant flow flooding regime is as good as, or better than, a reciprocating flooding regime in the aquaponic test system used.     

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.