Effect of Antarctic krillmeal on quality of farmed Atlantic cod (Gadus morhua L.)

Farmed Atlantic cod with a mean weight of 4.8 kg were maintained for 9 weeks in sea cages and fed diets where the dietary fishmeal component was substituted with increasing proportions (0%, 22%, 63% and 100%) of meal from Antarctic krill, Euphausia superba. Wild‐caught cod were included in the study as external control. At termination of feeding, all fish were slaughtered and muscle pH was immediately recorded. The fish were then stored on ice for three days and assessed for muscle pH and objective (skin and fillet colour, and fillet texture) and subjective (sensory evaluation) quality criteria. Replacement of fishmeal with krillmeal in the diets resulted in the skin colour above and below the lateral line to be more red than the control group without krillmeal substitutions, even though this difference was not significant, and with more yellow hue. Additions of krillmeal increased the muscle whiteness and yellow hue compared with the control group and wild fish. There was no difference in red hue between the groups. Muscle pH, texture or sensory attributes were unaffected by dietary inclusion level of krillmeal. Wild‐caught cod deviated in several aspects from the farmed cod. It is concluded that the replacement of fishmeal with Antarctic krillmeal in the diets two months before slaughter did not move the sensory attributes more towards wild fish.     

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.     

Mercury comparisons between farmed and wild Atlantic salmon (Salmo salar L.) and Atlantic cod (Gadus morhua L.)

Wild and farmed Atlantic salmon ( Salmo salar L.) and Atlantic cod ( Gadus morhua L.) were collected to assess changes in mercury with size in wild vs. farmed fish. Mercury concentrations were compared with Health Canada and United States Environmental Protection Agency consumption guidelines. Lipid dilution of mercury was examined by comparing lipid-extracted (LE) and non-lipid-extracted (NLE) flesh samples in both farmed and wild fish. Mercury concentrations in the flesh and liver of farmed salmon were significantly lower than concentrations in wild salmon of similar fork length ( P <0.001), possibly due to growth dilution in rapidly growing farmed fish. Mercury concentrations were higher in LE tissue compared with NLE ( P <0.05), suggesting lipid dilution of mercury in farmed fish with a high lipid content. Farmed cod, which do not grow more rapidly than wild cod, did not have significantly different flesh and liver concentrations compared with wild cod of similar fork length ( P >0.05). Between species of farmed fish, cod had significantly higher mercury concentrations than salmon ( P <0.05), but neither farmed nor wild salmon mercury concentrations exceeded federal consumption guidelines. These results suggest that rapid growth rates and a high lipid content may play important roles in regulating concentrations of contaminants such as mercury.     

Movements and spatiotemporal distribution of escaped farmed and local wild Atlantic cod (Gadus morhua L.)

Commercial farming of Atlantic cod (Gadus morhua L.) is now being developed in several countries. The ecological consequences of cod culture are poorly understood, but recent research suggests that Atlantic cod are more prone to escape from net pens than Atlantic salmon. Here, we describe the movements and the spatiotemporal distribution of farmed cod after escape relative to wild cod, both during and outside the natural spawning season. The experimental design included simulating escape incidents of farmed cod tagged with acoustic transmitters and using an array of automatic listening stations to monitor their dispersal and distribution. For comparison, local wild cod were monitored using the same array of receivers. The farmed cod dispersed rapidly after a simulated escape, they randomly distributed over large areas and their distribution overlapped with local wild cod. Moreover, escaped farmed fish were found at local cod spawning areas during the spawning season. The study also indicated that the recapture rate of escaped farmed cod was high compared with that of escaped farmed salmon. Thus, while our results showed that there is a considerable potential for ecosystem effects caused by escaped farmed cod, mitigating actions such as an efficient recapture fishery for escapees may be possible.     

Occurrence of Francisella piscicida in farmed and wild Atlantic cod, Gadus morhua L., in Norway

Francisellosis, caused by the bacterium Francisella piscicida , has become one of the most serious diseases in Atlantic cod production in Norway. The major aim of this study was to determine the distribution of F. piscicida in farmed and wild fish in areas with cod farming along the Norwegian coast, and its occurrence in cod from areas without cod farming. Two real-time PCR assays, targeting the 16S rRNA gene and the FopA gene of F. piscicida , were developed since sensitive and specific diagnostic tools are required for detecting asymptomatic carriers of the bacterium. A total of 422 wild cod from 13 sampling areas and 955 farmed cod from 10 areas along the coast of Norway were examined. Using the real-time polymerase chain reaction (PCR) assays, F. piscicida was detected in wild populations of cod from all counties examined south of Sogn og Fjordane in southern Norway (overall prevalence 13%, n  = 221). Wild cod north of Sogn og Fjordane were negative for the bacterium ( n  = 201). Farmed cod from most parts of Norway were F. piscicida positive. The apparent absence of the bacterium in wild populations of cod in the northern parts of Norway and its widespread occurrence in wild cod from southern parts of Norway is believed to relate to differences in seawater temperatures.     

Stress response to anthropogenic noise in Atlantic cod Gadus morhua L.

The potential effects of anthropogenic noise on the physiology of Atlantic cod have not been well described. The aim of the present study was to investigate the impact of anthropogenic noise on Atlantic cod stress response using cortisol as a biomarker as well as on broodstock spawning performance. Results showed that artificial noise consisting of a linear sweep from 100 to 1000 Hz can induce a transient and mild cortisol elevation with a clear noise intensity dose response. In all cases plasma levels returned to baseline levels <1 h post sound exposure. Daily exposure to a similar intensity and frequency noise range applied habitually to a broodstock population during the spawning window resulted in a significant reduction in total egg production and fertilisation rates thus reducing the total production of viable embryos by over 50%. In addition, a significant negative correlation between egg cortisol content and fertilisation rate was observed. These results confirm that cod can perceive noise generated within a frequency range of 100–1000 Hz and display a heightened cortisol plasma level. In addition, anthropogenic noise can have negative impacts on cod spawning performances.     

Effect of season, light regime and diet on muscle composition and selected quality parameters in farmed Atlantic cod, Gadus morhua L.

Effect of season and diet on muscle composition were evaluated in farmed Atlantic cod (Gadus morhua L.), fed varying levels of macro‐nutrients, and kept at two different light regimes during 1 year grow‐out in sea‐cages. The cod were fed seven different diets varying in protein, lipid and starch, in a mixture design. The diets spanned 4–20% starch, 8–26% lipid and 36–66% protein. Each dietary regime was subjected to two different light regimes: continuous light (24 h), or natural light (August 2001 to June 2002). Fish subjected to natural light started to mature in December/January and spawning was more or less completed during March/April. No maturation was registered in the continuous light groups at this point. No variation was found in muscle dry matter, protein or lipid concentration as a consequence of the dietary or light regime variations, except for the groups spawning in March. Glycogen varied from 1 to 6.5 mg g^?1 wet weight, without any correlation to the present dietary variations. At the June 2002 sampling all groups given a natural light regime showed almost twice the concentration of muscle glycogen compared with fish subjected to continuous light. Such clear results were not measured at the December or March samplings. Increased dietary lipid resulted in lowered muscle vitamin E concentration. A strong covariation between muscle vitamins C and E was found at all samplings, and these showed a negative correlation towards eicosapentaenoic (EPA) and docosahexaenoic acid (DHA), and the ratio n‐3/n‐6 in muscle. The highest dietary vitamin E resulted in the lowest muscle vitamin C concentrations. All dietary lipid added was identical and of marine origin; in addition, the wheat added as a starch source summed up parts of the dietary lipid fraction, resulting in slightly decreased sum of polyenes in the diets holding the highest levels of wheat. The lean cod muscle consists almost solely of membrane lipids. These were, however, highly influenced by the dietary lipid composition, especially as concerned the different monoenes. Two of the diets showed tendencies to increased thiobarbituric acid‐reactive substances (TBARS). This was not reflected in the muscle concentrations of vitamins E or C, and did not result in any changes in muscle TBARS values at any of the samplings, except for the fish spawning in March. The water‐soluble fraction of the muscle protein, pH range from 4.5 to 6, did show a difference in structure when comparing our experimental cod to wild cod. We could also identify a differential pattern between some of the experimental groups. The method used to identify this was, however, not quantitative, and further studies are needed. Taste panel evaluation and shear force measurements after final sampling in June concluded with minor differences between muscles from the different diet groups, except for fish given one diet with relatively high protein, intermediate lipid and low starch levels. Fish kept at continuous light was described as less firm (texture) compared with fish kept at natural light, explained partly by the different size of these two categories of fish. In conclusion, both diet and season (spawning or not) did influence several of the measured muscle parameters.     

Whole-body corticosteroid and plasma cortisol concentrations in larval and juvenile Atlantic cod Gadus morhua L. following acute stress

Methods were developed to assess whole‐body immunoreactive corticosteroid concentrations (IRC) and plasma levels of cortisol in Atlantic cod subjected to several common, acute stressors. A measurable corticosteroid stress response was observed at the first sampling in whole bodies of 8‐day post‐hatch (dph) larvae. Two groups of juveniles (～5 and 30 g) were subjected to a 30 s net stressor and whole‐body IRC and plasma cortisol levels were determined. Post‐stressor IRC in smaller fish rose approximately 14‐fold, peaked at 1 h, were sustained for 3–6 h and returned to pre‐stressor levels within 24 h. Post‐stressor plasma cortisol levels in larger fish rose approximately 18‐fold, peaked at 0.5–1 h, were sustained for 1–3 h and then returned to near pre‐stressor levels after 24 h. Immunoreactive corticosteroid concentrations appeared to remain elevated longer than plasma cortisol levels suggesting that steroids other than cortisol were contributing to total immunoreactivity in homogenates. Juveniles exposed to either a grading procedure or high density transport had maximal IRC and plasma cortisol levels within 90 min which returned to pre‐stressor levels within 24–72 h. A reduction in water salinity (20 g L^?1) did not moderate the corticosteroid response in juveniles. The results show that Atlantic cod respond to common, acute stressors in a manner similar to other teleosts. Whole‐body homogenates can be used to identify changes in IRC in response to acute stress in cod with the caveat that recovery IRC may differ from plasma cortisol concentrations.     

Dietary plant protein utilization in Atlantic cod, Gadus morhua L.

The present trials aimed to investigate the effects of replacing fish meal with plant proteins in diets for cod, using a regression design where fish meal constituted the control. The plant protein diets were formulated to meet the amino acid requirements according to NRC (1993) and contained corn gluten meal, soybean meal, a mixture of these, or a mixture of wheat gluten meal and soybean concentrate. The plant protein fraction constituted up to 440 g kg⁻¹ of the extruded diet. Two feeding experiments were conducted, one at high (11 °C) and one at low (6.5 °C) temperature. High growth and feed utilization were obtained in all diet groups at both temperatures. However, only in Exp.1 (11 °C), growth and feed utilization were linearly reduced by increases in dietary soybean and corn gluten meal, while no such effect was detected in Exp.2 (6.5 °C). Reductions in protein retention were seen at both temperatures with use of all evaluated plant protein sources. Due to large amounts of plant protein ingredients in diet, differences in diet amino acid composition were seen. These were partly reflected in the muscle free amino acid pool. Dietary plant ingredients did not affect whole body, liver or muscle proximate compositions, or liver indices. In both experiments, blood parameters were within the range of earlier reported normal values and indicated, together with low mortality, good fish health status. The results show that there is high potential to use protein‐rich plant ingredients in diets for Atlantic cod.     

Evaluation of Electrical Stunning of Atlantic Cod (Gadus morhua) and Turbot (Psetta maxima) in Seawater

The aim of this study was to assess electrical stunning of Atlantic cod and turbot in seawater to develop a protocol for the process of stunning and killing. An induced general epileptiform insult (unconscious) had a duration of 40 ± 27 s (n =14) in cod (2.6 ± 0.5 kg) and 34 ± 18 s (n = 19) in turbot (520 ± 65 g). Seven cod and 3 turbot displayed a physical reaction, and 11 turbot registered an electroencephalogram (EEG) response to pain stimuli administered 30 s post-stun. The heart rate was 32 ± 6 beats/min in cod and 25 ± 7 beats/min in turbot prior to stunning. Post-stunning, the electrocardiogram (ECG) revealed fibrillation and reduced activity post-stun. EEG, ECG recordings, and behavioral observations indicate that when a bipolar square wave current was applied with a frequency of 133 Hz and 43% duty cycle side to side (turbot) and at 170 Hz and 33% duty cycle (cod) head to tail, both species were stunned in seawater at current densities of 3.2 A/dm² and 2.5 A/dm², respectively. For turbot, a 5 s exposure to electricity followed by chilling in ice water for 15 min is sufficient to prevent recovery. For cod, a killing method needs to be established.     

Inclusion of camelina meal as a protein source in diets for farmed Atlantic cod Gadus morhua

Camelina meal Camelina sativa (CM) is a potential protein source in aquaculture feeds, because of its crude protein level (39%) and essential amino acids. Two feeding experiments were conducted with Atlantic cod Gadus morhua. Cod in Experiment I (19.4 g fish^?1) were fed diets with 0%, 12% or 24% CM for 9.5 weeks at 10°C; and cod in Experiment II (14.4 g fish^?1) were fed diets with 0%, 15%, 30% or 40% CM for 13 weeks at 10°C. Growth, lipid and amino acid tissue composition were compared amongst cod fed varying levels of CM. In Experiment I, cod could tolerate the highest level of CM inclusion (24%) without affecting growth compared to cod fed the control diet. In Experiment II, growth performance was significantly affected at 30% CM inclusion compared to the control treatment, and cod fed 15% CM displayed some signs of depressed growth (reduced feed intake and weight gain). Both treatment and duration were interacting factors (P = 0.015) that determined growth performance when comparing both experiments. Muscle tissue composition was relatively unaltered with less than 30% CM inclusion; however, multivariate statistics revealed significant differences in muscle tissue fatty acid composition between cod fed 40% CM and the control diet. The tissue amino acid profile was generally unaltered because the dietary amino acid profile was consistent after CM inclusion. A few antinutritive compounds in CM may have affected palatability in diets with greater than 30% CM inclusion, which may have resulted in reduced growth performance.     

Effect of sustained exercise on white muscle structure and flesh quality in farmed cod (Gadus morhua L.)

Growth performance and white muscle cellularity were investigated in Atlantic cod (Gadus morhua L.) (500–600 g) to determine if texture and thereby quality is affected by exercise. Three exercise regimes [low flow, 0.5 body lengths per second (BL s^?1) and 1.0 BLs^?1] were applied, and the experiment was run for 9 months, at which point the fish had reached around 2 kg body weight. There were no significant differences in fork length or condition factors between groups that could be attributed to the exercise treatment, but the 0.5 BL s^?1 group had significantly lower body weight performance than the control group. The average specific growth rate ranged from 0.37% to 0.42% per day. Muscle fibre diameter was positively correlated with fish weight. However, there was no difference in the distribution of muscle fibre diameters between treatment groups. A texture analyser, model TA‐XT2, equipped with a Warner–Bratzler shear blade was used to measure the texture of the fillet. No correlation was found between the shear force required to cut through the sample and the muscle fibre diameter, but the shear force decreased with increasing growth rate. A tristimulus colorimeter was used to measure the colour composition of the fillet instrumentally. Red/green chromaticity (a*) was positively correlated with the shear force. It was concluded that the exercise regimes tested were insufficient to influence either growth performance or muscle growth patterns. Individual variations in fibre diameter did not influence the colour or texture of the fillet.     

Efficacy of egg surface disinfectants in captive spawning Atlantic cod Gadus morhua L. and haddock Melanogrammus aeglefinus L.

In an effort to optimize the efficiency of high‐density incubation of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus eggs, the per cent hatch of eggs treated with four disinfectants (3% hydrogen peroxide, 1% polyvinylpyrrolidone iodine, 0.1% sodium hypochlorite and a 0.005% antibiotic solution – penicillin/streptomycin) was compared in both species. The per cent hatch of eggs of each species was greatest after a 24 h treatment with the antibiotic solution. The hatching success of eggs treated within the different disinfectant treatments depended upon the embryonic developmental stage in both species. The sodium hypochlorite treatment had the lowest % coverage of colony growth after disinfected haddock eggs were plated onto sterile agar media, highest survival rates to the end of the embryonic period, but the lowest per cent hatch.     

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.     

Investigation of the temporal effects of spawning season and maternal and paternal differences on egg quality in Atlantic cod Gadus morhua L. broodstock

A better understanding of the parameters affecting egg quality and larval survival is of importance for continued development of cod broodstock and efficient husbandry practices. Decision tree analysis (DTA) was applied to analyse 3 years of egg quality data in an effort to extract the most important variables (i.e. predictors) in explaining differences in egg quality. The effect of three predictors (spawning time, maternal and paternal differences) has been studied on early cleavage pattern parameters, egg diameters, fertilization and hatching rates and has shown that females are the dominant variable and that time has a limited and inconsistent impact on the data. When using maternal, paternal differences and batch number (instead of spawning time) as predictors, the results confirm that no particular relationship is found between batch order (i.e. order in time) and egg quality. Moreover, batches with a higher egg quality show a consistency in the parameters assessed (i.e. batches with higher rates of normality in any parameter tend to be normal for other parameters). This is confirmed by the significant correlations found between cleavage parameters. Our results highlight that spawning time is of less importance than female parent contribution in ensuring high rates of fertilization and larval hatch, and maximizing general egg quality.     

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.     

Pre-rigor filleting and drip loss from fillets of farmed Atlantic cod (Gadus morhua L.)

Improved slaughtering procedures of farmed fish may provide sufficient time so that filleting can be performed pre‐rigor while the muscle pH is still high. Such filleting not only reduces fillet gaping but also lowers the transportation costs and makes fresh fillets available to the markets at an earlier stage. The aim of our work was to determine the weight reduction of the fillets due to liquid loss and to study the proteins and enzymes in the drip. After 11 days of cold storage, fillets of farmed Atlantic cod produced pre‐rigor had a weight loss of 10% and a fillet contraction of 19% while for fillets produced post‐rigor the values were 5% and 4% respectively. At the same time, approximately twice the amount of proteins had been lost from the pre‐rigor‐produced fillets. Sodium dodecyl sulphate polyacrylamide gel electrophoresis analysis showed that the proteins in the drip were similar to the sarcoplasmatic proteins extracted from the muscle. Specific analysis of proteolytic enzymes indicated that they are less stable in the expelled liquid than in the flesh during storage. The extensive loss of weight and proteins from pre‐rigor‐produced fillets during subsequent storage must be taken into account if such processing is considered for farmed cod.     

Spawning‐related mortality in captive Atlantic cod (Gadus morhua L.)

Two year‐classes of Atlantic cod were reared in indoor tanks from November 2002 to November 2004. The average annual mortality was 21% and 36% for males and females respectively. The highest monthly mortality rates in females were observed around and during the spawning periods from March to July, maximally 7% and 20% for year‐class 2001 and 18% and 34% for year‐class 2000. The mortality in males was less dependent on season, reaching a maximum of 7% and 5% month^?1 for year‐class 2001 and 2000 respectively. The maximum mortality during spawning increased with age from: 7%, 19% and 34% per month for 2‐, 3‐ and 4‐year‐old females respectively. The average gonadosomatic indices in 3‐ and 4‐year‐old mature females that died during the spawning periods were 22% and 26% respectively and most of them were spawning when they died. The results suggest that egg‐retention may have been the main cause of death in females during spawning.