Territorial and agonistic interactions between farmed and wild cod (Gadus morhua)

Studies of contest competition between wild and farmed fish have mostly focused on fish with strongly territorial behaviour. Little is known about species with more plastic social behaviour, such as Atlantic cod (Gadus morhua L.), a species that can either aggressively defend territories or shoal. There is also concern that cod that escape from farms will compete with wild populations. We examined dyadic contest competition between wild and farmed juvenile cod using an intruder–resident experimental set‐up in the laboratory. No prior residency advantage was observed, but the differences between farmed and wild cod were clear. Farmed cod were more submissive than wild cod and fled earlier during contests, which suggests that wild fish often out‐compete farmed intruders. Both fish types initiated aggression earlier against fish of the same background. A multivariate analysis of 11 different behavioural traits indicated that a group of 59% of farmed fish were behaviourally very similar to the 55% most submissive wild fish. These results suggest that wild juvenile cod may be quite robust towards competition for food and shelter from juvenile farmed cod, but further research is needed to verify this pattern.     

Egg quality determinants in cod (Gadus morhua L.): egg performance and lipids in eggs from farmed and wild broodstock

Lipids and essential fatty acids, particularly the highly unsaturated fatty acids, 20:5n‐3 (eicosapentaenoic acid; EPA), 22:6n‐3 (docosahexaenoic acid; DHA) and 20:4n‐6 (arachidonic acid, AA) have been shown to be crucial determinants of marine fish reproduction directly affecting fecundity, egg quality, hatching success, larval malformation and pigmentation. In Atlantic cod (Gadus morhua L.) culture, eggs from farmed broodstock can have much lower fertilization and hatching rates than eggs from wild broodstock. The present study aimed to test the hypothesis that potential quality and performance differences between eggs from different cod broodstock would be reflected in differences in lipid and fatty acid composition. Thus eggs were obtained from three broodstock, farmed, wild/fed and wild/unfed, and lipid content, lipid class composition, fatty acid composition and pigment content were determined and related to performance parameters including fertilization rate, symmetry of cell division and survival to hatching. Eggs from farmed broodstock showed significantly lower fertilization rates, cell symmetry and survival to hatching rates than eggs from wild broodstock. There were no differences in total lipid content or the proportions of the major lipid classes between eggs from the different broodstock. However, eggs from farmed broodstock were characterized by having significantly lower levels of some quantitatively minor phospholipid classes, particularly phosphatidylinositol. There were no differences between eggs from farmed and wild broodstock in the proportions of saturated, monounsaturated and total polyunsaturated fatty acids. The DHA content was also similar. However, eggs from farmed broodstock had significantly lower levels of AA, and consequently significantly higher EPA/AA ratios than eggs from wild broodstock. Total pigment and astaxanthin levels were significantly higher in eggs from wild broodstock. Therefore, the levels of AA and phosphatidylinositol, the predominant AA‐containing lipid class, and egg pigment content were positively related to egg quality or performance parameters such as fertilization and hatching success rates, and cell symmetry.     

Quality of farmed Atlantic cod: effects of season and storage

Farming of Atlantic cod makes it possible to supply cod throughout the year. The aim of this study was to investigate the effects of season and postmortem ice storage on the quality parameters of farmed cod. Farmed Atlantic cod were sampled from the same pen in the period February 2006–March 2007. Cod whole body and somatic weight, condition factors and hepatosomatic index were all affected by season, all decreasing during maturation and spawning in the winter. Muscle pH and water were influenced by season, increasing during maturation and spawning and then decreasing after spawning. Season also affected the colour of the fillet. There was a significant increase in whiteness post‐spawning and a decrease during the second maturation. Water‐holding capacity (WHC), levels of trimethylamine oxide and trimethylamine, as well as microbiological data, did not vary due to season. Muscle pH, water content, WHC and microbial counts increased during storage for all samplings.     

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.     

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.     

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.     

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.     

Changes in Atlantic cod (Gadus morhua L.) sperm quality during the spawning season

The biology of cod reproduction is well described in the scientific literature. However, sperm biology and spermatozoa management are poorly studied in this species. Because of its recent farming expansion, a better knowledge of cod gametes is becoming especially useful. This work aimed at establishing tools to study sperm biology in cod, and also investigated the existence of changes in cod sperm quality during the spawning period. We showed that sperm concentration could be assessed using spectrophotometry at 260 nm. Sperm motility significantly decreased after a 168‐h storage at 4 °C. A 1:9 dilution of sperm in a non‐activating medium (1/3 seawater and 2/3 freshwater, osmotic pressure: 360 mOsm kg^?1) improved sperm storage. Sperm concentration, sperm velocity and storage capacity at 4 °C peaked during the medium period of the spawning season and then decreased to values close to those observed at the beginning of the reproductive period. The measured values of osmotic pressure, pH, protein, Na⁺, Cl^? and Ca²⁺ concentrations of the seminal fluid were modified along the spawning period. Cell damage was noted at the end of the spawning period: local blebs were observed on the flagellum but also loops at its distal part. On the other hand, spermatocrit did not vary with the sampling date. In conclusion, cod sperm quality is modified during the spawning period, the highest‐quality samples being collected during the medium part of this season.     

Differences in growth and chemical composition between male and female farmed cod (Gadus morhua) throughout a maturation cycle

The growth of farmed cod fed to satiation under natural conditions in the sea in North Norway (67°N) was followed over 1 year. At the start of the experiment in September 2003, the average weight was 1300 g. After 1 year, in September 2004, the average weight was 2800 g. From the end of November to June, the fish matured and spawned and there was little or no weight gain in the fish during this time. The male cod developed gonads earlier than the females. In February, the male gonad index was 19% and decreased to 7% by the end of April, whereas the female gonad index increased from 14% in February to 25% in April. The male liver index decreased by 2% in February and by another 3% in April, when it only decreased by 1% in the female cod. The muscle protein in the female cod was reduced by more than 20 g kg^?1 in April, and the difference between the muscle protein in the male and the female cod was still significant in September. No differences were found in the post‐rigour muscle pH due to the sex of the fish.     

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.     

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.     

The effects of stress and storage temperature on the colour and texture of pre-rigor filleted farmed cod (Gadus morhua L.)

This experiment deals with the effects of pre‐slaughter stress and storage temperature on muscle pH, fillet contraction, colour and texture in pre‐rigor filleted farmed cod fillets. The fish were either sedated with a low dose of MS‐222 (14.3 mg L^?1) (unstressed groups) or exposed to the air for 3 min (stressed groups) before being submerged in a benzocaine bath (150 g L^?1). The fish were then killed by a blow to the head, their gills cut, filleted and finally stored at either 4 or 20°C. The stressed groups had significantly lower pH values after slaughter (pH=7.0) than the unstressed groups (pH=7.3). This difference was maintained until post rigor for the fish stored at 4°C, but at 20°C it was immediately overshadowed by a decrease in pH caused by temperature‐dependent processes. The length contraction and changes in registered colour values were more pronounced at both the higher temperature and the higher level of pre‐slaughter stress. Again temperature dominated, but significant and consistent effects were registered from stress. No significant effects of stress on texture post rigor were observed. It is concluded that high storage temperature masks the majority of effects caused by pre‐slaughter stress on the measured variables. Stress management protocols, however, are important when the fillets are kept at the common storage temperature of 4°C.     

Reconstructing migrations of individual cod (Gadus morhua L.) in the Baltic Sea by using electronic data storage tags

A new methodology is presented to reconstruct migration pathways of individual fish inhabiting ecosystems with moderate‐to‐strong gradients in temperature or salinity. The method uses measurements of ambient pressure, temperature and salinity obtained from electronic data storage tags attached to individual fish and is particularly applicable in areas with negligible tides. We demonstrate the method with Baltic cod. Hydrographic fields obtained from hydrodynamic modelling were used as a geolocation database to identify daily positions of Baltic cod by comparison with the environmental data collected by the tags. Using randomly distributed individual parameter perturbations in the range of the instrument precision of the tag we simulated a cod migrating through the Baltic Sea. The distance between the prescribed and geolocated positions of this artificial cod was on average 2.9 ± 4.7 (SD) km. Subsequently, the method was used to reconstruct migration routes of 10 real cod tagged in the Bornholm Basin of the Baltic Sea in early spring 2003. Differences were compared between the tag data and the geolocation database. The uncertainty in geolocation at recapture day was on average 75 ± 23 (SD) km, as shown by comparison between geolocated position and recapture position.     

Optimized biophysical model for Icelandic cod (Gadus morhua) larvae

A characteristic of biophysical models (BPM) is that they contain a number of parameters that are poorly known or only known within a range of possible values. This paper describes an efficient optimized BPM developed to simulate the Icelandic cod pelagic 0‐group survey data. The method is based on presenting the results as a probability density function (PDF) that a particle released from a given spawning drifts downstream to a given grid location some time later. The model determines egg production model parameters (peak spawning time, spawning duration, number of eggs released) for each of 15 spawning grounds as the solution of a bound constrained optimization problem that minimizes model‐data misfits in abundance and age distributions. The model is applied to simulating the 2002 and 2003 summer survey data. The model does a reasonable job of simulating the observed inshore / offshore abundance gradient and spatial age gradient for each year. Problem areas are explained from the point of view of model limitations. We caution that the results from an optimized model should always be assessed with the model’s limitations in mind and with respect to whatever biological data are available.     

The effects of feeding frequencies on seasonal changes in growth rate and chemical composition of farmed cod (Gadus morhua)

Atlantic cod (Gadus morhua) were fed with dry feed, in sea net pens from May 2003 to September 2004, under natural conditions occurring in Northern Norway, with a sea temperature varying from 3.3 ° to 16.6 °C. The fish was either fed five times or three times a week, once a day to saturation until December 2003, and after that five times or twice a week, once a day to saturation until September 2004. Fish fed five times a week had a higher FCR 1.47 compared with 1.35 for fish fed three to twice a week for the entire period. No significant differences were found in growth or chemical composition in the cod due to the feeding intervals. The best growth of the fish was in the first summer from May to September 2003 with a specific growth rate (SGR) of 0.81% per day. From June to September 2004, the SGR was 0.42% per day. From December to May, the SGR was negligible caused by low‐water temperature, darkness, and sexual maturation. The liver index increased from 7% to a maximum of 16% during the growth period except during the sexual maturation period from November to April, where it shows a slight decrease. Maturation and seasonal variation are the factors having the largest influence on the growth and chemical composition of the cod.     

Digestive capacity and compensatory growth in Atlantic cod (Gadus morhua)

The objective of this study was to examine whether digestive capacity correlates with growth rate in Atlantic cod (Gadus morhua). Ninety fish (1035±240 g; 478±36 mm) were assigned to one of three treatments: deprived of food for 5 weeks and then re-fed for 24 days, deprived for 10 weeks and then re-fed for 24 days, or fed for 24 days without deprivation (controls). Three times a week during feeding, the fish were provided with meals of capelin (Mallotus villosus) in excess. Within each treatment, ten fish were sacrificed before and twenty after the feeding period. The relative masses of the pyloric caeca, intestine, and white muscle were determined. Cytochrome c oxidase (CCO), citrate synthase (CS), and nucleoside diphosphate kinase (NDPK) activities were assayed in the pyloric caeca, intestine, and muscle as was trypsin activity in the pyloric caeca. During the re-feeding period, fish that had been deprived of food for 10 weeks showed compensatory growth, growing 1.9 times faster than the controls (0.94±0.26 versus 0.50±0.31% body mass ⋅ day⁻¹). Fish that displayed compensatory growth had a lower relative white muscle mass than controls (66.5±4.1% vs. 69.6±7.4%) while the relative masses of the pyloric caeca (2.21±0.49% vs. 1.78±0.31%) and intestine (0.95±0.20% vs. 0.77±0.15%) were greater than the controls, suggesting that the sizes of these digestive tissues might correlate with compensatory growth capacity. Citrate synthase in the pyloric caeca was the only enzyme that showed higher activity during compensatory growth (8.57±0.94 U ⋅ g tissue⁻¹ compared to 7.13±1.03 U ⋅ g tissue⁻¹ in the control group). This suggests that aerobic catabolic capacity of pyloric caeca could be related to growth capacity during recovery, possibly via the energetic cost of digestive enzyme synthesis. This revised version was published online in July 2006 with corrections to the Cover Date.