Temperature-dependent development rates of cod Gadus morhua eggs

Temperature development relationships were determined for batches of Irish Sea cod Gadus morhua eggs incubated in flow-through incubators. Hatching began 16·4 days after fertilization (DAF) at 6° C, 10·3 DAF at 8° C, 9·4 DAF at 10° C and 7·4 DAF at 12° C. Egg mortality increased at the higher temperatures, but survival was >80%. Results were compared with published data at four comparable stage end points: the end of blastula, the end of gastrula, the point of growth of the embryo completely surrounding the yolk and the point when 50% of the eggs were hatched. All the studies showed a curvilinear relationship between age at stage and temperature. There was a 12 day inter-study difference in time to 50% hatch at 2° C and 4 day difference at 10° C. There were no consistent trends that differentiated eastern v. western, or northern v. southern populations. A single model for cod egg incubation time from fertilization to 50% hatch was derived based on data from six cod populations, but it is recommended that individual stock relationships should be used where possible.     

Nodavirus in farmed Atlantic cod Gadus morhua in Norway

Viral encephalopathy and retinopathy (VER) was diagnosed in 5 to 24 g sized farmed Atlantic cod Gadus morhua kept in sea cages at Parisvatn, Hordaland county, on the west coast of Norway. Moderate mortality (10 to 15%) was observed, along with anorexia and abnormal swimming behaviour, such as looping or spiral swimming and reduced coordination. Nodavirus was detected by 2 different real-time RT-PCR assays, and this was later confirmed by immunohistochemistry. This is the first report of an outbreak of VER in farmed cod in Norway, and the first report that VER affect cod exceeding 5 g in size.     

Stage-specific mortality of Baltic cod (Gadus morhua L.) eggs

A study on cod egg mortality was carried out in the Bornholm Basin (southern central Baltic Sea) toward the end of July 1996. An initial egg aggregation marked by a satellite‐tracked drifter buoy was sampled repeatedly over an 11‐day period; profiles of temperature, salinity and dissolved oxygen were concurrently recorded. Three replicate estimates of mortality were obtained for each pair of subsequent developmental stages from newly spawned eggs to early larvae. A consistent pattern of stage‐specific mortality coincided well with previous experimental observations. Average daily mortality rates were 7.2% (eggs IA/IB), 38.7% (eggs (IB/II), 25.6% (eggs II/III), 40.0% (eggs III/IV), and 42.3% (eggs IV/early larvae). The cumulative mortality until hatch amounted to 99.9%. Results from hydrodynamic modelling, however, indicated that the drifter's trajectory was influenced by wind stress. Hence, the mortality rates might be biased despite the short sampling intervals; a modification of the sampling design is recommended for future studies.     

Minisatellite DNA fingerprints of Atlantic cod (Gadus morhua)

The human minisatellite probes 33.6 and 33.15 cross–hybridized to Hae III and Hinf I digested cod DNA, revealing complex fragment patterns in both Arctic and coastal cod. The DNA fingerprints were highly polymorphic, individual specific and stable in the germline. The potential applications of multi locus probes in cod research are discussed.     

Behavioural responses of hatchery-reared and wild cod Gadus morhua to mechano-acoustic predator signals

The behavioural responses of wild (predator-experienced) and hatchery-reared (predator-naive) cod Gadus morhua to standardized mechano-acoustic (MA) stimuli were compared in the laboratory. Wild fish responded mainly with freezing and fast-start escapes away from the stimulus, whereas hatchery-reared fish often ignored or approached the stimulus. Wild fish also had stronger responses, turning faster during escapes and reducing activity immediately after the stimulus. Both fish types were less active on a 'risky' bare substratum after the stimulus. The antipredator responses of wild fish were consistent to repeated stimuli, whereas hatchery-reared fish that had generally only encountered harmless stimuli showed more variable responses with lower repeatability. This suggests that experience plays a role in shaping the behavioural response of fishes to MA stimuli.     

The effect of dietary lipid content and stress on egg quality in farmed Atlantic cod Gadus morhua

The present study assessed differences in fecundity and egg quality from Atlantic cod Gadus morhua fed isoproteic diets containing 13% fat (low fat, LF) or 20% fat (high fat, HF) and either stressed or left unstressed as a control over the spawning season. Each diet was fed to triplicate groups of G. morhua from June 2009, through to first maturation and spawning. In January 2010 sub-groups of G. morhua were moved to land-based spawning tanks where the experimental trial was carried out. At the start of the experiment, G. morhua fed the high-fat diet were significantly larger than G. morhua fed low-fat diet. These differences were maintained through the spawning season, although with a loss of mass in both dietary groups. Relative fecundity through the season was significantly lower in stressed G. morhua fed LF compared to unstressed G. morhua fed the same diet. Stressed G. morhua had a higher variability in weekly amount of eggs spawned, spawning occurred more irregularly, and the spawning period lasted longer than in unstressed G. morhua. Several egg quality variables were also affected: eggs from G. morhua fed LF and exposed to stress had lower fertilization and hatching rates compared to the unstressed G. morhua fed the same diet as well as all G. morhua fed HF. Gadus morhua fed a low-fat diet appeared less tolerant to stress than fish fed a high-fat diet.     

Ontogeny of redox regulation in Atlantic cod (Gadus morhua) larvae

The reduction potential of a cell is related to its fate. Proliferating cells are more reduced than those that are differentiating, whereas apoptotic cells are generally the most oxidized. Glutathione is considered the most important cellular redox buffer and the average reduction potential (E_h) of a cell or organism can be calculated from the concentrations of glutathione (GSH) and glutathione disulfide (GSSG). In this study, triplicate groups of cod larvae at various stages of development (3 to 63 days post-hatch; dph) were sampled for analyses of GSSG/2GSH concentrations, together with activities of antioxidant enzymes and expression of genes encoding proteins involved in redox metabolism. The concentration of total GSH (GSH+GSSG) increased from 610±100 to 1260±150 μmol/kg between 7 and 14 dph and was then constant until 49 dph, after which it decreased to 810±100 μmol/kg by 63 dph. The 14- to 49-dph period, when total GSH concentrations were stable, coincides with the proposed period of metamorphosis in cod larvae. The concentration of GSSG comprised approximately 1% of the total GSH concentration and was stable throughout the sampling series. This resulted in a decreasing E_h from −239±1 to −262±7 mV between 7 and 14 dph, after which it remained constant until 63 dph. The changes in GSH and E_h were accompanied by changes in the expression of several genes involved in redox balance and signaling, as well as changes in activities of antioxidant enzymes, with the most dynamic responses occurring in the early phase of cod larval development. It is hypothesized that metamorphosis in cod larvae starts with the onset of mosaic hyperplasia in the skeletal muscle at approximately 20 dph (6.8 mm standard length (SL)) and ends with differentiation of the stomach and disappearance of the larval finfold at 40 to 50 dph (10–15 mm SL). Thus, metamorphosis in cod larvae seems to coincide with high and stable total concentrations of GSH.     

Phenotypic flexibility of digestive system in Atlantic cod (Gadus morhua)

This study examined the restoration of the digestive capacity of Atlantic cod (Gadus morhua Linnaeus) following a long period of food deprivation. Fifty cod (48 cm, 1 kg) were food-deprived for 68 days and then fed in excess with capelin (Mallotus villosus Müller) on alternate days. Ten fish were sampled after 0, 2, 6, 14 and 28 days and the mass of the pyloric caeca, intestine and carcass determined. Two metabolic enzymes (cytochrome c oxidase and citrate synthase) were assayed in white muscle, pyloric caeca and intestine, and trypsin activity was measured in the pyloric caeca. A delay of 14 days was required before body mass started to increase markedly, whereas most of the increase in mass of both the pyloric caeca and intestine relative to fish length occurred earlier in the experiment. By day 14, the activities of trypsin and citrate synthase in the pyloric caeca as well as citrate synthase in the intestine had reached maxima. The growth of the digestive tissues and restoration of their metabolic capacities thus occur early upon refeeding and are likely required for recovery growth to take place. The phenotypic flexibility of the cod digestive system is therefore remarkable: increases in trypsin activity and size of pyloric caeca resulted in a combined 29-fold increase in digestive capacity of the fish during the refeeding period. Our study suggests that Atlantic cod are able to cope with marked fluctuations in food availability in their environment by making a rapid adjustment of their digestive capacity as soon as food availability increases.     

Maternal influence on the size and viability of Iceland cod Gadus morhua eggs and larvae

Size, condition, and age of female-Icelandic cod Gadus morhua were correlated to the size of their eggs and newly hatched larvae. A positive relationship was detected between egg size and some larval viability parameters, including the age at first feeding, successful development of a swimbladder, and specific growth rates during the first 15 days after hatching. These results reveal that the viability of cod larvae is related to attributes of the spawning females and that this information is important to our understanding of stock-recruitment relationships.     

A reappraisal of activity metabolism in Atlantic cod (Gadus morhua)

Atlantic cod ( Gadus morhua ) were forced to swim in a swim tunnel respirometer until fatigued; oxygen consumption rate (O₂) was measured during swimming at incremental speeds until the fish was exhausted and during recovery from exhaustion. Maximal oxygen consumption (O_2max) occurred during maximal activity as has been found for other fish species, but at odds with the current paradigm for Atlantic cod. Earlier experiments had drawn the conclusion that O_2max in Atlantic cod occurs during recovery from exhaustive exercise. We found no support for this paradigm in our experiments and we propose that the respiratory physiology of Atlantic cod is not unlike that of other fishes.     

Feeding behaviour of cod (Gadus morhua) in relation to spawning

The food consumption and egg production of 26 adult (13 female and 13 male) Atlantic cod ( Gadus morhua ) were monitored during prespawning, spawning and postspawning periods. Females spawned from late January to mid-April. Feeding activity occurred from December to early January and ceased for females, on average, 36 days (15–54 days) before the onset of spawning. The duration of spawning by females was, on average, 42 days (10–61 days) and feeding was suppressed by both sexes during the first three-quarters of each female's spawning period. Mature females went, on average, 70 days or 19% of the year without eating. An abrupt increase in feeding activity, particularly by females, occurred during the last quarter of spawning or shortly after the release of the last egg batch (on average, feeding started again after 91% of a female's eggs had been released or 82% of egg batches). Females consumed greater quantities of food than males during both winter and postspawning feeding periods. During spawning, females lost, on average, 29% of their body weight and males 14%. Fecundity ranged from 0.75 to 3.97 million eggs per female. The volume of eggs produced by four individual females (range = 1285–5995 ml in four to 11 batches) ranged from 99 to 195% (mean 150%) of a female's postspawning body volume. Six immature cod fed throughout the experimental period and gained, on average, 8% of initial body weight. Laboratory results were supported by stomach fullness index values of Georges Bank cod exhibiting different maturity states.     

Oxygen uptake in developing eggs and larvae of the cod, Gadus morhua L.

Unfertilised cod eggs showed a mean oxygen uptake rate at 5°C of 0.089 μl O₂, dry wt.⁻¹ h⁻¹; this gradually rose to 0.768 μl O₂ mg dry wt.⁻¹ h⁻¹ in eggs about to hatch. From hatching to complete yolk absorption larvae respired at 1.6 μl O₂, mg dry wt.⁻¹ h⁻¹. During starvation following yolk absorption, uptake fell significantly to 1.1 μl O₂, mg dry ⁻¹ h⁻¹. Much of this decrease in oxygen consumption was shown to be caused by reduction in activity. Loss of weight during the embryo and larval phases could not easily be reconciled with total oxygen consumption; it is suggested that cod embryos and larvae may not rely solely upon endogenous energy reserves during development.     

Purification and characterization of pancreatic elastase from Atlantic cod (Gadus morhua)

1. A pancreatic elastase from Atlantic cod (Gadus morhua) has been purified and characterized. 2. The enzyme is a very basic protein with an approximate mol. wt of 28,000. 3. The cod elastase has higher elastin specificity than porcine elastase, and it is inhibited by soybean trypsin inhibitor, which has no effect on porcine elastase. 4. The cod elastase expresses a higher turnover number (kcat) and catalytic efficiency (kcat/Km) than porcine elastase, but it is less thermostable.     

Fertility of gynogenetic Atlantic cod (Gadus morhua L.)

In order to investigate whether meiotic gynogenetic Atlantic cod is fertile and able to produce viable offspring, meiotic gynogenetic females were produced in spring 2010 by activating cod eggs using irradiated sperm. The extrusion of the second polar body was prevented by the application of hydrostatic pressure (56.6 MPa) 36 min after fertilization. In February 2012, their mean round weight was 972 g, and 2580 g in March 2013. In 2012, when the fish were 2 years old, about 52% were mature, 33% were immature, and 13% had undifferentiated gonads. One year later, 77% were mature, 11% were immature, and 11% had undifferentiated gonads. Several of the mature females had malformed gonads, with only one developed ovary lobe or with the two lobes fused. The mean gonadosomic index (GSI) of the 2‐year‐old mature females was 5.2%, with an estimated relative fecundity of 581 000 eggs kg ovary‐free wet weight^?1. Females were stripped for eggs when 2 and 3 years old (2012 and 2013), and fertilized with sperm from normal males. Offspring were obtained from 12 of 17 and 12 of 15 egg batches incubated in 2012 and 2013, respectively, proving that the gynogenetic females are fertile. Furthermore, larvae in all but one of the hatched groups from 2013 had commenced feeding 2 h after being startfed using rotifers 4 days after hatch, indicating viable offspring.     

Immune parameters of immunised cod (Gadus morhua L.)

The immune response of cod (Gadus morhua L.) is unusual in that specific antibody response is limited or absent. In the present study cod was immunised with haptenated and non-haptenated protein antigen at two different temperatures and the antibody response monitored over a period of 18 months. Other humoral parameters of immunological importance were also analysed, namely total immunoglobulin concentration, anti-protease and spontaneous haemolytic activity. No specific antibody response was detected but increased activity of non-specific anti-TNP antibodies was observed 10-12 weeks after immunisation, irrespective of the antigen used. This antibody activity was attributed to the adjuvant used (FCA) and did not cross react with other antigens tested. Other parameters were probably not influenced by the immunisation but seasonal fluctuations were indicated. The immunoglobulin level appeared to peak in August-September and the anti-protease activity and the haemolytic activity in October-January.     

The spawning activity of cod, Gadus morhua L.

Aspects of the reproduction of reared cod, Gadus morhua L., with special emphasis on the females, were studied under laboratory conditions. The fecundity and condition factor were 2–5 and 1–5 times, respectively, that of wild cod. A total of 18 spawning females were kept in separate tanks/ chambers, each with one or two males. Seven of the 18 females were classified as stressed, based upon behaviour, irregular spawning intervals and low fertilization rates of the eggs. The reared cod were found to spawn 17–19 batches. The number of eggs liberated in each batch normally followed a smooth, dome-shaped curve with time. The fertilization rate was normally 100%. Egg size decreased from first to last batch and the egg dry weight decreased by about 20–30%. The reared cod showed the same egg diameter to dry weight relation as wild cod. Egg diameter of first batch and maternal fish length were significantly positively correlated. The mean spawning interval for the female and the mean water temperature during its spawning were negatively correlated. The reared cod spawned in both the night and the day for about 50–60 days.     

Ecological implications of Echinorhynchus gadi parasitism of Baltic cod (Gadus morhua)

Of 533 Baltic cod (body length 21–93 cm) 530 were infected with Echinorhynchus gadi at infection intensities from 1 to 490 per host. The heavy infection of large cod (normally not feeding on amphipods) may be explained by a transfer of parasites from prey fishes to the predating large cod.     

Directional hearing of cod (Gadus morhua) under approximate free field conditions

Summary The cod,Gadus morhua L., can hear the direction of a sound source of 75 Hz in the transversal plane. At the experimental site in the middle of a fjord, local depth 35 m, the sound sources were situated at radial distances of 4 to 5.3 m from the subject in its netting cage. Both in two-alternative and four alternative choice experiments, in which an acoustic discrimination of separate similar sound sources was required, reward conditioning yielded positive results. The bearing of the sound source was decisive for the discrimination and not the identity of the sound sources used (intensity or timbre deviations). If the reverberation characteristics of the fjord at the receiving point were strongly dependent on the spatial position of the source, the results of the choice experiments might represent a demonstration of pseudo-directional hearing. However, from acoustic measurements of the time averages of the sound parameters such an anisotropy for sources with different bearing was not apparent (Fig. 5). Furthermore, in a well-trained cod, in which the pars inferior of only one single labyrinth was put out of function by surgery, the discrimination of the bearing of the sound sources was abolished but not the detection of sound as such (see Table 5). Detection of the direction of the sound therefore seems to be a function of the labyrinths and not of the lateral line system (directionalhearing). Estimates are given of the minimal angle that can be distinguished (about 22° for a 50% detectability likelihood; Fig. 6), and of the minimal acoustic level necessary for acoustic orientation (about — 13 dB re 1 bar; see Fig. 9). Finally an extension to an existing detection model of directional hearing is proposed. It receives empirical support in the next paper (Schuijf and Buwalda, 1975).Professor S. Dijkgraaf's readiness to develop the necessary operations and perform them in Norway in 1971 and 1972 is gratefully acknowledged. I thank Mr. G. Aase, Mr. K. Olsen and Dr. G. Sætersdal, Fiskeridirektoratets Havforskningsinstitutt (Bergen, Norway), for offering facilities to continue Kjell Olsen's study on directional responses of cod to sound. The contributions and the enormous efforts of my former student companions Drs M. A. van Arkel and Drs. M. E. Siemelink were decisive for success in the field. Guests from Utrecht supported the project in different directions, especially in constructing. I am grateful to Drs. R. J. A. Buwalda, Prof. S. Dijkgraaf, Drs. E. Meelis (statistics), Dr. F. J. Verheijen and Ir. D. W. van Wulfften Palthe (acoustics), all of whom made valuable comments on various aspects of the paper, and to Drs. J. W. Baretta for his aid enabling a quick start in 1971. The Netherlands Organization for the Advancement of Pure Research (Z.W.O.) for this study financed two journies to Norway and one to Scotland.