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  • 1
    Electronic Resource
    Electronic Resource
    Variability in condition and growth of Atlantic cod larvae and juveniles reared in mesocosms: environmental and maternal effects (2003)
    Oxford, UK : Blackwell Science Ltd
    Journal of fish biology 62 (2003), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Standard length, dry mass and RNA : DNA ratio measurements of 3876 Atlantic cod Gadus morhua larvae and juveniles from 26 families of recruit (fish during their first year of spawning) and repeat spawners (fish which were in their subsequent spawning season) reared in two mesocosms (2500 and 4400 m3) under semi-natural conditions were analysed over a period of 10 weeks using microsatellites. Larvae from recruit spawners were significantly longer and heavier at hatch and throughout the 10 weeks. RNA : DNA ratios from recruit spawner offspring were only significantly higher at week 1. The smaller (2500 m3) mesocosm was characterized by low plankton density during the transition from endogenous to exogenous feeding followed by a higher density during the metamorphosis period (weeks 4 and 5), with the reverse pattern evident in the 4400 m3 mesocosm. Patterns of larval growth followed patterns of zooplankton density. Significant differences in RNA : DNA ratios between the mesocosms at all comparable sampling dates were found and within each mesocosm individual fish exhibited a wide range of growth and condition responses under the same environmental conditions. RNA : DNA ratios as a function of size differed in the amount of variability between mesocosms, indicating that the higher food density led to a higher proportion of well-conditioned larvae in the first 3 weeks. Food availability probably has a major role in determining offspring growth and condition, with limited effects due to maternal effects in cases where the broodstock females are approximately of similar size and condition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1095-8649.2003.00060.x
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  • 2
    Electronic Resource
    Electronic Resource
    Effects of fish size on spawning time in Norwegian spring-spawning herring (2000)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 56 (2000), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: During October-January in a northern Norwegian fjord system and in February-March at the main western Norway spawning grounds, an index IG of gonad weight of spring-spawning herring Clupea harengus in percentage of expected gonad weight at full maturity for a given total length LT, tended to increase with LT in the range 27–31 cm as the proportion of recruit spawners decreased. Insignificant differences in IG were found between LT groups in the range 32–37 cm (repeat spawners). This is contrary to that suggested in other studies and signifies that generally the stock spawns in only two waves, repeat spawners first and the recruit spawners second. IG also increased with somatic and total condition factor, which signifies that spawning time may be influenced by summer feeding conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.2000.tb02107.x
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  • 3
    Electronic Resource
    Electronic Resource
    Differences in growth performance between Arcto-Norwegian and Norwegian coastal cod reared under identical conditions (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 49 (1996), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Offspring from seven family groups of Arcto-Norwegian cod (AN) and a genetically marked Norwegian coastal cod (NC) broodstock, were mixed at metamorphosis and raised in the same rearing unit. The fish were transferred subsequently to a net-pen and held under standard farming conditions. In December 1992, 466 cod juveniles were measured, weighed, and tagged individually. Length and weight changes were monitored until the fish matured (January 1994). Genotyping of each individual was performed using enzyme electrophoresis to identify the fish to strain. Prespawning females were examined for organ weights and stage of maturity. There were population specific Differences in growth performance. NC displayed significantly higher specific growth rate (SGR) and daily length increment (DLI) during spring/summer season. The AN had significantly lower hepatosomatic and gonadosomatic indices, and were thinner than the NC, indicating Differences in body form and energy allocation pattern between the two strains. All NC (both sexes) became sexually mature at the age of 2 years while 2% of fish in the AN group were still immature at the end of the experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1996.tb00008.x
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  • 4
    Electronic Resource
    Electronic Resource
    Buoyancy variations in eggs of Atlantic cod (Gadus morhua L.) in relation to chorion thickness and egg size: theory and observations (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 41 (1992), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Egg specific gravity is a function of egg volume, chorion volume, perivitelline space (PVS) and the specific gravity of chorion and ovoplasm. Experimental studies on cod (Gadus morhua L.), a marine multiple batch spawner, showed that approx. forty per cent of the chorion volume is incorporated between batches and that the chorionic material has a mean specific gravity of 1·20 (range: 1·14–1·35). The chorion volume and specific gravity are low at the end of the spawning period in extremely fecund fish. Specific gravity of ovoplasm was observed to be 1·017 in marine eggs and calculated to be about 1·008 in brackish water eggs. Inherent variation in PVS is found to have little influence on egg specific gravity. With the other variables constant, egg specific gravity is inversely related to egg volume supported by studies on single batches of eggs. Measurements on every batch of eggs reveal that the production of large eggs (〉1·3 mm) gives low density eggs initially and high density eggs towards the end of spawning, while the production of small eggs (〈1·3mm) gives moderate specific gravity. These observations are discussed in relation to vertical distribution of the eggs and reproductive success of recruit and repeat spawners. There are indications that repeat spawners produce larger eggs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1992.tb02685.x
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  • 5
    Electronic Resource
    Electronic Resource
    Oocyte size and structure in relation to blood plasma steroid hormones in individually monitored, spawning Atlantic cod (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 49 (1996), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: A series of seasonal blood and ovarian tissue samples were taken from five female Atlantic cod Gadus morhua at different times between various batches of eggs and subsequently examined for oocyte size, plasma steroid hormones and ultrastructure of the cortex oocyte cytoplasm and follicle layer. This detailed protocol demonstrated that the spawning cod ovary is vitellogenically very active. Individual oocytes were able to develop at different rates, and at least two cohorts of oocytes with a different developmental status were present simultaneously. The concentration of estradiol-17β varied between batches and decreased over the season. The levels of 17,20β-P and 11-deoxycortisol were generally low. Oocytes of an advanced developing cohort showed numerous coated vesicles and closely apposed oocytic and follicular microvilli located in the chorionic pores. The number of oocytic microvilli was estimated at about 1–8 million cell−1, which increased the oocyte surface area by a factor of 12. Furthermore, the oocytic microvilli were found to stretch during final maturation. This stage-specific mobility of microvilli is discussed in relation to further incorporation of yolk and the relevant actin-based cytoskeleton observed for other classes of animals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1996.tb01789.x
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  • 6
    Electronic Resource
    Electronic Resource
    Time of start of spawning in Atlantic cod (Gadus morhua) females in relation to vitellogenic oocyte diameter, temperature, fish length and condition (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 45 (1994), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Spawning time of female Atlantic cod (Gadus morhua L.) can be predicted to within 3 days using vitellogenic oocyte diameters measured over the last month before spawning. Simulations show that a l°C drop in temperature during vitellogenesis delays spawning by about 8–10 days. Similar results were obtained with fish on low ration for 4–8 months before spawning. Moderately fed females held over two to four consecutive spawning seasons demonstrated a significant delay (⋍3–40 days) in the date of first spawning as the fish grew older and more fecund. Forecasting studies on spawning migrating Arcto-Norwegian cod gave no evidence of a significant effect of fish length on the time of spawning. Warmer water temperature during vitellogenesis for the larger fish appears to compensate for the negative effect of fecundity On spawning time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1994.tb00939.x
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  • 7
    Electronic Resource
    Electronic Resource
    Physiological mechanisms of buoyancy in eggs from brackish water cod (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 48 (1996), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Newly fertilized eggs of brackish water (Gotland, Baltic Sea) and marine (Lofoten, Norway) cod were investigated with regard to specific gravity, wet and dry weight, water content, chorion weight, and content of protein, free amino acids (FAA), and ions. The eggs had neutral buoyancies equivalent to a salinity of 14.3% (range 11.5–16.2%) in brackish water, and 33.0% (range 31.8–34.5%) in the marine environment. A buoyancy model was developed and showed that this difference was mainly caused by differences in egg water content which was 96.6 ± 0.47% and 92.7 ± 0.45% in the brackish and marine eggs, respectively. The higher water content of the brackish eggs resulted from increased water uptake during final oocyte maturation due to higher intracellular contents of FAA, Cl – and NH4+. SDS polyacrylamide gel electrophoresis of eggs and oocytes, and measurements of egg protein content suggested that the FAA pool of both egg types originated from hydrolysis of specific yolk proteins. The main contributor seemed to be a protein with a molecular weight of 100 kDa.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1996.tb01440.x
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  • 8
    Electronic Resource
    Electronic Resource
    Oogenesis in cod, Gadus morhua L., studied by light and electron microscopy (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 34 (1989), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The development of the oocytes in the cod, Gadus morhua L., is described by light and transmission electron microscopy. The oocyte volume increases about 700 times during the stages preceding hydration. The size of the nucleoli increases 35-fold, the cortical alveoli increase 6-fold and the yolk granules increase 70-fold as the oocyte grows. The maximal number of yolk granules is nearly 100000 per oocyte; this number is reduced to less than the half prior to hydration. A significant reduction occurs also in the number of nucleoli at that time. The number of cortical alveoli increases steadily towards hydration. Yolk is deposited in the oocyte as crystalline granules. The lattice is broken down at hydration, leaving the egg transparent. Follicle cells go through a primordial stage and later change to a squamous and to a cuboids shape. The presence of lipid droplets in their interior and the virtual lack of interfollicular spaces are characteristic. The chorion grows to a tripartite structure: an outer thin porous layer, an intermediate homogenous layer and an inner thick helicoidally layer. A mucous substance covers the porous layer. The occurrence of the so-called lamellae in the helicoidal layer is considered a function of the orientation of its micro fibrils and the plane of sectioning, i.e., not caused by alternating chemical arrangements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1989.tb03353.x
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  • 9
    Electronic Resource
    Electronic Resource
    The spawning activity of cod, Gadus morhua L. (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 34 (1989), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1989.tb03302.x
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