Abstract: In the summers of 1981 and 2000, 288 goosanders Mergus merganser were sampled in the estuary of the River Tana in northern Norway, which has the largest wild Atlantic salmon Salmo salar stock in the world. Based on 2308 otoliths found in their stomach contents, sandeels Ammodytes sp. were found to be the dominant prey, and only one pair of Atlantic salmon otoliths was found. This suggested that goosanders were not significant predators on Atlantic salmon smolts in the estuary, probably because of the high abundance of sandeels and other marine prey fishes, at least in these particular years. (c) 2005 The Fisheries Society of the British Isles.
Abstract: Metamorphosis in cod, Gadus morhua , was investigated with respect to morphological changes in the alimentary tract (mouth, stomach and pyloric caeca) and external characters (finfold and fin development): Morphometric measurements and histological investigations were made on both larvae and juveniles and all fish lengths are given as standard length. Median unpaired fins started to develop when the fish were 9-10 mm in length. The larval median finfold was resorbed when fish reached a size of 11-15 mm. The vertebrae were formed in fish of 11-20 mm. The supraterminal mouth, seen in larvae < 12 mm, changed gradually to a sub-terminal position in juveniles larger than 65 mm. The development of the stomach and pyloric caeca started at 15 mm and the juvenile form was seen in fish of 40 mm. The ability to store and grind food particles in the stomach developed gradually as fish increased in size from 20 to 40 mm. Compared to many other teleost species, cod juveniles are relatively large when the stomach and pyloric caeca become fully developed. These findings may have important implications for both start-feeding of cod larvae and the weaning of juveniles on the artificial diets.
Abstract: Cod larvae were reared from the yolk sac stage in an enclosure system in northern Norway. After metamorphosis, the juveniles were offered a commercial diet in addition to the natural zooplankton available. From 3 wk after metamorphosis, there was a gradual increase of the commercial diet in gut contents, and by Day 55 most of the diet consisted of this food. Specific growth rate (SGR) was 19.5%/day during the first 3 wk after metamorphosis, but decreased to 6.5%/day during the subsequent 5 wk. Analysis of fish, zooplankton and the commercial diet indicated that the fatty acid composition of cod neutral lipids in particular, was significantly influenced by the polar lipid classes of the zooplankton during the first 3 wk after metamorphosis. Thereafter there was an increase in the fish triacylglycerols of 18:1 (n-9), 20:1 (n-9) and 22:1 (n-11) fatty acids which were typical for the neutral lipid classes of both the live food and the artificial diet. It is suggested that cod larvae and small juveniles may have low digestibility of neutral lipid classes due to a lack of suitable lipases, bile acids or both.
Abstract: Juveniles of several salmonid and freshwater species, have been marked in the vertebrae or otoliths by incorporating tetracycline into the food. The mark is detected by ultraviolet illumination of the skeletal structure. This marking technique has not been common for marking marine fish species, and an experiment was conducted to mark cod (Gadus morhua L.) juveniles. One experimental group (oxytetracycline (OTC) group) was fed a dose of 500 mg OTC kg-1 fish day-1 for a period of 10 days, while the control group received an OTC-free diet. The OTC was incorporated into the centra of the vertebrae during the month following the start of feeding giving an easily detected mark, but no marks were detected in otoliths. During the course of a 14 month on-growing period, there were no significant differences in growth or condition between the OTC group and the control group. The fish length at the time of marking could be predicted from the OTC mark diameter by a linear regression equation relating fish length at marking and OTC mark diameter. Release lengths could thus be estimated from measurements of the OTC mark diameter of recaptured OTC-marked fish if the juveniles were released about 1 month after the feeding of OTC.
Abstract: The effects of condition, sex and water temperature/season on the growth of large, sexually mature cod, Gadus morhua were studied throughout an annual cycle. Condition of the fish had a marked effect on growth, with fish of lowest condition factor showing highest rate of growth. This is interpreted as being an example of the phenomenon of recovery growth, in which previously fasted or poorly-fed animals show a marked growth spurt on return to good feeding conditions. Recovey growth was observed in the cod during the period immediately following capture from the wild and shortly after spawning. There were no differences in growth rates between sexes, except during early winter, and these differences in growth observed during December-January may have been related to sexual differences in reproductive growth and development. Water temperature had a marked effect on growth, with growth rates increasing with increasing temperatures. It is suggested that the optimum temperature for growth of large cod is sligthly lower than that for small cod.
Abstract: Feeding, growth, and survival of cod (Gadus morhua) larvae from hatching to metamorphosis were investigated in an enclosure system in northern Norway (70 degree ) in 1987. Yolk-sac larvae of cod were released into the basin in late April. Larval growth rate was low during first feeding in April, but increased in May and was comparable to the maximum growth rates obtained in enclosure systems in western Norway. The rates of deposition of lipid were related to the overall growth rates of he larvae. Survival was estimated to be 3% from larval release to metamorphosis. The results are discussed in relation to biotic and abiotic factors in the enclosure system.
Abstract: Large scale production of juvenile Atlantic cod (Gadus morhua ) has been carried out since 1980 in a saltwater pond. A break-through was obtained in 1983 with high survival rates of cod larvae to metamorphosis. In 1985 progress was made within two fields: reduced cannibalism and automatic harvesting. Juvenile cod formed large schools while fed dry pellets in the currents set up by five propellers. An underwater loudspeaker was programmed to give sound pulses just before feeding. During harvesting, dry pellets were released inside a fish trap while giving the sound signals the cod juveniles were conditioned to. The cod readily entered the trap and a computer-controlled fish pump transported the fish from the fish trap into a storing tank and grading grids. More than 80% or 100,000 juvenile cods were captured with the automatic harvesting system.
Abstract: The weekly amounts of roe and cod (Gadus morhua ) landed in the main spawning area at Vestfjorden are known since 1929. An index of the median of the spawning intensity curve was constructed on the basis of an analogy to the gonosomatic index using the weight ratio roe/cod. The index of median reveals a delay of about one week during the time period from 1929 to 1982. This method is compared to the median of spawning curves based on net hauls of newly spawned eggs from the traditional spawning grounds for the time period 1976 to 1982. Sources of errors in the fishery statistics are discussed.
