Torstein Pedersen

Growth rates of large, sexually mature cod Gadus morhua, in relation to condition and temperature during an annual cycle

Abstract The effects of condition, sex and water temperature/season on the growth of large, sexuallymature 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 rates 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. Recovery 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 temperature. It is suggested that the optimum temperature for growth of large cod is slightly lower than that for small cod.

The influence of dietary lipid classes on the fatty acid composition of small cod Gadus morhua L. juveniles reared in an enclosure in northern Norway

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 55most of the diet consisted of this food. Specific growth rate (SGR) was 19.5% · day−1 during the first 3 wk after metamorphosis, but decreased to 6.5% · day−1 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. During larval and early juvenile periods cod may have an absolute need for polar lipids both for the supply of energy and for essential fatty acids.

Effects of predation from juvenile herring (Clupea harengus) on mortality rates of capelin (Mallotus villosus) larvae

Predation has been suggested as a cause of substantial mortality of fish larvae to the degree that it might influence recruitment. This field-based study concludes that juvenile herring (Clupea harengus) as small pelagic predator can significantly affect mortality rates of the planktonic larvae of capelin (Mallotus villosus) in the Barents Sea. Surveys were carried out in the summers of 2001 and 2003. In 2001, juvenile herring were widely distributed and overlapped with capelin larvae over a wide area, whereas in 2003, the herring were more aggregated. The study focused on predation in the areas of predator–prey overlap. Capelin larvae were observed in the herring stomachs at 11 of 24 stations and at 8 of 16 stations where herring were caught in 2001 and 2003, respectively. At those stations, an estimated 7.3% and 9.9% of the capelin larvae were eaten by herring per day in 2001 and 2003, respectively. Statistical models revealed that density of capelin larvae and copepods and occurrence of euphausiids in ...

Optimal foraging in chick-raising Common Guillemots (Uria aalge)

The Norwegian population of the Common Guillemot Uria aalge has declined by >95% since the 1960s, and is classified as critically endangered in the Norwegian Red List. Much of the recent decline has been attributed to reduced food availability, but without extensive documentation of adult diet. Instead, chick diet has been considered a proxy of adult diet during the breeding season in many Norwegian studies. However, central-place foraging theory, especially for single-prey loaders, predicts that this may not be so, and this study directly compares the diet of adult and chick Common Guillemots during the breeding season at a colony in NE Norway. Whereas chicks were fed mainly capelin (Mallotus villosus) and sandeels (Ammodytes sp.), most of the adult diet consisted of the two youngest year classes of Gadidae, probably cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). A successful ecosystem management of seabirds is dependent on a full understanding of how prey quality, abundance and availability influence seabird populations and their viability, such that knowing the true diet of adult Common Guillemots has important implications in the modelling and management of the Norwegian populations.

Effect of parent type and temperature on vertebrae number in juvenile cod, Gadus morhua (L.), in Northern Norway

Abstract Cod juveniles settling in shallow water and recruiting to coastal cod (CC) stocks have lower vertebrae number than juveniles settling in deep water and recruiting to Northeast Arctic cod (NAC) stock. We investigated whether the differences in vertebrae number in CC and NAC were caused by genetic differences or by exposure to different temperatures during the embryonic period when the vertebrae number is determined. Egg groups from NAC and CC parents (parent groups), and egg groups from CC broodstocks (broodstock groups) were maintained at constant temperature during the egg stage and reared to the juvenile stage before their vertebrae were counted. Data on vertebrae number from year classes of CC and NAC were compared with the experimental results. The vertebrae number of the male parents had a greater influence on the vertebrae number of the offspring than the vertebrae number of the female parents, indicating that the parental effect was a genetic sex linked effect. NAC parents produced offspri...

Marking cod (Gadus morhua L.) juveniles with oxytetracycline incorporated into the feed

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.

Migration, growth and mortality of released reared and wild cod (Gadus morhua L.) in Malangen, northern Norway

In order to evaluate the potential of cod enhancement in a north Norwegian fjord, 4842 hatchery-reared cod juveniles (14-23 cm in length) were tagged and released into Stålvikbotn in 1988, a 10 km2 area sill fjord in Northern Norway. Wild cod were tagged to serve as a reference group for the hatchery-reared group. At the time of release, the size of the reared juvenile cod was about the same as that of the 1-year-older wild cod. The released fish showed a similar migration, growth and mortality pattern as wild fish of similar size. Both wild and released cod were mainly recaptured in Stålvikbotn. Large cod, short-horn sculpin and harp seal predated on the released reared juveniles. The recaptured reared cod were longer than average at release, indicating size-dependent predation mortality. The natural mortality rate (M) was estimated to be about 0.5 year-1 for cod longer than 30 cm and about 1.1 year-1 for both small wild and reared cod (14-30 cm). The fishing mortality rates were low, about 0.2 year-1 for cod longer than 30 cm. Only 2.4% of the reared cod were recaptured, and the yield was about 100 g per released cod, indicating low feasibility of release of small juveniles.

Prey partitioning between cod (Gadus morhua) and minke whale (Balaenoptera acutorostrata) in the Barents Sea

Abstract Stomach samples of minke whale (Balaenoptera acutorostrata) and cod (Gadus morhua) were collected on the northeastern parts of the Finnmark coast, northern Norway, in 1998 and 1999 to investigate prey preference, niche overlap and niche width for the two species. A survey to estimate prey abundance was also conducted. Prey preference, niche overlap and niche width were described using Manlys alpha (α), Piankas index (Ojk) and Levins index (B), respectively. The diet of cod consisted mainly of capelin, deep-water shrimp, gadoids and krill. The smallest cod (≤64 cm length) preferred capelin, whereas the largest cod (>64 cm length) preferred gadoid species, including cod. The minke whale diet consisted mainly of capelin (Mallotus villosus), herring (Clupea harengus) and krill, and showed a particular preference for herring and capelin. Krill were consumed in large quantities, but did not seem to be a preferred food item for either cod or minke whales. In contrast to cod, which fed very little on herring, the minke whale fed heavily on herring in some areas. The niche widths for both cod and minke whales were relatively low, and the diet overlap between minke whale and cod was low.

Spatial Scales of Movement in Northeast Atlantic Coastal Cod

The movement patterns of released hatchery-reared fish determine the geographical scale at which a population may be enhanced, while the movement patterns of wild fish affect the management strategies of wild populations. This article investigates (1) if movement patterns differ between hatchery reared and wild cod, and (2) if the movement patterns of coastal cod differ between regions. The results from a large mark-recapture experiment in northern Norway showed that displacement distances (DD) for both wild and hatchery-reared fish were highly skewed. The frequency distribution of DD for wild cod was well described by a log-normal distribution. While reared cod that were released at a small size (< 27 cm in length) and young age (< 1 year of age) had similar movement patterns to wild cod of similar size, reared cod that were larger and older at release dispersed rapidly after release. The frequency of long-distance migrations (DD > 50 km) for larger reared cod was similar, however, to that of wild cod. Fitting log-normal distributions to published mark-recapture data revealed that there was a latitudinal trend, with longer displacement distances in the north than in the south. Cod released in coastal bank areas and offshore island groups also had longer displacement distances than cod released at the Norwegian coast.