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Norwegian coastal cod form a stationary population of Atlantic cod Gadus morhua consisting of several genetically separated subpopulations. A small-scale differentiation in marine populations with pelagic eggs and larvae is made possible by local retention of early life stages in coastal environments. A numerical model was used to simulate the circulation in a fjord system in northern Norway over 2 years with different river runoff patterns. The dispersal of cod eggs was calculated with a particle-tracking model that used three-dimensional currents. The observed thickness of the low-salinity surface layer was well reproduced by the model, but the surface salinity was generally lower in the model than in the observations. The cod eggs attained a subsurface vertical distribution, avoiding the surface and causing retention. Interannual variations in river runoff can cause small changes in the vertical distribution of cod eggs and larger changes in the vertical current structure. Retention in the fjord system was strong in both years, but some eggs were subjected to offshore transport over a limited time period. The timing of offshore transport depended on the precipitation and temperatures in adjacent drainage areas. A possible match between maximized spawning and offshore transport may have a negative effect on local recruitment. publishedVersion

Contributors: Merete Fonn, Svein A. Iversen and Anders Thorsen

Contributor: Svein A. Iversen

Contributor: Richard Nash

Contributors: Merete Fonn, Svein A. Iversen and Anders Thorsen

Contributors: Merete Fonn and Svein A. Iversen

Contributors: Jan de Lange, Eilert Hermansen, Merete Fonn and Anders Thorsen

Contributors: Svein A. Sivertsen, Maria Krüger-Johnsen

Contributors: Petter Fossum

Contributors: Svein Iversen, Olav Kjesbu, Maria Kruger-Johnsen

Contributors: Jan de Lange, Eilert Hermansen

Contributor: Petter Fossum

Contributor: Svein A. Iversen

Embryological studies on living cod eggs in NUNC-trays showed that spontaneous malformations during embryo development are lethal. They are called natural egg mortality in contrast to external lethal factors, as predation, extreme physical conditions and pollution. The natural mortality in cod eggs, both coastal and Arcto-Norwegian, is significantly reduced from first to second spawning, from laboratory experiments. The same embryological method was used during field studies on natural egg mortality on planktonic cod eggs in the spawning area in Lofoten, Northern Norway, in the March - April years 2000 and 2001. The types malformations which was found in a fild also corresponding to certain morphogenetic movements in embryodevelopment and equel discover in the experimental work. Most part of lehal malformations (more than 50 %) is identified after hatching only. The pattern of types malformations differ between spawning grounds and to be consistant during two years. The frequencies of malformations to the end spawning period significantly reduced of 29,7 to 9,8 % for Vestfjord. To demonstrate the higher natural egg mortality in eggs from first time spawners, two strategies were followed: 1. Comparing natural egg mortality in egg samples from 2000 and 2001, collected during same period and from the same areas. 2.On the basis of the acoustic survey of the spawning population of Arcto-Norwegian cod in Lofoten, areas of high frequency of first time spawners were compared. Luckily, in the year 2001 the proportion of first time 2 spawners was specially high. A comparison between the years 2000 and 2001 demonstrated an increase in natural mortality from 23 to 31 %. In Vesterålen, the area of highest frequency of first time spawners, natural egg mortality increased from 31 to 46 %. Because of two systematic errors the numbers are minimum values.

Rapporten viser funn av fiskeegg, fiskelarver og dyreplankton på Draupnerfeltet (58°11'N, 02°28'Ø) i Nordsjøen, fra mars 1997 til juli 1998. Vannsøylen ble undersøkt med håv to ganger i uken. Egg og larver av fisk var til stede i vesentlig antall fra februar til oktober. Sandflyndre var den vanligste arten, med gytesesong fra midten av februar til slutten av august. Etter sandflyndre var øyepål, gapeflyndre, makrell og skrubbe de mest tallrike artene. Calanius finmarchicus var den viktigste kopepoden hva biomassen angår, og nærmere 300 000 individer /m² ble registrert i mai 98. Den hadde minst tre generasjoner i året :: This report is listing findings of fish eggs, fish larvae and zooplankton from Draupner in the North Sea (58°11' N, 02°28'E), from March 1997 to July 1998. Samples were collected with a WP2- net two times a week. Dab was the most numerous species, and eggs were abundant from February to August. Norway pout, long rough dab, mackerel and flounder were also common species. Calanus finmarchicus (copepoda) was numerous in the spring, reaching 300 000 individuals /m² in May 98. lt is assumed that C. finmarchicus produces 3-4 generations per year in the area.

Contributor: Svein A. Iversen

Contributor: Svein A. Iversen

Contributors: S. Bolla, Ingvar Huse, Lis Lindal Jørgensen, Elin Kjørsvik, Terje Kleppe, Anders Mangor-Jensen, T. Meeren, Kjell Emil Naas, Arnfinn Skadsheim, Y.G. Attramadal, Didrik S. Danielssen, R. Farestveit, K. Gravningen, J. Gulbrandsen, K. Hamre, I. Holmefjord, I. Lein, Erlend Moksness, J.R. Rainuzzo

1. The mean reduction of egg diameter from March to April is from 1.45 to 1.39 mm and 1.44 to 1.38 mm for artificially and naturally spawned eggs of Arcto-Norwegian cod, respectively. This means a reduction of volume of about 11%. 2. The neutral buoyancy of cod eggs ranges from 29.5-33.0°/oo salinity. Eggs of captive fish show a tendency of higher specific gravity. 3. The neutral buoyancy of the cod eggs is not correlated to egg size, but to the weight of the eggshell. 4. Computed ascending velocities in the natural environment of Lofoten range from 0.2 mm s⁻¹ to 1.7 mm s¹ , and show an approximately Gaussian distribution with a mean/ascending speed of 1 mm s¹ 5. Since the spawning of Arcto-Norwegian cod is strictly connected to the thermocline at temperatures of 4-6°C, spawning can occur both pelagically and close to the bottom. 6. Norway pout and other gadoids are also spawning in Lofoten, and occasionally their eggs are found in majority. Their vertical distribution have maxima in deeper water, indicating a higher specific gravity than for cod eggs. 7. During calm wind conditions the concentration of cod eggs increases rapidly towards the sea surface. Large variation in the upper centimeters of the sea is found during such conditionsm Increasing wind rapidly mixes down the eggs in the upper meters. As the wind speed maintains, larger parts of the water column are influenced by the wind mixing. At south-westerly wind of 10 ms⁻1 and at 24 hours duration more than 50% of the eggs are found below 30 m depth.

In 1960 cod eggs and larvae were more abundant at Skrova in the Lofoten area than in the previous year (Table 1). Maxima of spawning and hatching occurred 14 days earlier than in 1959. On the western side of the Lofoten islands, at Eggum, cod eggs and larvae were scarce in 1960. On 31. March and 1. April cod eggs and larvae were numerous in the inner part of the Lofoten area (Fig. 3). The Nansen net and the Clarke-Bumpus plankton sampler have proved to be equally effective in catching eggs and newly hatched larvae of the cod (Table 2). In March-April 196O eggs of cod were found in smaller numbers over larger areas of the west coast with moderate concentrations at the Møre and Halten banks (Fig. 3). Haddock eggs were scarce along the slope from Træna to Andenes. In April eggs of saithe and Norway pout occurred in moderate numbers in the Vestfjord, but major concentrations were only met with farther south, from the Halten bank to Stad (Fig. 4). In March-April 1960 herring larvae were found in the coastal area from south of Bergen to Andenes, with major concentrations on the Møre-, Frøya-, Sklinna- and Halten-banks, in the order mentioned, mean numbers per m^2 of sea surface 630-230 (Fig.s 5, 6). The statement by various workers that herring larvae are caught to a greater extent by night than by day in traditional nets, was confirmed. A comparison between the catches in hauls with Nansen net and Clarke-Bumpus plankton sampler (Table 3) showed that during the night the two gears caught nearly the same number of herring larvae, 8-13 mm in length, per m^2 of sea surface (Table 3). When all hauls were included, the Nansen net took more than ten times as many, Clarke-Bumpus sampler nearly twice as many herring larvae during the night, as during the day. If three night hauls with high numbers of larvae were excluded from the mean, the night to day ratio for the Nansen net was 4.5/1, for the Clarke-Bumpus sampler 1.3/1. It is therefore assumed that the Clarke-Bumpus sampler is more effective than the Nansen net in catching herring larvae of the size mentioned, and may be used with a certain degree of reliability also during the day. In the middle of May larvae of capelin were abundant near Northcape (Table 4). Redfish larvae were numerous along the coast in May-June (Fig. 7).

Hovedoppgave i fiskeribiologi, avlagt ved Norges fiskerihøgskole, Universitetet i Bergen