260 _______________________________________________________ Rapp. P.-v. Réun. Cons. int. Explor. Mer, 178: 260-267. 1981. SPAWNING PERIOD, TRANSPORT AND DISPERSAL OF EGGS FROM THE SPAWNING AREA OF ARCTO-NORWEGIAN COD (GADUS MORHUA L.) B. Ellertsen, P. Solem dal, T. Strøm me, S. Sundby, S. Tilseth, and T. W estgård Institute of Marine Research, Directorate of Fisheries, 5011 Bergen, Norway and V. 01ESTAD Department of Fisheries Biology, University of Bergen, 5011 Bergen, Norway Vestfjorden and adjacent areas include the most important spawning grounds of Arcto-Norwegian cod stock. The main part of the stock spawns in this area during February-April, after migration from feeding areas in the Barents Sea. Since 1975, spawning has been surveyed by sampling eggs with a Juday net in three locations, Henningsvaer- strømmen, Hølla and Ausnesfjorden. The selected stations appear to be representative of spawning in the inner parts of Vestfjorden where spawning occurs mainly in early April when fresh water runoff is low. At this time, however, meteorological factors such as wind and air temperature influence the vertical stability of the water masses and the horizontal circulation. When the current enters the fjord a homogeneous upper layer forms due to surface cooling that results in strong convection currents. In this situation, eggs and larvae become concentrated in a narrow zone in the cold outflowing water masses on the west side of the fjord. INTRODUCTION 6830’» The most important spawning grounds of the Arcto-Norwegian cod stock include Vestfjorden and adjacent areas where the stock spawns mainly during February, March and April after migration from the feeding areas in the Barents Sea and areas in the Norwegian Sea close to Spitsbergen (Fig. 1). The spawning is concentrated in a band along the west side SKROVA- of Vestfjorden and sometimes also along the west side of the archipelago Vaerøy-Røst. A hydrodynamic de­ 6 S °- scription of the entire region is necessary to under­ stand the transport and spreading of eggs and larvae ILÅPEN from the spawning grounds. IÖTÖY Vestfjorden is a triangular ocean bay about 100 km long and 50 km wide at the mouth (Fig. 1). The inner regions have depths of up to 700 m while the sill depth (VÆRÔY is 227 m. The sides of the fjord are steep except at the west side where the cod spawn on a shelf area. 13° H* The east side of Vestfjorden is connected to a Figure 1. Vestfjorden. • Juday net and hydrographical stations. number of large sill fjords. The west side is bounded by Meteorological stations. — Thermograph section. Locations: the archipelago of Lofoten and Vaerøy-Røst. Here the I Henningsvaerstrømmen fjords are small, and most of them without dominant II Hølla sills. III Austnesfjorden Transport and Dispersal of Eggs of Cod 261 The vertical distribution of hydrographic param­ Vestfjorden. eters in winter is described by Eggvin(1938,1939). The The 1977 spawning period was surveyed and eggs upper layer is a relatively homogeneous layer of cold were sampled every second day with two vertical coastal water of about 3°C. The thickness of the layer Juday net hauls, 30-0 and 100-30 m respectively, at varies considerably, but is usually between 30 and 90 each station. Eggs less than 2 days old were used to m. The warmer deep water of 7° C is of Atlantic origin, calculate the spawning curve. The age of eggs was and the transition layer between the coastal and Atlan­ determined from a key developed during laboratory tic water has strong temperature gradients. observations of eggs from fertilization until hatching Rollefsen (1931) reviewed research on the connec­ within the range of temperatures present in Lofoten, tions between sea temperature and abundance of 2° to 5°C (Strømme, 1977), a method comparable spawning cod. In 1892 Premier Lieutenant Gade of the with Westernhagen (1970). Royal Norwegian Navy discovered that cod were espe­ cially abundant at temperatures of 5°C (Gade, 1894). FRESHWATER RUNOFF AND WIND CONDITIONS This temperature usually occurs in the transition layer During the spawning season the total flux of fresh­ where the temperature gradient is very pronounced. water to the Vestfjord basin is about 250-300 m3 / sec. Both the thickness and the level of the transition layer Sundby (1978) computed values for the total heat have been shown to exhibit significant variations from budget for the inflowing coastal water during Febru­ year to year (Eggvin, 1933 and Gausdal, 1972), e.g. in ary, March and April and found a residence time of 1961 the transition layer was situated at a depth of about 10 days. This corresponds to a volume transport 220-240 m and in 1964 it was only 30-40 m. Thus the of coastal water of 105 m’/sec. Thus the freshwater distribution of cod varies both in the horizontal and discharge is about 0.25% of the flux of water out of the vertical direction. Some years the spawning is very fjord. Natural variation in monthly runoff may be poor on the shelf at the west side of the fjord because considerable. During winters with high air tempera­ the transition layer is much deeper than the shelf. The ture and precipitation, for example, runoff may be spawning then occurs mainly in the areas between 300% higher, while cold winters will give runoff values Vaerøy and Røst. as low as 20% of the mean value. The horizontal circulation in Vestfjorden is domi­ Wind conditions during winter (October to April) nated by the movement of the upper layer of coastal are dominated by two typical weather situations. The water. One branch of the Norwegian Coastal Current most frequent situation is drainage of cold continental turns into the fjord on the east side and comes out on air masses from the mountains with roughly 45% of the Lofoten side. This branch joins the main current the total wind energy related to this weather situation. out at Vaerøy-Røst where it heads north outside Lo­ The wind then flows out of the fjords and in the foten. Local cooling in Vestjorden makes this branch Vestfjord basin wind direction becomes easterly. Cy­ of the current about 0.5° to 1.0°C cooler Sundby, clone passages are the second most frequent situa­ 1978). tion. This results in southwesterly wind of 10-20 m/ sec. Wind speed increases toward the head due to MATERIALS AND METHODS constriction by the fjord. Duration may vary between The station grid in Vestfjorden is based on six sec­ 2 and 5 days. tions from the Lofoten archipelago towards the south­ SPAWNING PERIOD east, covering the spawning banks. In addition nine Juday net stations from the spawning grounds in Cumulative spawning curves are shown for the Austnesfjorden, Hølla and Henningsvaerstrømmen years 1975-77 in Figure 2. The curve is based on the were worked to cover the spawning period. The dis­ sum of eggs younger than 2 days at the nine stations. tance between stations near the coast was 1 km, During the observed three year period the spawning in increasing to 3.5 km at a distance of 10 km off the 1975 occurred latest. This year 50% of the eggs were coast. The station grid is shown in Figure 1. spawned about 12 April. In 1976 and 1977, the spawn­ Thermograph recordings from ship route crossings ing seemed to occur in practically the same period, between Stamsund and Bodø were used to examine with 50% spawning around 2 April. temperatures anomalies in the fjord. In addition, wind In Table 1 the spawning curves for each of the three data from the meteorological stations around the fjord areas have been integrated to determine if there were were used to examine the effects on the hydrography. any fluctuations in distribution of the spawning Wind was recorded four times a day. Data from between the areas. The numbers in the table are an selected water gauges and maps of the specific runoff index of the total number of eggs spawned in the area (Norges Vassdrags og Elektrisitetsvesen, 1958) have during the season, found by measuring the area under also been used to compute freshwater runoff around the spawning curve with a planimeter. It is important 262 B. Ellertsen, P. Solemdal, T. Strømme, S. Sundby, S. Tilseth, T. Westglrd, and V. Øiestad peak number of eggs at Hølla for the years available in 100- the period 1948-77. It indicates that the mean date for I- peak spawning is around 1 April. 2 UJ Historically, the main spawning area is located in O the inner part of Vestfjorden where the stations for 50- _-1975 ce monitoring spawning have been situated. Minor UJ 1976 Q. ....1977 aggregations of spawning fish also occur along the entire coast of Lofoten. The spawning in 1977 deviated from this pattern as a large part of the stock, mostly 10 20 30 10 20 30 first and second time spawners, began spawning at the MARCH I APRIL MAY mouth of Vestfjorden on the banks of Vaerøy and Figure 2. Cumulative spawning curves 1975-77. Røst, and at both sides of the central Lofoten Islands. The spawning curve in 1977 is thus only representative Table 1. Index of the amount of spawning at Austnesfjorden, for the inner part of Lofoten. Holla and Henningsvaerstrømmen 1975-77. The indices are only comparable within each area. HORIZONTAL DISTRIBUTION OF EGGS AND THE PHYSI­ CAL PARAMETERS Austnes­ Henningsvaer­ Sundby (1978) described the structure of the coastal fjorden Hølla strømmen Year water and the effect of meteorological factors on cool­ 1975 11.5 ing. Figure 4 shows the mean monthly temperatures at 1976 9.5 8.5 5.3 4 m depth in February, March and April for the years 1977 14.7 4.5 2.3 1936-77 in a section between Stamsund on the west side and Bodø on the east side.