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,

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 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 close to Spitsbergen (Fig. 1). The spawning is concentrated in a band along the west side - 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 . The west side is bounded by Meteorological stations. — Thermograph section. Locations: the archipelago of 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 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. The temperatures are to note that the numbers can only be directly com­ recordings from commercial ship route crossings of pared within each area, because of the different water the fjord twice a week, which are representative for volumes involved. The table indicates a decline in coastal waters. This section shows thef the cold out­ spawning at Hølla and Henningsvaerstrømmen dur­ flowing water concentrates in a narrow band along the ing the period, and an increase in Austnesfjorden. west side. The lateral extension of the outflowing cold The time of peak spawning in 1975 seems to be late compared with the reports of earlier investigations STAMSUND BODO (Wiborg, 1957; Smedstad and Øiestad, 1974). In 1956 i 100 km the peak spawning at Hølla was found to be around 15 0 50 March (Wiborg, 1957). In the years 1968-72 the time of peak spawning varied between 26 March and 3 April (Smedstad and Øiestad, 1974). In Figure 3 the season is divided into seven day groups and shows the frequency distribution for the

CE < “ 6H UJ U-

fe

o 2 - z MEAN YEAR — FEB O' T" 1936 -1977 - - MARCH 7 U 21 28 11 18 25 APRIL MARCH APRIL

Figure 3. Frequency distribution during the years 1948-77 of Figure 4. Mean monthly sea temperatures across Vestfjorden at maximum number of eggs at Hølla. 4m depth. (The thermograph section is indicated in Fig. 1). Transport and Dispersal of Eggs of Cod 263 water is normally about 15 km, as shown in Figure 4. Table 2. Positive mean progressive wind vectors, W * ^ x W;> In some cases it can be significantly wider. Figure 5 shows a thermograph record from a ship route cross­ IW1 ing 16 February 1976. This shows intrusion of warm during a 3 day period. Neumann stability parameter, B = ------= 1W1 water on the west side, indicating an opposite circula­ tion and probably also upwelling along the west side. 100% during the 3 day period at the meteorological stations. Table 2 shows that the mean progressive wind vector Skrova, Grøtøy and Glåpen. from the three previous days was steady from south­ west and had a wind speed of 13 m/sec. This weather Wind Mean wind Neumann situation started on 9 February and persisted until 18 speed direction parameter February. (m/sec"') (°) (%) The distribution of eggs and larvae is closely linked 14, 15, 16 Feb 1976 13.1 39 90.7 to the outflowing current in Vestfjorden. Thus the 15, 16, 17 Mar 1977 3.8 272 thermograph section is also an indication of the distri­ 82.3 bution of eggs and larvae. Wind effects on the hydro- 22, 23, 24 Mar 1977 5.4 89 84.0 graphic structure seem to be fully developed after a 2-5 30, 31 Mar; 1 Apr 1977 9.2 358 76.8 day period depending on the force and persistence of the wind. Table 2 shows the positive mean progressive 25, 26, 27 Apr 1977 4.6 254 94.1 wind vector and the Neumann parameter, which is a measure of the stability of the wind vector. This 17 MARCH 1977 (FIG. 6) parameter has been suggested for use in processing The wind field was dominated by drainage of cold oceanographic current meter data (Ramster et al., continental air from the mountains which resulted in 1978). It is here found to give valuable information on an easterly light breeze in Vestfjorden. This may be the degree of upwelling and the distribution of eggs. described as the normal wind field. The lateral exten­ The values tabulated are means for the previous 3 days sion of the outflowing coastal water was about 15 km. of each hydrographic section. The effect of the wind will be described for the following four situations: 26 9 5 km — 270'

STAMSUND B0DÖ EGGS /nr>2 I I 0 50 100 km 271 150

100- 5

200

■273

33.8

3.5 U

4.0

100 5.0

S %o 3U 5- 6 0 - 15-16 FEB. 1976

3 200-

Figure 6. Hydrographic section Stamsund-SE (see Figure 1). Figure 5. Sea temperatures across Vestfjorden at 4m depth 15-16 Number of eggs 100-0m, temperature, salinity and at. 17-18 February 1976. (The thermograph section indicated in Fig. 1). March 1977. 264 B. Ellertsen, P. Solemdal, T. Strømme, S. Sundby, S. Tilseth, T. Westglrd, and V. Øiestad

The transition layer seems to be horizontally leveled. 1 APRIL 1977 (FIG. 8) The distribution of cod eggs shows that the spawning represented by 0-1 day old eggs was poor at this point A southerly breeze induced strong vertical mixing. of time, while 3-5 day old eggs had a bimodal distribu­ The Neumann parameter was relatively low due to the tion. However, all the eggs were found in the cold change in direction from southeast to southwest on 31 outflowing water. March. This may indicate that the hydrographic sec­ tion shows an unstable situation. This is also indicated 24 MARCH 1977 (FIG. 7, 10, 11 AND 12) from the water level measurements which show a rapid fall from 31 March to 1 April. Upwelling also oc­ A westerly breeze caused upwelling near the coast­ curred. The bimodal distribution of the cold coastal line (Fig. 10). The transition layer turned up at the water again reflects the distribution of the eggs. Eggs coast. The maximum concentration of 0-1 day old of age 0-1 days had the highest concentration close to eggs indicates that the spawners migrated close to the the coast, while 3-5 day old eggs were displaced off the coast following the upwelling transition layer (Fig. coast. 11). The form of the coastal water was transformed from its usual wedge shape into a lens which was 27 APRIL 1977 (FIG. 9, 13, 14 AND 15) displaced about 5 km from the coastline. The 3-5 day old eggs had been transported off the coast with the A northeasterly breeze of very high stability ruled cold water (Fig. 12). the wind conditions (Neumann parameter 94.1). This The horizontal distribution from this situation (Fig. favoured an elevation of sea level towards the coast. In 11) shows the main spawning close to land in the the surface layers the cold water had a seaward extent central part of Lofoten. However, there was also a of only 6 km, but a downwelling seemed to occur locally concentrated spawning in Austnesfjorden, with which gave lateral spreading of the cold water at a high concentrations of eggs. The high concentrations depth of about 30-40 m (Fig. 13). According to this the were probably caused by the inward blowing wind transition layer bends downwards. Spawning had which accumulated the eggs in the fjord. nearly ceased by this time. The majority of the eggs

— 27 1

271 EGGS I m2 EGGS/m 2 27152715

0 -1 DAYS -- 0-1 0AYS 3-4.5 DAYS — 3-4 5 DAYS

272

200-

273- 273-

200 10 0' 200

3.3 3 2

3 5 — 3 5

1 0 0 -

6.0 - 5 .0 -

2 c-o-

Figure 7. Hydrographic section Stamsund-SE. (see Figure 1). Figure 8. Hydrographic section Stamsund-SE (see Figure 1). Number of eggs 100-0m, temperature, salinity, and at. 24-25 Number of eggs lOO-Om, temperature, salinity, and atl. 1 April March 1977. 1977. Tranport and Dispersal of Eggs of Cod 265

------0 -270 / \ 0 5 knr 2705 2705 ^ /

EGGS /m 2 2 7 l'

------3-45 DAYS » 1170 1120-5 ...... 12-17 DAYS 100-

V— 27.2—

Crt \---- V -2 7 3 - TOO

200'

X a ......

\ \ v - - 15 - , - —33.9

> x Ä P \ / ^ ^ ' " 3 4 . 0 -- 13° U" Figure 11. 0-1 day old eggs, number/m2, (lOO-Om). 25 March 1977.

5-0" t °C s %o y us- 68 ° V,6 0 ' 30'

Figure 9. Hydrographic section Stamsund-SE (see Figure 1). HO, Number of eggs 100-0 m, temperature, salinity and ox. 27 April

1977. J 10

68 ° 30' 10 m/s loo

36

13° 14° 15o 3.6 Figure 12. 3-4, 5 day old eggs, number/m2 (100-0m) 25 March 1977.

trated near the coast due to convergence towards the land.

CONCLUSIONS In this paper we have consciously avoided using the 13° U* 15° word “patch” in describing the distribution of eggs, Figure 10. Wind vectors at the meteorological stations Grøtøy, Skrova and Glåpen (see Figure 2) from 22 March 0100 to 25 because we feel that the use of this term implies “unex­ March 0700, 1977. Temperature (°C) at 40m depth, 25 March. plainable distributions”. We believe that these distri­ butions may often be a matter of scaling the station grid and the sampling frequency to the dimensions and were 12-17 days old. There was no separation between the time scale of the component physical processes. eggs of different age (Fig. 14 and 15). All were concen- The four situations described here all have common 266 B. Ellertsen, P. Solemdal, T. Strømme, S. Sundby, S. Tilseth, T. Westgård, and V. Øiestad

200

68“ 100 500

13' « • 15° 13° U ° 1 5 0 Figure 13. Wind vectors of the meteorological stations Grøtøy, Figure 15. 12-17 day old eggs, number/m2, (lOO-Om), 27 April Skrova and Glåpen (see Figure 2) from 25 April 0100 to 28 April 1977. 0100, 1977. Temperature (°C) at 10m depth, 27 April 1977. water (24 March). The opposite effect occurs when the wind is northeasterly (27 April). The transition layer then bends down indicating downwelling and the eggs and the cold coastal water concentrates close to the coast.

ACKNOWLEDGEMENTS Thanks are extended to Mrs E. Strand for her careful work with the egg samples and to Mr Kolbein- son for drawing the figures. 68° ' 200/ REFERENCES

Eggvin, J. 1933. De oceanografiske forhold i Vestfjorden og deres sammenheng med Lofotfisket 1933. Årsberetning vedkommende Norges Fiskerier 1933 Nr. 2: 94-102. Eggvin, J. 1938. OceanographicalconditionsinNorth-Norwaycon- nected with the cod fisheries. Fiskeridir. Skr. (Havunders.) 5(4): 23-47. 13” 15° Eggvin, J. 1939. Oceanografisk beretning, Fiskeridir. Skr. Figure 14. 3-4, 5 day old eggs, number/m2, (lOO-Om), 27 April Havunders.) 5(1): 27-40. 1977. Gade, 1894. Temperaturmålinger i Lofoten 1891—; 1892. Carl C. Werner & Co., Christiania. large scale features. The cold coastal water is located in Gausdal, T. 1972. Hydrografiske forhold i Vestfjorden i februar- the shelf area on the west side of Vestfjorden. This mars årene 1951-1969. Cand. real thesis. Geophysical Institute supports the considerations about the general circula­ University of Oslo, 1972: 39 pp 69 figs. tion in the fjord. The eggs are concentrated in this Norges Vassdrags. og elektrisitetsvesen 1958. Hydrografiske under­ water mass. However, on a smaller scale all situations søkeiser i Norge: Aschehoug & Co., Oslo 236 pp. 65 figs. Ramster, J. W., Hughes, D. G., and Fumes, G. K. 1978. A'Steadi- are different. This is explained by the effect of the wind ness’ factor for estimating the variability of residual drift in field. During weak and variable wind conditions the current meter records. Dtsch. Hydrogr. Z., 31(6): 230-236. transition layer is horizontally levelled and the lateral Rollefsen, G. 1931. Temperaturmålinger i Vestjorden under fisket extension of eggs and the cold outflowing water is 1931. Årsberetning vedkommende Norges Fiskerier Nr. 2:90-92. about 15 km (17 March). Winds from southwest and Smedstad, O. M., and Øiestad, V. 1974. Preliminary report of west cause upwelling on the spawning banks, distrib­ investigations of cod-eggs and larvae in the Lofoten area in the uting the eggs off the coast with the cold outflowing years 1968-1972. ICES CM 1974/F: 14, 37 pp. mimeo. Transport and Dispersal of Eggs of Cod 267

Strømme, T. i 977. Torskelavens lengde ved klekking og virkning av (Gadus morhua) Flunder (Pleuronectes ßesus) und Scholle utsultning på larvens egenvekt og kondisjon. En eksperimentell (Pleuronecles plalessa) unter kombinierten Temperatur und undersøkelse på Norsk-Arktisk torsk (Gadus morhua). Thesis. Salzgehaltsbedingungen. Helgol. Wiss. Meeresunters, 21: University of Bergen (in Norwegian). 21- 102. Sundby, S. 1978. In/out flow of coastal water in Vestfjorden. ICES Wiborg, K. F. 1957. Factors influencing the size of the year classes in C.M. 1978/C:51, 17 pp. mimeo. the Arcto-Norwegian tribe of cod. Fiskeridir. Skr. (Havunders.) Westernhagen, H. von. 1970. Erbrutung der Eier von Dorsch 11(8).