Investigations on diel vertical migration of 0-group fish in the Barents Sea

BELTESTAD, A. I<. , NAKKEN, 0. and SMEDSTAD, 0. M. 1975. Investigations on diel vertical migration of 0-group fish in the Barents Sea. FiskDir.Skr. Ser.HauUnders. , 16: 000-000. In August-September diel vertical migration of 0-group fish was studied in the Barents Sea. Redfish and capelin had different vertical migration patterns. 0-group redfish were found at the surface at dark and spread in the upper 50 m cluring the daylight period while the unpigmented C-group capelin stayed in small schools close to the surface by daylight and dispersed into a weak scattering layer at the depth of the thermocline (20-30 m) at dark. This behaviour might introduce large errors to the abundance indices of 0-group capelin. Comparative fishing with a pelagic trawl and a purse seine were carried out. The samples showed no significant differences in the length distributions of each species of 0-group fish caught with the two gears. I N T R O D U C T I O N During late summer and early autumn fish fry of many species occur pelagically in the Barents Sea. Since 1965 joint international surveys have been carried out annually in this area during 12-15 days in late August and early September. The primary aim of these surveys has been to map the geographical distribution and to obtain estimates of yearclass strength of the different species of 0-group fish. Preliminary results have each year been reported to the International Council for the Exploration of the Sea (ASON. 1965-1974 a). The distribution and density of the pelagic layers of 0-group fish have been observed with echo sounders, echo integrators and small meshed pelagic trawls. Details of the technique applied are given in DRAGESUND, MIDTTUN and OLSEN (1970), and a thorough description and discussion of the method was made by ANON. (1974b). The main results from these surveys are indices of yearclass strength (HAUG and NAKKEN 1973). These indices might be influenced both by the sampling efficiency of the pelagic trawls and by the behaviour of the fish. The vertical migration of the different species may thus effect observations both with echo sounders and pelagic trawls. Die1 vertical migration js a well known feature of the behaviour of most pelagic fishes. WOODHEAD (1966) and BLAXTER (1970) have reviewed the literature on the subject. Normally, pelagic fishes are found at intermediate depths during the day, they move towards the surface at dusk and descend at dawn. This general pattern will probably vary with fish size, species and environmental factors. Although a few investigations have been carried out on redfish larvae (KELLY and BAKER 1961 a, KELLY and BAKER 1961 b, MAGNUSSON, MAGNUSSON and HALLGRIMSSON 1965), the diel vertical migration pattern of the 0-group fish in the Barents Sea was considered to be insufficiently known. During the survey in 1974 experimental work was therefore conducted to study this subject, and the results from the experiments are reported here. MATERIAL A N D M E T H O D S The investigations were carried out with the 600 tons purse seiner crHavdr0n~ which is also fitted for pelagic trawling and equipped with Scientific sounder, EK 38, two echo integrators, QM, and a net sonde. Observations were collected in three different localities (Fig, 1) . The periods of observations were as follows: Area 1 : 27-29 August Area 2: 13 September Area 3 : 45 September Fig. 1. Localities where diel vertical migration pattern of 0-group fish were st~~clied in A~lgust-September 1974. 1) Area 1, 2) area 2, 3) area 3. Isolines i~ldicate the echo integrator deflectiolls observed during the interllatiolial 0-group survey. The selection of the areas were made on basis of information from the other vessels which participated in the survey. A standardized sampling procedure was followed in all three areas. An anchored bouy was used as a fixed position. Another bouy, attached to a free floating drogue in 25 m depth, was used as the point of reference for a small scale survey (Fig. 2). The grid of the small scale survey was continuously adjusted to keep the drogue at point A (Fig. 2). The arrows in Fig. 2 indicate c(Havdr~n>)s courses, and during one small scale survey the ship sailed 32 nautical miles. Between each completed survey one trawlhaul, one hydrographic station, one plankton haul and one series of light measurements were taken (Fig. 2). In each of the areas 1 and 2 ten small scale surveys were carried out while five completed surveys were found to give sufficient inforn~ation in area 3. When the xveather conditions permitted, the 0-group fish scattering layer was observed by scuba divers. The divers observed the water column from 0 to 30 meters once by daylight and once at dark in area 2, and once at dark in area 3. The four echo integrator channels were continuously monitoring the following depth intervals: 6-15 m, 15-30 m, 30-60 m, 60-200 m. Fig. 2. The small scale survey conclr~ctcd in the three areas. A) Start and end. I ) Hydrography and plankton station, 2) pelagic trawl and purse scine station, 3) drogue. Their outputs were read for each nautical mile and then averaged for each half completed small scale survey. The values were converted to R.V. c<G. 0. Sarsn values in order to be directly comparable to the figures in Fig. 1 (BUZETA et al. 1974). The fishing gears, which were used, were of commercial sizes with minor modifications for sampling of 0-group fish. The purse seine was a capelin net 470 meter long and 112 meter deep with a mesh size of 20 mm. I n the very end of the bunt a piece of webbing, 28 meter long with mesh size 5 mm, was inserted to prevent the smallest fishes to escape. The pelagic trawl was a capelin trawl, 1000 meshes in circumference with a mesh size of 400 mm, giving an approximate vertical opening of 10-14 meters at 3.5-2.5 knots towing speed. An inner net of mesh size of 12 mm was used in the codend. The hydrographic stations were worked with Nansen bottles and bathythermograph. Plankton samples were collected with a 180 ,u Juday net, hauled vertically from 50 meter to the surface. The light measurements were made at 1 m depth.