MARINE ECOLOGY - PROGRESS SERIES Vol. 45: 45-55. 1988 Published June 20 Mar. Ecol. Prog. Ser. Winter distribution of macroplankton and micronekton in Masfjorden, western Norway Stein Kaartvedt, Dag L. Aksnes, Agnes Aadnesen Department of Marine Biology, University of Bergen, N-5065 Blornsterdalen, Norway ABSTRACT: Masfjorden was investigated as a potential site for sea ranching of cod. The macroplankton and micronekton were studied in order to reveal the distributional patterns of species and biomass during winter. The biomass was dominated by the mesopelagic fishes Benthosema glaaale and Maurolicus muelleri, the pelagic shrimps Pasiphaea multidentata and Sergestes arcticus, and the euphausiid Meganyctiphanes norvegica. All species performed die1 vertical migrations. B. glaciale was almost exclusively found in the deepest locations (>400 m). M. muelleri was found throughout the fjord, but large individuals were most common at one of the deep stations. The shrimps were mainly confined to the deeper parts of the fjord, where the 2 species were prominent in separate basins. The largest individuals were found at the deepest locations. The distribution of M. norvegica seemed to be unrelated to the topography. There was a marked maximum of euphausiids at the head of the fjord; these were probably advected into the fjord system. The total standing stock of macroplankton and rnicronekton was estimated to be 19.5 tonne ash-free dry weight. We conclude that the amount of macroplankton and micronekton present in Masfjorden does not provide a food resource on which several hundred thousand codlings could rely during winter INTRODUCTION transfer of organic material in pelagic communities of fjords in western Norway (Gjaseter 1973, 1981, Mat- Masfjorden (Fig. 1) has been selected to study the thews & Bakke 1977). Both the biomass and ecology of potential of enhancing natural populations of coastal such organisms in fjords, however, are generally poorly cod Gadus morhua L. by means of releasing artificially known. reared young cod. In Scottish sea lochs, it has been This paper has 2 objectives. First, to contribute to the shown that the production of juvenile cod may be general knowledge of the macroplankton and micro- limited by the winter food supply (Hawkins et al. 1985). nekton in fjords, and second, to relate our results to the Therefore, as a measure of the carrying capacity of the cod release project. Spatial patterns of the community fjord, the standing stock of food organisms in winter composition in Masfjorden during winter will be may be of special interest. The cod stock in Masfjorden addressed, and the results are interpreted with regard at present feeds mainly on benthic prey and gobiid to the topography of the fjord, hydrography, advective fishes along the shallow sublittoral parts of the fjord. processes, and biological interactions. To evaluate the Gobiids are the most important prey for cod less than potential of macroplankton and micronekton as food 20 cm length (Salvanes 1986). Cod is, however, known for cod, estimates of the total standing stock of macro- to feed on a variety of prey (e.g.Rae 1968, Daan 1973, plankton and rnicronekton in Masfjorden are compared Klemetsen 1982). By significantly increasing the cod to food requirements of juvenile cod. stock in Masfjorden, it is conceivable that new areas would be inhabited and that predation pressure on the macroplankton and micronekton would increase. THE STUDY AREA Macroplankton and micronekton are important prey for cod in fjords of northern Norway (Falk-Petersen & Masfjorden is a 20 km long fjord situated north of Hopkins 1981, dos Santos & Falk-Petersen in press), Bergen, western Norway (Fig. 1). The fjord borders on and these groups may also play an important role in the Fensfjorden, through which it is connected to outer O Inter-ResearchIPrinted in F. R. Germany Mar Ecol. Prog. Ser. 45 45-55, 1988 Fig. 1. Masfjorden with bottom contours (m).Borders of Stns A, B + C, and D are given Sampling was performed along the deep mid-parts of the basins coastal waters. At times, intrusions of coastal waters MATERIAL AND METHODS may play an important ecological role. A large impact of advective processes on mesoplankton has been The main sampling was carried out on 10 to 14 Janu- demonstrated by Aksnes et al. (unpubl.),who conclude ary 1986 from the RV 'Hbkon Mosby'. Supplementary that the standing stock of mesoplankton in the fjord is samples were collected in December 1986 and January to a large extent a result of exchange of biomass across 1987. In January 1986 samples were collected by 2 the sill. Isaac-Kidd Midwater Trawls (IKMT), 1 large IKMT (L- Sill depth in Masfjorden is 75 m, and maximum depth IKMT) with a mouth aperture of 10 m2 (specifications is 494 m. The topography of the fjord is outlined in are given by Anonymous 1981),and 1 smaller IKMT (S- Fig. 1. At the head of the inner southern branch of the IKMT) with mouth aperture of 2/3 m* (a 3 ft IKMT, Aron fjord, there is a hydroelectric power plant, supplying the 1962).The net in the L-IKMT had decreasing mesh size fjord with freshwater at a maximum rate of 50 m3 S-' backwards, from 25 mm in the front to 1.15 mm at the This runoff causes a short residence time of the surface rear part. The net of the S-IKMT was 2 mm throughout. waters, resulting in ice-free condihons along the main On each of Stns A to D (Fig. l),oblique tows covering a axis of the fjord during winter (Sztre 1974).During our depth range of approximately 100 m were conducted. survey, the inner northern branch was covered with ice, The actual depth interval sampled was determined preventing sampling in this region. using a Benthos Time-Depth recorder. Due to the short Kaartvedt et al.: Macroplankton and micronekton in Masflorden 47 day at latitude 60°50'N during winter, time for sam- tion, counting, and measuring size. The other part was pling during daylight was limited. Thus, Stns A and D frozen for biomass determinations after having sorted were sampled during darkness only. Details are given the catch into the categories 'fish', 'shrimps', in Table 1. 'euphausiids', and 'other taxa'. Prior to biomass deter- The IKMT's were towed at a speed of 3 knot. Since mination, shrlmps and fishes were counted in the fro- they had no closing device, the speed of the ship was zen fraction. In December 1986 and January 1987, the kept lower during shooting and hauling to reduce fish- entire sample was frozen (split into categories as ing outside the desired depth interval. Nevertheless, above) after counting the shrimps and fishes on board. contamination from upper layers may have been Size of fish is given as standard length, i.e. excluding noteworthy, especially for the deepest tows. The the caudal fin. Carapace length in shrimps was meas- catches were therefore corrected by reference to the ured from the hind orbit of the eye to the mid posterior catches in the shallower tows, for the time spent fishing dorsal margin. Dry welght was determined by drying in the upper layer of water during shooting and hauling the samples at 60°C to constant weight. The samples (Table 1). Any differences in sampling efficiency du- were then burnt at 500°C for determination of ash ring shooting and hauling have not been accounted for. content. In December 1986 and January 1987, only the S- Differences in abundance between stations were IKMT was used. Each station was sampled by 2 oblique tested by single classification ANOVA on log-trans- tows per day and per night - one from near the bottom formed data from the night tows with the L-IKMT to the surface, and one from approxin~ately100 m to (Sokal & Rohlf 1981). Tests were done both per unit the surface (Table 2). The net was towed at 3 knot surface area and on densities (per m3). Given levels of throughout the haul. significance were satisfied in both tests. In the analysis, The January 1986 samples were split into 2 parts by a tows from different depths at a station were treated as modified Folsom splitter (Motoda 1959). One part was 'rephcates'. This introduces an extra 'within station' preserved in 4 % buffered formaldehyde for identifica- variance in species confined to restricted depth inter- Table 1. Sampling data from the survey in Masfjorden, January 1986 Distances towed in and out of the sample intervals are given. L-IKMT and S-IKMT: large and small Isaac-Kidd Midwater Trawl nets, respectively Stn L-IKMT S-IKMT Time of Date Tow Distance (m) Time of Date Tow Distance (m) tow (h) depth In Out tow (h) depth in Out (m) (m) A 22.07-22:39 10 Jan 80-0 2718 0 21 03-21:45 220-80 2698 1044 19.10-19:35 13 Jan 100-0 l743 0 19:54-20:30 120-0 3309 0 B 23:09-23.46 10 Jan 100-0 3269 0 01:50-02.20 13 Jan 150-0 2898 0 00:15-00.50 11 Jan 310-100 2559 898 03.55-04:39 200-140 2256 1932 02:08-02:40 380-120 2427 845 04:52-05:54 360-190 3666 2372 01:15-01:45 420-180 2254 1104 08:12-09:02 370-250 2360 2585 10:05-10:47 13 0-0 3701 0 10:06-10:37 14 Jan 100-0 2600 0 11:04-11:54 250-80 3316 997 10:55-11:30 180-90 2316 1091 12:12-12:58 430-170 2986 l177 11:55-12:37 280-200 1934 2052 13:17-14:15 400-190 3071 1589 12:50-13:49 400-300 2474 3410 C 18:25-19:03 11 Jan 95-0 3259 0 20:40-21:09 12 Jan 100-0 2737 0 19:30-20:20 230-80 3648 765 21:26-22:08 210-140 2369 1839 20.42-21:33 380-180 2725 1190 22:26-23:20 320-220 3080 2262 21:53-23:Ol 370-220 3509 2021 23:38-00:32 380-260 2995 2403 1O:OO-10:35 100-0 2990 0 10:03-10:32 13 Jan 110-0 2266 0 14:15-14.53 165-70 2283 888 10:50-11:31 190-100 2874 1111 10:52-11:40 280-85 3320 884 12:53-13:30 320-240 2178 1655 15.10-16.05 320-150 3120 1323 11 :49-12:42 350-250 2513 2848 D 07:19-07.55 12 Jan 140-0 31 17 0 19:19-1955 12 Jan 150-0 3440 0 18:15-18:55 150-0 3602 0 4 8 Mar.