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Climate, Cod, and Capelin in Northern Waters

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Climate, Cod, and Capelin in Northern Waters ICES mar. Sei. Symp., 198: 297-310. 1994 Climate, cod, and capelin in northern waters Svend-Aage Malmberg and Johan Blindheim Malmberg, S.-A., and Blindheim, J. 1994. Climate, cod, and capelin in northern waters. - ICES mar. Sei, Symp., 198: 297-310. The hydrographic conditions in the Iceland Sea reveal three different regimes on the North Icelandic Shelf: Atlantic, Arctic, and Polar. Thus, time series of temperature and salinity in the shelf waters show that Atlantic Water from the warm irminger Current dominated during a long period prior to the mid-1960s. During the latter half of the 1960s, however, there was a dramatic change to lower temperatures, as Polar surface waters from the East Greenland Current were carried into the area. As a result, the northern and partly also the eastern coasts of Iceland were frequently blocked by drift ice in the late winter and spring of these years. Oceanographic effects of this extremely cold period, the so-called “Great Salinity Anomaly”, were observed during the following years over wide areas in the northern North Atlantic. Since this event, conditions in North Icelandic waters have been alternating between cold and warm periods, but stable and warm conditions similar to those before 1965 have not returned. There have not always been Polar surface waters on the North Icelandic Shelf during later cold periods, sometimes Arctic Water from the Iceland Sea has occupied the area. These shifts in hydrographic conditions in the Iceland Sea have important ecological impacts, because the North Icelandic Shelf is a nursery area for the Icelandic cod and capelin populations. Thus the variable flow of Atlantic Water into the North Icelandic waters may influence the drift of larvae from the spawning areas on the South Icelandic Shelf to the nursery grounds. In the nursery area the changing oceanographic conditions further bring about varying living conditions for the larvae and juvenile fish. The data show that survival rate of cod has been highest in periods with warm water from the Irminger Current on the North Icelandic Shelf. The Atlantic, Polar, and Arctic conditions in North Icelandic waters also seem to influence the size of the Icelandic capelin stock, which was at its lowest during Arctic conditions. As the “Great Salinity Anomaly” could be traced as it advected around the Subpolar Gyre in the Northern North Atlantic, the ecological impacts of this event on several cod stocks can be compared, i.e. the stocks off Iceland, West Greenland, Newfound­ land, and Norway. On these fishing grounds the catches of cod declined from 1960 to 1990 by about 50%, recruitment by about 67% and spawning stocks by about 75%. This indicates an increasing fishing load from 1960 to 1990. Furthermore, improving hydrographic conditions in Icelandic waters after 1990 did not result in a new strong year class of cod. It is questioned whether this failure in recruitment was due to a critically small spawning stock. Svend-Aage Malmberg: Marine Research Institute, Skûlagata 4, PO Box 1390, 121 Reykjavik, Iceland. Johan Blindheim: Institute for Marine Research, Department of Marine Resources, PO Box 1870 Nordnes, N-5024 Bergen, Norway. Introduction 287-291). Capelin (Mallotus villosus), however, is a cold-water pelagic species which spawns in shallow The distribution of cod (Gadus morhua) in the northern waters along the northern boundaries of the warm-water North Atlantic is confined to shelf areas (as shown in drift, but feeds and grows up farther north in Arctic Fig. 1). The spawning areas of the stocks are reached by waters, occasionally even beyond the Polar Front. Cape­ relatively warm and saline water of the North Atlantic lin is an important food item for cod when the distri­ Drift, while stock distribution in some regions may bution of the two species overlaps during certain life extend into Arctic waters (see Fig. 1 in Jönsson 1994, pp. stages. This is also the case for the Icelandic cod and 298 S.-A. Malmberg and J. Blindheim ICES mar. Sei. Symp., 198(1994) Figure 1. Spawning areas of the different cod stocks in the northern North Atlantic and adjacent seas (ICES, 1991). capelin populations that will be discussed here. Spawn­ stages, variability in the oceanographic structure may ing of both cod and capelin takes place off the south largely influence recruitment to the cod stock. coast of Iceland in relatively temperate waters, while the The present paper deals mainly with relationships spawning products drift with the current to nursery areas between variability in the hydrographic conditions and on the North Icelandic Shelf. the state of the Icelandic cod and capelin populations, Primary production in this area is mainly concentrated particularly growth and reproduction. Emphasis is put in a spring bloom based on nutrients brought up to the on the North Icelandic Shelf waters, which is the nursery photic zone by winter convection, although there is a area for these species. Oceanographic structure and moderate blooming throughout summer. The intensity variability in this area depend on fluctuations in trans­ and development of primary production depends on ports and properties in the major current systems in the how stratification in the water column develops when region, i.e. the supply of warm water masses from the the surface layer is warmed during spring and summer. Irminger Current and cold waters from the East Green­ This will further be reflected in the distribution and land Current (Fig. 2). abundance of zooplankton and, hence, in feeding con­ Some thoughts are also given to parallel relationships ditions for plankton consumers like capelin and postlar­ in some of the other cod stocks in the northern North vae cod. As the strength of year classes in the cod stock Atlantic, mainly those off Labrador and Newfoundland, depends on survival rate during larval and postlarval life West Greenland, and in the Barents Sea. ICES mar. Sei. Symp.. 198 (1994) Climate, cod, and capelin in northern waters 299 68' S-3 66' ICELAND 64' 62' 30' 25' 20 ' Figure 2. Main ocean currents in Icelandic waters and location of sections and stations studied. Data The Iceland Sea General circulation The hydrographic data used in this paper are mainly time series collected in Icelandic waters by the Marine Iceland is situated in the junction between two large Research Institute, Reykjavik. Hydrography and plank­ submerged ridges, the Mid-Atlantic Ridge and the ton have been observed in standard sections since 1924. Greenland-Scotland Ridge. This major bathymetric Since 1950, data have been collected annually in spring; feature is decisive for the hydrography in the region. The prior to 1950 sampling frequency was more irregular. Iceland Sea, which covers the area between Iceland, Since 1970 a grid of standard sections around Iceland has Greenland, and the island of Jan Mayen, consists of been observed four times a year. The present study is mainly cold waters from the north (Stefânsson, 1962; based mainly on a standard section carried out in spring Swift, 1980; Swift and Aagaard, 1981), while the area northwards from Siglunes on the North Icelandic coast south of Iceland is characterized by the North Atlantic (Fig. 2), and time series from Station S3 from the coast in Drift with relatively warm waters of southern origin. this section are chosen as representing the conditions in Thus, the circulation is dominated by two major current North Icelandic shelf waters. This section and station systems (Fig. 2). To the south, the Irminger Current reflect the overall conditions in North Icelandic waters carries warm waters from the North Atlantic Drift to the throughout the year (Malmberg and Kristmannsson, area south of Iceland and with a transport of approxi­ 1992). mately 3 Sv it continues northward along the west coast Annual assessments of the cod and capelin stocks are of Iceland. It splits into two branches in the Denmark taken from annual status reports from the Marine Re­ Strait. One turns southwestwards to flow along the East search Institute (e.g. Anon., 1993). The time series used Greenland Shelf and slope, where it gradually becomes in the present paper for the cod stock cover the period a warm intermediate component of the East Greenland back to 1950, while there are data on the capelin stock Current. The other branch follows the Icelandic slope since 1978. and with varying transport of 1-2 Sv (Kristmannsson et 300 S.-A. Malmberg and J. Blindheim ICES mar. Sei. Symp., 198 (1994) al., 1989) it flows eastwards into North Icelandic shelf Intermediate water types formed during winter, partly waters where it ultimately disperses off the eastern coast in the Greenland Sea and partly in the northern and (Stefânsson, 1962). central Iceland Sea, are defined by Swift (1980) and The East Greenland Current carries cold and fresh Swift and Aagaard (1981). In the Iceland Sea these waters southward along the east coast of Greenland with water types are characterized by temperatures around an offshoot into the Greenland Sea - the Jan Mayen 0°C and salinities of between 34.7 and 34.9. Current - and a larger one - the East Icelandic Current - North Icelandic Winter Water, with temperatures of into the Iceland Sea. The major portion of its transport, 2-3°C and salinities from 34.8 to 34.9, is defined by however, flows out of the Iceland Sea through western Stefânsson (1962) to be formed on the North Icelandic Denmark Strait to continue southwards along the East shelf during winter by convective mixing of cold and Greenland shelf. The Polar Water is separated from the warm water masses. basin water masses in the Greenland and Iceland Seas by Arctic waters in the upper few hundred metres of the a zone with increased horizontal gradients, particularly southern Iceland Sea may be relatively pure forms of in salinity.
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