Climate, cod, and capelin in northern waters

Transcription

1 ICES mar. Sei. Symp., 198: Climate, cod, and capelin in northern waters Svend-Aage Malmberg and Johan Blindheim Malmberg, S.-A., and Blindheim, J Climate, cod, and capelin in northern waters. - ICES mar. Sei, Symp., 198: 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, Newfoundland, 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 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 The distribution of cod (Gadus morhua) in the northern North Atlantic is confined to shelf areas (as shown in Fig. 1). The spawning areas of the stocks are reached by relatively warm and saline water of the North Atlantic Drift, while stock distribution in some regions may extend into Arctic waters (see Fig. 1 in Jönsson 1994, pp ). Capelin (Mallotus villosus), however, is a cold-water pelagic species which spawns in shallow waters along the northern boundaries of the warm-water drift, but feeds and grows up farther north in Arctic waters, occasionally even beyond the Polar Front. Capelin is an important food item for cod when the distribution of the two species overlaps during certain life stages. This is also the case for the Icelandic cod and

2 298 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. Spawning of both cod and capelin takes place off the south coast of Iceland in relatively temperate waters, while the spawning products drift with the current to nursery areas on the North Icelandic Shelf. Primary production in this area is mainly concentrated in a spring bloom based on nutrients brought up to the photic zone by winter convection, although there is a moderate blooming throughout summer. The intensity and development of primary production depends on how stratification in the water column develops when the surface layer is warmed during spring and summer. This will further be reflected in the distribution and abundance of zooplankton and, hence, in feeding conditions for plankton consumers like capelin and postlarvae cod. As the strength of year classes in the cod stock depends on survival rate during larval and postlarval life stages, variability in the oceanographic structure may largely influence recruitment to the cod stock. The present paper deals mainly with relationships between variability in the hydrographic conditions and the state of the Icelandic cod and capelin populations, particularly growth and reproduction. Emphasis is put on the North Icelandic Shelf waters, which is the nursery area for these species. Oceanographic structure and variability in this area depend on fluctuations in transports and properties in the major current systems in the region, i.e. the supply of warm water masses from the Irminger Current and cold waters from the East Greenland Current (Fig. 2). Some thoughts are also given to parallel relationships in some of the other cod stocks in the northern North Atlantic, mainly those off Labrador and Newfoundland, West Greenland, and in the Barents Sea.

3 62' 30' 25' 20 ' ICES mar. Sei. Symp (1994) Climate, cod, and capelin in northern waters ' S-3 66' ICELAND 64' Figure 2. Main ocean currents in Icelandic waters and location of sections and stations studied. Data The hydrographic data used in this paper are mainly time series collected in Icelandic waters by the Marine Research Institute, Reykjavik. Hydrography and plankton have been observed in standard sections since Since 1950, data have been collected annually in spring; prior to 1950 sampling frequency was more irregular. Since 1970 a grid of standard sections around Iceland has been observed four times a year. The present study is based mainly on a standard section carried out in spring northwards from Siglunes on the North Icelandic coast (Fig. 2), and time series from Station S3 from the coast in this section are chosen as representing the conditions in North Icelandic shelf waters. This section and station reflect the overall conditions in North Icelandic waters throughout the year (Malmberg and Kristmannsson, 1992). Annual assessments of the cod and capelin stocks are taken from annual status reports from the Marine Research Institute (e.g. Anon., 1993). The time series used in the present paper for the cod stock cover the period back to 1950, while there are data on the capelin stock since The Iceland Sea General circulation Iceland is situated in the junction between two large submerged ridges, the Mid-Atlantic Ridge and the Greenland-Scotland Ridge. This major bathymetric feature is decisive for the hydrography in the region. The Iceland Sea, which covers the area between Iceland, Greenland, and the island of Jan Mayen, consists of mainly cold waters from the north (Stefânsson, 1962; Swift, 1980; Swift and Aagaard, 1981), while the area south of Iceland is characterized by the North Atlantic Drift with relatively warm waters of southern origin. Thus, the circulation is dominated by two major current systems (Fig. 2). To the south, the Irminger Current carries warm waters from the North Atlantic Drift to the area south of Iceland and with a transport of approximately 3 Sv it continues northward along the west coast of Iceland. It splits into two branches in the Denmark Strait. One turns southwestwards to flow along the East Greenland Shelf and slope, where it gradually becomes a warm intermediate component of the East Greenland Current. The other branch follows the Icelandic slope and with varying transport of 1-2 Sv (Kristmannsson et

4 300 S.-A. Malmberg and J. Blindheim al., 1989) it flows eastwards into North Icelandic shelf waters where it ultimately disperses off the eastern coast (Stefânsson, 1962). The East Greenland Current carries cold and fresh waters southward along the east coast of Greenland with an offshoot into the Greenland Sea - the Jan Mayen Current - and a larger one - the East Icelandic Current - into the Iceland Sea. The major portion of its transport, however, flows out of the Iceland Sea through western Denmark Strait to continue southwards along the East Greenland shelf. The Polar Water is separated from the basin water masses in the Greenland and Iceland Seas by a zone with increased horizontal gradients, particularly in salinity. This is the Polar Front, which is normally located along the shelf break, except where Polar Water to a varying extent spreads into the Jan Mayen and East Icelandic Currents (Malmberg, 1984, 1985). Water masses Important water masses with their sources outside Icelandic waters are brought into the area by this circulation system (Stefânsson, 1962): Atlantic Water, carried into Icelandic waters by the North Atlantic Drift and the Irminger Current, is the warmest water mass in the area, with temperatures ranging from 3 to 8 C and salinities of about 35. On entering the Iceland Sea it is typically at temperatures of 3-6 C and salinity Polar Water occupies upper layers in the East Greenland Current. It originates mainly from the Arctic Ocean, but in summer in particular it also contains some runoff from Greenland. It is generally colder than 0 C, while its salinity is below Arctic Intermediate Water occurs in several modifications and from various sources. An Arctic intermediate component of the East Greenland Current flows below the Polar Water. It has circulated from the West Spitsbergen Current into the northern Greenland Sea southward and along the East Greenland Shelf. The relatively high temperatures, between 0 and 2 C, indicate its Atlantic origin, but for the same reason its salinity is also high, about It is therefore heavier than the Polar Water. The Iceland Sea Deep Water, with temperatures close to 1 C and salinities of , occurs at depths greater than approximately 600 m in the Iceland Sea proper. It has partly been transported southward along the East Greenland slope with a major source in the Arctic Ocean (Aagaard and Carmack, 1989; Malmberg et al., 1990; Rudels and Quadfasel, 1991; Buch et al., 1992), and partly it arrives from the Norwegian Sea south of the Jan Mayen platform. Some water masses are also formed within the Icelandic waters: Coastal water on the shelf around Iceland, with temperatures varying seasonally between wide limits, is diluted by runoff to salinities below Arctic ICES mar. Sei. Symp., 198 (1994) Intermediate water types formed during winter, partly in the Greenland Sea and partly in the northern and central Iceland Sea, are defined by Swift (1980) and Swift and Aagaard (1981). In the Iceland Sea these water types are characterized by temperatures around 0 C and salinities of between 34.7 and North Icelandic Winter Water, with temperatures of 2-3 C and salinities from 34.8 to 34.9, is defined by Stefânsson (1962) to be formed on the North Icelandic shelf during winter by convective mixing of cold and warm water masses. Arctic waters in the upper few hundred metres of the southern Iceland Sea may be relatively pure forms of intermediate water either from the East Greenland Current or from the Iceland Sea, or they may be mixtures between these and the other water types in the area. It is hardly necessary in the present discussion to distinguish between these varieties, the term Arctic Water being used for all water types of temperature 0-3 C and salinity These Arctic water masses are found above the Bottom Water in the Iceland Sea. South of Jan Mayen, some water of Atlantic origin penetrates from the Norwegian Sea and goes into a cyclonic circulation in the eastern Iceland Sea (Stefânsson, 1962). The southern side of this circulation joins with the East Icelandic Current which brings Arctic waters through the southern Iceland Sea and, to some extent, into the southwestern Norwegian Sea. The border between the Arctic waters of the East Icelandic Current and Atlantic waters forms the Arctic Front. In Icelandic waters the location of this front may vary with fluctuations in volume transport in the North Icelandic branch of the Irminger Current as well as in the East Icelandic Current. In some years the front may be found off the shelf break north of Iceland. This was the situation for example in 1980 (shown in Fig. 3). In other periods, the shelf area too may be occupied by Arctic or Polar waters. During such periods there is no Arctic or Polar front north of Iceland. Instead it may hit land at about Kögur and leave land somewhere on the northeast or east coast, as demonstrated by the situation from 1979 in Figure 3. Polar Water is normally distributed over the East Greenland shelf and not far into the East Icelandic Current. Periodically, however, it spreads far into the Iceland Sea and, during the period from 1965 to 1971, in particular, Polar Water was carried into the North Icelandic Shelf. As a consequence, conditions were of a polar character during winter and spring, as drift ice frequently blocked the north and east coasts and even ice formed in North Icelandic waters (Malmberg, 1969, 1984). Hence, Atlantic, Arctic, and Polar regimes on the North Icelandic Shelf may be defined according to the dominating water masses: During an Atlantic regime, as

5 ic e s mar. sd. Symp., 198 (1994) Climate, cod, and capelin in northern waters /5-15/ t C, 2 0 m Z3/5-10/6 I960 Figure 3. Temperature at 20 m in Icelandic waters in May/June 1979 and 1980 (Malmberg, 1983). exemplified by the section from 1980 in Figure 4, there is Atlantic Water on the shelf and the front toward the Arctic waters to the north is found off the shelf break. In spring (May), the temperatures are above 3 C and salinities above 34.9 in the upper 200 m of the water column; surface layer temperatures may be 5-6 C. An Arctic regime, as shown by the section from 1981 in Figure 4, is characterized by Arctic waters on the shelf; temperatures everywhere are below 3 C and salinities are around During a Polar regime, see the section from 1979 in Figure 4, there is cold, low-salinity Polar Water in the

6 302 S.-A. Malmberg and J. Blindheim ic e s mar. s d. symp., 198 (1994) J 600 J 800 SIGLUNES spring 1979 TEMP nm SIGLUNES spring 1980 TEMP nm < nm SIGLUNES spring 1981 TEMP 100 Figure 4. Vertical distribution of temperature and salinity on a section in North Icelandic waters in May/June 1979, 1980 and 1981 (for location, see Fig. 2).

7 ICES mar. Sei. Symp., 198 (1994) Climate, cod, and capelin in northern waters , > 34,! SIGLUNES spring 1979 SALINITY 0 nm > 35, , , SIGLUNES spring 1980 SALINITY nm , SIGLUNES spring 1981 Figure 4. Continued SALINITY 0 nm 100

8 304 S.-Æ Malmberg and]. Blindheim ICES mar. Sci. Symp., 198 (1994) 1955 I t C Atl Pol. Pol. Pol Pol. (400) I06 0-Gr. I RECRUITMENT O-GROUP INDEX 1955 I Figure 5. 5 a, b. Temperature and salinity at 50 m depth at Station S3 in North Icelandic waters in May/June (for location, see 1Fig. 2). c. Recruitment of 3-year-old cod (year classes) in Icelandic waters in d. 0-group indices of cod in Icelandic waters surface layer with increased density gradients toward the deeper water mass, which may be Arctic or even Atlantic in character. The shelf waters may be covered by ice during winter and spring. Time series The time series of temperature and salinity observed at 50 m depth at Station S3 (Fig. 2), covering the period since 1952, are shown in Figure 5. Observations earlier than 1924 are not shown in the figure. All observations during the period indicated that temperatures varied between 4 and 6 C and salinities between about 34.9 and Although the observational frequency was irregular before 1950, there are several indications that there was a continuous Atlantic regime during this period. Hence, there have been no winters with drift ice on the coast during this period since 1918

9 ICES mar. Sei. Symp (1994) Climate, cod, and capelin in northern waters 305 (Sigtryggsson, 1972). Furthermore, on the island of Grimsey, on the North Icelandic shelf, sea-surface temperatures have been observed since 1871; there were no indications of Polar or Arctic water in the surface layer during the actual period as temperatures were generally above 3 C during winter and spring (Stefânsson, 1969). It is therefore unlikely that Polar or Arctic regimes occurred in years in which there were no observations. After this long and stable period with an Atlantic regime, conditions changed in the mid-1960s with a drastic cooling and freshening of the upper water column to the lowest values on record in 1969 with temperature close to 1 C and a salinity near 34 at Station S3. As a result, the north, east, and even southeast coast of Iceland experienced the worst ice years since early in this century. Since this cold period, which lasted through the rest of the 1960s, conditions have been more changing. There have been four short periods with an Atlantic regime, around 1973, 1980, 1985, and 1992, but temperatures have usually been below 5 C and salinities have hardly exceeded 35. Polar regimes occurred in four years, 1975, 1977, 1979, and 1988, while Arctic regimes occurred during and On average, the period from 1964 to the present has been considerably colder and fresher than the 40 years between 1924 and The Arctic regimes in and in were both related to low salinities in the Arctic Intermediate Water returned from the West Spitsbergen Current (Malmberg et al., 1990; Malmberg and Kristmannsson, 1992). In the low salinities were due to the return of the Great Salinity Anomaly (Dooley et al., 1984; Dickson and Blindheim, 1984; Dickson et al., 1988). Current measurements in the core of the eastern branch of the Irminger Current off Kögur during the years reveal stronger currents during the Atlantic period from 1985 to 1987 than during the Polar and Arctic regimes in the following three years. This indicates that the change from an Arctic to an Atlantic regime is not only due to changes in properties of the inflowing water masses, but also to larger volume transport in Atlantic inflow to the shelf north of Iceland (Kristmannsson, 1991). In general, the influence of warm Atlantic Water has decreased in the northern North Atlantic since its most extensive distribution during the period from about 1930 to 1960 (Smed, 1975; Rodewald, 1967, 1972). There has thus been a general cooling trend since the 1950s from West Greenland and Newfoundland waters in the west to the Barents Sea in the east (Malmberg and Svansson, 1982). In North Icelandic waters this has brought about more variable and, in general, weaker Atlantic inflow. Instead, periods with cold and fresh Arctic and Polar Water have been more prevailing, even with drift ice as in the late 1960s. The forcing of this changed oceanic circulation is not well understood, although the increased outflow of Polar Water from the Arctic Ocean during the mid-1970s anomaly (Great Salinity Anomaly) was linked to variations in the Greenland High and the Iceland Low (Rodewald, 1967, 1972; Dickson et al., 1975; Aagaard, 1972; Jönsson, 1991). The increased outflow of fresh Polar Water from the Arctic Ocean may also weaken the deep water formation in the Greenland Sea (Meincke etal., 1992), which again has an impact on the Atlantic inflow to the Nordic Seas. Cod and hydrography Icelandic cod matures to first spawning at an age of around 7 years (Schopka, 1994; Jönsson, 1982). Spawning takes place in warm water to the south and southwest of Iceland during the period March to May. Hatching occurs after 2 to 3 weeks, while eggs and larvae drift northward along the west coast. By August most of the spawning products occur as 0-group cod on the North Icelandic Shelf and, often, a smaller portion drift toward East Greenland in the western branch of the Irminger Current. Young cod is distributed on the Icelandic shelf, mainly off the northwest, north, and east coasts. Adult year classes feed largely in the same areas, but have spawning migrations back to the warm waters. Estimates of the recruitment to the Icelandic cod stock since 1952 are shown in Figure 5 given as assessments of year-class strength at the age of 3 years. The values, however, are entered in the years when the year classes were spawned. Indices on 0-group estimates of cod in August in Icelandic waters are also shown in Figure 5, but are not discussed further in this paper (see Astthorsson et al., 1994). The figure shows interannual variations in year-class strength throughout the period, but since 1970 there has been a decreasing trend in the weaker year classes. All strong year classes after 1966 were recruited either within Atlantic periods on the North Icelandic Shelf, or in years with increasing temperature prior to such periods. Hence, when conditions started to improve after the Polar period in the late 1960s, a strong year class was recruited in 1970, and the 1973 year class, which is the strongest in the time series, was produced in the warmest year of the Atlantic period from 1972 to Strong year classes were also recruited in 1983 and 1984 at the beginning of the Atlantic period from 1984 to 1987 (Malmberg and Kristmannsson, 1992). The fluctuations in year classes during the long Atlantic period before 1965 show that a warm environment is not the only prerequisite for the production of strong year classes. However, there does seem to be a closer relationship between Polar and Arctic periods and weak

10 306 S.-A. Malmberg and J. Blindheim ICES mar. Sei. Symp., 198 (1994) year classes as recruitment was generally low in the periods with such conditions after 1965, although there were exceptions in 1966 and 1975 when relatively strong year classes were recruited simultaneously with Polar regimes in North Icelandic waters. Standing alone, the relationship in the present, relatively short time series may not be absolutely convincing, but it is in agreement with similar relations in other cod stocks. In this context the most obvious relationship is observed in the West Greenland cod stock, which lives near the lower limit of the temperature range for the species. This population was reduced almost to depletion when temperatures decreased during the 1960s and 1970s (Blindheim, 1974; Hovgaard and Buch, 1990). In Northeast Arctic cod in the Barents Sea, Sætersdal and Loeng (1987) found similar relationships with most year classes of strong or medium abundance occurring in the early part of a warm period or shortly prior to a shift to a warmer regime. It therefore seems reasonably likely that the present time series indicates similar relationships in North Icelandic waters. The mechanism behind the survival rate of larval and early juvenile stages may lie in the feeding conditions in the nursery area. Ellertsen et al. (1989) suggested a temperature-induced timing of the first-feeding prey, the nauplii of Calanus finmarchicus, as a possible link between temperature and recruitment success for Northeast Arctic cod, which has its main spawning area in Lofoten. In this cod stock, spawning is fixed at the same time from year to year, while low temperatures may delay the spawning of C. finmarchicus, with a difference of up to 6 weeks between warm and cold years. As the survival of cod during its early larval stage depends on the presence of suitable concentrations of food items, e.g. nauplii, such time shifts in plankton spawning may result in serious mass starvation owing to lack of prey at the most critical life stage. Although conditions for the Icelandic cod stock are not identical to those of Northeast Arctic cod, the two stocks have several conditions in common. Both stocks live in outer branches of the North Atlantic Current system. Northeast Arctic cod depends on the Norwegian Atlantic Current and its branching into the Barents Sea, while Icelandic cod depends on the Irminger Current and its branch into North Icelandic waters. C. finmarchicus dominates the zooplankton population in both regions (e.g. Astthorsson et al., 1983; Blindheim and Skjoldal, 1993). It therefore seems likely that simultaneous production of cod larvae and Calanus nauplii is an important factor for the survival success of Icelandic cod too. An ideal situation would probably be one in which the planktonic spawning products and the fish larvae are advected in the same volume of water, where the cod larvae and their prey have a parallel growth. During Atlantic periods in North Icelandic waters, surface temperatures are relatively high and decrease gradually with depth. The mixed layer will deepen relatively slowly and supply nutrients to the productive layer over a relatively long period. The primary production will have a relatively high and long-lasting spring bloom with a more moderate production later in the season (Thördardöttir, 1977, 1986; Stefânsson and Ölafsson, 1992). This will be reflected in the secondary production of zooplankton (Astthorsson et al., 1983). During an Arctic regime temperatures are generally below 3 C and there is not much stratification in the water column during winter. Because of this the formation of a stratified surface layer by warming in spring may be somewhat slower than in an Atlantic regime, but still, development of blooming during spring and summer may not be much different from that in an Atlantic regime, although species composition may be different. Observations show that primary production may be large (Malmberg, 1986, 1988), but abundance of zooplankton is generally much lower than in Atlantic regimes. Furthermore, the zooplankton population will have a considerably larger component of Arctic species than in Atlantic periods (Astthorsson et al. 1983) and the spawning of C. finmarchicus will be delayed and early juvenile cod may not find suitable food items on the North Icelandic Shelf. In a Polar regime, conditions for plankton production will be different. Here there is a surface layer with low density and relatively strong density gradients before the warming in spring starts. The warming will therefore result in a shallow surface layer with a short and intense spring bloom (Thördardöttir, 1977; Stefânsson and Ölafsson 1992). The nutrients will be quickly depleted and deepening of the layer will be modest because of the sharp pycnocline below the mixed layer. Blooming later in the summer will therefore be almost negligible. The plankton fauna will have a relatively large component of Polar and Arctic species which may be less suitable food items for juvenile cod. In North Icelandic waters the decrease in abundance of zooplankton is considerable (Astthorsson et al., 1983). Also low temperatures as such may be an important factor. Cod as a species belongs in temperate and boreal regions and adult cod avoids low temperatures and seldom occurs in abundance in temperatures near 0 C. Although it is claimed that cod larvae and juveniles adapt better to low temperatures than adult fish (Goddard et al., 1992), there are also reports of increased mortality of 1- and 2-group cod during extremely cold winter temperatures (Ponomarenko, 1984). The overall effect of low temperatures, directly and indirectly, may be concluded to have an adverse impact on the recruitment.

11 ICES mar. Sei. Symp., 198 (1994) Climate, cod, and capelin in northern waters 307 Capelin and hydrography The Icelandic capelin spawns at the age of 3-4 years in late winter in the coastal waters south of Iceland (Vilhjâlmsson, 1983, 1994). From there the larvae drift westwards and northwards into the nursery grounds in the Denmark Strait and in North Icelandic waters. During the following years the capelin feeds mainly in the area between the Polar and Arctic fronts in the Iceland Sea, until it migrates southwards again, mainly along the shelf break north and east of Iceland, to the spawning areas. Capelin suffers mass mortality after spawning. Hydrographic fluctuations in North Icelandic waters also affect the living conditions of capelin. Assessments of capelin stock abundance are available for the period since 1978 and entered in Figure 6. Maximum salinity in the upper 300 m of the water column at Station S3 (for location, see Fig. 2) is also entered in the figure. Here salinities above 34.9 indicate Atlantic periods, while lower salinities represent Arctic Water. The salinity trends are similar to the salinity curve from 50 m, which is shown in Figure 5, but the low surface salinities in a Polar period do not appear to the same extent as at 50 m depth. The interrelationship between abundance of capelin and salinity is reasonably good, although not perfect, and a very low abundance of capelin during the Arctic periods and is clear. In in particular the abundance was at such a low level that fisheries were stopped in In these years the decline in abundance may have begun before the Arctic period, i.e. in the Polar years 1979 and 1988, as there are indications of low survival rate and reduced growth of 0-group capelin during Polar conditions (Vilhjâlmsson, 1983, 1994; Malmberg, 1986, 1988). Furthermore, the abundance of capelin is reflected in the growth of cod, which feeds on capelin (Pälsson, 1983). The weight of five-year-old cod is also entered in Figure 6 and the relationship with the abundance of capelin is fairly good. Although the temperature fluctuations will have a similar trend (Fig. 5), with a similar physiological effect on growth, it is unlikely that this alone will result in the differences shown in Figure 6. The most likely reason for the low abundance of capelin and, indirectly, the low growth of cod, is the reduced zooplankton biomass during Arctic periods (Astthorsson et al., 1983; Malmberg, 1986, 1988). Similar effects are also observed in the Iceland Sea, where adult capelin feeds during summer. Abundance and distribution of plankton are therefore probably important links between environment and abundance and growth of capelin (Jakobsson, 1992). Growth of cod, however, seems not to be very dependent on hydrographic conditions, but more on abundance of capelin (Malmberg, 1986; Steinarsson and Stefânsson, 1991). Effects of fishing in relation to environment There are indications that the widespread cooling since about 1960 has entailed adverse effects on cod stocks in the Northern North Atlantic as a whole. While this is most recorded for the West Greenland cod stock (Blindheim, 1974; Hovgaard and Buch, 1990), there have been decreases in the abundance also in other North Atlantic ,0-3,0 tonnes ,9-2,5 I --34, ,0 Figure 6. a. Maximum salinity observed in the upper 300 m at Station S3 in North Icelandic waters in May/June (solid thin line), b. The abundance of Icelandic capelin stock (t) in (solid thick line; Vilhjâlmsson 1994). c. Weight of 5-yearold cod (kg) in Icelandic waters in (broken line; Anon., 1993).

12 308 S.-A Malmberg and, J. Blindheim ICES mar. Sei. Symp., 198(1994) Table 1. Catches, recruitment, and spawning-stock biomass of cod in Northern Waters around 1960 and 1990 (Iceland, West Greenland, Newfoundland, and NE Arctic; from Jakobsson 1992). Catch 1031 Recruitment 106 n Spawning stock 103 t Iceland West Greenland Newfoundland / Northeast Arctic / Sum cod stocks over recent decades. This seems to be important especially for stocks which live near the lower limit of the temperature range for cod, e.g. off Labrador/ Newfoundland, West Greenland, and even North Icelandic waters. In these areas it seems that the environmental conditions again must improve before stocks and catches can be expected to be of the same strength as before the mid or late 1960s. Here it is pertinent to ask to what extent the decrease in the cod stocks is due to the deteriorating ocean climate or overfishing. Some indication of this is given in the relation between the decline in catches and the decreases in recruitment and spawning stock in these cod stocks (see Table). Summed up, for all these fishing grounds in the northern North Atlantic there has been a decline, from 1960 to 1990, in catches of about 50%, in recruitment of about 67%, and in spawning stocks of about 75%. The smaller decrease in catch than in recruitment and spawning stock, in particular, indicates an increasing fishing load from 1960 to Figure 7. Conditions of the Icelandic cod stock /92. a. Three-year recruits in millions by number, b. Spawning-stock biomass in thousand tonnes, c. Fishing-stock biomass (4-year-old and older fish) in thousand tonnes, d. Catches in thousand tonnes (from Anon., 1993). Shaded columns indicate periodic maxima observed and the arrows the time lack from 3-4-year-old recruits up to the year of maximum in fishing and spawning stocks as well as catches a few years later.

13 ICES mar. Sci. Symp., 198 (1994) Climate, cod, and capelin in northern waters 309 For Icelandic cod a similar indication appears from Figure 7, which shows recruitment by 3-year-old fish, spawning biomass, fishing stock biomass and catches since Before 1980 the recruitment of strong year classes was reflected in increased fishing-stock biomass and, but to a lesser extent, increased spawning stock. It is, however, noteworthy that strong 3-year-old recruiters in 1986 and 1987 (year classes 1983 and 1984) did not result in an increased spawning stock or catch a few years later despite Atlantic conditions in North Icelandic waters, but contributed in catch already in Consequently, a good part of these year classes were not allowed to develop to maturity. It is difficult to say which of the factors, fishery or environment, is the more decisive in relation to the decline in the cod stocks. The crucial difference between them is that while we are unable to do anything about environmental fluctuations, we should be able to manage the fishery. Careful regulation is particularly important when a fish stock is weak as a result of unfavourable environmental conditions. At worst, an efficient trawling fishery could easily exploit a spawning stock below a critical level of abundance, from which point it may take many years for the stock to be restored. Concluding remarks Time series of oceanographic observations reveal variability in processes such as stratification and advection over regional and large ocean areas in connection with air-sea interaction. They contribute to a better understanding of the processes underlying the oceanic and biological variability. The authors believe that a major factor concerning the variation in size and behaviour of fish stocks, at least along the Arctic and Polar fronts, is the maritime climate, which influences the living conditions including spawning, feeding, recruitment, and maturation. The mechanisms behind this may not be well understood. Temperature may have a direct effect on fish stocks or be associated with variable current activities such as climatic events and turbulence and other physical parameters, which again affect the multiple living conditions. The overall results of this paper may be summarized as follows: There has been a dramatic decline of cod stocks in the northern North Atlantic during recent decades. In parallel with climatic changes in the northern North Atlantic during the same period, the spawning, feeding, and fishing grounds of cod in these waters seem to have declined since the 1960s. Capelin stocks reflect great interannual variance in abundance and growth, depending on environment and feeding conditions. Growth of adult cod is dependent more on food supply, such as abundance of capelin, than on hydrographic conditions. Cold years in North Icelandic waters as well as in the waters off Labrador, West Greenland, and in the Barents Sea, seem generally to give weak year classes of cod. The critically small spawning stock in Icelandic waters during recent years has given weak year classes during both cold and warm years. References Aagaard. K On the drift of the Greenland Pack Ice. Sea Ice. Proc. Int. Conf. Rvfk R.r. 72(4): Ed. Th. Karlsson. Aagaard, K., and Carmack, E. C The role of the sea ice and other fresh water in the Arctic circulation. J. Geophys. Res., 94, CIO: Anon State of Marine Stocks and Environmental Conditions in Icelandic Waters Hafrannsöknir Fjölrit, 34: 154 pp. Astthorsson, O. S., Hallgrimsson, I., and Jönsson, G. 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