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. This is the Polar Front, which is normally intermediate water either from the East Greenland located along the shelf break, except where Polar Water Current or from the Iceland Sea, or they may be mix­ to a varying extent spreads into the Jan Mayen and East tures between these and the other water types in the Icelandic Currents (Malmberg, 1984, 1985). area. It is hardly necessary in the present discussion to distinguish between these varieties, the term Arctic Water masses Water being used for all water types of temperature 0- Important water masses with their sources outside Ice­ 3°C and salinity 34.5-34.95. landic waters are brought into the area by this circulation These Arctic water masses are found above the Bot­ system (Stefânsson, 1962): Atlantic Water, carried into tom Water in the Iceland Sea. South of Jan Mayen, some Icelandic waters by the North Atlantic Drift and the water of Atlantic origin penetrates from the Norwegian Irminger Current, is the warmest water mass in the area, Sea and goes into a cyclonic circulation in the eastern with temperatures ranging from 3 to 8°C and salinities of Iceland Sea (Stefânsson, 1962). The southern side of this about 35. On entering the Iceland Sea it is typically at circulation joins with the East Icelandic Current which temperatures of 3-6°C and salinity 34.9-35.1. Polar brings Arctic waters through the southern Iceland Sea Water occupies upper layers in the East Greenland and, to some extent, into the southwestern Norwegian Current. It originates mainly from the Arctic Ocean, but Sea. The border between the Arctic waters of the East in summer in particular it also contains some runoff from Icelandic Current and Atlantic waters forms the Arctic Greenland. It is generally colder than 0°C, while its Front. In Icelandic waters the location of this front may salinity is below 34.5. vary with fluctuations in volume transport in the North Arctic Intermediate Water occurs in several modifi­ Icelandic branch of the Irminger Current as well as in the cations and from various sources. An Arctic intermedi­ East Icelandic Current. In some years the front may be ate component of the East Greenland Current flows found off the shelf break north of Iceland. This was the below the Polar Water. It has circulated from the West situation for example in 1980 (shown in Fig. 3). In other Spitsbergen Current into the northern Greenland Sea periods, the shelf area too may be occupied by Arctic or southward and along the East Greenland Shelf. The Polar waters. During such periods there is no Arctic or relatively high temperatures, between 0 and 2°C, indi­ Polar front north of Iceland. Instead it may “hit” land at cate its Atlantic origin, but for the same reason its about Kögur and “leave” land somewhere on the north­ salinity is also high, about 34.95. It is therefore heavier east or east coast, as demonstrated by the situation from than the Polar Water. 1979 in Figure 3. The Iceland Sea Deep Water, with temperatures close Polar Water is normally distributed over the East to — 1°C and salinities of 34.90-34.92, occurs at depths Greenland shelf and not far into the East Icelandic greater than approximately 600 m in the Iceland Sea Current. Periodically, however, it spreads far into the proper. It has partly been transported southward along Iceland Sea and, during the period from 1965 to 1971, in the East Greenland slope with a major source in the particular, Polar Water was carried into the North Arctic Ocean (Aagaard and Carmack, 1989; Malmberg Icelandic Shelf. As a consequence, conditions were of a et al., 1990; Rudels and Quadfasel, 1991; Buch et al., polar character during winter and spring, as drift ice 1992), and partly it arrives from the Norwegian Sea frequently blocked the north and east coasts and even south of the Jan Mayen platform. ice formed in North Icelandic waters (Malmberg, 1969, Some water masses are also formed within the Icelan­ 1984). dic waters: Coastal water on the shelf around Iceland, Hence, Atlantic, Arctic, and Polar regimes on the with temperatures varying seasonally between wide North Icelandic Shelf may be defined according to the limits, is diluted by runoff to salinities below 34.0. Arctic dominating water masses: During an Atlantic regime, as ic e s mar. sd. Symp., 198 (1994) Climate, cod, and capelin in northern waters 301

75

27/5-15/6 1979

t°C, 20 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 An Arctic regime, as shown by the section from 1981 Atlantic Water on the shelf and the front toward the in Figure 4, is characterized by Arctic waters on the Arctic waters to the north is found off the shelf break. In shelf; temperatures everywhere are below 3°C and sali­ spring (May), the temperatures are above 3°C and nities are around 34.8. salinities above 34.9 in the upper 200 m of the water During a Polar regime, see the section from 1979 in column; surface layer temperatures may be 5-6°C. Figure 4, there is cold, low-salinity Polar Water in the 302 S.-A. Malmberg and J. Blindheim ic e s mar. s d . symp., 198 (1994)

200

400 J

600 J

800 SIGLUNES spring 1979

TEMP

1000 0 nm 100

200 -

400 -

600 -

800 - SIGLUNES spring 1980

TEMP 1000 0 nm 40 100

< - 1°

200

400

600

800 SIGLUNES spring 1981 Figure 4. Vertical distribution of tempera­ TEMP ture and salinity on a section in North Icelan­ 1000 dic waters in May/June 1979, 1980 and 1981 0 nm 100 (for location, see Fig. 2). ICES mar. Sei. Symp., 198 (1994) Climate, cod, and capelin in northern waters 303

34,5

200 -

4 0 0 -

> 34,!

600 -

800 - SIGLUNES spring 1979

SALINITY 1000 -

0 nm 20 40 60 80 100

> 35,0

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SALINITY

1000 -

0 nm 20 40 60 80 100

200 -

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SALINITY 1000 -

Figure 4. Continued. 0 nm 100 304 S.-Æ Malmberg and]. Blindheim ICES mar. Sci. Symp., 198 (1994)

1955 I960 1965 1970 1975 1980 1985 1990

t°C

Atl.

35.0 Pol. Pol. Pol Pol.

(400)

34.0

300 0-Gr. I09 I06

200

RECRUITMENT 100

O-GROUP INDEX

1955 I960 1965 1970 1975 1980 985 990

Figure 5.5Figure a, bb. Temperature and salinity at 50 m depth at Station S3 in North Icelandic waters in May/June 1952-1992 (for location, see 1Fig. 2). c. Recruitment of 3-year-old cod (year classes) in Icelandic waters in 1952-1992. d. 0-group indices of cod in Icelandic waters 1970-1992.

surface layer with increased density gradients toward the since 1952, are shown in Figure 5. Observations earlier deeper water mass, which may be Arctic or even Atlan­ than 1924 are not shown in the figure. All observations tic in character. The shelf waters may be covered by ice during the period 1924-1964 indicated that tempera­ during winter and spring. tures varied between 4 and 6°C and salinities between about 34.9 and 35.1. Although the observational fre­ quency was irregular before 1950, there are several Time series indications that there was a continuous Atlantic regime The time series of temperature and salinity observed at during this period. Hence, there have been no winters 50 m depth at Station S3 (Fig. 2), covering the period with drift ice on the coast during this period since 1918 ICES mar. Sei. Symp.. 198 (1994) Climate, cod, and capelin in northern waters 305

(Sigtryggsson, 1972). Furthermore, on the island of in the late 1960s. The forcing of this changed oceanic Grimsey, on the North Icelandic shelf, sea-surface tem­ circulation is not well understood, although the peratures have been observed since 1871; there were no increased outflow of Polar Water from the Arctic Ocean indications of Polar or Arctic water in the surface layer during the mid-1970s anomaly (Great Salinity Anomaly) during the actual period as temperatures were generally was linked to variations in the Greenland High and the above 3°C during winter and spring (Stefânsson, 1969). Iceland Low (Rodewald, 1967, 1972; Dickson et al., It is therefore unlikely that Polar or Arctic regimes 1975; Aagaard, 1972; Jönsson, 1991). The increased occurred in years in which there were no observations. outflow of fresh Polar Water from the Arctic Ocean may After this long and stable period with an Atlantic also weaken the deep water formation in the Greenland regime, conditions changed in the mid-1960s with a Sea (Meincke etal., 1992), which again has an impact on drastic cooling and freshening of the upper water the Atlantic inflow to the Nordic Seas. 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 south­ Cod and hydrography east coast of Iceland experienced the worst ice years since early in this century. Icelandic cod matures to first spawning at an age of Since this cold period, which lasted through the rest of around 7 years (Schopka, 1994; Jönsson, 1982). Spawn­ the 1960s, conditions have been more changing. There ing takes place in warm water to the south and southwest have been four short periods with an Atlantic regime, of Iceland during the period March to May. Hatching around 1973, 1980, 1985, and 1992, but temperatures occurs after 2 to 3 weeks, while eggs and larvae drift have usually been below 5°C and salinities have hardly northward along the west coast. By August most of the exceeded 35. Polar regimes occurred in four years, 1975, spawning products occur as 0-group cod on the North 1977, 1979, and 1988, while Arctic regimes occurred Icelandic Shelf and, often, a smaller portion drift toward during 1981-1983 and 1989-1990. On average, the East Greenland in the western branch of the Irminger period from 1964 to the present has been considerably Current. Young cod is distributed on the Icelandic shelf, colder and fresher than the 40 years between 1924 and mainly off the northwest, north, and east coasts. Adult 1964. The Arctic regimes in 1981-1983 and in 1989-1990 year classes feed largely in the same areas, but have were both related to low salinities in the Arctic Inter­ spawning migrations back to the warm waters. mediate Water returned from the West Spitsbergen Estimates of the recruitment to the Icelandic cod Current (Malmberg et al., 1990; Malmberg and Krist­ stock since 1952 are shown in Figure 5 given as assess­ mannsson, 1992). In 1981-1983 the low salinities were ments of year-class strength at the age of 3 years. The due to the return of the “Great Salinity Anomaly” values, however, are entered in the years when the year (Dooley et al., 1984; Dickson and Blindheim, 1984; classes were spawned. Indices on 0-group estimates of Dickson et al., 1988). cod in August 1970-1992 in Icelandic waters are also Current measurements in the core of the eastern shown in Figure 5, but are not discussed further in this branch of the Irminger Current off Kögur during the paper (see Astthorsson et al., 1994). years 1985-1990 reveal stronger currents during the The figure shows interannual variations in year-class Atlantic period from 1985 to 1987 than during the Polar strength throughout the period, but since 1970 there has and Arctic regimes in the following three years. This been a decreasing trend in the weaker year classes. All indicates that the change from an Arctic to an Atlantic strong year classes after 1966 were recruited either regime is not only due to changes in properties of the within Atlantic periods on the North Icelandic Shelf, or inflowing water masses, but also to larger volume trans­ in years with increasing temperature prior to such port in Atlantic inflow to the shelf north of Iceland periods. Hence, when conditions started to improve (Kristmannsson, 1991). after the Polar period in the late 1960s, a strong year In general, the influence of warm Atlantic Water has class was recruited in 1970, and the 1973 year class, decreased in the northern North Atlantic since its most which is the strongest in the time series, was produced in extensive distribution during the period from about 1930 the warmest year of the Atlantic period from 1972 to to 1960 (Smed, 1975; Rodewald, 1967, 1972). There has 1974. Strong year classes were also recruited in 1983 and thus been a general cooling trend since the 1950s from 1984 at the beginning of the Atlantic period from 1984 to West Greenland and Newfoundland waters in the west 1987 (Malmberg and Kristmannsson, 1992). The fluctu­ to the Barents Sea in the east (Malmberg and Svansson, ations in year classes during the long Atlantic period 1982). In North Icelandic waters this has brought about before 1965 show that a warm environment is not the more variable and, in general, weaker Atlantic inflow. only prerequisite for the production of strong year Instead, periods with cold and fresh Arctic and Polar classes. However, there does seem to be a closer re­ Water have been more prevailing, even with drift ice as lationship between Polar and Arctic periods and weak 306 S.-A. Malmberg and J. Blindheim ICES mar. Sei. Symp., 198 (1994) year classes as recruitment was generally low in the During Atlantic periods in North Icelandic waters, periods with such conditions after 1965, although there surface temperatures are relatively high and decrease were exceptions in 1966 and 1975 when relatively strong gradually with depth. The mixed layer will deepen year classes were recruited simultaneously with Polar relatively slowly and supply nutrients to the productive regimes in North Icelandic waters. layer over a relatively long period. The primary pro­ Standing alone, the relationship in the present, rela­ duction will have a relatively high and long-lasting spring tively short time series may not be absolutely convinc­ bloom with a more moderate production later in the ing, but it is in agreement with similar relations in other season (Thördardöttir, 1977, 1986; Stefânsson and cod stocks. In this context the most obvious relationship Ölafsson, 1992). This will be reflected in the secondary is observed in the West Greenland cod stock, which lives production of zooplankton (Astthorsson et al., near the lower limit of the temperature range for the 1983). species. This population was reduced almost to de­ During an Arctic regime temperatures are generally pletion when temperatures decreased during the 1960s below 3°C and there is not much stratification in the and 1970s (Blindheim, 1974; Hovgaard and Buch, water column during winter. Because of this the forma­ 1990). In Northeast Arctic cod in the Barents Sea, tion of a stratified surface layer by warming in spring Sætersdal and Loeng (1987) found similar relationships may be somewhat slower than in an Atlantic regime, but with most year classes of strong or medium abundance still, development of blooming during spring and sum­ occurring in the early part of a warm period or shortly mer may not be much different from that in an Atlantic prior to a shift to a warmer regime. It therefore seems regime, although species composition may be different. reasonably likely that the present time series indicates Observations show that primary production may be similar relationships in North Icelandic waters. large (Malmberg, 1986, 1988), but abundance of zoo­ The mechanism behind the survival rate of larval and plankton is generally much lower than in Atlantic early juvenile stages may lie in the feeding conditions in regimes. Furthermore, the zooplankton population will the nursery area. Ellertsen et al. (1989) suggested a have a considerably larger component of Arctic species temperature-induced timing of the first-feeding prey, than in Atlantic periods (Astthorsson et al. 1983) and the the nauplii of Calanus finmarchicus, as a possible link spawning of C. finmarchicus will be delayed and early between temperature and recruitment success for juvenile cod may not find suitable food items on the Northeast Arctic cod, which has its main spawning area North Icelandic Shelf. in Lofoten. In this cod stock, spawning is fixed at the In a Polar regime, conditions for plankton production same time from year to year, while low temperatures will be different. Here there is a surface layer with low may delay the spawning of C. finmarchicus, with a density and relatively strong density gradients before the difference of up to 6 weeks between warm and cold warming in spring starts. The warming will therefore years. As the survival of cod during its early larval stage result in a shallow surface layer with a short and intense depends on the presence of suitable concentrations of spring bloom (Thördardöttir, 1977; Stefânsson and food items, e.g. nauplii, such time shifts in plankton Ölafsson 1992). The nutrients will be quickly depleted spawning may result in serious mass starvation owing to and deepening of the layer will be modest because of the lack of prey at the most critical life stage. Although sharp pycnocline below the mixed layer. Blooming later conditions for the Icelandic cod stock are not identical to in the summer will therefore be almost negligible. The those of Northeast Arctic cod, the two stocks have plankton fauna will have a relatively large component of several conditions in common. Both stocks live in outer Polar and Arctic species which may be less suitable food branches of the North Atlantic Current system. North­ items for juvenile cod. In North Icelandic waters the east Arctic cod depends on the Norwegian Atlantic decrease in abundance of zooplankton is considerable Current and its branching into the Barents Sea, while (Astthorsson et al., 1983). Also low temperatures as Icelandic cod depends on the Irminger Current and its such may be an important factor. Cod as a species branch into North Icelandic waters. C. finmarchicus belongs in temperate and boreal regions and adult cod dominates the zooplankton population in both regions avoids low temperatures and seldom occurs in abun­ (e.g. Astthorsson et al., 1983; Blindheim and Skjoldal, dance in temperatures near 0°C. Although it is claimed 1993). It therefore seems likely that simultaneous pro­ that cod larvae and juveniles adapt better to low tem­ duction of cod larvae and Calanus nauplii is an import­ peratures than adult fish (Goddard et al., 1992), there ant factor for the survival success of Icelandic cod too. are also reports of increased mortality of 1- and 2-group An ideal situation would probably be one in which the cod during extremely cold winter temperatures (Pono­ planktonic spawning products and the fish larvae are marenko, 1984). The overall effect of low temperatures, advected in the same volume of water, where the cod directly and indirectly, may be concluded to have an larvae and their prey have a parallel growth. adverse impact on the recruitment. ICES mar. Sei. Symp., 198 (1994) Climate, cod, and capelin in northern waters 307

Capelin and hydrography growth of 0-group capelin during Polar conditions (Vil- hjâlmsson, 1983, 1994; Malmberg, 1986, 1988). The Icelandic capelin spawns at the age of 3-4 years in Furthermore, the abundance of capelin is reflected in late winter in the coastal waters south of Iceland (Vil- the growth of cod, which feeds on capelin (Pälsson, hjâlmsson, 1983, 1994). From there the larvae drift 1983). The weight of five-year-old cod is also entered in westwards and northwards into the nursery grounds in Figure 6 and the relationship with the abundance of the Denmark Strait and in North Icelandic waters. capelin is fairly good. Although the temperature fluctu­ During the following years the capelin feeds mainly in ations will have a similar trend (Fig. 5), with a similar the area between the Polar and Arctic fronts in the physiological effect on growth, it is unlikely that this Iceland Sea, until it migrates southwards again, mainly alone will result in the differences shown in Figure 6. along the shelf break north and east of Iceland, to the The most likely reason for the low abundance of capelin spawning areas. Capelin suffers mass mortality after and, indirectly, the low growth of cod, is the reduced spawning. zooplankton biomass during Arctic periods (Astthors­ Hydrographic fluctuations in North Icelandic waters son et al., 1983; Malmberg, 1986, 1988). Similar effects also affect the living conditions of capelin. Assessments are also observed in the Iceland Sea, where adult capelin of capelin stock abundance are available for the period feeds during summer. Abundance and distribution of since 1978 and entered in Figure 6. Maximum salinity in plankton are therefore probably important links be­ the upper 300 m of the water column at Station S3 (for tween environment and abundance and growth of cape­ location, see Fig. 2) is also entered in the figure. Here lin (Jakobsson, 1992). Growth of cod, however, seems salinities above 34.9 indicate Atlantic periods, while not to be very dependent on hydrographic conditions, lower salinities represent Arctic Water. The salinity but more on abundance of capelin (Malmberg, 1986; trends are similar to the salinity curve from 50 m, which Steinarsson and Stefânsson, 1991). 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 Effects of fishing in relation to capelin and salinity is reasonably good, although not environment perfect, and a very low abundance of capelin during the Arctic periods 1981-1983 and 1989-1990 is clear. In There are indications that the widespread cooling since 1981-1983 in particular the abundance was at such a low about 1960 has entailed adverse effects on cod stocks in level that fisheries were stopped in 1982. In these years the Northern North Atlantic as a whole. While this is the decline in abundance may have begun before the most recorded for the West Greenland cod stock (Blind- Arctic period, i.e. in the Polar years 1979 and 1988, as heim, 1974; Hovgaard and Buch, 1990), there have been there are indications of low survival rate and reduced decreases in the abundance also in other North Atlantic

3 --35,0 -3,0

tonnes

2 --34,9

-2,5

I --34,1

2,0 1980 1985 1990

Figure 6. a. Maximum salinity observed in the upper 300 m at Station S3 in North Icelandic waters in May/June 1978-1992 (solid thin line), b. The abundance of Icelandic capelin stock (t) in 1978-1992 (solid thick line; Vilhjâlmsson 1994). c. Weight of 5-year- old cod (kg) in Icelandic waters in 1977-1993 (broken line; Anon., 1993). 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 1960 1990 1960 1990 1960 1990

Iceland 450 300 300 100 700 200 West Greenland 400 100 400 100 1000 100 Newfoundland 600 300 1000 150/500 1000 300 Northeast Arctic 1000 200/500 1500 300 600 250 Sum 2450 1 200 3200 1000 3 300 850

cod stocks over recent decades. This seems to be import­ cation of this is given in the relation between the decline ant especially for stocks which live near the lower limit of in catches and the decreases in recruitment and spawn­ the temperature range for cod, e.g. off Labrador/ ing stock in these cod stocks (see Table). Newfoundland, West Greenland, and even North Ice­ Summed up, for all these fishing grounds in the landic waters. In these areas it seems that the environ­ northern North Atlantic there has been a decline, from mental conditions again must improve before stocks and 1960 to 1990, in catches of about 50%, in recruitment of catches can be expected to be of the same strength as about 67%, and in spawning stocks of about 75%. The before the mid or late 1960s. Here it is pertinent to ask to smaller decrease in catch than in recruitment and spawn­ what extent the decrease in the cod stocks is due to the ing stock, in particular, indicates an increasing fishing deteriorating ocean climate or overfishing. Some indi­ load from 1960 to 1990.

2 5 0 0

2000

Figure 7. Conditions of the Icelandic cod stock 1955-1991/92. 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. ICES mar. Sci. Symp., 198 (1994) Climate, cod, and capelin in northern waters 309

For Icelandic cod a similar indication appears from Growth of adult cod is dependent more on food supply, Figure 7, which shows recruitment by 3-year-old fish, such as abundance of capelin, than on hydrographic spawning biomass, fishing stock biomass and catches conditions. since 1955. Before 1980 the recruitment of strong year Cold years in North Icelandic waters as well as in the classes was reflected in increased fishing-stock biomass waters off Labrador, West Greenland, and in the and, but to a lesser extent, increased spawning stock. It Barents Sea, seem generally to give weak year classes is, however, noteworthy that strong 3-year-old rec­ of cod. ruiters in 1986 and 1987 (year classes 1983 and 1984) did The critically small spawning stock in Icelandic waters not result in an increased spawning stock or catch a few during recent years has given weak year classes during years later despite Atlantic conditions in North Icelandic both cold and warm years. waters, but contributed in catch already in 1986-1988. Consequently, a good part of these year classes were not References allowed to develop to maturity. It is difficult to say which of the factors, fishery or Aagaard. K. 1972. On the drift of the Greenland Pack Ice. Sea environment, is the more decisive in relation to the Ice. Proc. Int. Conf. Rvfk. 1971 R.r. 72(4): 17-22. Ed. Th. Karlsson. decline in the cod stocks. The crucial difference between Aagaard, K., and Carmack, E. C. 1989. The role of the sea ice them is that while we are unable to do anything about and other fresh water in the Arctic circulation. J. Geophys. environmental fluctuations, we should be able to man­ Res., 94, CIO: 14484-14498. age the fishery. Careful regulation is particularly import­ Anon. 1993. State of Marine Stocks and Environmental Con­ ditions in Icelandic Waters 1993. Hafrannsöknir Fjölrit, 34: ant when a fish stock is weak as a result of unfavourable 154 pp. environmental conditions. At worst, an efficient trawl­ Astthorsson, O. S., Hallgrimsson, I., and Jönsson, G. S. 1983. ing fishery could easily exploit a spawning stock below a Variations in zooplankton densities in Icelandic waters in critical level of abundance, from which point it may take spring during the years 1961-1982. Rit Fiskideildar, 7(2): 73- many years for the stock to be restored. 113. Astthorsson, O. S., Gi'slason, Ä., and Gudmundsdôttir, A. 1994. Distribution, abundance, and length of pelagic juvenile cod in Icelandic waters in relation to environmental con­ Concluding remarks ditions. ICES mar. Sci. Symp., 198: 529-541. Blindhcim, J. 1974. On the hydrographic fluctuations in the Time series of oceanographic observations reveal vari­ Labrador Sea during the years 1959-1969. FiskDir. Skr. Ser. ability in processes such as stratification and advection HavUnders., 16: 194-202. Blindheim, J., and Skjoldal, R. H. 1993. Effects of climatic over regional and large ocean areas in connection with changes on the biomass yield of the Barents Sea, Norwegian air-sea interaction. They contribute to a better under­ Sea, and West Greenland large marine ecosystems. Large standing of the processes underlying the oceanic and marine ecosystems, stress, mitigation and sustainability, pp. biological variability. The authors believe that a major 185-198. Ed. by K. Sherman, L. M. Alexander, and B. D. Gould. Am. Assoc. Adv. Sci., Washington, DC. factor concerning the variation in size and behaviour of Buch, E., Malmberg, S.-A., and Kristmannsson, S. S. 1992. fish stocks, at least along the Arctic and Polar fronts, is Arctic Ocean deep water masses in the western Iceland Sea. the maritime climate, which influences the living con­ ICES CM 1992/C: 2. ditions including spawning, feeding, recruitment, and Dickson, R. R., and Blindheim, J. 1984. On the abnormal maturation. The mechanisms behind this may not be hydrographic conditions in the European Arctic during the 1970s. Rapp. P.-v. Réun. Cons. int. Explor. Mer. 185: 201- well understood. Temperature may have a direct effect 213. on fish stocks or be associated with variable current Dickson, R. R., Lamb, H. H., Malmberg, S.-A., and Cole- activities such as climatic events and turbulence and brook, J. M. 1975. Climate reversal in the northern North other physical parameters, which again affect the mul­ Atlantic. Nature, 256: 479-482. Dickson, R. R., Meincke, J., Malmberg, S.-A., and Lee, A. J. tiple living conditions. The overall results of this paper 1988. The “Great Salinity Anomaly” in the Northern North may be summarized as follows: Atlantic 1968-1982. Prog. Oceanogr., 20: 103-151. Dooley, H. D., Martin, J. H. A ., and Ellett, D. J. 1984. There has been a dramatic decline of cod stocks in the Abnormal hydrographic conditions in the Northeast Atlantic during the 1970s. Rapp. P.-v. Réun. Cons. int. Explor. Mer, northern North Atlantic during recent decades. In 185: 179-187. parallel with climatic changes in the northern North Ellcrtsen, B., Fossum, P., Solemdal, P., and Sundby, S. 1989. Atlantic during the same period, the spawning, feed­ Relation between temperature and survival of eggs and first- ing, and fishing grounds of cod in these waters seem to feeding larvae of northeast Arctic cod (Gadus morhua L.). have declined since the 1960s. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 209-219. Goddard, S. V., Kao, M. H., and Fletcher, G. L. 1992. Capelin stocks reflect great interannual variance in Antifreeze production, freeze resistance and overwintering abundance and growth, depending on environment of juvenile northern Atlantic cod (Gadus morhua). Can. J. and feeding conditions. Fish, aquat. Sci., 49: 516-522. 310 5.-/4. Malmberg andJ. Blindheim ICES mar. Sci. Symp., 198 (1994)

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