ARTICLE IN PRESS

Deep-Sea Research II 54 (2007) 2456–2477
www.elsevier.com/locate/dsr2

Climate variability and the Icelandic marine ecosystem
Olafur S. Astthorssona,, Astthor Gislasona, Steingrimur Jonssona,baMarine Research Institute, Skulagata 4, P.O. Box 1390, Reykjavik, Icelandb
University of Akureyri, Borgir v/Norjurslod, 600 Akureyri, Iceland
Received in revised form 18 January 2007; accepted 31 July 2007
Available online 5 November 2007

Abstract
This paper describes the main features of the Icelandic marine ecosystem and its response to climate variations during
the 20th century. The physical oceanographic character and faunal composition in the southern and western parts of the
Icelandic marine ecosystem are different from those in the northern and the eastern areas. The former areas are more orless continuously bathed by warm and saline Atlantic water while the latter are more variable and inﬂuenced by Atlantic,
Arctic and even Polar water masses to different degrees. Mean annual primary production is higher in the Atlantic water
than in the more variable waters north and east of Iceland, and higher closer to land than farther offshore. Similarly,
zooplankton production is generally higher in the Atlantic water than in the waters north and east of Iceland. The main
spawning grounds of most of the exploited ﬁsh stocks are in the Atlantic water south of the country while nursery grounds
are off the north coast. In the recent years the total catch of ﬁsh and invertebrates has been in the range of
1.6–2.4 million ton. Capelin (Mallotus villosus) is the most important pelagic stock and cod (Gadus morhua) is by far themost important demersal ﬁsh stock. Whales are an important component of the Icelandic marine ecosystem, and Icelandic
waters are an important habitat for some of the largest seabird populations in the Northeast Atlantic.
In the waters to the north and east of Iceland, available information suggests the existence of a simple bottom-up
controlled food chain from phytoplankton through Calanus, capelin and to cod. Less is known about the structure of themore complex southern part of the ecosystem. The Icelandic marine ecosystem is highly sensitive to climate variations as
demonstrated by abundance and distribution changes of many species during the warm period in the 1930s, the cold period
in the late 1960s and warming observed during the recent years. Some of these are highlighted in the paper.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Biomass; Climate variability; Icelandic marine ecosystem; Production

1. Introduction
This paper presents a description of the main
components of the Icelandic marine ecosystem andtheir response to climatic variation. The physicaloceanographic character and faunal composition inCorresponding author. Fax: +354 575 20 54.

E-mail address: osa@hafro.is (O.S. Astthorsson).
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dsr2.2007.07.030

the southern and western parts of the Icelandic
marine ecosystem differ from those in the northern
and the eastern areas. The former areas are bathedby warm and saline Atlantic water while the latterare inﬂuenced by Atlantic, Arctic and even Polarwater masses, and subject to larger interannual
variation (Malmberg and Valdimarsson, 2003;
Jonsson and Valdimarsson, 2005a). The distributionof many marine organisms reﬂects the differenceARTICLE IN PRESS
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between the Atlantic and the Arctic environments(e.g., Gislason and Astthorsson, 2004), but others,
such as most of the commercially important ﬁshstocks, exploit both the areas by spawning in the
Atlantic water and growing up in the waters off thenorth and northeast coasts (Astthorsson et al.,1994). Therefore, in spite of the differences in thecurrents and water masses around Iceland, theycreate an environment that is generally quite
continuous in its nature (Astthorsson and Vilhjalmsson, 2002; Gaard et al., 2005).
The emphasis in this paper is on the ecosystemeffects of climate ﬂuctuations in the more variable
waters to the north of Iceland. However, due to the
distribution of many organisms within both the
Atlantic and the Sub-Arctic ecotypes and the lack ofdetailed regional information, the discussion is inmany cases in the wider context of Icelandic waters
as a whole. Even in that, basic information is stilllacking on some major components of the Icelandic

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marine ecosystem and their response to climate
variability. Some of these gaps in knowledge are
discussed in the concluding remarks of the paper
and will be addressed in the coming years as an
Icelandic component of the planned Ecosystem
Studies of Sub-Arctic Seas (ESSAS) Programme(Hunt and Drinkwater, 2005).
2. Oceanographic features
Iceland is located at the junction of the MidAtlantic Ridge and the Greenland–Scotland
Ridge just south of the Arctic Circle (Fig. 1).
The shelf surrounding Iceland mainly follows the
400–500 m depth contour with the total area within
the 500 m isobath being about 212,000 km2.
The shelf is narrowest off the south coast while
off the west, north and east coasts it is relatively
broad, extending for 100–150 km from the coast(Fig. 1).

Fig. 1. Surface ocean currents around Iceland. The green lines show hydrographic monitoring transects (Siglunes, north of Iceland;
Selvogsbanki, south of Iceland) and the red dot indicates the position of the meteorological station at Thverfjall. Modiﬁed from
Vilhjalmsson (2002).

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The deep Iceland Basin to the south of Iceland, isseparated in the west from the Irminger Sea by the
Reykjanes Ridge, part of the Mid-Atlantic Ridge,
and in the east from the Norwegian Sea by the
Iceland–Faroe Ridge. North of Iceland the Kolbeinsey Ridge is a continuation of the Mid-Atlantic
Ridge. It stretches to the Jan Mayen Fracture Zone,
between Jan Mayen and Greenland, which marks
the northern limit of the Iceland Sea and separates itfrom the Greenland Sea. South from Jan Mayen,
the Iceland–Jan Mayen Ridge extends to the
Iceland–Faroe Ridge and separates the Iceland
Sea from the Norwegian Sea to the east.
The submarine ridges inﬂuence the oceanic
circulation and water mass distributions around
Iceland (Stefansson, 1962; Jonsson and Valdimarsson, 2005a) and consequently the biological production and distribution of the marine populations(Thordardottir, 1994; Astthorsson and Vilhjalmsson, 2002; Gislason and Astthorsson, 2004; Gislason, 2002, 2005). The Greenland–Scotland Ridge isof particular importance since it limits the exchangeof water through Denmark Strait and over the
Iceland–Faroe Ridge, preventing direct connectionof waters below their threshold depths of 620 m and
550 m, respectively. Topography also determines the
position of the fronts separating the water masses
thereby inﬂuencing the associated biological processes and species composition, which tend to differ
on either side of the fronts (Poulain et al., 1996;
Valdimarsson and Malmberg, 1999; Gislason andAstthorsson, 2004).
Iceland is surrounded by two primary water
masses with very different origins and properties(Fig. 1). Originating far south in the North Atlantic,
warm and saline Atlantic water is brought towards
the southern shores of the country. It then ﬂows
westward as the Irminger Current and eventuallynorth along the west coast. Most of this water turns
west towards Greenland and subsequently ﬂows
southwestward along the slope off Greenland. A
smaller branch continues northwards onto the north
Icelandic shelf area as the North Icelandic Irminger
Current (Valdimarsson and Malmberg, 1999).
South of Iceland the temperature of the Atlantic
water ranges from 6 to 11 1C depending on theseason but the salinity is typically between 35.0
and 35.2.
The other primary water mass is Polar water,which originates in the Arctic Ocean. Relatively
fresh (So34.5) and very cold (To0 1C), it ﬂows outof the Arctic Ocean through Fram Strait between

Spitsbergen and Greenland as the East Greenland
Current. Below the Polar water the East Greenland
Current transports remnants of Atlantic water fromthe Norwegian Atlantic Current that crosses the
Fram Strait and subducts under the Polar water.
The East Greenland Current, which also carries sea
ice from the Arctic, ﬂows along the shelf andcontinental slope off East Greenland and mainly
exits the Nordic Seas through the Denmark Strait.
Mixing and cooling of different proportions ofAtlantic and Polar waters form almost all the otherwater masses in the area. An exception is the lowsalinity coastal water mass in summer caused by
freshwater run-off that circulates clockwise around
Iceland with the general circulation. The cold andlow-salinity East Icelandic Current, which ﬂows
southeastward along the northeast Icelandic continental slope, carries with it Arctic water that is a
mixture of water derived from the inﬂow of Atlantic
water to the Iceland Sea south of Jan Mayen, Polarwater from the East Greenland Current andAtlantic water from the North Icelandic Irminger
Current (Stefansson, 1962; S. Jonsson, 1992; Swiftand Aagaard, 1981). As a result of the deep-water
formation north of the Greenland–Scotland Ridge,
there are strong bottom currents through Denmark
Strait (Jonsson and Valdimarsson, 2004; Macrander
et al., 2005) and over the Iceland–Faroe Ridge eastof Iceland (Perkins et al., 1998). These currents areimportant constituents of the thermohaline circulation of the world oceans.
Temperature and salinity conditions are variable,
particularly over the shelf north of Iceland, where
the Polar Front separates the contrasting Atlantic
and Polar water masses (Stefansson, 1962; Malmberg and Valdimarsson, 2003). This is mainly due tolarge variations in the relative amounts of Atlantic
and Polar water over the northern shelf (Jonsson
and Valdimarsson, 2005b). Atlantic water is usuallyevident as a tongue of relatively warm and saline
water but the temperature and salinity decrease in
the direction of the ﬂow due to mixing with colder,less-saline water from the north and freshwater runoff from land. For over 50 years a hydrographic
section across the shelf north of Iceland (Siglunes
transect; Fig. 1) has monitored the marine climaticconditions in the area (Fig. 2). After a generally
warm period in the northern North Atlantic and in
the waters to the north of Iceland from 1920 to
1964, the period from 1965 to 1971 was characterized by low temperature and salinity and often
accompanied by sea ice on the northern shelf

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Fig. 2. Temperature and salinity deviations in late spring from the 1961 to 1980 average (3.288 1C and 34.727) on the Siglunes transect (seeFig. 1) to the north of Iceland (the deviation is based on ﬁve stations between 4 and 85 km offshore and depths of 0, 20, 50, 75, 100, 150
and 200 m). Based on data in Anon (2005b).

(Malmberg, 1986). Temperature differences in thewaters north of Iceland between these two periods
were up to 4 1C (Malmberg and Jonsson, 1997).
Between 1971 and 1995 ‘‘warm’’ (1972–1974, 1980,
1984–1987 and 1991–1994) and ‘‘cold’’ (1975, 1977,
1979, 1981–1983, 1989, 1990, 1995) periods of 1–4
years alternated. During 1996 temperatures north of
Iceland increased again and since then the temperature has been relatively high, indicating an intensiﬁed ﬂow of Atlantic water to the area (Malmberg
and Valdimarsson, 2003; Jonsson and Valdimarsson, 2005b).
In the warm Atlantic water south of Iceland
the marine climate is more stable, although temperature and salinity observations (Selvogsbankitransect; Fig. 1) have demonstrated some interannual variations (Malmberg and Valdimarsson, 2003).
For example, salinity was particularly low during
the periods 1974–1978, 1985–1988, and again

1992–1995. These periods have been related to‘‘Great Salinity Anomalies’’ in the northern North
Atlantic (Malmberg, 1985; Dickson et al., 1988;
Belkin et al., 1998; Belkin, 2004). Also, there has
been a gradual increase in salinity and temperaturesince 1996 (Anon, 2005b; Malmberg and Valdimarsson, 2003). The increased temperature andsalinity both north and south of Iceland in the
recent years are a part of a larger-scale change
observed in the North Atlantic Ocean (Anon,
2004a).
Atmospheric patterns inﬂuence the ocean circulation around Iceland. Northerly winds, as measured
at Thverfjall in northwest Iceland, tend to reduce
the ﬂux of Atlantic water through the Denmark
Strait and onto the north Icelandic shelf (Fig. 3),
whereas southerly winds increase the ﬂow (Jonsson
and Valdimarsson, 2005b). This is in accordancewith the strong correlation between the spring

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Fig. 3. Relationship (po0.01) between monthly ﬂux of Atlantic
water through Denmark Strait in Sverdrups (1 Sv ¼ 106 m3 s1)
and the monthly north-south component of the wind at Thverfjall(see Fig. 1 for location) for the period 1994–2001.

Fig. 4. Relationship between freshwater thickness in spring
within the East Icelandic Current and wind-stress curl over the
Iceland Sea averaged over 1-year period prior to the salinity
measurement. The numbers above the points indicate the year of
the salinity measurement. From S. Jonsson (1992).

temperature at Siglunes and the atmosphericpressure gradient across the Denmark Strait foundby Blindheim and Malmberg (2005). Olafsson(1999) earlier reported a signiﬁcant relationshipbetween hydrographic conditions in spring at the
Siglunes transect and the frequency of local northerly/southerly wind directions but no correlation
with the North Atlantic Oscillation (NAO) index.
S. Jonsson (1992) further demonstrated that theamount of freshwater within the East Icelandic
Current was related to the local wind-stress curlover the Iceland Sea (Fig. 4). This freshwatercontent is a measure of the amount of Polar water
within the Iceland Sea and affects the convection as
well as the biological processes, such as the onset
and magnitude of primary production north of
Iceland (Thordardottir, 1984).
The above examples indicate that the atmosphericforces driving the observed seasonal and interannual variations in the ocean climate north of Icelandseem to be more local in origin rather than being
associated with large-scale atmospheric patterns
such as the NAO. This is probably because theNAO index is mainly related to the westerly winds
blowing across the Atlantic at mid-latitudes to thesouth of Iceland.
Most of the sea ice that reaches Iceland originatesfrom the Arctic Ocean and is transported south-

wards by the East Greenland Current along the east
coast of Greenland, although during winter some
ice is also formed along the way (Wadhams, 1986).
In most years, sea ice on the Icelandic shelf is only atemporary wind-dependent feature (Jakobsson,2004), usually ﬁrst seen to the northwest and thentransported along the north coast as part of thegeneral clockwise circulation around the country.
A normal inﬂow of Atlantic water over the northern
shelf usually melts any ice quite quickly, but if the
ﬂow of Atlantic water is blocked and Polar water
persists over the shelf, sea ice can drift to the east
coast and in extreme cases as far as the south coast.
This occurred during the cold period between 1965
and 1971 with drastic effects on the ecosystem(Malmberg, 1986). Sea-ice records at Iceland during
the past 120 years generally indicate increased ice
coverage during cold periods and reduced ice inwarm periods (Figs. 2 and 5).
3. Phytoplankton
Diatoms of the genera Thalassiosira spp. and
Chaetoceros spp. typically dominate the phytoplankton spring bloom over the Icelandic shelf(Thordardottir and Gudmundsson, 1998). Duringsome years, however, the prymnesiophyte Phaeocystis pouchetti may be abundant in the waters to

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300

Koch´s index

250
200
150
100
50
0
1880

1900

1920

1940

1960

1980

Year
Fig. 5. Koch’s (1945) sea-ice index near Iceland 1880–1996. The index is the product of the number of weeks with ice per year and thenumber of coastal areas near which it was observed. Based on data from Thor Jakobsson, Icelandic Meteorological Ofﬁce.

25°

20°

15°

10°

68°

67°

66°

65°

64°

<75
75-100100-150150-200
200-300>300

63°

Fig. 6. Average annual primary production (gC m2 yr1) in Icelandic water based on data from 1958 to 1982. Redrawn fromThordardottir (1994).

the north of Iceland in spring. Dinoﬂagellates of
the genera Ceratium spp. and Protoperidinium spp.
increase in abundance after the spring bloom while
diatoms continue to be relatively abundant. In theautumn there is usually a second bloom of diatoms
although dinoﬂagellates may still be abundant(Thordardottir and Gudmundsson, 1998).
On the basis of long-term (1958–1982) production
measurements, Thordardottir (1994) estimated theaverage annual primary productivity in Icelandic
waters (Fig. 6). Over the southern and western
shelves the primary production tends to decreaseseawards while over the northern shelf the productivity decreases from west to east, i.e., in the

direction of diminishing admixture of Atlantic
water. Relatively high primary production is foundin the frontal areas southeast and northwest of
Iceland. The annual primary production is generally
higher in the warm Atlantic water off the south andwest coasts (200–300 gC m2 yr1) than in the colder
Sub-Arctic/Arctic water off the north and east
coasts (100–200 gC m yr1) (Fig. 6).
Variable hydrographic conditions, particularly
over the northern shelf, result in marked interannual changes in the spring development of thephytoplankton (Thordardottir, 1976, 1977, 1984,1986). Primary production estimates at stations withsalinity 434.5 and o34.5 demonstrate that the

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25

Productivity (mg C m-3 h-1)

Salinity > 34.5
Salinity < 34.5

20

15

10

5

0
61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93

Year
Fig. 7. Comparison of mean productivity (mgC m3 h1) in the
shelf region northeast of Iceland during spring 1961–1993 atstations with salinity 434.5 and stations with salinity o34.5.
Adapted from Gudmundsson (1998).

spring production is, on the average, about 2.5 times
higher at the high-salinity stations (Fig. 7). During
the years of limited Atlantic inﬂow, nutrienttransport to the northern shelf is less, the watercolumn stabilizes sooner after the initial spring
bloom due to less dense low-salinity water, nutrients
become exhausted and the bloom ends abruptly,
leading to low production during the summer(Thordardottir, 1977, 1984; Stefansson and Olafsson, 1991; Gudmundsson, 1998; Astthorsson andVilhjalmsson, 2002). These variations in primary
productivity seem to cascade to higher trophic levels
as demonstrated for zooplankton and capelin by
Astthorsson and Gislason (1998a).
4. Zooplankton
In terms of numbers of individuals, copepods
dominate the mesozooplankton of Icelandic waters
with Calanus finmarchicus being the most abundantspecies, often comprising between 60% and 80% of
net caught zooplankton in the uppermost 50 m
(Astthorsson et al., 1983; Gislason and Astthorsson,
1995, 1998, 2000; Astthorsson and Gislason, 1992,
1999; Gislason, 2002). Other copepod species
occurring regularly over the shelf around Iceland
are Pseudocalanus spp., Acartia longiremis andOithona spp., while some species are more conﬁned
to the Atlantic water (e.g., Temora longicornis,
Centropages hamatus) or to the Polar water (e.g.
Metridia longa, Calanus hyperboreus, Calanus glacialis) (Jespersen, 1940; Hallgrimsson, 1954; Gislason and Astthorsson, 1995, 1998, 2004; Astthorsson

and Gislason, 2003). The euphausiids Thysanoessa
raschi and Thysanoessa inermis are common around
Iceland, with T. raschi being mostly conﬁned to
the fjord areas while T. inermis is the dominanteuphausiid in the waters over the shelves (Einarsson, 1945; Astthorsson, 1990; Gislason and Astthorsson 1995; Astthorsson and Gislason, 1997). In
addition, the euphausiids, Meganyctiphanes norvegica and Thysanoessa longicaudata, are mainly
found near the shelf edge in oceanic water to thesouth and west of Iceland (Einarsson, 1945).
Since the early 1960s, monitoring of zooplankton
biomass in the upper 50 m in Icelandic waters has
been carried out on standard transects during
May–June (Astthorsson et al., 1983; Astthorsson
and Gislason, 1995, 1998a; Beare et al., 2000).
C. finmarchicus is the dominant species of theplankton community, and therefore the biomass
mainly reﬂects the biomass of this species. The
spring zooplankton biomass generally ranges
from ca. 1–10 g dry wt m2, with an average of
2–4 g dry wt m2 (Fig. 8). Higher biomass is usuallyobserved in shelf waters off the south and west
coasts, in the oceanic waters north and northeast of
Iceland where Arctic inﬂuence is greatest and large
Arctic species dominate (C. hyperboreus), and in
offshore waters of the Irminger and Norwegian
Seas.
Zooplankton biomass time series in the waters to
the north of Iceland show maxima occurring
approximately every 7–10 years (Fig. 9). Alsostriking is the collapse in zooplankton biomass
during the cold period in the North Atlantic and to
the north of Iceland in the 1960s and it was not until
the warm period in the 1990s that biomass levels
recovered (Astthorsson and Gislason, 1995; Beare etal., 2000). Astthorsson et al. (1983) further reported
that during the cold period between 1965 and 1971
the composition of the spring zooplankton north of
Iceland changed in such a way that C. finmarchicus
was to a degree replaced by C. hyperboreus,
Metridia longa and euphausiid nauplii.
Zooplankton biomass variability to the north of
Iceland is positively related to temperature (Fig. 10),which again reﬂects the inﬂow of Atlantic water into
the area. On the average, zooplankton biomass inthe ‘‘warm’’ years is about 2 times higher than in the‘‘cold’’ years (Astthorsson and Gislason, 1994,
1998a). Astthorsson and Gislason (1998a) andAstthorsson and Vilhjalmsson (2002) noted thatthe greater inﬂow of Atlantic water leading toincreased primary production, results in good

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35°

30°

25°

20°

15°

2463

10°
0 to 1

69°

69°

1 to 267°

2 to 467°

4 to 8

65°

65°

63°

63°
35°

30°

25°

20°

15°

8 to 16

10°

Fig. 8. Average biomass of zooplankton (mg dry wt m2) around Iceland in May–June 1960–2004. Contours are at 200 and 500 m. From
Gislason (2002) plus additional data.

1.0
0.8
log (dw+1)

Log (biomass+1)

0.8

0.4

0.6
0.4
0.2

1960

1970

1980

1990

2000

year
Fig.
9. Variationinspringzooplankton
biomass(log mg dry wt m2) in the waters to the north of Iceland during
1960–2001. The data point for each year is based on observations
from 26 stations on average. The curved line shows a 7-year
running mean.

feeding conditions for zooplankton. Further, thestronger inﬂow of Atlantic water may advect more
zooplankton from south and west, and also thewarm temperatures will promote increased growthand faster development times of zooplankton.
5. Exploited ﬁsh and invertebrate populations
5.1. Annual catch
The annual catch of ﬁsh from Icelandic waters
has increased from around 200,000 ton at the
beginning of the 20th century to about 2 million ton
at present (Fig. 11) and currently about 30 species ofﬁsh and invertebrates are exploited (Astthorsson

0.0
-3

-2

-1

0

1

2

3

Temperature deviation (°C)
Fig. 10. Relationship between spring zooplankton biomass northof Iceland and temperature deviations on the Siglunes transect.
The zooplankton data cover the period 1960–2001 and are basedon 26 observations per year on average.

and Vilhjalmsson, 2002; Anon, 2005a). During thelast ﬁve decades the annual catch of demersal ﬁsh
has ﬂuctuated between 450,000 and 850,000 ton.
Cod (Gadus morhua), haddock (Melanogrammus
aeglefinus), saithe (Pollachius virens), redﬁsh (mainly
Sebastes marinus) and Greenland halibut (Reinhardtius hippoglossoides) are the economically mostimportant demersal ﬁsh species in Icelandic waters
and of these, cod has been by far the mostimportant. Around 1950, pelagic ﬁsheries forherring (Clupea harengus) started to increase. Thisﬁshery, which was mainly based on the AtlantoScandian (also referred to as Norwegian spring
spawning) herring, reached a peak of about
600,000 ton just before a collapse in the late 1960s,

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Fig. 11. The catch of ﬁsh and invertebrates in Icelandic waters during 1905–2004.

which was due to the combined effect of overﬁshingand environmental change (Jakobsson, 1980; Dragesund et al., 1980). After the herring stocks
collapsed, the capelin ﬁshery expanded and in the
recent years has yielded about 1 million ton annually. A capelin collapse in the early 1980s led to aﬁshing ban for almost 2 years and also an abrupt
reduction in the catches was enforced for one season
about 10 years later. Except for these two occasions,the annual yield of the capelin ﬁshery has remained
fairly constant (Fig. 11). In the recent years theabundance of blue whiting (Micromesistius poutassou) has increased markedly in the waters around
Iceland and in 2004, the ﬁshery within the Icelandic
200 mile exclusive economic zone (EEZ) amounted
to about 300,000 ton. These intensiﬁed migrationsof the blue whiting to the waters around Iceland arethought to be partly connected to the climatic
warming that has taken place.
Only ﬁve species of invertebrates, northern
shrimp (Pandalus borealis), Norway lobster (Nephrops norvegicus), Iceland scallop (Pecten islandicus), ocean quahog (Arctica islandica), and common
welk (Buccinum undatum) are currently exploitedcommercially in Icelandic waters and of these theshrimp ﬁshery is the most important. Initially, an
inshore ﬁshery amounting to a few thousand tonnes
per year, during the late 1980s an offshore ﬁsherydeveloped on the outer part of the northern shelf.
Since then, the total shrimp catch has increased
from about 10,000 to 70,000 ton. Recently, however,
the offshore shrimp ﬁshery has declined. This has
been attributed to higher predatory pressure by cod(Anon, 2005a), but could possibly be caused by
reduced recruitment due to recent warming, as themain distribution of northern shrimp is typically inthe colder water on the shelf north of Iceland.
5.2. Capelin
Capelin is unique, being the only stock of highArctic origin exploited in large quantities in
Icelandic waters (Vilhjalmsson, 1994, 2002). It is
the largest pelagic stock in Icelandic waters and also
the most important food source for cod. Capelin isalso consumed in large quantities by most otherdemersal ﬁsh species, whales and seabirds. Capelinspawning takes place off the south and southwest

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break. During this phase, the capelin have therefore
been far less accessible to cod than they used to be
until the late 20th century. Furthermore, in the last
5 years, capelin larval drift routes have changed andthe 0-group drifted north and then west onto theGreenland plateau to the west of the Denmark
Strait instead of onto the north Icelandic shelf.
During this period, practically no capelin havegrown up in Icelandic waters and thus juvenile
capelin have not been available to juvenile cod(Anon, 2005a; Palsson, 1983; Vilhjalmsson, personal communication). Sæmundsson (1934) reported
that, during warming in the late 1920s and early
1930s, the Icelandic capelin became scarce on themain spawning grounds off the south coast of
Iceland while it spawned in great quantities off thenorth coast.

coasts of Iceland in February–March but thenursery and feeding grounds are on the shelves off
the northwest, north and northeast coasts. Feeding
migrations of adults to the deep Sub-Arctic andArctic waters of the Iceland Sea occur during
June–September (Vilhjalmsson, 1994; Astthorsson
and Gislason, 1998b). Upon returning to thenorthern shelf, and until spawning off the southwest
coast, capelin is preyed upon by cod in large
quantities (Palsson, 1983; Magnusson and Palsson,1991). These extensive capelin migrations transfer
large quantities of the zooplankton production in
the Iceland Sea into the Icelandic shelf ecosystem(Vilhjalmsson, 1997a, 2002).
The key role of capelin in the Icelandic marine
ecosystem is clearly demonstrated by the changes
that have occurred in the weight at age of codin response to ﬂuctuations in the capelin biomass(Fig. 12). During the near collapse of the capelin in
the early 1980s and 1990s, the weight of cod, inparticular among age groups 4–8, decreased byabout 25–30% (Vilhjalmsson, 1997a). The conditionof the cod in both periods only improved when thecapelin abundance increased. This indicates that, in
Icelandic waters, cod cannot fully substitute for theloss of the capelin by converting to other foodsources. Most recently, cod condition has beenpoorer than expected given the high capelin biomass(Fig. 12). This has been attributed to a more
northerly distribution and later arrival of adultcapelin to the north Icelandic shelf because of recent
warming north of Iceland. Also, the spawningmigrations northeast and east of Iceland have to alarge extent taken place in deep waters off the shelf5.3. Cod
Cod, the most important exploited demersal ﬁshstock in Icelandic waters, during the past 100 yearsof exploitation has undergone marked changes in
distribution and abundance. These changes are
considered to be caused by variability in recruitment, which is partly a response to environmental
conditions (e.g., Jakobsson, 1992; Astthorsson
et al., 1994; Schopka, 1994; Astthorsson and Vilhjalmsson, 2002; Jonsson and Valdimarsson, 2005b).
More recently, however, the ﬁshery and its effect on
factors such as the age and size composition of thestock seems to be becoming a more important factor
in deciding the development of the stock (Marteinsdottir and Thorarinsson, 1998; Anon, 2005a).

Capelin stock biomass
4.5

Weight 6 yr. Cod

4

1500

3.5

1000

3

500

2.5

0

2
03
20

00
20

97
19

94
19

91
19

88
19

85
19

19

19

82

Kg

2000

79

Thousand tonnes

2500

Fig. 12. Changes in capelin biomass and mean weight of Icelandic cod at the age of 6 years. From Astthorsson and Vilhjalmsson (2002)
plus additional data.

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The Icelandic cod stock spawns in April–May,
mainly in the Atlantic water to the south and westof Iceland. The eggs and larvae then drift withclockwise currents to the nursery and feeding
grounds off the north coast. During some years a
fraction of the larvae may also drift to Greenland
waters, where they spend their juvenile years before
returning to Icelandic waters as mature individuals
to spawn (Astthorsson et al., 1994; Marteinsdottirand Astthorsson, 2005).
Year-class strength (measured as numbers of
3-year olds) of the Icelandic cod stock, including
the component of the year class of immigrants fromGreenland, is presented in Fig. 13. All of the largest
year classes are due to immigrants from Greenland
while pure Icelandic year classes never seem toexceed 300–350 million individuals. Further, amarked change occurred around 1985 with subsequent years frequently producing lower than
200 million recruits. Five extremely poor year
classes (1986, 1991, 1994, 1996 and 2001) wereobserved, at levels previously never encountered(below 100 million individuals).

The warming of the northern North Atlantic in
the early 1920s extended the distribution area of cod
along the east and west coasts of Greenland (e.g.
Vilhjalmsson, 1997b). Combined with the drift oflarval cod from Iceland to Greenland and thesubsequent return of these ﬁsh as mature individuals, this warming is considered to be the main
reason for the outburst of superabundant year
classes at Iceland in the early 1920s (Schopka,
1994). Prior to 1985 immigration of cod fromGreenland took place at fairly regular intervals
while during the last two decades it has not been
observed (Fig. 13). Dickson and Brander (1993)
attributed increased larval exchange between Iceland and Greenland in the ‘‘warm’’ years to astrengthening of the Irminger/West Greenland
Current system. Schopka (1994) further considereda poorer state of the cod stock at Iceland to be apossible reason for less frequent drift of larvae fromthe Icelandic ecosystem and towards Greenland in
the recent years. In spite of favourable environmental conditions in Icelandic waters in the recent
years, and markedly so since 1996 (Fig. 2), theFig. 13. Recruitment (at age 3) and spawning stock biomass (SSB) of the Icelandic cod stock 1920–2002. The recruits are shown separately
for the Iceland (blue bar) and the Greenland (yellow bar) components of the stock. Based on Astthorsson and Vilhjalmsson (2002) plus
additional data.

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recruitment of the cod stock has been below thelong-term average. A stronger inﬂow of Atlantic
water onto the northern shelf has, however, resulted
in a more eastward distribution of the young year
classes (Anon, 2005a). The importance of the highfecundity, large females in the spawning stock forsuccessful recruitment of the Icelandic cod stock has
been noted (Marteinsdottir and Thorarinsson, 1998)
but currently they are only a tiny fraction of theirlevels during the early part of the last century(Anon, 2005a). The poor recruitment during recent
favourable environmental conditions is most probably due to the combined low stock size, young age
composition and high ﬁshing pressure. While thatsituation lasts, it seems unlikely that, in spite of a
warming climate, an extensive drift of larvae to
Greenland and return migration of mature ﬁsh to
Iceland will take place. It should be noted, however,that coinciding with increased temperatures at West
Greenland in the recent years some initial signs of
the recovery of the cod stock there have also been
observed (Anon, 2005c; Wieland and Storr-Paulsen,
2005).

2467

ﬁsh (Lophius piscatorius) are amongst the species
that have shown the largest distribution extension
and increase in abundance. Recruitment investigations on haddock have further demonstrated that,except for the 2001 year class, all year classes
between 1998 and 2003 have been strong. In fact,
the 2003 year class is estimated to be the strongest in
45 years. This increased recruitment and more
northward and northeastward distribution of haddock (Fig. 14) is believed to be related to thepositive temperature anomaly in the recent years(Anon, 2005a). Prior to 1985, monkﬁsh was mainly
conﬁned to the deeper waters off the south coasts(G. Jonsson, 1992) but in the recent years it has
increased in abundance and become distributed
along the whole of the west coast and onto thenorthern shelf (Fig. 15). Recent shifts in the
distribution of the Icelandic summer spawning
herring around Iceland have also been associatedwith the warming (Gudmundsdottir and Sigurdsson, 2004) and the same probably applies to theincrease of blue whiting in Icelandic waters.
6. Marine mammals

5.4. Other fish stocks
At a dynamic boundary area, such as that to thenorth of Iceland where the Atlantic and Polar water
masses converge, relatively small changes in the
distribution and properties of water masses canhave marked effects on the distribution andabundance of marine animals living there. Thus,Sæmundsson (1934) and Fridriksson (1948) reported on large distribution and abundance changes
for about 20 marine ﬁsh species associated with thewarming that occurred in the waters around Icelandin the 1920s and 1930s. Vilhjalmsson (1997b) also
reviewed the changes in the distribution andabundance of ﬁsh stocks in Icelandic and nearby
waters during the 20th century and concluded thatthe variations in the climate in the area may havelong lasting and far ranging effects on the marine
ecosystem.
Marked changes in the ﬁsh distributions andabundances also have been observed during therecent warming. These are particularly evident fromthe Icelandic groundﬁsh survey, which covers about
600 ﬁxed stations on the shelf around Iceland andhas been conducted annually since 1985 (Palsson
et al., 1989). According to Valdimarsson et al.
(2005), southern gadoids such as haddock, saithe
and whiting (Merlangius merlangus) and the monk-

About 18 species of cetaceans (whales, dolphinsand porpoises) and seven species of pinnipeds (seals
and walrus) inhabit the coastal and offshore waters
around Iceland and more than half of these species
have been exploited during the past centuries(Sigurjonsson and Hauksson, 1994).
On the basis of the extensive surveys conducted in
1987, 1989 and 1995 by the Marine ResearchInstitute in cooperation with adjacent nations in
the North Atlantic, Sigurjonsson and Vikingsson(1997) summarized the information on the abundance and distribution of cetaceans. Of the 18
species of cetaceans found in Icelandic waters, 12
are toothed cetaceans and six are baleen whales.
Many of these species are found all around Iceland
but generally are most common off the west andeast coasts. Most of the species are migratory tosome degree and occur in greatest abundances
around Iceland during the summer (Sigurjonsson
and Vikingsson, 1997). Minke whale (Balaenoptera
acutorostrata) is the most numerous stock of baleen
whales (ca. 60,000 animals), while the stocks of ﬁn(Balaenoptera physalus) and sei whales (Balaenoptera borealis) are also quite large (ca. 10,000 each)(Fig. 16). The populations of humpback (Megaptera
novaeangliae) and blue whale (Balaenoptera musculus) have increased in the recent years, because they

ARTICLE IN PRESS
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O.S. Astthorsson et al. / Deep-Sea Research II 54 (2007) 2456–2477

Fig. 14. Distribution and abundance (number per tow) of
haddock in the Icelandic groundﬁsh survey during 1985, 1992,
1999 and 2005 (based on ca. 600 stations covering the shelf areaaround Iceland).

have been protected from hunting since 1955 and1959, respectively (Sigurjonsson and Gunnlaugsson,1990). Of the toothed whales, the long ﬁnned pilot
whale (Globicephala melas) is the most abundant(ca. 58,000), followed by northern bottlenose(Hyperoodon ampullatus) and white sided dolphin(Lagenorhynchus acutus) (both at ca. 40,000).
The total annual food consumption of the 12
most common species of cetaceans in Iceland andadjacent waters is estimated to be about 6 million
ton (Sigurjonsson and Vikingsson, 1997) or about 3
times the total annual landing by the Icelandicﬁshing ﬂeet in the recent years. About 2 million tonof this consumption is estimated to be ﬁnﬁsh,mostly capelin. Humpback (M. novaeangliae) andminke whales consume the largest amount ofcapelin, respectively, 800,000 and 610,000 ton (Vilhjalmsson, 2002, personal communication). Due tothe large food consumption required by cetaceans
and their distribution, which often overlaps with theﬁshing grounds around Iceland, further studies into
their feeding ecology and role in the Icelandic
marine ecosystem are of great importance.
Studies on the biological parameters in ﬁn whales
based on material sampled between 1967 and 1989
demonstrated large ﬂuctuations in the age at sexual
maturity (Lockyer and Sigurjonsson, 1991; Konradsson et al., 1991). Age at sexual maturity was
found to decline steadily from 11 years in the year
classes from the late 1940s, to 8 years in those fromthe late 1960s. Then a reversal was observed, andthe year classes from the late 1970s became mature
at an increased age of about 10 years. For the period
above Sigurjonsson (1992) demonstrated negative
correlations between mean age at maturity of ﬁnwhales in Icelandic waters and the abundanceof euphausiids in continuous plankton recorder(CPR) samples from the deep waters southwest of
Iceland. This was considered to reﬂect that a higher
abundance of euphausiids would mean better
feeding conditions for the ﬁn whales and thus faster
growth rates and maturity at an earlier age.
Energetic studies on ﬁn whales also indicate acorrelation between body condition (blubber thickness, girth, chemical composition) and fecundity offemales supporting a close correlation between foodavailability and fecundity (e.g. Vikingsson, 1990).
Effects of climate on zooplankton populations can,through trophic interactions, inﬂuence the production of cetacean stocks.
Recent records of strandings on Icelandic shoresof striped dolphin (Stenella coeruleoalba) are of

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1990

1995

2000

2004

2469

>5
1-5
0

Fig. 15. Distribution and abundance (number per tow) of monkﬁsh in the Icelandic groundﬁsh survey during 1990, 1995, 2000 and 2004
(based on ca. 600 stations covering the shelf area around Iceland).

Number of individuals

70000
60000
50000
40000
30000
20000
10000

H

um

p

ba
Lo
ck
ng
B
lu
-fi
e
N
or nne
th
d
.b
pi
lo
W ottl
t
hi e
n
t
es o
H
ar
. d se
bo
ol
ur
ph
po
.
U
ni
r
de po
is
nt
W
.d e
hi
o
te
lp
h
b.
do .
lp
h
K .
ill
er
Sp
er
m

i
Se

n
Fi

M

in

ke

0

Fig. 16. Abundance of the 13 most common whale species in Icelandic waters. Based on Sigurjonsson and Vikingsson (1997) and G.
Vikingsson (personal communication).

particular interest in the context of distribution of
cetaceans in relation to climate. The ﬁrst stranding
record of this species was in 1984 (Bloch et al., 1996)
but during 1998–2005 another ﬁve strandings wererecorded (Vikingsson, 2004, personal communication;
Hjartarson and Olafsdottir, 2005). These ﬁve strandings were well separated in time and space showingthat they could not have resulted from a single
incidental migration event. The main distribution
area of striped dolphin is in the Atlantic Ocean far tothe south of Iceland and in the Mediterranean where

it is the most abundant dolphin species (Bloch et al.,
1996; Vikingsson, 2004). The increased frequency of
strandings of this southern species in Iceland andelsewhere in the North Atlantic may be result fromchanging environmental conditions (Bloch et al.,
1996; Vikingsson, 2004, Hjartarson and Olafsdottir,
2005). In Japanese waters, the distribution of striped
dolphin is mainly conﬁned to temperatures above
18 1C and the northern distribution boundary has
been observed to shift seasonally northwards inrelation to temperature (Perrin et al., 1994).

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O.S. Astthorsson et al. / Deep-Sea Research II 54 (2007) 2456–2477

Two species of seals, common seal (Phoca
vitulina) and grey seal (Halicoerus grypus) breed in
Icelandic waters, while ﬁve northern vagrant speciesof pinnipeds are found in the area (Sigurjonsson andHauksson, 1994; Hauksson, 1993, 2004). The
common seal is observed in the coastal areas allaround the country, while the grey seal is mainly
found off the west, northwest and southeast coasts.
The seal populations at Iceland have been
harvested through the centuries, both for food andfur. Hunting of both the species combined iscurrently limited to about 1000 individuals annually, well below the level of ca. 6000 animals of
several decades ago. Regular surveys of the common seal and grey seal populations have been
undertaken in Icelandic coastal waters since 1980
and 1982, respectively (Anon, 2005a). Stock estimates for both species have declined considerably
since initial surveys, common seals from about
30,000 individuals to about 10,000 and grey seals
from about 9000 to 6000. The steady decrease in the
population size of both the species is mainly
considered to be due to overexploitation (Anon,
2005a).
The total food consumption of common seals andgrey seals is estimated to be 28,000 and 30,000 ton,
respectively (Bogason, 1997; Hauksson, 1997). The
most important prey of both the species were codand sand lance (Ammodytes spp.), which comprised,
respectively, 42% and 14% of the total food intake
for the common seal and 24% and 22% for the greyseal.
7. Seabirds
Iceland and Icelandic waters are important areas
for some of the largest seabird populations in the
Northeast Atlantic with the largest seabird cliffs on
the Westfjord Peninsula on the northwest coast(Petersen, 1994). The distribution of the seabirds
around Iceland is mainly related to the marine
feeding conditions as breeding locations do notseem to be limiting (Gardarsson, 1995). The
proportion of seabirds is about 30% of theapproximately 70 species of all birds breeding in
the country. However, the importance of seabirds
in the Icelandic bird fauna is much greater based onabundance and biomass.
In total, the breeding populations of the 23
seabird species have been estimated to be about
7.7 million pairs (Anon, 2002). Assuming thatimmature birds are about 30% of the breeding

populations (Cairns et al., 1990; Furness, 1978)
total population size is about 20 million individuals.
The biomass of the breeding populations of theseabirds in Icelandic waters was estimated to be
11,000 ton (Anon, 2002) and when the immaturecomponent is added (Cairns et al., 1990; Furness,
1978; K. Lilliendahl, personal communication) thisvalue increases to a total of about 14,000 ton. A
more recent estimate of abundance and biomass ofseabirds in the Iceland and East Greenland area
combined suggests that the values presented here for
Iceland alone may be somewhat conservative(Anon, 2004b).
The Atlantic pufﬁn (Fratercula arctica) is thelargest seabird population with about 2.8 million
breeding pairs or about 40% of the total number ofseabirds at Iceland. This is followed by northern
fulmar (Fulmarus glacialis), with 1.5 million breeding pairs, and the common guillimot (Uria algae),with about 990,000 breeding pairs (Fig. 17).
Lilliendahl and Solmundsson (1997) estimated thatthe summer food consumption of the six mostnumerous seabird species of Icelandic waters was
about 442,000 ton. Sandeel and capelin were by far
the most important food items constituting 184,000
and 171,000 ton, respectively. The six largest seabird
populations consume about 8% of the total capelinbiomass in Icelandic waters during summer.
There is no information available on the recent
trends in the sizes of the major seabird populations
due to the lack of monitoring. However, the more
northern distribution of capelin during the recent
warming has been suggested to have contributed tothe massive mid-winter decline of alcids (Uria spp.)
along the west, northwest and north coasts ofIceland in the winter of 2001–2002 (Nielsen andEinarsson, 2004). Climate-related changes in prey
distribution are believed to have caused mortality inother seabird populations as well (Durant et al.,2004).
During the 20th century marked changes havebeen observed in the size of the breeding populationof the high Arctic little auk (Alle alle) to the north of
Iceland. It is the rarest of all Icelandic seabird
species and at the turn of the 20th century thebreeding population on the island of Grimsey(661300 N, 181000 W) to the north of Iceland wasconsidered to be only 150–200 pairs while breeding
also was known to take place at several otherlocalities on the north Icelandic mainland (Petersen,
1998). During the recent decades, Grimsey has been
the only known breeding site and its breeding

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2471

Number of pairs x 1000

3000
2500
2000
1500
1000
500

n

or
.

pu
ffi
N

tl .
A

fu
lm
C
om
ar
.g
ui
lle
B
m
l..
le
g.
k
i tt
B
ru
iw
. ´s
.
gu
ill
em
.
R
az
or
bi
ll
C
om
.e
id
er
A
rc
ti c
Le
te
ac
rn
h´
s
Eu
st
.p
ro
p.
et
.
st
or
m
O
th
pe
er
t.
s
(1
3
sp
p)

0

Fig. 17. Breeding pairs of seabirds at Iceland. Based on Petersen (1994) and Anon (2002).

population gradually declined to only a few pairs
during the 1990s, a single pair between 1993 and1996 and no breeding in 1997 and 1998. The main
reason for the decline in the little auk population
north of Iceland during the 20th century has been
considered to be the gradual climatic warming(Bardarson, 1986) but whether that effect is direct
through physiological processes or indirectly
through food availability is not known.
8. Total biomass and production
In an attempt to provide a more holistic view of
the Icelandic marine ecosystem and in order tofacilitate a comparison to other Sub-Arctic ecosystems, we present a summary of available information on biomass and production of the main
components or groups in the ecosystem. The
sources of information for the biomass estimatesand production calculations are summarized inTable 1. As biomass estimates for all of the ﬁshstocks, except mesopelagic ﬁsh, we have usedaverages for the period 1995–2004 presented inAnon (2004c), whereas for other groups they are
single estimates from the respective references listed
in Table 1. For C. finmarchicus the production was
calculated from egg production rates (Gislason,2005), whereas for other zooplankton species the
production/biomass ratios are used to calculate
production (Sakshaug et al., 1994). For ﬁsh, whales
and seabirds the production was calculated on thebasis of a size related production/biomass relationship presented by Banse and Mosher (1980)
combined with the relevant biomass information
similar to Skjoldal et al. (2004) for the NorwegianSea. The production estimates in terms of carbon

are based on wet weights assuming that, forzooplankton, dry weight is 20% of wet weight andC content is 50% of dry weight (Skjoldal et al.,2004), and for ﬁsh and higher animals dry weight is
33% of wet weight and C content is 40% of dry
weight (Sakshaug et al., 1994).
The estimated biomass values expressed as wet
weight were calculated for an area of 758,000 km2,
corresponding to the Icelandic 200-mile EEZ(Fig. 18). The total biomass of all the major
components in the Icelandic marine ecosystem isabout 56 million ton wet weight. Phytoplankton is
the largest component amounting to about 29 million ton or 51% of the total biomass. Zooplankton
is the second largest group amounting to about
17 million ton or 30% of the total. Of the zooplankton, C. finmarchicus is the largest single species/group amounting to about 7 million ton. No information is available on microzooplankton in
Icelandic waters and similarly amphipods, gelatinous medusae and squid are not included in thezooplankton/nekton biomass. All of these groups
have been found to be very signiﬁcant components
of nearby Nordic Seas ecosystems (Dalpadado
et al., 1998; Bjørke and Gjøsæter, 2004) andtherefore the zooplankton component presented
here is likely underestimated. Pelagic ﬁsh biomass
amounts to about 8.8 million ton or about half ofthe zooplankton biomass estimates. This number
includes an estimate of mesopelagic ﬁsh of
950,000 ton based on average oceanic biomass
values in Gjøsæter and Kawaguchi (1980), but can
only be regarded as an approximation for Icelandic
waters as no direct measurements of the biomass of
mesopelagic ﬁsh have been undertaken in the area.
The combined biomass of the largest demersal

ARTICLE IN PRESS
O.S. Astthorsson et al. / Deep-Sea Research II 54 (2007) 2456–2477

2472

Table 1
Sources of information for biomass estimates and calculation of production for the main components of the Icelandic marine ecosystem
Group

Biomass

Phytoplankton
Calanus finmarchicus
C. hyperboreus
Euphausiids

K. Gudmundsson, (personalcommunication)
Gislason (2002)
Own data
Own data

Other plankton

Own data

Capelin
Herring
Blue whiting
Oceanic redﬁshAnon (2004c)
Anon (2004c)
Anon (2004c)
Anon (2004c) and Th. Sigurdsson(personal communication)Gjøsæter and Kawaguchi (1980)
Anon (2004c)
Anon (2004c)
Anon (2004c)
Sigurjonsson and Vikingsson (1997)
Sigurjonsson and Vikingsson (1997)
Sigurjonsson and Vikingsson (1997)
Sigurjonsson and Vikingsson (1997)
Anon (2002)

Biomass (million tonnes)

Mesopelagic ﬁsh
Cod
Haddock, saithe
Other ﬁsh
Large baleen whales
Minke whale
Sperm whale
Other toothed whales
Seabirds

Weight at maturity

Production
Thordardottir (1994)

Anon (2004c)
Skjoldal et al. (2004)
Skjoldal et al. (2004)
Anon (2004c)

Gislason (2005)
Own estimate
Own estimate and Sakshaug etal. (1994)
Own estimate and Mauchline(1998)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)

Skjoldal et al. (2004)
Anon (2004c)
Anon (2004c)
Anon (2004c)
Sigurjonsson and Vikingsson (1997)
Sigurjonsson and Vikingsson (1997)
Sigurjonsson and Vikingsson (1997)
Sigurjonsson and Vikingsson (1997)
Anon (2002)Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Banse and Mosher (1980)
Sakshaug et al. (1994)

30
25
20
15
10
5

C

Ph
yt
o
. f pla
in
m nkt
o
a
E rc nO
th up hicuerhzo aus s
C op iid. h la
s
yp nk
er to
bo n
re
O
u
C
ce
ap s
a
M
ni
e
li
es c
op re ndela fis
B gic h
lu
e fis
w
h
hi
t
H ing
er
rin
H
g
La ad
d
rg
C
od
e ock
ba
,
le sa
en ithM w e
To ink hal
ot e w eshehadSp w le
h
er al
m es
w
Se hal
ab e
ird
s

0

Fig. 18. Estimated wet biomass of the main components within the Icelandic EEC based on sources in Table 1.

species, i.e., cod, haddock and saithe, is about 1
million ton (about 2% of the total biomass) or
similar to that of the baleen whales. The biomass of
the large baleen whales is about 900,000 ton whilethat of minke whales and toothed whales is eachestimated to be about 300,000 ton. The total
biomass of seabirds is about 14,000 ton and thatof seals only about 2000 ton (Fig. 18). Several
demersal species (e.g., Greenland halibut andseveral other ﬂatﬁshes), redﬁsh (S. marinus) andinvertebrates with an annual catch of about
100,000 ton in the recent years (Anon, 2004c) havenot been included in Fig. 18 as information on theirstock sizes is very limited.
Fig. 19 shows the calculated annual production interms of carbon per m2 for selected trophic levels in
the Icelandic marine ecosystem. The total annual
primary production within the Icelandic EEZ wasestimated by Thordardottir (1994) to be 1220 million ton or 160 gC m2 yr1. The annual productionARTICLE IN PRESS
2473

1000
100
10
1
0.1
0.01

er
sa

c
gi

m
de
O

th

er

th
O

lf
is
W h
ha
le
s
Se
ab
ird
s

h
C
od

fis

in
el
la
pe

nk
la

C
ap
er

us
an
zo
op

C
al
O

th

er

Pr
im

to
n
K
ril
l

0.001
pr
od

Production g C m-2yr-1

O.S. Astthorsson et al. / Deep-Sea Research II 54 (2007) 2456–2477

Fig. 19. Estimated production (gC m2 y1) for selected components of the Icelandic marine ecosystem based on sources in Table 1.

of Calanus (mainly C. finmarchcus) is estimatedto be about 7 gC m2 yr1 and the production ofother zooplankton is of a similar magnitude(6 gC m2 yr1). The combined production of
pelagic ﬁsh (in decreasing order: capelin, mesopelagic ﬁsh, blue whiting, oceanic redﬁsh (Sebastes
mentella) and herring) is estimated to be about
1.5 gC m2 yr1 and of these the production of
capelin is by far the largest (0.5 gC m2 yr1). The
production of cod is estimated to be about
0.04 gC m2 yr1, which is almost the same as thatof all other demersal species. The production ofother top predators, namely whales and seabirds, issmall in comparison to that at lower trophic levels
but, as pointed out above, their food consumption
is not.
As described earlier, capelin exploits the Atlantic
water south of Iceland for spawning but its main
distribution and feeding area is in the Sub-Arctic
waters to the north of Iceland. The actual area
where the main production takes place therefore
may be smaller than the one used to calculate theirproduction (The Icelandic EEZ) and thus the
production per unit area is likely to be higher thanthat presented in Fig. 19. The same also applies todifferent degrees to most of the other groups
considered in Fig. 19, but at present we feel thatthe spatial and temporal resolution of the data do
not allow analysis conﬁned to, for example, the SubArctic component of the ecosystem only. The
present estimates of production in Icelandic waters
must be considered rather uncertain. It is, however,interesting to note that the production estimates arein most cases similar to those reported by Sakshaug
et al. (1994) for the Barents Sea. This may indicatethat the main processes determining the productionin the two ecosystems are also somewhat similar andpossibly regulated in a similar manner.
9. Concluding remarks
In this paper we have attempted to summarize themain features of the Icelandic marine ecosystem andhow some important components have been affected by climate variability during the 20th century.
A marked warm period occurred between 1920 and
1940, a cold period was observed in the late 1960sand during the past decade marked signs ofwarming have again been observed. During all ofthese periods the environmental changes affected
the abundance and distribution of many commercial ﬁsh stocks and also other components of the
Icelandic marine ecosystem. Considering this, andalso model forecasts for future climatic scenarios(ACIA, 2004), it is very important for a ﬁsheries
nation such as Iceland to gain an understanding of
its marine environment and particularly so for thewaters to the north where the impact of climate
variation is likely to be greatest.
Astthorsson and Vilhjalmsson (2002) presented aconceptual model for the Sub-Arctic waters to thenorth of Iceland where the food chain fromphytoplankton-zooplankton–capelin and to cod
was considered to be bottom-up controlled andhighly sensitive to climate variability. We havefurther noted above the key role that capelin play in
the Icelandic marine ecosystem and their role intransferring zooplankton production from the Iceland Sea to the more southern part of the Icelandic
ecosystem. The growth of the capelin stock is

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believed to depend mainly on the environmental
conditions in the Iceland Sea (Malmberg andBlindheim, 1994; Vilhjalmsson, 1994; Astthorsson
and Gislason, 1998a) and the nearby waters, butthere are also certain indications that a small
spawning stock will lead to a poor recruitment(Vilhjalmsson, 2002). Furthermore, predation bywhales, ﬁsh and the ﬁsheries exploitation are
considered important factors in determining thedevelopment of the spawning stock. Since 1998,
the distribution and the main ﬁshing area of capelinin the Iceland Sea have changed and this has lead to
difﬁculties in assessing the size of the stock. Studies
to gain a further understanding of the Sub-Arctic
ecosystem north of Iceland (The Iceland Sea
Ecosystem Programme) are under way as an
Icelandic component of ESSAS. They are centredaround deﬁning and quantifying the environmental
and biological factors that determine the distribution and production of capelin in order to better
forecast the capelin stock abundance and distribution in a varying environment.
Acknowledgements
We thank H. Vilhjalmsson for providing Fig. 1
and H. Bjornsson for Figs. 14 and 15. Also
we thank D. Olafsdottir and G. Vikingsson forcomments on the paragraph on marine mammals.
Finally, thanks are due to K. Drinkwater,
H. Vilhjalmsson and S.A. Malmberg for many
useful comments.
This paper was ﬁrst presented in the GLOBECESSAS Symposium on ‘‘Effects of climate variability on sub-arctic marine ecosystems’’, hosted by
PICES in Victoria, BC, May 2005.
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