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Progress in Oceanography 68 (2006) 134–151
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The regime shift of the 1920s and 1930s in the North Atlantic
Kenneth F. Drinkwater
*
Institute of Marine Research and Bjerknes Center for Climate Research, P.O. Box 1870 Nordnes, N-5817 Bergen, Norway
Abstract
During the 1920s and 1930s, there was a dramatic warming of the northern North Atlantic Ocean. Warmer-thannormal sea temperatures, reduced sea ice conditions and enhanced Atlantic inflow in northern regions continued
through to the 1950s and 1960s, with the timing of the decline to colder temperatures varying with location. Ecosystem changes associated with the warm period included a general northward movement of fish. Boreal species of fish
such as cod, haddock and herring expanded farther north while colder-water species such as capelin and polar cod
retreated northward. The maximum recorded movement involved cod, which spread approximately 1200 km northward along West Greenland. Migration patterns of ‘‘warmer water’’ species also changed with earlier arrivals and later
departures. New spawning sites were observed farther north for several species or stocks while for others the relative
contribution from northern spawning sites increased. Some southern species of fish that were unknown in northern
areas prior to the warming event became occasional, and in some cases, frequent visitors. Higher recruitment and
growth led to increased biomass of important commercial species such as cod and herring in many regions of the
northern North Atlantic. Benthos associated with Atlantic waters spread northward off Western Svalbard and eastward into the eastern Barents Sea. Based on increased phytoplankton and zooplankton production in several areas,
it is argued that bottom-up processes were the primary cause of these changes. The warming in the 1920s and
1930s is considered to constitute the most significant regime shift experienced in the North Atlantic in the 20th
century.
2006 Elsevier Ltd. All rights reserved.
Keywords: Benthos; Climate changes; Cod; Ecosystem; Herring; North Atlantic; Regime shift
1. Introduction
A regime shift in marine ecology is ‘‘a persistent radical shift in typical levels of abundance or productivity
of multiple important components of the marine biological community structure, occurring at multiple trophic
levels and on a geographical scale that is at least regional in extent’’; distributional shifts are also often a characteristic of regime shifts (Bakun, 2004). The concept gained prominence in its application to the dramatic
abundance changes in sardines and anchovies across the globe (Lluch-Belda et al., 1989, 1992) and to salmon
and groundfish populations in the North Pacific during the mid-1970s (Venrick et al., 1987; Francis and Hare,
*
Tel.: +47 55236990; fax: +47 55238687.
E-mail address: [email protected].
0079-6611/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.pocean.2006.02.011
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
135
1994; Hare and Francis, 1995; Hare and Mantua, 2000). As Bakun (2004) points out, ecosystem regime shifts
are often linked to climate forcing but can also occur due to anthropogenic forcing, such as heavy fishing or
pollution (e.g. Steele, 2004).
In the North Atlantic, scientists generally have been much slower to adopt the idea of regime shifts compared to their colleagues working in the North Pacific, however, this appears to be changing. The majority of
papers identifying Atlantic regime shifts are associated with changes during the 1970s to 1990s in or around
the North Sea (Reid et al., 2001; Beaugrand, 2004; DeYoung et al., 2004). I would argue, however, that the
largest and most significant climate-induced regime shift of the last century in the North Atlantic occurred
earlier in the century and was much greater in geographical extent.
In the 1920s and 1930s, there was a dramatic warming of the air and ocean temperatures in the northern
North Atlantic and the high Arctic, with the largest changes occurring north of 60oN (Rogers, 1985; Polyakov
et al., 2003; Johannessen et al., 2004). This led to reduced ice cover in the Arctic and subarctic regions and
higher sea temperatures. Jensen and Hansen (1931) and later Jensen (1939, 1949) documented the expansion
of Atlantic cod (Gadus morhua) and halibut (Hippoglossus hippoglossus) along the west coast of Greenland in
response to the changes in the ocean climate. Other species were also observed to have undergone significant
abundance and distributional changes. This was a clear case of an environmentally driven ecosystem response
that became of paramount interest for fishery researchers at the time. This interest lead to the first scientific
meeting by ICES on climate change held in 1948 at Copenhagen (ICES, 1949) entitled Climate Changes in the
Arctic in Relation to Plants and Animals. Ahlmann (1949), in his introductory address, noted that the warming
had broad geographic extent with significant effects in the region: increasing air temperatures, receding glaciers, decreasing Arctic ice extent and thickness, decreasing water levels in lakes through increased evaporation, and high sea-level elevations due to melting ice.
This warming event was associated with atmospheric changes causing increased transfer of heat from low to
high latitudes (Brooks, 1938; Ahlmann, 1949; Rogers, 1985). Indeed, increased southerly winds pumped warm
air into the northern North Atlantic and also into the Arctic. Overland et al. (2004) showed that the Icelandic
Low was located farther to the east than usual in the 1930s with the result that Northern Europe was subsequently warmed by winds from the southeast. This is in strong contrast to its normal warming from the
southwest associated with a positive North Atlantic Oscillation (NAO) phase. In the Northwest Atlantic, a
high-pressure system over Greenland caused warm southerly flow over Baffin Bay (Overland et al., 2004).
Bengtsson et al. (2004) proposed that the temperature increase was related to enhanced wind-driven oceanic
inflow into the Barents Sea with an associated sea-ice retreat. Through feedback mechanisms, this in turn
generated and enhanced the cyclonic low pressure in the region and created a strong surface heat flux over
the ice-free areas, a mechanism previously proposed by Ådlandsvik and Loeng (1991). Modelling studies
suggest that these changes might be caused by internal, non-linear dynamics of the atmosphere (Delworth
and Knutson, 2000; Bengtsson et al., 2004).
Large and significant changes in marine ecosystems occurred as a result of this warming and many of these
changes were discussed at the 1948 Symposium (ICES, 1949) and in later papers (Beverton and Lee, 1965;
Cushing and Dickson, 1976; Cushing, 1982). The objective of the present paper is to provide a review of
the changes to the marine ecosystems of the northern North Atlantic during the 1920s and 1930s and to discuss them in the light of contemporary ideas of regime shifts. Following a brief review of the changes to the
climate, and in conjunction with the topic of this special issue, I will attempt to assess whether the ecosystem
changes were primarily a top-down or bottom-up response. The description of the ecosystem responses during
the warm period is presented by region and relies heavily upon information from fisheries, although not exclusively so.
Given the earlier reviews noted above, why is a new review needed? First, new information has become
available, most noticeably on phytoplankton and zooplankton, as well as on the response of fish stocks such
as capelin (Mallotus villotus) and herring (Clupea harengus). Second, our understanding and knowledge of
some of the physical and biological processes have increased so we are in a better position to determine what
likely happened. Finally, while many fisheries scientists working during that era were familiar with the event,
many of today’s marine ecologists and fisheries scientists have either forgotten or do not know about it. This
needs to be rectified given the important lessons it can teach us about what to expect under future climate
change.
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K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
2. Physical changes
In the North Atlantic (Fig. 1), air temperatures through the latter part of the 19th century and the early
20th century were relatively cool compared to years since then. During the 1920s, and especially after 1925,
average air temperatures began to rise rapidly and continued to do so through the 1930s (Fig. 2a–f). Mean
annual air temperatures increased by approximately 0.5–1 C and the cumulative sums of anomalies varied
from 1.5 to 6 C between 1920 and 1940 with the higher values occurring in West Greenland and Iceland. Farther south along the east coast of Canada and the northeastern United States, as well as in much of southern
Europe, no such temperature increase occurred (Fig. 2g–h). However, even farther south, for example around
Cape Hatteras and at Funchal in Madeira off the west coast of Africa, the large temperature increase was also
observed (Fig. 2i–j). Throughout the remainder of this paper, only the changes associated with rise in temperatures in the most northern regions of North Atlantic are considered.
Through the 1940s and 1950s air temperatures in the northern most regions varied but generally remained
relatively high (e.g. at Nuuk in West Greenland, Fig. 3). Thereafter, there was a rapid cooling trend with the
exact timing of the decline varying spatially. In the Northwest Atlantic, warm conditions remained through
most of the 1960s whereas in the Northeast Atlantic they began declining slightly earlier. The high temperatures recorded during the warm period from 1930–1960 match, and in some cases exceed, the present day
warming (Johannessen et al., 2004).
Sea temperatures also rose in the northern North Atlantic (Fig. 4). Jensen (1939) documented the increase
in sea surface temperatures (SSTs) off West Greenland while Scherhag (1937) and Smed (1947, 1949) did the
same for several areas of the northern North Atlantic. Smed’s (1947) analysis showed the largest increase was
Fig. 1. The map of the North Atlantic including the air temperature sites used in the study. The Kola Section in the Barents Sea and three
zooplankton transects off West Greenland are also marked.
0.5
0.0
-0.5
Nuuk
-1.0
1915
1925
Akureyri
1930
1935
1940
0.5
0.0
-0.5
Bodø
1920
1925
Haparanda
1930
1935
1940
0.5
0.0
-0.5
Torshavn
1920
1925
Uccle
1930
1935
1940
0
-1
1920
1925
1930
1935
1940
1945
1920
1925
1930
1935
1940
1945
1920
1925
1930
1935
1940
1945
1920
1925
1930
1935
1940
1945
1920
1925
1930
1935
1940
1945
d
5
4
3
2
1
0
-1
5
f
4
3
2
1
0
-1
2
g
Cummulative Sum of
Air Temperature Anomalies (˚C)
A i r T e m p e r a t u r e A n o m al i e s ( ˚ C)
1
-2
1915
1945
1.0
0.5
0.0
-0.5
Boston
1920
1925
Lisbon
1930
1935
1940
1
h
0
-1
-2
-3
-4
-5
-6
1915
1945
1.0
6
i
Cummulative Sum of
Air Temperature Anomalies (˚C)
A i r T e m pe ra t u r e A n o m a l i es ( ˚ C )
2
6
e
0.5
0.0
-0.5
Cape Hatteras
-1.0
1915
3
-2
1915
1945
1.0
-1.0
1915
4
6
c
-1.0
1915
b
5
-2
1915
1945
C u m u l a t i v e S u m of
Air Temperature Anomalies (˚ C)
Air Temperature Anomalies (˚ C)
1920
-1.0
1915
Air Temperature Anomalies (˚ C)
Cummulative Sum of
Air Temperature Anomalies (˚ C)
a
1.0
137
6
1.0
C u m m u l at i v e S u m o f
A i r T em p er a t u r e A n o m al i e s ( ˚ C )
Air Temperature Anomalies (˚ C)
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
1920
1925
1930
Funchal
1935
1940
1945
j
5
4
3
2
1
0
-1
1915
Fig. 2. The annual air temperature anomalies and their cumulative sums from 1920 to 1940 for locations in the northern North Atlantic:
Nuuk in West Greenland and Akureyri, Iceland (a,b); Bodø, Norway, and Haparanda, Sweden (c,d) and Tórshavn, Faroe Islands, and
Uccle, Belgium. Increasing values in the cumulative anomaly plot indicate positive anomalies (above normal temperatures). See Fig. 1 for
station locations. The annual air temperature anomalies and their cumulative sums from 1920 to 1940 for locations in the North Atlantic:
Boston, USA and Lisbon, Portugal (g,h); and Cape Hatteras, USA and Funchal, Madeira (i,j).
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K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
Air Temperature Anomaly (˚ C)
4
Annual Mean
3
10-yr Mean
2
1
0
-1
-2
-3
1860
1880
1900
1920
1940
1960
1980
2000
2020
Fig. 3. The annual and 10-year running mean of the air temperatures at Nuuk in West Greenland.
2.0
SST Anomaly (˚C)
1.5
a
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
1870
1880
1890
1900
1910
1920
1930
1940
1920
1930
1940
1.0
b
SST Anomaly (˚C)
0.5
0.0
-0.5
Annual Means
-1.0
5-yr Running Means
-1.5
1870
1880
1890
1900
1910
Fig. 4. The annual and five-year running means of the sea surface temperatures (SST) anomalies from (a) off West Greenland and (b) off
southwest Iceland. The anomalies are relative to the long-term averages from 1876–1915 for West Greenland (Smed, 1947) and from 1895–
1934 for southwest Iceland (Thomsen, 1937).
off West Greenland and Norway, although no data from north of the Arctic Circle were analyzed (see also
Beverton and Lee, 1965). A significant but lower sea surface temperature increase was observed off southwest
Iceland (Fig. 4; Thomsen, 1937). The increase at the Faroes was of the order 0.5 C and occurred in the early
1930s (Tåning, 1953), somewhat later than many other locations but consistent with the later rise in air temperatures observed at this site (Fig. 2). Sea temperatures also rose farther south in the North Sea, the English
Channel and the Baltic Sea (Beverton and Lee, 1965). Scherhag (1937) reported that Gulf Stream temperatures
were approximately 0.4 C higher in 1926–1933 compared to 1912–1918.
The rise in ocean temperatures was not restricted to the surface waters. Jensen (1939) and Dunbar (1946)
documented subsurface temperature increases down to 500 m in the waters off West Greenland. On the other
side of the Atlantic, the annual average temperature of the top 200 m in the Kola section in the Barents Sea
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
139
(Fig. 1) shows a sharp rise during the 1920s of over 0.5 C and remained high through to the early 1960s before
declining (Fig. 5).
There was a marked reduction in sea-ice extent that accompanied the warming. Polar ice or ‘‘storis’’ commonly flows from East Greenland around Cape Farewell and along West Greenland. The northernmost position of the storis off West Greenland, by month, and the number of months storis appeared between October
and the following September, for the years 1899/1900 to 1971/1972 were listed by Valeur (1976). May is typically the month with the maximum northerly extent of storis. During the period 1899/1900 to 1925/1926 the
ice penetrated farther north during May and lasted almost 2 months longer than for the period 1926/1927 to
1960/1961 (Fig. 6). Off Iceland, in the 1920s and 1930s, the ice edge retreated northward such that ice was
observed along the north coast for only a couple of weeks each year compared to 12–14 weeks in the late
1800s (Lamb and Johnson, 1959; Schell, 1961). In the Barents Sea, the ice edge moved north and east (Beverton and Lee, 1965). Around Svalbard it retreated an average distance of some 150 miles northward in the late
summer (Brooks, 1938). These retractions began in the Barents Sea and the Kara Sea in 1920 (Ahlmann,
1949).
5.0
Annual Mean
10-yr Mean
T em p er a tu r e (˚ C )
4.5
4.0
3.5
3.0
2.5
1890
1910
1930
1950
1970
1990
2010
Fig. 5. The annual and 10-year running mean of the 0–200 m average temperature along the Kola Section. The horizontal lines represent
the period means.
60
a
Relative Position
50
40
30
20
10
0
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980
12
b
# of months
10
8
6
4
2
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980
Fig. 6. The northern most location of the storis along West Greenland in May (a) and the number of months storis was observed
anywhere along West Greenland (b). The relative position index has a value of 0 in East Greenland and 60 at Disko Island.
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K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
There were also changes in the ocean circulation. The transport of the relatively warm eastward flowing
Irminger Current south of Greenland, which eventually forms the offshore branch of the northward moving
West Greenland Current, was above normal throughout most of the period from 1930s to the late 1950s, with
the exception of 1948–1955 (Beverton and Lee, 1965). Estimates of Irminger Current flow off Cape Farewell at
the southern tip of Greenland indicated that peak transports occurred during 1931–1938 (average 3 Sv. (1
Sv. = 106 m3 s 1)) and greater than three times those occurring either before and after. This is consistent with
the increased easterly winds to the east of southern Greenland during the warm period compared to the earlier
and later cold periods (Dickson and Brander, 1993; Kushnir, 1994). Such winds are qualitatively consistent
with an increased westward flow in the Irminger Current in the warm period and reduced flows in the cold
periods. Around Iceland, the flow of Atlantic water to the west and north of the island is believed to have
increased in the 1920s and pushed the colder East Icelandic Current farther offshore. This is consistent with
the sea-ice distributions and the increased southwesterly winds off northern Iceland during the warm period
(Stefánsson, 1954). The branch of the North Atlantic Current to the west of Svalbard, known as the West
Spitzbergen Current, also increased in strength, with the result that the surface layer of cold water in the Arctic
Ocean decreased from 200 to 100 m in thickness (Brooks, 1938). Finally, the inflow of Atlantic waters to the
Barents Sea is believed to have increased based on relationships between atmospheric patterns and inflow
revealed by recent modelling studies (Bengtsson et al., 2004).
3. Ecosystem responses
3.1. West Greenland
The most well-documented biological change that occurred during the warm event was the increased abundance of Atlantic cod off West Greenland. From the late 1910s to the early 1930s they not only increased in
numbers but also spread gradually northward from near the southern tip of Greenland to Upernavik, a distance of over 1200 km (Fig. 7; Jensen, 1939). While cod had always been present in the fjords of West Greenland, the large population increase in the early 20th century was due to their becoming established on the
offshore banks. The increased abundance led to the development of a cod fishery, which quickly replaced sealing as the main industry in West Greenland. That the increased landings of cod were simply due to increasing
fishing effort can be ruled out, since several expeditions, including ones as late as 1906 and 1908, found no
commercial concentrations and indeed few cod at all (see discussion by Buch et al., 1994). However, an expedition in 1909 found enough cod for commercial cod fishing to begin (Jensen, 1949).
The cod fishery yielded moderate landings through the 1930s (<105 mt) but this declined during the war
years (Fig. 8). Catches rose dramatically through the 1950s reaching a peak at close to 5 · 105 t in the early
1960s before declining rapidly later that decade. This decline came during a period of decreasing air and ocean
temperatures. Cod catches have remained relatively low since the 1970s. The abundance of cod larvae has also
been relatively low, at least for the 1970s to the mid-1980s when data were available (Pedersen and Rice, 2002).
Coinciding with the decrease in cod was an increase in northern shrimp (Pandulus borealis) and Greenland
halibut (Reinhardtius hippoglossoides). Indeed, the shrimp fishery replaced cod as the dominant industry in
West Greenland and remains so today.
The cod off West Greenland originated from Iceland. Their rather sudden appearance is believed to be due
to a combination of increased transport of larvae from Iceland, because of the increased flow of the Irminger
Current (see above), and better survival of larvae once they reached West Greenland waters (Jensen, 1949;
Dickson and Brander, 1993). Haddock (Melanogrammus aeglefinus) catches off West Greenland provide additional evidence of increased larval transport from Iceland to Greenland. This species does not spawn off West
Greenland so must have originated from Iceland (Dickson and Brander, 1993). The relative catches of halibut
mirror quite well those of the catch of cod, lending further support for the advection hypothesis (Dickson and
Brander, 1993).
Zooplankton samples were collected in June–July every year but four from 1950 to 1985 on transects across
three of West Greenland’s offshore banks (Fig. 1). Zooplankton abundance was much greater during the warmer 1950s and very early 1960s compared to remaining years (Pedersen and Rice, 2002). Identification of species in these samples began in 1956. Calanus finmarchicus is arguably the most important and dominant
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
141
Fig. 7. The cod distributions (in red) along West Greenland from 1900 to the late 1930s showing the northward extension during the
warming period.
Cod Catches (103 tonnes)
500
400
300
200
100
0
1900
1920
1940
1960
1980
2000
Fig. 8. The annual catches of cod off West Greenland Modified and updated from Dickson and Brander (1993).
zooplankton species in the North Atlantic and known to be critical for survival of young cod in several regions
of the North Atlantic (Astthorsson and Gislason, 1995; Sundby, 2000). Its abundance off West Greenland was
highly variable with no overall trend, but during the late 1950s it tended to be relatively high whereas none
were found in sampling conducted from 1973 to 1977 (Pedersen and Rice, 2002). These authors found that
approximately 25% of the interannual variability in cod larval abundance could be accounted for by either
the abundance indices of total zooplankton, total copepods or C. finmarchicus. While C. finmarchicus abundance was not significantly correlated to temperature, the total abundance of zooplankton was weakly correlated to temperature.
Jensen (1949) and Tåning (1949) documented changes in many other species during the warming period.
Like cod, spotted catfish (Anarchichas minor) and herring, as well as mussels (Mytilus edulis) and starfish
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K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
(Asterias) spread northward. Their abundances also increased noticeably along with those of haddock, coalfish (Gadus virens), Atlantic salmon (Salmo salar), picked dogfish (Squalus acanthias), dealfish (Trachypterus),
Caaing whale (Globiceps melas), and the common starfish (Asterias rubens). Some of these relatively warmerwater species including herring, coalfish and redfish reproduced successfully in areas north of their previous
range. On the other hand, colder-water species such as capelin no longer migrated as far south along the West
Greenland coast and their abundance in southwestern Greenland decreased while it increased northward as
far as Thule. Greenland shark (Laemargus microcephalus) retreated from the region off southwestern Greenland while densities in the colder, more northern regions increased. In northwestern Greenland, white whales
(Delphinapterus leucas) and narwhals (Monodon monoceros) arrived earlier and left later on their annual
migrations. New immigrants came to Greenland including tusk (Brosmius brosme), ling (Molva vulgaris), witch
(Pleuronectes cynoglossus) and the jellyfish Halopsis ocellata. It was suggested that most of these new species
probably arrived through advection from Iceland (Tåning, 1949).
3.2. Iceland
Cod is the dominant commercial fish in Icelandic waters. During the warming period of the 1920s catches
by both the Icelandic and foreign fleets rose rapidly and peaked in the early 1930s at almost 6 · 105 t. While
increased fishing effort also contributed to these record catches, there was very high recruitment in the 1920s,
with the highest recruitment on record being in the early 1920s (Schopka, 1994). This large abundance of cod
larvae is believed to have contributed significantly to the rise in the West Greenland cod population. Prior to
the 1920s warming, cod spawned almost exclusively off the south coast of Iceland. As the waters warmed, cod
spawning spread northward until there were major spawning locations completely surrounding Iceland (Sæmundsson, 1934).
Sæmundsson (1934) also documented changes in capelin, the major prey of adult cod off Iceland. Prior to
the warming they spawned during the spring (March–May) in the waters off southern Iceland where they
could find temperatures necessary for their eggs to hatch. Later in the season (May–July) spawning occurred
off the north coast. With the rise in ocean temperatures in the 1920s, they no longer needed to go so far south
to spawn and thus, became scarce on the south coast, which resulted in a decrease in the condition of cod in
the south, while those cod residing on the north coast were in good condition.
The increased influx of Atlantic waters to the north of Iceland is believed to have lead to an increase in
primary production off northern Iceland, based on more recent studies of the relationship between spring primary productivity and the presence of Atlantic waters (Thorardottir, 1984; Gudmundsson, 1998). These
waters provide for an enhanced bloom that lasts longer due to reduced stratification and higher nutrient concentrations than are found in Arctic waters. In these northern waters, zooplankton abundance is also found to
be significantly higher in warm years (i.e. more Atlantic waters) than in cold years (Astthorsson and Vilhjálmsson, 2002). Since C. finmarchicus constitutes 60–80% of the zooplankton biomass in spring, interannual variations mainly reflect variations in this species (Astthorsson and Vilhjálmsson, 2002).
Several warm-water species that visited Iceland occasionally prior to the warming in the 1920s became more
frequent or extended their range to include the usually cold regions off northern and eastern Iceland. These
included witch, turbot (Rhombus maximus L.), basking shark (Selache maxima), tunny (Orcynus thynnus),
Atlantic mackerel (Scomberus scomberus), Atlantic horse mackerel (Trachurus trachurus, formerly Caranx trachurus), sunfish (Orthagoriscus mola) and several other more minor species (Sæmundsson, 1934; Fredriksson,
1949). In addition, the number of sei whales (Balænoptera borealis Less.) around Iceland appeared to increase
(Sæmundsson, 1934). The changes were not restricted to fish and marine mammals. Sæmundsson (1934) also
noted the increase of three marine seabirds including the black-headed gull (Larus ridibundus L.), the lesser
black-backed gull (L. fuscus L.) and the herring gull (L. argentatus Brünn).
3.3. Barents Sea and Svalbard
With the warming in the 1920s and 1930s, cod appeared in high abundance on Bear Island Bank, resulting
in the reestablishment of a cod fishery there after an absence of almost 40 years (Blacker, 1957). Cod also
spread northward into the area off West Svalbard with sufficient abundance to support a fishery (Beverton
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
143
and Lee, 1965). Cod as well as haddock moved eastward reaching Novaya Zemlya by 1929–1930 (Cushing,
1982). There was a distributional shift in spawning with proportionately more cod spawning in the northern
regions of Norway (Lofoten and Finnmark) compared to southern Norway at Møre (Sundby and Nakken,
2004). During the colder periods before and after the warm period, the percent spawning at Møre was much
higher. There is a suggestion that the cod might also have spawned earlier during the warm period, based upon
timing of the ratio of the weight of the roe to the weight of the cod at Lofoten, (Pedersen, 1984). However,
since younger cod spawn later (Pedersen, 1984), the suggested later spawning of the population during the
colder period might equally be explained by fishing down of the older cod.
The stock size of Arcto-Norwegian cod in the Barents Sea and offshore Norway peaked in the 1930s
and 1940s (Hylen, 2002). Catch per unit effort (CPUE) also was significantly higher in the period 1925–
1960 than in the periods before or since, consist with an increase in abundance levels at that time (Godø,
2003). While this change coincided with the rapid development of the trawl fishery and increasing fishing
efficiency cannot be ruled out as a contributor to the increased CPUE, recruitment was higher during the
warm period than the subsequent cool period (Godø, 2003). High recruitment is believed to be, in large
part, a result of greater food availability (Sætersdal and Loeng, 1987; Ottersen and Loeng, 2000). The
mean weight of the cod in Lofoten rose rapidly in the 1920s into the early 1930s and remained high before
starting a general decline in the 1960s. The increase in weight between the pre-1920s period and 1930s–
1960s was over 50%.
Capelin is an important forage fish within the Barents Sea. The capelin feeding migration likely spread farther north and east in the Barents Sea during the warm period as they migrated to and from the Polar Front
that separates the cold, low salinity Arctic waters from the warmer, high salinity Atlantic waters. This change
in capelin migration behaviour has been inferred from studies conducted during more recent warm and cold
years (Vilhjálmsson, 1997a). 0-group and age 1–3 herring typically occupy the western Barents Sea. In the
1920s, they pushed farther eastward as evidenced by the development of a herring fishery along the Murman
coast of Russia, where previously this species was almost unknown (Beverton and Lee, 1965). Particularly
large catches were observed in the 1930s (Cushing, 1982). Also in the 1930s, Atlantic salmon, cod and herring
appeared in the Kara Sea and haddock catches were recorded in the White Sea for the first time (Cushing,
1982).
The changes to the ecosystem were not limited to fish. Russian studies revealed a retreat of Arctic-type benthic species along the Murman coast and an increase in the number of boreal species, doubled between the
period prior to and during the peak of the warming (Nesis, 1960). Gastropods (Gibbula tumida, Akera bullata),
hermit crabs (Eupagurus bernhardus L.) and cockles (Cerastoderma edule L.), all species normally associated
with Atlantic waters, were reported along this coast for the first time in the 1930s (Cushing, 1982).
Benthic ecosystem changes were also recorded to the west and southwest of Svalbard. Comparing the benthos prior to 1931 with that of the 1950s indicated that Atlantic species had spread northward by approximately 500 km (Fig. 9; Blacker, 1957, 1965). This was attributed to an increased influence of Atlantic
waters (Blacker, 1957) and is consistent with an increase of the warm north-flowing West Spitzbergen Current
noted by Brooks (1938).
3.4. Norwegian and Greenland seas and vicinity
One of the most important species in the Norwegian Sea is herring. This species underwent distributional
and abundance changes during and following the warm period. The abundance of Norwegian spring-spawning herring increased in the 1920s in parallel with the smoothed mean annual temperatures recorded at the
Kola section (Toresen and Østvedt, 2000; Fig. 10). High temperatures tend to favour high recruitment
although it is a necessary but not sufficient condition for high recruitment, i.e. at low temperatures recruitment
is always low but at high temperatures recruitment can be high or low (not shown). The herring stock
decreased during the cooling period but has risen again with temperature increases of the 1990s (Fig. 10). Fishing pressure also played a significant role in the decline of the spawning stock biomass (SSB) in the 1960s
(Toresen and Østvedt, 2000).
The stock underwent large distributional changes as well (Vilhjálmsson, 1997b). During the warm period,
herring migrated from their spawning locations on the west coast of Norway and around the Faroes and their
144
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
18
4.3
16
4.2
14
4.1
12
4
10
3.9
8
3.8
6
4
3.7
2
3.6
0
1900
1920
1940
1960
1980
Temperature (˚ C)
SSB (106 tonnes)
Fig. 9. The changes in the benthic species near Svalbard (taken from Blacker, 1965). The open circles represent Arctic species and the
triangles Atlantic species. The stippled area indicates where Atlantic species dominate and the hatched area where conditions can vary
from extreme Atlantic to extreme Arctic conditions (+5 C to near 2 C).
3.5
2000
Fig. 10. The estimates of the Norwegian spring-spawning herring stock biomass (SSB, solid line) and the 19-year running mean of
temperature from the Kola Section (dashed line) (taken and modified from Toresen and Østvedt, 2000).
nursery areas in the western Barents Sea to the feeding grounds off northeastern Iceland. That they were farther west than normal is confirmed by the first reports of herring at Jan Mayen in 1930–1931 (Cushing, 1982).
The overwintering grounds at this time were primarily to the east of Iceland and then in spring, the herring
migrated back to their spawning grounds. As the Polar Front between the cold polar waters to the northwest
and the warmer Atlantic waters to the southeast shifted southeastward during the colder 1960s, the herring
feeding grounds moved farther eastward and then to southwest of Svalbard. When the population declined
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
145
to drastically low levels, those individuals remaining no longer migrated out into the Norwegian and Greenland seas but stayed near the Norwegian coast to both feed and spawn (Vilhjálmsson, 1997b). In the 1990s, as
temperatures warmed and the population increased, they again began migrating back towards Iceland to feed
(Vilhjálmsson, 1997b).
There were distributional shifts in other species in the Norwegian and Greenland seas and around the Faroes during the early 20th century warming. Cod, like herring, were first caught off Jan Mayen in the early
1930s. The most noticeable biological response around the Faroes to the warming event was an invasion
by the Atlantic horse mackerel (Tåning, 1953). In addition, several other warm-water species became occasional visitors to the Faroe Islands, including swordfish (Xiphias gladius), twaite shad (Alosa finta) and pollock
(Pollachias pollachias) (Tåning, 1953) as well as thick-lipped grey mullet (Mugil chelo) (Tåning, 1943).
3.5. Other regions
Ecosystem changes to the warming of the 1920s and 1930s were not limited only to the regions so far
described. Ocean temperatures rose in the North Sea. Examining 31 species of demersal fish caught in
1919–1922 to those caught in 1949–1952 showed that southerly species generally increased while the more
northern ones decreased (Southward, 1963). Plymouth herring nearly disappeared by the middle of the
1930s and those on the Firth of Forth by the early 1940s. Southward (1963) suggested that the decline of
the Plymouth herring was an effect of the temperature increase and Cushing (1982) suggested that it was likely
due to competition with pilchard (Sardina pilchardus), a southern species that moved into the region as the
waters warmed. The distributional changes were not limited to fish but also occurred for phytoplankton
and zooplankton as more southern species invaded the region and more northern species declined (Cushing
and Dickson, 1976). In addition, several species of barnacles from the south spread along the coasts of France
and Britain (Crisp and Southward, 1958; Southward, 1967). The Russell Cycle, a term used to identify the
large changes in abundance and shifts in fish and plankton species that were observed in the English Channel
between the 1920s to the 1970s, can be interpreted as a response to the warming event (Cushing and Dickson,
1976; see detailed discussion by Cushing, 1982).
In the Northwest Atlantic, Tåning (1953) suggested that the cod off eastern Canada expanded northward to
northern Labrador, but he did not discuss the evidence for such a claim. Catches of northern cod off southern
Labrador and northern Newfoundland showed no large changes during the warm period compared to the
early 1900s. In the Bay of Fundy, several Arctic species of animals were recorded during the 1930s, indicative
of advection from the north, but during the same period several warm-water species were found in the Gulf of
Maine. This makes it difficult to understand whether any large-scale changes in this ecosystem took place at
this time or not. Ocean temperatures in this region show no signs of significant warming during the 1920s and
1930s consistent with air temperature trends (see Boston air temperatures, Fig. 2g–h). Major warming was
observed in the 1950s and cooling in the 1960s, which have been related to changes in the strength of the Labrador Current (Petrie and Drinkwater, 1993).
4. Discussion
This review, the results of which are summarized in Table 1, describes the significant changes in the marine
ecosystem in the northern North Atlantic that occurred during the 1920s and 1930s and how these were linked
to a general warming of the oceans. This warming was not due to the large rise in air temperatures alone, but
to an apparent change in ocean circulation that brought more warm water northward. This intensification of
northward flowing ocean currents is believed to be linked to changes in the atmospheric wind pattern. The seaice edge shifted northward as a result of the warming. The ecosystem changes included significant northward
shifts in distribution and changes in the timing and extent of the migration patterns of numerous species of
fish, marine mammals and some seabirds. The northward movement of many boreal and subtropical species
occurred concurrently with a retraction in the distribution of Arctic species. Spawning shifted northward and
in some areas, such as West Greenland and Iceland, new spawning sites were established farther north than
previously observed. Large increases in the biomass of several commercially important species, such as cod
and herring, occurred, driven by increased recruitment and improved growth. The changes in fish populations
146
Table 1
Summary of the physical and ecosystem changes off West Greenland, Iceland, in the Barents Sea and off Svalbard and in the Norwegian/Greenland seas
Annual air temp increase 1925–
1954 minus 1895–1924
Increase in cumulated air temp
anomalies 1920–1940
Increase in ocean temp anomalies
Sea Ice
Primary production
Secondary production
Benthic production
Fish production
Iceland
Barents Sea/Svalbard
Norwegian/Greenland Seas
0.4 C
0.5 C
0.3 C
0.3–0.4 C
5 C
6 C
4 C
4–5 C
SW Greenland, 0.9 C; 1925–
1939 minus 1895–1924a
Increased transport of warm
Irminger Current northward d
SW Iceland, 0.44 C; 1925–1936
minus 1895–1924b
Increased Atlantic flow eastward
along north coaste
Reduced amounts of storis ice
and less northward penetrationh
Much reduced ice on north coasti
Suggested increased productionk
Suggested increased zooplankton
productionn
Kola Section, 0.4 C; 1925–1954
minus 1904–1924c
Increased Atlantic inflow into the
Barents Seaf and northward off
West Svalbardg
North and eastward retreat of ice
edgeg,j
Suggested increased productionl
Increased zooplankton
productiono
Increased abundance of codu
Extension of Atlantic benthic
species northward off West
Svalbardq and eastward along
the Murman coast of Russiar,s
Increased abundance of codd,q,s
Increased zooplankton
abundance in 1950s and early
1960s relative to cooler period of
the late 1960s–1985m
Increased abundance and
northward spread of the common
starfishp
Increased abundance of cod,
haddock, herring, spotted catfish,
mussels, salmonp,t
Fish growth and condition
Fish distribution
Northward expansion of cod,
spotted catfish, herring, musselsp
Spawning
Cod spawned further northt
Increase in cod condition on
north coast but decrease off south
coast increased related to capelin
distributionw
Capelin tended to stay farther
northw
Increased numbers of cod
spawned off the north coast while
capelin spawned almost
exclusively off the north coast in
contrast to earlier spawning on
the south coastw
Increased Atlantic inflow into the
Norwegian Seaf
Increased abundance of herringv
Increase in cod growth and
conditiono
Codd,q,s and capelinx spread
northward off W. Svalbard and
north and eastward in the
Barents Sea. Haddock and
salmon also spread eastwards
Herring spread eastward along
the Murman coast and into the
Kara Seas
Increased proportion of cod
spawned farther northz
Herring spread westward
towards Iceland and Jan
Mayenx,y along with codq
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
Ocean circulation
W. Greenland
New immigrant species
Tusk, ling, witch, jellyfish
(Halopsis ocellata)p
Marine mammals
White whales and narwhals
arrived earlier and left later on
their annual migrationsp
Seabirds
Witch, turbot, basking sharks,
tunny, Atlantic mackerel, jack
mackerelw,aa
Increased number of sei whalesw
Atlantic horse mackerel,
swordfish, twite shad, and
pollock around the Faroesab
Numbers of black-headed gull,
lesser black-backed gull, and
herring gull all increased
significantlyw
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
Unreferenced results are from the present study and blank spaces indicates no available information at present.
a
Based on SST data from Smed (1947).
b
Based on SST data from Thomsen (1937).
c
Based on 0–200 m averages from Kola Section data supplied by PINRO, Russia.
d
Beverton and Lee (1965).
e
Suggested from Stefansson (1954).
f
Bengtson et al. (2004).
g
Brooks (1938).
h
Valeur (1976).
i
Lamb and Johnson (1959), Schell (1961).
j
Ahlmann (1949).
k
Based on studies by Thorardottir (1984) and Gudmundsson (1998).
l
Based on modelling results of Slagstad and Wassmann (1996).
m
Pedersen and Rice (2002).
n
Astthorsson and Vilhjálmsson (2002).
o
Sætersdal and Loeng (1987), Ottersen and Loeng (2000).
p
Jensen (1949).
q
Blacker (1957, 1965).
r
Nesis (1960).
s
Cushing (1982).
t
Jensen (1939).
u
Schopka (1994).
v
Toresen and Østvedt (2000).
w
Sæmundsson (1934).
x
Based on Vilhjálmsson (1997a).
y
Based on Vilhjálmsson (1997b).
z
Sundby and Nakken (2004).
aa
Fredriksson (1949).
ab
Tåning (1949, 1953).
147
148
K.F. Drinkwater / Progress in Oceanography 68 (2006) 134–151
had significant economic impacts, especially in West Greenland where there was a shift from a seal-dominated
economy to one dependent upon cod. Where warm Atlantic waters replaced the cold Arctic waters or became
relatively more important, primary and secondary production appears to have increased. Also, the benthos
changed with the increased influence of the warm-water currents. It is clear, based on the definition of Bakun
(2004), that this represents a large and important climate-forced ‘‘regime shift’’ in the northern North Atlantic.
This new regime lasted for approximately 30–40 years and covered a geographical distribution extending several million square kilometres.
As temperatures declined in the northern North Atlantic during the 1960s, ecological conditions often
returned to their previous state. In some regions, however, new regimes became established, e.g. off West
Greenland where shrimp biomass increased and became the dominant economic fishery, replacing cod. This
appears to be a new stable state. By the time of the change in the environmental conditions in the 1960s, expanding fisheries due to more extensive use of trawlers, the development of large long-distance foreign fishing fleets,
and the general increase in the number of fishermen after the last world war, lead to significant impacts on fish
populations. Thus, there has been much debate as to whether the observed decline in several fish species, such as
cod in the 1960s, was mostly due to fishing or to climate. It is clear that both played a significant role.
The last question to ask, and of particular relevance to this volume, is: Were the observed changes driven by
bottom-up process through increased production or was it dominated by top-down processes through predation from higher to lower trophic levels? A third possibility is a ‘‘wasp-waist’’ control when a forage species
such as capelin might determine the overall changes to the system. While it is probably true that all three processes were operating during the warming event of the 1920s and 1930s, the increases in phytoplankton and
zooplankton production observed off West Greenland and Iceland, which are consistent with recent studies in
the Barents Sea, leads me to think that the primary response during the warm regime was driven by bottom-up
processes. It has become clear that the increased presence of Atlantic waters contributed to higher primary and
secondary production. In addition, with the reduced extent of ice-covered waters, more open water allows for
higher production than in the colder periods. Modelling studies in the Barents Sea (Slagstad and Wassmann,
1996) suggests that primary production levels are as much as 400% higher in ice-free regions in a warm year
(1984) compared to when these same areas are ice-covered during a cold year (1981). Relative to the entire
Barents Sea region, reduced ice cover resulted in an approximately 30% increase in primary production. This
occurred due to a combination of higher light levels in areas of decreased ice extent, higher nutrient levels in
the Atlantic waters where they extended northward and eastward, and faster turn-over times due to the higher
temperatures. Similar increases in primary and secondary production during the warm period of the 1920s to
at least the late 1950s off Iceland (as suggested by the findings of Astthorsson and Vilhjálmsson, 2002) and off
West Greenland (by Pedersen and Rice, 2002) were also a result of higher light levels (in areas affected by seasonal ice coverage), higher nutrient concentrations and faster turn-over times.
Studying the response to the early 20th century warming is of practical significance, in that this information
may provide clues about what to expect under future climate scenarios. We are presently experiencing a warming trend, not only in the northern North Atlantic but globally. Several of the ecosystem changes observed
during the 1920s and 1930s in the northern North Atlantic appear to be repeating themselves, a detailed analysis of which will be the topic of a future paper.
Acknowledgements
I gratefully appreciate the many discussions with K. Brander, O. Nakken, and S. Sundby and the helpful
comments on an earlier draft by O. Nakken, S. McKinnell, J. Overland and an anonymous reviewer. Financial
support from the Research Council of Norway through the NESSAS and MACESIZ projects is acknowledged. This is publication Nr A 114 from the Bjerknes Centre for Climate Research.
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