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Progress in Oceanography Progress in Oceanography 68 (2006) 134–151 www.elsevier.com/locate/pocean 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. 136 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). 138 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. 140 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 142 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. 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