The influence of climate on marine populations which are also being heavily exploited by fisheries is a tough problem to quantify. It is doubtful that any attempt to assess the variations in population levels in terms of effects tied to environmental change versus fishing mortality can be successful without some further understanding of the fundamental processes involved with the dynamics of fish populations and their interaction with both the physical environment and the fishery. At the same time it is crucial to obtaining an understanding of population dynamics that both the natural and man-induced sources of population variation are considered. With this in mind it is worthwhile to consider the variations in stocks.
The variations in catch for a number of species of commercially utilized fish on Georges Bank is shown in Fig. 8-9. The extreme changes in catch reflect a combination of variations in species that fisheries target, the response of the populations to fishing pressure and variations in the environment. Fluctuations such as the peak in red hake catch in the mid-1970s can be attributed to changes in fleet deployments. Likewise the decline in cod and haddock in the late 1960s reflects fishing pressure. Survey cruises by NMFS, however, indicate that there have been major swings in year class strength which do not bear a direct relationship to fishing. The recovery of the cod and haddock for example follows a period of anomalously mild winters in the early 1970s. The effects of these winters and the transition to the harsh winters of 1977-78 show up clearly in long term weather records and in water masses throughout the Northwestern Atlantic (Talley and Raymer, 1982; Talley and McCartney, 1982). These changes are also reflected in large zooplankton standing stocks in 1973 which cannot be tied to the activities of the fishing fleets (Davis, 1987). The problem is to sort out the changes tied to climate variations, i.e. the physical signal itself and its manifestation in terms of the environmental needs of fish.
It is not the purpose of this portion of the report to consider approaches to the problem, but it is worth pointing out that the complexity of the issue demands a broad approach with a combination of techniques in the field, on the data archives and in both biological and physical models.
The North Atlantic Oscillation was first described by Walker (1924) and Walker and Bliss (1932) as a see-sawing in surface pressures between the Azores and Iceland (Fig. 8-11). It is associated with fluctuations in wind strength across the entire North Atlantic and in sea surface temperature anomalies (Bjerknes, 1962; Rogers and van Loon, 1979). The slow decline in the NAO index in the late 1960s is correlated with marked changes in the 18°C subtropical mode water at Bermuda (Talley and Raymer, 1982), the cessation of Labrador Sea Water formation (Lazier, 1981; Talley and McCartney, 1982) and a maximum in the extent of sea ice around Iceland (Lamb, 1977; Kelly et al., 1987) and Newfoundland (Hill and Jones, 1990) in the late 1960s. This period in the late 1960s also saw the onset of a pronounced freshening of the sea surface which has been traced around the entire subpolar Atlantic over the decade and a half following the NAO minimum in the late 1960s (Dickson et al., 1986). The overall connection between NAO, the sea ice which also involves interaction with the Arctic proper (and the solar cycle according to Hill and Jones, 1990) and the freshening of the subarctic Atlantic in the 1970-80s is still only partly understood. It is clear, however, that these changes in climate have profound effects on the marine environment and fisheries such as the cod. For example, there was a massive decline in the West Greenland cod fishery associated with the anomalous conditions around 1970 in the Labrador and Irminger Seas. In fact Koslow (1984) suggests that these conditions impacted cod stocks throughout the northwestern Atlantic. The exact relationship between climatic variables and cod stocks in this regard demands further study in order to understand the underlying mechanics behind these large scale covariations (Koslow et al., 1987).
Part of the problem with isolating causal relations between large scale climate signals and local stocks involves differences in the manifestation of the NAO and other climate changes in various local regions. For example, the effect of the NAO on the Scotian shelf is quite different than what is expected off Iceland. Typically more localized analyses such as that by Thompson et al. (1988) of eastern Canadian sea surface temperature anomalies and their relationship to atmospheric variables isolate fluctuations on other time scales which further complicate the picture. The biotic responses, of course, are expected to change markedly throughout the range of a species like cod or zooplankters such as Calanus finmarchicus which is closely linked to cod throughout its range. Unraveling the complicated relationship between these organisms and climate variability demands both detailed consideration of the processes involved with their interaction and relationship to local physical conditions as well as attempts to glean more information for the historical data sets.
One clue to the possible dynamics of these swings in the earth's climate involves an interaction between the atmosphere-ocean system that seems to be a robust result of certain computer simulations of the system. This involves global atmosphere circulation changes and a switch between two states in the oceans' density driven or thermohaline circulation (Manabe and Stouffer, 1988). The center of these changes occurs in the far North Atlantic, which is the primary site for the formation of nearly all of the worlds deep water characteristics in our present climate state (Reid and Lynn, 1971; Gordon, 1986). At present deep waters are formed through air-sea interaction in the Norwegian, Greenland and Labrador Seas. It is known that the formation rate in these waters has varied on time scales as short as decades in the available hydrographic database (Talley and McCartney, 1982). These switches in deep water formation are tied to salinity anomalies similar to the freshening observed in recent decades in the North Atlantic (Lazier, 1980). Essentially the formation of a layer of fresh, low density waters, known as polar water on the sea surface causes a cessation of deep convection associated with cooling and salinity increases in surface salinity. The salinity changes in the deep ocean are also reflected in the salinities in the Labrador Current, which affects the coastal regime as far south as Georges Bank.
Interestingly, models such as the GFDL (Geophysical Fluid Dynamics Laboratory of NOAA) global climate model are prone to the onset of a complete cessation of deep water formation (Manabe and Stouffer, 1988). Such an alternate model state can exist with initial conditions that vary only slightly from those which set up a global ocean-atmosphere system very close to that observed today. Manabe and Stouffer (1988) suggest these two possible climate states are at least qualitatively similar to the conditions associated with variations between the little ice age and today's climate and very possibly the difference between glacial and interglacial periods. Long term trends in state of the art climate models suggest that the ocean-atmosphere system is fundamentally susceptible to climate oscillations (Bryan, 1986).
The changes in conditions evident from direct observations, the geological record and model simulations all suggest large shifts in fisheries potential within the North Atlantic ecosystem. At the ice age extreme the habitat for cod was probably greatly compressed in latitudinal extent and viable local space because of the combination of sea surface temperature and salinity changes and the hundred meter drop in sea level that put areas such as Georges Bank and much of the Scotian Shelf above sea level. The fairly abrupt changes between glacial and interglacial periods with the accompanying fresh water surges through the Mississippi and St. Lawrence Rivers (Broecker et al, 1989), and sea level rise represent large short time scale swings in demersal habitat and probably eventually a wide expansion of the species such as cod to fill the expanding niches. The time scales for these changes are such that they sit between the time required for speciation and the shorter time frame adaptations in physiology and behavior through which a species can acclimate itself to global change.
An approach to the longer time scale issues in North Atlantic climate change requires a better understanding of the present ecosystem and its response to variations in physical forcing and an exploitation of models as well as a study of the full historical and geological record. Therefore, advances on the longer term question of the evolution of the ecosystem will be made through process oriented investigations aimed at shorter time frame questions. There should also be some consideration of the longer term aspects with a focus on the crucial multidisciplinary tools required for progress on reconstructing transitions associated with ice age cycles and anomalous periods such as the little ice age.
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