Influence of Long-term Changes of Sea-Surface Temperature on the State of Some Fish Stocks in the Northwest Atlantic

Abstract

The changes in spawning biomass of the northern stock cod in Canadian waters and cod biomass off West Greenland were compared with the changes of sea-surface temperatures (SST)in the Irminger Sea.The correlation between variations of SST and cod biomass during the period 1956–88 in these areas indicated the influence of climatic changes on the status of the spawning stock of cod in the Newfoundland area and the cod stock at West Greenland. Similar correlations were observed in the variations of SST during the period 1977–88 in the Scotian Shelf area and the status of the silver hake stock. Introduction Over the recent 30 years, considerable changes in the status of stocks of some commercial fish species in the Northwest Atlantic have been reported. The changes in the biomass of particularly the northern spawning stock of cod off Newfoundland (FO’S’CLE, 1988), cod stock at West Greenland (Schumacher, MS 1988) and the silver hake stock on the Scotian Shelf (Fanning et al. MS 1987; Noskov, MS 1987) have been of significant interest in recent years. In the late-1970s and early-1980s these stocks were reduced to their minimum levels, followed by marked increases in the mid-1980s. There have been differing viewpoints concerning the causes of these changes. While various abiotic and biotic factors are often considered to have effects on the status of the stocks, one of the causes is also thought to be the introduction in 1977 of the Extended Jurisdiction of Coastal States to 200-miles from the coastline and the subsequent fishery regulatory measures and fishing practices. The overall effect of such factors are difficult, if not impossible to evaluate, however, consideration of individual factors are often possible and practical. The present report considers the relationship of one environmental parameter, water temperature, in relation to the status of these three fish stocks. Materials and Methods Mean monthly sea surface temperatures (SST) from the centres of each 5° quadrants of latitude and longitude on the Scotian Shelf (40–45°N, 60– 65°W) and in the Irminger Sea (55–60°N, 30–35°W were obtained from the Hydrometeocenter of the Union of Soviet Socialist Republics. Information on the status of the three stocks was obtained from published literature. Silver hake biomass estimates were from Fanning et al. (MS 1987) and Noskov (MS 1987), while the biomass estimates of the northern spawning stock of cod off Newfoundland were from FO’C’SLE (1988) and the West Greenland cod stock were from Schumacher (MS 1988) and other papers. The SSTs are considered as indexes reflecting the interaction of oceanolographic systems. The systems relevant to this study consist of: the admixture of warm Gulf Stream waters and cold waters of the Labrador Current in the Nova Scotian Shelf area; warm waters of the North Atlantic Current merging with the Irminger Current and cold polar and Labrador waters in the Irminger Sea and West Greenland areas. Integral curves of the water temperature anomalies at the sea surface were therefore prepared. This technique is widely accepted in studies of global variations of the oceanolographic and hydrometeorlogical factors in the North Atlantic (Zverev, 1972; Girs, 1971). The integral curves are good for identification of intensifications (when positive values of anomalies are recorded), weakening (when negative values of anomalies are recorded) or stabilization (when small departures are recorded in the anomalies) of the SST. The increase of the sum of water temperature anomalies on the Scotian Shelf is considered to be the result of intensification of the Gulf Stream influence (approaching closer to the Shelf) while the decrease is assigned to increased influence of the cold Labrador Current. Similarly, the rise of the water temperatures in the Irminger Sea can be attributed to the increased effect of the warm Irminger Current and the movement of the subpolar front to the north and west, while the drop of temJ. Northw. Atl. Fish. Sci., Vol. 14: xx–xxx 2 J. Northw. Atl. Fish. Sci., Vol. 13, 1992 TABLE 1. Change of sum of SST anomalies in the Irminger Sea from 1977 to 1988. Month 1977 1978 1979 198