Environmental influences on fish and shellfish production in the Northwest Atlantic

Evidence is reviewed, linking physical oceanographic processes in the marine environment to changes in fish and shellfish stocks in the Northwest Atlantic. A case history study of the cod (Gadus morhua) stock of the northern Grand Banks and Labrador Shelf indicates a long slow amelioration of the environment between about 1945 and 1965, followed by a deterioration in the period 1965–1992. The most important environmental factors for the cod stocks appear to have been salinity and temperature. The trends can be traced back to climatic factors involving the Icelandic Low and the Azores–Bermuda High. When the atmospheric pressure difference in winter tended to be high, there was a progressive increase in the area of sea ice off Labrador and in the volume of cold water at depth. These factors have been shown to affect temperature and salinity conditions on the Grand Banks in spring and summer and are associated with poor growth and recruitment in the cod stocks. A similar case study of lobster (Homarus americanus) stocks indicates that temperature and river discharge are important environmental correlates, but neither can be shown to fully account for the recent trends in the stocks. Evidence is reviewed to show that physical environmental processes also influence recruitment and distribution of stocks of haddock, capelin, and squid. Some of the problems with correlational analysis are also discussed. It is recognized that factors other than the environment are influencing the stocks. Fishing mortality (detailed consideration of which is not included in this review) has clearly been important. Interactions between environmental factors and fishing mortality are probably of major importance.Key words: ocean environment, fish production, recruitment, northern cod, American lobster.