Climate, cod, and capelin in northern waters
The hydrographic conditions in the Iceland Sea reveal three different regimes on the North Icelandic Shelf: Atlantic, Arctic, and Polar. Thus, time series of temperature and salinity in the shelf waters show that Atlantic Water from the warm irminger Current dominated during a long period prior to the mid-1960s. During the latter half of the 1960s, however, there was a dramatic change to lower temperatures, as Polar surface waters from the East Greenland Current were carried into the area. As a result, the northern and partly also the eastern coasts of Iceland were frequently blocked by drift ice in the late winter and spring of these years. Oceanographic effects of this extremely cold period, the so-called “Great Salinity Anomaly”, were observed during the following years over wide areas in the northern North Atlantic. Since this event, conditions in North Icelandic waters have been alternating between cold and warm periods, but stable and warm conditions similar to those before 1965 have not returned. There have not always been Polar surface waters on the North Icelandic Shelf during later cold periods, sometimes Arctic Water from the Iceland Sea has occupied the area. These shifts in hydrographic conditions in the Iceland Sea have important ecological impacts, because the North Icelandic Shelf is a nursery area for the Icelandic cod and capelin populations. Thus the variable flow of Atlantic Water into the North Icelandic waters may influence the drift of larvae from the spawning areas on the South Icelandic Shelf to the nursery grounds. In the nursery area the changing oceanographic conditions further bring about varying living conditions for the larvae and juvenile fish. The data show that survival rate of cod has been highest in periods with warm water from the Irminger Current on the North Icelandic Shelf. The Atlantic, Polar, and Arctic conditions in North Icelandic waters also seem to influence the size of the Icelandic capelin stock, which was at its lowest during Arctic conditions. As the “Great Salinity Anomaly” could be traced as it advected around the Subpolar Gyre in the Northern North Atlantic, the ecological impacts of this event on several cod stocks can be compared, i.e. the stocks off Iceland, West Greenland, Newfoundland, and Norway. On these fishing grounds the catches of cod declined from 1960 to 1990 by about 50%, recruitment by about 67% and spawning stocks by about 75%. This indicates an increasing fishing load from 1960 to 1990. Furthermore, improving hydrographic conditions in Icelandic waters after 1990 did not result in a new strong year class of cod. It is questioned whether this failure in recruitment was due to a critically small spawning stock.
Article from Marine Science Symposia Vol. 198 - "Cod and climate change". Symposium held in Reykjavik, 23-27 August 1993. To access the remaining articles please click on the keyword "MSS Vol. 198".