CR 05:69-82 (1995)  -  DOI:

Climate, copepods and cod: some thoughts on the long-range prospects for a sustainable northern cod fishery

Conover RJ, Wilson S, Harding GCH, Vass WP

ABSTRACT: Cod Gadus morhua stocks on the Labrador Shelf and Grand Banks, NW Atlantic, are apparently at an all-time low. While overfishing has undoubtedly occurred, it is probable that other factors are affecting stock recruitment as well. Water temperatures and salinity are also unusually low, which may have affected the abundance of cod in some way. Northern cod begin spawning in March in most stock management zones around Newfoundland, Canada. First-feeding cod larvae survive in waters less than 2*C but require suitable prey, especially nauplii of the copepod Calanus finmarchicus, to grow well. The 'match/mismatch' hypothesis assumes that cod spawn at the same time each year, but spawning by copepods may or may not 'match' depending on the stimulus provided by the 'spring bloom' of phytoplankton, which can vary with environmental conditions by up to 6 wk. While C. finmarchicus is common in the Atlantic waters of the Labrador Sea and Grand Banks, zooplankton in the Labrador Current, especially during this period of low temperature and salinity, are dominated by 2 arctic copepods, C. glacialis and C. hyperboreus, which have different spawning seasons, and therefore probably would not provide a good nutritional match for young cod. Despite global warming, sea water supporting important demersal fish on the continental shelf off northeast Canada is presently colder and fresher than normal. Global warming may have induced melting of glaciers and sea ice and increased runoff in rivers entering the Arctic Ocean and its coastal drainage, reducing salinity at the sea surface, increasing stability and reducing deep convection, and hence upward heat transport. Several cold, fresh 'anomalies' have been observed in the last few decades and they have contributed to less favourable conditions at locations around the North Atlantic. These anomalies may be part of an interdecadal climate cycle of alternating warm and cold periods, the effects of which must be removed to clearly identify those associated with global warming. Whatever the causes, periods of 'ocean cooling', with potentially serious consequences for the cod fishery, may be predictable. Increasing amounts of ice in the eastern Arctic and Greenland Sea are easily monitored by satellite and appear to anticipate reduced temperature/salinity anomalies in the Labrador Sea by about 4 yr. Climatic impacts, whether cyclic or continuing, could be identified sufficiently far in advance to enable modifications in management of the fishery toward at least mollifying their effects, thus improving the prospects for long-term sustainability.

KEY WORDS: Recruitment · Calanus · Copepod nauplii · Cod Gadus morhua · First feeding · Spring bloom · Sea surface temperature · Salinity anomaly · Banks · Currents · Polar front

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