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

Abstract

Cod G. morhua stocks on the Labrador Shelf and Grand Banks, NW Atlantic, are at an all-time low. While overfishing has occurred, other factors are affecting stock recruitment as well. Water temperatures and salinity are also unusually low. Northern cod begin spawning in March. First-feeding cod larvae require suitable prey, especially nauplii of the copepod Calanus finmarchicus. 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. 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. Several cold, fresh "anomalies' have been observed in the last few decades. Periods of "ocean cooling', with potentially serious consequences for the cod fishery, may be predictable. -from Authors