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

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. Flrst-feeding cod larvae survlve in waters less than 2°C but require suitable prey, especially nauplii of the copepod Calanus finmarchlcus, 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 stabhty 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 temperaturekalinity 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.

Physical controls and interannual variability of the Labrador Sea spring phytoplankton bloom in distinct regions

“…The match/mismatch hypothesis between predator and prey cycles states that the survival of higher trophic levels with time-varying life stages depends on food availability at critical periods (Conover et al, 1995). Several physical effects are known to impact bloom timing or productivity, primarily light and stratification (Sverdrup, 1953).…”

“…Labrador Sea food chains tend to be short, enabling a description of the relationship between higher consumers such as cod, whale and bird populations to primary production (Conover et al, 1995;Laidre et al, 2007Laidre et al, , 2008. Recruitment of copepods, the most abundant mezozooplankton in the region, depends on the supply of food (Head et al, 2000).…”

Seasonal patterns in plankton communities in a pluriannual time series at a coastal Mediterranean site (Gulf of Naples): an attempt to discern recurrences and trends

“…In addition, inadequate spatial and temporal scales of sampling may fail to record all the phases of an annual cycle, or overestimate the importance of exceptional events or miss them altogether. Multiannual series of data represent a powerful tool for the reliable reconstruction of plankton seasonal cycles and of their driving factors (Conover et al, 1995;Le Fevre-Lehoerff et al, 1995;Sournia and Birrien, 1995;Southward, 1995;Buecher et al, 1997;Mozetić et al, 1998;Licandro and Ibanez, 2000). In fact, observations repeated over several years allow one to distinguish regular and recurrent patterns from occasional and exceptional events (Goy et al, 1989;Ménard et al, 1994Ménard et al, , 1997Shiganova, 1998).…”

Spatial patterns of the temporal dynamics of three gadoid species along the Norwegian Skagerrak coast

“…Variation in fish abundance is furthermore linked to abiotic environmental influences, such as changes in temperature, salinity, wind field and currents (e.g. Cushing & Dickson 1976, Southward et al 1988, Cury & Roy 1989, Ellersten et al 1989, Dickson & Brander 1993, Conover et al 1995, Ottersen & Sundby 1995. Human exploitation is another process that strongly affects fish dynamics and has been identified as the main cause of the collapse of several regional fish stocks (e.g.…”