ABSTRACT. Use of the atomic 0 : N ratio as a metabolic index requires clarification of the biochemical complexities of zooplankton intermediary metabolism. We review the processes responsible for natural and experimentally-induced changes in the ratio and discuss their relative importance in relation to a variable natural environment. During starvation the 0 : N ratio is clearly linked to the availability of energy reserves and the use of body protein. Using theoretical computations, it can be shown that pure protein catabolism will yield 0 : N ratios in the range 3 to 16, while equal amounts of lipid and protein catabolism will correspond to values between 50 and 60. Under natural feeding conditions, the value of the ratio depends on the use by the animal of each biochemical fraction assimilated. Others factors, such as seasonal events in the life cycle, biochenlical composition of the body and food quality are of importance for correct interpretation of the variations. The concepts behind the rates are discussed, suggesting that such measurements reflect the adaptability of the animals to their total environment.

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.

The natural h~story of ctenophores from the Resolute Passage area of the Canadlan High Arctic was examined in 1985 and 1986 The seasonal abundance of 3 ctenophore species, Mertensia ovum, Beroe cucums, and Bolinopsis infundibulum, are reported M ovum was by far the most abundant accounting for 88 9 % of all ctenophores collected, and because of its abundance received the most study The M ovum population developed similarly in both years reaching a maximum abundance of 911 m-2 in early June 1986 The lipofuscin age-pigment technique suggested that M ovum can l~v e for at least 3 yr Gut contents showed A4 ovum to be a n opportunistic feeder, but the copepods Pseudocalanus acuspes Calanus glacialis and C hyperhoreus and the amphipod Parathemlsto libellula formed most of the d~e t M ovum was estimated to consume between 4 and 10 % d.' of the standlng crop of macrozooplankton in Resolute Passage

The natural h~story of ctenophores from the Resolute Passage area of the Canadlan High Arctic was examined in 1985 and 1986 The seasonal abundance of 3 ctenophore species, Mertensia ovum, Beroe cucums, and Bolinopsis infundibulum, are reported M ovum was by far the most abundant accounting for 88 9 % of all ctenophores collected, and because of its abundance received the most study The M ovum population developed similarly in both years reaching a maximum abundance of 911 m-2 in early June 1986 The lipofuscin age-pigment technique suggested that M ovum can l ~ v e for at least 3 yr Gut contents showed A4 ovum to be a n opportunistic feeder, but the copepods Pseudocalanus acuspes Calanus glacialis and C hyperhoreus and the amphipod Parathemlsto libellula formed most of the d ~ e t M ovum was estimated to consume between 4 and 10 % d.' of the standlng crop of macrozooplankton in Resolute Passage

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