Late copepodite and adult stages of the copepod Calanus finmarchicus were collected north of Shetland, in the Faroe-Shetland Channel and West of Ireland on seven occasions from October 1993 to March 1996. In winter the population was found in highest concentrations at depths greater than 500 meters around the 0°C isotherm and consisted mainly of copepodite stage CV in a state of diapause. The project investigated seasonal fluctuations in dry weight, carbon, nitrogen, protein and RNA content and respiration rates. The study indicate that moulting, ovary development and ascent from overwintering depth to the surface waters in the spring is largely dependent on reserves stored from the previous summer. Oxygen consumption rates were very low at in situ temperatures in winter (7 - 30 μmol O2 gC-1 h-1 for stage CV, 29 - 148 μmol O2 gC-1 h-1 for stage CV, 332 μmol O2 gC-1 h-1 for females). Overwintering animals showed significant response to changes in temperature. The animals regulated their oxygen consumption with declining partial pressure of oxygen but oxygen consumption decreased when oxygen tension dropped below 80 Torr for stage CV and 100 Torr for females. Oxygen consumption tended to decrease with increasing body weight but he correlation was poor, possibly owing to the variable amount of stored fat. In March respiration rates of fed stage CV were consistently higher than for non-feeding specimens at all temperatures. Temperature constituted a major factor in the change in respiration rates while feeding increased the oxygen consumption only slightly. A model was constructed to estimate survival of overwintering animals with carbon content equivalent to that expected of animals in October. In order to survive during winter and have enough energy for moulting and ascent these animals have to live at temperatures close to 0°C and be in a diapause state
