The seasonal changes in the respiration of Calanus are considerable and are, on the whole, related to size and therefore to weight. Length alone is not enough to account for the differences since ripe females, although the same length as unripe, have a markedly higher respiration. In addition, by taking samples of large and of small Calanus of a single stage, it was shown that the difference in respiration was small. Neither is weight by itself enough to account for the difference between groups. Stage V Calanus are, for a given length, heavier even than ripe females and yet their oxygen utilization is low. In this instance, however, an important part of the weight consists of fat which is a food reserve and not actively metabolizing. The difference of weight between ripe and unripe females is not known, but ripe females must be heavier and this will account for their higher oxygen consumption.

Although the oxygen and therefore the food required during the spring months is high, at that time the phytoplankton is at its maximum and is probably sufficient to fulfil all needs. Egg-laying depends on the food supply and it is then that Calanus starts breeding. In winter, on the other hand, the Calanus is present as Stage V and oxygen consumption is little more than half what earlier figures suggested. No ‘hibernation’ seems to take place but the population is living in an economical way for Stage V use little oxygen, live in deep water and do not undertake diurnal vertical migration.

At 10° C ripe female Calanus will require daily from 3·9–7·2% of their body weight as dry matter in summer and from 2·8–6·7% in winter. Stage V will require 2·3–3·1% in summer and 1·4–3·3% in winter. The higher values are for carbohydrate and the lower for fat.

It is difficult to believe that Calanus in winter will be able to find enough food by filtration alone. The fact that in the winter months it depends more on predation may account for its survival.