Overwintering strategies in Calanus finmarchicus

The marine copepod Calanus finmarchicus is known to overwinter at depth in a state analogous to diapause in terrestrial insects, but the cues which cause individuals to enter and leave this state are the subject of speculation. In this paper we use a simple demographic model, driven by environmental data from 4 distinct locations in the organism's eastern range, to test the plausibility of a number of candidate mechanisms. We start from Miller et al.'s (1991; Mar Ecol Frog Ser 72:79-91) hypothesis that diapause is controlled by photoperiod, and show that this leads to unrealistic patterns of reproductive activity in northern waters. We extend the model to include the recently reported phenomenon (Hirche 1996a; Ophelia 44:111-128) of delayed gonad maturation in females experiencing low food concentrations. This yields more plausible annual patterns, but fails to reproduce observed geographical variation in the timing of the yearly onset of reproductive activity. Our final model dispenses with photoperiodic control and assumes that diapause duration is controlled by the normal development processes operating at a reduced rate. This model reproduces all the qualitative features of the patterns of reproductive activity observed at our 4 test sites, thus demonstrating that photoperiod cueing is not a necessary prerequisite for the regular re-ascension of C. finmarchicus into surface waters.