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 ca use individuals to enter and leave this state are the subject of speculatio n. 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 th e 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 contr olled by photoperiod, and show that this leads to unrealistic patterns of r eproductive 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 yie lds more plausible annual patterns, but fails to reproduce observed geograp hical variation in the timing of the yearly onset of reproductive activity. Our final model dispenses with photoperiodic control and assumes that diap ause duration is controlled by the normal development processes operating a t a reduced rate. This model reproduces all the qualitative features of the patterns of reproductive activity observed at our 4 test sites, thus demon strating that photoperiod cueing is not a necessary prerequisite for the re gular re-ascension of C. finmarchicus into surface waters.