Diel vertical migration behaviour of Calanus hyperboreus

Samples historically collected and analysed by the Continuous Plankton Recorder survey in the North Atlantic and adjacent seas were used to examine temporal patterns in the near-surface abundance of copepodite stages V and V1 Calanus hyperboreus (n = a total of 32836 samples; mean latitude of samples = 54" N. SD = 6.7'). The occurrence of CV and V1 C. hyperboreus was largely restricted to samples collected in April, May and June. During t h ~ s period there was a diel cycle in near-surface abundance which was consistent with a normal pattern of diel vertical migration. For example, CV-V1 C. hyperboreus were 1.9 times more abundant at night than during the day, a difference which was highly significant (t-test, t,,,;, 3.9. p c 0.001). The ratio of n1ght:day abundance was similar In CV-V1 C finmarchicus (night:day abundance = 1.4), but markedly higher in CV-V1 Metridia lucens (n1ght:day abundance = 12.3) and CV-V1 M. longa (night:day abundance = 15.0).