LONG-TERM CHANGES IN THE DIEL VERTICAL MIGRATION BEHAVIOR OF ZOOPLANKTON

Samples historically collected and analysed by the Continuous Plankton Recorder survey were used to examine long-term (1958 to 1994) pattern s in the normal diel vertical migration (NDVM) behaviour of 7 copepod taxa in the North Sea: Calanus finmarchicus C5-C6; Calanus spp. C1-C4; Centropages typicus; Centropages hamatus; Temora longicornis; Acartia clausii and Para-Pseudocalanus (this last group included all Paracala nus and Pseudocalanus species). The ratio of night:day abundance near the surface was used as a measure of the extent of NDVM. For all 7 tax a, the extent of NDVM between 1958 and 1994 co-varied with the abundan ce of herring Clupea harengus in the North Sea. Fisheries data show th at during this period the herring stock was a good indicator of the ov erall abundance of planktivorous fish in the North Sea. These results suggest that changes in the abundance of planktivorous fish in the Nor th Sea over recent decades have resulted in modifications in the NDVM behaviour of many zooplankton taxa.