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Much effort is presently expended in attempting to understand more fully the dynamics of pelagic communities and the role of phytoplankton, zooplankton and fish. Long-term changes in conlmunities may be a particularly enlightening way of determining the interactions which occur, and may be useful for predicting the consequences of environmental change. Recently, examinations of long-term changes of diel vertical migration behaviour of the calanoid copepod Calanus finmarchicus (copepodite stages C5 and C6) were completed by Hays (1995) and Hays et al. (1996) uslng data from over 3 decades, collected by the Continuous Plankton Recorder (CPR) survey in the North Sea. The results of their studies appeared to demonstrate that the diel vertical migration of this important zooplankton species was correlated with the biomass of the herring Clupea harengus, a species for which a major component of their diet may be, late stage C. finmarchicus (Last 1989). While herring stocks declined from 1960 to the mid 1970s, and then increased until 1990, C. finmarchicus (C5 and C6) appeared to demonstrate a decrease in the time spent in surface waters from the 1960s to the mid 1970s, and an increase until the 1990s. It was suggested that this copepod avoided predation from herring by avoiding surface waters during the day in years when this visual predator was more abundant. Such a large scale, long-term pattern in behavioural change would have important implications for our understanding of fisheries and zooplankton behaviour and dynamics. We will show, however, that the results and conclusions are based on an unsuitable test, and are unfounded. Previously the formula used to quantify diel vertical migratory activity by Hays (1995) and Hays et al. (1996) was of the form:

Long-term changes in the diel vertical migration behaviour of Calanus finmarchicus in the North Sea are unrelated to fish predation