Considerations Before Going into the Field

Abstract

If the depth of a zooplankton population in daytime differs significantly from the depth at night, the population is said to migrate. It has become customary to speak of diel vertical migration, thus of a phenomenon involving a 24-h cycle. In the older literature, the term diurnal was used, indicating that downward and upward swimming occurred in daytime. Since light was (and still is) considered important as a causal factor, the term diurnal was obvious. Sometimes migrations were called nocturnal if a single maximum at the surface was observed sometime between sunset and sunrise (Hutchinson, 1967). Also the term twilight migrations was used if it was thought that the vertical movements occurred at dawn and around sunset. I think a classification of migration patterns is difficult to make, hard to apply and not useful. The classifications suggest something about the underlying mechanism, for example, that the initiating stimulus in diurnal migrations operates during the daytime light period or during the night in nocturnal migrations but nearly always information from the field was not detailed enough and about the stimulus nothing was known. So the neutral term diel has come to be used but the experience from the past makes it clear that more information must be obtained than is possible with a few samples per day if DVM is to be understood.