It has long been known that certain fish, for example, herring and mackerel, usually approach the surface in large numbers only during darkness, and fishermen have adapted their techniques to match this feature of fish behaviour. Since the early days of plankton research it has also been observed that tow-net catches of zooplankton from the surface are usually richer and more varied during the night than in daytime. Poor daytime catches of zooplankton at the surface may be partly due to the ability of some of the larger and more active animals to see the slow-moving net and to avoid capture by swimming; but if repeated plankton hauls are made at a series of depths throughout the day and night, it becomes clear that many planktonts alter their depth dielly, moving nearer the surface during darkness and returning to the deeper water to pass the day. At night, these organisms are not found in samples collected at the deeper levels where they can be captured during the day, but are then found in samples taken nearer the surface from which they are absent in daytime.
These vertical migrations have often been termed diurnal. However, they are more accurately described as diel - events that occur with a 24-hour rhythm. Diel changes of distribution are shown by a great variety of organisms, both plankton and nekton, including medusae, siphonophores, ctenophores, chaetognaths, pteropods, copepods, cladocerans, amphipods, mysids, euphausids, pelagic decapods, some cephalopods and fish. In shallow water there are many creatures that live on or in the bottom during the day but leave the sea-bed at night and swim in the surface water.
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