Diel Vertical Migration
(^) Nineteenth-century studies of zooplankton in both oceans and lakes showed greater
abundance estimates near the surface at night than during daylight. For a time, two
competing hypotheses could have explained this observation (Franz 1912). Either
plankton moved upward at night and returned to depth during the day, or they avoided
nets better in daylight than in the dark. A clear test requires vertically stratified
sampling, that is, abundance estimates for animals filtered from restricted layers in the
water column. If animals depart the lower layers at night, as well as departing the
upper layers during the day, then an actual population movement occurs. Since many
kinds of animals are involved, it is not very surprising that each explanation applies to
some species. Daytime net avoidance is restricted to animals with advanced visual
systems such as euphausiids. It is not seen in those with simpler eyes such as
copepods and chaetognaths. A statistically strong data-set (Fig. 8.16) was provided by
Brinton (1967a & b), who examined the migration and net avoidance of euphausiids
in the California Current using opening–closing ring-nets. Species like Nyctiphanes
simplex are harder to catch during daylight, so the vertical integral during the day was
much less than that at night, but they showed no sign that deeper-living individuals
moved up at night. Presumably, those individuals near the surface see the nets coming
and move out of the way, while those in dim light at depth are less successful at
dodging. Species like Euphausia hemigibba had nearly constant vertical integrals;
apparently they did not interpret the oncoming net as a threat to avoid, but their modal
depth shifted strongly upward from day to night. For many years, such migratory
behavior was called diurnal vertical migration, but in the 1970s that changed to diel
vertical migration, since diurnal specifically refers to activity in daytime, while the
migratory behavior is characterized over the whole day–night cycle. Under either
terminology the behavior is often referred to by the acronym DVM.
Fig. 8.16 Comparisons at stations in the California Current of day and night net-tow
profiles for (a) Euphausia hemigibba, a vertically migrating euphausiid, and (b)
Nyctiphanes simplex, a non-migrating euphausiid that avoids nets during daylight.
Numbers beside the curves are vertical integrals (number m−2 )
(^) (After Brinton 1967b.)