Diel vertical Migrations
Diel vertical migrations
• Primary production confined to upper layers
• Visual predators forage most efficiently in upper
layers
• Both food supply and risks of predation decreases
by depth
• Vertical distribution depends on size; many small
forms (~< 1 mm) in upper waters throughout the diel
cycle
• Larger forms to upper layers under the cover of
darkness, recide in deeper waters during the day
• Scale: from a few meters to many hundred meters
Other reasons for DVM
• Migrations into warm upper waters at night
to speed up digestion
• Some organisms appear to exploit low
oxygen deep-water for predator avoidance
– ascent into more oxygenated upper
waters at night (we have projects in
Bunnefjorden and in the Benguela current
off Namibia)
The classic DVM: ascent at night;
descent at day
Note however:
• Many small forms in upper layers also at day
• Some organisms/(organisms on some
occasions) may perform inverse migrations (up
at day, down at night)
• Asynchornous migrations (e.g. individuals
making repetitive excursions between upper and
lower layers throughout the night) may be
important, yet little documented
Inverse diel vertical migrations
• If the main predator is a dielly migrating,
non-visual predator (invertebrates), then
inverse diel migrations may occur, i.e.
small plankton stays in upper layer at day,
and descend at night
Genotypic/phenotypic
What is genetically fixed, and how
flexible is DVM behavior?
Field experiment fresh water
• Some Alpine lakes are without fish, some
lakes stocked with fish for short periods,
and some with fish through centuries
• Has natural selection resulted in different
vertical distribution and DVM in these
lakes?
Cyclops sp
Vertical distribution of Cyclops sp day and night
in a lake without fish (left) and a lake stocked
with fish for millenia (right)
Vertical distribution of Cyclops in lakes stocked with fish
for 12 years, 35 years, and several centuries (note that
the copepods may dig into the sediment)
Conclusions:
• No DVM without predators
• More migration with increasing time periods
(evolutionary time) exposed for predators
• There has been a genetic selction in the
population
• Apparently a high degree of genetically
imprinted DVM in the ocean
• However:
Phenotypic flexibility?
• Genetically invariant behavior is profitable
with stable conditions of good growth, but
high predation risk in upper layers
• It is a waste to descend at day (leave food
and high temperature) if the threath of
predation is not real
• Flexibility has been tested in mesocosms
Plankton (copepods) in
mesocosms with, and without fish
Conclusion
• Always DVM in the lagoon outside the
mesocosms (fish in the lagoon)
• DVM in mesocosm with presence of fish
• No DVM without fish
If the predator is non-visual
• Experiments in mesocosms (freshwater)
with Chaoborus as predator and the
copepod Diaptomus being prey
Diaptomus
Vertical distribution of a copepod (shaded) day and night
with (left) and without a dielly migrating invertebrate
predator. Chla profiles suggest the access to food
Vertical distribution of a copepod at night in
water from a lake without (left) and with
”predator spiked” water
Conclusion
• The copepod responds to a chemical from
the predator (Chaoberus)
• Phenotypically indused reversed DVM
related to presence of a non-visual
predator
• The non-visual predator carries out normal
DVM; vulnerable to fish and forage on
other prey items than the reversely
migrating copepod
DVM Mesopelagic fish
Diel vertical migration of
mesopelagic fauna
• Many mesopelagic groups (e.g. fish and
shrimps) forage in upper layers at night
• Some forage during ”antipredation
windows at dusk and dawn”; other forage
throughout the night, or carry out
asynchronous migrations
• Apparently little DVM below 800-1000 m
Maurolicus muelleri
”mid-night” sinking and dawn
ascent
”Antipredation window”
• Visual predators like small fish have their own visual
predators (larger fish)
• Periods at dusk and dawn may represent sufficient
strong light to detect own food, while light is
sufficiently low to give relative protection against
predators
• Planktivores need less light than piscivores since
they are feeding on much more numerous prey (i.e.
do not have to look as far as piscivores)
• In this case, the planktivores stop foraging at night
(”mid-night sinking”), which is not always the case
Sometimes plankton is safer in
mid-waters than in deeper waters
Krill, saithe and blue whiting in the
Norwegian Sea
Implications presence of
piscivorous fish
• Blue whiting has large eyes and can likely
forage efficiently on krill in deep waters
• Predation pressure increases if it ascends
into more shallow waters ”patrolled” by
saithe
• Presence of saith makes life safer for krill,
and krill is safer in mid-waters than in
deeper waters
Benthosema glaciale
Northern lanternfish/nordlig
lysprikkfisk
Maurolicus muelleri
Laksesild
Mesopelagic fish Masfjorden
Conclusion mesopelagic fish
• Species of mesopelagic fish may show varied
and divers DVM-patterns, including normal DVM
(ascent in darkness; with and without midnight
sinking), reversed DVM (some ascent at day), or
no migration.
• Normal DVM is likely related to feeding in upper
waters; reversed DVM to diurnal feeding on
midwater prey, and non-migrating may be
satiated, or for other reasons lack ”motivation” to
migrate