Biological Oceanography

(^) Field data are always somewhat indirect, often incomplete in one way or another.
For example, in the study by Bollens and Frost (1989a) there were no actual fish
trawls in April 1985. Seeking more assurance, Bollens and Frost (1989b) ran an
experiment on the migrations of the copepod Acartia hudsonica in mesocosm bags
hung from a raft in a coastal lagoon. Some bags had just copepods, some had
copepods plus fish, and some had copepods plus fish confined in mesh containers
suspended near the surface. Fish free in the container induced copepod vertical
migration, while copepods alone or with caged fish did not migrate. The caged fish
show that chemicals from the fish do not mediate induction of migration; the fish are
detected from the effects of their motion. While this was true in that one marine study,
the results of many freshwater studies have shown that fish odor can be a strong
migration cue. For example, Loose (1993) pumped water from an aquarium with no
fish, then one or more fish, into the top of a tower tank holding a stock of the
cladoceran Daphnia and a food stock of algae. The tank was 11 meters deep with
algae mostly above a thermocline at 3 meters. In the absence of fish in the aquarium,
the Daphnia spread out through the layer above the thermocline. When fish were
placed in the aquarium, Daphnia spread through the upper layer at night, but sank to
the thermocline during daylight. With more fish, the effect was stronger. Since the fish
were not in the tank with the cladocerans, the transfer of information about predation
risk must have been relayed by something exuded by the fish and carried in the water.
Such substances are called kairomones, chemicals transferring information that the
fish would be better served to keep secret, as opposed to pheromones that an animal
secretes in order to transfer information that it is useful to make known to other
animals.
(^) While predation avoidance is reasonably accepted as the dominant adaptive value of
DVM, other adaptive purposes have been suggested. Before it was realized that many
species actually stop migrating when predators are absent, these alternative
explanations held considerable interest. It remains possible that they play a part for
some animals and circumstances. Alister Hardy pointed out that a vertical migrator
that finds its food in one layer of the ocean can shift position substantially (many
kilometers) by spending a period in a different layer going in another direction or at a
different speed. Ocean currents do vary directionally with depth, providing a
zooplankter with a means to search for “greener pastures”. Maximum benefit would
require that migration stop once a really good “grocery” had been located. No strong
observations have been reported of such cessation of migrations. It has been hinted
that the phytoplankton patches produced by iron fertilization experiments retain some
of the regularly migrating copepods like Metrida and Pleuromamma during daytime.
However, net-tow results (Zeldis 2001) from “SOIREE”, an iron fertilization
experiment in the Southern Ocean, showed little if any such effect. Ian McLaren
suggested that migrants might benefit from DVM because they switch between