5. VERTICAL DISTItIBUTION AND MIGRATION
171
temperatures. We are interested in how much this energy bonus might
be, and whether it can be directed into fecundity in such a way as to
overcome the retardation by low temperatures, and thus give migrants
an advantage over individuals which spend their whole lives in the
warmer surface waters.” Two main criticisms can be made of this
argument. The first is that not all migrating organisms obtain their food
exclusively or even nearly exclusively in the surface layers a t night;
as pointed out earlier in this chapter, euphausiids do not seem to feed
more intensively a t night than during the day. The second is that
McLaren probably does not pay sufficient attention to the relatively
large amount of energy which a migrating organism has to expend to
reach the surface layers and the further expenditure of energy involved
in swimming back down. Further, phytoplankton is available in the
surface waters during the summer when the period of darkness is
relatively short and the animals in temperate latitudes spend the least
amount of time in the surface waters, and so there is some doubt as to
whether they can filter enough water to obtain sufficient phytoplankton
for their needs during the night ; it is probable that feeding also takes
place during the day. Small et al. (1966) and Small and Hebard (1967)
measured the respiratory rates of Euphausia paci$ca a t different
temperatures and found that if they migrated to deeper and cooler
water during the day, their respiratory rates decreased but they point
out that the effects of pressure, not investigated in this study, might
tend to offset the effects of temperature. These criticisms do not
invalidate McLaren’s theory; they simply imply that any effects of
changes in temperature on growth are secondary to other reasons for
vertical migration, if such reasons can be recognized.
We consider that vertical migration probably confers a number of
benefits on the organisms rather than one of great significance. First,
these organisms are in a three-dimensional environment which has
vertical gradients of light intensity and pressure and may or may not
have a vertical gradient of temperature. Light and pressure gradients
are perceptible and useful controlling mechanisms for vertical movements of the organisms. All these organisms are directly or indirectly
dependent for their nutritional demands on the surface phytoplankton
and consequently, there must be considerable advantage in their
migrating to the euphotic zone during summer months when phytoplankton is available. Lateral movements brought about by differential
rates of movement of water layers enables parts of the population of a
species to reach patches of phytoplankton and the associated herbivores
and this is beneficial to the survival of the species. Further it disperses
the species throughout its geographical range and allows it to utilize the
171
temperatures. We are interested in how much this energy bonus might
be, and whether it can be directed into fecundity in such a way as to
overcome the retardation by low temperatures, and thus give migrants
an advantage over individuals which spend their whole lives in the
warmer surface waters.” Two main criticisms can be made of this
argument. The first is that not all migrating organisms obtain their food
exclusively or even nearly exclusively in the surface layers a t night;
as pointed out earlier in this chapter, euphausiids do not seem to feed
more intensively a t night than during the day. The second is that
McLaren probably does not pay sufficient attention to the relatively
large amount of energy which a migrating organism has to expend to
reach the surface layers and the further expenditure of energy involved
in swimming back down. Further, phytoplankton is available in the
surface waters during the summer when the period of darkness is
relatively short and the animals in temperate latitudes spend the least
amount of time in the surface waters, and so there is some doubt as to
whether they can filter enough water to obtain sufficient phytoplankton
for their needs during the night ; it is probable that feeding also takes
place during the day. Small et al. (1966) and Small and Hebard (1967)
measured the respiratory rates of Euphausia paci$ca a t different
temperatures and found that if they migrated to deeper and cooler
water during the day, their respiratory rates decreased but they point
out that the effects of pressure, not investigated in this study, might
tend to offset the effects of temperature. These criticisms do not
invalidate McLaren’s theory; they simply imply that any effects of
changes in temperature on growth are secondary to other reasons for
vertical migration, if such reasons can be recognized.
We consider that vertical migration probably confers a number of
benefits on the organisms rather than one of great significance. First,
these organisms are in a three-dimensional environment which has
vertical gradients of light intensity and pressure and may or may not
have a vertical gradient of temperature. Light and pressure gradients
are perceptible and useful controlling mechanisms for vertical movements of the organisms. All these organisms are directly or indirectly
dependent for their nutritional demands on the surface phytoplankton
and consequently, there must be considerable advantage in their
migrating to the euphotic zone during summer months when phytoplankton is available. Lateral movements brought about by differential
rates of movement of water layers enables parts of the population of a
species to reach patches of phytoplankton and the associated herbivores
and this is beneficial to the survival of the species. Further it disperses
the species throughout its geographical range and allows it to utilize the