128
'
=‘I‘HE SEAS
,
.
o our 'coasts it is necessary in the daytime to sh at a depth
"
of ten to fteen fathoms in order to get thelargest catches.
In thebpen ocean the depths at which the greatest assem—
plankton animals are f0und are generally consider—
'ably deeper.
N OW, as has been said, this is the case in the
'
daytime ; but it is a remarkable fact that at night matters
are
quite different, and a curious change comes over the
plankton distribution.
On the approach of dusk, just after
the sun has set, all the animals begin to swim in an upward
direction, so that by about nine or ten o’clock, in the summer
_
'
months, the surface layers, so barren in the daytime, have
been enriChed by animals that have swum up from the
deeper levels.
In Plate 49 is shown the actual results of
collecting at different depths in daylight and at dusk. It
,
can be clearly seen that whereas in— the daytime_ the layers
down to about ve fathoms are very poor in plankton
_
compared -with*the deeper” levels, at dusk the two upper
'
collections are quite as large as the deeper‘ones.
It will be
'
,
noticed that the bottom
is also
'
,
that taken in the daytime, this is because of the addition-of
large quantitieS' of animals that have come up from deeper
'
levels quite near the sea b0ttom ; in fact, there are certain
Small créatures that liVe actually on the bottom itself in the
daytime which move upwards towards the surface at
night ; they may never actually have time to reach the
surface itself, because at dawn all the animals begin to move
doWnwards again to take up their'abodes at their usual day
levels (Plate 48) . We all know that the herring shermen only
shoot
their drift—nets.
This is because at night, the
herring, like the plankton animals, also come to the surface.
This phenomehon of u”pward movement at night, or
_
vertical migration, throws' considerable light on why the
_
dierent“änimals prefer Certain depths in the
almost certain that the causes of. the
.'
'
=‘I‘HE SEAS
,
.
o our 'coasts it is necessary in the daytime to sh at a depth
"
of ten to fteen fathoms in order to get thelargest catches.
In thebpen ocean the depths at which the greatest assem—
plankton animals are f0und are generally consider—
'ably deeper.
N OW, as has been said, this is the case in the
'
daytime ; but it is a remarkable fact that at night matters
are
quite different, and a curious change comes over the
plankton distribution.
On the approach of dusk, just after
the sun has set, all the animals begin to swim in an upward
direction, so that by about nine or ten o’clock, in the summer
_
'
months, the surface layers, so barren in the daytime, have
been enriChed by animals that have swum up from the
deeper levels.
In Plate 49 is shown the actual results of
collecting at different depths in daylight and at dusk. It
,
can be clearly seen that whereas in— the daytime_ the layers
down to about ve fathoms are very poor in plankton
_
compared -with*the deeper” levels, at dusk the two upper
'
collections are quite as large as the deeper‘ones.
It will be
'
,
noticed that the bottom
is also
'
,
that taken in the daytime, this is because of the addition-of
large quantitieS' of animals that have come up from deeper
'
levels quite near the sea b0ttom ; in fact, there are certain
Small créatures that liVe actually on the bottom itself in the
daytime which move upwards towards the surface at
night ; they may never actually have time to reach the
surface itself, because at dawn all the animals begin to move
doWnwards again to take up their'abodes at their usual day
levels (Plate 48) . We all know that the herring shermen only
shoot
their drift—nets.
This is because at night, the
herring, like the plankton animals, also come to the surface.
This phenomehon of u”pward movement at night, or
_
vertical migration, throws' considerable light on why the
_
dierent“änimals prefer Certain depths in the
almost certain that the causes of. the
.'