6. VERTICAL DISTRIBUTION AND MIGRATION
149
when the intensity is still at moonlight or twilight levels-the intensity
of a false dawn ". As in the case of the " midnight sinking ) ) , he
produces many examples of species of different kinds of planktonic
crustaceans that tend to approach the surface once again at dawn.
Nyctiphanes simplex and Euphausia pacijca (Esterly, 1914b), E. tenera
and Stylocheiron carinatum (Lewis, 1954) increased in numbers in the
surface layers just prior to sunrise.
Both these phenomena are no doubt common features of the
vertical migration of euphausiid species but difficult to detect unless
hauls are made very frequently throughout the night, certainly at much
shorter intervals than 4 hr. In the analysis of the summer diurnal
migration of Meganyctiphanes norvegica (Fig, 5 5 ) when hauls were
made hourly throughout the night there is still no evidence of a
(I midnight sinking ) ) and " dawn rise )' but the data are more suggestive of a continuous slow movement upwards throughout the period of
darkness. One point of major interest is that the movement down to the
deeper layers starts before the observer is aware of sunrise ; they appear
to start downwards at the first lightening of the eastern sky-the false
dawn.
The amount of water filtered by each metre net during each of these
hauls was approximately 1 000 cubic metres. Most hauls indicate a
density of one specimen per 2 or 3 cubic metres, but densities of four
Thysanoessa raschii per 1 cubic metre and one Meganyctiphanes norvegica
per cubic metre were recorded at a depth of 100 m just after dawn and
confirm the idea that an isolume is descending in the sea as the sun
rises and " collects " the animals on its way down. That is, the animals
below the immediate surface layers do not descend until the layer of
optimum light intensity reaches and then passes them.
During the summer months, these animals live close to the bottom
all day but at dusk they rise towards the surface. The night is short,
darkness lasting only about 5 hr, because there is a long twilight period
in the Firth of Clyde when the sky is clear and even during the night
there is a noticeable lightness in the sky. The relative length of time
that they spend in the surface layers is therefore much shorter in
summer but otherwise the pattern of the migration of both species is
similar to that of the winter months ; that of Thysanoessa raschii is not
illustrated because it was of the same pattern as that of Meganyctiphanes
norvegica.
The vertical migrations of other species of euphausiids are similar
to those of M . norvegica and Thysanoessa raschii and the species whose
vertical migration has been examined are listed in Table IV. Just under
half the known species of euphausiids have been examined and all but
149
when the intensity is still at moonlight or twilight levels-the intensity
of a false dawn ". As in the case of the " midnight sinking ) ) , he
produces many examples of species of different kinds of planktonic
crustaceans that tend to approach the surface once again at dawn.
Nyctiphanes simplex and Euphausia pacijca (Esterly, 1914b), E. tenera
and Stylocheiron carinatum (Lewis, 1954) increased in numbers in the
surface layers just prior to sunrise.
Both these phenomena are no doubt common features of the
vertical migration of euphausiid species but difficult to detect unless
hauls are made very frequently throughout the night, certainly at much
shorter intervals than 4 hr. In the analysis of the summer diurnal
migration of Meganyctiphanes norvegica (Fig, 5 5 ) when hauls were
made hourly throughout the night there is still no evidence of a
(I midnight sinking ) ) and " dawn rise )' but the data are more suggestive of a continuous slow movement upwards throughout the period of
darkness. One point of major interest is that the movement down to the
deeper layers starts before the observer is aware of sunrise ; they appear
to start downwards at the first lightening of the eastern sky-the false
dawn.
The amount of water filtered by each metre net during each of these
hauls was approximately 1 000 cubic metres. Most hauls indicate a
density of one specimen per 2 or 3 cubic metres, but densities of four
Thysanoessa raschii per 1 cubic metre and one Meganyctiphanes norvegica
per cubic metre were recorded at a depth of 100 m just after dawn and
confirm the idea that an isolume is descending in the sea as the sun
rises and " collects " the animals on its way down. That is, the animals
below the immediate surface layers do not descend until the layer of
optimum light intensity reaches and then passes them.
During the summer months, these animals live close to the bottom
all day but at dusk they rise towards the surface. The night is short,
darkness lasting only about 5 hr, because there is a long twilight period
in the Firth of Clyde when the sky is clear and even during the night
there is a noticeable lightness in the sky. The relative length of time
that they spend in the surface layers is therefore much shorter in
summer but otherwise the pattern of the migration of both species is
similar to that of the winter months ; that of Thysanoessa raschii is not
illustrated because it was of the same pattern as that of Meganyctiphanes
norvegica.
The vertical migrations of other species of euphausiids are similar
to those of M . norvegica and Thysanoessa raschii and the species whose
vertical migration has been examined are listed in Table IV. Just under
half the known species of euphausiids have been examined and all but