146
THE BIOLOGY OF EUPHAUSIIDS
but this need not necessarily be true. Consequently, care is required in
the interpretation of data on vertical distributions of planktonic
animals when the data are obtained from " open nets ". Studies of the
vertical migration of species should be made several times throughout
the year to detect seasonal variations in behaviour. The frequency with
which hauls are made throughout the 24 hr period depends, to a large
extent, on the relative lengths of the periods of daylight and darkness
and on the vertical extent of the migration. The latter parameter also
influences the vertical intervals between nets. If the range of depth
of occurrence is 800 m as opposed to 100 m then the intervals between
nets could be 100 m instead of 20-30 m. If the night is long then hauls at
four hourly intervals probably provide sufficient details of vertical movements but during the summer when the night is short it is frequently
necessary to take hourly or even more frequent hauls to determine the
pattern of movement. It is usually advantageous to sample frequently
during the periods of dawn and dusk especially if accurate estimates of
the rates of upward and downward movements are required.
Mauchline (1960, 1966a) has made detailed studies of the vertical
migrations of Meganyctiphanes norvegica and Thysanoessa raschii in the
Firth of Clyde and a detailed examination of the results of these studies
is warranted before examining the less detailed data on other species.
Hickling (1925) and Macdonald (1927a) were the first authors to
describe a movement of Meganyctiphanes norvegica towards the surface
layers a t night, although Fowler (in Holt and Tattersall, 1905c)
obtained data which indicated such a movement. This species lives
close to the sea bottom during daylight hours, as was realized by
Hickling (1925), Macdonald (1927a), and Hjort and Ruud (1929), but
moves towards the surface layers at dusk. Bigelow (1926) says : " not
only have we taken it [ M . norvegica] right down to the bottom of the
deepest trough of the Gulf [of Maine], but it is only in the lowest strata
of the Gulf that it occurs regularly and in numbers throughout the year,
except in the Eastport region ". Meganyctiphanes norvegica lives close
to or immediately above the sea bed in the deeper areas of the Firth of
Clyde and a net carried on a sledge, which runs over the sea bottom,
often catches more animals than a similar net towed 20 m above the
sea bottom. The vertical migrations of M . norvegica and Thysanoessa
raschii were examined in the winter and summer (Fig. 5 5 ) to discover
the effect of length of night on the migrations. Migrations of males and
females were examined separately (Mauchline, 1960, 1966a) but their
behaviour was so similar that, for the purposes of this discussion, the
results have been combined in Fig. 55. The 0- and I-year groups are
mentioned in descriptions of these results ; the O-group consists of those
THE BIOLOGY OF EUPHAUSIIDS
but this need not necessarily be true. Consequently, care is required in
the interpretation of data on vertical distributions of planktonic
animals when the data are obtained from " open nets ". Studies of the
vertical migration of species should be made several times throughout
the year to detect seasonal variations in behaviour. The frequency with
which hauls are made throughout the 24 hr period depends, to a large
extent, on the relative lengths of the periods of daylight and darkness
and on the vertical extent of the migration. The latter parameter also
influences the vertical intervals between nets. If the range of depth
of occurrence is 800 m as opposed to 100 m then the intervals between
nets could be 100 m instead of 20-30 m. If the night is long then hauls at
four hourly intervals probably provide sufficient details of vertical movements but during the summer when the night is short it is frequently
necessary to take hourly or even more frequent hauls to determine the
pattern of movement. It is usually advantageous to sample frequently
during the periods of dawn and dusk especially if accurate estimates of
the rates of upward and downward movements are required.
Mauchline (1960, 1966a) has made detailed studies of the vertical
migrations of Meganyctiphanes norvegica and Thysanoessa raschii in the
Firth of Clyde and a detailed examination of the results of these studies
is warranted before examining the less detailed data on other species.
Hickling (1925) and Macdonald (1927a) were the first authors to
describe a movement of Meganyctiphanes norvegica towards the surface
layers a t night, although Fowler (in Holt and Tattersall, 1905c)
obtained data which indicated such a movement. This species lives
close to the sea bottom during daylight hours, as was realized by
Hickling (1925), Macdonald (1927a), and Hjort and Ruud (1929), but
moves towards the surface layers at dusk. Bigelow (1926) says : " not
only have we taken it [ M . norvegica] right down to the bottom of the
deepest trough of the Gulf [of Maine], but it is only in the lowest strata
of the Gulf that it occurs regularly and in numbers throughout the year,
except in the Eastport region ". Meganyctiphanes norvegica lives close
to or immediately above the sea bed in the deeper areas of the Firth of
Clyde and a net carried on a sledge, which runs over the sea bottom,
often catches more animals than a similar net towed 20 m above the
sea bottom. The vertical migrations of M . norvegica and Thysanoessa
raschii were examined in the winter and summer (Fig. 5 5 ) to discover
the effect of length of night on the migrations. Migrations of males and
females were examined separately (Mauchline, 1960, 1966a) but their
behaviour was so similar that, for the purposes of this discussion, the
results have been combined in Fig. 55. The 0- and I-year groups are
mentioned in descriptions of these results ; the O-group consists of those