162
THE BIOLOGY O F EUPHAUSIIDS
Gravid females of Meganyctiphanes norvegica are occasionally found
close to the surface during daylight hours in summer in the Firth of
Clyde, but so few have been found that they are considered exceptional and their breeding does not materially affect the vertical
distribution of the population.
Several papers have been published suggesting that euphausiids are
the source of the effect known as the deep scattering layer or sonic
scattering layer (Boden, 1962). This scattering layer is observed with
echo-sounding equipment and the source of the sonic scattering has
been ascribed to crustaceans, fish, or cephalopods (Hersey and Backus,
1962). Current ideas suggest fish to be the most likely, the sound being
reflected by the swim bladders of such deep-sea fish as myctophids.
Hersey and Moore (1948) however, were the first to suggest that euphausiids might be responsible because they perform a diurnal vertical
migration, as do the scattering layers. Further, these authors found
that a hundred euphausiids present in the sound beam will reflect sound
a t about 20 kc a t a distance of 6-8 ft from a projector-receiver. Moore
( 1950) further elaborated this suggestion that populations of euphausiids
can be the origin of a sonic scattering layer ; the circumstantial evidence
which he presents for this is impressive. Kampa and Boden (1954)
measured the intensity of illumination a t the level of a scattering layer
which performed a diurnal vertical migration. They concluded that
there were two parts to the layer, an upper part probably comprising
smaller organisms (invertebrates? )-and Saito and Mishima (1952) found
that a layer recorded on the echo-sounder near Cape of Esan in Hokkaido
consisted predominantly of E . pacifica-and a lower part of larger
organisms (fish?). Tucker (1951) concluded that the upper part of the
layer that he observed was composed dominantly of euphausiids
which were found t o live at depths where the isolumes were in
the light range 1 x
pW/cm2 and to follow these isolumes upwards and downwards in the sea during the 24 hr so that their
vertical position was correlated with the upper regions of the deepscattering layer recorded on the echo-sounder. Other organisms, such
as the lantern fish and sergestid shrimps, were responsible for the echos
received from the lower parts of the layer (Clarke, 1966). Boden and
Kampa (1967) studied a sonic scattering layer southeast of Fuertaventura in the Canary Islands and concluded that the diurnal vertical
migrations of the animals (unidentified) comprising the layer were
controlled primarily by changes in the intensity of daylight, the
vertical position of the layer a t any one time corresponding closely with
the depth of the 5 x
pW/cm2, 474 mp isolume. Kampa and Boden
(1957) and Boden et al. (1965) examined levels of intensity of bioto 1 x
THE BIOLOGY O F EUPHAUSIIDS
Gravid females of Meganyctiphanes norvegica are occasionally found
close to the surface during daylight hours in summer in the Firth of
Clyde, but so few have been found that they are considered exceptional and their breeding does not materially affect the vertical
distribution of the population.
Several papers have been published suggesting that euphausiids are
the source of the effect known as the deep scattering layer or sonic
scattering layer (Boden, 1962). This scattering layer is observed with
echo-sounding equipment and the source of the sonic scattering has
been ascribed to crustaceans, fish, or cephalopods (Hersey and Backus,
1962). Current ideas suggest fish to be the most likely, the sound being
reflected by the swim bladders of such deep-sea fish as myctophids.
Hersey and Moore (1948) however, were the first to suggest that euphausiids might be responsible because they perform a diurnal vertical
migration, as do the scattering layers. Further, these authors found
that a hundred euphausiids present in the sound beam will reflect sound
a t about 20 kc a t a distance of 6-8 ft from a projector-receiver. Moore
( 1950) further elaborated this suggestion that populations of euphausiids
can be the origin of a sonic scattering layer ; the circumstantial evidence
which he presents for this is impressive. Kampa and Boden (1954)
measured the intensity of illumination a t the level of a scattering layer
which performed a diurnal vertical migration. They concluded that
there were two parts to the layer, an upper part probably comprising
smaller organisms (invertebrates? )-and Saito and Mishima (1952) found
that a layer recorded on the echo-sounder near Cape of Esan in Hokkaido
consisted predominantly of E . pacifica-and a lower part of larger
organisms (fish?). Tucker (1951) concluded that the upper part of the
layer that he observed was composed dominantly of euphausiids
which were found t o live at depths where the isolumes were in
the light range 1 x
pW/cm2 and to follow these isolumes upwards and downwards in the sea during the 24 hr so that their
vertical position was correlated with the upper regions of the deepscattering layer recorded on the echo-sounder. Other organisms, such
as the lantern fish and sergestid shrimps, were responsible for the echos
received from the lower parts of the layer (Clarke, 1966). Boden and
Kampa (1967) studied a sonic scattering layer southeast of Fuertaventura in the Canary Islands and concluded that the diurnal vertical
migrations of the animals (unidentified) comprising the layer were
controlled primarily by changes in the intensity of daylight, the
vertical position of the layer a t any one time corresponding closely with
the depth of the 5 x
pW/cm2, 474 mp isolume. Kampa and Boden
(1957) and Boden et al. (1965) examined levels of intensity of bioto 1 x