158
THE BIOLOGY O F EUPHAUSIIDS
ment outside the wheel. They showed, by means of this apparatus, that
Meganyctiphanes norvegica can swim upwards through distances of the
order of 100 m at an average speed of about 90 m per hr but higher
swimming speeds were recorded over shorter vertical distances
-173.1 m per hr through 5.77 m upward movement. Similarly,
M . norvegica swam downwards through 130 m a t a speed of 130 m per
hr. They give no results for the upward rate of swimming of Thysanoessa
inermis but show that it can swim downwards through 40m a t a
speed of 40m per hr. The average vertical distance through which
species of euphausiids migrate is 100-300 m (Table IV) and they take
2-4 hr to travel this distance, times which agree with the potential
swimming speeds determined by Hardy and Bainbridge. One of the
main points made by these authors is that the downward movement at
dawn is not a passive sinking movement but an active period of swimming back down to the daylight depths of occurrence.
What factors modify the general pattern of migration? Mauchline
(1960) showed that age (or size) of Meganyctiphanes norvegica seemed
to determine how closely the individual animals approached the surface
a t night. The animals collected a t the different times and depths
throughout the 24 hr were measured and grouped according to size.
The mean depth of occurrence of each size group at any one time was
then calculated and the results are shown in Figs 57 and 58. There is
a definite correlation in the November samples between size of males and
females and the depth to which they rise during the hours of darkness.
This layering of size groups is not so regular in July but the population
now comprises two distinct year groups; the smaller animals, the
O-group and the larger animals, the I- and II-groups, produce an
almost bimodal population because the number of animals in the
population belonging to the PI-group is small and relatively insignificant
when compared to the numbers of 0- and I-group animals. There is a
difference in the behaviour of the 0- and the I-group animals because
(Fig. 58) the O-group approach the surface much more closely than do
the older animals. The size groups are not spaced out so regularly as in
the winter but the period of darkness is much shorter in the summer and
any effects of differential sinking rates between animals of different
sizes would not be so pronounced in the shorter time period. No such
relationship between depth of night occurrence and size was present in
the populations of ThysanoEssa raschii investigated in the Clyde
(Mauchline, 1966a).
Einarsson (1 945) shows that juvenile Meganyctiphanes norvegica live,
on average, closer to the surface than the adults and in an analysis
of the spatial vertical distribution of the adults he finds that the
THE BIOLOGY O F EUPHAUSIIDS
ment outside the wheel. They showed, by means of this apparatus, that
Meganyctiphanes norvegica can swim upwards through distances of the
order of 100 m at an average speed of about 90 m per hr but higher
swimming speeds were recorded over shorter vertical distances
-173.1 m per hr through 5.77 m upward movement. Similarly,
M . norvegica swam downwards through 130 m a t a speed of 130 m per
hr. They give no results for the upward rate of swimming of Thysanoessa
inermis but show that it can swim downwards through 40m a t a
speed of 40m per hr. The average vertical distance through which
species of euphausiids migrate is 100-300 m (Table IV) and they take
2-4 hr to travel this distance, times which agree with the potential
swimming speeds determined by Hardy and Bainbridge. One of the
main points made by these authors is that the downward movement at
dawn is not a passive sinking movement but an active period of swimming back down to the daylight depths of occurrence.
What factors modify the general pattern of migration? Mauchline
(1960) showed that age (or size) of Meganyctiphanes norvegica seemed
to determine how closely the individual animals approached the surface
a t night. The animals collected a t the different times and depths
throughout the 24 hr were measured and grouped according to size.
The mean depth of occurrence of each size group at any one time was
then calculated and the results are shown in Figs 57 and 58. There is
a definite correlation in the November samples between size of males and
females and the depth to which they rise during the hours of darkness.
This layering of size groups is not so regular in July but the population
now comprises two distinct year groups; the smaller animals, the
O-group and the larger animals, the I- and II-groups, produce an
almost bimodal population because the number of animals in the
population belonging to the PI-group is small and relatively insignificant
when compared to the numbers of 0- and I-group animals. There is a
difference in the behaviour of the 0- and the I-group animals because
(Fig. 58) the O-group approach the surface much more closely than do
the older animals. The size groups are not spaced out so regularly as in
the winter but the period of darkness is much shorter in the summer and
any effects of differential sinking rates between animals of different
sizes would not be so pronounced in the shorter time period. No such
relationship between depth of night occurrence and size was present in
the populations of ThysanoEssa raschii investigated in the Clyde
(Mauchline, 1966a).
Einarsson (1 945) shows that juvenile Meganyctiphanes norvegica live,
on average, closer to the surface than the adults and in an analysis
of the spatial vertical distribution of the adults he finds that the