10. GROWTH, MATURITY AND MORTALITY
311
crystallorophias, E . frigida, Thysanoessa macrura, and even T . vicina
and some of the subantarctic and subarctic species take 2 years to
mature sexually. No information on the maturation of tropical species
of euphausiids is available and it is, of course, possible that some of
these species, especially those which are of small body size, may mature
in less than 1 year so that not only may one female produce more than
one brood of eggs but two complete generations may be possible within
the 1 year. The general biology of tropical and subtropical euphausiids
has still to be studied. The majority of these species are of small body
size unlike subarctic, subantarctic, and polar species which are larger.
Similarly, almost nothing is known about the growth of the mesopelagic
and bathypelagic species (Table 111) because the numbers of these
animals obtained in any one haul are very small.
What is known about rates of mortality in euphausiids a t different
stages of their life history? There must be a heavy mortality of the
eggs, nauplii, and metanauplii because catches of calyptopes are
considerably fewer than of these previous stages. Zelikman (19 64)
counts, on average, 200-400 eggs per cubic metre of water from 50 m
depth to the surface in the Barents Sea but the numbers of calyptopes
were only about 10-100 per cubic metre. These are similar to counts
made in the Clyde between the sea bottom (180 m) and the surface.
The numbers of furciliae, especially the later furciliae, are fewer still
but they do occur in comparable numbers to the calyptopes so that the
mortality of these stages cannot be so great as that of the earliest
stages. The next period in the life when increased mortality takes place
is at breeding. The changing proportions of the different age groups
within the population of Meganyctiphunes norvegica are evident in the
population histograms of Mauchline (1960). I n March, when breeding
starts, the O-group animals represent two-thirds to one half of the
population, the I-group the remainder. I n April, the number of I-group
animals decreases, presumably because of mortality after breeding, and
the O-group now represents 7 5 4 0 % of the adult population. This
O-group (now breeding and becoming the new I-group) also suffers a
decrease in numbers. Thus, it is concluded that males and females born
in the spring and surviving to the autumn are likely to survive through
the winter in the Clyde to breed. After breeding, and so becoming the
new I-group, they suffer mortality, the magnitude of which is difficult
to estimate because the concentrated breeding population now tends
to disperse; it is probable from the present evidence, however, that
their numbers are reduced by about one half. The greater portion of
these seem to survive through the following winter to breed a second
time, but mortality at this breeding period, and after it, is greater and
A.M.B.-7
11
311
crystallorophias, E . frigida, Thysanoessa macrura, and even T . vicina
and some of the subantarctic and subarctic species take 2 years to
mature sexually. No information on the maturation of tropical species
of euphausiids is available and it is, of course, possible that some of
these species, especially those which are of small body size, may mature
in less than 1 year so that not only may one female produce more than
one brood of eggs but two complete generations may be possible within
the 1 year. The general biology of tropical and subtropical euphausiids
has still to be studied. The majority of these species are of small body
size unlike subarctic, subantarctic, and polar species which are larger.
Similarly, almost nothing is known about the growth of the mesopelagic
and bathypelagic species (Table 111) because the numbers of these
animals obtained in any one haul are very small.
What is known about rates of mortality in euphausiids a t different
stages of their life history? There must be a heavy mortality of the
eggs, nauplii, and metanauplii because catches of calyptopes are
considerably fewer than of these previous stages. Zelikman (19 64)
counts, on average, 200-400 eggs per cubic metre of water from 50 m
depth to the surface in the Barents Sea but the numbers of calyptopes
were only about 10-100 per cubic metre. These are similar to counts
made in the Clyde between the sea bottom (180 m) and the surface.
The numbers of furciliae, especially the later furciliae, are fewer still
but they do occur in comparable numbers to the calyptopes so that the
mortality of these stages cannot be so great as that of the earliest
stages. The next period in the life when increased mortality takes place
is at breeding. The changing proportions of the different age groups
within the population of Meganyctiphunes norvegica are evident in the
population histograms of Mauchline (1960). I n March, when breeding
starts, the O-group animals represent two-thirds to one half of the
population, the I-group the remainder. I n April, the number of I-group
animals decreases, presumably because of mortality after breeding, and
the O-group now represents 7 5 4 0 % of the adult population. This
O-group (now breeding and becoming the new I-group) also suffers a
decrease in numbers. Thus, it is concluded that males and females born
in the spring and surviving to the autumn are likely to survive through
the winter in the Clyde to breed. After breeding, and so becoming the
new I-group, they suffer mortality, the magnitude of which is difficult
to estimate because the concentrated breeding population now tends
to disperse; it is probable from the present evidence, however, that
their numbers are reduced by about one half. The greater portion of
these seem to survive through the following winter to breed a second
time, but mortality at this breeding period, and after it, is greater and
A.M.B.-7
11