10. GROWTH, MATURITY AND MORTALITY
303
Several growth curves for euphausiids in the northern and southern
hemispheres have been computed from statistical analyses of samples
collected throughout the year (Figs 115 and 116). The curves for 1- and
2-year-old Thysanozssa longipes are depicted separately because of the
sexual differences in growth rates in this species (Nemoto, 1957). The
differences between the growth rates of males and females of other
species are less marked and so average curves have been produced.
Several notable features appear in these analyses and some of these are
shown in Table XXX along with some data on other species whose full
growth curves have not been computed. The growth rates of the same
species in different sea areas are different as illustrated by the curves for
Meganyctiphanes norvegica, Thysanoessa raschii and T . inermis, and
T . longicaudata. Lower sea temperatures are associated with slower
rates of growth. Einarsson (1945) found the growth of T . inermis and
T . raschii to be so similar that he only presents curves for T . inermis.
It is these curves which are reproduced here for southern Iceland and
for north and eastern Iceland and western Greenland. These are
compared with the growth curve of T . raschii in the Clyde sea area.
Marr (1962) compares Bargmann’s growth curves for male and female
Euphausia superba with that of Ruud for the sexes combined (Fig. 116) ;
the three curves have overall similarities but the earlier part of Ruud’s
curve is based on too few data and rises too steeply. According to Ruud’s
curve E. superba attains a greater length by January/February of the
second year and also by January/February of the third year than is
indicated by Bargmann’s curves but Marr suggests that these differences
in the curves may originate from the sources of the samples ; most of
Ruud’s samples were obtained from the stomachs of whales whereas
Bargmann’s samples were obtained by nets from the sea. The whales
tend to select larger krill owing to the mesh of their baleen and so the
samples may not have been truly representative of the natural population but biased towards the larger size groups. Ruud’s and Bargmann’s
growth curves are based on material from the relatively warm zone of
abundance-the Weddell drift, the Bransfield Strait, and the South
Georgia whaling grounds-but Bargmann’s samples a t Stations 1359,
602, and 575 are in the East Wind Zone and the average lengths of
males and females in this area are lower than the values in the computed
curves for lower latitudes (Fig. 116). Nemoto’s curve, which tends to be
lower than the others, is computed from samples taken from the more
southerly East Wind Zone. There is an interesting difference between
the growth curve for E. triacantha and those for E. superba (Fig. 116).
Growth of E. triacantha appears to cease during the first half of the
second year of life while E. superba continues to grow in the early
303
Several growth curves for euphausiids in the northern and southern
hemispheres have been computed from statistical analyses of samples
collected throughout the year (Figs 115 and 116). The curves for 1- and
2-year-old Thysanozssa longipes are depicted separately because of the
sexual differences in growth rates in this species (Nemoto, 1957). The
differences between the growth rates of males and females of other
species are less marked and so average curves have been produced.
Several notable features appear in these analyses and some of these are
shown in Table XXX along with some data on other species whose full
growth curves have not been computed. The growth rates of the same
species in different sea areas are different as illustrated by the curves for
Meganyctiphanes norvegica, Thysanoessa raschii and T . inermis, and
T . longicaudata. Lower sea temperatures are associated with slower
rates of growth. Einarsson (1945) found the growth of T . inermis and
T . raschii to be so similar that he only presents curves for T . inermis.
It is these curves which are reproduced here for southern Iceland and
for north and eastern Iceland and western Greenland. These are
compared with the growth curve of T . raschii in the Clyde sea area.
Marr (1962) compares Bargmann’s growth curves for male and female
Euphausia superba with that of Ruud for the sexes combined (Fig. 116) ;
the three curves have overall similarities but the earlier part of Ruud’s
curve is based on too few data and rises too steeply. According to Ruud’s
curve E. superba attains a greater length by January/February of the
second year and also by January/February of the third year than is
indicated by Bargmann’s curves but Marr suggests that these differences
in the curves may originate from the sources of the samples ; most of
Ruud’s samples were obtained from the stomachs of whales whereas
Bargmann’s samples were obtained by nets from the sea. The whales
tend to select larger krill owing to the mesh of their baleen and so the
samples may not have been truly representative of the natural population but biased towards the larger size groups. Ruud’s and Bargmann’s
growth curves are based on material from the relatively warm zone of
abundance-the Weddell drift, the Bransfield Strait, and the South
Georgia whaling grounds-but Bargmann’s samples a t Stations 1359,
602, and 575 are in the East Wind Zone and the average lengths of
males and females in this area are lower than the values in the computed
curves for lower latitudes (Fig. 116). Nemoto’s curve, which tends to be
lower than the others, is computed from samples taken from the more
southerly East Wind Zone. There is an interesting difference between
the growth curve for E. triacantha and those for E. superba (Fig. 116).
Growth of E. triacantha appears to cease during the first half of the
second year of life while E. superba continues to grow in the early