312
THE BIOLOQY OF EUPHAUSIIDS
only about 5-10% of the breeding I-group (now becoming II-group)
animals survive to the following spring when they can breed for a third
time. Similar fluctuations in the proportions of the different age groups
of Thysanoessa raschii were found (Mauchline, 1966a). The O-group
animals which survive to the autumn are likely to survive the winter to
breed. The O-group (becoming the I-group) suffer mortality but
appreciable numbers, about 25%, survive the summer and autumn to
breed again the following spring. Mortality is evident at breeding and
the I-group animals (becoming the II-group) disappear almost completely from the population by the summer.
The mortality at breeding can sometimes be detected by examining
changing sex ratios within the population. The sex ratio of the population of Meganyctiphanes norvegica in Loch Fyne was 1 : 1 in October to
December but females became predominant in January, February, and
March, the time during which transference of spermatophores is made
from the males to the females. The following month, April, the ratio
changed completely and males predominated, presumably through
death of females after egg-laying. Marr (1962) found that after mating,
swarms of Euphausia superba may comprise only females, suggesting
that the males begin to die immediately they have transferred the
spermatophores.
Euphausiids die through natural causes and through the action of
predators. The mortality of adults during the breeding season is
probably caused by the state of health of the individuals but mortality
of eggs, larvae and of adult euphausiids during the summer and autumn
is more probably caused by predation. We shall discuss the predation
by other animals on euphausiids later.
THE BIOLOQY OF EUPHAUSIIDS
only about 5-10% of the breeding I-group (now becoming II-group)
animals survive to the following spring when they can breed for a third
time. Similar fluctuations in the proportions of the different age groups
of Thysanoessa raschii were found (Mauchline, 1966a). The O-group
animals which survive to the autumn are likely to survive the winter to
breed. The O-group (becoming the I-group) suffer mortality but
appreciable numbers, about 25%, survive the summer and autumn to
breed again the following spring. Mortality is evident at breeding and
the I-group animals (becoming the II-group) disappear almost completely from the population by the summer.
The mortality at breeding can sometimes be detected by examining
changing sex ratios within the population. The sex ratio of the population of Meganyctiphanes norvegica in Loch Fyne was 1 : 1 in October to
December but females became predominant in January, February, and
March, the time during which transference of spermatophores is made
from the males to the females. The following month, April, the ratio
changed completely and males predominated, presumably through
death of females after egg-laying. Marr (1962) found that after mating,
swarms of Euphausia superba may comprise only females, suggesting
that the males begin to die immediately they have transferred the
spermatophores.
Euphausiids die through natural causes and through the action of
predators. The mortality of adults during the breeding season is
probably caused by the state of health of the individuals but mortality
of eggs, larvae and of adult euphausiids during the summer and autumn
is more probably caused by predation. We shall discuss the predation
by other animals on euphausiids later.