12. PREDATORS AND PARASITES
357
I n the Antarctic regions, Euphausia vallentini, E. superba, E. crystalbrophias, and Thysanoessa rnacrura are all species that aggregate in the
surface layers and so are easily available t o the baleen whales.
Euphausia similis aggregates in the lower subantarctic zones where it
has been infrequently recorded in whale stomachs. The two species,
E. frigida and E . triacantha, and especially the latter (see Baker, 1959),
do not aggregate to the same extent as the other species and have not
been recorded as whale food. Similarly, E. longirostris and Thysanoessa
vicina have not been recorded in the stomachs of whales. A small species
such as Nyctiphanes australis, which is known to form ‘‘ surface rafts ”,
might be expected to be grazed by whales if they are in the sea area at
the right time but so far there are no records of this.
What is the extent of the predation by whales on euphausiids?
Bryde’s whale can have as much as 200 kg of euphausiids in its stomach ;
since the weight of a single specimen of Euphausia sirnilis, the species
in question, is about 0.2g, this weight represents about one million
individuals. Nemoto (1959) has estimated quantities of euphausiids
present in the stomachs of fin whales. A male of body length 62ft
and a female of body length 65 ft had 112.5 kg and 100 kg of euphausiids
respectively; another female of 64 ft had 10 kg of euphausiids. The
most detailed study of predation of an euphausiid by whales is that of
Marr (1962) on Euphausia superba. He argues that smaller size groups,
even the sixth furciliae, form a significant contribution to the diet of
whales, a fact which was not previously realized because it was thought
that only E. superba of more than 20 mm body length were of paramount importance. This probably devolved from the fact that the mass
of larger krill in the whale’s stomach masks the large number of small
krill mixed in it. Marr concludes that the whales feed indiscriminately
on all size classes of krill but because of the structure of their baleen
plates there is a tendency for them to catch larger krill more efficiently
than smaller krill. The whales are only feeding on E. superba throughout the Antarctic summer and migrate to warmer waters a t the onset
of the Antarctic winter. Consequently, the yearling krill (O-group) have
a good chance of surviving the grazing period of the whales after which
they continue to grow and develop to breeding condition when they
are once again subjected to grazing. Marr tries to estimate the amount
of krill consumed by the whales in unit time. He makes the following
assumptions : (1) that the average population of the large Antarctic
baleen whales was about 210000 over the period 1933-39, (2) that,
as a conservative average, each individual of this population spends
90 days on the feeding grounds, (3) that each individual fills its stomach
once per day and (4) that the average weight of the contents of a full
357
I n the Antarctic regions, Euphausia vallentini, E. superba, E. crystalbrophias, and Thysanoessa rnacrura are all species that aggregate in the
surface layers and so are easily available t o the baleen whales.
Euphausia similis aggregates in the lower subantarctic zones where it
has been infrequently recorded in whale stomachs. The two species,
E. frigida and E . triacantha, and especially the latter (see Baker, 1959),
do not aggregate to the same extent as the other species and have not
been recorded as whale food. Similarly, E. longirostris and Thysanoessa
vicina have not been recorded in the stomachs of whales. A small species
such as Nyctiphanes australis, which is known to form ‘‘ surface rafts ”,
might be expected to be grazed by whales if they are in the sea area at
the right time but so far there are no records of this.
What is the extent of the predation by whales on euphausiids?
Bryde’s whale can have as much as 200 kg of euphausiids in its stomach ;
since the weight of a single specimen of Euphausia sirnilis, the species
in question, is about 0.2g, this weight represents about one million
individuals. Nemoto (1959) has estimated quantities of euphausiids
present in the stomachs of fin whales. A male of body length 62ft
and a female of body length 65 ft had 112.5 kg and 100 kg of euphausiids
respectively; another female of 64 ft had 10 kg of euphausiids. The
most detailed study of predation of an euphausiid by whales is that of
Marr (1962) on Euphausia superba. He argues that smaller size groups,
even the sixth furciliae, form a significant contribution to the diet of
whales, a fact which was not previously realized because it was thought
that only E. superba of more than 20 mm body length were of paramount importance. This probably devolved from the fact that the mass
of larger krill in the whale’s stomach masks the large number of small
krill mixed in it. Marr concludes that the whales feed indiscriminately
on all size classes of krill but because of the structure of their baleen
plates there is a tendency for them to catch larger krill more efficiently
than smaller krill. The whales are only feeding on E. superba throughout the Antarctic summer and migrate to warmer waters a t the onset
of the Antarctic winter. Consequently, the yearling krill (O-group) have
a good chance of surviving the grazing period of the whales after which
they continue to grow and develop to breeding condition when they
are once again subjected to grazing. Marr tries to estimate the amount
of krill consumed by the whales in unit time. He makes the following
assumptions : (1) that the average population of the large Antarctic
baleen whales was about 210000 over the period 1933-39, (2) that,
as a conservative average, each individual of this population spends
90 days on the feeding grounds, (3) that each individual fills its stomach
once per day and (4) that the average weight of the contents of a full