FOOD AND FEEDING OF SEI WHALE CAUGHT IN
THE WATERS SOUTH OF 40°N IN THE
NORTH PACIFICAKITO KAWAMURA
Whales Research Institute, Tokyo

ABSTRACT

Results on the food and feeding conditions of sei whales caught by Japanese whaling fleets in the waters south of 40°N of the North Pacific during
the 1972 season were preliminary reported. In the region south of 40°N
the whaling ground was mainly found only in the limited sea area: in theenvirons of the southern tip of the Emperor Seamount Chain, although thefleets were in search of whaling ground between 165°E and 165°W. The
feeding conditions of sei whales in those regions in terms of feeding percentages
did not differ so much when compared with those have been found in thenorthern regions (1967-1971), i.e., 34.4% in the south of 40°N while 55.1 %in 40°-50°N zone, and 50.2% between 50°N and Aleutian Islands. Mostof sei whales in the south of 40°N fed exclusively on a copepod, Calanus pacificus along with several species of euphausiids and fishes. In view of the frequency of occurrence, Calanus pacificus population being consisted of copepodites IV and V, young fishes of Japanese mackerel, Scomber japonicus andJapanese sardine, Sardinops melanosticta were the most important foodstuff. By
examining prey organisms of these food fishes, it was found that most of themfed solely on C. pacificus or on a mixture of C. pacificus with C. plumchrus or with
Eucalanus bungii bungii. The whaling ground found in the south of 40°N can beinterpreted hydrobiologically as having been formed primarily by the richmass occurrence of C. pacificus by the aid of intrusion of migrating populationsof young fishes which were presumably in search of foods. In this connection
it was strongly suggested that two heterotypical communities of food organisms
are distinguished (see Figs. 4 and 6), and the whaling ground is divided bysuch ecological discontinuity which runs along the Emperor Seamount Chain.
This ecological discontinuity would possibly be correlated hydrologically with
the East Kamchatka Current Extension on which Uda (1971) has pointed out
its importance in relation to the formation of fishing ground in the region.
INTRODUCTION

Japanese pelagic whaling in the North Pacific Ocean have been restricted its operation area with the lines of 40°N or 45°N latitudes as its southern most limit by themeasures of Japanese Government in addition to the regulation by the IWC's arrangements on the whole. It was, however, decided to lift the ban to some extent
in 1972 and by this decision the sea region down to 20°N was newly opened chiefly
Sci. Rep. Whales Res. Inst.,

No. 25, 1973, 219-236.

220

KAWAMURA

in the zone of central Pacific between 159°E and 150°W (see Ohsumi, 1973).
Since the beginning ofJapanese whaling operation in the North Pacific in 1952
the main whaling grounds have been restricted almost in the northern North Pacificand Bering Sea (Nasu, 1966), and consequently our knowledge on the food of baleen
whales was also biassed (Nemoto, 1957, 1959). Under these circumstances little is
known on the food and feeding conditions of baleen whales in the southern sea regions above mentioned. The whaling ground south of 40°N may be one of the leaststudied regions in the North Pacific possibly due to its geographical position, thesouth of the Subarctic front. By knowing the opening of new whaling region, somebiological investigations including a collection of foodstuff of whales were established.
During the whaling season of 1972 Japanese fleets entered into those newly opened
region including 40°N chiefly in July and caught 884 sei, 5 bryde's and 8 fin whales
from which I got some amount of food samples. The results of examination andanalysis on the food and feeding conditions of sei whales are reported.
MATERIALS
Among many biological examinations on each whale carcases, kind of food organisms, an approximate amount and freshness of foodstuff in the first stomach were
examined by eyes on the ship's deck. A total of 34 food samples was also collectedfrom the whales caught in the south of 40°N and they were preserved in formalin.
Methods of observation and description on food organisms and feeding conditions in
the field were not different from those having been undertaken in the Antarctic (seeKawamura, 1970). Since many food fishes occurred were consisted of the wellknown species in the North Pacific region, they were remarked by the common namein addition to the records by ordinary classification of "Fish" by the observers.
Euphausiids are usually expressed as " Eu " by the three different sizes among which
Sergestes similis, a macruran shrimp, is also included by confusion. They are, however, reasonably distinguished by the size record of" Large" since there are no such
euphausiids as equivalent to this category in the North Pacific region (Omori et al,1972). Although the number of collected samples was slightly few against the number of whales, it was possible to estimate the kind of food organisms of each animals
by refering both to the results of identification on collected samples and catch records.
WHALING GROUND SOUTH OF 40°N IN THE NORTH PACIFIC OCEAN
Many whaling grounds which have been developed and exploited in the world arelocated chiefly, if not entirely, in the higher latitudes of the seas most of which show a
distinct fertility in standing crops of zooplankters and other organisms during warm
season. These whaling grounds are undoubtedly some peculiar regions formed
through productive food chains such they usually called as feeding ground. The
northern North Pacific and Bering Sea where many whalers have been in chase ofwhales do not differ from the others in its basal formation. From this point of view
pelagic whaling operations by the Japanese fleets since 1952 in the northern North
Sci. Rep. Whales Res. Inst.,
No. 25, 1973.

FOOD AND FEEDING OF SEI WHALE

221

Pacific between 50°N and 60°N must be quite natural as well as the case of manyother fisheries in that region. Much accumulations of whales are usually expectedin these seas during warm season.
Baleen whales, however, are distinct migrator that move between warm andcold seas by seasons with an approximately a year cycle, and such whale movements
as a unit of whole population do not always proceeded continuously with any completion by season since they are considered to show somewhat variable stream likemovements as its situation has been suggestively demonstrated by Mackintosh and
Brown (1956), and Mackintosh (1965). It may possible that when the whales
migrating in the head of a population will have already entered into the feeding
ground of higher latitudes while the others are still far outside from it. Althoughmany of the rest may enter soon or later into those food rich ground, this discrepancy
in time and space makes it difficult to know or predict exact movements of whales attheir feeding ground. The whaling operation in 1972 can be regarded to haveundertaken under these circumstances in general.
On the other hand, it is well known fact that the Subarctic boundary which lies
roughly along 40°N latitude divides the northern North Pacific into two regions ofdistinctly different waters both in physical and biological characters (e.g. Zenkevitch,
1963). Comparing latitudinal standing stocks of zooplankton in that regions,
Odate (1966) and Vinogradov (1968) demonstrated its difference in biologicalcharacters between boreal and northern temperate waters which prevail in bothregions: the faunistic abundance in the south of the subarctic boundary is hardly
comparable to that of boreal regions. In this connection the sea region lies in thesouth of 40°N-41°N in the North Pacific probably deviate from those general idea asa feeding ground of higher latitudes. However, the newly opened whaling groundwhere actually operated and caught the whales were found only in the west side of
180° longitude, and can not be considered analogous with any other areas south of
40°N since a distinct mixed waters of Kuroshio and Okhotsk Sea origin which aresupposedly more fertile prevail in the region west of 180° (Dodimead et al, 1963).
In such a whaling ground, whether or not any amount or kind of food organisms
which must be principally gregarious organisms along with considerable large
population size will be found, are undoubtedly much interesting in connection withboth the formation of feeding ground of whales and general zoogeography.
CATCH DISTRIBUTION OF WHALES

A rough sketch of whaling region operated by three Japanese fleets in the 1972
season is demonstrated in Fig. 1. The shaded areas in the figure indicate principal
whaling ground. As it is shown in the figure Japanese fleets slightly entered into
Bering Sea but caught a little. The main whaling ground was found in two areas,i.e., the whaling ground in the east longitudes and that in the west longitudes.
Zonal formation of main whaling ground like this pattern seems characteristic in
recent operations, and considerable heavy catch in the waters south of 40°N in 1972
might be rather the result beyond expectation. In spite of being opened the whaling
Sci. Rep. Whales Res. Inst.,
No. 25, 1973.

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KAWAMURA

region southerly down to 20°N the whaling ground actually operated in the south of
40°N was diminutive in July where the southern most position was 34°N in the zone
between l 70°E and 180°. No notable catch was recorded in the south of 40°N
region of west longitudes.
N

40°

30°

E

170'

160°

1so·

170°

160°

150°

140'

130°W

Fig. I. The North Pacific whaling ground operated by three Japanese fleets in
the 1972 season.
TABLE I. CATCH DISTRIBUTION OF SEI, BRYDE'S AND FIN WHALESCAUGHT IN THE WATERS SOUTH OF 41°N LATITUDE
IN THE NORTH PACIFIC, 1972.
NUMBER OF ANIMALS: SEI/BRYDE'S/FIN
Latitude (N)Longitude
165°E-170°E
l 71°E-l 79°E
l 72°W-l 77°W*
168°W-169°W
Total

34

35

36

31/-/l174/2/2

8/-f74/1/-

41/2/120/-/l

36/-/110/-/-85/-/108/-/2
21/-/l31/-/l174/2/2

82/1/-

161/2/l146/-/-

214/-/3

37

38

39

40

14/-/31/-/1
20/-/11/-/76/-/l* No catch in l 76°W zone.
In the region of south of 41°N sei, bryde's and fin whales were caught, and thenumber of animals caught by 10° longitude are given in Table 1. It is clear in thetable that the newly opened region benefited almost exclusively for sei whaling andthe catch of both fin and bryde's whales were sporadically. It is also shown that
latitudinal spread of sei whale ground is found between 35°N and 39°N, which ispresumably correspond to the general features of bottom topography, i.e., the presence of the Emperor Seamount Chain.
FEEDING BY SEI WHALE IN THE SOUTH OF 40°N

In the waters south of 40°N of east longitudes relatively warm waters high above
Sci. R.ep. Whales R.es. Inst.,
No. 25, 1973.

223

FOOD AND FEEDING OF SEI WHALE

l 4°C in the surface prevailed during July of 1972 at 40°N with remarkable temperature gradient of about l.6°C/l 0 lat. toward the south to show 21 "C or more at 35°N
(Fig. 2). The meandering isotherms run SW to NE direction on the whole, and the
intrusion of tongue like warm or cold w~ters from south or north between 165°E
and l 78°E was distinct. The overall surface sea conditions can be seen as those of
subtropical characters.
N

45°
9.0

-----.:::::,---11.0
12.0
13.0
14.0

40°

13.0

15.0

14.0

.K·'·

.......

"

35°

160' E

170'
Fig. 2.

TABLE 2.

180°

170°

w

Distribution of surface sea temperature (°C) in July, 1972.

STOMACH CONDITION OF SEI WHALE CAUGHT IN THE WATERS
SOUTH OF 41°N IN THE NORTH PACIFIC, 1972.

Longitude

Stomach with
food

Empty

165°E-170°E
171°E-179°E
l 72°W-l 77°W*
168°W-169°W
Total

68
195
18
11
292

116
453
23
592

No. of animal
examined
184
648
41
11
884

feeding
percentage
38.0
30. l
43.8
100.0
33.05 (Av.)*** No catch in l 76°W zone.
** 168°-169°W zone is excluded.

Feeding percentages, the ratio of food containing animals to the total animals
examined are demonstrated in Table 2. Although some figures may be unreliable
due to poor number of material source, stomach conditions of sei whale in terms of
the feeding percentages in the south of 40°N as far as the zone between 165°E andl 79°E concerned, could be considered to be about 30-40% with 33.05% on anaverage. Comparing these figures with those of averages obtained from the.Sci. Rep. Whales Res. Inst.,
No. 25, 1973.

224

KAWAMURA

northern whaling ground north of 40°N where 50% or more are expected (Table 3),
the whaling ground south of 40°N showed slightly poor feeding conditions on thewhole but could be regarded as still rich to some extent in availability of foodstuff.
Relatively poor feeding conditions in the south of 40°N are also shown by the fulnessof stomachs in terms of the amount of food as expressed in percentage figures (Table 4 ).
It is curious, however, that considerable high percentages of whales were found to becarrying well repleted stomachs in contrast to very slight numbers in northern
whaling grounds.
TABLE 3. LATITUDINAL CHANGE IN FEEDINGPERCENTAGES OF SEI WHALE.
Latitudes

Feeding percentage

50°-60°N I*
II**
40°-50°N
20°-40°N ***

21.83
50.19
55.01
34.40

Av. 1967/71
Av. 1972* Bering Sea.
** northern North Pacific between 50°N and Aleutian Islands.
*** Actual southern most position: 34°N.
TABLE 4.

STOMACH CONDITIONS IN PERCENTAGE FIGURESBY NUMBER OF ANIMALS
Amount of food

Latitudinal area
North of 40°N*
South of 40°N*
North of 40°N**

Few

Moderate

Rich

Full

60.2
43.9
63.2

26.8
26.3
25.9

9.6
12.7
6.2

3.4
17.1
4.7

* 1972** Average: 1967-1972

Characters of the southern whaling ground as feeding place were examined
geographically or quantitatively in view of the availability of foodstuff, and revealed
that the southern whaling ground by no means so poor in feeding conditions as
having been supposed in general. Table 5 shows one of another characters, thequalitative peculiarity of the region. It is noticed in the table that great deal offoodstuff in the south of 40°N are solely comprised of fishes while they are almost
less important in the northern whaling ground. On the other hand, copepods comprised only 7.2% among all foodstuff while they are 80% or more in the north.
These qualitative characters indicate that the formation of whaling ground in thewaters south of 40°N possibly relates to the accumulation of young fishes underfeeding migrations as well as whales.
KIND OF FOOD ORGANISMS
Table 6 demonstrates the kind of food organisms found in 1972 materials which inSci. Rep. Whales Res. Inst.,
No. 25, 1973.

FOOD AND FEEDING OF SE! WHALE

225

elude the food samples collected in the waters north of 40°N up to 50°N but a little.
Relatively large number of food species was found especially in euphausiids andfishes. To describe biological characters of the population of food organisms
something more in detail would be as follows: both Calanus cristatus and Calanus
plumchrus were entirely consisted of copepodite stage V which is only one copepodite
stage showing gregarious occurrence during warm season in the North Pacific as
having been reported in many previous works (e.g. Fae. Fish. Hokkaido Univ., 1961).
TABLE 5. A GENERAL FEATURES OF FEEDING CONDITIONS IN THE
NORTH PACIFIC SEI WHALE THROUGH PAST SIX SEASONS*
Whaling
Season
1967
1968
1969
1970
1971
1972

Feeding**
percentage

Food organism
Euphausiid

Copepoda

Fish

Squid

22.0
9.0
6.3
13.8
17.7
2.7

76.7
89.9
81.6
81.6
75.5
7.2

0.6
0.2
9.1
4.2
4.9
24.6

0.3
0.7
3.0
0.3
1.9

73.1***
60.9
60.8
64.1****
54.5
54.4* Number during 1967-1971 represents the whole area of the North Pacific north of 40°N,
and that 1972 represents only the area south of 40°N.
** No. of stomach with food/No. of stomach examined.
*** 0.4% of amphipoda is excluded.
**** 0.1 % of unlisted organism is excluded.
TABLE 6.

FOOD ORGANISMS OF SEI WHALE CAUGHT IN THE
NORTH PACIFIC WHALING GROUND, 1972.

COPEPODA
Calanus cristatus Kroyer
Calanus plumchrus Marukawa
Calanus pacificus Brodsky
EUPHAUSIID
Euphausia recurva Hansen
Euphausia pacifica Hansen
Euphausia diomedeae Ortmann
Euphausia tenera Hansen
Thysanoessa inermis (Kroyer) Hansen
Thysanoessa spinifera Holmes
Nematoscelis difficilis Hansen
Nematoscelis gracilis Hansen*

DECAPODA: Macrura
Sergestes similis Hansen
FISH
Scomber japonicus Houttuyn
Sardinops melanosticta (Temminck & Schlegel)
Engraulis japonica Houttuyn
Cololabis saira (Brevoort)
Maurolicus muelleri (Guerin)
Pseudopentaceros richardsonii (Smith)
CEPHALOPODA: Decabrachia
Two or three species unidentified* Identification is doubtful due to the damage of specimens.

Calanus pacificus was separated from Calanus finmarchicus or Calanus helgolandicus,
and established as warm water prefering species being prominent in Far Eastern
Seas by Brodsky (1948). Among the occurrence of C. finmarchicus and C. helgolandicus in the surrounding waters of Japan, some of which might be included C.
pacificus since the species prefers well much warmer waters than the two others. It
Sci. Rep. Whales Res. Inst.,
No. 25, 1973.

226. KAWAMURA

seems the first record that C. pacificus is reported as a principal food sources of baleen
whales in the North Pacific region. C. pacificus which was found to be the primalily
important foodstuff of sei whales in the waters south of 40°N, was also represented bycopepodite stage V but copepodite stage VI of both sexes also occurred. However,the number of adult males and females in the population was hardly comparable tothat of copepcidite V and .an approximate ratio of each stage was: CV: CVI(female): CVI(male)=60: 3: 1. There was a net plankton sample collected in thewhaling ground and it revealed that copepodites III and IV of C. pacificus also present
in the region with slightly larger numbers in the latter.
In euphausiids, Euphausia pacifica was found most frequently and most of themwere consisted of both adult and adolescent individuals some of which carried sperm
sacs on the belly. E. recurva and E. diomedeae were also found being consisted of·adult forms of both sexes with almost same number though slightly less female in thelatter species. Some females carried sperm sac. Nematoscelis spp. and others wereconsisted of both sexes with some mixture of adolescent forms. None of these
species were found carrying about sperm sac.
The only one macruran, Sergestes similis was in the body length of26.3-47.0 mm
though most of them were found to be larger than 40 mm. Their body lengths,
however, did not differ much by each collected samples, and this fact suggests thatmass occurrence in patchness would be consisted of the individuals of nearly samedevelopmental stages possibly due to a sort of segregation by year class.
Many individuals of Japanese anchovy, Engraulisjaponicus were 9.0-10.5 cm infork length and supposed to be a spring population of current year almost attained at
the maturity from their sizes (Kondo, 1971). Japanese mackerel, Scomber japonica
was the most dominant food fish, and all of them was sexually immature from theirbody length of 8.4-11.7 cm (Usami, 1968). Body length of Japanese sardine,
Sardinops melanosticta and Pacific saury, Corolabis saira, was B.3-11.6 cm and 10.517.7 cm respectively. They were ah;o sexually immature from their body length(Kondo, 1964; Hotta, 1964 ), and Pacific saury was possibly a offspring of the springin current year (Hotta, 1964). A Gonostomatiid fish, pearlsides (Maurolicus muelleri)
showed 4.7-4.B cm in their fork length.
Although there were many kind of food organisms as mentioned above, a bulkof these food organisms can be considered being consisted of zooplankton and fish
species which prefer rather warmer waters when compared with those having beenfound previously in the northern waters (see Nemoto, 1957, 1959), that is, Calanus
pacificus, Nematoscelis gracilis, N. dijficilis, E. diomedeae, E. tenera and some fishes such
as Japanese mackerel.
Food organisms in detail with corresponding number of whales are given in
Table 7. It is clear in the table that a typical cold water copepods, C. cristatus andC. plumchrus were exclusively fed by the whales in the northern waters north of 40°N,
while C. pacificus, the warm water copepod, was the representative in the south of
40°N. Table 7 also indicates that the fin whales feed more preferably on euphausiids
than copepods as Nemoto (1959) early reported their food preference. To see theoverall results it is no doubt that the essential diet of sei whales in the waters south of
ScL Rep. Whales Res. Inst:,

No. 25, 1973.

FOOD AND FEEDING OF SEI WHALE

227

40°N is formed almost solely by C. pacificus and small fishes. It is notable that bothC. cristatus and C. plumchrus correspond to C. pacificus in the north of 40°N, andeuphausiids and macruran, Sergestes similis (Kawamura, 1970, 1971; Omori et al.,1972) also taken over the role of small fishes. As it will be mentioned later the
majority of those food fishes prey upon C. pacificus. Euphausiids were also fed butvery few. This indicates that they are less important as whales food in the waters
south of 40°N. Among the food fishes both Japanese mackerel (Scomber japonicus)
and Japanese sardine (Sardinops melanosticta) were the most important.
TABLE 7.

NUMBER OF ANIMALS BY THE FOOD ORGANISMS IN SEI
AND FIN WHALES IN THE NORTH PACIFIC, 1972.
South of 40°N

Kind of organisms
sei
C. cristatus
C. plumchrus
C. plumchrns--C. cristatus
C. pacijicus
C. pacijicus-E. recurua
E. pacijica
E. recurua
E. diomedeae
E. tenera
Th. inermis
Th. spinifera
Th. inermis-Th. spinifera
N. gracilis ( ?)N. difficilis
S. similis
S. similis--C. cristatus
S. similis-E. pacijica
S. similis--C. plumchrus--C. cristatus
S. japonicus
S. japonicus-C. pacijicus
S. japonicus-S. melanosticta
S. japonicus-E; japonicus
S. melanosticta
E. japonicus
C. saira
M. muelleri
M. muelleri-S. saira-Squid
P. richardsonii larva {?)Squid

North of 40°N,-----"'--------,.

fin

sei

2

18

6
7*
1
3

3

2
1
3

2

1.

10
1
1
3

2

2

* One material collected by the net is included.

In summerizing the results it is confirmed that C. pacificus, S. japonicus and S.
melanosticta, all the warm water food organisms, essentially made the whales accumulate for feeding in the waters south of 40°N. A clear latitudinal succession
in the spec~es composition of food organisms or in its community also make us
Sci. Rep. Whales Res. Inst.,

No. 25;19}3.

228

KAWAMURA

confirm that the subarctic front had undoubtedly positioned in the vicinity of 40°N
in the east longitudes during July of 1972.
DISTRIBUTION OF FOOD ORGANISMS
Food organisms occurred can be divided into three major groups: copepods, euphausiids and fishes. Figs. 3 and 4 demonstrate a rough sketch of their distribution
being arranged by examining the food samples and the record of eye observations
made on the ship's deck. As the latter covers all of whales caught while the food

50°

I'

+

,.·· ··'

0

80

o

o+

.f
40°

• Calanus pacificus° Calanus cristatus+ Calanus plumchrus" Sergestes similis

30°
180°

170°

160°

170°

150°

E

160°

Fig. 3.

Di•tribution of food copepods and macruran shrimp, Sergestes similis.

N

.

50°

~

,.
I

• t.

..

140°

,,~

,.,. -···"'

'

+

~···

...

...

40'

w

0

Scomber japonicus• Engraulis japonica• Sardinops melanosticta+ Cololabis saira

30'

A

Maurolicus muelleri... Squid
E

160°

170°

180°

170°

Fig. 4. Distribution of food fishes and squids.
graphical discontinuity.

160°

150°

140°

w

Broken line indicates the zoogeo-

samples were collected sporadically, the spread of distibution range shown in the
figures does not follow exactly to the positions or the range by the food samples.
Fig. 3 shows the distribution of copepods and a macruran, Sergestes similis. It isclear in the figure that C. pacificus occurs only in the waters south of 40°N while C.
Sci. Rep. Whales Res. Inst.,.No. 25, 1973.

FOOD AND FEEDING OF SEI WHALE

229

plumchrus and C. cristatus predominate extensively in the northern waters. The
actual southern most position of the occurrence of C. pacificus was found in 34°N andthat of northern most was in 41 °20'N, 170°E. Taking some allowance into consideration on their distribution range, C. pacificus may occur up to 42°N or thereabouts with a considerable large size stocks so as to be fed by the whales. Sergestes
similis, on the other hand, occurred only in the waters of west longitudes with its
distribution center in 44°-46°N, 160°W. The occurrence of S. similis in this way isvery characteristic as having been reported and pointed out its importance as
whales food in these region (Omori et al., 1972). A majority of euphausiids in thenorthern waters were represented by Euphausia pacifica, Thysanoessa spinifera andTh. inermis though they might be less important in the region where mass occurrenceof S. similis is expected.
There are noticeably different features of distribution between Japanesemackerel and Japanese sardine (Fig. 4). In the waters south of 40°N Japanesemackerel occurred exclusively in 165°-l 73°E while Japanese sardine was found in
174°-l 78°E. Composition of whales food fish was relatively complicated in theregion east of 174°E where young Pacific saury of 10.5-17. 7 cm and pearlsides of
4.7-4.8 cm occurred along with Japanese sardine. Very small larvae presumably ofboar fish, Pseudopentaceros richardsonii, also occurred though sporadic. All these small
but swarm forming fishes did not occur in the waters north of 40°N except a very
few instances of Pacific saury. In general, population of fishes as food ofsei whale in
the waters south of 40°N can be considered to be consisted of two groups of food-,,.. .,.

N• f.

,,,,. ,.._

..

/,.

..

40°

Thysanoessa inermis
Thysanoessa spinifera"' Euphausia pacifica" Euphausia recurva• Euphausia diomedeae
0Euphausia tenera• Nematoscel is gracilis
0
Nematoscelis difficilis
4

@

30·
E

160°

170°

1so·

170°

160°

150°

140°

w

Zonation of Food Euphausiids
IIillIIlliliJ Thysanoessa zone
~ Euphausia pacifica zone
~Mixturezone of warm water

Euphausia

Fig. 5. Distribution of food euphausiids and their possible zonation of occurrence
as whales food. The mixture zone in the figure does not mean the interspecificmixing but the species occur in complete monotypic population under a condition of rather sporadic occurrence each other.

Sci. Rep. Whales Res. Inst.,
No. 25, 1973.

230

KAWAMURA

stuff of fishes: Japanese mackerel alone or it accompanying Japanese anchovy
population in the west side region, and Japanese sardine population with several
numbers of cold water or cosmopolitic fishes. These food fish populations divide
the region into two parts that different fish communities predominate roughly with
their occurrence border crossing northwest to. southeast direction between 170°E
and 180° as shown by a broken line in Fig. 4.. Recognition of whaling ground by
this way can also be interpreted by the pattern of occurrence in euphausiids (Fig. 5).
PREY OF FISHES FED BY WHALESAs it was mentioned in the preceding section a considerable part of foodstuff of sei
whales in the waters south of 40°N was consisted of small sized but school forming
fishes, and such sea conditions as feeding ground seem somewhat characteristic
when compared with those formed by the planktonic organisms since the fishes
above mentioned can be regarded as competitors to feed on the whales food. In
connection with this circumstances the whaling ground in the south of 40°N
would be regarded as that formed secondarily although there seems still in need offurther examination on foodstuff in the fishes fed by the whales.
As it is shown in Table 8 great deal of fishes found in the stomach of whales
had fed solely on copepods of one or two species. Among copepods fed by thefishes of whales food, C. pacificus was most predominant being followed by C. plumchrus and C. cristatus. It was also noted that one of the typical cold water copepod
species, Eucalanus bungii bungii which was completely absent from the foodstuff ofsei whale occurred along with C. pacificus. Some individuals of Japanese mackerel
TABLE 8.

STOMACH CONTENTS OF FISHES FED BY SE! WHALE.
Kind of food fishes*

Food organisms of fishes
Sj

Cs

Ej

Sm

Mm

C. cristatus-C. plumchrus
C. plumchrusc. pad.ficus

2

C. pacijicus-C. plumchrus
C. pacificus-E. bungii hungii
C. pacificus-C. plumchrus-amphipoda (Gammariid)
OthersUnknown****
Empty

2**
4
5

l
2

4***
2
3

* Sj: Scomber japonicus, Cs: Cololabis saira, Ej: Engraulis japonicus, Sm: Sardinops melanosticta,
Mm : Maurolicus muelleri.
** Euchaeta sp., Phronima sp., Sapphirina (?), Eucalanus sp., Oikopleura sp., Salpa (?).
*** Pseudocalanus elongatus, Eucalanus bungii bungii, Candacia colombiae, Oncaea sp., Euphausiids
furcilia, Amphipoda, Phronima sp.
**** decomposed.

and pearlsides (M. muelleri) fed on a mixture of many kind of zooplankters. These
fishes seem undoubtedly to have preyed upon the zooplankters being present inSci. R.ep. Whales R.es. Inst.,
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FOOD AND FEEDING OF SEI WHALE

231

discriminately without any selection. It is interesting that warm water zooplankton
species such as Phronima and Sapphirina, were found as a food of young Japanesemackerel which somewhat prefers warm water while M. muelleri, a cosmopolitic
species fed on cold water zooplankters. By examining on the foodstuff of bothwhales and their food fishes, it is confirmed that they are closely linked up solely
through C. pacificus. In these prey-predator relationships the whaling ground in
the south of 40°N can be recognized as being formed at its beginning by the distributional characters peculiar in C. pacificus, which would possibly related to somehydrodynamical processes of past seasons in the northern waters. In approaching
to the causation of aggregations of whales through the formation of their feeding
ground, manifold composition of foodstuff in pearlsides seems less important thanJapanese mackerel which feeds exclusively on monotypic swarms of copepods.

,

N50·.-------~----~-----~----~--~

,,.

•

,"

COLD

40°

WARM
TRANSITION
30°

E

160°

170°

1so·

17o·w

Fig. 6. Schematic zonation of whaling ground in the waters south of 40°N as
based on the distribution of prey organisms of sei whales and their food fishes.
'WARM' in the figure is the zone characterized by C. pacificus or its mixture
with various warm water copepods, and ' COLD ' is those by C. plumchrus or
its mixture with C. cristatus. Both 'WARM' and 'COLD' zones meet andmix with each other at the ' TRANSITION' zone. Arrow shows a possible
penetration of subsurface cold waters from the EKCE.

By plotting the occurrence of prey organisms of fishes fed by whales, two ecologically heterogeneous regions could be distinguished (Fig. 6). One of them ischaracterized by C. pacificus or a mixture of various warm water zooplankters, andthe other is by C. plumchrus or a mixture of C. plumchrus and C. cristatus populations.
It is also noticed that there are transision zone with some width where the preyorganisms of fishes intermingled with those in both regions each other. These distinguished regions agree well with the result obtained in food fish populations, but
Sci. Rep. Whales Res. Inst.,

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KAWAMURA

the location of transision zone seems likely to shift rather southward than the locationof ecological discontinuity which is shown in Fig. 4.
DISCUSSION
The region which lies in the south of 40°N of the North Pacific does not seem to be
kept under such a conditions of good feeding ground as make the whales, if notentirely, be accumalated for a while when considered from the point of views of anabundance in food organisms. This is the problem of gross availability of food organisms as it is generally indicated by feeding condition of whales. Feeding percentages, the ratio of food containing animals to the total examined in sei whales in
the waters north of 40°N through past five years were 62.7% on an average while
those in the south of 40°N revealed it remains at about 30-40%. Under a rather
barren sea conditions, however, the region lies between 160°E and 180° zone in
the south of 40°N seems to be distinct being covered with some peculiarities under
the influence of prevailing current.
Mentioning on the upwelling of suarctic intermediate waters in relation to thecharacter of subarctic front in the zone of east longitudes of the North Pacific, Uda(1971) proposed the presence of a southeasterly branch from the East Kamchatka
Current, which is called as the East Kamchatka Current Extension (EKCE), and it
presumably influences upon the sea conditions in the environs of 40°N, 160°-l 70°E
region. The EKCE which originates from the region about 50°N, 160°E extends
south to southeast direction, shifting its direction more easterly when it meets with
the Oyashio front at about 40°N. This cold southeasterly current must presumably be causative for the meandering in surface temperature distribution along with
the influence of a complex bottom topography by the Emperor Seamount Chain,and further, it possibly relates to the extension of mixed waters of the Kuroshio andOkhotsk Sea as Dodimead et al, (1963) figured out. Recent work presented by
Kishi and Uda(1973) makes me confirm those hydological peculiarities of the whaling ground under consideration. Analyzing on the depth distribution of 10°C waters
based on the enourmous amount of data during 1935-1969, they (Kishi and Uda,
1973) found a steady southerly intrusion of the cold EKCE waters into the south of
40°N region between 150°E and 160°E.
The whaling ground in the south of 40°N would be divided into two regions ofrelatively warmer and colder characters by the intrusion of colder waters from thenorth, and they are formed in the both southwest and northeast side of the region(see Figs. 5 and 6). In this connection the proposed ecological discontinuity which
was interpreted from the results of examinations both on whales food and the preyorganisms of the fishes of whales food agrees well with the hydrological characterson the whole, and supports Uda's proposal on the EKCE on biological basis although
the location of the discontinuity may shift to some extent by seasons and years. In
the populations ofJapanese sardine and pearlsides the latter fed exclusively on boreal
or cold arctic zooplankton species such as Pseudocalanus elongatus not with standing
the fact that both fish species occurred in almost the same region. On the other hand,
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233

it was more in the southerly waters that monotypic aggregations of C. pad.ficus were
found in the stomach of Japanese mackerel. Both C. cristatus and C. plumchrus as afoodstuff of sei whales never occurred in the waters south of 40°N while they were
found in the further south to some extent as a prey of food fishes of whales. These
circumstances seem to be discrepant but it would be explained as follows thoughstill have not proved yet: Both C. cristatus and C. plumchrus are the important foodstuff of baleen whales through widely in the northern North Pacific (Nemoto, 1963),
and they could occur by forming a dense swarms so as to be fed by the whales in thenorth of 40°N. In the south of 40°N, on the other hand, it is far out side from theirusual habitat, and as any of both species could not be present as swarms in the surface so were they in the northern waters. C. cristatus and C. plumchrus found in thestomach of Japanese mackerel might not be preyed upon their swarming populations
but possibly upon rather sporadic and dispersed populations having been carriedby the intermediate waters from the north. The general features in the zoogeographical successions of fish foods suggest that some mixing of water would takeplace in or near by those discontinuous borders mentioned previously, and it might
presumably be due to the influence of the EKCE, since cold waters widely foundover this region can be recognized as the subsurface Oyashio origin waters which
penetrated into the region at the depth ofabout 50-100 m. (Kishi and Uda, 1973).
According to Betesheva (1954) fin whales feed on anchovy in the waters ofKurile region during August and the anchovy population often accompanies
Thysanoessa raschii, a cold neritic species (Boden et al, 1955 ). Japanese anchovy
found in the North Pacific in 1972 occurred at the southern most part of its distribution in Japanese sardine-pearlsides populations. This fact suggests also thesuccession of water masses more colder toward north to northeast side in contrast to
warming toward south to southwest, that is, more stronger influences of warm
water in the south. In this respect Omori (1965) reported an interesting result,that is, C. pacificus distributes fairly wide in the zone of 40°-50°N of the North Pacificduring June to August but it is only in the west of 170°W that C. pacificus often
occurs being accompanied by warm water copepods such as Calanus tenuicornis.
This fact would explain the biological character of newly opened whaling groundand its peculiarity as mixing region in general.
Consulting with the general faunistic features around the region in the south of
40°N of the North Pacific (Zenkevitch, 1963), the whaling ground operated in thesouth of 40°N resembles well to the features usually found off Sanriku, the southeast
coast of northern Japan, and it is something likely to their extension with slight
addition of more boreal characters. Although a considerable number of C. pacificus
occurs far in the Gulf of Alaska (Omori, 1965), they are usually accompanied with
C. plumchrus. These difference in their specific combinations as a communities offood organisms, though it resembles at a glance, should be noted since it might
give the sea quite heterogeneous characters. In concluding on the whaling groundalong with its formation in the south of 40°N in the North Pacific, it is consideredthat the key factor is found solely in the distributional ecology of C. pacificus. However, the feeding ground of baleen whales as Kawamura (1973a, b) has firmly
Sci. Rep. Whales Res. Inst.,
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KAWAMURA

pointed out should not be considered solely on the basis of each food species alone
but of their community which embodied through the food chains. C. pacificus is
usually found widely over the northern North Pacific but no other staple feeding
ground of baleen whales as exploited in the zone of east longitudes would possibly
be found in the zone of west longitudes.
SUMMARY
1. In accordance with lifting the ban of whaling activities by the measures ofJapanese Government in the waters south of 40°N of the North Pacific in 1972 a
total of 34 food samples of sei whales was collected in the newly opened region, andtheir food and feeding conditions were examined.
2. The main whaling ground in the south of 40°N was located only in thezones between 165°E and 180° with 34°N as its southernmost position, where theEmperor Seamount Chain ends. The surface sea temperature of 14°-21°C
prevailed during July in the region with a remakable meandering pattern which
jaggs northwest to southeast direction.
3. The sea conditions were complex under a possible influence of the East
Kamchatka Current Extension, and the formation of whaling ground seems rather
the peculiar case in the waters south of 40°N.
4. The ratio of food containing animals to the total examined in percentage
figures showed 33.05% on an average while it was 50.0% or more in the waters
north of 40°N "through past five seasons.
5. In connection with the feeding percentages the fertility of the region southof 40°N as feeding ground of baleen whales is considered to be not so barren as having
been supposed previously.
6. The foodstuff of sei whales in the waters south of 40°N was almost solely
comprised of fishes (24.6%) and copepods (7.2%), while they showed less than 5%
and 80% or more respectively in the northern waters.
7. More than 20 species of food organisms were identified. A large number ofthem was comprised of warm or cold temperate species especially in euphausiids, but
only Calanus pacificus, Japanese mackerel (Scomber japonicus), Japanese sardine (Sardinops melanosticta) and Japanese anchovy (Engraulis japonica) were the main constituents of the food of sei whales.
8. The Calanus pacificus population was represented by the copepodites V and
VI of both sexes, and that of the fishes was by the young individuals of sexually
immature.
9. Distribution of food organisms revealed that the main whaling ground in
the south of 40°N is likely to be divided by a zoogeographical discontinuity into twoheterogeneous regions under different faunistic characters.
10. Examination on prey organisms of the fishes fed by sei whale prooved
that they also feed chiefly on C. pacificus or its mixtures with C. cristatus or C. plumchrus.
11. Above results seem to support again the presence of the ecological disSci. Rep. Whales Res. Inst.,
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FOOD AND FEEDING OF' SEI WHALE

235

continuity, and at the sametime, the intrusion of cold intermediate waters into thereg10n.
12. The whaling ground in the south of 40°N was presumably formed solely
by a bulk of mass occurrence of C. pcificus at its beginning, and the concentration ofyoung fishes under feeding migration also made the sea region so fertile as to be thestaple feeding ground of baleen whales.
13. It was known in this study that the feeding ground of baleen whales has
been usually recognized as being formed chiefly by the aggregations of primaryconsumers of planktonic crustaceans, but it would also be formed or at least supplemented considerably by the organisms of more higher trophic levels, and that thecondition of available foods for baleen whales in the region should not be consideredby each food species alone but more comprehensively by the community of organisms.
ACKNOWLEDGMENTS

The food samples examined in this study were collected through the courtesy ofJapanese whaling fleets of Taiyo Gyogyo K.K., Nippon Suisan K.K. and K.K.
Kyokuyo, Far Seas Fisheries Research Laboratory of the Fisheries Agency alsokindly admitted me to use a statistical materials on whales caught. In particular I
am much indebted to Associate Professor Tadashi Kubota of the Faculty of Marine
Science and Technology, Tokai University for identifying pearlsides Maurolicus
muelleri, a species of Gonostomatiid fishes. Mr. Saburo Machida of the Whales
Research Institute also kindly offered me his original figure of the temperature distribution as illustrated in text-fig. 2.

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Sci. Rep. Whales Res. Inst.,
No. 25, 1973.