Sei Whales in the Adjacent Waters of Japan.
II. Further Studies on the External Characters.
By
HIDEO OMURA and KAZUO FUJINO

Introduction

Omura, Nishimoto and Fujino (1952) stated, after investigating the
body proportions and other characters of the sei whales taken in thewaters adjacent to Japan, that two types of sei whales, namely thenorthern and the southern types, are both found in the adjacent watersof Japan. They concluded further that the former belong to the ordinary sei whales (Balaenoptera borealis) but the latter are more related tothe bryde's whales (B. brydei) than the ordinary sei whales. As themost distinct differences between the two types in the external charactersare (1) the shape and the quality of the baleen plates and (2) the extent
of the ventral grooves, the present authors made further investigations
mostly on these two points, results of which are reported in the present
account.
We are much indebted to the Japanese government whaling inspectors and the staff of the whaling companies who cooperated in thepresent investigations. Our sincere thanks are due to Dr. F. C. Fraserof the British Museum (Natural History), who kindly gave Omura achance to read some literatures kept in that museum while the latter
stayed in London.
Material

We made the following scheme of investigation prior to the commencement of the whaling season for baleen whales 1952 in the adjacentseas of Japan.
1. A biggest baleen plate should be taken from right or left row ofeach sei whale captured and such baleen plates should be sent to thelaboratory with the particulars of the said whale for the identificationof the two types and for the measurement of their length and breadth.
2. The extent of the ventral grooves of each whale should be observed,
classifying the following three classes.
The ventral grooves end(a) anterior to the umbilicus,(b) at the umbilicus, and(c) posterior to the umbilicus.

90

H. OMURA and K. FUJINO

Baleen plates were taken for the investigation from almost all sei
whales caught by the pelagic whaling in the waters around Bonin Islandand in the northern Pacific. However, regarding those of sei whales
processed at the various land stations only about forty per cent of thetotal catch were sent to the laboratory. Furthermore, some plates
were broken in their inner parts in the course of transportation, especiallyin those taken in the Bonin Island waters. Besides, in some plates themargin line of the gum were not clear because of the carelessness at
the time of sampling. Such baleen plates were of course available forthe identification of the two types, but were not of use for the measurements. Table I shows the total number of sei whales taken in 1952
and those of which baleen plates were measured.
Table 1.

Number of Sei Whales taken and Number of Baleen
Plates measured in 1952.

Geographical Areas

Number of Sei Whales
taken

Number of Baleen Plates
measured
Males

Females

Total

411

164

101

265

390

665

94

141

235

8

6

14

4

4

8

553

537

1,090

262

246

508

Males

Females I Total

Bonin Island(Pelagic)

270

141

Coastal

275

Northern Pacific(Pelagic)
Total

As shown in this table, about a half of the total sei whales catchin the waters of Japan, Bonin Island and northern Pacific (pelagic) were
measured of their baleen plates.
Observations on the extent of the ventral grooves were made by
Mr. K. Ozaki and Mr. I. Hasegawa, whaling inspectors of the Japanese
Government on the factory ship which operated in the Bonin waters.
In the northern Pacific (pelagic), such observations were made by Fujino.
We were not able, however, to send any biologist for a long period to
any land stations, so we asked to the staff of the each land station to
make such observation.
Analysing these data obtained, we classified at first the sei whales
into two types based on the observation of the baleen plates, because
this character was deemed as first essential, and then we investigatedthe extent of the ventral grooves. We rejected such data of theventral grooves of whale which was not observed firstly by ourselves for
identification of the two types according to baleen plate, in the course

91

Sei Whales in the Adjacent Waters of Japan.

of the analysing. We only used, therefore, such data of whales ofwhich baleen plates were observed by ourselves and sametime the observation of the ventral grooves were available. Table 2 shows thenumber of sei whales on which the both observation were made.
As shown in tables 1 and 2, only 14 per cent of the total sei whales
processed at the whaling land stations were observed on the both characters, while almost all sei whales were investigated in the Bonin Island
waters and in the northern Pacific (pelagic).
Table 2.

Number of Sei Whales of which Baleen Plates and the Extent of
the Ventral Grooves were observed in 1952.

Geographical Areas

Males

Females

Total

Bonin Island (Pelagic)

268

139

407

Coastal

43

49

92

Northern Pacific (Pelagic)

8

6

14

Total

319

194

513

Baleen Plates

Omura, Nishimoto and Fujino (1952) describe that the baleen platesof sei whales taken in the Bonin Island waters are shorter in length,
coarse in texture, greyish black in colour and with thick bristles ascompared with those of ordinary sei whales. It was thought at thattime that there might appear some intermediate type of baleen plates
if we conduct further investigation in future. While investigating,however, the material of
1952, we could not find anyof such intermediate type.
All baleen plates were identified clearly as northern type(sei whale type) or southern
type (bryde's whale type) Fig. 1 Measurements of baleen plates. Dotted line
shows the margin of gum.
without any exception. No
a : length along the outer margin
hesitation was felt whileb: breadth at baseidentifying the both types.
The length and the breadth of each baleen plate was measured incentimeter. As shown in Fig. 1, the length was measured along theouter edge of the baleen plate from the point on the outer edge at theinsertion to the gum to the tip of the plate, excluding the length of

92

H. OMURA and K. FUJINO

the bristles. The breadth was measured by straight line between thetwo points on the outer and inner edge at the insertion to the gum.
The results of the measurements were dotted in Figs. 2 and 3,
separately by sex.
x NORTHERN TYPE
60 - • SOUTHERN TYPE x

60

55

So

x

so-t

so

2

~

45

u

u 45

z
I

t5

NORTHERN TYPE

• SOUTHERN TYPEz40i:} 35 ~..

.. • j ... :. .

I

t5:: :.'t!!?l1;:!:. : •• .
...
:. ........ ..
... : :.. :. .:

_J

~
35
_J

•,

30 ~

40

30

25~

JI.

• • • • • • _;••••. : •• rr'·•.a.· H ... •.1.:·!i~¥.

25
I

I

I

IS

20

25

BREADTH !N CM
Fig. 2. Relation between length andbreadth of baleen plates in sei whales.
Males

15

20

25

BREADTH IN CM
Fig. 3. Relation between length andbreadth of baleen plates in sei whales.
Females

As shown in these figures, the two types of northern and southern
are almost completely separated into different groups in the shape ofthe baleen plates, both in males and females.
Then we calculated the quotient of the length divided by the breadthin each baleen plate. Table 3 shows the occurrence of each of suchquotient (L/B). Figures 4 and 5 were drawn based on the data listed
in table 3.
Judged from the above table and figures, the two types of northernand southern are also divided into two different groups nearly completely
and we can say in general that the length of the baleen plate is over
two times of its breadth in the northern type, while below two times
in the southern type. No difference is seen between male and female.
Further, a difference is seen in the shape of the palate :between
northern and southern types.
As clearly shown in Figures 6 and 7, the breadth of the palate is93

Sei Whales in the Adjacent Waters of Japan

broad and slightly concaved inward in the middle part of it in thesouthern type, but the breadth is very narrow and convexed outward
considerably in its posterior part in the northern type sei whale. This
Table 3. Quotient of Length/Breadth of Baleen Plates in Sei Whales.
Southern Type
L/B
1. 20-1. 29

Males

I

J

Northern Type

Females [ Total

I

Males

Females [ Total

0
0
0

0
0
0

0
0
0
0
0

0
0
0
0
1

0
0
0
0
1

11
5
0
0
0

1
0
6
13
22

3
5
14
29
17

4
5
20
42
39

0
0
0
0
0

0
0
0
0
0

21
17
8
4
0

30
22
11
2
3

51
39
19

0
0
0

0
0
0

1
0

0
2
1

1
5
1

166

106

272

96

140

236

164
2
0

101
5
0

265
7
0

0
92
4

0
136
4

0
228
8

1.30-1.39
1.40-1.49

1
6
9

0
2
6

1
8
15

0
0
0

1.50-1.59
1.60-1.69
1. 70-1. 79
1.80--1. 89
1. 90-1.99

24
43
40
18
11

22
30
24
13
7

46
73
64
31
18

2.00-2.09
2.10--2.19
2.20-2.29
2.30-2.39
2.40-2.49

10
4
0
0
0

1
1
0
0
0

2.50-2.59
2.60-2.69
2.70-2.79
2.80-2.89
2.90--2.99

0
0
0
0
0

3.00-3.09
3.10--3.19
3.20-3.29

000

Total Number
Geographical areas:Bonin I.
CoastalN. Pacific

I

J

I

I

I
I

I

3

I

6
3

difference is seemed a very essential external character beside otherssuch as the baleen plate and the extent of the ventral grooves, and not
the individual difference, though it is not recorded in quantities.

94

H. OMURA and K. FUJINO

X.

I

I

I

I•··· .X

I

2.5

3.0

VALUE OF L/B
Fig. 4. Value of Length/Breadth of baleen plates in Sei whales.

Males.

x· x NORTHERN TYPE

SOUTHERN TYPE
)\
:

·.

x

~

·)t·····X

15Fig. 5.

20

25

3.0
VALUE OF L/BValue of Length/Breadth of baleen plates in sei whales.

Females.

Ventral Grooves

As already stated, we classified at first the sei whales taken in 1952
into two types according to their baleen plates and then we studied theextent of the ventral grooves, classifying in three groups. Table 4shows its result.
According to table 4, the ventral grooves end anterior to umbilicus
in all of the northern type sei whales and no exception is seen. The
fact that the ventral grooves end anterior to umbilicus is one characterof Balaenoptera borealis and our data well agree with those reported by
Matthews (1938) concerning the sei whales in the southern hemisphere.
Andrews (1916) describes, after investigating the sei whales of Japan,that the ventral furrows of B. borealis are present on the throat, breast
and abdomen, but instead of extending posteriorly to the umbilicus,

. P
Fig.6
tTaiyoAkkeshi
i whale. Processeda
ealateofas. Northern Type
n
o
i
t
a
t
lands

(Cl

"
'

g−omロ
︒ E言︒口許可35︒
吾
H
こ88

e
:
. SouthernTYI
n
o
i
t
a
t
lands

. P
Fig.7
tTaiyoAyukawa
iWhale. Processeda
e
faS
eo
t
a
l
a

白色巧

96

H. OMURA and K. FUJINO
Table 4. Extent of Ventral Grooves in S9i Whales.
Northern Type

Extent of Ventral
Grooves

Males

Anterior to the
Umbilicus

II

At the Umbilicus

I

Posterior to the
Umbilicus
Geographical areas:Bonin I.
CoastalN. Pacific

51

I

Males

I

0

0

I Total
0
I

4

0

I

0

264

141

405

I Females [ Total
I

53

·,

1.

104

I

0

0

0

0

I

Southern Type

I
i

0

I

[Females

4

0

0

0

268

139

407

43

47

0

2

2

8

6

90
14

0

0

0

they end half way between the tip of the pectoral fin laid back andthe umbilicus. His observations were confirmed by our present data.
In the southern type sei whales, however, the ventral grooves run
posteriorly beyond the umbilicus with only one per cent exceptions, inwhich the ventral grooves end at the umbilicus. No whale with its
ventral grooves end anterior to the umbilicus is observed.
In table 5 of our former report (1952), such point is not so clear,because in that report the whales were only classified according to thegeographical areas. When the whales were classified after the observation of their baleen plates, as mentioned above, the relation between
the end of the ventral grooves and the umbilicus become so distinct.
However, it is thought that there may be some exceptions, in which
their ventral grooves end at the umbilicus, in the northern type sei
whales in the future investigations as Matthews (1938) reports in thesouthern hemisphere.
Discussion

The so-called sei whales in the adjacent waters of Japan may beclassified into two distinct types in the following three points, asmentioned already:
1. Quality and shape of the baleen plate and the thickness of its
bristles2. Shape of the palate.
3. Degree of the extent of the ventral grooves.
Besides, some differences are seen in the body proportions, as alreadystated in our former report (1952). However, in the body proportions
except measurement No. 12 (Notch of flukes to end of system of ventral

Sei Whales in the Adjacent Waters of Japan97

grooves) there exist some individual differences, which make it difficult
to identify the two types only by such body proportions. We can conclude that there are such differences between the two types only ingroup, not in individual. Measurement No. 12 is in other word theextent of the ventral grooves. So we conclude that the most distinct
external characters in the two types are above mentioned three points.
Our osteological study has not been completed yet. However, it
was made clear that there exist distinct differences in the external
characters between the two types of so-called sei whales in the adjacentwaters of Japan. And we consider at this stage of the investigation,
the two types belong to the different species, and not to the different
races in the species B. borealis.
The northern type sei whale is, as already stated, identical with
B. borealis, but the characters of the southern type sei whale are wellagree with those which hitherto reported as the identical characters of
B. brydei. Olsen (1913) states many external characters of B. brydei but
most important points distinguishing B. brydei from B. borealis are theextent of the ventral fullows and the character of the baleen, as alreadypointed out by Andrews (1916). The shape of the plate should be added
to these two external characters.
In conclusion of the above statement, it might be concluded thatthe southern type sei whale is identical with B. brydei.
Concerning the osteology of B. brydei, the number of vertebrae isreported as 52-53 (Lonnberg 1931, Junge 1950). We examined thenumber of vertebrae of the both type sei whale in 1953, which is shownin Table 5.
The vertebral formula given by Andrews (1916) for B. borealis in
the Japanese waters is C7+T14+L13+Ca23=57 and the total numberof vertebrae given by Lonnberg for B. borealis is 56-57. Our data concerning the northern type sei whale agree with those statements, however, those concerning the southern type sei whale does not only agreewith those figures, but also with the figures given by Lonnberg andJunge for B. brydei. According to our data the total number of vertebrae
is 54-55 in the southern type sei whale, namely two more than thosegiven by Lonnberg or Junge. However, Junge describes concerning thePulu Sugi whale, from which his conclusion was drawn, that certainly
one, perhaps two of the last caudals have been lost. So his figure of
total number of vertebrae cannot be said as accurate. Future investigation on this point is needed for B. brydei in all areas concerned. The
strong backward inclination of the spinous processes of the dorsal and98

H. OMURA and K. FUJINO
Table 5. Numbar of Vertebrae in Sei Whales of Japan.

Type

s
"
"
"
"
"
"

"
"

"
"
"
"
N

"

Whale Body
No.
length Sex
1)
T. 57
T. 60
T. 63
T. 64
T. 65
T. 66
T. 67
T. 70
T. 71
2)
Ki. 32
Ki. 33
Ki. 34
Ki. 35
T. 69
T. 75

Remarks.

Date of
catch

37
35
42
41
40
45
47

r

3)

ft.
42
44
40
41
40
37
43
43

I Position of Catch Number of Vertebrae
I N. Lat. IE. Long. c. j T. L. f ca.f Total

'53
'53
'53
'53
'53
'53
'53
'53
'53

37°-41'
36 -54l'l Aug. '53
17 Aug. '53
17 Aug. '53

17 Aug. '53

38 -0737 -2437 -21
37 -18¥ 116 Jui. '53£f- 11 Aug. '53

37 -5438 -02t>'l Jui.

¥11 Jul.
13 Jul.
13 Jul.
13 Jul.
13 Jul.
13 Jul.
20 Jui.
20 Jul.

t>
t>
t>
¥

t>
¥
¥
¥
¥
¥
£f-

_I

142°-28'
142 -31
142 -32
142 -33
142 -2236 -5036 -5436 -5236 -5736 -50
36 -07
36 -06

I

7
7

14
14
14

142 -25
142 -29
144 -20

7
7
7
7
7

14
14
14
14
14

34*
33*
33*
34*

144 -18

7

14

33*

144 -00
143 -187
7

143 -15

7
7

14
14
14

143 -18

7

14

141
I

145 -381_7_

I 145 -32

7

14

34*
34*
34*
34*

55
!35

55
55
55
54
54
55
54

14
14

19

54

20

13
13

21
20

55
55
54

35*
14121

56
56

1) T ...... Taiyo Ayukawa Land station.
2) Ki. .... Kinkai Ayukawa Land station.
3) * ...... not distinguished into lumbar and caudal regions

lumbar vertebrae, an important difference which separate B. brydei
from B. borealis according to Junge, is also seen in the southern typesei whale. Our osteological study on the southern type sei whale will
be reported in detail in future when the work is completed.
Hitherto B. brydei has been reported to inhabit in a narrow areaof waters around Cape of Good Hope. However, Kellogg (1931) describesthat in the returns made by the Norwegian companies operating off
Lower California 34 bryde's whales were caught in 1926. Junge (1950)
notes that a whale cast ashore on the coast of Pulu Sugi, near Singapore,
and B. edeni, as reported by Anderson (1878) are both identical with
B. brydei and concluded further that this whale occurs in South African
waters as well as in the waters of South-eastern Asia. Ruud (1952) reports that 42 sei whales taken in 1952 in the waters of French Equatorial
Africa were all bryde's whale after investigating their baleen plates
and he further states that we must assume that the bryde's whale
has a much wider distribution than previously known and we may
expect that this species can be met with in subtropical and tropicalwaters in all oceans.

Sei Whales in the Adjacent Waters of Japan99

In the waters near Japan the bryde's whale inhabit mostly in the
Bonin waters.
It migrates further north as far as off Sanriku(N-E part of Japan Proper)
but we assume at the present
stage of our investigation thatit never go north beyond thepolar front.
Regarding to the distribution of the sei and bryde's whales
of Japan we found recently some
unpublished data concerning the measurements of body proporI .
tions.
These data
11------1------1-1 40'
were collected during
the period from Novem ber 1935 to July
1936 at the various:~ g~CJFIC

OCEAN- - - r - - - 1------+-----+----t------+-- --+----+-30°

e BONIN ISLAND
t----+-----+----t---1-----+----t-----~"+-----+------+---+-2~

128"E

130°

136°

142°

N

.144'

14B°E

Fig. 8. Location of the Whaling land stations of Okochi, Oshima, Bonin Is.,
Ayukawa and Kiritappu.

H. OMURA and K. FUJINO

100

land stations of Japan, of which location are shown in Fig. 8. Somebody proportions of the socalled sei whales processed at these land stations
were measured.
Seven measurements
were made to each sei
e OKOCHI
whales processed, but mostx BONIN ISLAND
important one in this con0 OSHIMAo AYUKAWA
nection is the length from
• KIRITAPPU
thetip of the lower jaw
61
to the end of the system
060-~---- --o-- __._ _- - - - -- - - -- --- - - - - - -- -- - - ofthe ventral grooves.
059
the measurement
Though00cannotbesaid as accurate,
SS 00
because this was done by0
57
D
the staff of the each landDstations, instead of the56
biologist, however, we can
55- ----------------x----------------------assume that these data are0
54
still available to get roughDD
53
idea of the distribution ofo.Dx
the both species.
52 xxxD
In Fig. 9 the lengtho B x xx
5I 0
0x
from the tip of the lower
so-... -- --- - --- ----- - - 0----~x---x-x-- ---- ----- jaw to the end of the
O
00 D
B. X
systemof the ventral
49
,
0
x
xx
El xx
groovesareshown in per~ XX Dx
48
centage against the total
0xx
47
body length, separately
0xaccording to each land
I
I
I
I
40
45
50
35
station. We can see two
BODY LrnGTH IN F.
groups in this figure, one ·Fig. 9. From tip of lower jaw to posterior end ofbeing over 55-56 per centthe system of ventral grooves (shown as percentages
and another one is below
of body length)
this figure. The former
means the whales, of which ventral grooves extend posteriorly far beyond
the middle of the body, which is one important character of the bryde's
whale. The latter means that whale belongs the ordinal sei whale. All
whales processed at Okochi and Oshima land stations belong to the bryde's
whale and those processed at Bonin Island, Ayukawa, and Kiritappu
land stations belong to the ordinal sei whales with some exceptions.

•

•

•

•

I

I

I

I

•

•

Sei Whales in the Adjacent Waters of Japan

101

Andrews (1916) observed some sei whales at Oshima land station,
but no record of measurements was given by him. So we cannot conclude which type of sei whale he observed there.
As shown in table 3 and 4, all sei whales taken in the Bonin waters
belong to the southern type, namely B. brydei. Also in our former
account (1952), all whales were southern type with a few exceptions.
However, our present data show that all sei whales processed in 193536 belong to the northern type with only one exception, which wecannot conclude to which type it belongs.
How can be occur such difference?
We should assume that is because the Table 6. Monthly catch of Sei Whalesin the Bonin Waters in the Seasonsdifference of the seasons. The seasons
1935-36 and 1952.
of the Bonin Island whaling in the post1935-36
1952
Monthswar days are quite different as compared(N-Type)
CS-Type)with those in the pre-war days. In the
Nov.
10
pre-war days the whaling was mainly
Dec.
10
operated in the period of January to
Jan.
0
0
Feb.
2
0
April. In the post-war days the seasons
Mar.
11
0
were commenced later and later, and now
1)
12
Apr.
0
it has been designated by the Japanese
2352 )
May
0
Government that no baleen whales should
Jun.
0
176
be taken prior lst May. Consequently,
now the season is quite different from
Notes: 1) Season closed on 8
Apr. 1936.
that in the pre-war days.
2) Season opened on 1
Table 6 shows the monthly catch in
May 1952.
1935-36 and 1952. The whaling wascommenced from lst May in 1952, but it was ended on 8th April in
1935-36.
Judged from the above fact, we assume that a herd of B. borealis
comes near to Bonin Island in the period from December to the middleof April, and after the herd passed north another herd of B. brydei
approaches to the island from the middle or end of April. This herdof B. brydei seems also go north towards the end of June as far as off
Sanriku, but one part of that seems to remain longer in the Boninwaters. According to the Japanese Fisheries Agency (1953), a whale
marking trip was conducted in the Bonin waters from 27th June tolOth July 1952 and 74 sei whales were sighted in this period.
As mentioned above, many B. brydei have been taken yearly in theseas around Bonin Island, but it is the fact in the post-war days.
Namely, it can be said that it was made clear that a new herd of

102

H. OMURA and K. FUJINO

B. brydei approaches to that island in the period in which hitherto rn
whaling had been conducted, and consequently a new whaling for thii
herd had inaugurated quite recently.
It is very interesting to investigate the ratio of intermingling OJ
the both species in the waters off Sanriku, especially in connection witl
theoceanographic
conditionsof the se~
SANRIKU
HOKKAIDO
150
water, because thE
two currents of warn:
and cold water inter·(/) 120
mingle in these area~
w
..J
~ 110
of the sea. We col·~lected some data ir
25 1001952, but it was sc
ffi 50few to get any con3z 40
clusion on this point,
30.
as shown in Fig. 10.
20
Many baleen pla10
tes have been sent to
X
VVlVIlVIITIXX
the laboratory in
MONTH
1953. They are now
Fig. 10. Number of sei whales taken and number
under investigationof baleen plates examined in 19132 in Sanriku andand the result will be
Hokkaido.
published later.

0!Il

Conclusion
1. The two types of sei whales in the adjacent waters of Japan, as
reported in our former account (1952), are separated clearly by thefollowing characters.
a. Quality and shape of the baleen plate and the thickness of its
bristle.
b. Shape of the palate.
c. Degree of the extent of the ventral grooves.
2. Though the osteological study has not been completed yet, there aresome differences between the two types in the total number of vertebrae,etc.
3. The conclusion from above is that we can consider the southern
type and B. brydei conspecific and the northern type is identical with
B. borealis.

Sei Whales in the Adjacent Waters of Japan

103

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