On the Sexual Maturity of the Sei Whaleof the Bonin Waters(Received March 6, 1953)

By
MASAHARU NISHIWAKIThe Whales Research Institute

TAKASHI HIBIYAandSEIJI KIMURA
The Laboratory of Fishery Zoology, Faculty of Agriculture,
Tokyo University

CONTENTS
Introduction ................................................................ 165
Chapter I. Material and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
Chapter II. Sexual Maturity of the Male .................................. 1'10
Chapter III. Sexual Maturity of the Female ...................•.......... 173.
Discussion and Conclusions ................................................. 175
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Introduction

Since 1923 Japanese whalers have been operating in the Bonin Islands
waters, and the land stations on Chichi-jima was the base of the operation prior to the World War II. As this land station has been lost as
the result of the war, the whaling has been conducted under the floating
factory system since the 1946 season, taking 300 to 400 whales per season(Fig. 1). The catch in those postwar seasons consists mainly of sei andsperm whales, and the former species amounts to more than 50% of
the catch in very season. Since the 1950 season this species have beenaccounting for even more than 80% of the catch, because of the changeof operating season due to the amendment of the International Whaling
Regulations and the favorable price of this whale. In recent years, therelative importance of the whaling in the Bonin waters to the Japanese
coastal whaling have increased, partly because the productivity of theformer operation has been augmented by the adoption of the floating
factory system and the efficient catcher boats, and partly because theproductive whaling grounds in the Kuril and Korean waters was lost
because of the war (Table 1).

M. NISHIWAKI, T. HIBIYA and s. KIMURA

166

In keeping with the increasing importance of the sei whale of theBonin waters as the fishery resource, the biology of this whale is investigated actively. With the progress of the study, it has become
more probable that this whale is not the true sei whale, Baraenoptera
borealis, but the Bryde whale or it very close relative.
- - - Total catch
Catch of se1 whale

Table 1. Whale Catch by the Japanese
Coastal 'IVhaling and its Part Taken in
the Bonin Waters

400

'

,

'1
I

'
I

300

I
I

'

if!

0"'
s

I

I

I

I

I
I

'
'

b zoo

I

'

Japanese
Bonin Waters
CoastalWhaling(Number) (Number) (Percentage)

I

,'

I
I

1937

2050

160

1951

2300

340

8.2

I

\

.c:

Season

''

',,'

I

14.8

Several authors have hitherto
investigated in the sexual maturity'zof the sei whales of the adjacentwaters of Japan. Andrews (1916)
100
reported a pregnant fem ale of 25 ft.
in length. This individual, however,should be regarded as an exceptionalcase, if it was not a mink, but really
/9Lf6
1950
a sei whale. Hayashi (1927) conYear
cluded, on the basis of his anatomical
Fig. 1. Number of Catch by year
study of the ovary, that the fem ale
attain sexual maturity at lengths greater than 12 m. (40 ft.). According
to Matsuura (1935), however, the length of the female at sexual maturity
is not more than 40 ft., because pregnant females range between 40 and
43 ft. in length. Kasahara (1950) studied the sexual maturity of themale as well as of the female, and also distinguished the individuals
caught in the Bonin waters from those caught off the pacific coasts ofSanriku (northern Honshu) and Hokkaido. Based on the graph in which
the length of the male is plotted against the weight of the testes andon the presence or absence of corpus luteum, he approximately estimatedthe lengths of the two sexes at sexual maturity as follows (male: female):
41 ft.: 41 ft. for the Bonin group and 43-44 ft. for the Sanriku-Hokkaido
group. He ascribed this regional difference in the length at sexual
maturity to the fact that the whaling is operated in the Bonin waters
about a half year earlier than in the Japanese coastal waters, and con'-QJ

.0

E

::J

I

I

On the Sexual Maturity of the Sei Whale of the Bonin Waters

167

sidered that the whales grow by that length while they are migratingfrom the former region to the latter. Along the same line of reasoning, he estimated the growth rate of the sei whale at 2-3 ft. in 5-6
months.
Omura (1950) determined the length of both sexes at sexual maturity
for the three different whaling grounds, namely Bonin Islands, Sanriku(Ayukawa to Kamaishi) and Hokkaido (Kushiro to Kiritappu). His
result follows:
Bonin Islands
Sanriku
Hakkaido
40 ft.
Male
42 ft.
43 ft.
Female41 ft.
44 ft.
45 ft.
On the basis of the herein indicated geographic differences in thelength at sexual maturity, he presumed that the sei whales of the
Hokkaido waters and those of the Bonin Islands waters belong to different
populations, and that the two populations mix in the Sanriku waters.
In his study, females with any corpus luteum and males with a testis
weighing 1.0 kg. or more are regarded as sexually mature.
Since Omura published this study, a group of Japanese biologists
have collected, under his leadership, the various evidences which support
his theory and distinguished the sei whales in the adjacent waters ofJapan into the northern and southern types. The northern type is considered as preponderant in the Hokkaido waters, and the southern type,in the Bonin waters. The most notable characteristic distinguishing
the two types are the colour and structure of the baleen plate::.
Chapter I.

Material and Methods

The material for this study consists of the 694 sei whales taken in
the whaling area round the Bonin Islands in the 1951 and 1952 seasonsby the whaling fleet of the Kyokuyo Whaling Co., Ltd. accompanied by
the floating factory Baikal Maru. The operating period was May 1-June
10, 1951 in the former season, and May 1-June 20, 1952 in the latter.
In the two seasons the whaling fleet operated in the nearly same area
(Fig. 2). The sex composition and average lengths of different sexesof these whales are shown in Table 2. Length frequency distributionof these whales is illustrated in Fig. 3.
That all these whales belong to the aforementioned "southern type"has been ascertained from the characteristics of the baleen plates.
Sample baleen plates were collected from all individuals and are preserved.
Of course, it is nearly impossible to sample a whale stock accordingM. NISHIWAKI, T. HIBIYA and s. KIMURA

168

to a well-defined plan. In order to compensate for this disadvantage,
the sample size, i. e. the number of the whales to be examined, as large
as possible is desired. From this standpoint, the catches of the two
seasons are combined and subjected to analysis, because they consisted
exclusively of the southern type, and were taken in the nearly same
area and in the nearly same
145°E ~
season of the year during a
relatively short period oftime, (i. e. one and a halfmonths). The difference in
35°N
the length composition between the catches of thetwo seasons has also beentaken into account.
Pacific Ocean
Sexual maturity of the
30°N
male is determined by thehistological method.
Immediately after the carcassof a male is dismembered,a sample piece of about 1
cm. cube is taken from the
25°N
gland tissue about 1 cm. unBONtN /[derneath the free surface of
14-0"E
14 'E
the central part of eachtestis, and fixed in the alcoFig. 2. Whaling Ground of Bonin Area
hol-formalin solution (1 partof 10% formalin solution in 9 parts of 60% alcohol). At the same,each testis is weighed. In the laboratory, the sample piece is sectioned,
Table 2. Sex Composition and Mean Length of the Sei Whales
used as the Material
FemaleMale
SeasonCaughtLength (ft.)
NumberMeanStandard
Deviation

Sex Ratio

Length (ft.)
NumberStandard
Deviation

Male : Female

Mean

1951

157

41.7

1.56

126 (1)

42.3

1.39

57.6 : 42.l

1952

270 (3)

41.7

1.25

141 (7)

42.5

1.52

65.7 : 34.3

Total

427 (3)

41.7

1.56

267 (8)

42.4

1.41

61.7 : 38.3

On the Sexual Maturity of the Sei Whale of the Bonin Waters

169

double-stained by haematoxylin and eosin and examined under the microscope for the presence of spermatozoa. If completely metamorphosed
spermatozoa is found in the sample piece, the testis, hence the whale,from which the sample piece is taken is reKarded as mature.

50

1951

40

JO
20
10

40

3'J

<f)
QJ

45

--

0

_c

:;;:
't0

L
QJ

_o

80

1952

Mole

70
Female

E

::J

z

60
50

40

,,.,... _30,r--_.,,

20

,' ,

- ',
.......

I
I
I

10

I
I
I
I

35

40
Body length in feet

45

Fig. 3. Frequency Curve of Body Length.

It may happen that the testis which is determined as immature bythis method is actually mature, containing metamorphosed spermatozoa
in its other part than the sample piece. This possibility limits, to someextent, the applicability of the result of the determination. For practical
purposes, however, the result of this method can be applied with little,
if any disadvantage, because neither the length of the whale at sexual
maturity nor the weight of the testis at its maturity can be estimatedsmaller than the truth.

M. NISHIWAKI, T. HIBIYA and s. KIMURA170

In the female, sexual maturity is determineil on the basis of thefemale, sexual maturity is determined on the basis of the corpus luteum
evidence.
In the female, sexual maturity is determined on the basis of the
corpus luteum evidence.
The collection of the material, i. e. weighing the testis, taking andfixing the testis sample, and examining the ovaries for the presence ofcorpus luteum, is carried out by the government inspectors and thebiologists on board the floating factory.
Chapter II.

Sexual Maturity of the Male

Based on the results of the examination of all the testis samples,the percentage maturity of the testes is calculated for each testis weightclass, and graphed in Fig. 4.
on -010 Maturity
In the graph, the testis weightcorresponding to the percentage ma~~~=Fiducial range/
turity of 75% is 0.74 kg, its fiducial
range being 0.63-0.95 kg. at thesignificance level of 5%. Hence,statistically the testis of the sei
whale of the "southern type" becomes mature at the weight between
b 50
0.63 and 0.95 kg, and in average at
0.74 kg.
''In Fig. 5 the weight of the'25,
testisis plotted against the length,of the whale, and discrimination is,/made between mature and immature___ .
testes. The general trend suggested
1.0
0.5
by the graph, though not very disWeight of, testis in kg.
tinct because the dots are not sufFig. 4. Maturity of Testis.
ficiently many, is that the relative
frequency of the mature testes ismore closely correlated to the weight of testis than to the length of
the whale. And the relatively extensive scattering of dots in the direction of the length of the whale probably represents the considerable
variation of the length within each age-class.
In Table 3 the male whales are classified in respect to the relative
weights of their right and left testes. The statistical analysis of the
I

I

/

I

o~....---~~-.--.._,....__,.__,.-...,~.,..--.-

On the Sexual Maturity of the Sei Whale of the Bonin Waters

171

result shown in Table 3 indicates that there is no such tendency thatheavier testis occurs on the definite side of the body. This fact suggeststhat there is little sense in segregating the right testis from the left
for the purpose of making•
2.5
comparison between rightor left testis only, and•that the combined weight•of the right and left testes•
2.0•
is a more suitable measure•
for comparison than the••weight of the testis of one ~•definite side.
c
1.5
<I>
•
In l!'ig. 6, the corn- t;•-2:!
bined weight of the pair 't••of testes is plotted against :1::
••
•
1.0
•
the length of the whale.
•
Only a few whales are•0•
smaller than 40 ft., be~cause this is the minimum•
length limit provided by•
the regulations. In Fig.
6, in contrast with the
0 .
similar graphs for other
40
41
42
43
Body lenth in feetwhales, it is difficult todefine such a whale length. • Matureo Immature
above which sexually
Fig.
5.
WeightofTestis and Body Length, stained
mature individuals preas the Maturity of Testis.
dominate and below which
immature whales are do- Table 3. Number of the Male as Classified According to the Relative Weights of the Right
minant. (This fact may
and the Left Testis.
be interpreted to indicatethat the present length
Left Testis Both Testes Right Testis
SeasonEqual inCaughtHeavier
Heavier
Weightlimit is stringent enoughto protect the immature
1951
44
31
82
male.)
Although the
93
116
1952
58
weight of the testis is
Total
137
147
140
positively correlated toIthe length of the whale,
the relationship between the two factors is much obscured by the considerable dispersion of the dots, particularly in the lower length classes.

..

f.. -.. -- -.. -.. ....
..

-

'}~

I

i

M. NISHIWAKI, T. HIBIYA and s. KIMURA

172

Therefore, it may well be said that the length at which the male sei
whales of the Bonin waters attain sexual maturity can hardly be estimated correctly by the simple observation of the graph in which theweight of the testes is plotted against the length of the whale.
- - Average curve
lnd1v1duol
10

...

cri

-"'

.JI. t.
...
.. .Is

c

I

<f)

+:
</)

_g;
'+0

~ I

5

+_c:

1-.& ..m... :f!t

CJ)

QJ

S:

r

.Tr t:e- t~ ~~

y

'II

40

45

49

Body lengtn in feetFig. 6.

Weight of Testes and Body Length.

With the view of overcoming these limitations of Fig. 6, the percentage maturity is calculated for each length class, and used for thedetermination of the length at sexual maturity. Since it has beenstatistically shown that a testis weighs an average of 0. 74 kg. at its
maturity, a whale is regarded as sexually mature if its pair of testes
weighs more than twice as much, i. e. 1.5 kg or more. The result is
illustrated in Fig. 7.
Statistical computation shows that the length of the whale corresponding to the 75% maturity in Fig. 7 is 40.4 ft. and its fiducial range
is 39.8-40.9 ft. at the level of significance of 5%.
To sum up, the length of the male sei whale of the Bonin waters
at sexual maturity is estimated at 40.4 ft. on the basis of the measurements taken in May and June.

On the Sexual Maturity of the Sei Whale of the Bonin Waters

173

Of the males examined in this study, i. e. those taken in the 1951
and 1952 seasons, about 90% were sexually mature. Accordingly, one
may consider that the majority of the male sei whales taken in the
Bonin waters are mature, as far as the present regulations are operative.
--Mature--•--Immature

100

100

75

,,
Vl

--Percentage maturity

.<lJ
..__
::J

+a

E
2'

0

..c
$
'+0

50

~~-_-]Fiducial range

0

~

QJ
_D

50

E

::J

::J

x

z:

<lJ
</)

40

~

"'

25

',

',,

O"-~--~\-'~,J~~~-=-~~~-=-~•~-~>~~or-40

40
Body length in feet

Body length in feetFig. 7. Sexual Maturity of Male

45

Fig. 8. Frequency Curve of Matureand Immature Male

Fig. 8 shows the length composition of the sexually mature andimmature groups of the males taken in the 1951 and 1952 seasons.
The modal and the mean length of the mature group are 42 and 41.9
ft., respectively.
Chapter III.

Sexual Maturity of the Female.

Fig. 9 shows the relation between the length of the female andthe number of the corpora lutea in its both ovaries.
A female is regarded as sexually mature if any corpus luteum isfound in its ovaries. And the percentage of the sexually mature in-M. NISHIWAKI, T. HIBIYA and s. KIMURA

174

..
•

•

•

.... •
•

15

•
•
•

c
~

10

::J

0
L
0

e-

100

...

.. .... •• ••• • - ... - .
•- .. ..a....- ·0--..aa.- ·-..

0

u

•

<t0

'--

<U

.0

E

•

- - ·- ·-..
.. - - --..
•

::J

?

• -.me .mi •

•

'"

=-··
··-

•

·- ..aa.

0

,,

•
•

40

<l)

L
;:;)

+-

Cl

E
2' 50
0

I

i
I

;:;)

''

x

I/

QJ
<f)

•

'

)

<tCl

-''
C';-

'

'I

- - Precentage
maturity~::J Fiducial range

,'1

:I

25

I

i

~Immature

0

45

40

Body length in feelFig. 9. Number of Corpora Lutea andBody Length

Fig. 10. Sexual Maturity of Female--Mature-----Immature
50
<f)

<l)

0

-C

S:

b
L
<l)

D

E

',

::J

z

A5

Body length in teet\I --,--,~~"--1--~~-_ _ _ -: :.=--",--~------·~
40
45

o.......

Body length in fee[

Fig. 11. Frequency Curve of Mature and Immature FemaleOn the Sexual Maturity of the Sei Whale of the Bonin Waters

175

dividuals, or the percentage maturity, is calculated for each length classof the female. The result is illustrated in Fig. 10.
By the statistical method, the length corresponding to the 75%
maturity in Fig. 10 is estimated at 41.1 ft., and within the fiducial
range of 39.9-41.9 ft. at the 5% level of significance. This fiducial
range is wider than was estimated for the male in the preceding chapter,probably because fewer females are examined than males. (259 females
are examined as against 424 males.)Of the females taken in the 1951 and 1952 seasons, 84% were
sexually mature. This percentage maturity is lower than that of themales taken in the same seasons.
Fig. 11 shows the length composition of the sexually mature andimmature groups of the females taken in the 1951 and 1952 seasons.
The modal and the mean length of the mature group are 42.0 and 42.5
ft., respectively.
Discussion and Conclusions

Since the appropriateness of determining the sexual maturity of
the female whale on the basis of the corpora lutea evidence is almost
established, it may be said that maturity or immaturity of each individual
fem ale has been determined with great certainty in this study. In
estimating the length of this sex at sexual maturity, a much large
sample has been used than in the preceding studies. (Omura's (1950)
sample was the largest in the past, containing 93 females, while oursample consists of 259.) Nevertheless, the fiducial range of the estimate
has been considerably wide. These facts seem to suggest that a stilllarger sample is necessary for a narrower fiducial range.
The determination of sexual maturity of the male is much moredifficult than in the female. The most reliable method over derived
for this determination is probably such histological one as was used inthis study. If well developed spermatozoa are detected in the histologicalsample of a testis little problem occurs in interpreting the result. If
spermatozoa are not detected in the sample piece, two interpretations
are possible: (1) the testis is immature and (2) the testis is mature,and contains well developed spermatozoa in some other part of it than
the sample piece. The latter possibility can not be denied entirely,
although the maturity of the various parts of a testis was comparedin the preliminary study and the histological sample for the present
study was taken from one of those parts where maturity is attained

176

M. NISHIWAKI, T. HIBIYA and s. KIMURA

earliest. As a counter-measure, the juice squeezed from the testis was
smeared on the slide glass and examined microscopically. But thismethod did not prove to be effective. Consequently, we have decided
to regard a testis as immature if completely metamorphosed spermatozoa
was not found in its histological sample, and to content ourselves withthe fact that the percentage maturity computed according to this criterion increases sharply above certain length of the whale or weightof the testies.
Regarding the accuracy of the estimated lengths of the whale atsexual maturity, following two points should be mentioned. Firstly,
these estimates are based on the whales taken by the commercial whaling
operations, and these whales are not warranted to be a representative
sample of that stock of the whale of which we wish to know. Secondly
the length of the whale is not measured very accurately.* The accuracyof the aforementioned estimated length is limited, in either sex, bythese factors, however carefully the data may be analyzed.
In this study, the weight of the pair of testes of 1.5 kg. has beenused as a criterion to distinguish sexually mature an.d immature males.
If 1.2, 1.4, 1.6, 1.8 or 2.0 kg. is used as the criterion instead of 1.5 kg.
and the same process of computation is followed as in Chapter II, thelength of the male at sexual maturity is estimated respectively at 40.2,
40.3, 40.5, 40.6 or 40.7 ft., or with in the fiducial range that was computed with 1.5 kg. as the criterion. This fact may be interpreted tosuggest that the weight of the testes of 1.5 kg. is an appropriate criterion. In the light of the results of the present study, e.g. the resultof the histolOgical examination and the estimated length at sexual
maturity, the weight of the testis of 1.0 kg. seems to large as a criterion
for distinguishing mature and immature males. And it is our opinion
that, while the weight of the single testis is a good index of the maturityof the testis, the weight of the both testes is superior to the former
as an index of the sexual maturity of an individual, and that the doubleof the weight of a single testis at its maturity is to be used as thecriterion for distinguishing the sexually mature and immature males.
The results obtained in this study are summarized as follows.
1. The sexual maturity of the sei whale of the so-called "southern* This does not mean that whalers make false statements, for their convenience,of the length of the whales they have caught. The measurement of the length of thewhale become "inaccurate", because it is difficult to take the measurement strictly
in compliance with the definition ("on the level in a straight line"), because the whaleon the dismembering deck is seldom straightened and seldom in the position that isnormal when it is alive in the sea, and because the reading under I ft. is rounded up.

On the Sexual Maturity of the Sei Whale of the Bonin Waters

177

type" is studied with the material secured in the Bonin whaling groundin May and June.
2. On the basis of histological examinations, the weight of a single
testis at it maturity is estimated at 0.74 kg., its fiducial range being
0.63-0.95 kg. at the 5% level of significance. A male is regarded as
sexually mature, if its both testes, in combination, weigh more than
1.5 kg. (the double of 0.74 kg.) The length of the male at sexual
maturity, which is defided as the length corresponding to the percentage
maturity of 75%, is statistically estimated at 40 ft. and within thefiducial range of 39.8-40.9 ft. at the 5% level of significance.
3. Whether a female is sexually mature or immature is determinedfrom the presence or absence of a corpus luteum in its ovaries. The
length of the female at sexual maturity, which is defined in the samemanner as that of the male, is estimated at 41 ft. and within the fiducial
range of 39.5-41.9 ft. at the 5% level of significance.
4. The present regulations forbid the floating factory whaling
catcher to take sei whales under 40 ft. long in the Bonin waters. Thissize limit is considered to be more stringent and protecting the whale
stock more effectively, if not perfectly, than any other size limit now
applied to various species of whales.
In conclusion, we wish to express our sincere gratitude to the crewof the Baikal Maru fleet of the Kyokuyo Whaling Co., Ltd. for supplying
the material for this study, and to Messrs. S. Nishimoto and K. Ozaki,
government inspectors, and Messrs. T. Kawakami and K. Fujino, biologists, for collecting the material in the field.
References
Japanese Whaling Association (1951). The Whaling in the Bonin Waters. (In Japanese)
Kasahara, H. (1950). The Whaling and the Whale Resources in the Adjacent Watersof Japan. Report of the Institute of the Japan Marine Products Company
No. 4. (in Japanese)
Matsuura, Y. (1925). On the Reproduction of the Sei Whale. Plants and Animals
Vol. 3, No. 5. (in Japanese)
Matsuura, Y. (1935). On the Distribution and Habit of the Sei Whale. Ibid. Vol. 3,
No. 5. (in Japanese)
Omura, H. (1955). Whales in the Adjacent Waters of Japan. Sci. Rep. of Whales
Res. Inst. No. 4.
Omura, H: Nishimoto, S. and Fujino, K. (1952). Sei Whales in the Adjacent Watersof Japan. Fisheries Agency, JapaneS3 Government.