YEARLY CHANGE IN AGE AND BODY LENGTHAT SEXUAL MATURITY OF A FIN WHALE
STOCK IN THE EASTERN NORTH PACIFIC*SEIJI OHSUMI**
ABSTRACTThe age and body length at sexual maturity were studied on the eastern
stock (east of 180° Longitude) of the fin whale (Balaenoptera physalus) in the
North Pacific by means of two independent examinations of a) proportionsof sexual maturity in each age and body length classes and b) age and bodylength distributions of females with one corpus luteum or corpus albicans
in ovaries. Thus, yearly change in these two kinds of biological parameters
was analysed by use of data collected from fin whales caught during 22
years from the start to the end of fundamental exploitation of this stock.
Statistically significant yearly change was observed in the ages at sexual
maturity which were obtained from both the 50 % maturity and averageage of females with one ovarian corpus. The age at sexual maturity wasestimated to have changed from 12 to 6 years in females and from 11 to
4 years in males, respectively, from the beginning to the end of the periodsof exploitation. The average age of females with one ovarian corpus was
0.5-1 year older than the average age at sexual maturity of females which
was estimated from 50 % maturity.
However, no statistically significant trend was observed in the yearlychange in the average body length at sexual maturity, and this parameter
kept 57.2 and 60.6 feet (17.4 and 18.3 m) for males and females, respectively,
during these 22 years. The average body length of females with one ovarian
corpus was 0.9 feet (0.3 m) larger than the average body length of femalesof 50 % maturity.
These phenomena represent that the growth pattern of this whale
stock changed from the start of fundamental exploitation. The change inthe feeding environments along with the decline of total biomass of baleen
whale populations may have affected the growth and maturity of younger
animals more strongly than the older.

* A draft paper was presented to the International Whaling Commission/Scientific Committee/Minke
Whale Ageing Workshop, Cambridge, April 1983 (Document No. SC/A83/AW7).

** Far Seas Fisheries Research Laboratory, 5-7-1 Orido, Shimizu-ski, 424 Japan.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986, 1-16

2

OHSUMIINTRODUCTION

The age at sexual maturity is recognized as an important biological parameterfor the population assessment of whale stocks, and many papers have beenreported on this matter, especially since the discovery of teeth (Nishiwaki andYagi, 1953) and earplugs (Purves, 1955) as age characters for the cetacea in
1950s.
There are several methods to estimate the age at sexual maturity of
whales such as: 1) proportion of sexual maturity at different ages, 2) relation
between age and corpora numbers in both ovaries, 3) growth curve and bodylength at sexual maturity, 4) age distribution of females with one ovarian corpus
and 5) age at transition phase in earplug. Among them the study of transition
phase by Lockyer ( 1972) provided a new field of studies on the age at sexual
maturity.
Possibility of the decline in age at sexual maturity of whale stocks was firstsuggested by Laws (1962) without direct evidence on the Southern Hemisphere
fin whale, and Utrecht-Cook (1966) indicated this kind of phenomenon on
the same fin whale stock by use of baleen plates which are recognized not tobe so good age character. Lockyer (1972) proved this phenomenon directly
on fin whales from the Southern Hemisphere based on the age at transition
phase in earplugs. Her paper stimulated whale biologists strongly, and thismethod has become main tool to study the yearly change in the age at sexual
maturity of whale stocks since then. Lockyer ( 1974) reported the lowered age
at sexual maturity in more recent year classes on the Southern Hemispheresei whale by use of transition phase in earplugs. Masaki ( 1979), Best ( 1982)
and Kato (1983) all reported the same phenomenon by use of the transition
phase of earplugs of the Southern Hemisphere minke whale. However, Cookeand de la Mare (1983) criticized these reports, and they claimed the apparent
decline in the age at sexual maturity might be an artifact of the data. They
also threw a question on the use of transition phase for studies of the age atsexual maturity of whales.
On the other side, trials to apply other methods than transition phase as
indicators of sexual maturity have been continuing (Kato, 1983, Masaki, 1976;
1979, Lockyer, 1983) on the Southern Hemisphere minke, sei and fin whales.
This paper aims to find an evidence of yearly change in the age at sexual
maturity in the whale without use of the transition phase which has been inlarge discussion. As such a whale stock, a fin whale stock in the North Pacific
was picked up to solve the question.
Fin whales which are distributed in the eastern North Pacific (waters eastof 180° Longitude) have been recognized by the International Whaling
Commission (IWC) as a stock based on the papers such as Fujino (1960) andOmura and Ohsumi (1964) by means of some methods including blood typing
and whale marking. This whale stock began to be exploited along the coasts ofCalifornia, British Columbia and Alaska from the beginning of this century,
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

CHANGE IN SEXUAL MATURITY OF A FIN WHALE STOCK

3

but the fundamental exploitation started by pelagic whaling in the waters of
the Bering Sea and around Aleutian Is. in 1954 (Japan Whaling Association,1955). Since then, this fin whale stock continued to be exploited by Canada,Ja pan, USA and USSR whalers till 197 5 when the IWC decided to stop the
exploitation. According to Chapman (1976), this stock decreased to 38% of
the initial population level by that year. Therefore, this whale stock will be
useful for examination of change in population parameters accompanying
with the change in population level, because the population level of this stock
was estimated to change gradually and largely during relatively long time. In
addition to this feature, detailed biological data were collected on each whale
caught by Japanese pelagic whaling from the beginning to the end of thefundamental exploitation by the cooperation of many Japanese whale biologists,
national inspectors and whaling workers. These data enable us to examine theyearly change in the biological parameters thoroughly.
I would like to express my sincere thanks to these people including
technicians who prepared biological materials for this study. I am also indepted
to Mr Hidehiro Kato of the Whales Research Institute for his kind review of
my manuscript.
MATERIALS

All the materials used for this work were biological data on the fin whalescaught in the waters east of 180° Longitude from 1954 to 1975. The following
kinds of biological data were used in this study:
Position caught, 1954-1975
Whale species, 1954-1975
Sex, 1954-1975
Body length, 1954-1975 (excluding 1961)
Ovarian corpora numbers: 1954-197 5
Weight of testis of each side, 1954-1975
Total age, 1957-1975 (lack of data in 1965)
The position of catch was recorded for each whale in degree and minute.
The body length was measured from the tip of snout to the notch of flukes
in unit of foot till 1959 and 0.1 m from 1960. Then, foot unit was used in thiswork converting 0.1 meter unit into foot unit. The females from which bothovaries were collected were used to identify an individual which had one corpus
luteum or corpus albicans in total. The sexual maturity of a female wasdetermined as an individual which had at least one corpus luteum or corpus
albicans in the ovaries. The sexually immature female was determined as anindividual which had neither corpus albicans nor corpus luteum in both ovaries.
Other animals were excluded for determination of sexual maturity of females.
The male sexual maturity was determined by means of weight of testis, andan individual which had a testis of 2.5 kg and over in any side of testis was
regarded as sexually mature based on Ohsumi (1964). If testis of one side wasSci. Rep. Whales Res. Inst.,
No. 37, 1986

4

OHS UM I

TABLE 1. NUMBER OF MATERIALS WHICH WERE USED FOR EXAMINATION OF AVERAGE AGE
AT SEXUAL MATURITY OF THE FIN WHALE IN THE EASTERN NORTH PACIFIC
Males
Year

Females

Immature

Mature

Total

Immature

MatureTotal

1957-58
1959-60
1961-62
1963-64
1966-67
1968-70
1971-73
1974-75

93
219
183
109
128
46
18
10

139
257
357
236
110
128
81
69

232
476
540
345
238
174
99
79

35
298
255
130
150
74
33
20

71
271
333
227
93
94
60
42

106
569
588
357
243
168
93
62

Total

806

1,377

2,183

995

1, 191

2,186

TABLE 2. NUMBER OF MATERIALS WHICH WERE USED FOR EXAMINATION OF
AVERAGE BODY LENGTH AT SEXUAL MATURITY OF
THE FIN WHALE IN THE EASTERN NORTH PACIFIC
Females

Males
Year
1954
1955
1956
1957
1958
1959
1960
1962
1963
1964
1965
1966-67
1968-69
1970-71
1972-73
1974-75
Total

Immature

Mature

Total

Immature

Mature

Total

71
122
65
83
156
114
169
110
104
92
141
215
43
69
18
21

289
506
363
511
385
254
378
501
437
417
367
354
225
254
81
93

360
628
428
594
541
368
547
611
541
509
508
569
268
323
99
114

103
168
105
130
170
157
230
114
126
120
156
289
81
101
28
34

288
407
286
385
298
229
369
401
332
315
293
329
143
180
57
65

391
575
391
515
468
386
599
515
458
435
449
618
224
281
85
99

1,593

5,415

7,008

2,112

4,377

6,489

Sci. Rep. Whales Res. Inst.,
No. 37, 1986

CHANGE IN SEXUAL MATURITY OF A FIN WHALE STOCK

5

not collected and obtained was less than 2.5 kg in weight, such an individual
was excluded from the data for determination of sexual maturity of males.
The total age was represented as the total numbers of growth layers in earplug
which was read by naked eyes or binocular dissecting microscope after the
preparation of the material. The individual of which neonatal line or germinal
layer was lost from an earplug was not used in this study. The growth layers in
earplug of the fin whale are clearest and easiest to read among all baleen whales.
Tables 1 and 2 show the number of materials used in this study by sex,
by maturity and by year.
RESULTS

Proportion of sexually mature whales at each age
Age distribution of sexually immature and mature whales were tabulatedon each sex and in each year, and proportions of sexually mature whales atdifferent ages were calculated and plotted in Figs 1 and 2. A curve of change
in the proportion was drawn by eye on each graph. Then, the average age atsexual maturity was determined as the age at which 50 % of individuals were
sexually mature.
In the early period of exploitation till 1958, sexual maturity started fromabout 7 years of age and finished by about 16 years of age in the case of males,
although the feature in the earliest period was unknown because of no collectionof earplugs till 1956. Accompanying with the development of exploitation ofthis whale stock, the starting age decreased gradually, and it became to beabout 2-3 years in the latest period of 1974-75. However, the finishing ageof sexual maturity remained almost the same till the middle period of
exploitation of 1968-70, and then it became suddenly younger to about 9
years of age in the latest period of 1974-75. The average age at sexual maturity
represented as the age of 50 % of sexual maturity gradually decreased from
12.4 to 4.3 years during the periods.
Similar patterns were observed for females. The sexual maturity started
from about 9 years of age and finished by about 15 years of age in the early
period of exploitation of 1957-58. The starting age of sexual maturity gradually
decreased since then, and it became to be about 3 years of age in the latest
period of 1974-75. On the other hand, the finishing age remained at almost
the same age till the middle period of exploitation of 1968-70, and then it
decreased rapidly. It became to be about 12 years of age at the latest periodof 1971-75. The average age of females at sexual maturity decreased graduallyfrom 11.2 years in 1957-58 to 5.8 years in 1974-75.
Fig. 3A illustrates yearly change in the average age at sexual maturity inboth sexes. The trend in decrease of the age at sexual maturity is clear in both
sexes, and there is a trend that they have changed parallelly in each other sex
with the difference of about 1.5 years of age. Thus, the figure of males in
1957-58 should be larger than the actual figure. In the case of females theSci. Rep. Whales Res. Inst.,
No. 37, 1986

6

OHSUMIr-·-·-

100.

1957-58-- - -- --- ----

100
1966-67--;{. .

50 ---------------.---

.

!

10

s

50

----------------!

50

100

::i

.

:
j

. .--___. .....
15

20

5.9

25

....._._....

1971-7350-------------

.

15

20

1963-64

50

-

25
100

__
50

--------------- •

15

20

25

A g e

. ·-·-·-·------·152025--------- •

a:

100

10

100

1961-6250.

25-------------

:

.."
0

.

20

15

__..!---·-·-

1968-70

9.5

.....

25

1959-60

.

-"'

20

100

E
..

15

100

>
;

~

(·--·-·--·--·4.9

10

15

20

25

4.3

10

15

20

25

....

( y r·s )Fig. l. Average age at sexual maturity of the male fin whale in the eastern North
Pacific in each year.

age was about 11 years in 1957-58, and it became to be about 6 years in the
period of 1974-75. In males it was about 10 years in 1957-58, and it became
to be 4 years in 1974-75. The trend in decline of the average age at sexual
maturity was detected statistically with 1 % of risk of error for both sexes.
Although there are no data at the start of fundamental exploitation in 1954,
the average age at sexual maturity in the period can be estimated to be about
11 and 12 years for males and females, respectively, from a backward
extraporation of the line of trend in Fig. 3A.
Age distributions of females with one ovarian corpus
As the sexual maturity of the female is determined as the onset of ovulation,a cetacean female which has one corpus luteum or corpus albicans in ovaries
can be regarded as a pubertal or just matured individual. Thus, age distribution
Sci. Rep. Whal,es Res. Inst.,

No. 37, 1986

7

CHANGE IN SEXUAL MATURITY OF A FIN WHALE STOCK;·-·-·-·-· ·:: ....:., -------~·-·-

100
1957-58'" --------------1\

__

.:

'
15

25

20

.. .

.-·----~·-·-·

100
1959-60

15

25

20

100
1968-70>.~

50 --------------------.

50

'

~

•:I

'

~

"'E

'

:

7..-..-._._.._._. . .
15"'::1100=

1961-62•~. ·--------·----.._

25

20

15

100

•

20

25

1971-73

Cl)~----------------:

I'" --------------1r. . .. .
•100
1963-64

50

50 ---------------------------~

:·'

15

20

.

25

__.J---•-·-·-·---

5

50

20

10

1974-75

•

15

l.J.

100

----·---·15

20

25

15

20

25

--------- ---25

10Age(yrs)

Fig. 2. Average age at sexual maturity of the female fin whale in the eastern North
Pacific in each year.

of these animals will indicate the age at sexual maturity independently fromthe proportions of sexually mature individuals, although the former method
will give higher figure, because it includes all females between the first andthe second ovulation. On the other hand, it is difficult in the case of males to
identify the similar stage to one ovarian corpus like females by means of testis
weight, because there is large individual variation in the weight of testis at
sexual maturity, so that it is impossible to apply this kind of method for males.
Fig. 4 shows age distributions of females with one ovarian corpus in eachperiod of exploitation. The age showed a normal distribution in the earlier
period of exploitation. The attainment of this stage started from 8-9 years ofage, and finished by about 20 years of age in the same period. Then, the startingage gradually moved to younger ages, and it became to be about 4 years in
the latest period of exploitation. However, the .finishing age of this stage
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

8

OHSUMI

14

at sexual maturity

12
en 10

...

> 8

Cl)

CJ>
et 6

4
1955
62

1960
1965
1970
1975
-"' B. Average body length at sexual maturity
L:r- - -l:r-- ---6- - - 6 -!:.- - -b- - - --......,

...
Cl)
Cl)

.... 60

I

p
Q,

,'
'

/'\ ,"£r-"_ -1r-\

---c.

\
b..__

,-.o. ...

,.o-0.,-<:f"O.._O._dI"0-'... '()-__ .0----0.. ---o

...

~

C)

c: 58
Cl)

>
'O

0 56

Ill

1955

1960

1965

1970

1975

Ye a rFig. 3. Yearly change in average age and body length at sexual maturity of the fin
whale in the eastern North Pacific. Open circle and broken line: Females, Closed
circle and solid line: Males, Cross and chain line: Females which have one corpus
luteum or corpus albicans in the ovaries.

remained at almost the same age till the middle period of exploitation, andthen it rapidly moved to younger ages in the late stage of exploitation. The
range of distribution was relatively narrow in the early period of exploitation,and it became gradually wider to the end of middle period of exploitation.
Finally it became again narrow in the latest period. The distribution pattern
became gradually skewed to younger side, and the average age of the
distributions moved gradually to younger ages accompanying with the
exploitation. These phenomena are almost the same as those of the proportionof sexual mature whales as examined in the previous section.
The average age at sexual maturity in each period of exploitation thus
obtained in Fig. 4 was plotted on Fig. 3A as same as those in the previous section.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

CHANGE IN SEXUAL MATURITY OF A FIN WHALE STOCK

9

40
30
1957-58
20
10

::~
20

20

10

1961-62

15

1
~
>
c:

~.

I.I
Q)

:I
C'

..

15

20

20
1966-67

Cl)

LI..

20
1963-64

10
15
20

20
1968-70

10
20

::~

1971-73

I

15

20

30
1974-75
20
10

7.0

1--f--l101520

Agelyrsl
Fig. 4. Age distribution of female fin whales which had one corpus luteum or albicans
in the ovaries.

Clear trend in decline in the average ages of females with one ovarian corpus
was observed, and it was statistically significant within 1 % of risk of error. Theline of average ages runs parallel with that of average ages of females of 50 %sexual maturity, and the former is about 0.5-1 year older than the latter. Thismeans that the average age of females with one ovarian corpus is about 0.5-1
year older than the age at sexual maturity.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

10

OHSUMI

From these two independent examinations it can be concluded that thetrend in decline of age at sexual maturity was proved for the fin whale stockin the eastern North Pacific.
Proportion of sexually mature individuals in each body length
Size distributions of sexually immature and mature whales were tabulated
by sex and by year, and proportions of mature whales in each body lengthclasses were calculated and plotted in Figs 5 and 6 for males and females,
respectively. Then, a curve of change in the proportion was drawn by eye oneach figure, and the body length at 50 % level of sexual maturity was determined as the average body length at sexual maturity.
Patterns of the curves are almost sigmoidal and similar each otherthroughout the whole periods of exploitation in both males and females.
Males began to be mature from 55-56 feet, and finished maturation by 60-62
feet in body length. As the legal size limit of the North Pacific fin whale
maintained at 55 feet for the pelagic whaling throughout the whale periods,
this factor will not affect this analysis. The average body lengths of males
during the whole periods ranged between 56.2 to 58.2 feet with the averageof 57 .2 feet (17.4 m). Females began to be mature from 56-57 feet in bodylength, and most females attained sexual maturity by 63-64 feet. The average
body lengths ranged from 59.5 to 61.5 feet throughout the periods of
exploitation with the average of 60.1 feet (18.3 m).
Fig. 3B shows yearly change in the average body length at sexual maturityof both sexes. Although some declining trends seem to be seen for both sexes,
the trends were not detected statistically within 5 % of risk of error. Therefore,
it can be estimated that the body length at sexual maturity was constantthroughout the whole periods of exploitation, in spite of the existence of afeature that the age at sexual maturity moved to younger age during the periods.
Size distributions of females with one ovarian corpus
Fig. 7 shows size distributions of females with one ovarian corpus during
periods from 1954 to 197 5. The smallest body length of these females was 55
feet, and the largest was 69 feet. The distribution patterns of the frequency
were almost normal through the all periods, and modes ranged between 60 and
62 feet. All the average body lengths were in a range between 61. l and 61. 9
feet with the average of 61.5 feet (18.6 m). This average figure was 0.9 feet
(0.3 m) larger than the average of the average body lengths of females at 50 %maturity as examined in the previous section. This feature coincides with thefact that the mean age of females with one ovarian corpus was higher thanthat of 50 % maturity. Furthermore, the values of standard deviation were
almost the same (1.8-2.7 feet) during all periods.
The average body length in each period was plotted in Fig. 3B to examine
the trend in the average body length of females with one ovarian corpus. It
was found that there was no trend in change in these values within 1 % of risk
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

~-

~

~

~

~.~

""~
..........
""~

~~

a:

ea

CD

..

0

...."'

CD

)(

50

100

50

::I 100

.

E

::I

....

.. 50

>

~

100

100

I
I

I

60

65

60

65

50

100

50

100

50

100

50

100

•

I

58.6

55

I

2.:.£

I

§.Li

i

•

60

58.7
-·'

60

_,:Ji.,_ r
55

1960

55

Body

65

65

;/. !
/".-y-65....--·

65

·-·•

/.

--------/·

1959

55

1958

•

l

1964

1963

56.8

~

60

............

60

1'

65

--.-·--

65/•--.--;-•

•/

f

I
I
I

:

I

,.,_.,__ i

60

55
~
iength (feet)

50

100

~

,• :

65

65

, ._ ,:·:r~
_ ._,
f
100

50

100

100

50

100

56.8

56.5

•

1972-73

56.6

1970-71

56. 2

l'

_:::_

•

60

60

•

60

60

65

65

65

65

,____.

•••

--

. --1

501------··

100

50

100

50

100

Fig. 5. Average body length at sexual maturity of the male fin whale in the eastern North Pacific in each year.

56.3

65

/~

;·

Ii

_____

1957

55~

e

/i

'

60

•

60

-/ __ ._..!:~~

I-----j

1956

55

55

--·---·-::i::

t'1

--

~

ll
:;<:

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~
::i::

z

'"rj

>

'"rj

0

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;;i::l

c::

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~

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t""

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VJ

z

t'1

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>
z

ll

&.:

..""~
""
?i
.....
'O ~

.~ 1'"
"'

~

~

~

~

~

-100

a:

50

100

., 50

..

0

.,_

><100

.,.,

:I

..

E

:I

....

>

0

1954

m

55

1957

55

1955

55

65

65

65

65

---·-..-.

1958

1959

55

.

;·:

60.6

1

50

100

50

100

. :i

I

60.3

65

55

60.0

:
Body

65

_ ·:·--~r~

55

./·,A

65

55

55

1966

55

1965

I

I

'

•

'

§Q.:1.

~

i

60.4

·-.
65

65

65

65

•--.

;·-.-.-·§Q_,]_

ilength(feet)

50

100

50

100

55

• '

• ·:-;-1968-69

..

o

Io

•

65

r·.·- :'" ----------- '
I
/.. :

100

1972-7355-

1970-71

55

•

'

I

59.6

•

50

100

55

1974-75

55

59.6

59.9

I

I

.

59.8

----- -'

50 1-------------·-

100

50

100

I'

-........

65

65. ·----·

65r--·.. ... ,- .- .
1 ~------- .1·

!'

I

.

(

50 ~---------------

100 ..
1964/.-•-.

61.5~-------------!
·:

1960

55

1963

'", ______,_____ ..

100..

65

7·.-·-

so~---- ---- -------1.·l

100

50 ·-------------

100

Fig. 6. Average body length at sexual maturity of the female fin whale in the eastern North Pacific in each year.

59.5

59.6

§Q.:1.

50 ·---------- ---

100

~

"'c:

0
::c:

-

N)

~ ?.

""::0

ft:

~

::0

~

~~

.......

.'l~

:I

Ill

>
(,)
c

10

5

10

15

I

t

10

lH-

10

15

~ 15t
u..

-

....0

0

10

ll

55

""

JJ

,.

61.9

\

I

60

'
61.6

\/ \

61.4 \

f\c

vu

--

\
65

u~

u"

65

u"

-

70

/U

/U

70

Body

1962-63]QfiO-fil

1958-59

/U

l

t

r.

r
55

55

I

55

55

I

/

length (feet)

5

10

15

20 I-

25 I-

5[

10

15

5

10

15

5

10 I-

15

I

.

0

\

·A·
I

61. 7

\

I

I

\

65

I

60

I

I

A

I

65

I

I

1070- 7?

1968-69

70

70

I

70

.k,7~

I~
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14

OHSUMIof error, and the line was almost parallel to those of body length of femalesof 50 % maturity. Thus, this is the evidence that the body length at sexual
maturity was constant even in the condition showing the age at sexual
maturity declined.
DISCUSSION

This study provided an evidence of yearly change in the age at sexual maturity
in a baleen whale stock independently from the use of transition phase in ear
plugs. In other word, this supports the reality of similar phenomenon on theage at sexual maturity as examined by means of transition phase, and proves
the reasonability of the transition phase as a mark of the sexual maturity as
proposed by Lockyer (1972).
It was found from Figs 1 and 2 that the youngest limit of age of mature
whales changed rapidly to younger ages from early period of exploitation, but
the oldest limit of age at sexual maturity remained unchanged for relatively
long periods of exploitation. The similar phenomenon was also observed in
the age distribution of females with one ovarian corpus. Fig. 8 shows a schema
to illustrate the changing pattern of proportions of sexually mature individuals
at different ages during the periods of exploitation as observed in thisexamination. A similar pattern of age at sexual maturity was observed between
African elephants populations in different environment (Laws, Parker andJohnstone, 1975). These phenomena indicate that the change in the habitatof an animal stock affects on the growth and maturity of younger animals
more strongly than the older animals. Thus, younger animals attain sexual
maturity rapidly, when their habitat has become better than the past. On theother hand, the older animals which had spent their young stage in the past
inferior environment remained in the immature stage even in the improved
habitat changed.
The analyses of age at sexual maturity were made in this paper based on
years when animals were caught. If the same analyses are carried out basedon year classes as examined by Kato (1983) for the Antarctic minke whales,
clearer result than this paper will be obtained, considering the above possibility.
Further studies will be needed on this matter.
An evidence was provided in this study that the average body lengths at
sexual maturity kept almost the same size during whole periods of exploitation,in spite of the condition that the average age at sexual maturity decreased
significantly in the periods. Lockyer ( 1972) reported the same phenomenonon the Southern Hemisphere fin whale, but she reached to this result by review
of several papers which described on the body lengths at sexual maturity fromvarious regions. The present paper examined this matter by use of the sameseries of data collected from the same stock. In this sense, this paper will provide
the first evidence on this feature in the cetaceans.
Although the average age and body length of females with one ovarianSci. Rep. Whales Res. Inst.,

No. 37, 1986

15

CHANGE IN SEXUAL MATURITY OF A FIN WHALE STOCK
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Fig. 8. Schematic figure to represent the change in pattern of proportions of sexuallymature whales in each age accompanied with the change in environment of a whale
stock. The approximate correspondence to actual seasons is, A: 1957-58, B: 1959-60,
C: 1966-70, D: 1971-73

corpus were regarded as one of the indicators of those at sexual maturity, it was
found in this paper that they were larger than those of 50 % sexual maturity.
The differences were 0.5-1 year and 0.9 feet for age and body length,
respectively. The age and body length of 50 % maturity is regarded as themean value at the instance of sexual maturity, whereas females with one
ovarian corpus have been already matured before their catch, so that it will
be natural to reach the above result. Anyway, we must recognize that theresults of both approaches are somewhat different, but the application ofboth of these independent methods will be useful to confirm the results ofanalyses.
The change in age at sexual maturity and no change in body length at
the same developmental stage mean that the growth pattern of this whale
stock changed during the periods of exploitation, and the whales of this stockgrew faster accompanying with the decrease of population level. Furthermore,
a whale stock has its fixed body length at sexual maturity, and when an
individual which grows fast attains the fixed size of body, it becomes sexuallymature, even if it is still young. Contrary to it, an old individual does not
attain sexual maturity, unless it grows to the fixed body length.
The fin whale stock dealt in this study decreased its population level
rapidly during 22 years from 26 to 10 thousands (Chapman, 1976). Blue,
humpback and sei whale stocks which have similar feeding behavior to the fin
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

16

OHS UM I

whale decreased also in the same waters during the periods (Chapman, 1976).
The clear decline in the age at sexual maturity and change in growth patternof the eastern North Pacific fin whale stock reflect change in total biomass ofbaleen whale populations similar to the Southern Hemisphere fin whales(Gambell, 1975). The decline of total biomass of baleen whale populations
provided gradually better habitat and larger amount of intake of food perhead of the fin whales, and better nutrition and less stress improved the growthand maturity.
The catch history and change in population level and its habitat are
different from stock to stock even in the same whale species. The comparisonof the change in biological parameters including the age at sexual maturity
will be interesting to be studied in future in conjunction with the differenceof biomass and their habitats.
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