EARPLUG TRANSITION PHASE AS AN INDICATOR
OF SEXUAL MATURITY
IN FEMALE ANTARCTIC MINKE WHALES*SEIJI OHSUMI**
ABSTRACTThe transition phase in the earplug of the minke whale (Balaenoptera
acutorostrata) was examined on its meaning as an indicator of the sexual
maturity by use of earplug materials of 360 females caught in the Antarctic
Areas III and IV in 1971/72-1981/82 Seasons.
It was confirmed from seven kinds of examinations that the transition
phase can be regarded as an indicator of the sexual maturity of the minke
whale. However, few whales form the transition phase in sexually immature
stage, when they grow near the body length or age at sexual maturity. The
transition phase becomes to be identified fully by the time of accumulationof 4-5 growth layers after its formation. Ten to fifteen percent of sexuallymature whales do not form identifiable transition phase in earplugs fortheir life.
Annual ovulation rate was estimated from age at capture, age at
transition phase and number of ovarian corpora to change from 1.9 in theearly stage of sexual maturity to 0.9 in the oldest stage of life in recent years.
INTRODUCTION

Growth layers of the earplug have been used as the best age character ofbaleen whales since the discovery of the layers by Purves (1955). Ichihara( 1966) suggested for the fin whale that the age when width of growth layers
suddenly declines might indicate the age at sexual maturity. Lockyer (1972)
confirmed this through the comparison of growth layers and biological dataof the Southern Hemisphere fin whales, and named the phase where thegrowth layer width changes abruptly as 'transition phase'.
Masaki (1973) found the presence of the transition phase in earplugs of
the Antarctic minke whale as already reported on other Balaenoptera whale
species. Masaki ( 1979) further studied the age at sexual maturity of the
Antarctic minke whale, and indicated that the age at sexual maturity suggestedby the transition phase agreed well with the age obtained from the data of* A draft paper was presented to the International Whaling Commission/Scientific Committee/Minke
Whale Ageing Workshop, Cambridge, April 1983 (Document No. SC/A83/AW8).

** Far Seas Fisheries Research Laboratory, 5-7-1 Orido, Shimizu-ski, 424 Japan.
Sci. Rep. Whales Res. Inst.,

No.37,1986,17-30

18

OHSUMI

reproductive tract. Kato ( 1983) used the trans1t1on phase to estimate theyearly change in the age at sexual maturity of the Antarctic minke whale, andcompared the results obtained from other independent methods. Best ( 1982)
confirmed the correlation between sexual maturity and presence of transition
phase in earplugs of minke whales off Durban, and he also studied yearlychange in age at sexual maturity by use of the transition phase.
Although these authors indicated the usefulness of transition phase as amark of the sexual maturity of the Southern Hemisphere minke whale, it has
been understood that there are some need to examine further on this matter
through the discussion in the Scientific Committee of the International Whaling
Commission in the beginning of 1980s (Anon., 1983). This paper will examine
if the transition phase in the earplug can be used as an indicator of the sexual
maturity of the Antarctic minke whale. As the materials only females wereused, because the sexual maturity can be more easily identified.
Mr Hidehiro Kato of the Whales Research Institute provided me withearplug materials and biological data for this study. I would like to express mysincere thanks to him for his endeavor to select these materials. I am largely
indebted to Dr Toshio Kasuya of the Far Seas Fisheries Research Laboratory
who kindly criticized my manuscript and made many suggestions.
MATERIALS AND METHODS

The materials used in this study were earplugs of 360 female minke whalescaught by Japanese whaling expeditions in the Antarctic Areas III (0-70°£)
and IV (70-130°£) in 1971172-1981/82 Seasons and stored in the Far Seas
Fisheries Laboratory. Mr H. Kato selected these materials under the principle
agreed by the Scientific Committee for use in the Minke Whale Ageing
Workshop which was held in Cambridge in April 1983 (Anon., 1984). The
selecting procedure of these earplugs was described in detail by Kato (1984).
In this case stratified sampling of two readability classes and nine corpora
count classes (18 strata) was adopted: 20 samples were allocated to eachstratum. The earplugs were read by myself for total number of growth layers(age at capture) and the number of growth layers before the transition phase(age at transition phase). Each half-sectioned earplug was put in water andread for these two kinds of layers under a binocular dissecting microscope forthree times without referring to any biological information of the whale, andfinally decided the best numbers of these layers on each whale as the average.
The body lengths were measured in 0.1 m unit on board of the whaling
factory ships. The numbers of corpora lutea and corpora albicantia in theovaries were counted in the Far Seas Fisheries Research Laboratory or by
biologists on board of factory ships. These data have been stored in the Far
Seas Fisheries Research Laboratory, and used to determine the sexual maturityof the materials. The sexual maturity was determined by the presence of corpus
luteum or albicans in the ovaries. Among them a female having one or two
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

19

TRANSITION PHASE OF THE ANTARCTIC MINKE WHALE

TABLE 1. APPEARANCE OF TRANSITION PHASE ACCOMPANIED WITH SEXUAL MATURITY
AND AGE AT CAPTURE OF THE ANTARCTIC MINKE WHALES
Age

Immature

+
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26±

4
5
9
7
4
6
2

Mature

+

Total

4
5
9
7
4
6
2

3
2
3
2
4
5
4
2
2
3

10
9
9
69
4
267

319

2
l

3
9
11
14
10
17
15
13
17
14
10

3

8
10
11

4
3

7

38

2

40

52

+

Total

3
3
5
3
7
14
15
16
12
20
16
14
18
14
11
11
10
11
14
9
12
76
5

l

Unknown
Total

Total

4
5
12
9
7
8
6
5
5
2
2
3

3

4
5
12
l

10

2

11
10
9
9
69
5

9
9
9
15
16
16
12
20
16
14
18
14
11
11
10
11
14
9
12
76
6

269

359

3
10

11
14
10
17
15
13
17
14
10
8
10

4
3
7

90

Total

Remarks: - : Transition phase not appeared, +: Transition phase appeared.

ovarian corpora was recognized as a whale just after the attainment of sexual
maturity, and used as an indicator of a stage of attainment of sexual maturity.
RESULTS

Transition phase and sexual maturity
Among 360 females used in this study, an earplug of a whale had anabnormal pattern of growth layers, and it was impossible to read age at capture
or at transition phase. This whale was omitted from the later analysis. Earplugsof other 6 whales were incomplete missing either germinal or neonatal layer,
and their age at capture was not estimated, although some of their transition
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

20

OHS UM I
TABLE 2. APPEARANCE OF TRANSITION PHASE
ACCOMPANIED WITH ACCUMULATION OF
CORPORA LUTEA AND ALBICANTIA IN THE OVARIESOF THE ANTARCTIC MINKE WHALES
Number ofcorpora

+

Total
40
11
17
9
9
18
13
11
20
9
13
21
7
10
16
16
18
10
12
16
15
48
359

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21±

38
9
7
2

4
2
5

2
2
10
7
8
13
12
11
19
7
10
21
7
10
15
12
15
9
11
12
13
43

Total

90

269

5

1
2
3

1
4
3

Remarks: - : Transition phase not appeared,+: Transition phase appeared.

phase could be identified (Table 1).
As shown in Tables 1, 2 and 3, 40 whales were sexually immature. Amongthem two (5.0 %) had transition phase, and others (38 whales, 95.0 %) did
not. This indicates that most of the immature whales have no transition phasein the earplugs, but few immature whales have already formed it.
These tables also show that 267 whales (83.7 %) have transition phaseamong 319 sexually mature females, but it is not detected on remaining 52
whales ( 16.3 %). Some of the latter may have transition phase actually, but the
post-transition phase could not be thick enough to be identified, because
transition phase has just formed on the individuals which were soon after theattainment at sexual maturity. In addition to it, there are old whales of whichtransition phase was not detected at all.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

21

TRANSITION PHASE OF THE ANTARCTIC MINKE WHALE

TABLE 3. APPEARANCE OF TRANSITION PHASE ACCOMPANIED WITH SEXUAL MATURITY
AND BODY LENGTH OF THE ANTARCTIC MINKE WHALES
Body length

Immature(m)+

6.9- 7.0
7.1- 7.2
7.3- 7.4
7.5- 7.6
7.7- 7.8
7.9- 8.0
8.1- 8.2
8.3- 8.4
8.5- 8.6
8.7- 8.8
8.9- 9.0
9.1- 9.2
9.3- 9.4
9.5- 9.6
9.7- 9.8
9.9-10.0
10.1-10.2

l
2
5
4
8
2
5
3
5
3

Total

38

Total

Mature

+

Total

3
5
4
8
2
5
3
6
3

2
8
7
4
16
3
3
6
2

40

2

52

+

Total

3
6
8
32
38
54
48
32
25
14
3
2

3
8
16
39
42
70
51
35
31
14
5
2
2

267

319

l
2
5
4
8
4
13
10
9
19
3
3
6

2

90

Total
l
3
6
5
11
10
21
42
48
73
57
35
31
14
5
2
2

l
3
6
8
32
39
54
54
32
25
14
3
2

269

359

Remarks: - : Transition phase not appeared, +: Transition phase appeared.

TABLE 4. COMPARISON OF PRESENCE OF TRANSITION PHASE AMONG SOME REPORTS
Sexual maturity
classification

Lockyer (1972)
Fin whale
Females
Sample size

Immature
Pubertal*
Other mature

27
47
177

%
0.0
19.2
96.5

Best (1982)
Minke whale
Both sexes
Sample size
21
12
94

Present paper
Minke whale
Females

%
4.8
16.7
83.0

Sample size
40
28
291

%
5.0
42.9
87.6

Remarks:*: One to two corpora in ovaries or both mature and immature tubules in testes.

Correlation between ovarian corpora number and transition phase
Table 4 shows that the proportion of whales having transition phase is42.9 % in early stage of mature females (one or two ovarian corpora), which
is lower than that of older mature whales (87.6 %). This suggests that there
will be some correlation between ovarian corpora number (or time after
maturation) and proportion of individuals having transition phase.
Fig. lB shows this correlation using data in Table 2. The proportion of
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

22

OHSUMIr·----@_____@
___ @
-

100
80
60
Q)

C/I

40

Q,

20

ea
.s::.

o

/}I

/,'/
0

/

c:

I

,, ,•

............_.....
o~"_.__.___.___.__.__.__.__...._..__.__.__.__.__"-..L.....L.....L....._ __ ___j___

0

5

....
C/I

c:

10

15

Total

age (yrsl

•

•

~ 100

....
Q)

>
ea
.s::.

B

(,.)

•/ /.......

80
I

60

I

•

I
I

5

0

.
0

10

100
80
60

0

0

21+

20

0

40

20

.

0

@

@

-----------0--.,....------------®,. ,.

.

0

c

Q,

e
a.

15

Number of corpora in ovaries

~

0

I

•
...................._._...L......._.l....-L....1-.L........_._...L......._.l....-L....l-L-L.....1........L..---____J__

Q)

c:

.

I

ea
.s::.
0

.

I

20

C/I

.....

•

25+

I

40

.s::.

~

.

20

--------------•-------•--

I

.s::.

@

@

A

,.,
,.
.,,,
•,.,. ........

/
/

,,, "

/'

,,, ,.

@

,/'
0

•

.................................._.__..__._......._..._,,__L......IL-l--L....J......L....1....-L-..L-J......~--...,L_
7.5
8.0
8.5
9.0
9.5+

Body length (m)Fig. 1. Proportions of whales which have transition phase in earplug to total whales
in each growth stage. A: Based on total ages, B: Based on corpora number in ovaries,
C: Based on body length. Open circle and solid line: Sexually mature whales, Closed
circle and broken line: Total whales.

individuals which have transition phase in earplug increases rapidly tillaccumulation of 4 or 5 corpora, then it becomes constant at a level of 87 .9 %individuals with five corpora. This indicates that the transition phase is difficult
to be identified at the beginning of its formation, although it is formed aroundSci. Rep. Whales Res. Inst.,

No. 37, 1986

TRANSITION PHASE OF THE ANTARCTIC MINKE WHALE

23

the attainment of sexual maturity, but it can be detected fully after the stageof 4 or 5 corpora number, if it is formed in an individual.
Correlation between age at capture and transition phase
Table I shows frequency distributions of ages at capture of 359 females
used in the present analysis. Ages of sexually immature whales are distributedin the range of 3 to 11 years. Two immature whales have transition phase.
One of them is 10 years old. The age of another whale is unknown, because
germinal layer was missing from the earplug. This whale is estimated to be atleast 14 years old from remaining part of the earplug. These two whales indicatethat the transition phase may be formed in some whales after a certain age
when most whales attain at sexual maturity, even if the whale remains in theimmature stage.
At the ages of 6-9 years, all the 19 immature whales had no transition
phase, but seven individuals in 18 mature females had it. This means that thecorrelation of transition phase with age is lower than that with sexual maturity.
Although sexually mature whales were found from 5 years of age, sexuallymature whales with transition phase appeared from 6 years of age. This isreasoned by that the transition phase is difficult to identify at the beginningof its formation.
Fig. IA shows age related change in proportion of whales with transition
phase in earplugs. The proportion increases from the age of 5-6 years to 13-15
years, and then it becomes stable at a level of 89.8 % over 14 years as in thecase of corpora numbers mentioned above. The increase in the proportion
with the age (5 to 14 years) will be related with the increase in the proportionof sexually mature whales as well as the increase of detectability of transition
phase. The proportion is higher than that calculated including immature
whales. This means that the formation of transition phase is related morewith the sexual maturity than the age. However, even in the sexually mature
whales the proportion of whales having transition phase increases with age thesame as in the case of ovarian corpora.
Ages of full identification of transition phase
Due to the definition of the earplug transition phase, it is reasonable toconsider that the detection of transition phase becomes easier after several
growth layers have been accumulated after the phase.
Fig. 2 shows a frequency distribution of times after transition phase (ageat capture minus age at transition phase). The recognized shortest time after
transition phase is one year which is represented by only three whales, but thehighest frequency is at 5 years. This indicates that the presence of transition
phase can be rarely recognized when one growth layer is accumulated after
the phase, but most usually recognized after the deposition of 2 to 5 layers. The
transition phase can be fully identified, if it exists, when more than 4 layers
are accumulated after its formation.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

24

OHSUMI

0 15
...
Q)

.lJ

E
:s 10
c:
>

tJ

c:
Q)

:s

5

C'

...

Q)

LL

5

10

15

Time after transition

20

25

30

phase (yrs)Fig. 2. Frequency distribution of ages after transition phase in earplugs of the female
Antarctic minke whales.

Correlation between body length and presence of transition phase
Table 3 shows frequency distributions of body lengths of the present
material. Whales without identifiable transition phase are found in all sizeclasses. Fig. IC shows the relationship between body length and proportionof whales having transition phase. In the length range of 7.4 to 8.6 m, theproportion increases with increasing body length. Above 8. 7 m of body length
the proportion is constant at the average of 84.5 %.
Fig. 1C also shows the proportion of whales with transition phase calculatedusing only sexually mature whales. Although the sample size is small (11whales) for whales below 8.0 m, it is considered that there is no correlation
between the proportion and body length, suggesting that the formation oftransition phase is not directly related with the body length.
Comparison of age of whales just after sexual maturity and age at transition phase
Table 5 shows distribution of age at capture of whales with one and two
corpora in ovaries. The range of the ages is from 5 to 15 years with averageof 9.4 years. Among 28 whales which have one or two ovarian corpora, 12
whales are identified with the transition phase in earplugs, and the distributionof ages at transition phase of these whales is also shown in Table 5. The ages
ranged from 4 to 10 years with an average of 7. 7 years. This figure is less than
the average of ages at capture of these 12 females, because these whales havepassed some time after the attainment of sexual maturity before the capture
Sci. Rep. Wha/,es Res. Inst.,

No. 37, 1986

25

TRANSITION PHASE OF THE ANTARCTIC MINKE WHALE

TABLE 5. AGE DISTRIBUTIONS OF THE ANTARCTIC MINKE WHALES IN SEVERAL CASES
Age

Total ages

(yrs.)
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Total
Average
S.D.

Ages at transition phase***:S 13 yrs

2
1
1
1
2
2

3
2
4
2
3
4
4
1
2
2
1

2
10
11
8
16
3

11
8.55
2.54

28
9.39
2.85

50
7.70
1.36

:s 15 yrs
2
3
11
13
14
27
9
2
1

82
8.04
1.61

Total yrs.
3
4
21
23
34
53
44
25
21
16
12
3
5
1
2
1
268
9.73
2.65

**
1
1
1
3
4
2

12
7.67
1.89

Remarks: *: One corpus in ovaries, **: One or two corpora in ovaries.

but the age at transition phase can be closer to the age at the attainment ofsexual maturity. However, the deviation between the two is 1.7 years.
In this table frequency distributions of ages at transition phase are shownfor 82 whales of age at capture of 15 years (upper limit of ages at capture ofwhales with one and two ovarian corpora in Table 5) and younger. The
average of the ages at transition phase in this case is 8.0 years with range of
4 to 12 years. This is almost the same as that (7.7 years) of the same kind ofages in the case of whales with one and two ovarian corpora, and 1.4 years lessthan the average age at capture of females with one and two ovarian corpora.
Comparison of estimated and read total ages
Lockyer ( 1972) examined the meaning of transition phase of fin whales
by comparison of total earplug growth layers with ages of the animal at capture
calculated using age at transition phase, number of ovarian corpora and averageannual ovulation rate of the stock. The same method is applied in this study.
The age of a female at capture is calculated from following equation:te = tt + (cir) ..... (1)Where, te is calculated age at capture, tt is age at transition phase, c is ovarian
corpora number and r is annual ovulation rate. The data of tt and c are available
from the carcass. The problem is the value of annual ovulation rate. According
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

26

OHSUMI

.ui

>

Q)

7
6

C'>

ea

5

"C

..
ea
Q)

4

....0
Q)

C'>

2

"C

1

I

\

Q)

....ea

,

0

E

....

----1;/~-

\

I

I

I

I
I

I

-1

Q)

....0 -2

''

I

\I

I

I

\

-l,'----,--1

Ill

c:

\
:
-\-,'_ -- -- l
,.

ea

\
\

\
\

,,

-''
\

\
\
\

\

-3

\
\

.... -4
ea

>
Q)

c

'

\

0

' f ,/
'

I

',

I

-5

5

10

Age

15

at

20

25

30

capture (yrs)Fig. 3. Comparison between estimated and read total ages for the Antarctic minke whale.
Closed circle: Average deviation of estimated age to read age, Range: Standard
deviation.

to Masaki ( 1979), individual variation of annual ovulation rate ranges from
0.835 to 1.401 in the Antarctic minke whale. Best (1982) estimates the mean
ovulation rate of minke whales off Durban at 0.811. Kato (1983) reports thatthe ovulation rate of the Antarctic minke whale increased from 0.77 (1952-54)
to 1.25 ( 1967-69). Recently Kato ( 1985) showed a figure of 1.17 as the annual
ovulation rate, although he indicated the rate declines with age.
If the age at capture thus calculated is close to the total growth layers of
the same individual, the counted age at transition phase is proved to be close
to the age at sexual maturity under the condition that the annual ovulation
rate is true. A series of simulations was carried out using various values ofovulation rate to obtain the best match between total earplug age and thecalculated age at capture. The best match was obtained assuming an annual
ovulation rate of 1.4 as shown in Fig. 3. In this case average deviation between
these two kinds of ages was around zero till the earplug age at 19 years,
although there are large individual variations. This examination suggests thatthe age at transition phase can be regarded as the age at sexual maturity andSci. Rep. Whales Res. Inst.,
No. 37, 1986

TRANSITION PHASE OF THE ANTARCTIC MINKE WHALE

27

the annual ovulation rate of the Antarctic minke whale is 1.4 for individuals
below 19 years, which is larger than those ever estimated. The average deviation
changes gradually to lower figures in the ages older than 19 years. Thisphenomenon means that the annual ovulation rate is not constant throughout
all ages, and the ovulation rate of older age or earlier born generations had
been smaller than 1.4. When an annual ovulation rate is assumed to be 1.0
for the age class of 26-28, the deviation of calculated and counted ages at
capture becomes to be close to zero.
DISCUSSION

Presence of immature whales which have transition phase
Two sexually immature females were found among the present materials
to have transition phase. There is a possibility of miss-collection or misspreparation of earplug of one of the whale (No. W-293), because this whalewas 7 .1 m long (most individuals of this size are sexually immature as shownin Table 3), its age at capture was 14+ with the age at transition phase of 6
years. However, another one is difficult to assume such a possibility, for it is
8.5 m long and 10 years old at capture. Both of these figures leave for thefemale possibility of being sexually immature. As shown in Table 5, Best
(1982) already reported immature minke whales having transition phase.
Therefore, it can be concluded that some females will form transition phasein earplug while they are sexually immature, when they attain at certain age
and body length. On this connection, Lockyer (1972) and Kato (1985) examined
the growth of skull in relation to mechanism of the formation of transition
phase in earplugs of the fin and minke whales, respectively.
However, as examined in this study, the sexual maturity seems to correlate
in many individuals with the formation of the transition phase, even if they
are very small or very young.
Identification of transition phase
All the sexually mature female minke whales do not have always transition
phase in earplugs. There are two cases of the absence of transition phase insexually mature stage. One comes from the difficulty of identifying it soonafter its formation. The low proportion of whales with transition phase infemales just after the attainment of sexual maturity reflects this (Table 4). The
transition phase becomes identifiable after the accumulation of 4-5 growthlayers of post-transition phase as examined in this study.
Another is the case where transition phase cannot be recognized for life
due to the gradual change of the layer width. These whales comprise 17 %(Best, 1982), or 12.4 % (this study) of adult females. The corresponding figure
is only 3.5 % in fin whales (Lockyer, 1972). The frequency will be different
between whale species.
Sci. Rep. Whales Res. Inst.,

No. 37, 1986

28

OHSUMIUse of transition phase for estimation of ovulation rate
Using several methods, it was confirmed in this study that the formationof transition phase approximately coincides with the attainment of sexual
maturity of the Antarctic minke whale. Then, the time after sexual maturity
can be estimated from age at capture and age at transition phase. On theother hand, the data on ovarian corpora numbers are available for each whale
independently. Then, the mean ovulation rate of each whale is estimatedfrom these data as follows:r= c/(ta - tt) ..... (2)Where, ta is earplug age at capture, and other symbols are the same as thoseof the equation 1.
Fig. 4 shows that the mean annual ovulation rate declines with increasing
age at capture. Although the lower limits of the standard deviation are almost
constant (0.75 corpora annually) through the age, the upper limits of thestandard deviation decrease largely with age. Thus, the standard deviation
decreases with age at capture. The mean annual ovulation rate decreases
from 1.9 at the beginning of sexual maturity to 0.9 in the oldest ages. The
declining trend of ovulation rate has been reported for the minke whale by
Masaki (1979), Best (1982) and Kato (1985). The apparent decline of ovulation
rate will reflect two causes. One is the true age dependent change, where theovulation rate decreases with age even under constant environment. The second
cause will be the year factor. The cha:r:ige in the environment of the Antarctic
minke whale could have been continuously increased the annual ovulation
rate of females especially of young females and decreased the age at sexual
maturity. Both factors may affect this whale population, and obtained thisapparent result, but it is difficult to separate these two factors from the result.
Kato, Shimadzu and Kirishima (1985) tried to introduce the change in age atsexual maturity and pattern of change in ovulation rate with age to explain
the apparent relationship between the age after maturation and ovarian
corpora number.
The present analysis suggests that the annual ovulation rate of the Antarctic
minke whale changes from 1.9 in the beginning of sexual maturity to 0.9 inthe oldest stage in recent years. As examined in previous section, anotherkind of examination indicates that the mean annual ovulation rate keeps avalue of 1.4 till 19 years of age. Kato et al. ( 1985) show a graph of change inovulation rate with age of the same whale species that it changes from 1.6 at
the beginning of maturity to 0.2 in 40 years of age. On the other hand, Best( 1982) has a doubt on the possibility of the existence of ovulation rate of 1.56
and over per conception, because a failure rate will be very high in such acase, if the true pregnancy rate is about 78 %. However, we cannot explain
the actual phenomena unless we consider the values of the ovulation rate
which was estimated in the present study.
Sci. Rep. Whales Res. Inst.,
No. 37, 1986

29

TRANSITION PHASE OF THE ANTARCTIC MINKE WHALE

...ea

QI

3

A

f:. ~:rr:1:tr-JTJ~:J:-Y£f:~Jc0...ea 2
:I

>

0

ea

,, ' '

1

:I

c
c

<
5

...

10

20

15

25

30

(I)

B

~ 16
QI
(I)

ea 14
.c

c. 12
c

0

... 10(I)c.........ea
ea
QI

Cl

8
6

4

<(

L-1-----'-----'1------11----1-----1----- - - - 1 5

10

20

15

Ag e atc a pt u re

25

30

35+

( y r s)Fig. 4. Estimation of trends in annual ovulation rate and age at sexual maturity under
the condition that the transition phase is a mark of the sexual maturity. A: Relation
between total age and annual ovulation rate, B : Relation between total age and age
at transition phase. Closed circle: Average, Range: One standard deviation.

Transition phase to estimate yearly change in age at sexual maturity
Fig. 4B shows mean and standard deviation of age at transition phasecalculated in accordance with age at capture. The mean age at transition
phase clearly increases with increase of age at capture. The average age at
transition phase in whales of 35 years or over at capture is 13.5 years, but it
is 7 years or less in whales captured before 10 years of age. Although thisstudy did not deal with the data based on the year class as examined by Masaki(1979), Best (1982) and Kato (1983), similar values and trend as these papers
were observed in this way.
REFERENCES
ANON., 1983. Report of the Scientific Committee. Rep. int. Whal. Commn, 33: 43-190.
ANON., 1984. Report of the Minke Whale Ageing Workshop. Rep. int. Whal. Commn, 34: 675-699.
BEST, P.B., 1982. Seasonal abundance, feeding, reproduction, age and growth in minke whale off Durban.
Rep. int. Whal. Commn, 32: 759-786.

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

30

OHSUMIICHIHARA, T., 1966. Criterion for determining age of fin whale with reference to ear plug and baleen plate.
Sci. Rep. Whales Res. Inst., 20: 17-83.
KATO, H., 1983. Some considerations on the decline in age at sexual maturity of the Antarctic minke whale.
Rep. int. Whal. Commn, 33: 393-399.
KATO, H., 1984. Sampling procedure for earplugs provided to the Minke Whale Ageing Workshop. Rep.
int. Whal. Commn, 34: 717 - 718.
KATO, H., 1985. Further examination of the age at sexual maturity of the Antarctic minke whale as determinedfrom earplug studies. Rep. int. Whal. Commn, 35: 273-277.
KATo, H., Y. SmMADZli and K. KIRISHIMA, 1985. Further development of a biological simulation to examine
historical change in age at sexual maturity of the Antarctic minke whale. Rep. int. Whal. Commn,
35: 291-298.
LOCKYER, C., 1972. The age at sexual maturity of southern fin whale (Balaenoptera physalus) using annual
layer counts in the ear plug.]. Cons. int. Explor. Mer., 34: 276-294.
MASAKI, Y., 1973MS. Some notes on age estimation of baleen whales based upon the earplug. Doc. No.
SCIRJ presented to the Meeting on Age Determination in Baleen Whales, Reykjavik, April 1973.
MASAKI, Y., 1979. Yearly changes of the biological parameters for the Antarctic minke whale. Rep. int. Whal.
Commn, 29: 375-395.
PURVES, P.E. 1955. The wax plug in the external auditory meatus of the Mysticeti. Discovery Report,
27: 293-302.

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