EXAMINATION ON AGE DETERMINATION
OF THE FIN WHALE
SEIJI OHSUMIINTRODUCTION

As the age characters of the fin whale (Balaenoptera physalus L.), the following havebeen reported hitherto. They are body length, stage of growth, number of ovulation, white scars (Mackintosh & Wheeler, 1929), baleen plate (Ruud, 1940),
christalline lens (Nishiwaki, 1950), and ear plug (Purves, 1955). Among them,laminations presented in core of the ear plug is considered as the best age character,because the lamination is considered to be formed periodically, is used for both
sexes, and is recognized for all life span. Furthermore, ear plug is relatively easy
to be collected and prepared to examine the number of laminations in the core.
Number of corpora albicantia accumulated in overies has some weak points as
an age character. There is individual variation in the accumulation rate of corpora,and number of corpora is applicable as an age character for only sexually mature
females. However, it is useful for age character next to ear-plug lamination, andthe data of corpora count have been collected for long seasons. Furthermore theannual accumultion of corpora is also useful for the study of reproductive physiologyof the whale.
Ridges presented on the baleen plates is considered to be the most excellent
age character of the baleen whales in younger age groups, but unfortunately, it is
not able to be used for older age groups.
Now, it is very important to determine age of whale from above age characters,
and many hypotheses have been published on the age determination.
Concerning the ear-plug laminations, Purves (1955) first considers that theperiod of arrested growth corresponds to the migratory periods when physiologicaldemands of active swimming coincide with complete absence of food, so that eachlamination represents a growth period of 6 months. Laws & Purves (1956)
estimate that the biannual formation of lamination is correct, compared the ear
plug with baleen plate for small samples of fin whales. Nishiwaki (1957), Chittleborough (1959), Dawbin (1959) and Laws (1961) did not denied the hypothesis.
The hypothesis of biannual formation oflamination now influences on the cetacean
biology and population dynamics. However, recently long term marked whales
have been recovered (Brown, 1962) and the investigation of ear plug of the recaptured fin whales give a question on the annual accumulation rate of ear-pluglamination (Ohsumi, 1962). Purves & Mountford (1959) state that it is very
doughtful whether environmental conditions have any direct influence of the timeof formation of the laminae of the ear plug in whales. Since there is strong evidence(not given by them) that lamina formation is in inherent moulting cycle, it is very
probable that it takes place whether the whale migrates or not, and Ichihara (1959)
49

50

S. OHSUMI

discribed by the histological examination of ear plug that the ·effects of hormone
and enzyme on the degenerations of epithelial cells of glove-finger were regarded
initially, besides, the effect of the environmental factors were preseumed indirectly.
There are also many hypotheses on the accumulation rate of corpora albicantia
in ovaries. Laws (1958, 1961) reviewed the previous works on the problem. On
the fin whale, Mackintosh & Wheeler (1929) plotted the frequencies of corpora,found peakes occurred at 4-5, 12 and 19 corpora, and suggested that these represented the increase in number of corpora at intervals of 2 years. ·wheeler (1930)
developed this hypothesis by more material, and concluded that about 5 ovulations occur each breeding cycle, or in average 2.5 per year. Peters (1939) reported
that he had been able to establish morphological and histological criteria for distinguishing the corpora albicantia representing pregnancy and ovulations, and be
estimated that there is an average of0.9 in a year by counting the pregnant corpora
albicantia and assuming a 2-years reproductive cycle. Mackintosh (1942) shmved
by a marked fin whale recaptured that the annual increment of corpora albicantia
had possibility to be less than one. Using the baleen plates as a new method forage determination, Ruud (1940, 1945) suggested that in the breeding season, up
to 6-7 ovulations are possible before pregnancy inpervenes. Nishiwaki (1952)
assumed that the annual increment of corpora albicantia after the first sexuallymature age should be 1.5, combining baleen plates, crystalline lens, number ofovulation and sexual and physical maturity.
Since ear plug was discovered as a new method of age determination by Purves(1955), many authors have reported the relation between ear-plug lamination andnumber of corpora in the ovaries. Nishiwaki (1957) drew a figure on this relation
for the Antarctic fin whales and got a result that in average corpora accumulation
was 1.2, if ear-plug laminations are formed biannually. Nishiwaki, Ichihara andOhsumi (1958) drew similar figure as above paper and got that there was individual
and racial variation in annual accumulation rate of corpora and in average theannual corpora accumulation rate was 0.9 in the Antarctic fin whale and about
0.8 in the North Pacific fin whales, if ear-plug lamination was formed biannually.
Laws studied the annual accumulation rate of corpora albicantia in his valuable
paper (1961) from several aproaches, and got the annual accumulation of corpora
albicantia is 1.43 in average.
And he compared the ages of the fin whales between the age from ear plug(biannual accumulation) and the age from ovary (annual accumulation rate is1.4), and got the exact agreement between both ages. Fujino (1963) also calculate
the corpora accumulation rate per 4 ear-plug laminations from the relation between
ear plug and corpora for the Antarctic fin whales separating the areas, and got theresult that the rate was 2.80 in Area-II (Laws's result), 2.31 in Area-III west, 1.91
in Area-III east, 1.84 in Areas-IV, and 1.60 in Areas-VI and I respectively. Then,the difference of the annual rate of corpora between Nishiwaki et al. ( 1958) and
Laws ( 1961) will be caused by the difference of races.
On the age determination of baleen plates, Ruud (1940, 1945), Tomilin
1940, 1945), Nishiwaki (1950 a, 1950 b, 1951, 1952, 1957), Chittleborough (1954,

AGE DETERMINATION OF FIN WHALE

511959), Laws & Purves (1956) and Laws (1961) all recognize that one ridge grows
in one year. Especially Nishiwaki (1952) tried to estimate the annual growth ofbaleen plates.
Although there are many hypotheses on the age determination of the baleen
whales, I think the age determination has not been established. Recently theage characters have been obtained from the long term recaptured fin whales(Ohsumi, 1962). Direct age of recaptured whale is able to be checked by the timefrom marking till recovery. And examinning the recaptured fin whales, above
hypotheses are not able to explain some of the actual data. We must re-examine
the age determination to solve the actual results on the age characters.
MATERIAL AND METHOD

Several methods were shown to examine the age determination of the whales
by Ohsumi, Kasuya & Nishiwaki (1963). Among them breeding experiment as usedby Sergeant (1959) and the time marking in the age characters of living animals as
used by Nishiwaki & Yagi (1953) are not able to use for the fin whale because of
the difficulty of the breeding of the whale. And I have no material for study ofthe seasonal growth of the age characters as used by Ohsumi, Kasuya & Nishiwaki.
(1963) on tooth of the sperm whale (Physeter catodon).
I examined the age determination in this report by the following three methods.
One is the examination of age characters from the recaptured fin whales. The
second method is the comparison between each age characters. The third
is a population dynamical examination on the age composition based on the
ear-plug laminations. Combining the results obtained from above methods, I
will examine whether the previous hypotheses are contradictory or not.
Biological material obtained by Japanese fleets from recovered fin whales are
based on the data by my previous paper (Ohsumi, 1962).
Baleen plates were collected with ear plug from 74 fin whales in the Antarctic
and northern part of the North Pacific, and the ridges appeared on the plates arecounted by eye. Ear-plug laminations are counted after the same preparation as
by Nishiwaki, Ichihara and Ohsumi (1959).
Ear-plug laminations are counted by the saffs of our Institute using dissectingmicroscope. Standardization of the reading ear-plug lamination is need as trying by Ichihara ( 1963). If the standardization is established and the reading isdifferent to the present reading, my following examinations are needed to be reexamined.
Number of corpora lutea and albicantia were counted with naked eye by slicing
the left and right ovaries.
Sexual maturity is determined by the investigation of ovaries or testes. For
females, those who have at least one corpus luteum or albicans in their ovaries are
determined to be sexually mature. For males, those whose weights of larger sideof testes are 2.5 kg or more than 2.5 kg are recognized as sexually mature.
Material for the comparison between ear-plug lamination and other age

52

S. OHSUMI

characters are all obtained from fin whales caught by Japanese fleets in the
Antarctic Areas IV and V in 1958/59 and 1959/60 seasons and northern part of the
Pacific during 1959 to 1961 seasons.
Ear-plug laminations are counted from 978 female fin whales caught in thenorth of east Aleutian Islands in seasons from 1959 to 1961, and the age compositions are drawn in the two cases of annual accumulation rate.
Estimated number of marked fin whales in the north of east Aleutian Islands
and the number of recaptured whale from the marked whales are gathered fromJapanese whale marking and recovery records during 1954 to 1962.
Number of female fin whales of sexually mature, immature and pregnant arecited from annual reports on the biological investigation of whales caught in the
North Pacific from 1954 to 1961.
EXAMINATION OF AGE DETERMINATION BY MEANS OF WHALE
MARKINGSOME POINTS TO BE CONSIDERED IN THE MARKING METHODFOR EXAMINATION OF GROWTH AND AGE OF WHALES

On the whale marking investigations many reports have been published. However, most of them are on the migration and movements. And the reports on theexamination on the age and growth by means of whale marking are relatively few,
and all of them are fragmentary. Mackintosh (1942) discussed the rate ofaccumulation of corpora albicantia in the ovaries on a recaptured fin whale. Omura &Kawakami (1956) studied the growth of the marked sperm whales. Ohsumi,
Nishiwaki & Hibiya ( 1958) discussed the growth and age of the fin whale in thenorthern Pacific from the data of whales recaptured by Japanese whalers. Chittleborough (1959) used results of marking for the study of age in the humpback
whale (Megaptera novaeangliae). Dawbin (1959) also studies whale marking as theevidence on growth rate of the humpback whales. Laws (1961) examined theannual corpora accumulation rate from 10 recaptured fin whales.
In the whales, it is entirely difficult to use the similar marking method as is
usually used in other animals liked as fishes or birds. That is to say, in the whales
except some porpoises and dolphins, we cannot catch and examine biologicallythe individual on board of ship before the marking and discharge. Therefore,
one of the weak points in the whale marking is the difficulty of the biologicalinvestigation at the time of marking. Although it is not so weak point for the
purpose to examine the movement of whales, it is the biggest defect to the study ofthe growth and age of the whales.
We usually estimate the species and body length of the whale at marking. At
that time, the estimation involves more or less errors, because we measure witheye the whales moving in the water. Sometimes we make mistakes in estimationof species. For example, a Japanese mark No. 4267 whales was recorded as a sei
whale (Balaenoptera borealis) at the marking, but it proved to be a fin whale in therecovery. It is difficult to distinguish between a sei whale and a Bryde's whale

AGE DETERMINATION OF FIN WHALE

53(B. edeni) at the time of marking, therefore, Bryde's whales were recorded as se1
whales in the marking records of our country. The mistake of this sort affects tothe calculation of population size by means of the marking method. It is also difficult to estimate the sexes of the whale at marking, except adult killer whale orsome other species. Furthermore, we cannot estimate the age of a whale at marking. When a calf with its mother is marked, we can estimate its approximate
age. But in other cases, we cannot estimate the age and collect the age characters of a whale at marking. Whale mark is only used as a correct artificial age
character from marking till recovery.
The next point is the scantiness of the marked and recovered whales. For the
purpose of the study of growth and age of whales by means of marking, the morerecovered whales we get, the better results are obtained. But, at first, we cannot
use many marks, for the marking investigation is very expensive. On the second,
the rate of hit on whales is relatively low. On the third, the rate of recovered whales
per marked whales is also relatively low.
The third point to be considered in the marking method for the study of growth
and age of the whale is the influences to the whale body by the marking. Whalemarking which we use is not what is called a marking but a sort of tagging, and a
25-28 cm long metal tube is inserted in the body of whales. The body parts of theinsertion is mainly the muscle of back, but sometimes the mark destinates the vertebrae or internal organ. Therefore, sometimes abnormality must be consideredin the growth and life of whales by marking. The example that 3 sperm whales
which had been marked in the adjacent waters to Japan were found floating deadshortly later shows a fatal influence of marking to those whales (Omura & Kawakami, 1956). The marks were inserted in the body cavity of those whales. There
are other some examples in which the marks influenced badly on the growth ofwhales.
The fourth point is the difficulty of investigation of recovered whales. The
best condition to investigate the marked whale is to discover the mark before theflensing and for that purpose streamer mark was made to try (Ruud, Clarke &Jonsgard, 1953; Omura & Kawakami, 1956), and a modified mark was tested
by Chittleborough & Godfrey (1957), because the earlier the mark is discoveredin flensing, the better investigation is made on the recovered whales. ·when
the mark is recovered in flensing, meat cutting or bone cutting, the mark is usuallychecked with the marked whale and we can investigate and collect the necessary
data and material for the examination of the age and growth. But if a mark is
recovered in cooker or reflegarator vessel, we cannot connect the mark with themarked whale. Fortunatley, in Japanese fleets, we flense the whales thoroughly,
because we use the whale for food, and we can recover many marks in the placeoutside of the cooker.
Marking is considered to be an artificial age character, and although marking
has a weak point that we cannot know the age of the whale before the time of marking, it has an advantage that we can confirm the elapsed time from marking
until recapture.

54

S. OHSUMI

If an age character increase in arithmetical progression, age at the time ofmarking is symbolized as x, elapsed time as t, and the annual accumulation rate isb, the size of age character at the time of recapture (y) is:y = b(x+t)

Then,

The smaller x is, and the larger t becomes, the smaller x/t becomes. Then, y/t becomes near b. Therefore, the useful material for estimation of the accumulation rate of age character by means of marking is desireble to be long elapsed timeand younger age until the time of marking.
EAR-PLUG LAMINATIONS OF RECAPTURED 'v\"HALES

Tables 1 and 2 show the number of ear-plug laminations, elapsed years and number of laminations dividing by the elapsed years for 59 fin whales recaptured by
Japanese fleet in the Antarctic and the northern part of the North Pacific, which
are listed in the paper of Ohsumi (1962).
If a whale mark fit a new-born whale, we can get an actually annual accumulation rate of ear-plug laminations, but this case is practically impossible. All whales
will be marked at some time after birth. And if we can record the time of marking in an ear plug, we shall get an accurate number of laminations per year bycounting the number of lamination from the record, but such recording is now
also impossible.
Then, as the laminations accumulated in the time from marking until recovery
are added to the laminations which were accumulated by the time of marking, themean values which are got dividing the total number of ear-plug laminations by
the elapsed time come to be more than the actually annual number of laminations.
The longer the elapsed time is, the shorter relatively is the time from birth
until marking. Therefore, the mean number of laminations per year approaches
to the actually annual increment of laminations as the period from marking until
recovery increases.
Fig. 1 shows the relation between the elapsed time and the mean number of
ear-plug laminations, and in addition shows the typical curves of mean number of
ear-plug laminations for the case of 4 whales which are, respectively, just born
(D, D'), 10 laminations (C, C'), 25 laminations (B, B') and 50 laminations (A, A')
at the time when they are marked. Broken lines (A', B', C', D') show the casein which the annual increment of ear plug is 2 laminations, and solid lines (A, B,
C and C) show the case in which the annual increment is one lamination.
In the whales which were recovered within one year, the apparent mean
numbers of laminations per year are relatively large. But in those which wererecovered after more than 2 years, we get relatively smaller figures.
There is a North Pacific fin whale (No. J. 6817) of which mean number ofAGE DETERMINATION OF FIN WHALE

55

TABLE 1. DATA FOR EXAMINATION OF ACCUMULATION RATE OF EARPLUG LAMINATION BY RECAPTURED FIN WHALES IN THE
ANTARCTIC (DISCOVERY MARKS)Mark No.

17860
22236
22417
22517
22526'30'33
24038
24083
24202
24361
24501
22517
24064
24349
24547
17876
17476
17903
198I3
20944
21077'90
2116I'72
20927'813347
198II
2I08I'5
21 I90
I7516
I4610
I7894
I7945
18049
18330
I8573
I6309
I4700
I2761
I4779
1462I
12798
I2677
30I3
3I84
2825
2932
3I71
5763

Sex
M
M

F
F
M
M
M
M
M

F
M

F
F
M
M
M

F
M

F
F
F
F
M
M
M
M

F
M

F
M
M
M

F
F
F
M

F
F
F
F
F
F
F
M

F
M

Elapsedtime (A)(Yr.-Mon.)

0-2
0-1
0-0
0-1
0-1
0-1
0-1
0-0
0-1/2
0-0
0-2
0-2
0-2
0-3
1-1
1-2
I-I
1-1
I-1
I-1
1-1
1-2
2-4
2-2
2-I
2-I
3-I
3-1
3-2
3-1
3-2
3-1/2
3-3
4-0
4-I
4-2
4-3
5-2
5-4
6-4
24-11
24-II
25-0
25-0
25-0
24-lI 1/2

Estimated
No. ofbody length
Body length
ear-plugat marking (ft.) at recapture (ft.) laminations (B)

59
66-67
65
70
67
60
70
70
62
66
66-65
70
55
70
58
63
70
74
65
67
54
65
65
63
68
64
64
75
70
66
67
62
66
68
62
63
65
68
70-75

57
60
66
73
69
63
64
65
66
68
62
70
62
70
63
67
69
69
75
74
71
64
64
63
66
62
72
66
67
67
65
67
68
73
77
67
73
69
73
72
70
69
71
66
72
69

7
8
18
22
31
10
8
10
16
24
6
16
7
53
24
16
10
I5
60
22
19
6
8
9
15
9+ (3)
18
21
2I
14+ (2)
57
I2
24
42
29
I4
67+
I5+ (2)
48
3I
33
36
38
38+ (3)
28
34

B/A

22.2
I3.7
9.23
I3.8
55.4
20.3
I7.5
5.15
3.44
4. I5
7.20
5.76
5.84
6.8I
6.63
5.17
18.0
3.96
7.38
I0.5
7. I I
3.36.
15.8+
3.29
9.00
4.90
1.33
1.45
1.52
1.64
1.12
1.37

S. OHSUMI

56

TABLE 2. DATA FOR EXAMINATION OF ACCUMULATION RATE OF
EAR-PLUG LAMINATION BY RECAPTURED FIN WHALESIN THE NORTHERN PART OF THE NORTH PACIFIC(JAPANESE MARKS)Mark No.

Sex

7968
8416
8433
8434
8567
8618
8632
7988
6801'2
6799
7962
6834
6831'7
7975
7686
4552
6264
6807
6939
7708
6923
5979
8089
6808
4558
7026
7200
6274
6835
7170
6504
6782'53243
6471'7'8'80
6498
7705
7727
7737
7541
6817
6977
3391
6044
7025
4393
6052
6098
4615
4452
6172
4564
6097
5960
4146
3287'3343
4651

F
F
F
M
F
M
F
M
F
M
F
M
M
M
F
F
M
F
F
F
F
M
F
F
M
M
M
F
M
M
F
F
F
F
M
F
F
M
F
M
M
M
F
M
F
F
M
M
M
F
F
M
M
F
M
M

Elapsedtime (A)(Yr.-Mon.)

0-0
0-0
0-0
0-0
0-0
0-0
0-1/2
0-0
0-10
0-10
0-11
0-9
0-10
1-0
1-1
1-10
1-10
1-10
1-11
1-11
2-0
2-0
2-1
2-9
2-10
2-10
2-10
2-10
2-10
2-11
2-11
2-11
3-0
3-0
3-0
3-0
3-0
3-1
3-1
3-10
3-10
3-11
4-10
4-10
4-11
4-11
4-11
4-10
5-0
5-1
5-9
5-9
5-11 1/2
6-0
6-11
6-11

No. ofEstimatedbody length
Body length
ear-plugat marking (ft.) at recapture (ft.) laminations (B)

62
62
65
53
60
63
66
64
60
60
61
69-70
61-62
64
62
60
60
56-57
62
55-57
62
60
73
65-66
65
60
54
60
60-61
55
63
60
(70)
54-55
61
66-68
58
60
60
57
55
63
65
55
60
62
52
56
60
62
50
60
65
65

66
57
63
61
69
61
73
59
58
59
64
61
58
61
57
62
56
57
64
62
64
58
68
64
62
62
58
62
61
59
58
61
55
57
60
60
65
64
58
59
61
64
67
63
65
67
62
61
59
62
56
65
64
68
64
67

9
4
9
6
31
19
12
6
7
13
12
28
14
50
10
II

8
5
41
8
32
9
13
15
21
14
8
8
16
9+
7
8
13
12
12
13
19
10
6
6
12
27
21
13
21
9+
9
12
10
II

10
23
15
16
16
43

B/A

8.43
15.7
13.2
37.3
16.9
50.0
9.23
6.01
4.37
2.73
21.5
4.19
6.00
4.50
6.25
5.45
7.42
4.95
2.83
2.83
5.65
3.08+
2.40
2.74
4.33
4.00
4.00
4.33
6.33
3.25
1.95
1.57
3.13
6.89
4.35
2.69
4.27
1.83+
1.83
2.48
2.00
2.16
I. 74
4.00
2.52
2.67
2.31
6.22

AGE DETERMINATION OF FIN WHALE

57

laminations per year is 1.6, although it was recovered only 3 years and 10 months
later. According to the record at marking, the whale was estimated as 57 feet long,and it might be over one year old at marking. When the age at marking is added on the elapsed time, mean number of laminations per year becomes under
1.3.

IOI
I I

20

"'
0

\

H· \

(})

I I

\o I

I

-0

19 I

I

>(})

"'

Q.

0

(})

15

~

'\
18 I

(})

Q.

"'

c

I

u

·E

~

tI \I \I

Ol

\

\

;;;
::>

c

0

',
',

.....

\

J ""''-....,~

lo •a

(l)

E

\
\

\

0

..0

\

0

\
•

~

0

\

I
\

10

::>

\

5

o
<f,

'

',,

''A-!__------------.a,, "'~'--Ji~_...._ • ....._ c'~--------------·-=-::::-,,-:--.-----=~a • •o"-;!2._ - - _ _-~-1.-).~D'~-----------------

t--~~~~~-~~~~~~~~~~~~~~~---'~~~D

5

15

20

25

Elapsed years from marking

Fig. 1. Change of number of ear-plug laminations per elapsed years according to theelapsed years from marking until recapture.
A, A': 50 laminations at marking, B, B': 25 laminations at marking, C, C': 10 laminations at marking, D, D': 0 lamination at marking (birth at marking), A, B, C, D:
One lamination accumulate a year, A', B', C', D': Two laminations accoumulate ayear. Open circle: Antarctic fin whale, closed circle: North Pacific fin whale.

In the North Pacific, Nos. J. 7541, J. 6098 and J. 4564 whales are also calculated the mean number oflaminations per elapsed years as under 2, notwithstanding their elapsed times are within 6 years. Further, in the Antarctic, 6 long termmarked fin whales from which ear plug were collected had been recaptured by
Japanese fleets by 1961/62 season. The mean numbers of laminations per elapsed
year of the 6 whales are all under 2 (1.33 in No. E. 3013, 1.45 in No. E. 3184,
1.52 in No. E. 2825, 1.64 in No. E. 2932, 1.12 in No. E. 3171, and 1.37 in No. E.
5763). The tip of ear-plug core of No. E. 2932 whale is broken, but the broken
laminae layers are estimated to be less than three, and the total lamination areS. OHSUMI

58

estimated to be 41.
There is no whale of which mean number of ear-plug lamination is calculated
as under 1.0 per year.
The mean number of ear-plug laminations per year calculated in this way,should be more than 2 for all recaptured whales, if they are actually laid down
biannually. However, in view of these examples, the annual increment of ear-pluglaminations is estimated to be under 2 for the marked fin whales, perhaps near one
lamination per year. The solid lines in this examination also support the estimationthat the accumulation rate of ear-plug lamination must be under two annually,
and may be near one per year.
TABLE 3. ESTIMATION AND COMPARISON OF THE AGE COMPOSITION
OF RECAPTURED FIN WHALES AT MARKING BETWEEN TWO CASES
OF ANNUAL ACCUMULATION RATE OF EAR-PLUG LAMINATION
Annualaccumulation
rate ofear plug

Elapsed years(actual figures)
Age atmarking

f
One

Two

l

51<
26-50
11-25
6-10
0-5
0>
Total

51<
26-50
f 11-25
6-10
0-5
0>

l

Total

0

1-2

2
7

Elapsed years(per cent)

5-7

Total

0

1-2

3-4

4
10
10
2

3-4
2
2
12
7
6

3
4
6
3

4
11
33
34
12

4.6
9.1
31.8
50.0
4.6
0.0

3.7
14.8
37.0
37.0
7.5
0.0

6.9
6.9
41.4
24.2
20.7
0.0

5-7
0.0
18.8
25.0
37.5
18.8
0.0

22

27

29

16

94

1
4
8
8
6

2

2
7
11

3.7
14.8
29.6
29.6
22.2
0.0

6.9
3.4
27.6
31. 1
24.2
6.9

0.0
12.5
25.0
6.3
43.8
12.5

27

4
9
27
29
21
4
94

4.6
9.1
31.8
50.0
4.6
0.0

22

2
4
1
7
2
16

11

8
9
7
2
29

Table 3 and Fig. 2 show the estimated age composition at the time of marking
by recaptured fin whales. Estimated age at marking is calculated from numberof lamination at recapture, elapsed time and the annual accumulation rate of earplug laminations. I calculated the estimated age in two cases where annual accumulation is one and two. The comparison of age composition at marking is
made in elapsed year classes 0, 1-2, 3-4 and 5-7. According to Fujino (1960),
the estimated size distribution of marked whale at the time of marking is not so
different from that which were caught, although young whales distribute little rather
more than the whales caught and the markings of whales are considered to be made at
randum from the stock in the whaling ground. If so, the estimated age distribution
at the time of marking must be not so different in each season. Now, as shown in
Fig. 2, the age composition at the time of marking is not so changed according to
the elapsed years for the case of one-lamination age. Ratio of the age class of 0
to 5 at the time of marking increases according to the elapsed years. This can

AGE DETERMINATION OF FIN WHALE

59

be explained that small whales are not caught soon after marking, but they growto captuable size as the time goes by. On the contrary, in the case of two-laminations age, estimated age composition of marked whale at the time of marking changes
by the increment of elapsed years. And in 5 to 7 elapsed years classes, the composition of under 5 years old class exceed the half of total whales. This is not able to be
explained, because the age distribution of marked whales must not be so different
from the whales caught at the time of marking. Besides, there exist the whales
under 0-year old at the time of marking in the case of two-laminations age. They
must be marked before birth. This is not practical.

75

11-25

75
l l-25

'J!_
c
0

50

50

·~

0
Q.

E

3

6-10
0-5

ID

"' 25

<(

25

0-5

0

2

4

6
Elapsed years from

0

2

4

6

marking

Fig. 2. Change of estimated age composition of recaptured fin whale at the time ofmarking according to the elapsed years from marking. Left figure : Annual rate of
ear-plug accumulation is one lamina, Right figure : Annual rate is two laminae.

However, Chittleborough (1959) reported that a male humpback whale wasmarked when its body length was approximately 30 feet (he estimated to be anyearling) and it was recaptured 4 years and 10 2/3 months later. The ear plug
contained as 12 laminations. From the above result, he agreeded with the biannual
formation of ear-plug lamination. I am afraid that the whale was truly yearling.
The estimate body length is sometimes different with the actual length. And
there is individual variation in the body length. Dawbin (1959) also reports amarked humpback whales which had been at least one year old at marking andwas recaptured 19 months later. Examining its baleen plate and ear plug, hestates that the result are consistent with the view that ear plugs develope twolaminations per year.

60

S. OHSUMI
EXAMINATION OF THE AGE AT SEXUAL MATURITY
BY THE MARKED WHALES

Sexual maturity is one of the remarkable stages in the life history of whales.
Besides, it is used for classification of broad age groups. Therefore, it is considered
to be a kind of age character. On the age at sexual maturity of the fin whale,
Mackintosh & Wheeler (1929) state that an examination of sizes of young whales atdifferent times of year suggests that sexual maturity is reached about 2 years
after birth. Studying the baleen plates, Ruud (1945) describes that it is apparent
that majority of the northern fin whales attain sexual maturity at an age of 3
years. From the similar method, Nishiwaki (1952) states that females reach tosexual maturity in the pairring season following the fourth birth day and themales at the age of about 3 years and a half in the case of the southern fin
whales. In the modified method of surveying baleen plates, Van Utrecht (1956)
obtained a result that the age at which the female becomes sexually mature lies
between 5 and 6 years.
TABLE 4. SEXUALLY MATURE AND IMMATURE FIN WHALES IN
EACH ELASPED YEARS FROM MARKING UNTIL RECAPTURE(INCLUDED DATA FROM TABLE BY LAWS, 1961)
Elapsed years from marking

I

Males

{
{s
{s
s

M
I

Females

M
I

M+F

M
Total

0

2

8 4
7 2
8 8
8 8
5 4
13 6
16 12
12 6
21 14
49 32

2
3
2
1
8
4
I
II
16

3
3
5
5
4
6
6
7
JI
24

4

5

6

7

11

12

17

19

20

21

23

24

25

3
2
2

3

5
7

2

5
4
3
6
13

2

4
I
6
8

4

2
2

5

Total
19
15
32
25
16
57
44
31
90
164

Remarks: I: Sexually immature (under 2.4 kg in larger testis for male, 0 ovulation for female).
S: Soon after sexual maturity (2.5-5.0 kg in testis for male, 1-2 ovulations for females).
M: Sexually mature (over 5.1 kg in testis for male, over 3 ovulations for females).

Since the discovery oflaminations in ear plug, Laws & Purves (1956) reported
that sexual maturity was reached at the age from 4 to 6 years in the male from theNorth Atlantic. Nishiwaki (1957) learned in the same method the age of sexual
maturity of the southern fin whale was 4 or 5 years. Ohsumi, Nishiwaki & Hibiya( 1958) state that if two laminations are formed every year, the fin whale in the northern Pacific will attain at sexual maturity in early term of seventh year after birth, andthe precocious will be mature sexually by the fifth year, furthermore, by the eighthyear all the fin whales will attain at sexual maturity. Purves & Mountford (1959)
give a figure of 4 and a half years for males and 5-6 years for females. Laws ( 1961)
also shows the percentage frequency of sexually mature females in different agegroups from ear plug (biannual rate), and get that the age corresponding to 50%
of mature females is about 5 years.

AGE DETERMINATION OF FIN WHALE

61

Above results are not lead with the absolute age of whales. Table 4 shows
the number of animals which are determined their sexual maturity among the recaptured fin whales. Data used for this purpose are from Ohsumi (1962) and Laws
( 1961). I divided sexually mature whales into two groups, that is, soon after
maturity and fully mature groups.
100

80
Mature'2Z600<.!":·

9
E

0

40

v

Soon after maturity""---..:~
'-.
'-..

20

'-..

'-..

'--,
'-.

Immature

'-.

'--,
'-..

2

3

4

5

6

7

8

9

l0

11

Elapsed years from marking

Fig. 3. Change of composition of sexually mature and immature fin whales recaptured
according to elapsed years, and estimation of age at sexual maturity. Chain line :Regression line of immature rate, Broken line: Estimated decreasing line of immature
rate assuming the age at sexual maturity as 5 years.

And Fig. 3 shows the change of the composition of sexually immature andmature animals according to the elapsed years calculated from Table 4. In 0year group which were recovered within 3 months later, the rate of sexually immature whales is 32.7%. This is almost the same ratio as the usually caught whalesin the factory ship whaling. But as the time elapses, the rate of immature whales
gradually decreases. There is a sexually immature female whale (J. 4564) which
was recovered 5 years and 9 months later. This whale certificates the existence ofan individual which is sexually immature in the age of at least 5 years and 9 months. This whale had been estimated to be 60 foot long at the time of marking.
Considering the error of the estimation, the whale is thought to be at least 1 or 2
years old at that time. If so, this whale must be still sexually immature in theage of 7 or 8 years.
There were several recovered whales which are estimated to be soon after
sexual maturity more than 3 years later from marking. One of them is a female
recovered 3 years later (J. 3243). It has only one corpus albicans in the ovaries.
Considering from the existence of vaginal band and immature mammary gland,
this whale is assumed to be soon after sexual maturity (puberty as defined by Chittle-

62

S. OHSUMI

borough, 1954).
Regression line is calculated from the elapsed time and the composition
equation of immature whales. It is:

Y = -3.68X +36.49
Where, Y is a percentage of immature whales, and X is the elapsed years. From thisequation, following values are calculated : The percentage of immature whales is36.49 at the time of marking (X =0), and the elapsed time is 9.93 years when percentage of immature whale is 0.
Thus, we can estimate that the age at sexual maturity is about 10 years fromthe examination of age at sexual maturity by the recaptured fin whales. Broken
line in Fig. 3 is a estimated declining line of sexually immature ratio, when the age
at sexual maturity is considered to be 5 years as many authors reported. This
broken line does not fit the actual change of composition of sexually immature
animals. We must consider that there is individual variation in the age at sexual
maturity, but usually whales are marked somewhile later from birth. And there
are 5 immature animals (25%) among 20 animals which were recaptured after 4
and 5 years from marking. Even among 10 recaptured animals after 6 and 7 years'
elaption from marking, there is one immature animal (10%). This phenomenon
may mean that the age at sexual maturity is longer in fin whale than those which
are recognized today by us, and I estimate from the change of composition of sexually immature animals according to elapsed years that the age at sexual maturity
is about 10 years.
On the humpback whale, Chittleborough (1959 b) reported an sexually immature male which was recaptured 4 years and 1 month later.
ANNUAL ACCUMULATION RATE OF CORPORA ALBICANTIAIN THE RECAPTURED FIN WHALES

Chittleborough (1959 a) reportes "One female humpback whale marked when
it was accompanied by a calf was killed 9 years later. The ovaries then obtained 10
corpora albicantia, so that if this female had been marked at the end of its firstpregnancy, the average rate of ovulation would have been 1.0 ovulations per year."And calculating from the data by Chittleborough (1959 b), a mean number ofovulations per year for a marked humpback whale recovered 17 and a half years
later is 0.9 ovulations.
By a result of a fin whale recovered 6 years and 2 months later, Mackintosh(1942) states that one cannot draw any final conclusion, but in this one, since it
had only 8 corpora lutea, the rate of accumulation (mean number of ovulations is
1.3 per year) cannot have been much more than about one a year, and since there
was no clue to the whales age at the marking, the rate of accumulation may havebeen even slower.
Laws (1961) checked the corpora accumulation rate by 10 recaptured fin
whales. He calculated the mean elapsed period as 13.8 years (ranging from 2 to
24 years) and the mean corpora as 15.7 (ranging from 3 to 40). Then he gets theAGE DETERMINATION OF FIN WHALE

63

TABLE 5. DATA FOR EXAMINATION OF ANNUAL ACCUMULATIONRATE OF CORPORA ALBICANTIA IN THE MARKED FIN WHALESCAUGHT IN THE ANTARCTIC (DISCOVERY MARKS)Mark No.

24350
18087
21196'98
14576
19843
20502'4
21187'21200
24064
24349
19837'9
20735
17903
19746
19835
19840
20927'8
15388
17839
17457
17516
18073
19921
13278
13481
6472
3507
3103
2627
5594
*10504*12870* 696* 7972* 4938* 6818* 1199'1203}* 1300Elapsed time
(Year-Month)

Estimated bodylength atmarking (ft.)

0-0
0-1
0-1
0-2
0-2
0-2
0-2
0-2
0-2
0-3
0-3
1-1
1-1
1-1
1-1
1-2
1-2
1-2
2-1
3-1
3-3
3-2
5-2
6-2
20-1
20-11
24-11
24-1
22-11
2.23
3.23
6.21
I 1.13
12.06
17.05
19.12

* Cited from Table 30 by Laws (1961)

61
66
70
60
66
62
70
70
55
66
67
70
60
63
67
65
62
65
64
62
60
68
60-65

Body length
at recapture (ft.)

No. ofcorporaNo. of corpora
per elapsed
year

63
73
64
69
64
66
73
70
62
64
66
69
65
69
69
64
65
66
71
72
68
63
73
73
71
70
70
69
74
71
70
76
69
73
75

19
18
0
0
0
1
0
2
6
0
2
0
6
3
2
0
15
13
9
23
15
32
12
10
3
8
5
14
40

0.60
1.33
0.53
4.48
0.93
1.29
0.45
1.16
2.33

76

II

0.58.

0
12
4
3
0

0.92
0.00
1.85
5.53
0.00
I. 72
0.00
2.88
0.97
0.60
0.00
2.90
2.11
0.45
I. JO

64

S. OHSUMITABLE 6. DATA FOR EXAMINATION OF ANNUAL ACCUMULATIONRATE OF CORPORA ALBICANTIA IN THE MARKED FIN WHALESCAUGHT IN THE NORTHERN PART OF THE NORTHPACIFIC (JAPANESE MARKS)Mark No.

Elapsed time
(Year-Month)

Estimated bodylength atmarking (ft.)

Body length
at recapture (ft.)

4266
7968
8416
8433
8632
4684
4802
7720
8567
4270
4658
4543
6815
6843
6801'2
7962
4552
6042
G995
6939
7708
6807
7686
4267
6923
6274
6504
6782'5
6808
8089
3243
3213
64 71 '7'8'80
7705
7270
7727
7541
4393
6044
6052
6172
4564
4146

0-0
0-0
0-0
0-0
0-1/2
0-1
0-1
0-1
0-0
0-2
0-11
0-11
0-10
0-10
0-10
0-11
1-10
1-8
1-10
1-11
1-11
1-10
1-1
2-0
2-0
2-10
2-11
2-11
2-9
2-1
3-0
3-2
3-0
3-0
3-2
3-0
3-1
4-11
4-10
4-11
5-1
5-9
6-0

68
62
62
65
66
65
71
64-66
60
68
61
62
62
62-63
60
61
60
55
57
62
55-57
56-57
62
45
62
60
63
60
65-66
73
(70)
60
54-55
66-68
60
58
60
60
65
62
56
60
60

62
66
57
63
73
61
65
66
69
63
58
66
60
56
58
64
62
64
65
64
62
57
57
63
64
62
58
61
64
68
55
63
57
60
60
65
58
65
67
67
62
56
68

No. ofcorpora

1
0
0
4
2
1
7
11
0
0
9
0
0
7
2
1
9
15
14
0
0
0
3
8
0
0
6
3
1
16
0
3
4
0
5
7
2
0
0

No. of corpora
per elapsed
year

0.00
9.81
0.00
0.00
8.08
2.18
0.55
5.40
8.27
7.30
0.00
0.00
0.00
1.50
4.00
0.35
0.00
0.00
2.18
1.44
0.33
5.06
0.00
1.00
0.32
1.33
0.00
1.02
1.45
0.41
0.00
0.00
0.17

AGE DETERMINATION OF FIN WHALE

65

result that mean increment of corpora is likely to be 1.14 per year if fin whales aremarked on average at about the time of puberty.
Table 5 and 6 show data for examination of corpora accumulation rate byrecaptured fin whales. Most of data are obtained by Japanese fleets in the Antarctic and the northern part of the North Pacific which are listed in the paper of
Ohsumi (1962). In addition to them, I cited the data from Table 30 of the paperby Laws (1961).
I
10

I
I
I

I

.1

I
I
I

8

I

\

.~

I
\

\

I

\

~

\

I
I
I
I

6

\
\

0. \\

\

\

.

'\

\

:t( \

•

4

\

0

2

•

',

'

',
0

•

'O

''

'-......
0','- ------',..._ /{

.....

--

B - -~--------:t(__, ____ e---,x-------------- ,.•

0

0

-----

-

---

0

0

M

---

•..
. .~
.
ft'•.,- ~-·5.

-----~.,., -----------~
D~--------

c

o~

-----

-------- ------

c'

:a:
10

D

15

20

25

Elapsed years from marking

Fig. 4. Change of number of ovulations per elapsed year according to the elapsed years
from marking until recapture. A, A': 25 ovulations at marking, B, B' : 10 ovulations
at marking, C, C': Just sexually mature at marking, D, D': Just birth at marking,A, B, C, D : Age at sexual maturity is 5 years, and 0.9 ovulations per year (Nishiwaki
et al., 1958), A', B', C', D': Age at sexual maturity is 5 years, and 1.43 ovulations per
year (Laws, 1961).

If the time of marking coincides with the attainment of sexual maturity of themarked whale, we can get the actual annual-increment of corpora albicantia,
dividing the number of ovulations by the elapsed years. But it is a very rare case,and most whales will be marked after or before the time of sexual maturity. Whena sexually mature whale is marked, the mean number of ovulations per year, given
by dividing the number of ovulations at recovery by the elapsed years, is more than
the actually annual number of ovulations. On the contrary, when a sexually im-

66

S. OHSUMI

mature whale is marked, the mean number of ovulations per year should be lessthan true annual number of ovulations. However, as the elapsed time from marking
increases, the number of ovulations accumulated before marking becomes relatively
small, and· mean number of ovulations calculated must approach to the actuallyannual rate of ovulations.

10

0

8

">-

-a·

• •
•

Q)

Q_
0

Q)

6

(])

0..

"'c

0

~
=>

4

>
0

0

0
(])

...n
E
=>
c

2

•

!\'

•

'6 "' •
0-:ci:•915> .. '4>

•••

•

P'.

B"

~c--~~~~~~~o~~-o~~~-_..•____.,;Jr."'-~i---.c"--_~-------------_-_-D-"-:=_-:=_-:=_-:=_-:=_:_~~:..._---'--o-0

5

10

15

20

25

Elapsed years from marking

Fig. 5. Change of number of ovulations per elapsed year according to the elapsed years
from marking until recapture. A": 25 ovulations at marking, B": 10 ovulations at
marking, C": Just sexual maturity at marking, D": Just birth at marking. A", B",
C", D": Age at sexual maturity is 10 years, and 0.5 ovulations per year.

Fig. 4 shows the relation between the elapsed years from marking till recapture
and the mean number of ovulations per elapsed year. The range of the mean
number of ovulations in marked whales which were recaptured shortly after, arelargely distributed. The reason is that the number of ovulations before marking
greatly influences the value obtained. However, with the lapse of time, the range
becomes narrow, and more than 19 years later, mean annual number of ovulations in 6 Antarctic fin whales are within the limits of 0.45 and 1.33. The meanvalue of the 6 individuals is 0. 77.
The broken line (A', B', C', D') in Fig. 4 is a case where the age at sexual
maturity is 5 years and the annual accumulation rate of corpora is 1.43 as calculatedby Laws (1961), and solid line (A, B, C, D) is another case where the age at sexual

67

AGE DETERMINATION OF FIN WHALE

maturity is 5 years and the annual rate of corpora is 0.9 as calculated by Nishiwaki
et al. ( 1958) from the relation between the number oflamination in ear plug and thenumber of ovulation when annual accumulation of ear-plug lamination is assumed
as two. A, B' ; B, B' ; C, C' and D, D' is the cases of 25 ovulations, 10 ovulations justpuberty and just birth at the time of marking. According to this figure, brokenlines do not fit the actual values, and there are 8 individuals which are under D' line.
This means that the individuals had been marked before birth. This hypothesis isinconsistant with the actual data, for they must have been marked after birth. Solidlines are more suitable than the broken line, but they have the same inconsistanceas broken line. Therefore, above two theories are not able to explain the actual
figures obtained from recaptured fin whales.
TABLE 7. ESTIMATION AND COMPARISON OF OVULATION COMPOSITION
OF RECAPTURED FIN WHALES AT MARKING BETWEEN TWO CASES OF
ANNUAL RATE OF OVULATION
Annualrate ofovulation

0.50

1.43

Elapsed years (actual figures)
Ovulation
at marking.

(
J

l

26<
11-25
6-10
1-5
0
Total

26<
11-25
6-10
1-5
0
Not birth
Total

0

1-2

4

2
6
6
10
24

I

8
8
21
4
8
8

2
4
5
13

3-4

5-7

I

7
8
16

1
14

3
4
9

5
2

21

24

16

9

10-25

Elapsed years (per cent)

0

Total

1-2

3-4

5-7

10-25

}

2
2
2
2
9

19.0 8.3 6.3 11.1 30.0
1010
} 42.9 50.0 43.7 44.4 40.0
26
32
38.1 41. 7 50.0 44.4 30.0
79

3
5
9

8.3 6.3
10.0
8 } 19.0
6} 42.9 37.5 6.3 22.2 0.0
1538.1 54.2 87.4 55.6 30.0
43
0.0 0.0 0.0 22.2 60.0
779

Fig. 5 shows the change of mean number of ovulations per elapsed year in thecase where age at sexual maturity is 10 years as shown in previous section and theannual accumulation rate is 0.5 as discussed in the following chapter. According
to this hypothesis, the curves (A", B", C" and D") fairly fit the actual values.
Age composition of marked whales is shortly less than the whales caught (Fujino,1960). According to Laws (1961) and Mackintosh (1942), the average numberof corpora of fin whale is calculated as 12.5 in adult female in 1939""41 season andthe ratio of immature whales is 15.1%in1936-37 Antarctic pelagic whaling. Therefore, the mean number of corpora including immature whales in 1936""40 season
is calculated to be 10.6. If the mean number of corpora in ovaries of marked fin
whales is 10 in the Antarctic pre-war seasons and the annual rate of corpora accumulation is assumed to be 0.5, mean corpora divided elapsed years after 20 to 25
years' elaption is calculated to be 1.0 to 0.9. This mean values agree relatively
well with the mean number of corpora calculated from 6 recaptured fin whales

68

S. OHSUMI

after 19"'25 years elaption (0.77). If annual accumulation rate is 1.43, the former values must be comes between 1.93 and 1.83. This is larger than the actual
values.

75

75
1-10 ov.

"J'!.
c

~
0

"-

50

50

E
0

u
c

~
0

"0 25

25

oov.

2

' 4

6

10-25

2

4

6

10-25

Elapsed years from marking

Fig. 6. Change of ovulations composition ofrecaptured fin whales at the time of marking
according to the elapsed years from marking. Left: Annual rate of ovulation is 0.5 andthe age at sexual maturity is 10 years, Right: Annual rate is 1.43 and age at sexual
maturity is 5 years.

Now, Table 7 and Fig. 6 show the estimated corpora composition at the timeof marking by recaptured fin whales. Estimated number of corpora at marking is
calculated from number of corpora at recapture, elapsed time and the annual rateof corpora accumulation. And I calculated the estimated corpora number in twocases where annual accumulation rate is 0.5 and 1.43. The comparisons of corpora
composition at marking is classified in elapsed year classes 0, 1-2, 3-4, 5-7 and 10-25.
As repeatedly said, the estimated size distribution of marked whale is not so different from that which caught. If so, the estimated corpora distribution at the time ofmarking must be not so different in each season. According to Fig. 6, the estimatedcorpora composition is not so changed by 7 years of elaption from marking in thecase of annual accumulation rate is 0.5. In the 10-25 elapsed years group, estimatedimmature whale is smaller than the former years classes. But this can be explainedthat they are pre-war marked whales, and in pre-war seasons the composition oflarger whales must be larger than recent season. And the estimated compositionof immature class of pre-war marked whale (22%) fairly agrees with the immature
rate (15.1 %) of pelagic catch in 1936-37 season obtained by Mackintosh (1942).
On the contrary, in the case of which annual accumulation rate is 1.43, estimatedAGE DETERMINATION OF FIN WHALE

69

corpora composition at the time of marking changes remarkably by the incrementof elapsed years. And over 4 elapsed years classes, the percentage of immature
whales attaines over 75°/o. This is not able to explain the actual age compositionof the whales in the sea. Besides, there exist the whales before birth at the time ofmarking in the case of annual rate of 1.43, and the rate attains to 55% in 10-25
elapted years class. This is not practical. Thus, this examination also support
the estimation that the annual rate of corpora accumulation must be under 1.43,
and may be near 0.5 per year.
However, above examination of annual rate of ovulations is considered as meanvalue. And we must remember that there are indivdiual variations in the ovulation rate as shown by Nishiwaki et al. (1958), and racial variation as shown by
Fujino (1963).

RELATION BETWEEN EAR-PLUG LAMINATION AND
OTHER AGE CHARACTERS
RELATION BETWEEN EAR-PLUG LAMINATION
AND NUMBER OF OVULATION

Fig. 7 shows the mean number of ovulations (corpora lutea and albicantia) in eachclass of ear-plug lamination for the fin whales caught in Antarctic Areas IV and
V in 1958/59 and 1959/60 seasons.
As shown in this figure, there is linear relation between number of laminations
and number of ovulations except the old age. An equation is given from this line.
It is
Y = 0.49X-4.28

Where, Y is the mean number of ovulations, and X is the number of ear-plug laminations. This equation is similar with that obatined by Nishiwaki, lchihara &Ohsumi (1958).
Thus, coefficient of X is given as 0.49. If the accumulation rate of ear-pluglamination is one per year, the coefficient, that is, 0.49, shows the annual ovulation
rate. Notwithstanding two ovulation rates are calculated independently by examination of ovulation of recaptured whales and by relation between number of
ear-plug laminations and number of ovulations, the both agree closely with eachother. This is, in other word, one of the certification that accumulation rate of
ear-plug lamination is one per year.

SEXUAL MATURITY AND NUMBER OF EAR-PLUG LAMINATIONS

Fig. 8 shows the change of ratio of sexual maturity according to increment of earplug laminations for the Antarctic fin whales caught in Areas IV-V in 1958/59 and1959/60 seasons. Individual stages of sexual maturity have been estimated fromthe standard described in previous chapter.

S. OHSUMI

70
30

25
.,.,
c

g_
a

20

=>

>

a

0
ill

_o

15

E

=>
c

c

a
QJ

2'

10

5

10

20

30

50

40

60

90

70

Number of ear-plug laminations

Fig. 7. Relation between number of ear-plug laminations and the numbr of ovulationsin the Antarctic fin whale in Areas IV-V.

--

JOO

•
/
0

75
I

I

I

I

I

I

I

/

• ,.......-o
5'"

0

/

/

h

50 -------------------------------.- ----i
I
I

I

a

=>
x
QJ

(/)

/

o /
/
_,,.,,

2

4

6

/

"?

:

:

'I

/

I

lI

I

I

I

I

I
I
I

I
I
I

l
t

l
I

I

.,,,,,./
O

8

/1
I

: /(:,

0? ~I

•

•

25

I

I

I

10

12

14

16

18

Number of ear-p.J.ug laminations

Fig. 8. Percentage of sexual maturity in successive ear-plug laminations, on the Antarctic
fin whales in Areas IV-V. Open circles and broken line: Females, Closed circles andsolid line : Males.

AGE DETERMINATION OF FIN WHALE

71

Males start to attainment of sexual maturity from 4 laminations, and by 14
laminations all individuals attain to maturity. Females attaines to maturity later
than the males, they start from 6 laminations and end by 15 laminations. The
number of ear-plug laminations corresponding to 50% of maturity is 9.4 in males
and 10.7 in females. In Fig. 7, the number oflaminations at the point where one
corpus level accross the average line is 11 laminations.
On the relation between number of ear-plug laminations and sexual maturityof fin whales, Laws & Purves (1956) compared testes weights and ear-plug laminations, and they concluded that sexual maturity is attained at 8-12 laminations in
the case of males in the North Atlantic fin whales. Nishiwaki ( 195 7) compared thenumber of laminations with number of corpora in ovaries of 34 Antarctic fin whales
and he found that 10 laminations corresponded to one corpus. Nishiwaki, Ichihara& Ohsumi (1958) tablated the frequency of sexually immature and mature fin
whales caught in the Antarctic and North Pacific in different number of ear-pluglaminations in Apendix. And they said that male and female fin whales matured
at about 11 laminations. Purves & Mountford (1959) draw growth curves ofmales and females in the Antarctic areas I and II where ages are based on ear-pluglaminations, and estimating body lengths at sexual maturity as 66 feet in females
and 63 feet in males, they concluded that the age at sexual maturity is 10-12 laminations in the females and 9 laminations in the males. Laws (1961) shows the percentage of sexually mature females in successive age groups based on ear-pluglaminations, and he obtaines that age corresponding to 50% of mature female isabout 5 years (10 laminations).
Summarizing the above results, the number of ear-plug laminations at sexual
maturity may be concluded to be 10-11 in the females and 9-10 in the males respectively in the case of fin whales.
Now, in the examination of age at sexual maturity by recaptured fin whales,
it is estimated to be about 10 years. This result is based on absolute time as elapsed
years from marking until recapture. On the other hand, the number oflaminationsat sexual maturity is obtained independently as 9-11 laminae. Connecting these
two results, I can get a conclusion that the annual accumulation rate of ear-pluglaminations must be near one. This conclusion also coincides with the results
obtained by the examination of annual rate of ear-plug laminations from recaptured
fin whales.
RELATION BETWEEN EAR-PLUG LAMINATIONS AND RIDGESON BALEEN PLATE

On the relation between number of ear-plug laminations and the number of ridges
on baleen plates of fin whales, Laws & Purves (1956), Nishiwaki (1957) and Laws
(1~61) examined, and Chittleborough (1959) also examined the relation between
ear plug and baleen on the humpback whale. They all recognized that two laminations corresponded to one ridge. However, for example, in Fig. 55 (p. 467) on thereport by Laws, there are individuals which are over the 45° line. If two laminations correspond to one ridge, all individuals must be plotted on or right side of the

72

S. OHSUMI

line. And in the Fig. 4 of the paper by Chittleborough (1959), also there aremany individuals of which number of ridges more than half of the number of earplug laminations. And from these figures, we can consider that there is possibility of the existence of individual of which one ridge on baleen plate correspondto less than two laminations.

8
2
0

6

0

0

Q_
c

§0

"'

~
0

0

..n
c
0

4

"'

Ol

~

0

"'E

-"

2

z
5

l0

15

20

25

30~

Number of ear-plug laminations

Fig. 9. Relation between number of ear-plug laminations and the number of ridges onbaleen plate for fin whales. Open circle: Antarctic, Closed circle : North Pacific, Solidline: One lamina correspond to one ridge, Broken line : Two laminae correspond to
one ridge.

Fig. 9 shows the relation between number of ear-plug laminations and baleenridges on 53 Antarctic and 21 North Pacific fin whales. In this figure, broken line
is base on hypothesis that two laminations correspond to one ridges, and solid line
is based on another hypothesis that one lamination corresponds to one baleen ridge.
As mentioned above, if former hypothesis is true, all individuals must be on thebroken line or right side of the line, because baleen plate become to wear at the
tip from relatively young ages (Ruud, 1950). However, there are many individuals
which are on the left side of the broken line. Then, we must abandon the former
hypothesis. On the contrary, almost of all points distribute on solid line or rightside of the line. I have scanty of data on the material less than 3 laminations, but thelatter hypothesis approaches to the truth more than the former hypothesis. That is
to say, there is possibility that one ear-plug lamination corresponds to one baleenridge in the first few ages. As reviewed in Introduction, nowaday, it seemes to berecognized that annual increment of baleen is equivalent to one ridge (Ruud, 1940,
1945; Nishiwaki, 1951 ; Chittleborough, 1959). If it is correct, one ear-pluglamination corresponds to one year. This conclusion agrees also with the resultsin former sections in the present paper. However, I must notice that there are
3 individuals which distribute on the left side of the solid line. Ichihara ( 1964)
confirms the prenatal formation of ear plug. But there is no report on the forma-

AGE DETERMINATION OF FIN WHALE

tion of ear plug chiefly in calf stage.
problem.

73

Furhter examinations are needed for thisEXAMINATION OF AGE COMPOSITION BASED ON EAR-PLUG
LAMINATIONSAGE COMPOSITIONS OF FIN WHALES CAUGHT IN NORTHOF EAST ALEUTIAN ISLANDS

By immunologic and marking approaches, Fujino (1960) reported that almost of
the fin whales caught in the north of east Aleutian Islands are composed withpure Population-II. And as Omura ( 1955) reported, the whaling in this area had
not been operated on a large scale before the time when Japanese fleets began tooperate in 1954.
TABLE 8. AGE COMPOSITION OF FEMALE FIN WHALES CAUGHT IN THE
NORTH OF EAST ALEUTIAN ISLANDS DURING 1959 TO 1961
Annual accumulation rate

Annual accumulation rate
Age
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

One lamination Two laminations

2
5
29
59
69
90
64
50
51
36
30
27
27
15
22
20
17
17
12
28
18
18
20
15

7
88
159
114
87
57
42
42
34
40
36
35
37
23
18
25
17
19
13
17
8
5
2
8
4

Age

One lamination

Two laminations

26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51<
Total

17
20
12
11
9
9
12
13
7
10

2
2
6
4
4
3
3
4

II

8
6
7
8
9
4
4
2
3
2
2
6
2
43
978

2

2
3
978

Table 8 and Fig. 10 show the estimated age compositions of female fin whalescaught in the north of east Aleutian Islands during 3 years from 1959 to 1961 basedon ear-plug laminations. In this figure, the age distributions are converted on both

74

S. OHSUMI

assumptions of biannual (two-laminations age; broken line) and annual (one-lamination age; solid line) accumulation rate of ear-plug laminations. Frequencies areshown in logarithm.
Modes of frequencies exist in 7 years in the case of one lamination age and in
3 years in the two-laminations age. This is because size limit is prescribed as 55
feet for northern hemisphere fin whale in the factory ship whaling, and younger
animals are protected from whaling.

100
80
60
40
~

<JJ
0
-"'

5

20

0

1'

~ I

;;;

I
I

_Q

E
~

z

10
8

I

~

x
I\

I

\

6

'1

I\

f

\ :

~

jl
JI

I J

I

\

t f \\

4

11

1J I

f~

JI

1

/ \

\ I

\I \ I: I

k-1

'f

/

v \xxf I1'I

2

x

iil/
l/

l/

v

*
5

10

15 - 20

25

30

xx x

35

40

x

45

50

55

60

65

70

Age !years I

Fig. 10. Age compositions based on ear-plug laminations of the female fin whales caught
in the north of east Aleutian Islands during the seasons from 1959 to 1961. Opencircles and solid line: Annual accumulation rate is assumed as one, Cross andbroken line : Annual accumulation rate is assumed as two.

Now, as shown in Fig. 10, catch curves are seemed to be separated to two straightlines excluding older age respectively in two accumulation rates. The cross pointof two straight lines is estimated to be 15 years for the case of one-lamination age
and 7 years for the case of two-laminations age.
If there is a population of which natural mortality coefficient (M) is constantand a constant fishing (coefficient is F) is operated to the population (more thanA-age individuals are catchable), the logarithmic catch curves must be changed by
years as shown in Fig. 11. Catch curves are separated to two straight lines. And
right line has the same coefficient as M. On the other hand, left line has a coeffici-

75

AGE DETERMINATION OF FIN WHALE

ent which is Z ( = M + F). Cross point of these two lines moves according to thefishing years, and after t years, the cross point must become to (A+t)-age.

·"

.i
0

OJ

.2

\

\

t\
A age

\

A+ I age\

i

\
5

\

25

20

15

10

Age (years)

Fig. 11. Modelal catch curves according to the change of years, when a constant fishingeffort (F) is given to a population of which natural mortarity (M) is constant and recruit
is constant.

vVhen this modelal catch curves are able to adapt the actual catch-curves, thefollowing factors must be chequed:
1) Natural mortality rate is constant: We cannot cheque on this factor.
But in the one-lamination age, mortality is seemed to be constant during theages from 15 to 40.
2) Recruitment to the age catchable is constant: We cannot know on thisfactor.
3) Fishing effort is constant: Table 9 shows the C.D.W. (catchers days
work) in the operation in the north of east Aleutian Islands from 1954 to
1961. During the 8 years, C.D.W. are not always constant, and they vary
between 110 and 350. But there is no tendency in the variation. And
average of C.D.W. is 286.8.
Although there are above weak points, the actual curve is seemed to be adapted
the modelal catch-curve.
On the cross point of two straight line, following result is obtained :A
One-lamination age
Two-laminations age

7
3

A+t

6
6

13
9

Actualcross point
15
7

76

S. OHSUMI

As actual catch-curve is given from sum of 3 seasons 1959-1961, this is considered
to be a catch curve in 1960. Therefore, t is 6 years. From above result, I cannot
determine which curve is more suitable to truth, but deviation between actual crosspoint is 13% in one lamination age, on the other hand, it is 29% in two laminations
TABLE 9. CATCH EFFORTS FOR FIN WHALES.IN THE NORTH OF
EAST ALEUTIAN ISLANDS (Areas P 25 BS AND P 26 BS)
AverageNo. of fin
YearCatchers*
Days work
C.D.W.
tonnagewhales caught

1954
1955
1956
1957
1958
1959
1960
1961

5
9
10
10
10
10
10
10

260
450
330
220
110
290
370
260

52
50
33
22
II

29
37
26

460.4
504.0
572.7
632.0
657.2
673.5
688.8
690.7

587
1177
773
408
299
705
948
585

* Including scouting boats
age. More accuarate result will be obtained in future on this subject by examining the tendency of change of age composition.
NATURAL AND FISHING MORTALITY RATE OBTAINED FROM AGE
COMPOSITION BASED ON EAR-PLUG LAMINATIONS

From Table 9, natural mortality coefficient (M) and total mortality coefficient (Z)are calculated on two cases of annual accumulation rate of ear-plug laminations asfollows:
Annual accumulation rateof ear-plug lamination
One (one-lamination age)
Two (two-laminations age)

z

M

0.192
(7-15)
0.336
(3-7)

0.067
(16-50)0.150(8-25)) : Range of age classes in the calculation

The values ofZ and Min the latter case are about twice of the former.
mortality coefficient (F) is calculated from Zand M values. That is,

Fishing

F=Z-M

So, F values are given as 0.125 and 0.186 respectively for above two cases.

Then

total mortality rate ( 1-S), unconditional natural mortality rate ( D =1-S))and exploitation rate ( E =~ (1- S)) are calculated as follows :Annual accumulation rate
One lamina
Two laminae

1-S

D

E

0.174
0.285

0.061
0.127

0.113
0.158

1: (

AGE DETERMINATION OF FIN WHALE

77

RELATION BETWEEN NATURAL MORTALITY RATE AND RECRUIT RATEIn the virgin stock, if natural mortality rate and pregnant rate in mature stage areconstant throughout life, recruit rate is calculated presumably by the mortality rate
and pregnant rate.
In the north of east North Pacific, the pregnant rate are obtained from matureand pregnant female whales caught as shown in Table 10.
Pregnant rates vary between 43.9% and 68.l %, and average rate during 8
years is 55.5%.
TABLE 10. PREGNANT RATE OF FIN WHALE CAUGHT IN THE NORTHOF EAST ALEUTIAN ISLANDS DURING 1954 TO 1961
Year

Mature females

Pregnant whales

Pregnant rate (%)

1954
1955
1956
1957
1958
1959
1960
1961
Total

224
395
269
140
64
204
337
160
1793

139
219
118
85
41
139
164
88
996

62.1
55.4
43.9
60.7
64.1
68.1
48. 7
55.0
55.5

Vv e consider that above pregnant rates do not show the true value, and I guess
the rate is higher than the true value, because lactating animals are protected fromcatch by International Whaling Regulation, and so, most of them are not included
into the figures in Table 10.
Now, recruit rate (R) are calculated in Table 11, when the first recruit number(number of whales born) is 1, sex ratio of females is 0.5, natural survival rate is S,
age at sexual maturity is m, and pregnant rate is P, as follows :R = P/2 (S

m

+S

m+l

oo

+ ...... + S )00

P • Sm ( 1- S
2
1-S

)

Then S is smaller than 1,
p. Sm
R = 2(1-S)

Age at sexual maturity (m) is considered to be 5 years in the case of two-laminations age and 10 years in the case of one-lamination age. And true pregnant rate
is not known, but I set the rate as 0.30, 0.40, 0.50, and 0.60. Then, the recruit
rates are calculated as shown in Table 11.
If survival rate does not vary throughout life, in the range in which R is smaller
than 1, the number of whales born become to be less than the previous number, andsuch a population cannot maintain the number of stock. Then, in general, theS. OHSUMI

78

mortality rate in calves is conciderable to be more than the adults, so, the R-value
calculated by above process must be fairly more than I in practice.
In one-lamination age, survival rate in which the stock continue the size are
more than 0.93 and 0.90 respectively in the cases of which pregnant rates are 0.30
and 0.60. On the contrary, in two laminations age, they are more than 0.9I and
0.86 respectively.
TABLE 11.

Age at sexual
maturitys.r.\p.r.
0.97
0.96
0.95
0.94
0.93
0.92
0.91
0.90
0.89
0.88
0.87
0.86
0.85

RECRUIT COEFFICIENT OF THE FINWHALE CHANGING THE
SURVIVAL RATES AND PREGNANT RATES

5 years old

10 years old
0.50

0.40

0.30

0.60

0.50

0.40

7.38
5.00
3.60
2.69
2.08
1.63
1.30

6.15
4.16
3.00
2.25
I. 73
I.36

4.92
3.33
2.40
1.80
1.39

3.69
2.50
1.80
1.35

1.09

0.82

1.08

0.86

1.04

0.87

8.60
6.13
4.65
3.67
2.98
2.51
2.08
1. 77
1.52
1.32
1.15

7 .17
5.11
3.88
3.06
2.48
2.09
1. 73
1.48
1.27

5.73
4.09
3.10
2.44
1.99
1.67
1.39
1.18

1.10

0.88

0.60

0.85
0. 70
0.53
0.47
0.39

1.04

0.30
4.30
3.06
2.33
1.83
1.49
1.25
1.04

0.89

1.02

0.96

1.01

0.88

s.r. : Survival rate, p.r. : Pregnant rate

Now, the natural mortality rates were shown in previous section that 0.06I(survival rate is 0.939) in the case of one lamination age. This value satisfy the Rvalue even if pregnant rate is 0.30, and when pregnant rate is more than 0.30, themortality rate is suitable for the continuance of the population size. On the other
hand, the natural mortality rate in the case of two-laminations age is 0.127 (survival
rate is 0.873). In this case R-value is lower than I when pregnant rate is lower
than 0.50. Even if the pregnant rate is 0.60, R-value is I. I. This value will be
not safe for the continuance of population size, considering the higher mortality ratein the young stage.
Therefore, when the annual accumulation rate of ear plug is assumed to be
two, the natural mortality rate calculated from age composition is too high for maintain the population size. On this point of view, the hypothesis of two-laminations
age should be abandoned.
Purves & Mountford (I959) calculated the mortality rate from the age distribution based on ear-plug lamination on the fin whales in the Antarctic area I.
And they got mortality coefficient. They are 0.133 in the female and 0.106 in the
male. The figures are based on an assumed rate of formation of 2 laminations per
year. Although Purves & Mountford did not consider the effect of whaling on
the age distribution, they seem to recognize the figures show the natural mortality

AGE DETERMINATION OF FIN WHALE

79

coefficient. The coefficient in the female is similar to my result in 2 laminationsage.
Laws ( 1961) calculated the mortality rate from the age distribution based on
number of corpora in the ovaries on the female fin whale in the Antarctic area IIin pre-war period when, as he estimated, fishing mortality was relatively small. The
mortality coefficient is 0.0973 corresponding to annual mortality rate of 9.2%.
In the calculation, the annual accumulation rate of corpora is assumed to be 1.43.
However, Doi (in press) calculates the natural mortality rate comparing the changeof number of fin whales catch per unit effort between the Antarctic pre-war andpost-war seasons independently to the age distribution. And he got the result thatthe natural mortality rate is 5-7%.
CALCULATION OF FISHING AND SURVIVAL RATEBY THE RECOVERY OF MARKED FIN WHALE

There are many undetermined factors for adaption of marking to population dynamics (Ricker, 1958). And I don't want to examine thoroughly the marking method
for population dynamics. In this section, I examine the marking result only forcheck the fishing rate and total survival rate calculated by the age composition basedon ear-plug laminations.
Table 12 shows the estimated number of fin whales marked in the north of east
Aleutian Islands and number of whales recaptured among them during 1954 to 1962
seasons. Number of whales recaptured are variable according to the number ofwhales caught. Then, the number of whales recaptured are reviced assuming that1000 whales are caught in every year. Number of whales marked (M) and recaptured (R) are so small that the average value is considered to be more suitable forthe exmaination.
Survival rate S is estimated by following formula (Ricker, 1958) :S=

Ri +R2+ ......Rn
Ro+R1 + ..... Rn-1

The number of recaptured whales in the same year of marking have many discussablefactors, so I consider the data of the recovery in the same season must be excluded
for this calculation, so the formula is reviced as following :S = R2+R 3 + ..... Rn
R 1+R 2+Rn- 1
The results are shown in Table 13. The calculated survival rate becomes
more correct by the increment of elapsed years. In the last 4 cases in the table,
survival rates are 79-86%.
The survial rate S calculated from age composition is 82.6% in the case of one
lamination age, and 71.5% in the case of two laminations age. Then one lamination
age corresponds to the result obtaned from marking experiments. And the survival
rate attained from age composition of two laminations age is considered to be too high.

80

S. OHSUMI

Fishing mortality rate is calculated briefly as follows :j - Ro-M

They are 5.1 % and 8.3 % respectively in actual and reviced. But whaling andmarking are operated almost parallelly in practice. So the twice of the calculated
TABLE 12. NUMBER OF FIN WHALES MARKED AND RECAPTURED IN THE
NORTH OF EAST ALEUTIAN ISLANDS DURING 1954 TO 1962
Actual figure.
No. ofwhalesYearmarked
A.

1954
1955
1956
1957
1958
1959
1960
1961
1962
Tota!
average
B.

210
33
57
41
28
15
II
23
6
424
47.1

Elapsed years from marking till recapture
0
7
2

2
7 (1)
1 (1)

3 ( 1)
I

4

7 (2)
I
2

2
6(3)
I
3

5
22(1)
2.44

2
20(4)
2.50

I
13(3)
1.86

3

4

3
6
2 (I)
I
14(1)
2.33

1 ( 1)
1
1
4(1)
0.80

5

7

6

5(2)
I
2
7 (2)
I. 75

2
0.67

2
1.00

8

Total

No. ofwhalescaught

7
9(1)
3(1)
10(4)
11 (2)16(2)11(1)
12( 1)
584(12)

585
1177
773
601
455
705
1066
587
284
6233

12.0
7.7
3.9
16. 7
24.2
22.7
10.3
20.4
17.6
135.5

1000
1000
1000
1000
1000
1000
1000
1000
1000
9000

Reviced figure when 1000 whales were caught in every year.

1954
1955
1956
1957
1958
1959
1960
1961
1962
Total
average

210
33
57
41
28
15
11
23
6
424
47.1

12.0
I. 7
5.0
2.2
5.7

6.0
1.3
15.4
1.4
1.9

2.6
10.0
2.2
4.3

8.5
35. l
3.91

7. l
33. l
4.14

3.5
22.6
3.23

I. 7
2.2
4.3
5.6
3.4
3.5
20.7
3.45

2.2
7. I
0.9
0.9
I. 7
I. 7
3.5
9.7
8.3
1.66 2.42

0.9
I. 7

3.4

2.6
0.87

3.4
I. 70

) ; Number of whales recaptured in other whaling areas or whales which were not known the position
recaptured.
TABLE 13.

TATOL SURVIVAL RATE OF MARKED FIN WHALES CALCULATED
1954-1957

1954-1962

Elapsed years (n)ActualRevicedActualReviced

2
3
4
5
6
7
Fishing mortality rate

0.800
0.852
0.684
0.805

0.841
0.794
0.678
0.785

0.062

0.102

0.744
0.961
0. 757
0.912
0.811
0.857
0.111

0.780
0.906
0. 771
0.862
0.781
0.845
0.194

AGE DETERMINATION OF FIN WHALE

81

f will become near the truth.

Then, they are 10.2% and 16.6% respectively. As
the data concerning recoveries in the same year of marking have many discussive
factors, the following formula (Ricker, 1958) are suitable for this purpose:f =R 1 +R2+ ...... +RnSM(l+S+ ...... Sn-2)

The results calculated from above formula is :sfD

1954-1957
Reviced
0.785
0.102
0.113

Actual
0.805
0.062
0.133

1954-1962__._
ActualReviced
0.845
0.857
0.111
0.194
0.032
-0.029Fishing mortality rates are situated between 0.062 and 0.194. The calculated
fishing mortality rates from age compositions are 0.113 and 0.158 respectively in
one lamination age and two laminations age. The former rate corresponds to reviced result of 1954-1957 and actual result of 1954-1962. And the latter is near
the result of reviced data 1954-1962. However, in the latter case the natural
mortality rate obtained from marking experiment is smaller than 0 (-0.029). Thiscannot explain the actual phenomena, because the natural mortality rate should
be more than 0. Therefore the fishing mortality rate of 0.194 is higher than theactual states of the whaling. Then, the one-lamination age agrees more with examination of fishing mortality rate from marking than the two laminations age.
CHANGE OF MATURITY RATE ACCOMPANIED WITH WHALING

Sexual maturity rate changes accompaniing with the increasement of whaling years.
Table 14 and Fig. 12 show the change of maturity rate of female fin whales caught
in the north of east Aleutian Islands from the begining of whaling in 1954 to 1961.
The rate decrease by the increasement of years.
Now, taking following two model-age-compositions, and the change of sexual
maturity rate are calculated from the begining of whaling in Table 15.

Natural mortality rate
Fishing mortality rate
No. of whales in 0 year classAge at sexual maturity
Age at recruitment for whaling
No. of whale caught before racruitment

Model I(One lamination age)
0.061
0.113
10,000107No. of whales in age 7 X 2Model II
(Two laminations age)
0.127
0.158
10,0005
3No. of whales in age 3 X 1

In Model I, number of mature females at the begining of whaling is 87,363.
If a quater of the mature females is the number of female whales which are born,the number is 21,841. Whereas the first assumption of females which are born is
10,000. From this calculation we can recognize that the natural mortality rate inyoung stage is fairly large. By the way, in Model I, whaling does not influence toS. OHSUMI82

the recruitment at least until 7th years after the beginin of whaling. In practice,there are whales caught before the age at recruitment for whaling, as shown Table
15, so, I assumed the whales as the twice of the number of whales at the age ofrecruitment (12,873). In Fig. 12, the change of maturity rate of Model I is similar
to the actual change of the maturity rate.
TABLE 14. ACTUAL NUMBER OF SEXUALLY MATURE AND IMMATURE
FEMALE FIN WHLES CAUGHT IN THE NOTH OF EAST ALEUTIANISLANDS DURING THE SEASONS FROM 1954 TO 1961.
Year

Immature

MatureMature rate (%)

1954
1955
1956
1957
1958
1959
1960
1961
Total

80
166
103
70
77
153
223
135
1007

224
395
269
140
64
204
337
160
1793

73.7
70.4
72.3
66.7
45.4
57 .1
60.2
54.2
64.0

TABLE 15.

CALCULATED NUMBER OF TOTAL AND MATURE FEMALESCAUGHT IN THE CASES OF MODELS-I AND -II.

Model-I: D=0.061, E=0.113 (One-lamination age).
Total
Year
lst
2nd
3rd
4th
5th
6th
7th
8th

118391.2
106468.896621.988483.7
81765.2
76287.771589.867842.9

MatureMature rate (%)

87362.6
76849.9
67601.859465.6
52747.1
47269.6
42611. 738824.8

73.8
72.2
70.0
67.2
64.5
62.0
59.5
57.2

Model-II: D=0.127, E=0.158 (Two-laminations age).
Year

Total

MatureMature rate (%)

lst
2nd
3rd
4th
5th
6th
7th
8th

59041.4
50764.844846.8<40614.8<37589.3<35425.8<33879.4<32773.1

39926.6
32701.1
26782.7
22551.0
19525.5<17362.0<15815.6<14709.3

67.6
64.4
59.7
>55.5
>51.9
>49.0
>46.7
>44.9

However, in model II, even in the virgin stock, the number of whales born islower than the firstly assumed number of whales born which can maintain the
population size. Then the whaling will influence to the number of recruitment for
whaling after 3 years.
Assuming the number of whales caught before the age of recruitment to be as

AGE DETERMINATION OF FIN WHALE

83

the same as number of whales at the age of recruitment, the maturity rates are lower
than the actual figures.
After 3 years from the start of whaling, the number of recruit becomes to besmaller than the virgin stock. Then, as the number of immature whales become relatively smaller, the maturity rate must be higher than the calculated rate.
In Fig. 12, the estimated changes of maturity rate are shown. This estimated change
in Model II does not correspond the feature of the actual data.

"'Q)
0

E

70

Q)

Q)

::J

0

E

60

0

c
Q)

u

© 50

0..

'54

'56

'58

'60

Year

Fig. 12. Change of sexual maturity rate of the female fin whales for actual and twomodels. Open circle and straight line: Actual data from the female fin whales caught
in the north of east Aleutian Islands, Closed circle and solid line : Model I, Cross andbroken line: Model II.

In conclusion, the actual change of maturity rate correspond fairly with ModelI (the accumulation rate is one annually), but those of which Model II is not consistent with the actual change.
DISCUSSION

By some examinations which were described above, I conclude that the annual
ear-plug accumulation rate must be under two which is nowaday recognized by us,
and I showed some examples for estimation that the annual accumulation rate may
be near one.
Indeed, ear plug is the best of age characters in the fin whale, but there are many
points which must be investigated further. One of them is a standardization of lamination count. As Ichihara (1963) reported, there is individual discrepancy in lami-

84

S. OHSUMI

nation count. And the photometric method which lchihara (1963) manufactured
for trial must be developed further for this purpose. When standardization of ear
plug reading is estabished, and if the new reading becomes different to the present
reading, the above examinations should be re-examineed. The second point to
be investigated in future is the periodicity of laminae formation. Purves (1955)
considered the migratory period as the relation of laminae formation. However,
Clarke (1962) reported the feeding of fin whales off the coast of Chile in October andNovember. And according to Mackintosh (1942) and Nemoto (1962), food of whales
are scanty in the lower latitude, and the baleen whales do not feed so much in winterseason. If the mechanism of formation of ear-plug lamination is coincided to thenutrition, the formation of ear-plug lamination must have more relation with the winter(reproductive season) or summer (feeding season) than the migratory seasons. Ridges of baleen plates are also considered to have relation with nutrition (Ruud, 1940,1945). If so, the periodicity of formation of ear-plug lamination and ridges of baleen
plates should be the same each other. My result of correlation between baleen ridges
and ear-plug lamination briefly support on the same mechnaism of formation between two characters. On the lamination formation, Purves & Mountford (1959)
and lchihara (1959) discribed on possiblity as shown in Introduction. Furthermore, Mr. lchihara suggests me the possibility of the change of periodicity of earplug lamination by the age. We must investigate the formation mechanism of earplug lamination. Then the factors conserning the formation of laminae will beknown, and it will be solved the annual rate and whether the periodicity of formation are variable or constant.
My second conclusion is on the annual ovulation rate of the fin whale. The
rate must be less than 1.4 which as Laws (1961) studied throughly. I got a resultthat it may be near 0.5 although there are individual variations and the difference
among the different stocks as shown by Nishiwaki, lchihara & Ohsumi ( 1958)
and Fujino (1963), and Kimura (1956) reported the tendency of multiple ovulationsin the course of increase of age by ths study of twinning.
The annual ovulation rate is very important not only for the age determination
but also for the reproductive physiology of the whales. According to my result,the fin whale ovulates in average one every two years. Then pregnancy rate
may be under 50%, because there are evidence that some corpora albicantia are
non-pregnant (Laws, 1961). Apparent pregnancy rate is 55% in my material,
but I consider that they are higher than the true rate. Although Laws (1961)
considered that over 80 % of females wean their calves before they enter the
pelagic whaling grounds, Chittleborough (1958) reported that the weaning periodof humpback whales is 10 months. And there are other observations on the suckling of dolphins. According to Tavolga & Essapian (1957) and Nakajima et al.
(1962), bottlenose dolphin (Tursiops sp.) suckles for over 1 year in aquarium. Although we have no data on direct observation of weaning period in the fin whale
and above data are given by other species, I think the lactating stage of the fin whale
will be longer than previous descriptions (Mackintosh & Wheeler, 1929; Laws,
1961). Mackintosh & Wheeler (1929) suggestthatforthenursing period the lactat-

AGE DETERMINATION OF FIN WHALE

85

ing females are segregated from the main herds, so that all lactating females do notappear on the whaling grounds. And in the whaling grounds, the taking of calvesor females accompanied with calves are prohibited by the International Whaling
Convention. Therefore, the pregant rate calculated by actual data are considered
to be higher than the true rate.
Concerning this subject, Laurie (a letter to Purves & Mountford, 1959) also
discussed and he wrote, if so, annual increment of corpora in the blue whale must
be lower than 1.13 and connected as 0.69. Further ecological knowledges are needon this problem. I think that the two-years breeding cycle will be common in thefin whale, but there will be some individuals whose breeding cycle is more than two
years.
In the examination on the relation between ear-plug laminations and baleen
plate ridges, there are some material which are not able to explain by the correspondence one to one. The number of ridges is more than the number of ear-plug lamination. As one of the caution, the discount of ear-plug lamination or baleen ridges
is considerable, but more collection and comparison are needed on this subject especially on the calves.
SUMMARY

1. Age determination of the fin whale (Balaenoptera physalus L.) was examinedby some methods.
2. Considering the elapsed time from marking, the annual accumulation rateof ear-plug lamination of marked whales recaptured should be less than two, andthere is no whale of which number of ear-plug lamination devided by elapsed years
is less than one.
3. Sexually immature rate of recaptured whales decrease with the increasementof elapsed years. And from the tendency of the decrease of immature rate, the age
at sexual maturity is estimated to be about 10 years.
4. Numbers of corpora lutea and albicantia in the ovaries of recaptured fin
whales devided by elapsed years are able to be explained more easily in the assumed
case which the annual ovulation rate is 0.5, and age at sexual maturity is 10 years.
5. Relation between the ear-plug lamination and mean number of corpora
shows that annual accumulation rate of corpora is 0.49 in the fin whales caught in
the Antarctic areas IV and V, if one lamination accumulates in a year.
6. Percentage of sexual maturity in successive the number of ear-plug lamination shows that the number of ear-plug lamination corresponding to 50% of sexual
maturity is 9-.4 in males and 10. 7 in females.
7. Before the tip of baleen plates begin to wear, number of ear-plug lamination coincide the number of ridges on baleen plates.
8. Age compositions of the female fin whales caught in the north of east
Aleutian Islands based on ear-plug lamination are drawn in following two cases.
And the natural mortality rate (D) fishing mortality rates (E) and total mortality rate(Z) are calculated as follows:S. OHSUMI86
Annual rate
One lamination
Two laminations

z
0.174
0.285

D
0.061
0.127

E
0.113
0.158

9. In the virgin stock, relation between natural mortality rate and recruit
rate is discussed. And natural mortality rate in the case of two laminations age isconsidered to be too high for maintainance of the stock size.
10. Examining the number of fin whales marking and recovery in the northof east Aleutian Islands, survival rate is calculated to be 79""'86 %· And the fishingmortality rate is also calculated from the data. It is 10-12%.
11. Yearly change of sexual maturity among the actual, Models I and II
populations are compared. And the change of Model I (annual rate is one lamination) is suitable to the actual change. On the contrary, Model II (annual rate is
two laminations) is not able to explain the actual change.
12. Thinking collectively above examinations, it will be concluded that theannual accumulation rate of ear-plug lamination must be less than two, probably
near one, and average annual ovulation rate will be under one, probably near 0.5,
although there are individual and racial .variations. In addition, the average age
at sexual maturity will be older than 5 years, probably 9""' 11 years. Of course there
are individual variation in the age of sexual maturity.
13. The standardization of the reading of ear-plug lamination should beestablished, and after then above examinations must be re-examined.
ACKNOWLEDGEMENT

Marking material used in this paper are based on the endeavers by Discovery Committee and National Institute of Oceanography which have been engaged to the whale
marking in the Antarctic from 1932, International Whales Marking Program in
the Antarctic from 1954 and the Fisheries Agency of Japanese Government whichhave marked into the whales in the North Pacific since 1949. Unless the endeavers
by above Agencies, present examinations would not be studied. I am indepted toabove Agencies for use the material. I would like to thank Mr. S. G. Brown who
kindly checked the marking data in the Antarctic.
And this paper could not be written unless the strenuous efforts of the biologists
who investigated, coll~cted and examined the material under the leading by Dr.
Omura, Director of the Whales Research Institute. In particular, Dr. M. Nishiwaki, Dr. K. Fujino, Dr. T. Nemoto, Mr. T. Ichihara and Mr. K. Nasu of the
Whales Research Institute. I due especially to Mr. T. Ichihara and Dr. M.
Nishiwaki, on the reading of ear-plug laminations. I wish to thank them.
My sincere thanks are due to Dr. H. Omura and Dr. M. Nishiwaki of the\!\Thales Research Institute and Dr. T. Hibiya, Assistant Professor of the Universityof Tokyo for their guidance throughout the work.
Dr. F. Nagasaki of the Fisheries Agency of Japan and Dr. N. Doi of Tokai
Resional Fisheries Research Laboratory kindly suggested me the method for analyzing
the population dynamics. Dr. R. M. Laws of the former staff of National InstituteAGE DETERMINATION OF FIN WHALE

87

of Oceanography kindly corrected some parts of my draft and gave me suitable
cnt1c1sm. I have benefited from discussions with Mr. T. lchihara. I should liketo thank them.
Finally, I am much indebted to many members of the Japanese whaling companies for the help to investigation and collection of the materials.

REFERENCES
Brown, S. G. (1962 a). The movements of fin and blue whales within the Antarctic Zone. Discovery Rpts.
33: 1-54.
- - (1962 b). Whale marks recovered during Antarctic seasons 1960/61, 1961/62 and in South Africa
1962.
Norsk Hvalfangst-Tid., 51 (11): 429-34.
Chittleborough R. G. (1958). The breeding cycle of the female humpback whale, Megaptera nodosa (Bonnaterre). Aust.]. Mar. Freshw. Res., 9 (1): 1-18.
- - (1959 a). Australian marking of humpback whales. Norsk Hvaifangst-Tid., 48 (2): 47-55.
- - (1959 b). Determination of age in the humpback whale, Megaptera nodosa (Bonnaterre). Aust. ].
Mar. Freshw. Res. 10 (2) : 125-43.
- - ( 1960). Marked humpback whale of known age. Nature 187: 164.
Chittleborough, R. G. & Godfrey, K. (1957). Review of whale marking and some trials ofa modified whalemarks. Norsk Hvaifangst-Tid., 46 (5): 485-508.
Clarke, R. (1962). Whale observation and whale marking off the coast of Chile in 1958 and from Ecuador
towards and beyond the Garapagos Islands in 1959. Norsk Hvalfangst-Tid., 51 (7): 265-87.
Clarke, R. & Brown, S. G. (1957). International co-operation in Antarctic whale marking from 1945 to
1957. Norsk Hvaifangst-Tid., 46 (9): 513-7.
Dawbin, W. H. (1959a). Evidence on growth-rates obtained from two marked humpback whales. Nature
183: 1749-50.
- - (1959 b). New Zealand and South Pacific whale marking and recoveries to the end of 1958. Norsk
Hvaifangst-Tid., 48 (5): 213-38.
Doi, T. (1964). Estimation of coefficient of natural mortality rate of fin whale population in the Antarctic.·
Bull. Takai Reg. Fish. Res. Lab., 38 (In press)
Fujino, K. (1960). Immunogenetic and marking approaches to identifying subpopulations of the North
Pacific whales. Sci. Rep. Whales Res. Inst., no. 15 : 85-142.
- - (1963). Population genetics of whales with reference to blood types. Bull. Jap. Soc. Sci. Fish. 29(12): 1133-8
Hylen, A.,Jonsgard, A., Pike, G. C. & Ruud, J. T. (1955). A preliminary report on the age composition of
Antarctic fin whale catch 1945/46 to 1952153 and some reflections on total mortality rates of fin whales.
Norsk Hvaifangst-Tid., 44 (10): 577-89.
Ichihara, T. (1959). Formation mechanism of ear plug in baleen whales in relation to glove-finger.
Sci.
Rep. Whales Res. Inst., no. 14: 107-135.
- - (1963). Photometric method for counting laminae in ear plug of baleen whale. Sci. Rep. Whales Res.
Inst., no 17 : 37-48.
- - (1964). Prenatal developement in ear plug of baleen whale. Sci. Rep. Whales Res. Inst., no. 18: 29-48
Kimura, S. (1957). The twinning in southern fin whales. Sci. Rep. Whales Res. Inst., no. 12: 103-25.
Laurie, A. H. (1937). The age of female blue whales and the effect of whaling on the stock. Discovery
Repts. 15 : 223-84.
- - (1959). See Purves & Mountford (1959).
Laws, R. M. (1960). Problems of whale conservation. Trans. N. Amer. Wildlife Conf. 25: 304-19.
- - (1961). Reproduction, growth and age of southern fin whales. Discovery Repts. 31: 327--486.
Laws, R. M. & Purves, P. E. (1956). The ear plug of the Mysticeti as an indication of age with specialreference to the North Atlantic fin whale. Norsk Hva{fangst-Tid., 45 (8): 413-25.

88

S. OHSUMI

Mackintosh, N. A. (1942). The southern stocks of whalebone whales. Discovery Repts. 22: 197-300.
Mackintosh, N. A. & Wheeler, J. F. G. (1929). Southern blue and fin whales. Discovery Repts. 1:
257-540.
Nakajima, M., Takahashi, K., Ogura, M. & Sawaura, K. (1963). On the growth of infants of the small size
toothed whales. ]. Jap. :{,oo and Aquarium. 5 (1): 16-22 (In Japanese)
Nemoto, T. (1962). (Food of baleen whales) Geiken Sosk_yo no. 4: 136 pp. (In Japanese)
Nishiwaki, M. (1950a). Determination of the age of Antarctic blue and fin whales by the colour changes incrystalline lens. Sci. Rep. Whales Res. Irist., no. 4: 115-61.
- - (1950 b). Age characteristics in baleen plates. Sci. Rep. Whales Res. Inst. no. 4: 162-83.
- - (1951). On the periodic mark on the baleen plates as the sign of annual growth. Sci. Rep. Whales
Res. Inst., no. 6: 133-52.
- - (1952). On the age-determination of Mystacoceti, chiefly blue and fin whales. Sci. Rep. Whales
Res. Inst., no. 7: 87-119.
- - (1957). Age characteristics of ear plugs of whales. Sci. Rep. Whales Res. Inst., no. 12: 23-32.
Nishiwaki, M. & Yagi, T. (1953). On the age and the growth of teeth in a dolphin (Prodelphinus caernleoalbus) (1). Sci. Rep. Whales Res. Inst., no. 8: 133-46.
Nishiwaki, M., Ichihara, T. & Ohsumi, S. (1958). Age studies of fin whales based on ear plug. Sci. Rep.
Whales Res. Inst., no. 13: 155-69.
Ohsumi, S. (1962). Biological material obtained by Japanese expedition from marked fin whales. Korsk
Hvalfangst-Tid., 51 (5): 192-8.
Ohsumi, S., Nishiwaki, M. & Hibiya, T. (1958). Growth of fin whales in the northern Pacific. Sci. Rep.
Whales Res. Inst., no. 13: 97-133.
Ohsumi, S., Kasuya, T. & Nishiwaki, M. (1963). The accumulation rate of dentinal growth layers in the
maxillary tooth of the sperm whale. Sci. Rep. Whales Res. Inst., no. 17 : 15-35.
Omura, H. (1955). Whales in the northern part of the North Pacific. Norsk Hvalfantst-Tid., 44 (6): 323-45,
44 (7) : 395-405.
Omura, H. & Kawakami, T. (1956). Japanese whale marking in the North Pacific. Norsk Hvalfangst-Tid.,
45 ( 10) : 555-63.
Peters, N. (1939). -Ober Grosse, Wachstum und Alter des Blauwales (Balaenoptera musculus (L)) und Finnwales (Balaenoptera physalus (L)). :{,ool. Anz. 127: 193-204.
Purves, P. E. (1955). The wax plug in the external auditory meatus of the Mysticeti. Discovery Repts. 27:
293-302.
Purves, P. E. & Mountford, M D. (1959). Ear plug laminations in relation to the age composition of a
population of fin whales (Balaenoptera physalus). Bull. British Mus. (H.N.). :{,ool. 5 (6): 6-161.
Rayner, G. W. (1940). Whale marking. Progress and results to December 1939. Discovery Repts. 19:
245-84.
Ricker, W. E. (1958). Handbook of computations for biological statistics of fish populations. Bull. Fish.
Res. Board Canada no. 119: 300 pp.
Ruud,]. T. (1940). The surface structure of the baleen plates as a possible clue to age in whales. Hvalradets
Skrifter nr. 23: 24 pp.
- - (1945). Further studies on the structure of baleen plates and their application to age determination.
Hvalradets Skrifter nr. 29: 69 pp.
Ruud, J. T., Clarke, R. & Jonsgard, A. (1953). Whale marking trials at Steishamn, Norway. Norsk
Hvalfangst-Tid., 42 (8): 429-41.
Sergeant, D. E. (1959). Age determination in Odontocete whales from dentinal growth layers. Norsk
Hvalfangst-Tid., 48 (6): 273-88.
- - (1962). The biology of the pilot or pothead whale, Globicephala melaena (Traill) in Newfoundland waters.
Bull. Fish. Res. Board Canada no. 132 : 84 pp.
Tavolga, M. C. & Essapian F. S. (1957). The behavior of the bottle-nosed dolphin (Tursiops truncatus):
Mating, pregnancy, parturition and mother infant behavior. :{,oologica 42 (1): 11-31.
Tomilin, A. G. (1945). (The age of whales as determined from their baleen apparatus). C.R. Acad. Sci. Dok!.
Acad. Nauk .. SSSR. 49 (6) : 460-3.
Wheeler, J. F. G. (1930). The age of fin whales at physical maturity. Discovery Repts. 2: 403-34.