DISTRIBUTION OF SPERM WHALE STOCKSIN THE NORTH PACIFICTOSHIO KASUYA * AND TO MIO MIYASHITA*
ABSTRACTTwo contrary opinions on the segregation pattern of sperm whale
stocks in the North Pacific, i.e. longitudinal vs. latitudinal, were tested
using whaling operation data, results of recent sighting cruises, andmovements of marked whales.
While Japanese coastal sperm whaling was exploiting the breeding
schools, the operation gradually changed from summer to winter and from
north to south. In the late 1950s and 1960s most of the whales were taken
during summer in the cold Oyashio Current area. During the 1970s
whales were taken in the warm Kuroshio Current area in summer and in
the Oyashio area in other seasons. In the 1980s most of the whaling occurred in the Kuroshio Current area during the winter. The last phase iscertainly undesirable for the industry using whaling stations opened forthe northern ground. In recent years sperm whale sightings are rare insummer in the previous whaling grounds north of the Kuroshio Current
and its extension.
These changes in the whaling operation and the whale distribution are
explained by assuming two latitudinally segregating sperm whale stocks
and earlier depletion of the northern one. In the summer adult males
move to colder waters segregating from their breeding population as wellas adult males of different stocks. This is supported by additional information from blood types, movement of marked whales, operation of Japanesepelagic whaling, and recent whale sightings. We further deduced that theeastern North Pacific is probably inhabited by a single sperm whale stock,
although presence of some local stocks is not fully excluded.
The present study also indicates the importance of ocean currents andwater masses for the evolution of sperm whale stocks, and provided an
example that a habitat vacancy produced by the depletion of one stock is
not easily reoccupied by nearby stocks. The successive exploitation ofdifferent components of one stock or exploitation of more than one stockwith overlapping time period could have masked possible changes in the
population structure and some density dependent life history parameters.

* Far Seas Fisheries Research Laboratory, 5-7-1, Orido, Shimizu-ski, Shizuoka-ken, 424 Japan
Sci. Rep. Whales Res. Inst.,!Vo.39,1988,31-75

32

KASUY A AND MIY AS HITA

Sea of Japan

Pacific Ocean•Bonin130°140°Fig. 1. Japanese whaling stations that have been used for processing sperm whales
during the 1955 to 1986 seasons.

INTRODUCTION

Two contradictory opinions have been published on the segregation of sperm
whale stocks in the North Pacific. One assumed that stocks were segregatedlongitudinally. The others believed that the stocks were segregated latitudinally, although recognizing the presence of some longitudinal segregation.
Although Kasahara (1950) did not clearly state about the stock identity
in his study on whale stocks and whaling in Japan, he drew a diagram ofsperm whale migration in the western North Pacific. It shows as if he believedthat most breeding schools of sperm whales winter in the Bonin Islands
Sci. Rep. Whales Res. Inst.,

No. 39, 1988

NORTH PACIFIC SPERM WHALES

33

waters and summer off Sanriku/Hokkaido waters, but that some females
winter off Sanriku/Hokkaido waters and summer in the central and northern
Kuril Islands area. Then Klumov (1955) presented a hypothesis that there are
two breeding stocks one summering in the northern Kuril Islands and theother in the southern Kuril Islands, with similar latitudinal stock segregationin the eastern North Pacific. An exwhaler Watase (1963) published a similar
hypothesis. He thought that the breeding schools of the southern stock segregate to the south of the Kuroshio Front with a northern range reaching 40°N
in summer (off Sanriku, see Fig. 1) following the shift of the Kuroshio Front,
while those of the northern stock principally inhabit waters north of the
Oyashio Front. He thought the breeding schools of the two stocks intermingle
in the summer in latitudes between 39° and 40°N off the Pacific coast ofJapan, although the northern stock will move further south in winter. He also
reported an autumn migration of a limited number of singletons or groupsof large males to the coastal Oya~hio Current area, but its relationship tothese stocks was not explained. In the same year, Fujino (1963) found asignificant difference in the blood type composition between sperm whales
taken by Japanese coastal fishery (both sexes, taken from June to November)and males taken by the pelagic fishery in the central Aleutian Islands/Bering
Sea area (51°-53°N, 172°E-165°W) in May to August, and concluded that thewestern North Pacific is inhabited by two sperm whale stocks and the northernstock migrates in autumn to Japanese waters but the southern stock does not
migrate to the Aleutian waters. He also analyzed the movement of marked
whales and concluded that sperm whales in the Bonin Islands area belong to
the southern stock. Although he does not seem to have clearly mentioned thatthe northern stock migrating off the Pacific coast of northern Ja pan includes
both sexes, an important factor for the interpretation of the result, his data
support that case (see Discussion).
Other studies have proposed a single sperm whale stock in the entire
latitudinal range off the east coasts of Ja pan and Kuril Islands. Masaki (1970)
analyzed mark-recapture data, blood types, catch distribution, size composition,and density hiatus appearing in the whale sightings, and assumed threelongitudinally segregating stocks in the entire North Pacific, with boundaries
at around 170°E-180°E and 150°W-160°W. Later, Ohsumi and Masaki( 1977) recognized a hiatus in the female catch in Japanese pelagic whaling inlongitudes around 160°W-175°W, and considered it to represent a stockboundary. Although such information was not available to the west of 160°E,
they considered from mark recapture data that sperm whales migrating off
the Pacific coast of northern Japan and Kuril Islands represent a single stock(western North Pacific stock). They ignored an apparent density hiatus offemales appearing in their data at 35°-40°N and west of 165°W, and concluded
that there were two (eastern and western) sperm whale stocks in the entire
North Pacific. Bannister and Mitchell (1980) analyzed the catch of 19th century
whaling between 20° -40°N to find a density hiatus at l 45°E, and assumed
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

34

KASUYA AND MIYASHITA

three longitudinally segregated sperm whale stocks, i.e. to the west of l 45°E,
in 145°E-l 79°E, and in l 79°E-150°W. The current management of the
sperm whale population by the International Whaling Commission (IWC) isbased on the assumption of two whale stocks in the entire North Pacific, a
distribution which has been adopted since the 1979 season (IWC, 1979) andis close to Ohsumi and Masaki (1977).
Ohsumi ( l 980b) indicated historical changes in the season and fishingground of Japanese coastal whaling during the past 29 years and attributed
them firstly to oceanographic changes, but later to the combination of changes
in the oceanography, selectivity between whale species, and decline in sperm
whale availability due to the operation of USSR pelagic whaling off Japan(Ohsumi, 1981 b ). A major oceanographic factor known to affect the Japanese
coastal whaling is the fluctuation of the Kuroshio Current as indicated by Uda(1954) and Ohsumi (1980b). The cycle, however, is believed to be 8 to 10
years (Uda, 1954). Since the studies of Ohsumi (l 980b, l 98lb), sperm whaling
has continued in Japanese coastal waters and resulted in a further southward
shift of the fishing ground and delay of the peak season (USSR pelagic whaling
lasted till 1979, and exploitation of North Pacific sei whales till 1976). If suchan uni-directional long time change of the fishery is attributable to change insome oceanographic condition, the total of such change must have been
extremely large and should have caused a significant climatic change in theregion. Such a change has not been detected.
Under these circumstances, the present study attempts to examine ifeither of the above two groups of hypotheses on the sperm whale stock off the
Pacific coast of Japan can better explain the observed changes in the fisheryand in the distribution of sperm whales. In addition we use analogy with thewestern North Pacific to consider the sperm whale distribution in the eastern
North Pacific.
MATERIALS AND METHODS

Statistics of Japanese land based fishery
The position, number and date of sperm whale catch by the Japanese
coastal whaling are used to analyze the historical change in the distribution of
the species off Japan. Catch positions of individual whales are available forseasons 1973 to 1985/86, but the corresponding data for 1955 to 1972 seasons
are only available by months and by whaling stations (compiled by the Japan
Whaling Association). We quoted from Kasahara (1950) the sperm whale
catch statistics before World War II, when the species was hunted without
regulation of fishing season or of catch quota. These are used in Tables 2 to
11 and Figs 2 to 4. Catch was regulated by national quota in 1959 to 1966,
and by agreement between related nations or by IWC since 1970 (Ohsumi,
l 980c). The actual figure of the quota varied by the government policy,
agreement between countries, or allocation of IWC quota between coastal andSci. Rep. Whales Res. Inst.,
No. 39, 1988

NORTH PACIFIC SPERM WHALES

35

pelagic fisheries. The reported catch agreed with the quota with a small
number of differences. These statistics are used as an indicator of peak monthof sperm whale catch, although we believe (without documented evidence)that there has existed unknown number of significant under-reporting in the
postwar coastal operations under quota system. Sex ratio or body lengthcompositions are not analyzed in detail. The body length must have beenskewed by the misallocation of body length due to the body length regulation(:2:30 feet in 1938 to 1948, :2:3S feet in 1949-1972, and :2:30 feet since 1973,
Ohsumi ( l 980c)). Sex ratio of the catch will not represent the ratio in the
population in the whaling ground due to the size selection of the fishery orcatch quota by sex (Table 1).
Catch by Kuril Islands land based fishery
Sperm whale catches by the USSR land based fishery summarized bymonth and land station during five years (l 9SO, l 9S 1, and 1962 through1964) are used to see the difference of migration timing between sexes (Fig.
8). The IWC body length regulation was the same as for Japanese coastal
whaling during the period, although the selectivity for larger whales or between sexes could be different.
Catch by Japanese pelagic fishery
Catch of sperm whales by sex, year, and two degree squares of latitude
and longitude have been cited from Smith (1980) and Shimadzu (1987), andused to find density hiatus or geographical variation in sex ratio. These data
are used in Figs 9 and lS. The range of Japanese pelagic whaling was modified several times by the Government during the period (Ohsumi, 1973). The
limit was ( 1) north of 4S 0 N in l 9S2-1966, (2) north of S0°N (west of 1S9°E),
north of 4S 0 N (1S9°-l 70°E) and north of 40°N (east of 170°£) in 1967 to
1971, (3) north of S0°N (west of 1S9°E), north of 4S 0 N (1S9°-l6S 0 E), northof 20°N (16S 0 E-1S0°W), and north of 3S 0 N (east of 1S0°E) since 1972. In
addition to these regulations Japanese factory ship operation was prohibited,since 1972, after 1 August in a small area off Japan surrounded by 20°N,
4S 0 N, l 6S 0 E, and 170°£. The size limit was 3S feet in 1949-19S l, 38 feet in
1952-1972, and 30 feet since 1973 (Ohsumi, 1980c), but there are somedoubts on the reliability of the length statistics near the above size limits(Allen, 1980).
Mark-recapture data
Data include all the available records of whales marked by Canada,
Japan, USA or USSR and recovered before 1987 by these countries. The
female records were provided by Shimadzu (1987), and male records werecompiled from published records of Omura and Ohsumi (1964), lvashin andRovnin (1967), Ohsumi and Masaki (1975), lvashin (1983), and Japanese and
USSR progress reports to IWC. Only recoveries made on 11 th day or later
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

36

KASUY A AND MIY AS HITA

from the date of marking are used (Figs 10 to 14).
Whale sighting data
Sightings of sperm whales were recorded during eleven whale sighting
cruises conducted in early summer to autumn in 1983 to 1986 using charteredwhale catcher boats Toshimaru No. 15 (September to October in 1985 and1986), Toshimaru No. 18 (June to September in 1986), Toshimaru No. 25 (Juneto August in 1983, June to September in 1984 and 1985, May to June in1986), Shonanmaru (June to July in 1983, June to August in 1984, June to
September in 1985), and Kankimaru No. 58 (September to October in 1986).
The method of sighting is described in Kasuya (l 986a), and is identical to thatused in the Antarctic minke whale assessment cruises (Best and Butterworth,1980). The vessels surveyed 74,274 nautical miles of prefixed trackline spending a total charter period of 761 days. Only presence of sighting effort insquares of one degree longitude and latitude, position of sperm whale sighting
and the estimated body length are used in the present study.
RESULTS

Changes in the fishing season
Ohsumi (l 980c) summarized the history of legal regulation of the sperm
whaling season. Until 1951 season, there was no regulation of sperm whaling
season in the Japanese coastal waters. The season was set since 1952 at eightmonths of April to November, and then altered to May-December (19661975) or August-March (1976177 to present) (Table 1). The industry could
operate whaling in any particular period described above. Each whaling
company owned several land stations at different latitudes along the Pacificcoast of Japan, and operated simultaneously one or two of them according to
the seasonal movement of the whaling ground and target species. Althoughno sperm whales were landed at stations in Kii from 1964 to 1975 (Table 1),
the stations were used during the period for short Bryde's whale seasons.
Four land stations are currently in use for sperm whaling, i.e. at Ohsawa(Nitto Whaling), Ayukawa (Nihon Whaling), and Wadaura (Nitto Whaling
being lent by Gaibo Whaling), and Taiji (Nihon Whaling).
Annual trend in the season of the sperm whale catch from 1955 to 1972
is shown in Table 2 (data of several years are grouped according to the change
in the regulation of the fishing season) and that from 1973 to 1985/86 in Fig.
2. The peak catch gradually moved from August/September in 1955-1960 tci
October/November in 1970-1972 seasons. Even after 1973 season, when
detailed catch data became available including the positions of individual
whales, this trend in the annual shift of the fishing season further continued,
to reach a peak in March in recent years (Fig. 2). It is notable that such
uni-directional change of the peak season preceded the alteration of legal
regulation. Possibly the legal regulation followed the need of the industry.
Sci. Rep. Whqles Res. Inst.,
No. 39, 1988

37

NORTH PACIFIC SPERM WHALESTABLE 1. NUMBER OF WHALING STATIONS** THAT OPERATED SPERM WHALING OFF
THE PACIFIC COAST OF JAPAN AND RELATED WHALING REGULATIONS
Kii
33.5°N

Year
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976177
1977178
1978179
1979/80
1980/81
1981/82
1982/83
1983/84
1984/85
1985/86

Boso
35°N

Sanriku
38°-40.5°N

Hokkaido
43°-43.l 0 N

FishingseasonCatch*

8
8
8
8
8
8
8
8
6
5
4
4
4
4
4
4
4
4
4
4
4
4
2
2
2
2
2
2
2
2
2

4
5
5
5
5
5
5
5
5
5
4
2
2
2
2
1

Apr. -Nov.
Apr.-Nov.
Apr.-Nov.
Apr.-Nov.
Apr. -Nov.
Apr.-Nov.
Apr.-Nov.
Apr.-Nov.
Apr. -Nov.
Apr. -Nov.
Apr.-Nov.
May -Dec.
May -Dec.
May -Dec.
May -Dec.
May -Dec.
May -Dec.
May-Dec.
May -Dec.
May -Dec.
May -Dec.
Aug.-Mar.
Aug.-Mar.
Aug.-Mar.
Aug.- Mar.
Aug.- Mar.
Aug.-Mar.
Aug.- Mar.
Aug.-Mar.
Aug.-Mar.
Aug.-Mar.

1,506
2, 125
2,361
2,588
2,111
2, 121
2,110
1,685
1,715
1,800
1,800
2,101
2,635
3,747
3,667
3,484
3,328
3,020
2,436
2,361
2,307
2,487
2,017
1,730
1,269
1,192
869
439
393
400
400

2
2
I

2
2
2
3
3
2

l***

* Take of females exceeding 15% of the quota has been prohibited since 1979/80 season.
** Given for geographical regions.
*** Operated using a station in Bonin Islands (26.6°N) instead of the Taiji station on the Kii Peninsula.
TABLE 2. MONTHLY CATCH OF SPERM WHALES IN JAPANESE COASTAL
WHALING OFF THE PACIFIC COAST OF JAPAN, EXPRESSED AS THE
PERCENTAGE OF ANNUAL CATCH
Month
Year
1955-60
1961-65
1966-69
1970-72

Apr.

May

June

July

Aug.

Sep.

Oct.

Nov.

6.5
1.4

6.3
3.4
1.4
1.5

6.8
5.5
3.4
1.6

9.4
9.0
2.9
3.4

22.2
20.6
16.5
8.7

26.6
34.5
24.8
20.8

13.6
17.3
22.I
29.3

8.6
8.4
19.l
22.7

* In parentheses are total number of catch representing the monthly catch.
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

Dec.

Total*

9.8
12.0

JOO (12,837)
JOO ( 9,099)
100 (12,150)
JOO ( 9,831)

38

KASUY A AND MIYAS HITA

20
10 1985/86

0 1-------------J---I
10 1984/85
01----------w:-----:---::-",~---;~

10 1983/84

Ot----l 0 1982183~

c 10 1981/82
QI~
QI

01-----a.

10 1980/81

c

0

:c 10

-----1

1979 /80

Ill

.c
~

.....

10 1978/79

0

0

0

10 1977178

zT

01------h1~~-1976/77_ _ _ __,

10 1975176

0~--------1

10 1974175

0 µ;:::::::;:::::.,==1::::__;
10 1973/74

M J

J

A S 0 N D J
Month

F M A

Fig. 2. Monthly distribution of sperm whale catch by .Japanese coastal whaling.

The shift of the peak fishing season continued even after the ban on catching
sei whales in 1977.

Whaling ground
The whaling companies gradually closed their stations in northern Japan
from 196_3-.through 1977, and several major companies ceased coastal whaling
during the period, although this change might not be attributable solely toavailability change of sperm whales but also to that of sei whales or to preferSci. Rep. Whales Res. Inst.,
No. 39, 1988

39

NORTH PACIFIC SPERM WHALESTABLE 3. NUMBER OF SPERM WHALE LANDINGS BY REGION AND WHALINGSEASONS, EXPRESSED AS THE PERCENTAGE OF THE ANNUAL CATCH(BASED ON THE SAME DATA AS TABLE 2)
Land stations
by region

Kii
33.5°N

Boso
35°N

Sanriku
38°-40.5°N

Hokkaido
43°-43.l 0 N

Total

1955-60
1961-65
1966-69
1970-72

1.9
0.7
0
0

0.4
0
0
0

32.l
46.3
71.4
88.5

65.6
53.0
28.6
11.5

100
100
100
100

ence of industry for pelagic whaling over land based operation. Since 1977,
coastal sperm whaling has been operated by two companies using four stations(Table 1).
Most of the sperm whale catch (65%) was landed at stations in Hokkaido(north of 43°N) during 1955-1960, but the proportion continued to decline
after this and landings in Sanriku (38°-40.5°N) increased to attain 88% of thetotal sperm whale catch in 1970-1972 seasons (Table 3). This trend further
continued in 1973 through 1985/86 seasons. In 1973 the major ground wasin the latitudes of 34°-39°N with a peak in 37°-38°N (Figs 3 and 4). In 1977/
78 season the catch occurred also in the additional latitudes of 32°-33°N or
between Boso and Bonin Islands. Then during 1978179 through 1982/83
seasons catch in the northern ground (34°-39°N) declined and became almost
negligible since 1983/84 season. The operation in 1981/82 season was exceptional because a station in the Bonin Islands was used only for one season.
Recent catches occur in winter and mostly in 29°-33°N and 130°-145°£,
which is quite different from the operation in the 1950s and 1960s done insummer and in higher latitudes.
The above change is hard to attribute to short cyclic fluctuations inoceanographic conditions because it lasted too long (over 30 years), nor to aselectivity change between whale species because the change continued both
before and after the ban on catching sei whales in 1977. If we consider thefact that the two whaling stations (Taiji and Wadaura) in the south can process
only meat, and that the other parts of the carcass (viscera, blubber and bone)
have to be transported for the final processing to the northern stations inSanriku (Ayukawa and Ohsawa), which in a straight line are 500 to 700km
apart from the corresponding southern ones, we can reasonably presume thatabove mentioned change in the fishing ground is due to a decline in availability of sperm whales in the northern ground.
Seasonal and latitudinal changes in the coastal operation
Latitudinal and monthly distributions of the catch are shown in Tables
5 to 11. Several fishing seasons are combined according to the above mentioned
changes in the regulation of whaling season and latitudinal range of operation.
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

40

KASUYA AND MIYASHITA

30

1985/86

20
10
0
10

1984/85010

1983/840
101962183..... 0
c 10
Cl.I

u
....

0

Cl.I

a. 10 1980/81
c
0

l/I

Cl.I

ili
.!:.

10

~

0

0

1979/800
10 1978179

0

z 10 1977/78
0
10

1976/770
10

1975/760
10

1974/7501001973174

25

30

-'~
35

1~1:::::1::1,

40

Latitude in degree
Fig. 3. Latitudinal distribution of sperm whale catch by Japanese coastal whaling.

Some catches for research purposes are included.
Data from biologists indicate that the sperm whale catch was composed
mostly of females in summer off Hokkaido (>43°N) and Sanriku (38°-41°N)(Table 4) or in winter Qanuary and February) in 36°-40°N (Ohsumi and
Satake, 1980). These agree with the analysis of prewar statistics by Kasahara(1950; see Table 5) or postwar statistics by Ohsumi (l 980b), and indicate that
Sci. Rep. Whales Res. Inst.,

No. 39, 1988

"[

?:

~
::i::i

Co ""

~~

_l.O~

\JJ

::i::i

<: ~
~·

?

"
"

...

--

-"'!~

~'to

-~

en

CD

'

~

CD

;:;;

---il~~~~~-+~~~~~~1-~~

-,.

~~

-

rn

0

0

0

w
0

0

....
0

Fig. 4. Position of sperm whale catch by Japanese coastal whaling in some selected years. Each mark represents one sperm
whale. Numerals (1 to 9) and alphabets (X, Y, Z) indicate month of the year Qanuary to September, October to December).
Every second individual has been plotted (being selected by the order of processing) for 1973 and 1976/77 seasons, but all
the catches are plotted for other seasons. Each mark represents one sperm whale.

....
0

0

w

Ill,.,...... ""'""'-"'Col ...

0

0

-

....

0

~

-

•,..-:J._.-:...., ... ~~-... ~-- ~-"""" ... -•~~~ ~-;,:.JP..J,

-~

0

mi-;~

.......-" it • ~·-t,•• 'dlE".
x

-

0

.::

-

0

w

0

&;1--+-~~~~~+-~~~~~+-~~~~~~1--~-;

0

~

w

<{:

0

w
~

-

0

w

CD
...,

.+:-

-

~
r:Jl

>

::c:

~

~

"'

r:Jl

c;

'-.j

~......

::c:

~

z
0

42

KASUYA AND MIYASHITA

the fishery exploited the breeding schools.
In the late 1950s, sperm whaling started in April and slightly over halfof the total catch was obtained off Sanriku (April to June), then the groundshifted to Hokkaido to the north Quly to October), but this pattern changed
from the late 1960s so that greater catches were made off Sanriku in any
month (Table 6). And in the three years of 1973 to 1975 when individual
catch positions became available, the season started in May at 34°-39°N, thenthe area of peak catch shifted in summer slightly to the south (33°-36°N), andagain returned to the north (37°-39°N) in autumn (Table 7). The last feature
was the same in the 1976177 and 1977178 seasons when the fishing season was
extended to March (Table 1). Most of the winter (December-March) catch
TABLE 4. PROPORTION OF FEMALES IN THE CATCH OF JAPANESE COASTALSPERM WHALING EXAMINED BY BIOLOGISTS, 1960 THROUGH 1965Land
stations

Sex

May

June

July

Aug.

Sep.

Oct.

Nov.

Hokkaido

malefemale

0
0

8
0
0

39
64
63

246
371
60

444
1345
75

200
691
78

20
73
79

957
2544
73

16
4
20

52
77
60

111
130
54

218
351
62

93
202
68

49
124
72

15
78
84

554
966
64

(%)
Sanrikumale
female(%)

Total

TABLE 5. PROPORTION OF FEMALES IN THE CATCH OF JAPANESE COASTAL SPERM
WHALING IN 1940 THROUGH 1948, WHEN THERE WAS NO REGULATION OF
SEASON OR QUOTA (EXTRACTED FROM TABLE 29 IN KASAHARA, 1950)
Month
Kuril Is

Jan.

Feb.

Mar.

M
F

%
Hokkaido

July

Aug.

Sep.

Oct.

Nov.

144

440
62
12

558
101
15

189

40
4
9

2
0

0

0

0
35
11
24

44
9
17

27
16
37

33
10
23

268
130
33

257
179
41

71
105
60

16
18
52

421
661
61

330
528
62

448
534
54

422
522

55

255
151
37

2

14

0

0

0

100
124
55

73
69
49

75
37
33

90
38
30

155
143
48

357
221
38

444
526
54

16
4
20

20
3
13

11
3
21

43
24
36

56
13
19

12
1
8

12
5
29

M
F

17

%

0

55
56
50

148
209
59

116
196
63

17
65
79

M
F
M
F

Dec.

338
11
3

14
13
48

%
Bonin Is

June

8

%
Boso
&Kii

May

6
3
33

M
F

%
Sanriku

Apr.

3
0

0

0

M: number of males, F: number of females, %: female proportion in %.
Sci. Rep. Whales Res. Inst.,

No. 39, 1988

43

NORTH PACIFIC SPERM WHALES

occurred at 36°-40°N (Table 8).
Off the Pacific coast of Japan the fronts of the cold Oyashio Current andwarm Kuroshio Current situates at around 35°N and 40°N, respectively, andthe surface water temperature in the intermediate area alternates seasonallyclose to either of the two currents with considerable annual and local variations
TABLE 6. LATITUDINAL DISTRIBUTION OF THE SPERM WHALE
CATCH BY JAPANESE COASTAL WHALING IN THE PACIFIC,
1955 TO 1972 SEASONS
Wh~ling

stations

Apr.

May

June

July

Aug.

Sep.

Oct.

Nov.

1955 to 1960
Hokkaido
Sanriku
Boso
Kii

341
314
40
144

280
500
8
21

264
548

781
420

2211
625

2792
612

1419
332

334
770

66

8

7

8422
4121
48
246

1961 to 1965
Hokkaido
Sarn:iku
Kii

44
37
45

31
269
10

155
336
5

416
401
2

770
1103

1883
1255

1061
515

466
295

4826
4211
62

1966 to 1969
Hokkaido
Sanriku

5
161

81
330

52
306

235
1765

880
2138

1086
1603

896
1425

236
951

3471
8679

1970 to 1972
Hokkaido
Sanriku

17
128

51
109

53
283

65
792

126
1920

414
2458

376
1848

16
1163

1130
8701

Dec.

Jan.

Feb.

Total

TABLE 7. LATITUDINAL DISTRIBUTION OF THE SPERM WHALE CATCH BY
JAPANESE COASTAL WHALING IN THE PACIFIC, 1973 TO 1975 SEASONS
Latitude

May

June

July

Aug.

Sep.

5
5

8
2
4

7
2

14
14
I

43°-44°N
42°-43°N
41°-42°N
40°-41°N4
2

39°-40°N
38°-39°N
37°-38°N
36°-37°N
35°-36°N

3
12
16
26
33

10
21
18
57

2
8
56

4
29
56

34°-35°N
33°-34°N
32°-33°N
31°-32°N

19
4

97
16

34
17

Total

120

231

133

Sci. Rep. Whales Res. Inst.,
No. 39, 1988

Oct.

Nov.

Dec.

Jan.

7
8
3
23
151
297
867
184

40
14

127
28
5

37
19

154

496

1596

67
228
645
1036
75

98
283
861
1078

10
33
25

8

2051

2339

Mar.

Total
8
43
30
19

11

4
3
26
91
183

Feb.

196
688
1882
3186
669
362
98
6

68

7188

44

KASUY A AND MIYASHITA(Mizuno and White, 1983), and possibly with a cyclical fluctuation of 8 to 10
years (Uda, 1954). The boundary area probably corresponds to the westernpart of the Subarctic Boundary of Favorite, Dodimead and Nasu (1976). The
seasonal shift of the sperm whaling ground shown above is the reverse of theseasonal movement of the oceanography, and can be explained only by
assuming that there were two groups or stocks of female sperm whales mainly
inhabiting each in the Kuroshio and Oyashio Currents. The industry exploited
more northern stock individuals in summer and autumn in postwar period
TABLE 8. LATITUDINAL DISTRIBUTION OF THE SPERM WHALE CATCH BY JAPANESE
COASTAL WHALING IN THE PACIFIC, 1976177 AND 1977178 SEASONS
Latitude

May

June

July

Aug.

Sep.

Oct.

Nov.

Dec.

0

40°-4l N
39°-40°N
38°-39°N
37°-38°N
36°-37°N
35°-36°N

63
3

34°-35°N
33°-34°N
32°-33°N
31°-32°N
30°-3l 0 N

3
11

43
7
25

116
270
18

51
23
18

18
34
20

7
38
385
647
3

Jan.

Feb.

Mar.

Total

51

6

9

66

315
146
95
416
53

75
182
213
562
8

26
129
203
216
8

429
498
1058
2118
115
95
83
49

26
22

2

2

29°-30°N
28°-29°N
27°-28°N
Total

7

14

173

478

1080

1124

1046

592

4514

TABLE 9. LATITUDINAL DISTRIBUTION OF THE SPERM WHALE CATCH BY JAPANESE
COASTAL WHALING IN THE PACIFIC, 1978179 THROUGH 1980/81 SEASONS
Latitude

May

June

July

Aug.

Sep.

Oct.

Nov.

Dec.

Jan.

0

40°-4l N
39°-40°N
38°-39°N
37°-38°N
36°-37°N
35°-36°N
34°-35°N
33°-34°N
32°-33°N
31°-32°N
30°-3l 0 N
Total

7

Mar.

Total

77

15

92

27
83
251
57

41
91
113
101
102

60
130
244
131
34

102
276
531
652
309

2

l

3

4

25
43

23
86
143
66

3
5
4

47
61
13

41
112
86
15
3

26
71
37
9

188
150
87
6

118
441
161
l

497
949
497
57
26

575

561

956

1335

3988

4

2

Feb.

I

7

ll

4

16

74
109
109
6
3

28

155

371

Sci. Rep. Whales Res. Inst.,
No. 39, 1988

45

NORTH PACIFIC SPERM WHALES

before the 1960s, and started in the early 1960s to take a significant catchfrom the southern stock on the summering ground and from the northernstock on the wintering ground. This interpretation have some agreement withTABLE 10. LATITUDINAL DISTRIBUTION OF THE SPERM WHALE CATCH BY JAPANESE
COASTAL WHALING IN THE PACIFIC, 1981/82 AND 1982/83 SEASONS
Latitude

May

June

July

Aug.

Sep.

41°-42°N
40°-4l 0 N

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

Total

2

39°-40°N
38°-39°N
37°-38°N
36°-37°N
35°-36°N

2
12
8
11

4

4

34°-35°N
33°-34°N
32°-33°N
31°-32°N
30°-31°N

8
12
21
33
5

6
10
8
26
19

29°-30°N
28°-29°N
27°-28°N
26°-27°N
25°-26°N

2
7
17

113

19
5
35
21

16
41
8
24
11

21
3
4
10

10

9

3
2

107

96

3

32
42
25
30
13

43
60
36
34
22

55
72
47
34
27

35
79
24
2

97
203
91
150
69

22
36
10

22
47
34
17

50
48
6

41
27
133
147
38

177

298

439

1234

24°-25°N
Total

4

TABLE 11. LATITUDINAL DISTRIBUTION OF THE SPERM WHALE CATCH BY JAPANESE
COASTAL WHALING IN THE PACIFIC, 1983/84 TO 1985/86 SEASONS
Latitude

May

June

July

Aug.

Sep.

Oct.

Nov.

Dec.

Jan.

40°-41°N
39°-40°N
38°-39°N
37°-38°N
36°-37°N
35°-36°N

534°-35°N
33°-34°N
32°-33°N
31°-32°N
30°-31°N

2

20
14
12

12
31
21
33
29

3
22
8
47
75

23
25
4
46
30

29°-30°N

20

17

7

Total

151

175

181

Sci. Rep. Whales Res. Inst.,
No. 39, 1988

53
18
29
35
31

166

Feb.

Mar.

To~al

7

34

41

19

19

3
5
5

7

30
21
23

7
65
42
36
42

67
60
28
49
41

165
221
132
246
248

2

46

307

1192

212

46

KASUYA AND MIYASHITA

the previous analyses of fishery oceanography. Uda (1954) states that sperm
whales are scarce in the middle of the Kuroshio Current System (Kuroshio
Current and Kuroshio Extension), but are dense on both sides of the current.
Several authors indicated bimodal catch of sperm whales off the Sanriku coast;
July-August and October-November (Kasahara, 1950), or July-early September
and early October-late November (Uda and Dairokuno, 1957). Although it isunclear if their ocean-ographical terminology accurately corresponds to ours,
there seems to be some similarity with the result of the present study.
In the 1978179 season the industry started operating both in the southern(32°-35°N, autumn) and northern (36°-39°N, winter) latitudes (Table 9), andthis pattern lasted till the 1980/81 season. Although this pattern was principally
retained in the 1981/82 season, the industry took sperm whales in autumn/
winter using a land station in the Bonin Islands with resultant catch in 25°300N only in this particular year (Table 10 and Fig. 4). Sperm whales takenfrom the northern stock on their wintering ground (35°-40°N) continued tillthe 1982/83 season, but it became negligible from the 1983/84 season when
the fishery expanded to the southwest (Table 11 and Fig. 4).
The above analysis is summarized as follows. Although both stocks had
been hunted by Japanese coastal whaling since before World War II, in the
postwar period before the early 1950s most of the sperm whales were caught
from the northern stock. A significant number of whales started to be takenfrom the southern stock probably in the late 1950s in the northern part ofits summer range, while operations in other seasons took mainly from thenorthern stock. However, the proportion of whales from the southern stockprobably did not show rapid increase till the 1978179 season, because the takefrom the northern stock on the wintering ground continued by extending thefishing season to March. Exploitation of the southern stock for the entire
whaling season started in late 1970s, and the hunting of the northern stock
became negligible since the early 1980s.
Other information on sperm whale distribution
Fig. 5 shows the distribution of sperm whales taken in Japanese waters
by American whaling operations during the middle 18th to early 20th centuries(Townsend, 1935). The catch off the Pacific coast of Japan occurred in thosedays in two isolated areas, one in 35°-40°N and west of 150°E, and the otherin an area extending from 28°N, 135°E to 30°N, 180°. The main season for
the former ground was May-July, and that for the latter was April-July (westof 143°E) or June-September (east of 143°E). The former ground probably
represents sperm whales of the northern stock which winter there and havebeen hunted by modern coastal whaling in the early summer of the 1973197 5 seasons and in the winter of the 1977-1983 seasons. However, there
remains a possibility that some individuals from the southern stock areincluded in the periphery of the main range. The hiatus in distribution
between the two fishing grounds roughly corresponds to the low density area
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

47

NORTH PACIFIC SPERM WHALES.

.--~~~~~~~~-.---r:=-~~~~-,,~-,50°N

o•Apr. to July
Aug. to Oct.

0 'Fig. 5. Distribution of sperm whale catches from the records of i 9th century American
whale fishery. Plots represent positions of vessels on days when one or more sperm
whales were taken (modified partial reproduction of Townsend, 1935).

in the middle of the Kuroshio Current System indicated by Uda (1954).
Although the northern range of the southern stock of females approximately
coincides with the Kuroshio Front, its primary habitat will be to the south of
the Kuroshio Current and its extension.
Recent whale sighting cruises in summer off the Pacific coast of Japan
indicate that most sperm whale sightings are limited to the south of the frontsof the Kuroshio Current System, and that the density is high to the south ofthese currents, which agrees with the feature observed in America whaling
and recent Japanese coastal whaling operations. Spending a considerable
sighting effort off the Pacific coast of northern Japan, we have confirmedthat recent sperm whale density is extremely low in summer to the northof 35°N (past coastal whaling grounds off Sanriku and Hokkaido) (Figs 4,
6 and 7).
In the northern North Pacific and Bering Sea only 15 sperm whales in
4 schools have been sighted during five Dall's porpoise sighting cruises witha total period of 202 days from May to October, i.e. 17 August to 19 September
1982 (Kasuya, l 982a; Kasuya and Ogi, 1987), 10 August to 11 September
1983 (Ogi, 1983; Kasuya and Ogi, 1987), 10 May to 20 June 1984 (Miyazaki,
Fujise, Komuro and Taketomi, 1984), 6 August to 16 September 1985 (Ogi,
Tanaka, Kuramochi and Yamamoto, 1986; Kasuya and Ogi, 1987), and 9
August to 6 October 1986 (Yoshioka, Ogura and Shikano, 1987). Because
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

48

KASUYA AND MIYASHITA

30°N

20°N

120°E

130°E

140°E

150°E

160°E

Fig. 6. Distribution of sperm whales observed during the whale sighting cruises in1983 to 1986, May to July. Star indicates school containing individuals 46 feet orover in body length, and closed circle school of smaller whales. Shaded area
represents one degree squares of latitude and longitude not surveyed.

Dall's porpoises are distributed to the north of the Subarctic Boundary(Kasuya and Jones, 1984), the survey area covered past grounds for sperm
whales to the north of 40°N. Although all the cruises had experienced whale
biologists on board, only the 1982 cruise had an observer with experience ofcommercial whaling. The target species of the cruise was the Dall's porpoise
which was usually sighted within 0.5 nautical miles from the trackline(Bouchet, Ferrero and Turnock, 1986; Kato and Miyazaki, 1986). These
factors could have caused a lower sperm whale sighting efficiency of thecruises compared with those off Japan (Figs 6 and 7). However, we still
consider that these data indicating low sperm whale density in the previous
whaling grounds are significant, because the number was less than one fourthof the sum of the blue, fin, sei and humpback whales identified to species (51individuals in 20 schools). The historical southward shift of the sperm whaling
ground (Ohsumi and Masaki, 1977) will therefore indicate the density decline
in the northern grounds.
It will be concluded from this that the sperm whales that once wintered
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

49

NORTH PACIFIC SPERM WHALES

Aug. to Oct.

30°N

20°N
120°E

13Q0 E

150°E

160°E

Fig. 7. Distribution of sperm whales observed during the whale sighting cruises in1983 to 1986, August to October. For marks see Fig. 6.

in the western North Pacific north of 35°N and were hunted by the Japaneseand USSR coastal and pelagic whaling operations have been extremely
depleted.
Segregation of adult males
Table 12 shows the frequency of estimated body lengths of sperm whales
listed in Figs 6 and 7. The majority of them were individuals below 40 feet(12.lm) and about 5% of the individuals with body length estimates werecalves (~20 feet), indicating that the North Pacific area south of the Kuroshio
Current System is inhabited in early summer to autumn season by breeding
schools of the sperm whale.
However, some sperm whales have been estimated at over 46 feet
(13.9m) in body length, which are expected to be mature males in the North
Pacific and Southern Hemisphere (Ohsumi, 1966; Best, 1979) (Table 13).
These individuals were singletons or found with relatively large individuals(over 42 feet or 12.7m), suggesting that adult males were solitary or formed
schools with pubertal males but they were not in breeding schools. Thisagrees with the observation of sperm whales off the Galapagos Islands thatthe mature males are associated with a particular group of females only fora periods of hours at a time (Whitehead, 1987). Although all six schools ofadult males in June and July were sighted in latitudes similar to that of thebreeding schools (Fig. 6), many of those in August tended to be segregatedSci. Rep. Whales Res. Inst.,
No. 39, 1988

50

KASUYA AND MIYASHITATABLE 12. COMPOSITION OF ESTIMATED BODY LENGTHS (IN FEET) AND NUMBER
OF CALVES IN THE SPERM WHALE SIGHTINGS IN FIGS 6 AND 7
Body
lengthMay toJuly

Augustto Oct.

Body
lengthMay toJuly

Augustto Oct.

:'513
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35

8

4

2

2

36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
Total
Calves*

40
40
45

6
8
8

42
8
27
5
3
4
5

8

I

3
3

5

13

2
7
2
3
18

2
2
17
3
31
16
24
25
19
35

40
15
32
57
34
78

3
1
2

543
62

3
6
2
4

6
5
8
2

256
34

* Number of calves, includes both those having body length estimation and listed in the frequency andthose without length estimation.

TABLE 13. POSITION AND SCHOOL STRUCTURE OF LARGE MALE SPERM WHALES(~46 FEET) IN TABLE 12, FIG. 6 AND FIG. 7
Date

Position

15 June '83
23 June '86
3 June '84
9 July '83
16 July '84
22 July '84
6 Aug. '86
13 Aug. '84
13 Aug. '84
18 Aug. '84
18 Aug. '84
18 Aug. '84

30°29'N, 137°32'E
24°59'N, 147°57'E
31°31'N, 143°26'E
22°33'N, 158°05'E
12°36'N, 147°4l'El5°5l'N, 158°47'E
26°37'N, 142°44'E
34°23'N, 152°36'E
34°29'N, 152°24'E
38°ll'N, 157°26'E
38°03'N, 157°15'E
37°57'N, 157°21'E

School
size

1
5
9

9
9

2

Body len~th
composition (ft)53
48
50
46X2, 48X2, 50
42X3, 44X3, 46X3
49
50X4, 51X3, 52X2
50X2, 51, 52X4, 54, 55
53
52
51, 52
53
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

51

NORTH PACIFIC SPERM WHALES

20

100

10

50

0

0

....0 20

200

CJ)
(b

.,,
E

....
(b

iU

<II

.,,Ol 10

-

100

(b

.....
(b

£
~

....0

c

u

CJ)
(b

0

0

Cl.

-

20

0

z
100

10

50

A

M

J

J

A

0

Month
Fig. 8 Catch of sperm whales by USSR coastal whaling off the Kuril Islands in 1950,
1951, and 1962 through 1964, expressed by sex, month and position of land stations(compiled from USSR IWC/NP Forms). Closed circle and solid line: proportion of
females in the catch; open circle and dotted line: number of female catches (actual
number); triangle and dotted line number of male catches (x0.25 for 46°-5G°N,
xo.5 for other areas).

to the north (34°-40°N) of the breeding schools (Fig. 7).
The breeding of sperm whales is seasonally diffuse, and in the northernhemisphere conceptions occur most frequently in April-May and least in
October-November (Ohsumi, l 96S; Shimadzu, 1987). The distribution of
adult males appearing in our sighting records suggests that adult males of thesouthern stock leave the breeding ground after the mating peak and migrate
to northern latitudes. However, we consider from their presence in 34°-40°N
in August that they may not migrate so far north as the Bering Sea/AleutianIslands waters (>S0°N) but probably remain during summer in the Oyashio
Current and its eastern extension (Subarctic Current) in 40° -S0°N, or just to
the north of the front of the Kuroshio Current System at 3S 0 -40°N.
Sexual segregation of sperm whales in the Oyashio Current area is seenin the operations of USSR coastal whaling off the Kuril Islands (Fig. 8). The
season of female catch was shorter, and the female proportion in the catch
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

52

KASUY A AND MIYAS HITA

was lower in the northern Kuril Islands. The peak of female catch in thenorthern Kuril Islands was slightly earlier than that of the males in the same
region (50°-51°N). Such features are expected when females are segregatedto the south and arrive later and return to the south earlier than the males.
The seasonal change in the sex ratio is apparent only in the northern part,suggesting that male density is probably higher off the northern Kuril Islands
than the south. Off the Pacific coast of northern Japan and during the same
period (early 1960s), the female proportion was slightly higher off the northern(Hokkaido) land stations than those off the southern (Sanriku) ones, and it
increased from summer to autumn/early winter in both areas (Table 4). The
similar seasonal trend is observed in the prewar catch in the 1940s (Table 5).
The absolute value of the sex ratio is not directly comparable between the
Kuril and Japanese coastal fishery, or between prewar and postwar operations
off Japan, because both the selectivity of sexes and availability of whales
might be different. However, the above feature suggests that one of theconcentrations of female sperm whales in the western North Pacific exists inthe summer season off the Kuril Islands area, and that it migrates off Hokkaido
and Sanriku waters in autumn/winter season. The period of high maleproportion was probably longer off Hokkaido (May to August) than off
Sanriku (March to April) during the 1940s (Table 5). This suggests that moremales were distributed off Hokkaido (and possibly to further north) than off
Sanriku.
Supporting evidence for this is found in the catch of sperm whales byJapanese pelagic whaling presented by Smith (1980). If the catch between
l 58°E and 180° is totalled latitudinally from his data, there appears in both
sexes two major latitudinal concentrations of the catch (Fig. 9). There is asmall hiatus in the catch, especially of females, in 50° -52°N and west ofl 72°E. We consider this relates to a local intrusion of cold water in the region(see below). Ignoring this small hiatus, we find larger hiatus in the catch inlatitudes 32°-44°N. The latitudes of low sperm whale catch correspond to avery distinct hiatus in whale sightings in 30°-40°N and 154°E- l 7 5°E apparent
in Ohsumi, Masaki and Wada (1977), and to the waters between the front of
the Kuroshio Extension (Mizuno and White, 1983) and the Subarctic Boundary(Favorite et al., 1976). The proportion of females in the catch is high on bothsides of these latitudes, or in 45° -55°N and south of 30°N.
The distribution of the catch of female sperm whales by the Japanesepelagic fishery (Smith, 1980; Shimadzu, 1987) also shows geographicalconcentrations, two in the western North Pacific and one in the eastern North
Pacific (Fig. 15). The two in the western North Pacific correspond to theabove mentioned latitudinal concentrations of female sperm whales. Although
the south western extension of the northern female concentration is unclear in
Smith ( 1980) and Shimadzu ( 1987) due to the prohibition of pelagic whaling
in the eastern area by the Japanese Government (see Materials and Method),the above analysis of the catch of USSR and Japanese coastal fisheries suggests
Sci. Rep. Whales Res. Inst.,

No. 39, 1988

53

NORTH PACIFIC SPERM WHALES
2000

No. males

1500

1000

500

300
200

100

0~---.1.----'-~-'-~0El::"-----'-~-'-~-'-~~

60
40
20
OL-__JL--__i~--'-~-r-c:~-'-~--'-''---'-~'~

200

30°

40°

50°

60°

LatidudeFig. 9. Latitudinal distribution of sperm whale catch by Japanese pelagic whaling in
1966-1977, between l58°E and 180° longitude (compiled from Smith, 1980).

that it will have extended to the southern Aleutian Islands (Fig. 15).
From these analysis we conclude that there are two concentrations offemale sperm whales in the western North Pacific, one in the Western North
Pacific Gyre to the north of the Oyashio Front or Subarctic Boundary and theother to the south of the front of the Kuroshio Current System. Ohsumi andNasu (1970) indicated that the northern limit of summer range of the former
concentration coinsides with the surface water temperature of 9°C. Adult
males are segregated in non-mating season to the north of these female
concentrations. There is some overlap between the ranges of adult males andtheir breeding population even in the non-breeding season. The summer
ranges of adult males of the southwestern North Pacific stock and adultfemales of the north western North Pacific stock overlap considerably, althoughSci. Rep. Whales Res. Inst.,
No. 39, 1988

54

KASUY A AND MIYASHITA

150°£

160°£

Fig. 10. Movement of marked female sperm whales, USSR marks only. Includes only
marks recovered over 10 days after marking. Closed circle represents position ofmarking, open circle position of recovery, and numeral by the circle month ofmarking or recovery. For movement outside of this range see Table 14.

the overlap between individuals of the corresponding sex or growth stages
from different stocks seems to be less common. This segregation will befurther analyzed using information on movement of marked whales (seebelow).
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

NORTH PACIFIC SPERM WHALES

55

Fig. 11. Movement of marked female sperm whales, Canadian, Japanese, USA and
USSR marks (excluding those listed in Fig. 10). Recoveries within 10 days or lessfrom marking or movement within the dotted area are excluded. For movement
outside this range see Table 14, and for symbols see Fig. 10.

Evidence from the movement of marked whales
If the widely accepted assumption of a single sperm whale stock in thewestern North Pacific is correct, there will be a significant number of marked
whales moving between north of 40°N and south of 30°N. However, if there
are two latitudinally segregated sperm whale stocks, we will see no mixing
between the two isolated latitudes. We do not consider the movement within
30°-40°N latitudes or between these latitudes (30° -40°N) and either of theranges of the two hypothetical stocks to be meaningful. The intermediate
latitudes can be seasonally inhabited by the two stocks, or simultaneously byschools of the two stocks. It is also important to note that since such marking
data have been collected over many years they exaggerate the intermingling
between stocks in a season of a particular year.
Large numbers of recaptures of marked females off Hokkaido andSanriku (Figs 10 and 11) are due to a high sperm whale catch in the area.
Their movement indicates that breeding schools taken off the Pacific coast ofJapan have a connection with those in the northwestern North Pacific orsouthwestern North Pacific including those in the Bonin Islands waters. The
females off Japan and the Kuril Islands waters migrate to 160°W-l 70°W,
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KASUY A AND MIY AS HITA

while females from the eastern North Pacific waters migrate l 70°E-l80° in
higher latitudes (40°-50°N) or between 160°W-l 70°W in lower latitudes(20° -30°N). No direct connection of female sperm whales is indicated between
western and eastern North Pacific. This does not support the assumption of
two stocks in lower latitudes of the western North Pacific (Bannister and
Mitchell, 1980), but agrees rather well with the conclusion of Ohsumi and
Masaki (1977) that sperm whales in the western and eastern North Pacificintermingle in the central North Pacific, although their deduction of a single
stock in the entire latitudinal range of the western North Pacific disagrees
with ours.
Available data indicate no female movement between north of 40°N andsouth of 30°N. We consider that this is indirect support of the presence oflatitudinally segregated sperm whale stocks in the western North Pacific.
There is one female that was marked by Japan in May 1967 in 48°00'N,
177°49'E and recovered in August 1976 in 25°0l'N, 166°27'W. Although thismight appear to represent an example that violated this hypothesis, we interpreted this in another way. It was recovered with a female that was marked
by USSR in August 1960 in 43°40'N, l 37°44'W, thus the school could havebelonged to the eastern North Pacific stock of Ohsumi and Masaki (1977),
which might have a wider latitudinal range (see below).
Movement of female sperm whales from the summering ground off theKuril Islands to the wintering ground off Hokkaido and Sanriku (north western
North Pacific stock) is indicated by numerous whale mark recoveries. Movement of females of another stock (southwestern North Pacific stock), which
may winter in the Bonin Islands waters or further south, to Sanriku/Hokkaido
waters for summering is indicated by six females (Figs 10 and 11).
Ohsumi and Masaki (1975) considered a Japanese whale mark No. 7741
to indicate female movement between the Bering Sea and the Gulf of Alaska.
However, accepting that such a rare case may occur, we ignored this particularmark return and considered that there are currently no reliable evidence
supporting such movement. This mark was recorded to have been fired at afin whale but reportedly recovered from a female sperm whale by a USSR
factory ship which processed both fin and sperm whales on that day (Shimadzu,
1987). We also ignored another Japanese mark No. 8394 apparently indicating
movement of a sperm whale of unknown sex between the Bering Sea (52°15'N,
I 76°00'W) and the central North Pacific (37°11 'N, l 76°10'E). This was firedin the Bering Sea in 1961 at an 11 m sperm whale and recovered by a USSR
fleet in 1972, but no information was provided on the sex, body length, or
how it was recovered. It is possible that the mark was recovered from a cooker.
The movement of this mark, if it is accepted, does not change our conclusion.
Since the distribution of adult males is different from that of the pubertal
or juvenile males (Best, 1979; Ohsumi, 1966), we analyzed movement by
growth stages, roughly classifying the maturity from body length at recapture,i.e. 2::46 feet (13.9m) as mature and ~45 feet (13.6m) as immature. There are
Sci. Rep. Whales Res. Inst.,
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NORTH PACIFIC SPERM WHALES

40°N

1500E

170°E

180°

1600W

Fig. 12. Movement of marked adult male sperm whales (~46 feet at recapture),using Canadian, Japanese and USSR marks recovered over 10 days after marking.
For symbols see Fig. 10.

20 whale marks recovered from adult male sperm whales, five from theBering Sea, 10 from the Kuril Islands and Pacific coast of Japan, and fivefrom the eastern North Pacific. And there are no mark returns indicating
movement between any of these areas (Fig. 12). We consider this significant
as an indication of segregation between adult males of different stocks.
Two concentrations of adult males identified in the summer season in
the western North Pacific and Bering Sea are located to the north of thesummer range of breeding schools of the corresponding female populations
mentioned above, i.e. the Bering Sea adult male concentration to the northof the females of the northwestern North Pacific stock summering in thewestern Aleutian/Kuril Islands area, and the Kuril Islands/Hokkaido/Sanriku
concentration to the north of the females of the southwestern North Pacificstock summering in the Kuroshio Current System (Fig: 15). This suggests that
adult males from each of the two western North Pacific stock migrates after
the mating season to the north of the female range. This will function to
decrease competition for food resources between adult males and breeding
schools of the same stock (Best, 1979).
The summer range of the adult males from the southwestern North
Pacific stock mostly overlaps with that of the females of the northwestern
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KASUY A AND MIYAS HIT A

Fig. 13. Movement of marked immature male sperm whales (::=;45 feet at recovery),using Japanese marks recovered over JO days after marking. For movement outside
this range see Table 14, and for symbols see Fig. 10.

North Pacific stock. We do not consider that this will mean free interbreeding
between stocks (see Discussion).
The movement of immature males (:=;45 feet or 13.6m) in the western
North Pacific has characteristics of both females and adult males, or in otherwords it is intermediate of the two in many cases. Many of such male recaptures
show movement within the range inhabited by the females, i.e. between
Sanriku/Hokkaido area and south of the western Aleutian Islands area or
between Sanriku/Hokkaido area and Bonin Islands/western Taiwan area.
However, some of the immature males showed movement between the Bering
Sea and female range south of the western Aleutian Islands or movement
within the Bering Sea as seen in the adult males. Another seven immature
males probably of the northwestern North Pacific stock moved to waters
surrounding the eastern Aleutian Islands near to l 70°W. These indicate assuggested by Best ( 1979) that males leave the breeding school at the puberty
and tend to migrate to higher latitudes, and provides support for the assumption that the adult male sperm whales migrating into the Bering Sea are thosefrom the western North Pacific stock.
Sci. Rep. Whales Res. Inst.,

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NORTH PACIFIC SPERM WHALES

so•N

30°N
180°

Fig. 14. Movement of marked immature male sperm whales ($45 feet at recapture),using USSR marks recovered over 10 days after marking. For movement outside
this range see Table 14, and for symbols see Fig. 10.

Although further accumulation of data, which is unlikely given thecurrent situation of whaling, may show some adult or pubertal males migrating
between the Bering Sea and the Gulf of Alaska through the eastern AleutianIslands, currently available data do not indicate such movement.
Identity of eastern North Pacific sperm whales
This subject is analyzed using only the catch distribution of sperm whales
by Japanese pelagic whaling reported by Smith (1980) for 1966 to 1977 andShimadzu (1987) for 1966 to 1979, the movement of marked whales, and theanalogy with the stocks in the western North Pacific.
The female catch by Japanese pelagic whaling in the eastern North
Pacific (June to September in 1966-1979) concentrated in two latitudinalzones centered at 43°N and 52°N, which merged at the eastern part (Fig. 15).
The presence of two latitudinal concentrations does not seem to be theariifact of biased fishing season, because this pattern appears in July andAugust of the monthly total of the 14 years catch or yearly totals of the seven
whaling seasons when a considerable catch has been made in the eastern
North Pacific (Appendix Figs 1.1 to 2.15 of Shimadzu, 1987). Rather, weconsider that this female concentration relates to the relatively warm area of
the Alaskan Stream, and that the low catch of females in the western part ofSci. Rep. Whales Res. Inst.,
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KASUY A AND MIYASHITA

Fig. 15. Distribution of three sperm whale stocks in the North Pacific suggested in
the present study. Solid line: northwestern North Pacific stock; Dotted line:
southwestern North Pacific stock; Chain: eastern North Pacific stock. For eachstock, thick line indicates a total geographical range for the stock (based on markrecapture data), thin line northern or western limit of females in summer season(based on mark-recapture data), shaded area major concentration of adult males insummer after mating season (based on mark-recapture and sightings data), dotted
area high concentration of females in summer season (constructed from Smith,
1980, Shimadzu, 1987, USSR coastal fishery off Kuril Islands, and whale sightingdata).

the Alaskan Gyre relates to the low water temperature (Fig. 16). We find asimilar hiatus in female distribution in the western part of the Western North
Pacific Gyre (see above) and will interprete it in the same manner as the effectof the intrusion of cold water indicated by Favorite et al. (1976).
From analogy with the western North Pacific female concentrations, wepresume that the summer concentration of females in the Alaskan Gyre
represents the breeding population of a sperm whale stock in the eastern
North Pacific. Shimadzu (1987) reported a small scale concentrations offemale catch in July 1978 around 25°N, 130°W and 25°N, 152°W (Fig. 15).
We wondered if these female concentrations represent a breeding populationof another sperm whale stock in the eastern North Pacific. However, there isno supporting evidence at present for that assumption. Rather, the currently
available marking data of females and the movement of an immature malethat wintered in the southern Mexican waters ( l 7°N) and summered in the
Alaskan Gyre (46°N) (Table 14) suggest a single sperm whale stock distributed
widely in the eastern North Pacific, i.e. breeding schools moving between
Mexican waters, Hawaiian ground, Alaskan Gyre, and waters to the south of
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NORTH PACIFIC SPERM WHALES

60°

50°

40°
30°
20°

60°

50°

40°
------26

30°
28

20°
140°

160°

180°

160°

140°

Fig. 16. Top. Major current systems in the North Pacific in July through September(based on Dodimead et al., 1963; Favorite et al., 1976; Maritime Safety Agency,1982). Keys: !=Okhotsk Sea Gyre, 2=Western North Pacific Gyre, 3=Bering Sea
Gyre, 4=Alaskan Gyre, A=Alaskan Stream, C=California Current, E=Kuroshio
Extension, K=Kuroshio Current, NE=North Equatorial Current, NP=North
Pacific Current, O=Oyashio Current, S=Subarctic Current, T=Tsugaru Current.
Bottom. Fifteen years (1966-1980) mean of the surface water temperature0( C) in the North Pacific in August (after Maritime Safety Agency,1982). The
surface water temperature in February will be about 10°C lower than that in Augustin the middle latitudes (30° -40°N).

the eastern Aleutian Islands.
Quite different from the two sperm whale stocks in the western North
Pacific, the movement of marked whales in the eastern North Pacific does notdemonstrate post-mating latitudinal segregation between adult males andbreeding population, but suggests segregation of adult males to the west of
the concentration of the breeding schools. This agrees with the fact that theSci. Rep. Whales Res. Inst.,
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KASUY A AND MIYASHITA

62

latitudinal segregation of males was unclear in the catch of the Japanese
Pelagic fishery in the eastern North Pacific but the proportion of males washigh in the western part of the Gulf of Alaska (Fig. 17). This segregation
pattern has been mentioned by Ohsumi and Nasu (1970). As mentioned
above, the western part of the Alaskan Gyre is covered, in July to September,by water which is a few degrees colder than the surrounding area of the Gulf
of Alaska (Fig. 16). We consider that the above adult male segregation relates
to the presence of this cold water area and has the same biological significanceas in the north/south segregation of sperm whales in the two western North
Pacific stocks (see above). The range of pubertal males of this stock probably
extends further to the west along the southern coast of Aleutian Islands(indicated by 44 feet and 35 feet males in Fig. 13). Although the AleutianIslands may not absolutely inhibit the migration of sperm whales (see above),the relatively shallow water depth (<500m) may decrease the chance of theirmigration between the Bering Sea and the North Pacific through the eastern
Aleutian Islands.
Two whales (a juvenile male and a female) marked in the northernhemisphere (< 10°N) in March and May were recovered in the southernhemisphere (4°-5°S) in November and March, respectively (Table 14). We do
not know the relationship between these individuals and the eastern North
Pacific sperm whale stock, but there may remain some possibility that there
is one sperm whale stock in equatorial waters or that the range of a southernhemisphere stock seasonally crosses the equator in the eastern Pacific.

TABLE 14. MOVEMENT OF MARKED SPERM WHALES IN THE EASTERNNORTH PACIFIC NOT LISTED IN FIGS 10 THROUGH 14
Marking

Recapture

Mark no.
date

position

date

positionbodylength29/05/1965
1011211965
10112/1965
19/08/1963
08/05/1963
07/05/1975

54°10'N, 141°50'W
35°58'N, 120°43'W
35°58'N, 120°43'W
40°00'N, 145°08'W
49°53'N, 129°20'W
02°52'N, 094°55'W

26/06/67
17/0717146°20'N, 134°26'W
48°35'N, 131°26'W

ll.6m
ll.6m

10/06/68
22/07/65
20/03176

43°37'N, 131°19'W
48°55'N, 127°57'W
04°40'S, 082°50'W

I0.5m
I0.7m
9.8m

18/08/1963
01/03/1966
29/03/1975

39°23'N, 144°27'W
17°1 I 'N, 102°32'W
09°07'N, 093°55'W

30/06/66
25/06/66
09/11175

44°ll'N, 142°22'W
46°43'N, 141°27'W
04°12'S, 083°20'W

ll.4m
I 1.2m9.8m

Ffmales

J9729
A439*
A444*
R610187
R610852
RE1725
Males

R610203
R6!0675
RE1885*: recovered from same individual. A: US mark. J: Japanese mark. R: USSR mark.
Sci. Rep. Whales Res. Inst.,

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63

NORTH PACIFIC SPERM WHALES

80

f\

60

!
j

40

:

'.

I

l

!\

:

1

800

\

R

1

\b. ..d' }' \ p'I \I/:!' "

f\

•

I ~

u.,n

/'r'
,'

600

'' '\

\..

'\ '

: \

\!

d

400
'

'

~

20

\

iii

E

....~

0

0

200

'o
170°

160°

150°

140°

130°

w

0

Longitude

0

z

40
20

40°

50°

LatitudeFig. 17. Sexual segregation of sperm whales in the eastern North Pacific appeared in
the catch of Japanese pelagic fishery from 1966 through 1977 (data are based on
Smith, 1980). Open circle and dotted line indicate total number of sperm whalescaught, closed circle and solid line percentage of females in the catch.
Top. Longitudinal segregation of female sperm whales in the eastern North
Pacific north of 40°N (Gulf of Alaska area).
Bottom. Latitudinal segregation of female sperm whales in the principal
range of the eastern North Pacific stock (Pacific areas east of l 70°W and north of
40°N, and east of 160°W and south of 40°N).

DISCUSSION

Stock boundaries
Klumov ( 1955) assumed two breeding stocks of sperm whales summering
off the northern and southern Kuril Islands, His southern stock might
correspond to some females of our southwestern North Pacific stock migrating
to southern Kuril Islands north of its ordinary summer range, and thenorthern stock to our northwestern North Pacific stock. However, it is also
possible to presume that he had just identified two latitudinal concentrationsof females of the northwestern North Pacific stock (Fig. 15). Our study failed
to find support for the two female stocks considered by Klumov (1955) to besegregated latitudinally in the eastern North Pacific.
In an isozyme analysis of North Pacific sperm whales, Wada (1980)
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KASUY A AND MIYASHITA

compared the gene frequency of the North Pacific sample from the central
sectors with several subsamples extracted from it, and concluded that thecentral North Pacific sample represents more than one stock. This supports
our contention that the central North Pacific is a mixing area for eastern andwestern stocks.
Our conclusion on sperm whale stocks in the western North Pacificagrees with the hypothesis presented by Watase (1963). Although he did notpresent the basic data used to construct the hypothesis, he wrote that the sizeof breeding schools is larger in his "northern stock" than the "southern stock".
He told us that the southern stock schools were frequent in the protrusionof the Kuroshio Current into the Oyashio Front and that of the northernstock to the north of it. The southern individuals had light brown color whilenorthern ones dark brown body (he was uncertain if it reflected the real bodycolor difference or difference in the color of sea water). Such difference willappear even from different degree of diatom infection. He also stated that thenorthern stock schools were tighter than those of the southern stock whenchased by the catcher boat, although he was not sure if it reflects the real
behavior difference of whales or difference in the ocean structure and soundconductivity (S. Watase, pers. commn in 1987). After mentioning the loose
dispersed schools of the southern type which might correspond to the above
mentioned school difference of Watase, Mr K. Yamamura of the Japan
Whaling Association stated that it was the general belief of his colleagues(confirmed by his observations made while he was working at an Onagawa
land station on the Sanriku coast) that individuals of the southern type had
a greater number of healed or unhealed oval scars than the northern type
and the skin looked unclean (pers. commn in 1987). Mr Y. Yanagihara,gunner of the Sanyo Whaling, believed that the southern type has slenderbody and is hard to harpoon because of the smartness and/or the school
members that tend to scatter during the chase (pers. commn in 1987). Thiswas partially confirmed by Dr H. Kato, the Whales Research Institute, that an
adult male (16.8m) of the southern type identified by the station staff had
smaller mandibular teeth and much slender lower jaw (0.86 time in the absolute
diameter at mid-symphysis) than the northern type adult male (17.6m) takenfrom different herds and processed on a same day in November 1987 at the
Ohsawa Station (pers. commn in 1987).
We presume that their southern type corresponds to the southern stockof Watase (1963) and that the scars would be equivalent to bites of the cookiecutter shark, lsistius brasiliensis, known on various cetaceans in warm waters
Qones, 1971). The recognition of two stocks (or types) of sperm whales off the
Pacific coast of Japan was probably common among whalers, although it was
not always clear from their description if the characteristics were a valid basis
for separating the stocks. Yamamura also stated that the southern type arrived
off Sanriku in early summer but that the date was delayed about two weeks
every year in the early 1970s. This agrees with our analysis of catch statistics,
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NORTH PACIFIC SPERM WHALES

65

and probably reflects a change in the timing of migration accompanied by thedensity decline in the peripheral habitat. We consider that the two sperm
whale types described by them represent two stocks identified by us in thewestern North Pacific. Adult males, solitary or in small schools, reported by
Watase (1963) to migrate off Hokkaido in autumn possibly represent adultmales from the southwestern North Pacific stock that summered in the cold
Oyashio Current area.
Blood type analysis by Fujino (1963) was probably the first to give ascientific basis for distinguishing two sperm whale stocks off Ja pan. Using 198
blood samples (all males) from the Bering Sea/Aleutian Islands waters (>51°N)
and 121 samples off the Hokkaido/Sanriku area (26% were males and 72%
were taken in August/September), he found that the former sample contained
no Ju2-positive individuals but the latter contained about 42% of such individuals. He further confirmed that blood type frequency did not significantly
differ between the sexes of the pooled Japanese coastal sample, and that the
proportion of Ju2-positive individuals in the Japanese coastal sample
decreased from 46/87 (53%) in August/September to 4/29 (14%) in October/
November. Finally he concluded after examining mark recapture data thatmembers (sex is not stated by him) of the stock in the Bering Sea/Aleutian
waters migrated in autumn to northern Japan, and that adult males of theJapanese coastal stock (with high Ju2-positive frequency) would migrate to
Kuril and Kamchatka waters but not to the central Aleutian waters.
Examination of data presented in Tables 4 and 6 of Fujino (1963) reveals
that his October/November sample contained a minimum number of 5 females
in 1959 and 14 in 1961 and 2 of these were Ju2-positive, i.e. the minimum
number of Ju2-negative females was 17 and the positive ones were actually 2
females. For safety we compared this minimum ratio (17:2) for females with
41 :46 (negative:positive) of the August/September sample (which includes
both sexes, and was collected in 1962 except for 2 in 1961) to find a highly
significant difference (Chi-square test, p<0.001). Thus we conclude that data
presehted by Fujino ( 1963) support our conclusion on the presence of two
breeding stocks of sperm whales off the Pacific coast of northern Japan.
We have concluded that many female sperm whales summering off theSanriku coast winter in the lower latitudes (20°-30°N) of the western North
Pacific and belong to the southwestern North Pacific stock, which contains ahigh proportion of Ju2-positive individuals (Fujino, 1963). If adult males
from this stock had migrated to the Aleutian Islands area, Fujino (1963)
should have detected some Ju2-positive individuals in the sample from theregion. The absence of such individuals in his sample must indicate thecorrectness of our conclusion on the segregation pattern of adult males of thetwo sperm whale stocks in the western North Pacific. At the same time thisindicates the presence of some yet unknown mechanism that inhibits freeinterbreeding between males of southwestern North Pacific stock and femalesof the northwestern North Pacific stock in their overlapping summer range
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KASUY A AND MIYAS HIT A(small scale interbreeding may occur between stocks).
The first of the possible factors will be the timing of migration andbreeding peak. The northward migration of adult males and the overlap of
distribution between sexes of different stocks occurs after the conception peakof the species (see above). This alone can decrease the chance of interbreeding.
The second is the social barrier inhibiting the free interbreeding between
stocks. Killer whales, Orcinus orca, off Vancouver Island are known to havethree communities each consisting of 3 to 12 pods and having occasionallyoverlapping ranges (Bigg, 1982; Ford and Fisher, 1983). Although pods
within community regularly associate with one another and may interbreed(cows in a pod without bull produced calves), pods in different communities
do not presumably due to cultural or behavioral differences (Bigg, Ford andCraeme, 1985). If this happens to the sperm whale, it will contribute to inhibit
free interbreeding between different stocks. The third possibility relates to thesynchlonize of migration and the cycle of reproductive activity in individual
males. The social structure of elephants has some similarity to that of the
sperm whales (Best, 1979). Adult males of the Asiatic elephant, Elephas
maximus, live apart from matriarchal group of cows and calves, and join it only
when it passes his territory while he is in heat which is thought to comeannually or biannually (Eisenberg, 1981). Dugongs, Dugong dugon, haveno sharply defined breeding season, but the males are not continuously inbreeding condition and their reproductive activity in a population is notsynchronized (Marsh, Heinsohn and Marsh, 1984). Although nothing is known
on the reproductive cycle of individual males of sperm whales in the North
Pacific, these example will suggest a possibility that males migrating to thehigher latitudes are in cycle of lower reproductive activity, and that they may
return to the breeding ground at the arrival of the heat as suggested by oursighting data.
A possible ratio of the two stocks constituting the Japanese coastal catch
can be estimated using data from Fujino ( 1963) and with the unconfirmed
assumptions mentioned below. The proportion of Ju2-positive individuals in
his combined Hokkaido/Sanriku sample was 2/5 in June and July, 46/87 inAugust and September, and 4/29 in October and November. Assuming as
suggested from his sample that the northwestern North Pacific stock contains
no Ju2-positives, assuming as an unconfirmed extreme case that all the individuals from the southwestern North Pacific stock are Ju2-positive, and using
the monthly catch reported by the industry (Table 2), we roughly estimate thecontribution of the southwestern stock in the late 1950s as (0.065 + 0.063 +0.068 + 0.094) x 2/5 + (0.222 + 0.266) x 46/87 + (0.136 + 0.086) x 4/29
= 0.40, and that of the northeastern stock as 1 - 0.40 = 0.60. Since the blood
type composition was probably different between Hokkaido and Sanriku
samples, further improvement of above estimate requires larger sample andcalculation by each geographical area. If Ju2-negative individuals exist in thesouthern stock, the above calculation will underestimate its contribution to
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NORTH PACIFIC SPERM WHALES

67

the catch. The proportion of the northwestern North Pacific stock in thecatch was probably higher in earlier years when the industry landed most of
the sperm whales in Hokkaido, and it could have increased again during the1976177 to 1977178 seasons when they hunted sperm whales wintering off
Sanriku coast. It must have become almost negligible since the 1983/84 seasons(see above).
Oceanographic factors
The effect of oceanography on migration and segregation of toothed
whale stocks has been demonstrated by several studies in the northwestern
North Pacific. With the exception of finless porpoises, N eophocaena phocaenoides,
which live in coastal waters of wide thermal range (Kasuya and Kureha,1979), most other toothed whales in the area inhabit either warm waters
under the influence of Kuroshio Current or cold waters such as the Oyashio
Current area (Kasuya, 1980; l 982b).
The Dall's porpoise, Phocoenoides dalli, is one of the best studied cold
water species. Its distribution is limited to north of the Subarctic Boundary,and the breeding population of each stock segregates into particular water
domains such as the Okhotsk Sea Gyre, Oyashio Current, Western North
Pacific Gyre, Bering Sea Gyre, and Alaskan Gyre (Kasuya and Ogi, 1987;
Miyashita and Kasuya, 1987; Yoshioka, Ogura and Shikano, 1987). One
population of the Dall's porpoise in the Oyashio Current has successfully
expanded its habitat to the Japanese coastal waters between the fronts of cold
Oyashio and warm Kuroshio Currents, but is still unable to intrude into theproper Kuroshio current area. The Baird's beaked whale, Berardius bairdii,
was considered from its breeding season probably to have evolved in higher
latitudes (Kasuya, 1977), which was supported by the distribution in thenorthern North Pacific. This cold water species in the North Pacific hasdeveloped a local stock in Japanese coastal waters near the southwestern
range, or between the fronts of Kuroshio and Oyashio Currents (Kasuya,
l 986a). Although these whales commonly occur in an area with high (>20°C)
surface water temperature in summer, they are still unable to expand theirhabitat in the main Kuroshio Cusrent area and their range is limited to thearea north of 35°N.
An opposite example is found with the short-finned pilot whales,
Globicephala macrorhynchus, inhabiting tropical and temperate waters of theworld. In the northwestern North Pacific, the majority of the individuals(southern form) live in a vast range to the south of the fronts of Kuroshio
Current and its extension. However the species has developed a morphologically distinct local form (northern form) in colder waters off Japan between
the fronts of Kuroshio and Oyashio Currents (Kasuya, l 986b; Kasuya,
Miyashita and Kasamatsu, 1988). Northern forms are not distributed in the
Oyashio Currents and its eastern extension. Although an apparently similar
form (having distinct saddle mark) has been sighted in the northeastern
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KASUYA AND MIYASHITA

North Pacific from Washington coast to Baja California, the distribution isapparently discontinuous between the two sides of the North Pacific andfurther study is needed on the systematic relationship between them (Kasuya
et al., 1988).
These examples suggest that the Kuroshio and Oyashio Fronts haveacted as strong barriers to the distribution of toothed whales, and that it has
not been possible to cross both of the fronts and to enter into quite different
water masses even for species that succeeded to overcome one of the two
fronts. For this reason if such a boundary is once passed it could have contributed to the development of isolated whale populations.
Several gyres to the north of the Subarctic Boundary probably haveworked also in a similar, but weaker way. The distribution of three sperm
whale stocks in the North Pacific seems to be closely related to the gyres in
the North Pacific, i.e. the concentration of breeding populations of the three
sperm whale stocks has a tight connection each with the Western North PacificGyre, Alaskan Gyre, and with the wide circulation area centered at theKuroshio Counter Current area and surrounded by Kuroshio Current,
Kuroshio Extension, North Pacific Current, and presumably North Equatorial
Current. The presence of only one recognized sperm whale stock in most of
the latitudinal range of the eastern North Pacific probably relates to theabsence of distinct ocean fronts in the region. Although there seem to be no
distinct oceanographic barriers between female ranges of the southwestern
North Pacific stock and eastern North Pacific stock, a significant isolation
could probably have been maintained because (1) the major concentration of
the latter stock occurs in the Alaskan Gyre and the low sperm whale density
in the lower latitudes of the stock's range decreased the chances of mixing, (2)the segregation of eastern North Pacific stock females to the east of malerange decreased chances of mixing (this also apply to isolation from thenorthwestern North Pacific stock), and (3) the mixing was limited due tothe direction of seasonal migration that tended to be north/south rather thaneast/west.
The pantropical distribution of sperm whales suggests the tropical originof the species. We believe that the sperm whales in the Okhotsk Sea or Sea
of Japan can be excluded from the possible origin of the northwestern North
Pacific stock, because the species is uncommon in these waters (Berzin, 1972),
and they do not seem to have established their niche in the region. Thus, thenorthwestern North Pacific stock of the sperm whales could have probably
originated either from the southwestern North Pacific stock or the eastern
North Pacific stock. Since the north/south temperature boundaries seem tohave blocked the dispersal of toothed whales more effectively than theboundary between the Western North Pacific Gyre and Alaskan Gyre (seeabove), we consider that the eastern North Pacific stock is more plausible as
the origin of the sperm whale stock in the northwestern North Pacific.
There are two possibilities on the origin of the eastern North PacificSci. Rep. Whales Res. Inst.,

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69

stock of the sperm whale. One is to assume that the sperm whales in the
Kuroshio Current System intruded into eastern North Pacific and establisheda niche in the Alaskan Gyre and the California Current possibly after the
Wurm Glacier Period which was the last glacier period ended about 10,000
years ago. Another possibility is that sperm whales immigrated in the cold
California Current area across the equator from the South Pacific. Southernhemisphere sperm whales could have immigrated to the northern hemisphere
more easily across the colder eastern equatorial waters during a period of cold
climate. Similar incidents have been suspected for several cetaceans (Davies,
1963; Kasuya, 1975; Brownell, 1983). Although the chance of such immigration could have certainly occurred during the Wurm Glacier Period, thechance probably lasted till the very recent time for such pantropical species assperm whales. The movement of two juvenile sperm whales that were marked
in the northern hemisphere in northern spring and recovered in the southernhemisphere in austral fall or spring provides an evidence of small scale (7° to
13° in latitude) transequatorial migration of breeding schools in the eastern
tropical Pacific (Table 14). Larger scale transequatorial movement (from 21°N
to 33°S in 4 years 3 months) of an immature male sperm whale was known
from the eastern Atlantic (lvashin, 1967), where the oceanographic structure
is comparable to eastern tropical Pacific. These indicate that the large scaletransequatorial immigration of sperm whales was possible in some geologicalage. Fujino (1963) indicated a highly distinct difference in blood type frequencies between two sperm whale stocks in the western North Pacific (noJu2-positives vs. 53% or more). This leaves a possibility for eastern South
Pacific sperm whales to be the origin of eastern North Pacific stock. Further
analysis of genetic similarity between sperm whale stocks in the North Pacificand those in the eastern South Pacific will throw some light on the evolutionof these stocks.
·
The niche previously occupied by the northwestern North Pacific stockof the sperm whale has become almost vacant after its depletion. However, so
far we do not find the niche being reoccupied by sperm whales from somenearby stocks, e.g. the southwestern North Pacific stock or stock in the eastern
North Pacific. This will be an example indicating that such vacancies are hardfor other nearby stocks to reoccupy, especially when the nearby stocks are also
depleted to some unknown degree. It can, however, reasonably be deduced
from the above analyses that the range of each sperm whale stock andmovement of the member whales are controlled by oceanographic conditions.
We have no data to indicate the possibility of a small local stock inJapanese coastal waters between the fronts of Kuroshio and Oyashio Currents
as observed for Baird's beaked whales and short-finned pilot whales. Development of such a local sperm whale stock could have been inhibited by theearlier establishment of a Baird's beaked whale stock in the near shore deep
waters or the current two sperm whale stocks partially overlapping off Japan.
However, even if such a small local stock had evolved in a geological age
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KASUY A AND MIYAS HITA

either from the southwestern or northwestern North Pacific sperm whale
stock, it could have been exterminated by the intensive operation of Japanesemodern whaling in coastal waters.
Significance for management
IWC (1980) identified two human induced factors expected to cause
pregnancy rate change in the sperm whale population, i.e. a decrease in thenumber of adult males relative to that of adult females, and a decrease in thefemale density. The first effect, decrease in pregnancy rate accompanied bya decrease in the adult male density, has not been detected in any sperm
whale stocks, but the second has been detected in sperm whales off South
Africa.
Best (1980) reported an increase in the apparent pregnancy rate ofsperm whales that followed exploitation off Durban from about 17% in theearly 1960s to 21 % in the middle 1970s. Later, Best, Canham and Macleod(1984) further refined the analysis to reinforce the conclusion that the pregnancy rate increase was due to the improvement of reproductive potential of
females of wide age range and not to the increased proportion of young adultfemales of high reproductive potential.
In the North Pacific, however, several attempts to detect the historicalchange in the sperm whale pregnancy rate were unsuccessful (Beddington,
1980; Masaki, 1980; Ohsumi, l 980a; Ohsumi, 198 la; Shimadzu, 1987).
Several factors could have masked possible change in the pregnancy rate.
They include (1) a very slight change in the timing of the sampling period,
which included both mating and parturition seasons, could have easilychanged the proportion of females in early or late pregnancy, (2) the difficultyin distinguishing females in early pregnancy and those having corpus luteumof ovulation, and (3) the proportion of unrecorded take of lactating females
could have changed between years, fishing grounds and whaling fleets.
One of two additional factors could be that the whaling operation moved
from north to south and from west to east. Thus the proportion of females
from the more depleted stock gradually decreased in the catch and that of theless depleted one increased, and during this period the depletion of the latterstock could have progressed. Many authors have attempted to overcome thisproblem by dividing the female sample into 10 degree (or larger) cells oflatitude and longitude, without recognizing the latitudinal segregation of thestocks. If the sample was divided into small areas or short time periods(month), it inevitably decreased the sample size in each cell and caused low
precision, but too coarse cell divisions also could have masked the possible
change in the pregnancy rate. A similar kind of effect is expected in the analysis
of many population parameter changes expected to accompany exploitation(with exception of growth rate increase (Kasuya, unpub.)).
Another factor that could have caused an apparently stable pregnancy
rate would be the combined effect of adult male depletion and the depletion
Sci. Rep. Whales Res. Inst.,

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NORTH PACIFIC SPERM WHALES

71

of females. Since adult males segregate, in summer, to the north or west of
the female range of the corresponding stock, and whaling commenced earlier
in the north/west and selectively hunted adult males which were larger inbody size, the availability of adult males for reproduction could have decreased
and the pregnancy rate could have declined before the large scale exploitationof females in the corresponding stock started. For example, hunting of adultmales of the southwestern stock segregating in summer off Hokkaido andKuril Islands continued from the start of Japanese coastal whaling in the
1910s (Table 5) to the end of pelagic whaling in the late 1970s (Ohsumi,
l 980c). Although the exploitation of breeding females of the stock started at
the same time off Sanriku (probably mainly in summer) and other Japanesewhaling station to the south of Sanriku region, most intensive exploitation
probably started in the late 1960s with the pelagic and coastal fisheries. The
pregnancy rate analyses for this stock used data collected since 1972 (Masaki,
1980; Ohsumi, l 980a). Therefore, the annual trend in pregnancy rate would
be hard to analyze without catch statistics by sex, maturity and stock, and thedetection or prediction of the change would be extremely hard due to the
opposite effects of the change in male availability and female density. The
similar change could have occurred on the eastern North Pacific stock.
If the present hypothesis on the range of North Pacific sperm whale
stocks is correct, the stock boundary currently in use for the management will
have to be adjusted. The boundary between the eastern North Pacific stockand two stocks in the western North Pacific will be the Aleutian Islands andline connecting points 52°30'N, l 75°E and 20°N, 160°W. The latter passes
middle of the overlapping area of the stocks. We have no data on the boundary
to the south of 20°N, but l 60°W longitude will probably be adequate. The
boundary between the breeding populations of two western North Pacificstocks will be at around 35°N in winter and 40°N in summer. Latitudinalboundary between the adult male populations of two western North Pacificstocks will be at about 50°N in summer and perhaps at around 35°N in winterto spring (mating season), but nothing is known on the boundary in theintermediate seasons. To make the problems more difficult is the range ofpubertal males, which are expected to be distributed in either of the adultmale or breeding female ranges and also in areas between the two ranges
depending on their age. Thus, if a sample is collected in summer in thewestern North Pacific between 40°N and 50°N, it will be mainly composed ofadult males of the southwestern North Pacific stock, and pubertal males andmembers of breeding schools (females and immature males) of the northwestern North Pacific stock. However, it will probably include also somemembers of breeding schools of the former stock in the southern part of thesampling area and some members of adult males from the latter stock in thenorthern part of the latitudes. Further study is needed for more precise
determination of stock boundaries and their seasonal movement.
Sci. Rep. Whales Res. Inst.,

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72

KASUYA AND MIYASHITAACKNOWLEDGEMENTS
We would like to thank many scientists who helped us in the preparation ofthis study. Drs R.L. Brownell Jr and P.B. Best critically read the manuscripts
and corrected English. Drs S. Ohsumi, N. Miyazaki, and I. Ikeda made usefulcomments at the various stage of preparation of this manuscript. Drs K.
Mizuno and S. Matsumura advised us in oceanography. We are grateful to
Dr Y. Shimadzu for providing data on marked whales in his unpublisheddoctorial thesis. Ms M. Ikeya and Ms Y. Yamamoto helped us in data analysis
and drawing.

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