Sei Whales in the Adjacent Waters of Japan

III. Relation between Movement andWater Temperature of the Sea
By
HIDEO OMURA, TAKAHISA NEMOTO

In our former reports (1952, 1954) we reached to a conclusion that
there are two kinds of species in the so-called sei whales in the waters
adjacent to Japan, one being Balaenoptera borealis and others B. brydei.
The latter, brydei, has been taken mostly in the waters around Bonin
Island, but it moves northward from there up to the ground off Sanriku, where the warm current from the south meet the cold current
from the north. It is assumed, however, also in the former report(1954) that it does not go far north beyond the polar front. In thisreport, the movements of both species are studied in relation to thewater temperature of the sea, based on the data collected in 1953.
In 1953, 584 sei whales were caught in the coastal waters of Japan,
in which the baleen plates for 343 whales were forwarded to us fromthe various landstations for the identification of both species. In table
1, the catch of both species in every ten days of the season are shown,separately in two regions of Sanriku and Hokkaido, classified according
to the location of the landstations, in which the whales were processed.
Among examined 343 whales, 296 were borealis and 47 were brydei.
Only one brydei was taken from the landstation in Hokkaido, and therest in Sanriku, in the period from the 3rd decade of June to the lst
decade of October, mostly in July and August.
In Figs. 1-18 the positions of the catches of both species areshown with isotherms of surface water temperature of the sea, whichwere supplied by the Oceanographic Section of the Tokai Regional
Fisheries Research Laboratory, Tokyo. One may get general idea on
the relation between movements and water temperature from thesecharts, though the actual water temperature at the position of thecatch may differ slightly from the temperature shown in these charts.
It is clearly shown in these charts that brydei moves in this regionof the sea with the warm current and retreats from this ground withthe prevalence of the cold current. In May, the water temperature isstill low and the isotherm of 20°C is lying further south from thisground. No brydei is taken in May, and it is only in the 3rd decadeof June that the first brydei was caught in this ground, in the waters

Hokkaido

Sanriku

Regions

Sf.

'(!)-

Sex

Total

Not
examined

B. brydei

B. borealis

Total

Sf.

MayJune-1

July

AugustSeptember

Catch of Balaenoptera borealis and B. brydei in Japan, 1953.

lst I2nd I 3rd 11st I2nd I 3rddecade IIIIIIIIII-2)-l~I

br~dei -1 '(!)-

Not
examined- B.-

Species

Table 1.

'16 : 8584

3669199

174'0""~0z~218
7101,..,
124c::.:?

::r:c;;::
2125

2154

67

27

3

IITotalcatch

1 i 2Octoberccc

Sei Whales in the Adjacent Waters of Japan

81of about 21°C. We have two records of brydei in May. They weretaken on 31 May 1952 and 4 May 1954 in the waters off Oshima,south-most part of Japan proper. The water temperature there is over20°C even in May. As in July and August the water temperature inthe ground off Sanriku is high enough to afford a room to the immigration of brydei, most of them are taken in this period. Some brydei are caught also in September and October, however, they weretaken in the waters of about 20°C. From the 2nd decade of Octoberthe cold current regains its force and no single brydei was taken inthis g1ound, suggesting that they had already retreated from thisground and went further south.
Andrews (1916) describes that his own study of the genus Balaenoptera has led him to conclude that the temperature of the water isof comparatively little importance in determing their movements, butwe think that the temperature of the water is a very important factor
which affects the movement of the whale, at least for Balaenoptera
brydei. Uda (1954) states that there is a close connection between theprosperous whaling ground and the oceanographic conditions of the sea,
chiefly from the viewpoint of the structure of water masses. To ourregret, brydei is not specified from borealis in his paper. We may add
more knowledge to this field of science if the staffs of whaling landstations could cooperate to us also in future, as they did in 1953.
Contrary to brydei, most of borealis are taken in the cold waters
below 20°C, though some are taken also in the more warmer waters than20°C. Nozawa, Takayama and Nemoto (1954) report that 98 sei whales
which were taken by the whaling factory ship "Baikal Maru" in thenorthwestern part of the North Pacific in 1953 were all borealis. Thewater temperature at the position of these catches were about 11°C.
Although it is likely to happen that borealis may be taken in the morecolder waters than this, however, it seems to us that borealis in theNorth Pacific do not migrate to the seas of so high latitude, as thatin the Antarctic.
In our former report (1954), it is assumed that borealis comes nearto Bonin Island in the period from December to the middle of April,
and after it moved up to the north brydei approaches to that islandfrom the middle or end of April. The water temperature in thesewaters in the former period is up to about 20°C and in May about
24 °C or more, though it varies according to the oceanographic conditions in that year.
Kawakami and others (1953) sighted 74 sei whales in these waters
on the whale marking trip, carried on from the end of June to thebeginning of July 1952, in the waters of 28-29°C. These sei whales

82

H. OMURA, T. NEMOTO

are thought to be brydei.
It will be generally concluded from the above, that Balaenoptera
brydei is living in the warm waters of about 20°C or more, and B.
borealis in the colder waters below this temperature, though the twospecies intermingle each other in the waters around 20°C in some cases.
Literature Cited
Andrews, R. C. 1916: Monographs of the Pacific Cetacea. II. The Sei Whale (Balaenopter·a borealis Lesson). Mem. Amer. Mus. Nat. Hist.
Kawakami, T.; Otsuru, K., Watase, S., and Iguchi, K. 1953:Report on Whale Marking inJapan, No. IV. Japanese Fisheries Agency.
Nozawa, Y., Takayama, I., and Nemoto, T. 1954: Biological Investigation of the Northern
Pacific Baleen Whales Caught by the Japanese Whaling Fleet in 1953. Fisheries
Agency of Japanese Government.
Omura, H., Nishimoto. S., and Fujino, K. 1952: Sei Whales (Balaenoptera borealis) in
the Adjacent Waters of Japan. Fisheries Agency of Japanese Government.
Omura, H., and Fujino, K. 1954: Sei whales in the Adjacent Waters of Japan. II. Fur_ther Studies on the External Characters. Scientific Reports of the Whales ResearchInstitute, No. 9.
Uda, M. 1954: Studies of the Relation between the Whaling Grounds and the Hydrographical Conditions (1). Scientific Reports of the Whales Research Institute, No. 9.

45•NFig. 3.

.
h I . the Adi"acent Waters of Japan
Se1 W a es 1Il
45•N-~~-.-~----7/:r--l

3rd Decade, May, 1953
x B. borealis

Fig. 2.

Znd Decade, May, 1953
x B. borealis.

. . 4.
F 1glst Decade, June, 1953
x B. borealis

8384

H. OMURA, T. NEMOTO

Fig. 5.

2nd Decade, June, 1953
x B. borealis

Fig. 7. lst Decade, July, 195~
x B. borealis • B. brydei

F 1g.
. 6. 3rd Decade, June, 1953.
x B. borealis • B. brydei. 8 . 2nd Decade, July, 1953
F 1g.
.
x B. borealis • B. brydei

Sei Whales in the Adjacent Waters of Japan150"f_140"EFig. 9.

3rd Decade, July, 1953
e B. brydei

x B. borealis

Fig. 11. 2nd Decade, August, 1953
x B. borealis e B. brydei

Fig. 10. lst Decade, August, 1953
x B. borealis • B. brydei

Fig. 12. 3rd Decade, August, 1953
x B. borealis e B. brydei

85

86

H. OMURA, T. NEMOTO

I,--x~5~15t

140'E

ISO'E

Fig. 13. lst Decade, September, 1953
x B. borealis e B. brydei

Fig. 14. 2nd Decade, September,1953
x B. borealis e B. brydei

Fig. 15. 3rd Decade, September, 1953
x B. borealis e B. brydei

Fig. 16. lst Decade, October, 1953
x B. borealis e B. brydei

Sei Whales in the Adjacent Waters of Japan
45°N

Fig. 17.

45°N

2nd Decade, October, 1953
x B. borealis

Fig. 18.

3rd Decade, October, 1953
x B. borealis

87