Food of Sei Whales (Balaenoptera borealis) caught in
the Bonin Island Waters

BY
SETSUO NISHIMOTO, MASAAKI TOZAWA and TAKEHIKO KAWAKAMI(Received June 20, 1952)

I.

Introduction

The factory whaling was conducted from February to June every
year after the World War II, and the biological investigations were putin practice when the carcases were treated on board the factory ship,
but the stomach contents of sei whales had never been completelystudied. The stomach contents of whales in these warm waters, farfrom Japan; seemed different from those in the adjacent waters ofJapan, so we gathered every kinds of stomach contents when the carcases were treated on board th~ "Baikal-maru" in 1951.
On returning home, we requested the classification of them to thezoologists of the research institutes. Now we have got the answer ofthem, and wish to report on them in brief along with other descriptionof research about the food of whales.
In the previous reports on the stomach contents of sei whalescaught in t:ffese waters, including that in 1951, the species of them
are found to be incorrect and must be corrected as follows.
Previous reportsEuphausia sp. or Eu.
Lantern fish or sardine

Be corrected
Thysanoessa gregaria G.O. Sars.
Y arrella sp.

In the following descriptions, we are indebted to Dr. Shigeru
Motoda of the Hokkaido University on Thysanoessa gregaria and Dr.
Tokiharu Abe of the Tokai Regional Fisheries Research Laboratory andMr. Masao Watanabe of the Research Institute for Natural Resources
on fishes, and moreover the crew of the factory ship "Baikal-maru".
To all, we extend our sincere thanks.
ll. Kind of Food1.

Thysanoessa gregaria G.O. Sars. Fig. 1.

These were found in large quantity in the stomachs of the sei
whales caught in these waters. Total length is about 20-25 mm. It

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FoodofSei1
司Thalescaughti
ntheBonin Island九＇Taters

81(Alcock)andi
sknownfromoffKochi, Kumanonada, Natal, Andamans,southernwaterso
ffAu
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Int
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ymixedwithThν
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αnoe
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αThedatao
ffou
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e
swereasfollows.

．

Datewhenthewhales
werecaught

WhaleLocation

15‑
30, 5MayS
ei,female41ft.

26‑04N
143‑
31E

fl

male 40ft.

26‑
16N
143‑35E

15‑00,10Mayflmale 44ft.

￡5‑43N
143‑46E

Thys
αnoessα

14‑00, 14Mayflt.
male 42f

24‑
58N
143‑40E"

16‑55,21 May" male 42ft.

25‑
41N
143‑
55E

none

12‑
15,26Mayt.
" female44f

25‑50N
143‑36E11‑
05, 9June

骨male 41ft.

25‑
41N
144‑02E

14‑25,fl

Otherfoodnone
flTh
匂Sαnoessα，none

3. Myctophum (Myctophum）αspeγumR
ichar
dson(Myctophidae)Fig.3.

Alineoflu
minousorgans(about25)are on thev
entralsur
face,and9onthes
idescatteredirregularly. Dorsalfin:about12softrays.
Pectoralfin:about13softrays. Ventralfin:about8sof
trays. Anal
fin:about22softrays. 39scalesin thelateralline. Thisspecies

Fig.3. Myctophum(Myctophum）αspe
1・wnRichardson （×1/1.
1)

.NISHIMOTO, M.ToZAWAandT.KAWAKAM1s28‑
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ewhaleshDatewhent
werecaught

Whale

Location

Otherfood

0May,1
51‑
70.
t. male 43fie
S

1N
5‑
527E
4‑
341，
αsseoαnThyslf.
tmale 42f

4N
3‑
520E
5‑
31‑
1n（iα，sseαnThysthe2ndstomach)

0May,254‑
70lf.
t5f
male 425‑37N
0E
4‑
341

α
Thysanoess,
03‑
70lf.
t4f
female43N
4‑
520E
5‑
341,0111『lflflf.
.4gi)FeadiloMsNardo(upy. RαMαMα t4h abouttgnellato. Tnosaen the seicns was found oeicepssih
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atilopomso. Cw
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Thedatumo

Food of Sei Wh'.l.hn caught in the Bonin Island Waters
Date when the whalewas caught

Whale

06-40, 19 May

5.

Sei, male

42 ft.

83

Location

Other food25-27 N
143-42 Enone(a) Argyropelecus sp. (Sternoptychidae)(b) Polyipnus sp. (Sternoptychidae)

Both of (a) and (b) were found only once in this season.
were so damaged that their species were unknown.

TheyIll. The State of Taking Food
Concerning the stomach contents, the first stomachs of all whalescaught were investigated. The degree of quantity is classified into fourclasses; full (R), rich (rrr), moderate (rr), few (r) and empty (0).
The following table shows the quantity of food by month and by
species in this whaling season, but fishes except Yarrella sp. are omitted
as their numbers were very small.
IIIIIMayIJuneTotalII

iT .sp.1 Y.sp.1 Total (%) IT .sp. Y .sp.1 Total (%) T .sp.1 Y .sp.1 Total (%)
517 9.7#
12 !12 9.8#
1759.4#
I1--IJRrrr

16

I -16 13.1#

rr

41

I (2)*

41

I 50 I (~)*

r33.6jf

53 43.4#

No. _of whales! 119 ' 3 [ 1 22 59.8eatmg food(3)*%" 2.5 I
97I.o c2.5)*1

14 I -

611.3#

1426.4jf272852.8#

I 77(l)*5369.7 I1171

I (3)*
46i

1II152I1i 98.11

2255

II-22 12.6jf
55 31.4jf(2)*--- -4-81

46.3#

175 62.5

I 97. 71 ({_~*1.9I

Empty

I 82 40.2

123Total

I 204 100.0

176 100.0Note.

30.31105 37.5
J

280 100.0

R ........... full
rrr ........ richrr ......... moderater ......... few
T.sp ....... . Thysanoessa gregaria
Y.sp ....... Yarrella sp.
* ........... Figures in parentheses show the number or% of whales eatingYarrella sp. mixed with Thysanoessa gregaria.
# •.••••••••• % to number of whales eating food.

Thus the main food was Thysanoessa gregaria, Schizopoda. 98 %of whales having food in their stomachs had eaten Thysanoessa gregaria
and whales eating Y arrella sp. only were 2 % and those eating bothThysanoessa gregaria and Yarrella sp. were 4%.

s. NISHIMOTO, M. TOZAWA and T. KAWAKAMI

84

Number of whales eating food was superior to that of whales
whose stomachs were empty, 62.5% of all through the whaling season.
The following table shows the number of whales eating food andits percentage to all whales caught by ten days.
l-'10
May

11-20
May

21-31
May

1-10Jun\!

Total

Number of whales
investigated

5072

82

76

280

Number of whales
eating food

31

35

56

53

175%

62.0

48.6

68.3

69.7

62.5

From this table it is found that the percentage was smallest in theperiod of 11-20 May, below 50%, and afterward it increased to near
70%. Regarding of the quantity, rather few food were found in thestomachs in most cases as shown in the previous table. Full stomsachs
were only under 10 % in each month.

IV. Weight of Food m Stomach
As mentioned before, the quantity of food in the lst stomach wasobserved, but it had not been weighed in these waters yet. So thistime it was weighed on 8 sei whales.
The method: the food out of the lst stomach was put in an empty
tin of kerosene, weighed and summed up.
I
Whale
i
No. --~--~B~-ll.
s ex lengthody
S pec1es
1

II

1
2
3
4
5
6
7

8Sei.

Ifemale I

"""

I/""
"
""
male""Note.

40ft. I
42 !
42 ;
I
43 I
40 I
I
40 I
4041
1I"
IIiI

Contents of the lst stomach

Time and Dateof capture
06-00.
10-10.
11-00.
12-30.
07-50.
14-10.
09-50.
11-55.

. I Weight. I Quantity
Mark used
KmdFreshness
T.sp.

7 June"20 May7 June2 June6 June4 June I
20 May I

T.sp .... . Thysanoessa gregaria
rr ....... moderater ..... few
ff ....... a little digested

I

203.9 kgl
156.0 i
I
155.0
129.0
I
I 121.0 I
I 95.9
80.5
I66.0

" I"
" I"
//
II//IlR"//II
I

"/!

"//"
rrr

ff fII//Irr

IF
ff"fff!

R ..... fullrrr .... rich
F ..... very fresh
fff .... fresh
f ..... nearly digested

In the upper table, the marks of quantity such as R, rrr, rr andr were determined by observation before weighed. This fact showsthat the quantity was marked as R when the contents of the firstFood of Sei Whales caught in the Bonin Island Waters

85

stomachs of the sei whales in these waters were more than 100-120 kgs.
And the stomach content of No. 1 whale in the upper table seemed
to be the fullest condition.
V.

RefferencesMizue, K. : Food of Whales. The Scientific Reports of The Whales Research InstituteNo. 5. 1951.
- - - : Factory Ship Whaling around Bonin Island in 1948. The Scientific Reportsof the Whales Research Institute No. 3. 1950.
Norman, J. R. : Oceanic Fishes Collected in 1925-1927. Discovery Report Vol. 2. 1930.
Omura, H.: Whales in the Adjacent Waters of Japan. The Scientific Reports of theWhales Research Institute No. 4. 1950.
(Fisheries Agency of Japanese Government)