FOODS OF BALEEN WHALES IN THE
NORTHERN PACIFIC
TAKAHISA NEMOTO
It is well known that the food item of whales is one of the most important problems on the biology of whales. Many studies on foods ofwhales have been carried out up to the present in the Atlantic andAntarctic waters by many excellent biologists. After Mackintosh &Wheeler (1929) investigated the foods of southern blue and fin whales,
Ruud (1932) and Hardy & Gunther (1935) made also comprehensive
studies of biology of food planktons. Recently, Peters (1955) discussed
some biology of Euplmusia superba, the main food of whales in the
Antarctic baleen whales, and Marr (1956) also discussed the relation
between Euphausia superba and surface currents of the sea in his preliminary paper. In the northern Atlantic, Hjort & Ruud (1929) and
Einarsson (1945, p. 159-160) described the importance of copepods andeuphausiids as foods of whales refering to many previous papers. On
the other hand, though considerable attentions have been paid to thefoods of whales, comparatively little is known of the problem in the northern Pacific before the year 1942. The previous nots on the problems
are found in papers by Zenkovitch (1937), Hollis (1939), Ponomareva(1949) and some others. Recently, useful works have been carried out
one after another by many biologists to which I refer in the suitable
columns of this paper as occasion demands.
In the summer of 1952, the staffs of the Whales Research Institutein Tokyo entered into the studies on foods of whales in order to study
the biology of whales and planktons. Thus during the last six years, alarge amount of data on foods of whales have been collected throughJapanese whaling expeditions. In addition, plankton samples collected
in vertical hauls with plankton nets also amount to a considerable
number. The present studies is designed to describe the outline on therelation between whales and their foods mainly based above samples.
Some biology of euphausiids which consist important parts of foods ofwhales is also investigated to some extent in this paper.
I would like to express my sincere thanks to Dr. Hideo Omura, thedirector of the Institute for suggesting this investigation as well as forconstant guidance. Thanks are also due to Dr. Yoshiyuki Matsue, Professor of the University of Tokyo and Mr. Yuzo Komaki for valuablesuggestions in the course of this work. I am also indebted to Dr.
33

34

T. NEMOTO

Albert, H. Banner and Dr. Brian, P. Boden for sending me kindly
valuable reprints on euphausiids and kind personal communications.
MATERIAL AND METHOD

The present paper is mainly based on stomach samples of whales captured in the northern part of North Pacific for three years since theyear 1954 to 1956, and data on quantity and freshness of stomach contents collected by inspectors and biologists through Japanese whaling
expeditions since the year 1952 to 1956. The main part of above
materials have been collected by following inspectors and biologists onboard.
1952 Haruyuki Sakiura, Katsunari Ozaki, Kazuo Fujino.
1953 Yasutake Nozawa, Iwao Takayama, Takahisa Nemoto.
1954 Setsuo Nishimote, Tamenaga Nakazato, Takehiko Kawakami, Ikuyo Hasegawa,Kazuo Fujino, Seiji Kimura.
1955 Yasutake Nozawa, Saburo Ikeda, Kenichi Iguchi, Kazuo Fujino.
1956 Heihachiro Kawamura, Sumio Matono, Sadao Ishii, Seiji Kimura.

The method of observation on stomach contents has followed the one
adopted by previous works (Mizue, 1952 and others). Stomach contents
m the first stomach are classified into following species.
Euphausiids
Copepods (mostly Calanoids including Metridia species in rare cases)
Fish
SquidsThen, the quantity of stomachs is divided into following classes.
Classes
Condition of stomachs

0
Empty

r
Few

rr
Moderate

rrr
Rich

R
Full

The freshness of the stomach contents is also determined by following grades.
fGradesCondition of contents Nearly digested

ff

fff

F

A little digested

Fresh

Very fresh

Above classifications are made by naked eyes, and not so exact instrict sense. The stomach samples have been collected from a part ofcontents on board, washed for some times and preserved in 10 percent
formalin sea water. The sampling of zooplanktons by plankton nets
in the whaling grounds has started in the spring of 1953. Present
study is mainly based on the materials collected during the year 1955,
through the whale marking cruise by 'Konan Maru No. 5 '. All thesamples have been taken in vertical hauls with the special net for
zooplanktons: Mouth diameter- 45 cm ; length 80 cm, shape conical,
materical synthetic-resin-processed silk grit gauze 54 (aperture 0.33mm).
All plankton samples have been preserved also in 5-10 percent formalin

FOODS OF BALEEN WHALES

35

sea water. The sample divider has been used for the fractioning ofabove samples. The plankton number in a sample is obtained fromthe multiple inverse proportional to the fractioning. On some of zooplanktons body length is measured for further investigations. Euphausiids are measured from the tip of the rostrum to the end of the telsonwith an accuracy of 1 mm or 0.5 mm for the smaller specimens, beingstraightened out on a measuring glass. Copepods are mesured the
cephalothorax length with a built in micrometer with bioculer microscope.
The papers and books used for the identification of plankton species arelisted in the last part of this paper.
WHALING GROUNDSHYDROGRAPHY

The whaling grounds where now Japanese pelagic whaling operates are
all feeding area type whaling grounds. Almost all whales swarm on
their foods, and the concentration of whales for their reproducting
never be considered. Mating grounds, such as whaling grounds off
Lower California for grey whales, is considered to be in far south
regions. And all Japanese pelagic whaling grounds are situated at thenorthern part of North Pacific.
Oceanographical studies on these parts of the North Pacific have been
carried out by Uda (1935), Barnes & Thompson (1938), Mishima &Nishizawa (1955) and some other workers. Fleming (1955) has advanced,
in recent years, a general summery of the oceanographycal conditionsof the North Pacific. This conception will surely prove to be of thegreatest use for the comprehension of the biological conditions in thewhaling grounds. Brief quotations of the review may, therefore, be ofinterest. I will chiefly use quotations from the paper by Fleming on
the point.
Fleming has divided the northern pacific to 3 zone, Boreal zone,
Subarctic zone and Central zone. Japanese whaling grounds lies in
Boreal and Subarctic zones after his divisions. Fleming, further, points
out characteristics of above divisions of zones. Boreal zone is divided
into five regions as shown in :figure 1. These five regions are as follows:
1. Kamchata-Kurile coastal region-Southerly flow of cold, dilute, nutrient-rich. Mostly
ice-covered in winter.
2. Western gyral region-Irregular currents but average counterclockwise circulation.
Very high nutrient content. Strong mixing between Aleutian Islands. Includes part of
Alaskan shelf.
3. Alaskan coastal region-Northerly flow of warm, dilute, medium nutrient-content
water. Mostly Ice-covered in water. Shallow area having an irregular coast with many
rivers.

36

T.NEMOTO

4. American coastal region-Northerly flow north of about 50°N and southerly in lower
latitudes. Salinities low because of local precipitation and runoff. Temperatures relatively
warm in northern part. Nutrients variable but usually moderate to high. Generally irregular coast.
5. Alaskan gyral-Subarctic water that turns northward and forms a counterclockwise
gyral. Salinity moderate. Temperature relatively high. Divergence supplies nutrients sothat content is generally high. Precipitation high. Deep area.

~-~--------·----­ '•.

SSN

sON

SUBARCTIC REGION
180°

Fig. 1.

160W

Natural regions of the northern Pacific Ocean; Broken line-February
ice limit. (Divisions follow the figure by Fleming, 1955).

Main Japanese whaling grounds locate along the boundary regions
between Kamchata-Kurile coastal region and western gyral region, between Alaskan coastal region and Alaskan gyral. Other main whaling
grounds locate in the adjacent waters to Aleutian Islands and along theslope of the continental shelf of Alaska. The boundary between subarctic region and boreal zone may have also some value for our pelagic
whaling though there has been no observation for whales. The central
zone called by Fleming is considered to has no weight for Japanesewhaling. From the year 1952 to 1953, Japanese expeditions operated
chiefly in the waters of the south-western side of the Aleutian Islands.
On the sea condition of the area in early summer, Mishima & Nishizawa (1955) describe that, 'A warm water mass of low salinity is found
to flow east to west. It reaches as far west as longitude 165° east, on
its way spreading several branches into the Bering', and 'A large clockwise eddy of this water is thus formed to the south or south west of
Attu Island'. Whaling grounds lie along above eddy, the boundary be-

FOODS OF BALEEN WHALES

37

tween the cold current of low salinity along the eastern side of Kamchatka Peninsula. And a whaling grounds is formed on the branch ofabove stated cold current bent east from the southern end of thatpeninsula and flows along the Aleutian Ridge. Japanese whaling factory
ship 'Kinjo Maru' operated in early summer in 1954 on the branchA

whales B

Vl
L
Q)

~

E

3.4<

1 st.decaae..c~July, 1953Q)

soor-~~~~~~~~~~~~~~~---

0

c

D

whales swarming

3.0
1001-----2

.s---

I/)

----~--3.0-

l...
GJ

~

GJ

1:===:::::---3.5' -- - - - -3.0---1- ------.:.::-----33. 65-- ----

Ec

2nd. decade

..c

3.4

~

~ 500

3.9----,(

June, 1953
3.7--....J

L-~--=-__...=-~~..c_...JL.<<L..L~_,_,.~,~~~~~~~~-3-.5-~~
Fig. 2. Vertical distribution of temperature in sections A-B and C-D
illustrated in figure 1. (Drawn by K. Nasu).
Crosses-Whales are swarming

and caught considerable number of fin whales. From the data of temperature obtained by the 'Tenyo Maru' cruise in summer of 1953
(Watanabe, 1954), two vertical sections of isotherms are drawn by Keiji
Nasu of the Whales Research Institute. In section A-B, the surface
water temperatures are higher than 4°C in first decade of July, andthe subsurface layer (sensu Mishima & Nishizawa, 1955) is found at
100 m level. The boundary of such cold layer, that is, the sea region
under where the cold layer is found, is considered to be good whaling
grounds (Uda, 1953; 1954; Uda & Nasu, 1956). In fact, the tendency
that whales swarm in such regions has been observed as shown in figure
2. I have observed above tendencies also in the data of the 'Takunan

38

i'. NEMOTO

Maru No. 6' in the adjacent sea waters to Japan in 1955. Some biological result of the latter cruise is also discussed in this volume. In
addition to this type of whaling grounds Ruud (1932) fully discussed
the general summery to the whaling grounds. Ruud describes, ' if,therefore, an area of production is to possess any importance as arendezvous of whales there must be a concentration of Krill there.
Such concentrations are found in the area of convergence, in backwaters, in the vortices of mixed layers, and at the centre of areas
where there is a cyclonic movement'. Such centre area of cyclonicmovement is the most favourable whaling ground also in northern hemisphere as discussed by Uda (1954).
Barnes & Thompson (1938) made compehensive study on the northpart of the eastern Aleutian Islands and Bering Sea. By their studies
the surface currents of north of the Aleutian Ridge, parallele the ridge
towards the east near Bogoslof Island, then swing north in the vicinityof Unalaska Islands as the water met the continental shelf and thendouble back along the shelf as it heads to the north-west just south of
the Plibilof Islands. Thus, the upwelling current along the continental
shelf by the currents, and backwaters between above currents and thewater from Bristol Bay and Yukon Delta, are valid causes for the formation of whales' swarming.
The ice covers the northern half of Bering Sea in winter (Fleming,
1955; Pilot chart of the North Pacific Ocean, 1955), where considerable
number of fin, humpback and gray whales are swarming. This migrations of fin, humpback and gray whales to the arctic sea through Bering Strait is proved by the catch data of Japanese whaling expeditionin 1940. Whales in these area in summer must retreat to south watersfrom there before ice prevailing the area exept few whales which inhabit among the broken ice or narrow uncovered sea areas.
WHALING GROUNDS AND CATCH

Japanese northern Pacific pelagic whaling expeditions have been operating since the year 1952. Outlines of the whaling in the North Pacificis discussed by Omura (1955). He also disscusses on the brief historyof pelagic whaling in the northern part of the North Pacific. So, I only
state short review of the problem here.
Whaling grounds. Japanese main whaling grounds lie along AleutianIslands, Komandor Islands and off Kamchatka Peninsula as shown infigures 1 and 3. These whaling grounds may be divided for convenience
into four grounds. Namely, A ground: the south part of Komandor,
off Kamchatka Peninsula and west south of Attu Islands; B ground;39

FOODS OF BALEEN WHALES

the north part of Komandor Islands; C ground: the north part of theeastern Aleutian Islands; D ground: the south part of the eastern
Aleutian Islands. The first whaling grounds divided, by further observations, into two subdivisions. The longitude line 168° east may be theappropriate line by which the first whaling grounds is divided. Japanese whaling expeditions operated only in A ground in years 1952 and1953. Successive expeditions in 1954, and 1955 operated also in C and
D whaling grounds. In 1956, Japanese whaling have operated in B
ground besides above A, C and D grounds.
no'E

180'

Whaling ground
B

170"w

BERING SEA" Pribilof I.
\)

Komandor I.
----~,i.:,~

Whaling ground~;;:q&••.

.......

I.A:
Attu I. =---..: - leutian Isfap-d.~~!

Wha ling ground
A I

I

.

I

18d'E

Fig. 3.

,oc:J-f. . . . 'Ll •-Ll _ _,~2 L 0-'i7.17t?·--

l.

•

I

So"N

~c

C

,...........
_:.:-.Yo--·-Whaling ground
D

I

sON

I

PACIFIC OCEAN
170°E

180°

170'w

Whaling grounds in the northern part of the North Pacific.

Catch. Japanese pelagic whaling has captured such number of baleen
whales as shown in table 1 since the year 1952. In these five years,
Japanese whaling expeditions have caught 429 blue, 4771 fin, 309 sei
and 368 humpback whales. When these catch data are divided byabove classifications of whaling grounds, some peculial features in themare observed. The species of baleen whales differ considerably in each
localities. The considerable difference in the catch is also observed between the catch of two subdivisions of whaling ground A as statedabove. In west area of 168°E in whaling ground A, fin whales are
dominant in number with considerable catch of blue and sei whales as
described in table 1. But fin whales are dominant with some humpback
whales in the east of 168°E, and blue and sei whales are captured in
far smaller number. In the whaling ground B, the north part ofKomandor Islands, fin whales are only dominant whales though very
few sei whales are caught in this water. Blue and humpback whales
have never been caught by the operation. Especially the fact that no40

T. NEMOTO

blue whales has been caught by previous Japanese expeditions in waters
north of Komandor Islands suggests that blue whales seldom migrate
to these waters (Omura, 1955). On the south area of the middle Alutian Islands, I find no peculiar feature in the composition of whalescaught if this area is separated from other parts. So I did not deal
with this area as a division. The catch composition of whales in whaling grounds along the east Aleutian Inslands shows remarkable difference from western regions. Only fin whales are caught in the north
TABLE 1. NUMBER OF CATCH BY JAPANESE EXPEDITIONS IN THE
NORTHERN PACIFIC SINCE THE YEAR 1952 TO 1956
Whaling ground A (West of l68°E)
Year/---------------~-~----

Whale species

1952

1953

1954

1955

1956

Blue
Fin
Humpback
Sei

29
130
11
9

83
273
17
96

28
442
15
67

23
87

1
186
1
29

20

Whaling ground A (East of 168°E)
Year,,,-----~-----______- -/--- -------~,

Whale species

1952

1953

1954

1955

1956

Blue
Fin
Humpback
Sei

26
83
25
5

6
197
17
2

122
1
21

61
18

154*
34
13

* Including 1 whale lost.
Whaling ground B

Whaling ground C

Year

Whaling ground D

Year

Year
/~----

Whale species
Blue
Fin
Humpback
Sei

*

1956
255*
5

1954
587
6
1

1955
1177
10

1956
774*

---

--~-

1954

1955

1956

117
165
114
40

47
35
89
1

69
46
2
1

Including 1 whale lost.

waters of the eastern Aleutian Islands, though 16 humpback and 1 sei
whales have been caught in a ambiguous position between Unimak andAtka Pass. As in the northern part of the western Aleutian waters,
no blue whale has been caught also in waters north of the eastern
Aleutian Islands and in one north of Pribilof Island. On the other
hand, comparatively many blue and humpback whales have been caught,
and fin whales are not so dominant in the south area of the eastern
Aleutian Islands. Blue and humpback whales are important catch in

FOODS OF BALEEN WHALES

41

this water. Besides above whaling grounds, the 'Tonan Maru' operated
in the arctic sea through the Bering Strait in 1940, and caught fin,
humpback and gray whales on which, to my regret, no biological collection is remained. So the discussion on whales in these waters is eludedin this pa per.
FOOD OF WHALESSTOMACH CONTENTS OF WHALES

Generally speaking, baleen whales in these waters take mainly zooplanktons as in other parts of the world. And some other foods, such as
squids and fish are also occasionaly found in stomachs of them. On
this subject, it is proper to treat it by respective whale species as disscussed by previous workers.
TABLE 2. STOMACH CONTENTS OF BALEEN WHALES CAUGHT BYJAPANESE WHALING FLEETS FROM 1952 TO 1956 IN THE
NORTHERN PART OF NORTH PACIFIC,..,,-------

Kinds of stomach contents

Whale species
--

Blue

Fin

Sei

Humpback

196
2

1674
102
2
3

4

201
2
1
11

667
1
3

107
4
4
1
12
173
305
4

Euphausiids
Eu. & Copepods
Eu. & Squids
Eu. & Fish
Eu., Fish & Squids
Copepods
Co. & Squids
Fish
Fish & Squids
Squids228
Empty
No. of stomachs examined 426
3
Not examined

10
2292
4755
16

45
1
107
368

Blue whales. Blue whales are famous for the plankton feeder, only
take euphausiids in the Antarctic waters though some rare appearances offish, amphipods have been observed (Mackintosh & Wheeler, 1929;
Mizue & Murata, 1951). Blue whales feed mostly on EuphauB'ia superba
in the Antractic, also on E. crystallorophias (Marr, 1956) and Thysanoessa
macrura (unpublished data by Japanese whaling expeditions in 1956).
Thysanoessa inermis and Meganyctiphanes norvegica are their favourite
foods in the Atlantic (Hjort & Ruud, 1929). Rough classifications ofstomach contents of whales examined on board are described in table
2. The table shows blue whales feed only on euphausiids with exceptional whales feeding on the mixture of euphausiids and copepods. Thisapparently indicates that blue whales are real euphauB'iids feeder in the42

T. NEMOTd

North Pacific as considered until now. Matsuura & Maeda (1942) also
state blue whales feed on euphausiids and blue whales are not polyphagous. While there are also some different data and con~lusions by
Mizue (1951.) and Sleptzov (1955). Blue whales have sardines and squids
respectively in their stomachs (Mizue, 1951), and Sleptsov (1955) describeson blue whales in Kurile waters that they feed sometimes not only on
zooplanktons but also on small gregarious fish, whenever blue whales
meet those fish. Indeed, 6 blue whales out of 15 whales fed on fish
after his data. Perhaps, the Kurile waters are less productive as compared with the northern waters for zooplanktons, so blue whales in the
Kurile waters feed on fish for want of their favourite foods of euphausiids. Foods of blue whales investigated by Sars (1874) in the Atlantic
are all krill (Thysanoessa inermis), and lodde or capelin has never beenfound in stomachs of them.
Fin whales. It is thought that fin whales are not so regulate inseasonal migrations as blue whales because fin whales are polyphagous,
being able to take their foods anywhere that planktons, fish or squids
are abundant. Their staple foods have been considered to be not so
restricted as blue whales, though many fin whales are also planktonophager. Fin whales take Euphausia superba, E. cristallorophias, andThysanoessa macrura in the Antarctic waters like blue whales. In the
Atlantic, fin whales feed not only on euphausiids, Thysanoessa inermis,
Meganyctiphanes norvegica, but also on swarming fish, such as Sild and
Lodde. Copepod, Calanus finmarchicus, is also considered one of thestaple diet in some seasons in the Atlantic (Hjort & Ruud, 1929).
In the northern Pacific waters, Zenkovitch (1934) describes that herrings are found in stomachs of fin whales and fin whales persue thoseswarms of herrings in Bering Sea. Matsuura & Maeda (1941) examined stomach contents of fin whales and observed that the most of fin
whales in the waters off Kamchatka feed on euphausiids, E. pellucida(E. pellucida is canceled by Hansen in 1905). Besides, 2 whales feedon Calanus cristatus and 5 whales feed on cods. Kasahara (1950) consideres from above facts and data by Mizue (1952) on the whales in
the Japanese waters, that euphausiids in the northern Pacific are rather
poor as the foods of fin whales. And the polyphagous habit of fin
whales may be due to above scantiness of eupnausiids. In recent years,
Banner (1949) reports, that 27 fin whales from Akutan Island, feed only
on a euphausiid, Thysanoessa inermis, but Sleptsov (1955), Kleinenberg &Makarov (1955) also describe that the considerable parts of food of fin
whales are occupied by fish and cephalopods. Sleptsov (1955) statesfurther that cephalopods is confirmed as one of the staple food of fin
whales in Aleutian waters. Fish are also considered to be the staple

FOODS OF BALEEN WHALES

43

rood for fin whales by the latter warkers. Indeed, far many fin whales
take fish and squids after them as compared the Japanese data illustrated in table 2. The Japanese data show, the most fin whales takeeuphausiids and copepods as staple foods though squids and fish arealso found in some occasions. However, they never be considered to be
the favourite food for fin whales. In the adjacent waters to AleutianIslands, fish and squids are considered to be only the makeshift foods
for fin whales when they meet no swarm of zooplanktons. Collett (191112) describes some observation in the Atlantic that, when fin whales has
to choose between fish and euphausiid diet, they choose the euphausiids.
It is often observed that fin whales or sei whales taking fish are suffered by the parasitic nematods in their stomachs. As the some larvaeof those nematods are considered to originate in fish (Margoris & Pike
1956), the ichthyophager of fin whales may be an acquired taste ofsome unusual fin whales from the weaning. The fact that some fin
whales take fish along Aleutian Islands, is apparently due to that swarming fish are very aboundant as compared with euphausiids or copepods,
favourite foods of fin whales.
Humpback whales. Euphausiids are the main food of humpback whales.
But humpback whales feed on swarming fish as well as euphausiids insome cases. They take herrings commonly (Zenkovitch, 1934) and considerable number of them feed on fish, cods, sardins, herrings also inrecent studies (Sleptsov, 1955; Kleinenberg & Makarov, 1955). In theadjacent waters to Attu Islands, they mainly feed on atka mackerels
which constitutes large swarms of themselves by Japanese observations.
Copepods and squids are scaresly observed as shown in table 2. Thus,
the value of copepods and squids as foods for humpbacks are considered
to be very few. Only 2 whales feeds on the mixture of copepods witheuphausiids and 2 whales feeds on squids and the mixture of squids
and euphausiids. To my regret, the data by Mizue (1951) and the discussion by Kasahara (1950) can not be refered on this point because
their division of food 'Krill' contains two different groups, copepods
and euphausiids, and their observations are not so accurate. Howell &Huey (1930) describe some foods of whales from California waters, andsuggest that 16 humpbacks feed on shrimps (perhaps Euphausia pacifica)
and 5 whales on sardines. From above many observations, only two
different groups, euphausiids and fish are considered to be the favourite
foods for humpback whales.
Sei whales. Very famous works have been carried out on foods ofsei whales in the north Atlantic until now. The migration of sei whales
was also studied in connection with the conditions of zooplanktons (Hjord& Ruud, 1929). Sei whales are noted to favour copepods, caianw;

44

T. NEMOTO

finmarchims in the Atlantic, so that abundance of Calanus finmarchims
in whaling grounds directly influence the number of whales whichswarm in the area. In the northern Pacific the data by previous workers
show sei whales take fish and squids as well as 'Krill' (Mizue, 1951)or copepods (Sleptsov, 1955). Sei whales mostly feed on copepods asshown in table 2 in Japanese whaling grounds as in the Atlantic. Only
4 whales take other zooplankton euphausiids only. This number is far
smaller when I compare with those in adjacent waters to Japan where
many sei whales feed on a euphausiid, Euphausia pacifica. From thedata of the Whales Research Institute, the favourite foods of sei whalesin the Japanese waters are euphausiids, mainly Euphausia pacifica sometimes Thysanoessa inermis or T. longipes in the cold waters. A copepod,
Calanus finmarchims is also found from early spring to summer in thesewaters (unpublished data of the Whales Research Institute). The indistinct species 'Krill' described by Kasahara (1950) and Mizue (1951) must
be corrected by above described species. Besides copepods, 12 whales
take squids and 4 whales on fish only. But these foods are consideredalso incidental appearances in the northern part of the North Pacificwhere copepods are abundant.
SPECIES OF FOODS

Planktons
The species of food planktons for baleen whales in the sea adjacent toJapan is fully discussed by Nakai (1954), and this paper shows somedifferences between foods of whales in the northern Pacific and Japanesewaters. From summerized review of this survey (Nakai, 1954) and mydata, following plankton species are considered as staple foods for baleen
whales in the northern part of the North Pacific. As stated in above
chapter, main foods of baleen whales in these waters are euphausiids
and copepods, and dominant species of them are restricted to somespecies mostly common in the sea. Some other less significant species
are discussed last part of this chapter.
Euphausiids

Euphausia pacifica Hansen
Copepods Calanus cristatus Krrpyer
Thysawessa inermis (Krq)yer)
Calanus plumchrus Marukawa
Thysawessa longipes Brandt
Calanus finmarchicus (Gunner)
Thysanoessa spinifera Holmes
Metridia lucens (Boeck)

Euphausia pacifica Hansen. In spite of the fact that E. pacifica is
one of the most important euphausiid in the adjacent waters to Japanand Korean waters (Nakai, 1942, 1954), very few observations has been
made by Japanese workers as disscussed by Nakai (1942). He insistson the importance of it as foods of whales and fish. Indeed, E. pacifica

45

FOODS OF BALEEN WHALES

is the most dominant food in the adjacent waters to Japan, off Sanriku(the north east part of Japan) and Hokkaido. E. pacifica is also notedby Howell & Huey (1930) to play some part of foods of gray, fin andhump hack whales in the Californian waters.
The northern distribution of E. pacifica is considered from Japanesedata as north as Aleutian Islands where considerable number of it foundin stomachs of whales. In the north parts of Aleutian Islands, thoughmany specimens are collected by tow nets, it vanishes as dominant
species in stomachs of whales. So it is considered the importance of
E. pacifica as food of whales in Aleutian waters is not so heavy as inJapanese waters. Only 9 specimens out of 126 collected euphausiids
samples are filled with dominant patches of E. pacifica as shown in
TABLE 3. DOMINANT APPEARANCES OF EUPHAUSIIDS
IN COLLECTED SAMPLES
Whale species~~

Species of euphausiids

E. pacifica
T. inermis
T. longipes
T. spinifera

Blue

Fin

2

6
65
21
1

8

2

Humpback

Sei

1
8

1

1

Total

9
92
24
1

TABLE 4. APPEARANCES OF EUPHAUSIIDS
IN COLLECTED SAMPLES
Whale species,,---------~--~

Species of euphausiids

E. pacifica
T. inermis
T. longipes
T. spinifera

Blue

Fin

Humpback

2
9
5
1

14
76
57
15

1
10
2
3

Sei

2

Total

17
95
66
19

table 3. Especially, in the north part of the eastern Aleutian Islands
where many whales feed on T. inermis, E. pacifica has scarecely been
observed from Japanese collections. The larval form of E. pacifica described by Boden (1950) also seem to be common in southern waters of
Aleutian Islands in summer. However, furcilia stages of it do not occur
in stomachs of whales as following euphausiids, although some incidental
appearances of noupli and furcilia larva of them are observed. Ruud
(1932) also describes such conclusion on E. superba. The cancelled
species Euplwusia pellucida quoted by Mizue (1952) and Kasahara (1950)
perhaps mean this E. pacifica, the most dominant species in the waters
to which they ref ere d.
Tkysanoessa inermis (Kr<j>yer) Hansen. It is one of the most famousfood euphausiid as 'small krill' in the Atlantic. It is so important in46'r. NEMOTO

some seasons as the migrations of blue and fin whales are affected by
the conditions of swarming of it (Hjort & Ruud, 1929). In thenorthern Pacific, perhaps Banner (1949) is the first to describe thisspecies from the fin whales of adjacent waters to Akutan Islands. He
again describes T. inermis as dominant foods of whales referring to the
distribution of T. inermis of those waters (1954). Many collections of
the Whales Research Institute show that T. inermis is the most important species of euphausiids, as swarms of T. inermis have beenfound as stomach contents of whales in Bering Sea, adjacent waters of
Aleutian Islands, Kurile waters and also in Okhotsk Sea. T. inermis•Pribilof I.

BERING SEA•' \K~mandor I.

."' "' "'··"'. !,

••

~-

I OE

I 0°E

•
•

+
e+

,r,++
+$:::..-...

,,, +.,_Attu I.

.

"' .....

'N
+

• •
"'" +
Ae
e ,. :.: ""_,,,_ .. +l,.
• "'lrJ,.:

"'

•

•

0

•

I 0°

I OW

SON

Fig. 4. Main occurrences of some euphausiids: Open circls- Thysanoessa longipes
spineless form; Solid circles-Thysanoessa longipes; Crosses-Thysanoiissa spinifera;
Open triangles-Euphausia pacifica.

A.

Fig. 5. Abdominal spines of Thysanoessa inermis (Kr\f>yer) Hansen.
Two spine form. B. One spine form with rudimentary spine on 5th segment.

distributes widly in the whaling grounds and the utmost concentrationof it is found in C whaling ground. Fin whales in C whaling groundtake almost only these swarms of T. inermis.
Ten furcilia stages of T. inermis (Einarsson 1945), have not beenfound as foods like E. pamfica by my studies. Above fact may be dueto the want of storage of fat in larval stages of euphausiids and different biological conditions from adults in depth of distributions or
degree of their congregations. The density of swarms of euphausiids is· considered to has . great significance for feeding habits of whales. I
suppose if the patch of euphausiids is not so dense, a:s in larval stages

FOODS OF BALEEN WHALES

47

whales perhaps take no notice of them. And sparse patches of
euphausiids may be saved from whales' swallowing.
In some taxonomical points of T. inermis differences have been discussed on the Atlantic specimens. Hansen (1915) reports the presenceof a spine in the fifth abdominal segments on the Pacific specimens.
On the contrary, Einarsson (1945) states 'None of the numerous specimens examined by me have shown eaven the slightest sign of a spineon the fifth segment' on the Atlantic specimens. In vast collections at
my hand, the fifth abdominal segments also have usually abdominal spine
as described by Hansen (1915). The case wanting the spine is rarely
observed. I count these two formes on some collections and the twospined form is dominant in each 100 specimens of collections. As compared with Atlantic specimens discussed by Einersson (1945), one characteristic features of Pacific specimens of T. inermis is considered to
TABLE 5. NUMBER OF ABDOMINAL SPINES OFT. INERMIS
Year

Samples' no.

One spined form*

Two spined form

1953
1954
1955

475
K728
118
393
1655
1811
1848
1849
1871
1879
1885

6
10
13
7
8
19
30
11

94
90
87
93
92
81
70
89
100
98
87

f!
f!
f!
f!
f!
f!
f!
f!

2
13

* Including those with rudimentary spines in the 5th segment.

bear two abdominal spines. The distribution of these two form show
some differences even in northern Pacific. However, further discussion
on this point needs more examinations.
Thosanoes,sa longipes Brandt. T. longipes is also the most common
euphausiid alike T. inermis in northern part of the North Pacific. So it isconsidered to bear considerable significance as the food of whales(Ponomareva, 1954), though T. longipes has occured less in number as
dominant foods of whales as shown in table 3. The cases that swarmesof T. longipes appeares in dominant number are about one-third of T.
inermis in the collections. On the contraly, T. longipes is the mostabundant and frequent in samples collected by surface plankton nets
from 200 m. Above data suggests that adults and adolescents of T.
longipes distribute scatteredly in the surface waters of the sea, not·. soconcentrated as T. inermis in every times. T. longipes is also found in

48

T. NEMOTO

the Okhotsk Sea and the adjacent waters to Japan as whales' foods.
On the taxonomical points of T. longipes, Banner (1949), and Boden,
Johnson & Brinton (1955) describe the smaller form of T. longipes
which lacks the conspicuous abdominal spines. Those spineless form
inhabits the whaling grounds in considerable number in the same surface waters. The spineless form has appeared in 2 stomachs of whales
dominantly in my collections, and swarms of spine form have been
mingled with it.

Fig. 6. Thysanoessa longipes Brandt. Upper; Adult female of spine form from theleft side. Middle; Adult male of spineless form from the left side. ( x 4) Lower;
Juvenile form of spine form about 7 mm.

The eyes of spineless form is larger than the original form in somespecimens as compared the spine form as shown in figure 6. Some ofthem possess a greatly enlarged eye (Boden, Johnson & Brinton, 1955),
which has occurred more frequently in eastern side of the North Pacific inrough speaking. This variation in the size of eyes is formarly notedby Banner. Banner (1949) states 'Both forms of T. longipes are fragile.
Especially so is the spineless form*. This special feature is well observed in first stomachs of whales. The eyes of T. longipes are almost

FOODS OF BALEEN WHALES

49

broken by the digestion of whales though eyes of other euphausiids
such as T. inermis or Euphausia paC'ifica are never broken in the same
conditions. The spineless form is more fragile in such cases as statedin Banner's paper.

B

Fig. 7.

Male copulatory oagans of Thysanoessa longipes Brandt.
C. Spine form. ( x 50)

Fig. 8.

A.B.

Spineless form.

Carapace of spineless form of Thysanoessa longipes Brandt.

( x 10)The position of the lateral denticle of carapace in spineless form differsfrom that of the original form. The lateral denticle of spine form islocated in about middle of the lateral margin of carapace, a little tothe back. On the other hand, the lateral denticle of spineless form is* On the spineless form of Thysanoessa longipes, Drs. Boden and Brinton kindly sentme a letter that ' At present it is best to consider the spineless and spined specimens as" forms " of the same species', and 'It is possible that further information on distribution
etc., may cause us to revise our present opinion '. I also consider this spineless form as
a form of Thysanoessa longipes in the process of my examinations. But, spineless form
differs from spine form of T. longipes in some points, such as rostrum, carapace denticles,
eyes, some body proportions, body length at the sexual maturity and distributions. So
further examination may be able to devide above two forms of T. longipes. Here I describe
this spineless form as a 'form' of T. longipes after the descriptions by Dr. Banner and"Drs. Boden and Brinton.

T. NEMOTO

50

located in the far back position. The length ratio, from the anterior
spine to the lateral denticle: lateral margin of carapace, is ranging 5060% in the spine form, while ranging 70-80% in the spineless form.
On the whole, lateral denticles of euphausiids are in fixed positions foreach species. Thysanoessa raschii has a pair of well developed denticles
always anterior to the middle of the margin, and E. pacijica bears
strong denticles a little anterior to the middle of lateral margin of thecarapace. So the difference between above two forms is the very
interesting feature in variations of euphausiids. On this point, thoughsuch variation of position of lateral denticles of Thysa.noessa species has
not been noticed, Hansen (1911) describes the denticle on the lower
margin of the carpace of Nematoscelis species shows some geographicalvariations.
20 .
l/)

i::
Cl)

E

~ 10

0.
Ul

"O

0

''
'

'

z

/

0 ,'
55

''

60

'
65

70

75

Fig. 9. Length ratio, from the anterior spine to the lateral denticle: lateral marginof carapace, of the Thysanoessa longipes Brandt. Solid line-Spineless form. Broken
line-Spine form.

In some larval form (over 7mm) of T. longipes the denticle is locatedin the middle of the lateral margin of carapace suggesting that it may
become the spine form when it will be mature, though the juvenilestages of T. longipes bear only dorsal keels on the third, fourth andfifth abdominal segments, and never have acute abdominal spines, and this
distinction is the same as the adult spineless form. Moreover the sizeof such juvenile stages is also in the same range of the latter. In thisdevelopmental stages, the position of lateral denticles may be sufficientevidence to divide two forms of T. longipes.
On the distributional range of two forms of T. longipes. Boden,
Johnson & Brinton (1955) state, 'The large specimens of T. longipes,
which bear abdominal spines, are rarely taken south of 50°N, whereas
the southern limit of the range of the smaller form is about 40°N '.
Main occurrances. of T. longipes in my data generally coincide with above·st~tement, exe'pt some of those found in adjacent waters to· Japan.
Considerable number of fin and sei whales take sometimes swarms of

FOODS OF BALEEN WHALES

51

spine form of T. longipes at about 40°N, 150°E. The spineless form
has not been occurred in adjacent waters to Japan and Okhotsk Sea
owing to the scanty collection of stomach samples.
The adolescents of T. longipes bear no sings of conspicuous abdominal
spines in length about 10 to 12mm. They bear only dorsal keel andthe spine of last abdominal segments like adult spineless forms. But
spineless forms have often short spines on fifth abdominal segments.
The eyes of them are also easy to be broken by any causes.
Thysanoessa raschii (M. Sars) Hansen. T. raschii has not appeared
as foods of whales in samples gathered from 1952 to 1956 in the whaling grounds though recent study by Sleptsov (1955) shows that T. raschii
is fed by whales in the Kurile Islands waters. According to Banner's
description (1949, 1954), T. inermis is dominant food for Akutan whales
against to T. raschii is the only euphausiid found in the cod and pollack
stomachs. Banner suggests (1954) that, 'such difference may be due tothe fact that T. raschii is an inshore species, whereas T. inermis is
more commonly found the margin of the continental shelf in the Akutan
waters '. By Japanese operations, whales mostly have been caught along
the margin of continental shelf or in boundaries of different water
masses far in the offing, and the catch within the margin of continental
shelf are very scarece. Thus, T. raschii may be rare as foods of whales
in Akutan waters. In the unpublished data from the Okhotsk Sea, considerable specimens of T. raschii are found among other euphausiids,
Euphausia pacijica, Thysanoessa longipes, and T. inermis. This occurances of T. raschii may be due to the sea depth of whaling grounds.
The sea depth of Okhotsk whaling ground is far shallower than thedepth of Akutan whaling grounds exept some areas where fin whalescaught within the Alaskan continental shelf.
Thysanoessa spinifera Holmes. This species has been found in lessnumber as foods of whales. Only 1 fin whale took it dominantly, and
it is thought T. spinifera is not so important as T. inermis or T.
longipes. Hollis (1939) describes that T. spinifera occurred as foods offin and humpback whales dominantly in Bering Sea. He states further,'The egg masses were particularly numerous in the stomach of a
humpback shot on August 26, and would lead one to believe that overa short period of time they may be of some importance as foods'. His
suggestion that egg masses may be the food of whales is very interesting. I would like to treat this problem in following chapter. The distribution of T. spinifera holds something to be examined. By Hansen(1915), the most western records is taken at 179°0!7'30'"E-54°12'N and
Boden, Johnson, & Brinton (1955) describe, 'It occurs along thewestern coast of North America from northern Baja California to the52

T. NEMOTO

south-eastern Bering Sea, usually within 200 miles of shore '. In thecollected samples, T. spinifera has occured mostly along the margin ofcontinental shelf. The most western occurrence is about 170° W-54 °N
and T. spinifera mostly converges on the north waters of Unalaska
Islands as shown in figure 4. So, it may be concluded that T. spinifera
inhabits only eastern side of the North Pacific from these observation. In
other words, T. spinifera inhabits mainly in natural regions Alaskan
Coastal Region, Alaskan Gyral, and American Coastal Region called by
Fleming (1955). Some larval form of T. spinifera may be transportedby the sea current from the Pacific to Bering seas, because the Pacificwaters set north into the Bering Sea at velocities up to 0.4 knot between Unimak and Unalaska Islands (Barnes & Thompson 1938).
Calanus cristatus (Krcpyer). Usually, C. cristatus is the most famous
copepod whaling grounds in the North Pacific. 'Calanus' or 'Red rice'
called by whalers means usually this species in the whaling grounds.
Fin whales take Calanus cristatus most favourably. This is clearly
illustrated in table 2. Cope pods fed by fin whales in table 2 are almost
all Calanus cristatus and occurrences of other copepods are only in someoccasions as disscussed in the description of C. plumchrus. The developmental stage of Calanus cristatus is almost all the copepodite 5, andvery few exeptional copepodite 4 is found among the former. But noadults has been found in Japanese collections. The patch of Calanus
cristatus copepodid 5 is considered to be extremely dense in the surface
waters. But towards the biological autumn of these areas, Calanus
cristatus disappeared from the surface waters. Such phenomenon is
also fully described by Nakai & Honjo (1954) and Bogorov & Vinogradov(1955). For example, fin whales caught at the south-west area of
Attu Islands in June and July in 1953 took no other foods than
Calanus cristatus. Whereas, when the time of their swarming passed,
euphausiids take the place of copepods in late August to September.
Bogorov & Vinogradov (1955-a) also examined the distribution of Calanus
cristatus in Kurile waters in 1953, and lead to the same results. Calanus
cristatus is important as foods of whale also in the Olyutorskiy Bay.
Brodsky (1950) describes that, 280 fin whales out of 304 whales caught
in Olyutorskiy Bay took unmixed patches of C. cristatus.
All specimens of Calanns cristatus belong to the copepodite 5 stage,and Calanns cristatus means this copepodite 5 stage in following discussions. The harvest of C. cristatus is abundant or poor according to theoceanographical conditions year to year. For example, C. cristatus isextremly abundant in 1953, while it is scarce in other years. I would
call the year when Calanus cristatu,s is abundant as ' Calanus year ',and ' Krill year ' when euphausiids are abundant.

FOODS OF BALEEN WHALES

53

Calanus plumchrus Marukawa. Calanus plumchrus has been discussed
by many biologists from the taxonomical point of view. It has been
considered as the synonym with Calanus tonsus Brady (Campbell, 1930;
Tanaka, 1954) or Calanus tonsus f. plumchrus (Brodsky 1950; Marshall& Orr, 1955). Brodsky suggests (1950) further that C. plumchrus may
be a seasonal form of C. tonsus f. typica. Recently, Nakai (personalcommunications) and Tanaka (1956) have studied these two species andcome to the conclusion that Calanus plumchrus is a characteristic copepodof the North Pacific, and is distinct from Calanus tonsus Brady of the
Antarctic. Detailed description on this point will be found in the discussion by Tanaka (1956), and I use the specific name Calanus plumchrus
in this report.
TABLE 6. OCCURRENCES OF CALANUS PLUMCHRUS IN
D WHALING GROUND IN 1954

Decadeslst. decade June
3rd.
"lst. decade JulyFin whales

Sei whales

C. plumchrus C. cristatus

C. plumchrus C. cristatus

1
6

2
12

13

4
2

Calanus plumchrus is considered to be very abundant in those northernpart of the North Pacific. The number of C. plumchrus is far numerous
than any other macro copepods in samples collected by plankton nets.
While, C. plumchrus is not observed so often as. C. cristatus dominantly
in stomachs of whales. I have noticed no dominant specimens of C.
plumchrus in data of fin whales of 1952 and 1953, exept sei whalescaught in August in A whaling ground fed on C. plumchrus. In D whaling ground in 1954 only 1 fin whale take C. plumchrus in first decade ofJune when 33 fin whales caught in the decade. Similarly,. 6 whales outof 64 fin whales fed on C. plumchrus and 12 whales fed on C. cristatus
in third decade of June, and 1 whale in July. In this season, sei whales
also swarm on the patch of C. plumchrus in this sea area. C. plumchrus
is fed by 13 sei whales out of 15 whales which took copepods in third
decade of June.
It may be suggested by above facts that Calanus plumchrus neverswarm so markedly as Calanus cristatus in these waters. Only thosewhales that skim their foods, such as sei whales or right whales (Ingebrigtsen, 1929), may easily take the sparse patch of Calanus plumchrus.
Calanus finmarchicus (Gunner). The most famous copepod C. finmarchicus as foods of whales in the Atlantic has been layed aside because
it occures not so frequent is Japanese waters. As stated in the partof sei whale, occasionaly it has been taken by sei whales in the adjacent

54

T. NEMOTO

waters to Japan. However, C. finmarchir:us is considered not so important in the northern part of the North Pacific as in the Atlantic. It
has only occurred with other copepods, C. cristatus or C. plumchrus
though considerable number of them has appeared in samples by plankton nets. Besides, the copepods in the samples are not always the
typical form of C. finmarchicus. Some of them rather resembles to
Calanus helgolandicus. The relation between above two forms may be
the most interesting subject on which many studies have been carriedout by many biologists. Extensive discussions of this problem will befound in the papers by Rees (1949), Brodsky (1950) and Marshall &Orr (1955). Including C. heligolandir:us only the specific name C. finmarchir:us is used in this paper.
Metridia lucens Boeck. This fine species is not so important as the former
3 species. Matsuura & Maeda (1942) describe this from stomchs of sei
whales in the waters off Kamchatka Peninsula, and I observed the stomachof 1 fin whale caught at 55°38'N, 169°00'W with Metridia lucens. Other
Metridia species such as M. okhotensis or M. pacijica described by Sleptsov(1955) as foods of whales have not been observed in my collections as
dominant foods of whales though they are found in few number.
FishSome baleen whales in the northern part of the North Pacific take swarming fish too. As shown in table 2, humpback whales in sometimes
undoubtedly ichthyophager. A few fin and sei whales also take fish as
discussed by many research workers. These fish species are listed
following.
Gadus macrocephalus
Theragra chalcogramma
Eleginus navaga gracilis
Atka mackerel Pleurogrammus monopterigius
Sand lance
Amodites hexapterus hexapterus
Capelin
Mallotus catervarius
RockfishSe9astodes polyspinis
Saury
Cololabis saira
See lamprey
Entosphenus tridentatus
Cod
Whiting

Pleurogrammus monopterigius and Cololabis saira are most commonly
found in stomachs of humpback and sei whales respectively. Especially
one of favourite foods of humpback whales is Atka mackerels. Humpback
whales take mainly it in two regions, the west waters of Attu Islands
and the south waters of Amchitka Islands. They have taken no otherfoods than Atka mackerels in these waters. Atka mackerels may beswarming in large number along the offshore of these Islands, andhumpback whales flock togather to take them. To the interest, otherjFOODS OF BALEEN WHALES

55

fin and sei whales seldom take Atka mackerels although they are swarming in the same waters. The data shows that only 2 fin whales takeAtka mackerels at the same time in the sea area. Since the year 1952,
3 fin whales have taken fish dominantly, and 4 whales fish with euphausiids or squids. The latter whales may take those fish which were in
taking their foods with swarms of euphausiids. The fact that stomach
of fish are satiated by euphausiids suggests those fish are involved inswallowing of whales.
It is often observed that sei whales have taken sauries in the adjacent waters to Japan (Mizue, 1951) and 5 sei whales has been foundto take them through this survey. The locations of such whales caught
are considered to be limited to the western side of the North Pacific. Thoseswarmings of saury looking for light of the ship have often been observed from the factory ship in night in the western whaling grounds.
TABLE 7. OCCURRENCES OF SWARMING FISH FOUND IN STOMACHS
OF HUMPBACK WHALES
Year~-----

Kinds of fish

1953

Atka mackerel
Cod
Capelin
Sand lance
Unknown

11

1954

---

---~--~

1955
13

1956
21

2
3
2
1

2

Sand lance is one of the favourite foods of little piked whales of adjacent waters to Japan (Omura, 1956). Few of them are sometimes
found in stomachs of humpback whales in whaling ground D.
Cod and rockfish are considered not to swarm so closely like Atka
mackerels as to stimulate whales' appetites. Few baleen whales take codor rockfish as compared with other fish above described, while many sperm
whales take them in these waters (Betesheva & Akimushkin, 1955).
From above observations, I consider that fish are only makeshift foods
for blue, fin and sei whales and rather important for humpback whalesin the northern part of the North Pacific where zooplanktons are moreabundant than any other southern waters.

Squid
Large squids, Ommastrephes sloani pacifieus is the most important,and some small squid (Watasenia scintillans) and opalescent squid (Loligo
opalescens) are also considered to be appeared in some cases.
OthersOther organisms, such as Themisto sp. Sagitta sp. are occasionally

56

T. NEMOTO

observed among euphausiids and copepods. These trespassers are soabundant as planktons that whales can hardly help swallowing a certain
number as discussed by Mackintosh & Wheeler (1929). Some live
specimens of Pandalus shrimps are collected in a stomach of a fin whale
through Japanese investigations. To the interest, they have survived
other euphausiids in the same stomach by about six hours.

b D

Sd'N

·~
o

170'£

...

ATTU

~.

i'

17S'E

IQO'

Fig. 10. Occurrences of fish eaten by baleen whales: Solid circles-Pleurogrammus
monopterigius eaten by humpback whales; Open circles-Pleurogrammus monopterigius eaten by fin whales; Solid triangle-Cololabis saira eaten by fin whales; Open
triangles-Cololabis saira eaten by sei whales; Open squares-Gadus macrocephalus
eaten by fin and humpback whales.

FEEDING HABITS OF WHALES

Preferences for foods by whale species. From the observations on
stomach contents of whales, considerable differences are noticed among
favourite foods of each whale species as stated in foods of whales. The
data at my hand accordingly somewhat differ from those of U.S.S.R.
collected in Kurile waters. Sleptsov (1955) states that baleen whales
are polyphagous, they take any foods whenever they meet zooplanktons,
cephalopods, and swarming fish. But it is considered baleen whales
possess remarkable preferences for foods after my observation. The
main causes for this phenomenon are considered as follows.
First, the differences of baleen plates may be attributable to theirselection for foods. As baleen plates differ in length, breadth, andthickness. The degree of luxuriance, size and length of baleen fringes,
these characteristic dispositions of baleen fringes have direct effect tofoods of whales. Whales with fine baleen fringes, such as sei whales orright whales (Balaena glacialis) are fitted to skim micro-zooplanktons
mainly copepods. Blue whales which have fairly rough baleen fringes
prefer euphausiids. Marshall & Orr (1955) suggest that Calanus
finmarchicus possibly may escape capture by some species of whales
with rough baleen finges, as Calanus is much smaller than euphausiids.
It is an interesting illustration on this point that Mizue (1951) states,

FOODS OF BALEEN WHALES

57'The whales having large sized baleen eat more homogenious food consisting only of small food', that is, 'larger the sized of the whales are
the fewer the varieties of food they eat'. However, the degree of
luxuriancy of baleen fringes is considered to be not so important asdecides food species of whales.
The preference of whales for foods are considered to be affected partly
by the ecological condition of food planktons, fish or squids in the sea
waters. The condition of density in the sea differs markedly by eachspecies, for example euphausiids bears different form of swarmings from
copepods. This must effect the whales in selection of their favourite
foods. Heretofore, two types of feeding habits of whales have been
considered, Skimming and Swallowing. Sei whales are observed to'skim ' the food (Ingebrigtsen, 1929), while blue, fin and humpback
whales turn over, aften with part of the head above water swallowing
foods (Ingebrigtsen, 1929). If some swarmings of zoo-planktons are notso crowded, whales such as blue, fin and humpback whales may pay
little attention to such swarming of foods. While some observations in
the North Pacific show that sei whales often take copepods, Calanus
plumchrus or C. finmarchicus of comparatively small quantity. The
cases, Calanus plumchrus found in the west-south of Attu Islands in
1953, or C. plumchrus in the eastern-south waters of Aleutian Islands
in 1954 are the facts illustrative of above observations. On the whole,
the fed copepods are fresh in the first stomachs suggesting that sei
whales took them a little while prior to that time of capture. From
above observations, it may be concluded that sei whales have skimed
the waters to take copepods. They can take the swarm of C. plumchrus
which is less crowded swarming sparsely as not to stimulate fin whales'
appetite. Ingebrigtsen (1929) describes the feeding habit of sei whales
as follows: 'It swims at great speed through the swarms of copepods,with half open mouth, its head above water to just behind the nostrils.
The copepods rush in with the water and are filtered from the waters
by the whalebone plates. When a suitable mouthful of copepods has
been taken the whale dives, shuts its mouth and swallows the food'. I
observed this skimming of sei whales in morning in the adjacent waters
to Japan in 1955. If sei whales take their foods in this method, they
can take rather rough swarms of copepods as above described.
Kitou (1956) observes many patches of C. heligolandicus at the surfaceof the sea. Many orange coloured patches of C. heligolandicus, each ofthem covering an area of 1 to 4m2 and 1 to 2m deep, distribute so far
as 15 miles. These swarms may be favourable condition for sei whales.
Calanus cristatus, one of the favourite food of fin whales, are always
found in a large quantity in stomachs of whales, suggesting that swarms

58

T. NEMOTO

of Calanus cristatus are more crowded than C. finmarckicus or C.
plumckrus in these waters. The catch of fin whales in August 1953 in
the whaling ground A decreased markedly as compared with the catchin June and July. The phenomenon is considered to be due to the poorof favourite foods, Calanus cristatus or euphausiids in this mouth. Fin
whales must have gone from these waters to another where theirfavourite foods were more abundant. In the case that sei whales havetaken C. plumckrus in the eastern-south of Aleutian Islands in 1954,
sei whales have shoaled in large number. These also comparatively
many fin whales were found, which scarcely take C. plumckrus dominatly.
They take mainly euphausiids, thus sei whales and fin whales must havedifferent preferences for food planktons. Hollis (1939) describe very
phenomenal occurrences of eggs masses of euphausiids from stomachs of
Alaskan whales. If the egg masses of euphausiids is enermous, it isprobable that whales are attracted to them.
In next, the depth of distribution of zooplanktons and other organisms
must be discussed in relation to the whales' feeding habits. Sei whales,
sometimes, have taken squids with copepods. The same freshness ofabove two species suggests that they have been fed at the same time.
This means that the depth of distribution is perhaps same for two
species. The sei whales may take squids which come up from the depth
to take copepods, because one of favourite foods of squids are copepods
in these waters (Sleptsov, 1955).
Feeding activity in a day. When Japanese biologists examine stomachs
of whales they describe usually only quantities of stomach contents by
the classifications of four grades (R, rrr, rr, r). There are some works
carried out to weight the contents (Nishimoto, Tozawa & Kawakami,
1952; Betesheva 1955), however, I have no accurate data if above fourclassifications mean real volume of contents. Such classifications may
be affected by sizes of whales, species of whales, and the decision by
naked eyes is of course not so accurate. In addition, the fact, thatwhales often disgorge their stomach contents when they are attacked
by harpoons, has been sometimes observed. For these reasons, thediscussion on the data is not so stable. While some interesting tendencies
are derived from them.
When I classify the fullness of stomach contents by the :time ofwhales caught, it is indicated that baleen whales caught in the morning
take more foods in quantities than those caught in daytime or the afternoon. There is also interesting tendency, whales with stomach contents
again increase in number from the evening to night. Figures drawn
after the data are shown in figures 11 and 12. The most remarkables of
fin whales are found in August and September in 1952, 2nd decade of

FOODS OF BALEEN WHALES
%

59

%
100

100

3 r.d.

Q)

bO

May
1954
A

Ill

t

50

50

'

Q)

u

~ 20
~
"U

•,

'•,

Au g.

L

20
10
0

1952

10
O>----50

(JJ
Q)

so

·lL

20
10
0

Sept.
4-6

8-10

12-14

16-18

20-22

.. -...... ..... \

10
0

%

,'

.

··."······

'\.-'

'

so

1 st.
June

:'"·

~

20

hr.

'

3 rd.

VJune

50

20

/
_,
.··__

~ 10
<tl

20
10
0

1953

t

0
100

Q)

u
L

<U
Q_

~ 50

so

1 st.
July

-u

3rd.

<U
<U

20

lL

'

10
0

June
1 st.
July

\/

.-.
___ ··..... ~
.....

2 nd.
so

July

20
10
0

........
--------···-----·

bO

50

"

Q)
Q)

lL

3 rd
Jui

20
10

0

4-6

8-10

12-14

16-18

20-22

hr.

%
50

Q)

bO
Ill

20
10
Q>-----

3rd.
Aug.

~ 50
<U

~ 20

3 rd.
Aug.

c

g_ 10
0
~so

50

-u

2nd.

~

Sept.

<U

20
10

0

3rd.

Sept.

20
10
0 '--4:"_-:-6-~-:::s~-1-=-o~-:-12~-~14-~-16~--1s-~2-o~-2-2~_Jhr.

8-10
12-14
16-18
20-22 hr.
4-6
Fig. 11. Feeding percentages of fin whales in A and D whaling grounds. Solid lineThe percentage of the number of fin whales with foods in their first stomachs morethan r for all whales caught at that time. Broken line-Those more than rr.

T. NEMOTO

60
%

%

" JOO

Fin

. bo

whales

<I
+.>

Blue

3rd .

whales

June

c

"
'0

-I•'\\

~ 50

I

I

\

I

20
~ 10
0

u.

\

\

I

',

I

/

1954

,

'

',

4-6

3 rd.
June
1954

20
10

D

\

I
-0

50

8-10

12-14

16-18

c

0
100

20-22 hr.

%

JOO

Fin

whales

Q)

1 st.
19 55

A

50

.,

+-'

,

~

~

20
10
0
0
'- JOO

'

/"

~ 20
L

bo
<I

I st.

,,,,

bO
<tl 50

+>

bO

"

-0

"

lL

' ..\

/

10

July
\

/

Of---L'--------"----~-~-----j
100

c

Q)
Q)

lL

50

2nd.
July

bO

50

-0
Q)

......... ,,

Q)

1'

u.

,'
,''

20
10
0
50

"
'

' ..,

..................

,,,----"

,/

20

10

I t
s .
Aug.

01--~~.,.,,,....,~~~~~~~~~~~~~-j

JOO

c
3rd.

20
10
0

so

Aug.
4-6

8-10

12-14

16-18

20
10.

20-22 hr.

0

%

so

lO

\

''

0
Q)

bO

<I

'\

'\ \
3rd.
July

\

' .....
\

Q)

20

100

Humpback
wh a I e s
I

Q)

'

a

I)

16-18

hr.

%

1953

\

50

+>

'-

12-14

8-10

4-6

3rd.
Aug.

20

1955
A

c

lO
lL
0
Q)

1954

....,'...., __ _

A

bo

<d
+-'
c

I

bo 100

bO

I

I

1954

I

I

D-

\

I
I

I

"u 2010

Q)

I

I
I

L

lL

c

so

3rd.

,'\

I

I

I

I

I

I

I

I

I

0

I

100

c
-a

-0

"'
Q)

u..

50

3 rd.

Q)
Q)

June

LL

1954

50

D
20

10
4-6

8-10

12-14

16-18

20-22 hr.

20
10
0

4-6

8-10

12-14

16-18

Fig. 12. Feeding percentages of fin, sei, blue and humpback whales in A, B, C and D
whaling grounds. Solid line-The percentage of the number of whales with foodsin their first stomachs more than r for all whales caught at that time. Broken line
- Those more than rr i.

FOODS OF BALEEN WHALES

61

July in 1953, and 3rd decade of July in 1953. The :figures in someother decades show somewhat indistinct decrease in daytimes. Genellary
speaking, feeding activities are comparatively heigh though in the daytime especially in whaling grounds C, where a euphausiid T. inermis isabundant along the margin of continental shelf. T. inermis in thiswaters may have more chance to stay at the surface waters owing to
the upwelling currents along the margin of the continental shelf. Thus
whales may take more foods in the daytime than other whaling grounds.
On blue and fin whales in the Antarctic waters, Nishiwaki & Oye (1952)
also have noticed the stomachs of whales caught mainly in the afternoon
were often vacant. They conclude that more whales take their foods inthe morning, and a clear peak in the morning in feeding percentages
show that they take their foods once a day. Although they do not allude
the slight ascent of feeding percentage is also observed in their :figures.
Mooth
April
May
June

J,ly

A"g
Sept.
Oct.

0

Fig. 13.

6

7

8

9

13
Hour

14

16

17

18

19

20

21

22

23

24

The duration of twilight of the latitude 50 North after the abridged
nautical almanac 1956.

The brief review of the research work undertaken by Slava whaling fleetof the U.S.S.R. (a pamphlet for the International Whaling Commission
7th meeting in Moscow, July, 1955) also alludes to such tendency.
If whales take more foods in the morning or evening than in thedaytime, it is considered intuitively that the tendency is partly due tothe clear diurnal migrations of zooplanktons. Ingebrigtsen (1929) states'skimming of sei whales takes place especially in the evening or earlyin the morning when the copepods are most at the surface of the sea'.
The diurnal migrations of zooplanktons are well known by many excellent biologists. The research for copepods, mainly Calanus finmarchicus, are fully discussed and summerized by Marshall & Orr (1955).
From the results of many those works, Marshall & Orr states 'It is
now generally agreed that the immediate stimulus to diurnal migration
is light, perhaps modified in extreme cases by temperature '. In thewhaling ground A, fin whales take Calanus cristatus dominantly in June
and July as stated before. The feeding percentages in these seasons

T. NEMOTO

62vary very remarkedly as shown in figure 11. All fin whales caught between four to six o'clock take their foods, and the feeding percentageof whales caught in the next time section between six to eight o'clock
suddenly has decreased. As the sun rises at about four o'clock in Julyin these waters and whaling catchers have commenced their chasing
with sunrise, whales which captured at four to six o'clocks must havetaken easily swarms of Calanus cristatus concentrated at the surface
waters in the morning. And feeding percentages have fallen accordingwith the sunrises and Calanus cristatus swim down to avoid the light.
Marr (1955, 1956) describes that the adolescents and adults of Euphausia
superba are mostly limited to the surface waters mainly above 10 meter
depth. And typical diurnal migration of Euphausia superba is not observed by Hardy & Gunther (1935). By their discussions the behaviourof E. superba appeared to be very erratic, but far more specimens are-- ........................ . .

100............. ~--------,

<ll
OJ

'

<O

.µ

c

<ll

~

<ll
0.

''

'

'

\

'\

\ \ ,,, ,

50

OJ

c

-0

<ll
<ll

LL

0

2

4

6

8

10

12

14

16

18

20

22

24 hr.

Fig. 14. Feeding percentages of fin whales in the Antarctic in 1955. Solid line-lst
decade of January. Chain line-2nd decade of January. Dotted line-3rd decade ofFebruary.

taken at the surface during the hour of darkness than during daylight.
They suggest that the migration may be less marked when the lightbecome weaker as in the late of March and April in the Antarctic
waters. The Japanese investigations on board show that the typicaldecrease in the feeding percentage through daytimes also occurres in the
Antarctic. As shown in figure 14, whales caught in the first decade ofJanuary show sudden decrease in feeding percentage from four o'clock.
While, feeding percentage of morning shows a low pitched decrease in
2nd decade of January. In 3rd decade of February, the feeding percentage is still heigh between ten and twelve o'clock. As discussed by
Hardy & Gunther (1935), the light become weaker towards the end of
the summer in the Antarctic. The low pitched decrease correspondswith the intensity of the light in the sea.
The diurnal migrations of zooplanktons are considered to be also affected by the depth of the sea, stages of their biological development

FOODS OF BALEEN WHALES

63(Marshall & Orr, 1955) and hydrographical condition of the sea. Some
biology of Calanus cristatus is made by Bogorov & Vinogradov (1955a).
They suggest the vertical distribution of C. cristatus show pecurial f eature in some part of the North Pacific where the intermediate cold waters
are found. C. cristatus is scarce in the intermediate cold layer, butvery abundant above the layer. In waters where the intermediate coldlayer vanished, C. cristatus is not so restricted at the surface waters.
Accordingly C. cristatus above the intermediate cold layer swim down
not so deep to the cold layer though in the daytimes.
Sei whales often take Calanus plumchrus also in daytimes in June in
1954. In other words Calanus plumchrus is considered to show not so
distinct diurnal migrations because C. plumchrus is collected by verticalplankton nets more than other Calanus in daytimes' towing. Although
sei whales are considered to dive not so deep as other fin and blue
whales (Ingebrigtsen, 1929) they must be easy to skim C. plumchrus in
daytimes if only C. plumchrus is limited to the surface waters in someseasons in the North Pacific.
The vertical migrations of zooplanktons are fairly speedy in somecases. Big krill Meganyctiphanes norvegica swims vertically about 128
meters for an hour and is capable of bursts of 271 meters in an hour
(Bainbridge, 1953). It is thought by some observation Calanus aentus,
a big Antarctic copepod, migrates vertically 50 m or more in an hour.
Recent investigations also show that the deep scattering layer, considered to be of zooplanktons also migrates fairly speedy. The diurnal migration of euphausiids' layer has not been examined in these whaling
grounds. But, in the adjacent waters to Japan, Saito & Mishima (1953)
observed the deep scattering layer consists of Euphausia pacijica in thewater off Hokkaido by the echo-sounder. They state the deep scattering
layer is observed 50 to 60m deep from the surface waters at 35 mintes
past 4 p.m. Then, it come up gradually and it come up to the surface
after the sun -set.
A pending question, how deep whales dive usually below the water
surface, has not been dissolved successfully to this time. Ommanney(1932) states, 'It may be said, then that a whale probably does notdescent to depth much greater than 130 feet, but can remain below forperiods of up to half an hour', from the view of the danger of caisson
disease. As adult specimens of C. cristatus have been found usually indeep waters below 500 meters in northern part of the North Pacific (Nakai& Honjo, 1954; Anraku, 1954; Nakai & Honjo, 1954) suggest, the fact
no adult specimens of C. cristatus is found in stomachs of whales
means baleen whales dive not so deep as 500 meters. Of course, adultspecimens of C. plumchrus have been found at the surface waters in

64

T. NEMOTO

northern parts of Bering Sea above 150 meters the presence of themnever means such conclusion as C. cristatus.
Recently, Owatari, Matsumoto & Kimura (1954) investigated feeding
habits of some dolphins, and presume that ' the dolphins do not likely
to swim deeper than 40 meters at any time nevertheless there are many
sardins escaped from above waters in the deeper waters of 40 meters.
The dolphins swim streight rising and falling at the surface waters
above 40 meter depth, and when they meet their foods they take foods
swimming hither and thither'. Of course, it is true that other sperm
whales and haired beaked whales take foods in far deeper waters as
described in the paper by Laurie (1933). Sperm whales caught along
the Aleutian Islands have often taken deep-sea fish and deep-sea crabs.
In contradiction to this, Matsushita (1955) has examined stomach contens of sperm whales in the Antarctic during the years 1953-54, andstates that sperm whales caught at night are less in number but fed
better than those caught in daytimes. By his observation, sperm whalescaught at early in the morning took abundant foods, but those caught
in daytime fed less, and whales fed regained in number in the night.
He suggests from above findings, the most favourite food of sperm
whales, gigantic deep-sea cephalopods and fish may come up to the sea
water surface through night and be caught by sperm whales. If it istrue, sperm whales need not dive so deep to take deep-sea cephalopods
as considered to this time by Iwai (1956) and others. Sleptsov (1955)
also consideres, many deep-sea cephalopods come to the surface throughnight with other oceanic deep-sea fish, forming good feeding grounds
for whales. One of causes for such phenomenon must be the intensityof the illumination by daylight, and the next their main foods, smaller
zooplanktons ·also come up to the surface waters through the night.
On some other marine mammals feeding habits also have been examined. Taylor, Fujinaga & Wilke (1955) describe that feeding activity of seals is probably a response to the upward migration of lanternfish and squids at night. They state that though seals take their foodsin night than in daytimes. Main foods of seals are those lantern fish
belonging to Myctophidae (Taylor, Fujinaga & Wilke, 1955). Thus they
conclude seals feed more actively before and during sunrise than during
daylight. Alike above seals, sei whales in Bonin waters (Nishimoto,Tozawa & Kawakami, 1952) or in waters off Japan (unpublished dataof the Whales Research Institute) take many lantern fish also in twilighttime of a day. From above many observations, it may be concluded,
feeding activity of whales must be partly affected by vertical diurnal
migration of crowding patches of zooplanktons, fish and squids.

65

FOODS OF BALEEN WHALESTHE INFLUENCE OF CHASING TIMES TO STOMACH CONTENTS

The review of U.S.S.R. (1955) shows that the stomach quantity ofcaptured whales are also affected by the time of chasing. Whales caught
with short chasing have a few foods at least. On the contraly, the longtime chasing causes vacant stomachs of whales. That is, the longer thetime of chasing, the fewer whales which have foods in their stomachs.
TABLE 8.

FRESHNESS OF THE STOMACH CONTENTS OF
FIN WHALES IN 1954
Freshness of stomach contents---Time of chasing(minutes)

0- 30
31- 6061- 9091-120121-150
151-180
181-210211-240241-270271-300
301-330331-360
...;
c
0
u

..c
u
tO
E
0

Cope pods

Euphausiids
f

ff

fff

13
19
31
27
20
6
7

9
40
26
26
14

14
22
12
10
5
4
3

11

1

7
6
3

2

1

F

f

ff

5
2
5
1

5
9
6
2
7
2

8
7
7
2
4

240-270

300-330360-390

fff

F

4
3
2

2
1

%
60

40

.µ

U>

20
(})

-----.......

Q)......... __

....

tO

..c

?3:

0-30

60-90

120-150

180-210420-450

480-510

The time of chasing (m)Fig. 15. The variation in stomach quantity of fin whales in 1954 by chasing intervals
in minutes. Solid line-More than r. Chain line-More than rr. Broken line-Morethan rrr.

The quantity of stomachs of whales decreases in accordance with prolongation of chasing as shown in figure 15. This phenomenon at the sametime, suggests the whales with full stomachs are more easily caught thanwith vacant stomachs. The result is essencially similar to that describedby Ingebrigtsen (1929), 'When these whales have no copepods they areoften so shy and difficult to approach within shooting range, that they66

T. NEMOTO

may be chased all day without being shot '. The freshness of thestomach contents also declines, as shown in table 8 with prolongations ofchasing. As the freshness of foods suffer no peculial change while waiting to be fiensed by my observations, the foods is considered to bedigested during chasing.
BIOLOGICAL DAT A ON FOOD PLANKTONSAGE AND GROWTH OF EUPHAUSIIDS

The growth of various species of euphausiids has been studied by Ruud
(1932) and Einarsson (1945). Euphausia superba is biennial in the
Antarctic water (Ruud, 1932), and Thysanoessa inermis, the most famousfood for baleen whales in the Atlantic, is annual in southern localities,
biennial in north and Icelandic waters, and some specimens in West
Greenland waters are considered to be triennial (Einarsson, 1945). To
the northern Pacific, above conclusions are applied in various points. In
the materials composed of stomach samples, I have measured about 30
specimens of each sample as possible at the same time examining thematurity of the external and internal sexual organs. The maturity ofthe external sexual maturity is determined by the formation of theendopodite of first and second pairs of pleopods in male, and of thethelycum and the presence of spermatophores in females.
The internal sexual maturity is determined by the examination ofovary and the presence of loose spermatophores in the spermatophoresac. As for the grades of maturity of euphausiids, I use the classification described by Einarsson. Those are the following groups: ' 1. The
larval and early post-larval stages, showing no sign of external characters.
2. Juveniles and adolescents showing various degrees of developmentof the external sexual characters, but not showing mature characteristics.
3. Adults with the external sexual characters fully formed, the maleswith loose spermatophores in the spermatophore sac, and the great
majority of the females fertilized, i.e. with spermatophores inserted into
the thelycum. 4. Specimens which are larger than the usual maturesize of the species in a certain area, but showing immature external
sexual characteristics'. The results of my observation and measurements
are shown in following figures. Specimens belong to 1 group and show
slight development of external characters, such as only swelling has
appeared in the first endopods, are put into 1 group.
Euphausia pacifica. As E. pacijica appears in less number among thematerials, the exact life cycle is not able to be illustrated. The fertilized
specimens of females with spermatophores in their thelycum are also

FOODS OF BALEEN WHALES

67

rarely found. However, distinct two size groups are observed as illustrated in figure 16. From June to September, the larger group, perhaps
belong to 1 or 2 year group is usually found throughout the summer inthe whaling ground A. The smaller groups, ranging from 6 to 12 mm,
is found in September in the whaling ground D. They are considered
to be 0 year group as sizes of them are suggesting that they have
hatched in this spring or early summer. Boden (1950) also describes
the latter larval stages of E. pacijica are abundant from spring tosummer in southern California waters. It is probable that these larvalstages may develop to length about 10 mm in autumn, though the growthof euphausiids is completely differs in the locality of them. Thesejuveniles and adolescents may attain to about 20 mm in next year, and
m.m.

D

A

20
.t:

~
c
~

....,

"'

~ 10

-

- - ,.... -%% % %3 %

%

%

0
0

10

0

10

0

10

5

0

5 0

5

5

%
0

% % 1% %
10 0

5

0

0

5

0

Number of specimens

Fig. 16. Size distribution in Euphausia pacifica in the whaling grounds A and D.
Oblique shading-The larval and early post larval stages, showing no sign of external
characters. No shading-Juveniles and adlescents showing various degrees of development of the external sexual characters, but not showing mature characteristics.
Blacked-Adults with the external sexual characters fully formed, the great majorityof the females fertilized, i.e. with spermatophores inserted into the thelycum. Solidline-Females, broken line-Males.

E. pacifica is considered to reach the sexual maturity in full one year

or more. Females collected on 9th in September in the whaling groundA bear spermatophores inserted, eggs of which have also fairly fertilized.
But, this is perhaps the rare example, as the spawning of E. pacifica
may occur in the more early season of a year from above results.
E. pam:jica of about 15 mm in length collected in the wamer waters
bears full sexual characters. E. pacifica may reach to the sexual
maturity within a year in the far southern waters of Japanese coast
like other euphausiids described following.
Thysanoessa inermis.
T. inermis in the North Pacific whaling
grounds is divided to three type by developmental conditions. In thewest whaling ground A, the specimens with spermatophores are only

68

T. NEMOTO

found m first decade of June. And adolescents with almost grownexternal sexual characters are found during June to September. Males
and females are perfectly classified by the external characters, and thedifference in size between males and females is observed from 18 mm
in length. These adolescents are considered 1 or 2 year groups. One
male specimens of 15 mm in length is collected in the far south waters
at 42°28' N, 149°48' E in August through 'Takunan maru' cruise, whichshows full grown external sexual characters and has loose spermatophore in the spermatophore sac. From the body length, it may belong
to 1 year group, judging by comparison with the Atlantic specimens described by Einarsson (1945).
TABLE 9. OCCURRENCES OF FEMALE SPECIMENS OF
T. INERMIS WITH SPERMATOPHORES
With spermatophores
Whaling area
Year
Date
Total no.
D

1954

"

"

"
A
"

D

ff

1955
ff

"

5 July19 ff
30 ff
2 June6 "1 July

1000
500
400
280
170
290

200
100
250
80
130
200

The collections from the whaling ground C and D show some differencesfrom those from the whaling ground A. Many fertilized specimens arecollected in July in the whaling ground D, while all other materials collected after July show no fertilized character. Thus it is considered
the mating season of T. inermis in the south waters of eastern AleutianIslands comes to an end in July. Perhaps the mating season of T.
inermis in this waters begins in early spring, larvae grow about 10 mm
or more in next year, those are 1 year group, then it grows in second
year about the length 20 mm to 26 mm and spawns. Specimens which
are larger than those fertilized mature in sizes, but showing immatureexternal sexual characters are found on 9th September sizes of whichare 24 mm to 28 mm, It is not certain if the larger specimens will bemature in the third year. Einarsson (1945) consideres, T. inermis inWest Greenland waters may be adult in the third year at about 28 mm
in length. Thus some specimens of T. inermis in this waters may beconsidered triennial.
In the north parts of the eastern Aleutian Islands the whaling ground
C, T. inermis shows interesting features. None of specimen in thesamples shows any external and internal characters fully developed.
While the body length of some of them are exceedingly larger thanthose collected in the west or southern waters. The figure gives themeasurement of these specimens caught in the north part of the eastern

69

FOODS OF BALEEN WHALES

Aleutian Islands. The larger materials collected on 13th September areabout 27 mm in length showing no sexual character fully formed. These
specimens may develope a little more and spawn in next year. The
m.m.

,---,---r,-~---,,--,--,----,,--,-.._..,.-...--r-r----r-,,---,~-~--~~---.~

30

A

>.

"CD
0

%

10

%
10

0

%'
JO

50

%

'.57
10

5

0

10 0

%

%

Ya

2){

10

10

JO

2Ys
0

JO

m.m.

30

c

.<::

+-'

""cru
>,

ro" 0
J

m.m.

,---.--,.---,--~--.--,---.--,--,--,-,--,---,.--...-~r---.-...--,-,----,---r~--~-~

30

j
- -~

20

0 0~~-J~o--'o-~-1Lo_o'--~--'-10-'-~s_J_~---"10'--~-1~0'---.s_J.o__,_.L_~-~10J__~---'10-o.L_J

Number of specimens

Fig. 17. Size distribution in Thysanoessa inermis in the whaling grounds A, C and
D. Double shading-Specimens which are larger than the usual mature size of thespecies in a certain area, but showing immature external sexual characteristics.

smaller group, about 10 mm to 18 mm also occurres in the late of summer.
As compared these size distributions with those reported by Einarsson
in West Greenland waters, the T. inermis in the whaling ground C,

70

T.NEMOTO

may be generally triennial as the specimens in West Greenland waters.
But some of them are considered to be biennal as the speciemens insouthern waters. In either case, the complete sexual maturity in twocases is attained in two years at least.

B

Fig. 18. Immature male copulatory organs of Thysarwessa inermis (Kr<f>yer) Hansenof comparatively large specimens.
A, 26mm.
B, 24mm x50.

Thysanoessa longipes. The materials of T. longipes in my samples arecomparatively abundant as compared with other euphausiids, and themore detailed disscussion can be obtained. In whaling ground A, thefirtilized female specimens, such as with spermaphores inserted into thethelycum or eggs are fully developed are found only in June, owing to
the scanty data before then. These full developed specimens have sizes
from about 20 to 28 mm in female and 18 mm to 24 mm in males. The
juvenils and adolescents are suceessively found from June to September.
Specimens about 13 mm in length are found 19th July most of whichshow no sign of external development of sexual characters of males.
These specimens may be considered to be 1 year group. In the collections following these, there are many specimens considered to be 1 year
group about 20 mm in length. They have well developed external
characters, but the thelycum of females is not full grown and therefore
it is empty. The differences of size distributions between males andfemales is observed from this developmental stage as illustrated in figure
19. In the late August, the smaller size group appears in collections
by plankton nets. Some furcilia larvae have mingled with them. These
specimens are apparently 0 year group, which have hatched in spring of
this year. With allowance for the scanty collection of 1 year group in
spring, above 0 year group developes to about 15 mm in next spring, andgrow about 20 mm in summer successively. Thus it may be concluded
T. longipes in this waters does not reach sexual maturity before it is
two years of age.
The samples from C and D whaling ground show somewhat defferent
manner of growth. The fertilized specimens of T. longipes have beenfound only 2 cases in C and D whaling grounds. Specimens collected
on 7th July in C whaling ground and 7th July in D whaling ground are

FOODS OF BALEEN WHALES

71

all full mature. It is interesting that some full grown specimens arefound in summer, which have no signes of mating or spawning though

>-,

"

0

co 10

- - - -

m.m . .---.-,--.,,--.-,.--.--.---,-.--.--rr-,,-.--.--,--.-,-,--r-..,..._,.,--,--_,

30

~
0

co

JO - -- -m.m.

-- - -- - -.--.,..--.-.--r---m-~-.,,--.,..--,----,-.,..--,--,--.--,--,---,,,--,-,.--r-o--r--ri

3o C~

D

Spineless form

00505

510

JO

0

5

0

5 0

Number of specimens

Fig. 19. Size distribution in Thysanoessa longipes in the whaling grounds A, C and D.

they have full developed thelycum and the male copuratory organs. They
may survive the winter and spawn in the next spring as the 3 year
group.

T.NEMOTO72

The smaller specimens are also found in the summer season, so twosize groups of immature are observed through the latter whaling seasonfrom July to September as shown in figure 19. As to the development of
the spineless form of T. longipes, I would condider it is usually annual inthese waters. Because it have full grown sexual characters at about 15mm
to 18 mm in length, and the larger specimen than 18 mm has never beencollected. Female spineless forms collected on 7th in August have
TABLE 10. OCCURRENCES OF FEMALES SPECIMENS OF
T. LONGIPES WITH SPERMATOPHORES
With spermatophores
Total no.
Whaling area
Year
Date
1954
1955
1956

B

c

A
I/

I/

I/

I/

I/

I/

I/

I/

I/

I/

25
20
30
60
18
18
5
6

30
30
100
440
20
20
45
200

7 June
6 I/
13 I/
13 I/
14 I/
16 I/
17 I/
19 I/

m.m.

-------tJ,

A- Ground25-------:----------;--' ------

20

:

..c:
.....
bo

~ 15

/
.
-!----I

,

,'

*"'

,... ....

.,.,

4

6

Fig. 20.

8

10

'
'

.

1
I
I
I

I

'

12

.

!

I
!

_/(Ge •

5----

Spawning

2

2-Gr.

1-Gr.
4

6
Month

B

10

12

2

4

6

8

The life cycle of Thysanoessa longipes in A whaling ground.

spermatophores inserted into the thelycum, while the original form among
above spineless form is not full mature. As the furcilia stage of T.
longipes has the lateral denticles of carapace in somewhat behind position
from adult specimens, spineless forms may make rapid progress in genital glands with some features such as the lateral denticle remained aslarval form by some stimulant. Above suggestion, however, is uncertain
because there is no valid pr,ove for it now.
Thysanoessa spinifera. T. spinifera only distributes in the eastern sideof the North Pacific as considered from the stomach contents of whales andFOODS OF BALEEN WHALES

73

samples by plankton nets, and some interesting biology on T. s'[li,nifera
was described by Hollis (1939). Hollis (1939) describes that the egg
masses of T. s'[li,nifera were particularly numerous in the humpback
whales shot on August 26 in Alaskan waters. This result suggests thatthe spawning of T. s'[li,nifera may occurres in summer. Some specimens
in my materials also bear such fertilized features. In nine cases fromstomach contents females of T. s'[li,nifera with spermatophores inserted
TABLE 11. OCCURRENCES OF FERTILIZED FEMALES OFT. SPINIFERAWITH SPERMATOPHORES INSERTED INTO THERYCUM IN C GROUND
Year

Date

Total no.

With spermatophores

1954

7 June5 July
5
"
30
"
10 ff
11 ff
14 ff
31 It
1 Aug.

1
14
5
6
14
6
70
30
60

1
12
1*
6
10
6
48
25
50

If
If
If

1956

"

"

ff

It

* Only one spermatophore.
m.m.
-

30

.c

- -

- - - -

20

µ

.,""c

Number of specimens

Fig. 21. Size distribution in Thysanoessa spinifera in the north-easternpart of North Pacific.

into the thelycum as shown in table 11. The egg is considerably developing especially in specimens found on 7th June, 5th, 30th July, andalso 2nd September besides the f ormar nine _cases.
The first specimen is considered to be full mature, with full grownegg masses in the ovary, though the size of body is 20 mm, rather
smaller as compared the latter grown specimens. The latter cases comprise specimens of length between 20 and 28 mm. Males found withabove fertilized females also bear the external sexual characters fully

74

T. NEMOTO

formed and with loose spermatophores in their spermatophore sacks. As
stated on T. longipes males are far smaller than adult females, sizes ofmales is usually ranging between 15 and 24 mm. Above stated result
completely coincides with the one reported by Hollis (1939) that T.
spinifera may spawn in summer from June to September in Alaskan
waters, the whaling grounds C and D.
Juveniles and adolescents bearing no full grown external sexual characters are also found in summer in these waters. Specimens found fromwhales' stomachs caught on 18th July and 12th September have little
signes of such characters. From these samples figures of the size distribution and life cycles of T. spinifera may be illustrated. It is, thus
considered that T. spin1"fera is biennial, reaching about 10 mm in length
in the first year and 20 to 24 mm in the second year when it becomesmature. The difference between males and females in the growth isobserved from the first year, and external sexual characters, in endopods
of first and second pleopods also begin to grow in the same year. There
are some large specimens, about 26 to 30 mm, with mature external
sexual characters with no spermatophors. But, I am not sure if thelarger specimens of T. spinifera in the samples obtained on lOth andllth July in these waters grow further in the third summer and spawn
like T. inermis in the Atlantic (Einarsson, 1945, p. 150).
SEX RATIO OF EUPHAUSIIDS

Einarsson (1945) examined the sex ratio of euphausiids and states, 'as
the sexes were on the hole found in similer numbers and of similer size,
I have not considered it necessary to divide the catches as to sex '.
But some different results are obtained through this study. As describedin the paper by Ruud (1932), Sars found no males of Euphawda antarctica, (synonym E. superba juv .) owing to only use the male external
character of the endopodite of first and second pairs of pleopods as themarks to divide both sexes. The classification on the sex, therefore,should be ascertained by the presence of spermatheca of females bisides
male characters (Ruud, 1932).
The sex of the euphaussiids in my samples, are determined by carefulconsideration to the points, and males and females are found not insimiler numbers. Sizes of specimens by each sex differ also considerably
in some species as stated before. Generally speaking, more females arefound than males. I count 100 specimens of each adult euphausiids,
then females are observed dominantly in almost all samples as shown intable 12. The total occurrences of females are also prevalent in the
recent study of euphausiids by Boden (1955).

75

FOODS OF BALEEN WHALESZOOPLANKTONS COLLECTED IN WHALING GROUNDS

Besides the stomach samples of whales, many euphausiids and copepods
are collected by plankton nets. I would state short review on thosesamples here. Further discussions on the materials will be given inanother article in future. The plankton samples are only restricted tothe surface waters' samples collected by vertical hauls from 50, 100, 150
and 200 meters to the surface.
TABLE 12.

OCCURRENCES OF MALE EUPHAUSIIDS IN EACH 100 SPECIMENS
Species----No. of males

10-14
15-19
20-24
25-29
30-34
35-39
40-44
45-50

E. pacifica

T. inermis

T. longipes

5
3
3
8
3

1

---------~""

T. spinifera

4
2

1

1
1

1

2

TABLE 13. TOTAL OCCURRENCES OF EUPHAUSIACEA PREPARED BY THE
FIRST SURVEY OF' W. SCORESLY', 1950, IN THE BENGUELA CURRENT*
Species of euphausiids

Thysanopoda microphthalma
Nyctiphanes capensis
Euphausia hanseni
E. lucens
E. recurva
E. similis var. armata
E. tenera
Nematoscelis megalops
Thysanoessa gregaria
Nematobrachion boiipis

Adult male

23
40
41
1

Adult female
1
75
4
304
46
1
2

3

7

2
2

* Figured up from the data by Boden, 1955.
Main species collected by plankton nets are nearly the same as previous
reports in this water (Brodsky, 1950; Banner, 1949; Anraku, 1954;
Boden, Jhonson & Brinton, 1955). Euphausiids in our samples are as
followings :Euphausia pacijica; Thysanoessa longipes; T. inermis; T. raschii ;T. spinifera; Thessarabrachion oculatus.
T. longipes is the most dominant euphausiid collected by plankton nets
in the samples and T. inermis is less in number on the contrary to theresult that T. inermis is the most dominant foods for baleen whales.

76

T. NEMOTO

T. raschii and T. spimifera are chiefly found in hauls within the marginof Alaskan continental shelf.
The adult Thessarahrachion o<mlatus is usually found below 100 meters(Boden, Jhonson & Brinton, 1955). Only one female of T. oculatus iscollected by the haul from 50 meter to the surface at the position
51°17'N, 162°56'E.
The main copepods are as follows:
Calanus cristatus Kr<pyer
Calanus plumchrus Marukawa
Calanus finmarchicus (Gunner)
Calanus helgolandicus (Claus)
Eucalanus bungi bungi Jhonson
Pseudocalanus elongatus (Boeck)
Pseudocalanus gracilis Sars
Centropages abdominalis Sato
Aetideus armatus (Boeck)
Euchaeta japonica Marukawa

Gaidius brevispinus (Sars)
Gaidius tenuispinus (Sars)
Scolecithricella minor (Brady)
Heterorhabdus papilliger (Claus)
Candacia columbiae Campbell
Metridia lucens Boeck
Pleuromamma robusta (Dahl)
Acartia clausi Giesbrecht
Oithona similis Claus

Calanus plumchrus is the most dominant Calanus in for ' Calanus 'species. Calanus jinmarchicus has not been reported by Japanese
workers from the whaling ground A (Anraku, 1954). But 'Calanus'
with typical 5th foot of C. finmarchicus has been occasionaly found in
my samples. Many other 'Calanus' are rather related to C. helgolandicus
as described by previous studies (Mori, 1937; Anraku, 1954). The smaller' Calanus ' Calanus paeiji<ms is reported by Brodsky (1950) from Pacificwaters. By his descriptions, Calanus pacificus is smaller than C. helgolandicus, and the endopod of the left leg reaches only to the distaledge of the second segment of the exopod in the male fith foot.
'Calanus' with such fith pair of foot has also observed in the samples.
Tanaka (1956) states on this point, the fith pair of foot of C. helgolandicus
is very variable, so it is not proper to attach importance to only the fith
foot of Calanus related to C. finmarchicus.
C. plumchrus is the most dominant copepod in the samples collectedby plankton nets, the adults of which are considered to distribute indeep waters blow 150 m (Anraku, 1954). By my studies it is observedthat the adults occurred in the vertical haul from 150 meter to the surface on 23 August at the position, 53°03'N, 166°58'E. Many adults arecollected by the vertical haul from 100 meter to the surface at the position 57°09'N, 173°44'W. Other many specimens are found in verticalhauls from 200 meter to the surface. In the northern area, above facts
suggest that the adult Calanus plumchrus appears commonly in the surface waters in the northern waters of Bering Sea. In the southernwaters along the Aleutian Islands, Calanus plumchrus may be commonly
found below 150 meter as described by Anraku (1954).

FOODS OF BALEEN WHALES

77

Calanus finmarchicus, Calanus plnmchrus and Metridia lucens are themost important foods for oceanic fish in these waters. Though they are
not so important as whales' foods, Pacific salmons in these waters takemany C. plumchrus as well as C. cristatus. The sparse swarms of C.
plumchrus are considered to be su:fficent to stimulate appetites of such
fish. Calanus pacijicus Brodsky is considered as one of the main foodsof baleen whales in the Pacific (Sleptsov, 1955). But, the swarms of
Calanus pacijicus is also considered to be not so congregated as C.
cristatus in northern part of the North Pacific. Consequently, the swarms
of C. pacificus in stomachs of whales never have been found by Japanese
investigations.
Many specimens of Encalanus bungi bungi are observed in plankton
samples, though I have few specimens in stomach samples. Sleptsov(1955) describes, Eucalanus bungi bungi constitues some part of foods ofwhales, but it has not been appeared as foods of whales by my observation. This fact suggests, E. bungi bungi swarms not so closely asother food copepods, C. cristatus and C. plumchrus, and not forms
huge biological masses. Accordingly, E. bungi bungi is considered to beof little importance as foods of whales. Stored fatty nutrients in bodyof Eucalanus are also not so abundant as other calanoid copepods.
Metridia lucens is also very abundant in plankton samples. It isobserved from the stomach of a fin whale caught at the position 55°38'N,
169°00'W with euphausiids. This is a only example for the dominantMetridia appeared in my observation. The sizes of the Metridia lucens
are comparative smaller than other locations in the whaling grounds.
As they are found in the warm backwaters off the Alaskan continental
shelf. These Metridia may have developed in such warmer backwaters,
and are remained in smaller sizes.
DISCUSSION

The whaling ground of northern Pacific is far smaller as compared with
Antarctic one, but it has some peculial features from the view of distribution of foods and oceanographic conditions. The stomach contentsof whales show some variations from year to year especially among fin
whales. In the whaling ground A, E. pacijica, T. longipes and C.
cristatus are main foods of baleen whales from early summer, T. inermis
is rather less in this water. Generally speaking, euphausiids are more
important than C. cristatus. While in Calanus year as 1953, C. cristatus
is more dominant from early May to July. Fin whales swarmed in thisseason to take C. cristatus; and many fin whales were caught in from
May to July in Calanus year. On the other hand, blue whales nevert.NEMOTO

18

migrate to the grounds if euphausiids are not abundant.
Wheneuphausiids are abundant as Krill year 1954, blue whales arrive at the
100

,
,,

-- - --- - -r- - -·.

~-'----.

80

(I)

bO

Calanus cristatus

<O

~ 60
QJ
u

,I
,

,,

,
/

t 40
0..

Euphausiids

20
May

June

July

Sept.

Aug.

The transition of foods of fin whales from C. cristatus to euphausiids
in the whaling ground A.
Broken line-1952. Solid line-1953.

Fig. 22.

100
rn

80

(I)

-;a
...£:

3 60

'to
40

.I..
Q)

..0
E

..........

-----·

20

:J

z

May

Fig. 23.

June

July

Aug.

Sept.

Number of fin whales caught in the whaling ground A.
Broken line-1952. Solid line-1953.

c
10·

f
.....111
::s
.I..

Q)

a.

E
Q)

-

5·

May

Fig. 24.

June

July

Aug.

Sept.

Mean surface temperature at mid noon position of the factory ship in
the whaling ground A. Broken line-1952. Solid line-1953.

whaling ground A already in June from southern waters. In 1953, someblue whales were caught in first decade of July, and many blue whales

FOODS OF BALEEN WHALES

79

were captured in September. The height of 2 year group euphausiids
was observed in late July, and of 1 year group in September in 1953.
Blue whales should swarmed on each euphausiids' swarms. Generally,
blue whales approach the cape Kamtchatke Peninsula or waters off
Kurile Islands from the south eastern warmer waters. Then blue whales
migrate along the Islands towards north-east. As to the migratoryroutes to north waters, it is considered blue whales follow the routes
much further off the coast, while they come back to the south waters
along the shore (Omura, 1952). Perhaps it is due to the distribution ofeupahusiids.
TABLE 14.

STOMACH CONTENTS OF FIN WHALES JN WHALING GROUND A
Year~---------~- - - -~-~

Kinds of stomach contents

1952

1953

1954

1955

1956

Euphausiids
Eu. & Copepods
Eu. & Squids
Eu. & Fish
Cope pods
Co. & Squids
Fish
SquidsEmpty
No of stomachs examined
Not examined

79
4

83
15

182
18

32
1

149
17
1

19

1
105

92

48

272
564

67
148

47
1
1
7
114
336

1
1
110
213

252
457
13

In June and July, the water south-west of Attu Islands at about 52°N,
170°E, attracts many fin whales every year as stated above. It is themost static whaling ground for fin whales in the North Pacific. Those
fin whales mostly take C. cristatus. When the biological autumn comesin this sea region, C. cristatus subsides to deep waters to be mature.
Thus C. cristatus becames scarce at surface waters in the late of Julyand euphausiid gradually take copepods' place though euphausiids arestill not so abundant in July. The catch of fin whales decrease in number consequently in August in this waters as in 1952 and 1953 as shownin figure 22. Especially only those fin whales with empty stomachs arecaught in August of 1953. Bogorov & Vinogradov (1955a) describe thatC. cristatus was very abundant during May to July in the surface waters
off Kamtchatka and Kurile Islands in 1953. Whereas it showed phenomenal decrease in August and vanished from the surface waters in
September. This fact endorses above observation obtained throughJapanese whaling expeditions. Accordingly whaling grounds consist of
C. cristatus may be said to be passing prosperity.
One of the causes may be the transition of the water temperature.

soT. NEMOTO

The surface water temperature at mid noon position of the factory ship
alteres markedly and rise rapidly during the first and second 10 days
period of July as shown in figure 24. This marked change in watertemperature is regarded as one of the contributing causes of the change
in principal whale foods. As C. cristatus distributes mainly in the lower
temperature waters, Nakai & Honjo (1954) state the sudden rise ofwater temperature hastenes the subsiding of C. cristatus. Bogorov &Vinogradov (1955 b) also observe, the rise of surface temperature causes
the subsiding of C. cristatus and C. tonsus in the north-west part ofPacific. In August, considerable swarms of fin whales are caught in
the whaling ground B, the north waters of Komandor Islands in 1956.
TABLE 15.

SIZE OF EUPHAUSIIDS MEASURED ON BOARD IN 1954

C whaling ground
Length (mm),----~

Decade5-15

lst June
3rd II
lst July
2nd II
lst Aug.

15-20

20-30

Unknown

1
1

16
29
64

2

D whaling ground
Length (mmJ
Decadelst June
3rd II
lst July
2nd ''

5-15

15-20

20-30

Unknown

2
7
20

6
11
13
4

7
38
51
44

20
9

Those fin whales may come from southern waters to take foods, because
the southern waters, the whaling ground A are unproductive in August
and there are few C. cristatus in surface waters in 1956. On the other
hand, C. cristatus is considered to be at the height in this northernwaters, the whaling ground B, just in August about a month late.
In the north waters of east Aleutian Islands, fin whales take C. cristatus
in off waters from continental shelf slope, while they take T. inermis
mainly distributing along the continental shelf slope. This fact alsosuggests that the deep waters is necessary for C. cristatus to be adults
as considered to these days. On the other hand T. inermis spawns on
the bank of continental shelf (Hjort & Ruud, 1929; Einarsson 1945)and drift to the surface strata where the larva develops (Einarsson, 1945).
So, T. inermis circulates in the sea waters from bank of the shelf to offwaters and vice versa. The distributions of T. inermis as illustrated in

FOODS OF BALEEN:}WHALES

81

figure 25, show apparently this circulation. T. raschii and T. spinifera
are more coastal form as considered to this time, and they never spawn
in so off waters. Because no fertilized specimens of above two species
have been collected in off waters. The size measurement of euphausiids
also has been carried out on board. In 1954, the comparatively much

1000

L

"
"E

+'

56•N; 172'w
00000000000000000000

-'=
+'

~IOOO

Cl

2000

B

3000

.!O

i1000

"

Cl

Fig. 25. The distributions of Calanus cristatus and Thyanoessa inermis in relation
to the sea depth of whaling grounds. Open symbols-Calanns cristatus. CrossesThysanoessa inermis. C figure,-Cut the center position at an angle of 45° from
north·west.

data are obtained as shown in table 15. The larger euphausiids are
more dominant in C and D whaling grounds than the smaller one. This
smaller group, ranging 5 to 15mm is directly considered to be 1 year or
O year groups. I observe the rare occurrences of such 0 year groups in
C and D whaling ground chiefly consist of T. inermis and T. longipes.
The furcilia larva of T. inermis is observed on 29th July in the stomachof 1 fin whale caught at 54°33'N, 171°43'W.

82

T. NEMOTO

Rund (1932) and Marr (1956) also consider the occurrences of furcilia
stages of E. su,perba in the Antarctic. But the dominant foods for
Balaenopteridae whales are those over 20 mm or more in length. And
as a rule Furcilia stages of euphausiids may not be able to be favourite
foods of whales as discussed in the special part on euphausiids.
The whaling ground C has large continental shelf in the north-east
part. As winds blow usually to north-east in summer, some physical processes of enrichment of surface waters are considered (Cooper, 1952).
Cooper states in his summary that, 'Winds blowing intermittently towards a continental slope may produce vertical oscillations which bringabout spillage of deeper nutrient-richer water from the ocean to the continental shelf'. With the upwelling current caused by sea currents met
the continental shelf from south-west along the Aleutian ridge, the borderline of Alaskan continental shelf should be very powerful productive
region. Swarms of euphausiids on the border line is also due above
causes. Hardy & Gunther (1935), also describe the distribution of blueand fin whales is deduced from the phosphate values of waters by theexamination on the waters of South Georgia.
It is often observed by the whalers that, the state of whaling grounds
has changed during the stormy weather. They say whales swam in anarea migrate to another waters after the storm in some occations. If
above observations is true, it may be attributable to the change ofaspects in their food planktons by storm. However, Hardy & Gunther(1935) state that their results do not confirm the supposition that plankton
organisms tend to sink from the surface layers during stormy weather.
And stormy weather does not influence the state of swarms of zooplanktons.
Uda & Nasu (1956) describe the relation between the whaling condition
and the cyclone. They conclude that 'Shoaling condition of whales on thedays after the passage of cyclone was better than that on the days before
the passage of cyclone'. But by my observation, whales never shoal or
disperse as a rule according with any weather conditions. Whales are considered to only swarm or disperse on their foods or for the reproducting.
The rendezvous of whales is found along the boundary between two
currents in the whaling ground A. The center of rendezvous of whales
lies in south-east waters in June, whereas the center ascends to northin July. It descends again to south from August to September according with the variation of boundary region. The intensity of the cold
current along the Kamchatka Peninsula and the current along the southof Aleutian Islands decide above boundary. The migrating routes ofbaleen whales appear in the zone of abundant food such as above boundaries, and it seems that the migration of whale schools are subjectedby the north and south movement of the front of abundant food zone

FOODS OF BALEEN WHALES

83(Uda, 1954). The whale schools in the north waters of the eastern
Aleutian Islands may attain there along those routes of boundaries.
When the examination on zooplanktons of stomach contents is carriedout, the simplicity of dominant species is more often observed thanthose mingled with two or three species at the same rates. Nakai &Honjo (1954) state those simplicity of foods' species suggest that they
apt to form the swarm of single species in the sea. In other words,
each zooplanktons tend to form the swarm of themselves. This characteristic feature of planktons may partly due to the different ecologicalor the different spawning seasons of each species. As stated before,
Thysanoessa spi,n1j'era may spawn in summer, and Euphausia pacijica,
Thysanoessa longipes and Thysanoessa inermis spawn during spring to3rd. Ju!Y((
lst. Au!g/ /,' --':)

2 nd.Ju~ __ ,_,---lst.June

--

/

ll

I

so°N
I

3rd.Au91

II

1/ '~ --Ist. Sept_.

v / / ' ,/f ,/2 nd. Sept.
'/

:

I
I

1

lst. Ju!Y lst June
160°E
I s°E

Fig. 26.

Attu I.
~.

I a°E

The centre lines of the whaling ground in 1953 in the whaling ground A.

early summer in these waters. If euphausiids swarms for spawningas considered by Hjort & Ruud (1929), respective eupeausiids swarm
separetely for their reproducting. Generally speaking T. raschii andT. spi,nifera are coastal forms, usually found within the continental shelf.
T. inermis is more commonly found beyond the margin of the continental
shelf (Banner, 1954). And above respective euphausiids may be carriedby water currents of different conditions to the same waters. However,they may never dismiss their swarms.
Each swarms of zooplanktons must have peculial characters as biological masses, and preferences of whales for foods are due to such differences of biological masses. Very rare example, that blue whales took
copepods in considerable quantity, are observed in two cases. In bothcases, copepods are mingled with euphausiids at about the same rate.

T.NEMOTO

84

Humpback whales, that never take copepods, also take the mixture ofeuphausiids and copepods ... Above facts suggest that, the swarming
mixtures of copepods and euphausiids bear the characteristic features of
euphausiids' swarms, and blue and humpback whales take the swarms
of copepods and euphausiids for the swarms of euphausiids. It is a very
interesting fact that the mingled swarms of euphausiids and copepods
stimulate blue and humpback whales' appetite, which have poor appetites
for swarms of copepods.
In the first stomachs of whales, it is occasionaly found that foods aredigested different grades in the first stomachs of whales. Further observations reveal that the foods of different digested grades are also considered different swarms of euphausiids. Because the species of eachswarms are different from each other. I use every attention in dealing
with such samples of different groups, as the simplicity of swarms isvery important for the biology of euphausiids, and it is considered, whales
should have taken successively such swarms of different species in theneighbourhood in the sea waters.
SUMMARY

The first summarized study on foods of baleen whales in the northern
Pacific is stated. The essential points are concluded as follows.
1. The hydrographic conditions of the whaling grounds are discussed
referring mainly to the papers by Barnes & Thompson (1938), Mishima& Nishizawa (1955), and Fleming (1955).
2. Generally speaking, baleen whales are planktonophager in thenorthern part of the North Pacific. Blue whales feed only on euphausiids,and fin whales feed mainly on euphausiids. When those zooplanktons
are poor fin whales take fish or squids instead of zooplanktons. Only
humpback whales take fish as well as euphausiids, but they never takecopepods and squids favourably. The most favourite foods of sei whales
is copepods though some of them take euphausiids, fish and squids.
Baleen whales are not polyphagous animals in the northern part of
the North Pacific.
The swarms of zooplanktons have peculial features according with thespecies of zooplanktons. Their characteristic features must have influenceon the preferences of whales for their favourite foods.
3. Then, the important foods of baleen whales is as follows :Euphausiids

Euphausia pacifica
Thysanoessa inermis
Thysanoessa longipes

FOODS OF BALEEN WHALES

85

Copepods

Calanus cristatus
Calanus plumchrus
Fish, Atka mackerel Pleurogrammus monopterigius

Thysanoessa inermis and Calanus cristatus are the most important foods
among them. The harvests of above two species in each whaling grounds
control the migrations of whales, and whales never migrate to such area
as their favourite foods are scanty. The spineless forms of T. longipes
have also appeared as a large swarms of euphausiids in two occasions.
For baleen whales two forms of taking foods are considered. 'Skimming ' by sei and right whales, and ' Gulping ' or ' Swallowing ' by blue,
fin and humpback whales. The former method is able to take any sparsepatches of zooplanktons in the sea. The preferences of baleen whales
for foods are affected by the degree of the congregation of zooplanktons.
If swarms of zooplanktons are sparse, the Swallowing type of whales
can not take them so successfully as Skimming type of whales.
4. Feeding activity of whales may partly be a response to the upwardmigration of zooplanktons, fish and squids. More whales take their foodsin the morning or in the evening than in daytimes. The quantity ofstomach contents decrease in accordance with prolongation of chasing.
The freshness of stomach contents also declines with prolongations ofchasing by catcher boats.
5. The age and growth of euphausiids in the northern part of theNorth Pacific may be summarized as follows :Euphausia pacifica becomes mature at the age of two year about 20 mm
in length in adjacent waters to Aleutian Islands. Spawnings take placein early spring to summer. Thysanoessa inermis is also biennial in theadjacent waters to Aleutian Islands. In far southern localities it becomesmature in one year and spawns at a length 15 to 18 mm. Some specimensof T. inermis may be triennial in the north waters eastern AleutianIslands. T. longipes becomes mature at the age of two years and spawns
at a length of 20 to 28 mm. Spineless sorms of T. longipes matures inon year and spawns at a length of 15 to 18 mm. Some of the originalforms of T. longipes may survive in the third year and spawn. Mating
and spawning take place in late spring to early summer. The spawningof T. spinifera takes place in summer in Alaskan coast waters. As tothe sex ratio of euphausiids, males are dominant usually in the swarmsof euphausiids.
6. In the northern part of the North Pacific, and the Bering Sea, following species of zooplanktons are commonly found in the surface waters.
Adolescents and juveniles of Thysanoessa longipes are the most abundant in the surface water in summar. T. inermis and Euphausia pacifica
come next. Thysanoessa spinifera and T. raschii are only found in the

86

T. NEMOTO

coastal waters. Among copepods, Calanus plumchrus, C. .finmarchicus,
C. helgolandicus, C. cristatus, Eucalanus bungi bungi, Metridia lucens
and Oithona similis are the main constituents of the samples collectedpalnkton nets. The typical form of C. jinmarchicus is also found in thesamples. Other organisms Sagitta elegans, Tomopteris pacijica, Limacina
helicina, and Themisto sp., are also very abundant in the surface samples
collected by plankton nets.
7. It is considered that there are Calanus year when copepods areabundant, and Euphausiid year when euphausiids are abundant. Fin
whales stay in the whaling ground consists of Calanus for a long datein Calanus year, and if there is the transition time from Calanus cristatus
to euphausiids as in 1953 in the whaling ground A, fin whales leave thewaters to seek their foods. Thus, the catch of fin whales in the whaling ground A is inconstant in the each period of seasons. The whaling
ground consists of C. cristatus may be passing prosperity, on the other
hand, the whaling ground consists of euphausiids along the margin ofcontinental shelf is far stable through out the season.
8. The migrating route of baleen whales appears in the zone ofabundant foods, and the whaling grounds are mainly situated along theboundary of different water masses, along the slope of Alaskan continental shelf. The concentration of euphausiids and cope pods by currents
are found in the areas of convergence, in backwaters and at the centerof areas where there is a cyclonic movement.

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