FOOD AND FEEDING ECOLOGY IN THE
SOUTHERN SEI WHALE*
AKITO KAWAMURA
Whales Research Institute, Tokyo

ABSTRACTThe sei whale is an important stock of whales which has currently been
hunted in the Antarctic region taking over the decreased catch quotas of fin
whale. However, its biology and ecology as a whole are poorly studied being
compared with those in another whale species due to relatively recent development in commercial whaling.
This study aims to add some habitual and ecological knowledge on thefood and feeding in southern sei whale being based on the materials obtainedby the Japanese floating factories and by the research ships through 1967/68
to 1971/72 seasons.
The sei whaling ground is formed throughout the circumpolar seas, but
it is centered approximately between 40°S and 50°S under the surface sea
temperature of about 8.0-18.0°C, particularly in a zone along the Subtropical Convergence of under 10°C or more in temperature where more than
70% of whales is caught.
A total of 23 species of food organisms was found, of which the following species being arranged with the order of importance consisted main foodstuff of sei whale: Calanus tonsus, Euphausia vallentini, Parathemisto gaudichaudii,
Calanus simillimus, Euphausia superba, Drepanopus pectinatus, Clausocalanus laticeps,
and Euphausia lucens; and, the following 7 species were newly known as sei
whale food in the Antarctic region: C. tonsus, Cl. laticeps, Thysanopoda actifrons,
E. diomedeae, E. lucens, Penaeus sp. and Scomberesox saurus though Penaeus sp. is
still doubtful in identification. However, C. tonsus of copepodite V stage was

fed by more than 70% of600 animals examined, and its nutritional importancealong with its influence on the formation of whaling ground would be distinct.
The food organism changes successionally with the shift in whaling groundfrom northern extreme along 40°S toward south by following the order ofcopepods, small sized euphausiids or amphipods, then again euphausiids.
The baleen plates furnished in a row as feeding structure of filtering
apparatus are formed by the various sized baleen plates which are furnishedwith 0.93-1.4 plates/cm, and the figures do not vary with the body lengthand the sex. The averaged number of bristles on the baleen plate was 45.5
bristles/cm. An example of a total area for the filtering apparatus was 3.59 m 2
in the animal of 14.4 m in body length, and relative ability in filtering thewater by a row of baleen plates was considered to be placed somewhere between that in balaenopterid whales other than sei whale and balaenid whales.

* This work was submitted in partial fulfillment of the requirements for the degree of Doctor of Fisheries
Science at the Hokkaido University in 1974.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974, 25-144

KAWAMURA

26

Comparison of the filtering ability in sei whale with others revealed the sei
whale to be poor in those ability being 1/5 of the fin whale, and 1/8 of thebolting silk cloth GG54 (0.33 mm mesh aperture, 46% in porosity).
The analysis on the fulness of stomach contents by hours revealed that
20-30% of their amounts reduced within 5-6 hours, from which it is supposed that the sei whale feeds fully once in a day. The amount of stomach
contents usually found in fully repleted sei whale was 150-200 kg but wasless than 100 kg when they were consisted by Calanus tonsus. The daily rationin sei whale was calculated as 4.43% of body weight by using the formulation; Heart weight X 100/body weight· I/daily ration"=;:0.11 (const.), thenaverage sized sei whale requires about 900 kg of food daily. The figures bothamout of stomach contents and daily ration in sei whale lead to a consideration that the whale would hardly be filled its nutritional requirements by C.
tonsus food alone, that is, the whales seem to be kept under starvation to someextent in the natural environment.
A possible population density in C. tonsus patches being based on both theamount in the stomach and the filtering ability was calculated as approximately 103 -10 4 inds./m 3 , and it was proved in .this calculation that feeding byskimming type would be much advantageous for sei whale than by swallowing
or gulping type feeding. After examining on the net samples of C. tonsus
patches being composed of similar population structure, it was proved thatthe population density under patchiness was 2.4 x 104 inds./m3, which agreed
quite well with the figures assessed by the theoretical calculation.
The distribution of whales in connection with the environmental conditions would be understood by a high concentration of phosphate alongwith the presence of rather deeper layered thermocline where larger standing
crops of organisms were found. However, since the swarming of zooplankton
into patchiness would be essentially important character as the food of whales,
none of organisms which widely occur with larger biomass such as tunicatesand chaetognaths induce the concentration of whales. The geographicaldistribution of C. tonsus may essentially be influenced by the physico-chemical
conditions of the sea. They may, however, be of no use for considering on
the causation of swarming phenomenon of, say, the patchiness of prey organisms since aggregating habits in animal including zooplankton seem to
be strongly controlled or released by some less known biological factors suchas the density effect under crowding.
CONTENTS

l.
2.
3.
4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Material and method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biological observations of whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Recent catch and whaling ground of sei whale . . . . . . . . . . . . . . . .
4-1. An outline of the whaling ground . . . . . . . . . . . . . . . . . . . . . . .
4-2. Catch distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-3. Surface sea temperature in sei whaling ground . . . . . . . . . . . .
Food of sei whale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-1. Kind of food organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28
32
34
36
36
37
40
45
46

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FEEDING HABITS OF SEI WHALE

6.

7.

8.

9.

10.

11.

12.

5-2. Composition of the stomach contents ................... . 48
5-3. Geographical distribution of food organisms ............. . 53
5-4. Latitudinal change in the distribution of food organisms ... . 56
5-5. Distribution of food organisms by the whaling area ....... . 58
Distribution of whales, and its relation to physical, chemical andbiological environment factors ............................... . 61
6-1. Distribution of whales and the environmental factors in the
Pacific and Antarctic Ocean ........................... . 61
6-2. Whaling ground of sei whale and the distribution of zooplankton ........................................... . 64
Body size of Calanus tonsus ................................... . 72
7-1. Composition of body size in C. tonsus population ......... . 72
7-2. Discussion .......................................... . 74
7-3. Patchiness of C. tonsus and the density effects ............. . 76
Feeding apparatus of sei whale ............................... . 77
8-1. Oral cavity ......................................... . 78
8-2. Baleen plates ....................................... . 79
8-3. Bristles on baleen plates ............................... . 81
8-4. Area of filtering apparatus ............................ . 82
Filtering ability in the row of baleen plates ................... . 84
9-1. Porosity in the meshes of filtering apparatus of sei whale ... . 84
9-2. Relative filtering ability in a row of baleen plates ......... . 85
Amount of the stomach contents and the population density of foodorganisms ............................................... . 87
10-1. Influence of chasing time to stomach contents ........... . 88
10-2. Amount of stomach contents ......................... . 89
10-3. Daily variation in feeding activity ..................... . 91
10-4. Possible cause induces a decrease in the fulness of stomach
contents ........................................... . 94
10-5. Daily ration in sei whale ............................. . 96
10-6. Estimation of population density in the food organisms ... . 101
Swarms of Calanus tonsus Brady ............................... . 105
11-1. Zooplankton swarms as whales food ................... . 106
11-2. Material and method ............................... . 107
11-3. General hydrology in the investigated region ........... . 110
11-4. Shape, color, and size of C. tonsus swarms ............... . 110
11-5. Distribution of C. tonsus swarms ....................... . 112
11-6. Frequency distribution by hour ....................... . 115
11-7. Population density in the C. tonsus patches ............. . 116
118
11-8. Zooplankton abundance in the back ground waters
11-9. Some biological characters in the C. tonsus patches ....... . 119
11-10. Possible cause of swarming in zooplankton ............. . 121
General discussion and conclusion ........................... . 125
Summary ............................................... . 129
Acknowledgments ......................................... . 133
References ............................................... . 134
Appendix ............................................... . 143
Plates ................................................... . 144

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27

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KAWAMURA

1.

INTRODUCTION

The way to the modern pelagic whaling from the early North Atlantic whaling inSpitzbergen through those sperm whale whaling by the American ships in the southseas was a story of exploring the unknown seas by mankind who pursued for the
undeveloped stock of whales which move in search of rich food concentrations in
the vast ocean. In the ecological views it was a activity of mankind to increase
the knowledge by the aid of accumulated experiences on the breeding and feeding
behavior of whales, since those two are the essentially important factors in controlling the whale movements in the sea.
The famous whaling grounds which are formed during warmer season havebeen found where rich accumulation of food organisms is expected, or whale
movements and the formation of whaling ground were recognized generally as
a function of the distribution of food organisms in the sea. For instance, Scoresby(1820) in his cruise to the East Greenland waters early noticed that a "small crustaceans " was fed by the bowhead whale Balaena mysticetus, which indicates thewhaleman at that time had been interested in whales food in relation to the whale
movements although his observation was not biologically tangible. According
to Collett (1886), however, the food of sei whale caught in Varangerfjord, Finmarkin July 1885 was consisted of Calanus finmarchicus and of also Euphausia inermis, andthey might be an important diet of sei whale in some seasons while fishes were lessavailable as their food. Collett (1912) suggested an importance of the oceanic
front as whaling ground where rich planktonic organisms occur due to confront
the warm Atlantic and cold Arctic waters, and later Lillie (1915) called the sei
and blue whales in the North Atlantic would be a "plankton feeder". In the
North Atlantic the distribution pattern of herring and euphausiids, or and copepods
during fishing season closely relates to the movements of baleen whales which
feed on those organisms. Concerning with feeding habits of baleen whales, it has
been also known that they follow to two different ways to collect the food organisms,
say, "skimming" and "swallowing" or "gulping" by boltering the body whilefeeding, and they are so shy and agile when there are no foodstuff in the stomach(lngebrigtsen, 1929).
Returning from the 2nd exploring voyage into the southern seas during 17721775, Captain]. Cook brought an information that the bulk of pinnipeds and whales
were accumulated in the Ross Sea region, and since then the Antarctic region
became an indispensable whaling ground after passing several tenth years as heavy
sealing places (Fraser, 1964). When C. Darwin made a famous voyage on boad of
the H.M.S. "Beagle", he sighted of many whales in the vicinity of Tierra del Fuego,South America where the surface of the sea was discolored into red-brown by the hugeswarming of small crustaceans (Darwin, 1906). At this sight he (Darwin, 1906)
thought that those swarming crustaceans must be the food of whales. According
to Hinton (1925), Hamilton in his investigations at South Georgia had took notethe importance of planktonic " swarming life " as Risting early had pointed out
the matter in relation to whales food. It is important to take into account that
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FEEDING HABITS OF SEI WHALE

29

the movement of whales had been considered as a function of food available there,
especially as its distinct distributional pattern of so-called the plankton patchiness(Hardy and Gunther, 1935), and these recognitions of whales and their movements in connection with food organisms must be highly valuated ecologically apartfrom a qualitative knowledge on the whales food by species. In the investigation
on southern whale stocks, much are owe to the works done by the Discovery
Committee, especially to its bulk ofresults having been accumulated, and also to theestablishments in methodology for biological examination of whale carcasses (e.g.
Kemp et al, 1929; Hardy, 1967). In the matter of whales food, the records ofthe stomach contents by defining the size of food organisms present an importantmaterials for considering geographical distribution of food organisms and theirdistribution pattern (Mackintosh and Wheeler, 1929; Marr, 1962; Mackintosh,1973). By examining the stomach contents of blue and fin whales in the South
Georgia whaling ground and in Nathal, South Africa, Mackintosh and Wheeler(1929) found only E. superba as a pricipal foodstuff, and has long been believed that
only above species was the food organisms of southern baleen whales (Harmer, 1928,1931). However, E. superba population which is actually fed by the whales is consisted of both matured adults and small or medium sized immature and adolescent individuals, and they occurred different ways by the places, seasons and by the kindof whale species which prey upon them. By noticing these difference, Norwegianwhaleman called them from their experiences as " blue whale krill " for small
sized E. superba and " fin whale krill " for larger sized one. This would be analogous in the case of Meganyctiphanes norvegica and Thysanoessa inermis which occur in
the Norwegian waters, and they have called them as "star-krill" and "smaakrill" respectively. These also suggest that the seasonal movements of whales andtheir spring run close to their own shore have been considered in connection with thecharacteristic occurrence of food organisms. Hamilton who conducted biologicalobservations on whale carcasses in 1913-14 season at South Georgia found" shrimp"
in the stomachs of fin and humpback whales (Hinton, 1925). It is note worthy
that Hamilton clearly separated them from usual krill, and this leads to a supposition that his " shrimp " might had not been larger E. superba but Galatheid shrimp
such as Munida gregaria (Matthews, 1932; Dawbin, 1955; Tabeta and Kanamaru,1970). In 1913-14 season at South Georgia a total of 90 sei whale was caught(Hinton, 1925). There were, however, no notable biological observations on thisspecies except a mere note as " plankton eating species " since sei whale had lesscommercial value at that time. The food habits of sei whale have not known well
until recent years by reason of little catches as above mentioned (Mackintosh,1947). During the whaling seasons from 1960/61 to 1964/65 a total of 1223 sei
whale was caught at South Georgia whaling ground, and food organisms such as
Parathemisto gaudichaudii, Electorona sp. were newly found in addition to E. superba(Brown, 1968). He (Brown, 1968) also mentioned on the feeding habits of sei
whale being based on the observations on the stomach contents.
With development of pelagic whaling operation in contrast to declining inprincipal whale stocks, the whale species commercially hunted for were greatly
Sci. Rep. Whales Res. Inst.,.No. 26, 1974

KAWAMURA

30changed from fin to sei whales after 1964/65 season. This caused also the geographical change and expansion of the whaling ground itself, and the knowledge
on the biological characters of sei whale gradually increased in relation 1.o theneeds for stock assessment. The kind of food organisms of southern sei whale which
known recently in addition to E. superba are:- Euphausia vallentini (Marr, 1956;
Mackintosh, 1960), E. crystallophias (Marr, 1956), Thysanoessa macrura (Nemoto andNasu, 1958), E. recurva, E. diomedeae (Bannister and Baker, 1967), Parathemisto
gaudichaudii (Nemoto, l 962a ), Calanoides acutus (Peters, 1955 ), Drepanopus pectinatus(Tanaka, 1964), Ca/anus propinquus and E.jrigida (Pervushin, 1968), C. tonsus (Best,
1967; Kawamura, 1970a), Clausocalanus laticeps (Kawamura, 1970a) and so forth.
In this report more several species of food organisms would be added. Among
those above mentioned records, one by Best (1967) would be interesting since hismaterials were obtained in South African waters far outside from the so-called feed-

25
SEI

"" 20

;::

x

Fig. 1-1. Total catch of fin, blue, humpback, and sei whales in the Antarctic
pelagic whaling during the past 28 seasons. Catch of sei whale by the Japanese
fleets is shown by the hatched area. Whales taken under special permit after
1966/67 season; 3 humpback whales in 1971/72, 7 blue and 5 humpback whales
are excluded.

ing ground in southern oceans. He (Best, 1967) reported a total of more than 44
species of food organisms in sei whale stomachs. Although the principal food was
represented by a small number of species such as Calanus tonsus and some euphausiids,
his result suggests us that sei whale would feed on even the organisms which do notswarm patchily in those temperate waters. Nemoto (1959) early suggested thatsome copepods would be fed by the sei whale in some place in the Antarctic region,
but his suggestion was proved earlier than was expected as I have described above.
Recent study on the food and feeding habits of whales tends towards the analysisof both the formation of whaling ground and whale movements along with the
distribution of their food organisms (Slepzov, 1955; Nemoto, 1957, 1959; Pervushin,
Sci. Rep. Whales Res. Inst.,
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31

FEEDING HABITS OF SEI WHALE

1968; Kawamura, 1973a), and nutritional physiology based on feeding rate ofwhales (Klumov, 1961; Sergeant, 1969; Nemoto, 1970; Kawamura, 1970a, 1971a).
Commercially important whale species in the Antatctic region have been
blue, fin, sei, humpback, right and minke whales respecitvely. However, recent
status of some those stocks are not always kept under hopeful level, and only three ofthem are currently opened for regal catches. Fig. 1-1 demonstrates the numberof catches in the Antarctic by species after 1945/46 season. The figure of the fin
whale clearly shows its distinct importance among the total catches but it was took
over by the sei whale since 1964/65 seasons. In these figure the history of the Antarctic whaling could be seen in another way as a history of shifts in commercially
important species such the circumstances as shown schematically in Fig. 1-2. The

Fig. 1-2. A rough figure showing the change in whale species which have beencaught in the Antarctic.

TABLE 1-1. CATCH OF SEI WHALE AMONG TOTAL ANIMALS IN THE PAST 28
ANTARCTIC SEASONS EXPRESSED IN PERCENTAGE FIGURES.
Seasons

Percentage

Seasons

Percentage

1945/46
1946/47
1947 /48
1948/49
1949/50
1950/51
1951/52
1952/53
1953/54
1954/55
1955/56
1956/57
1957 /58
1958/59++++

1959/60
1960/61
1961/62
1962/63
1963/64
1964/65
1965/66
1966/67
1967/68
1968/69
1969/70
1970/71
1971/72
1972/73

8.9
11. l
12.7
18.3
28.6
63.3
71.9
61.2
68.7
50.3
49.0
50.9
47.4
39.3

0.3
1.2
0.1
0.4
0.8
0.4
0.8
2.2
6.6
3.9

'+' less than 0.1 percent
Discovery Committee aimed to manage and control the Antarctic whale stocks
under scientific basis (Kemp et al, 1929). In the days of blue and fin whaling,
Matthews (1938) noted the forth comming situation that large catch effort would
be added onto sei whale in the furture, and stressed the necessity of biologicalinvestigation on this species as much as possible. When we consider the present
whaling status his anticipation must be greatly valuated. In 1964/65 season the total
number ofsei whale catch (19874 animals) exceeded that of fin whale, and showed
63.3 % among total catch in that season. In 1971/72 season the minke which
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KAWAMURA

32

had not entirely been regarded as commercially exploitable resources turned tosome importance for whaling by the pelagic floating factories, and 3000 animals
were caught successfully although it was still exploratory whaling. Under thesecircumstances in the Antarctic whaling as having been mentioned above the purpose and reasons which made me study the sei whale could be summerized asfollows:1) Food and feeding habits in the animals can be generally recognized asa whole as one of the most important event along with their breeding behavior
throughout their life history. The baleen whales are very characteristic in ecological niche through marine food chains where the very primary herbivorous organisms are connected directly with the animals in the highest trophic levels. In
connection whh these circumstances the sei whale was in need of knowledge about
its food and feeding habits and ecology since these factors in general strongly relate
to the change of animal population and the movements of whales.
2) Catch of sei whale in enormous numbers began relatively recently, andthe biological characters were poorly known particularly in sei whales. There
have been done almost no observations and analysis on sei whales food but abrief discusion by Nemoto (1962) since then the main whaling ground in the
Antarctic region shifted to further northern waters up to the Subantarctic region.
3) In connection with the shift and vast expansion in the region of main
whaling ground, several kind of food organisms began to occur newly in the dietof sei whale (e.g. Kawamura, 1970a).
4) It is hoped to recognize the mechanisms of formation of whaling ground
not only by the hydrodynamical process but also biologically through the foodconditions of baleen whales.
5) It is expected to make clear the actual distribution characters of planktonic
food organisms, especially their uneven distribution in the feeding places throughthe formation of dense swarms so-called plankton patchiness, which seems to be
ultimately responsible for the formation of feeding spot of whales.
6) The knowledge of patchy distribution of some planktonic organisms would
be useful for developing the plankton fisheries (e.g. Hempel, 1968; Parsons; 1972).
7) The study of sei whale food is desirable to be conducted while larger
number of catch of this species would be expected since the history of shifting
the whale species which have been commercially hunted teaches us the importance of properly partinent investigations.
2.

MATERIAL AND METHOD

The material used for analytical treatments were obtained from various source,
namely, pelagic whaling both in the Antarctic and North Pacific, and some werefrom land based whaling in the Japanese coastal waters. One of others wereobtained in the cruises on board of the research ships. The material can be divided
into two principal categories, namely, row material of both whale food and plankton
samples, and data in the field chiefly compiled by the Fishery Agency, Ministory
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FEEDING HABITS OF SEI WHALE

33

of Agriculture and Forestory and by the personnel on the floating factories.
Although the material and the method of analysis in particular will be discribed
more in detail elsewhere in the following each sections, the principal sources ofmaterial obtained are as follows:
The stomach contents of sei whale caught in the Antarctic waters were collected continuously through 1967 /68 to 1971 /72 seasons by the Japanese floating factories
and those materials were treated chiefly for examining the kind of food organisms,
their geographical and seasonal distribution. Some results obtained on the material
in 1967/68 season concerning to the food habits of sei whale has already beenreported by the author (Kawamura, 1970a).
In the seasons of both 1967/68 and 1971/72, I was on boad of the floating
factories, " Nisshin Maru " of Taiyo Gyogyo Co. and " Tonan Maru No. 2 " ofNippon Suisan Co. respectively, and engaged in general biological observationsof whale carcasses as routine work and also collected the samples of the first stomach
contents of sei and fin whales.
In the course of this study I made theoretical estimation on the population
density of Calanus tonsus, the principal prey crustacean of the sei whale, when it forms
the patches being based on both the amount of this organism found in the firststomach and relative filtering ability in the row of baleen plates (Kawamura, 197la).
To prove this result I cruised down into southern waters off Western Australia in
1971/72 season on boad of M.S. " Eiho Maru " of Hokoku Suisan Co., andmade quantitative sampling of C. tonsus by spotting its patches in the surface waters.
After finishing these investigations, I moved to the F.F. "Tonan Maru No. 2" asabove mentioned when both ships met each other on high seas.
The row materials concerning to examining the relation between the distributionof whales and of their prey zooplankton along with sea conditions were obtained in
1968/69 season by the R.V. "Hakuho Maru" during her KH 68-4 cruise down to the
Antarctic (Ocean Research Institute, University of Tokyo, 1970). With this material a quantitative and qualitative study on the food organisms of sei whale in
the Pacific sector of the Antarctic and Subantarctic waters was made, and at thesametime the distribution of sei whale and the occurrence of the DSL in relation
to that of physico-chemical elements in the region were discussed. A part ofthese results were preliminary reported (Kawamura, 1969a; 1970b, c; Kawamura
and Kureha, 1970).
Mechanisms of feeding as a function of the structure of filtering apparatus ofsei whale and its food habits were also studied. In order to estimate the capacityof oral cavity in sei whale the amount of air used for making afloat the whale
carcasses was measured on the whale boat, "Toshi Maru No. 18" which belongs
to the Nisshin Maru fleet of Taiyo Gyogyo Co. in the summer of 1969 when she
engaged in the North Pacific whaling operation, and a row of baleen plates wascollected too by that fleet to assess its relative filtering ability of the water. Whether
or not the whale vomits its stomach contents under continuous chasing was an important matter which have to be clarified when we consider the amount of stomach
contents in carcasses, and the observations for this purpose were made by six whale
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KAWAMURA

34

boats, of which each two belonged to K. K. Kyokuyo, Taiyo Gyogyo Co. andNippon Suisan Co. respectively (Kawamura, 1971a).
To know the daily ration of sei whale theoretically the heart weight which
was proposed by Sergeant (1969) as a useful index for this purpose was weighedin 1970/71 Antarctic season at the floating factories, "Tonan Maru No. 2" and"Nisshin Maru No. 3."
There is bryde's whale Balaenoptera edeni Anderson (Olsen, 1913), which isvery similar to sei whale dealt in this study though the former has currently been
separated clearly as completely different species (Omura, 1966). According toOmura (1966) the bryde's whale distributes in the offshore waters of South Africa,
Brazil, Japan and western Australia, and a considerable number of this whale wascaught in the north-western North Pacific in recent operations. Japanese whalemen have been noticed some specific differences by calling it as the sei whale of"southern origin " or " southern type " which means the warm water species, but
it has not been treated separately in the old catch statistics of Japanese coastal
whaling. The bryde's whale are usually found in the warm waters of sea temperature higher than 20°C and its food habits also differs to some extent from thesei whales (Omura and Nemto, 1955). Best (1970), on the other hand, observed
two allopatric forms in this species in the South African waters, namely, "inshore " form which distributes in the waters within 20 miles of the coast whereas
the " offshore " form is found 50 miles or more from the coast. They differ eachother in some biological characters such as external appearance, breeding andfeeding habits.
Accordingly, it is supposed to be caught the bryde's whale along with thebulk of sei whale catch especially in the whaling area III in the Antarctic since
the sea conditions in that area are much complicated, and sei whale is widely
pursued for by the whaling fleets. Of 945 sei whales taken by the Russian fleets in
1972/73 season 5 bryde's whales were included (Intern. Whal. Stat., 1973). However, the morphological and ecological characters of bryde's whale as mentioned
above are well known currently by the whalers, and sei whale treated in thisstudy can be considered as B. borealis Lesson exclusively.
3.

BIOLOGICAL OBSERVATIONS OF WHALESThe Japanese whaling fleets which engage in the Antarctic and North Pacificwhaling operate under an obligation to record and collect the materials of every
whale carcasses treated after the instructions of whaling inspectors on boad; theactual items and the way of the observations are essentially based on those establishedby the Discovery Committee (see Mackintosh and Wheeler, 1929). The personnel
who engages in this scheme is a governmental whaling inspectors and some deck
workers of the whaling company, and the following items are observed: that is,whale species, sex, body length, foetus (body length and sex if present), stomach
contents (kind of organisms by rough grouping, the amount of food and freshness),condition of mammaly gland whether lactating in any degree or not, and its thickSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

35ness), thickness of blubber, weight of both testes, number of corpora albicans andcorpora lutea (but they are collected for later examination if counting them on
the deck was impossible), collection of the ear plug on every carcasses and thestomach contents about every tenth of carcasses if any food present. In addition
to these, the position, date and time of catch and treatment as supplemental data.
All these data are compiled in the seibutsu-chosa-daicho, a kind of official fieldnote of the Fishery Agency, which can be reffered for statistical analysis.
The data treated in this study other than those obtained on row materials,
chiefly based on this field note. The position of catch, however, is not actual but
represented by the noon position of the mother ship. Accordingly, the positionsof catch may differ to some extent particularly those in the morning and evening.
It is, however, recognized empirically that the difference in the position is within
tenth miles, and none of those seems to cause any confusion in analysing the data
except particular cases such as the catch occurred fairly close to the convergencezone.
It is recognized anatomically and histologically that the stomach of whale iscomposed of four compartments arranged in series: the first compartment or
stomach with no digestive glands, being a kind of dilated sack of the oesophagus
origin to the fouth stomach by the extraordinarily developed ampulla (Slijper,
1958a; Hosokawa and Kamiya, 1971). The second and the third stomachs arereal gastric compartments provided with digestive glands. Accordingly, thestomach of whale carcasses is usually examined only on the first stomach since thecontents remain being undigested for long time and the second or the thrid stomachs
are examined only when there was no food remains in the first stomach. In thiscase, however, the condition of the stomach contents is recorded as " empty "( =0) in the field note above mentioned.
The food conditions of the first stomach in the whaling ground give an important suggestions for whalers to judge whether or not the whaling ground isunder suitable sea conditions as long lasting whaling ground. In the observations
on the kind of food organisms in the field, the following rough calssifications andabbreviations are adopted as have been described previously by Nemoto (1959)
and Kawamura (1970a), namely, 'Calanus' (Ca), 'Amphipoda' (Am), 'Euphausia' (Eu), 'Fish' (F), 'Squid' (Sq), 'Munida' (Mu) and so forth. Since
the stomach contents are consisted of so many species of food organisms, it is hardto distinguish all of them after the classifications especially in ' Calanus ' and ' Euphausia ', then both of which actually mean copepods and euphausiids respectively.
As for' Euphausia 'the group is divided into three sizes by the body length; ' Large '(L) for larger than 5 cm, 'Medium' (M) for 4-5 cm, and 'Small' (s) for smaller
than 4 cm., and they are recorded by one of a dominant forms in the official record.
This definition for classifying the size of euphausiids was supposedly adopted after
the way having been employed by the Discovery Committee. In those days there
had not been the problems of mixing the food organisms with more than two species
since only E. superba had consisted of foodstuff in higher latitudes (Mackintosh andWheeler, 1929). As mentioned before the sei whale is currently the most important
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

36

species, and they seem to prefer copepods or small sized euphausiids such as E.
vallentini being found in more northern waters. It may hard to distinguish thesekind of small euphausiids from young E. superba or from some another euphausiids
species expressed as 'Eu-S' in the data record. However, it is possible with considerable preciseness by reffering the result obtained on row food materials to identify
the kind of food organisms in those field record. There are no euphausiid species
that correspond to the body length of' L' and 'M' in the northern North Pacific,
but the occurrence of ' L ' and ' M ' of Euphausia was recorded in 1969 season, andwas proved later by examining the row materials collected that they were a kindof Sergestid shrimp, Sergestes similis Hansen (Kawamura, 1970d; Omori et al,1972). This fact suggests that the observations by defining the size as ' L ', ' M'
and ' S ' would be still methodologically efficient for the field work.
One of another items of observations is the fulness of stomach with food against
the capacity of stomach itself. This is recorded by five different degrees alongwith the freshness; these are, ' empty' (0 %), 'r' (less than 25 %), 'rr' (25-50 %),'rrr' (50-75 %) and' R' (75-100 %) respectively. The freshness of food organisms
therefore usually corresponds well to the fulness of stomach by its order such as'R' for 'F ', very fresh.
Food organisms as row material were collected on the every tenth of flensed
carcasses when the stomach contents were kept under good freshness for the later
examination in the laboratory. The collection of row material, however, is also
conducted to make the material characteristic in time and space when the kindof food organisms seemed different from others, when the floating factory changed
its position, and when the sei whale was caught sporadically during the period of
fin whale hunting in higher latitudes. A considerable part of this study is greatly
based on the statistical material having been compiled like this way by the J apanese floating factories during 1967/68 to 1971/72 seasons.
TABLE 4-1.
Area

Longitude

Main land and ocean included

I
II
Ill
IV

120°W- 60°W
60°W- 0°0° - 70°E
70°E -130°E
130°E -170°W
l 70°W-120°W

Eastern Pacific, South America, Falkland Is.
South Georgia Is., South Sandwich Is.
Bouvet Is., Marion Is., Crozet Is.
Kerguelen Is., Heard Is., Western Tasman Sea
Tasman Sea, New ZealandCentral Pacific

v
VI

4.

4-1.

THE ANTARCTIC WHALING AREAS ROUND THE SOUTH POLE CAP.

RECENT CATCH AND WHALING GROUND OF SEI WHALE

An outline of the whaling ground

The Japanese pelagic whaling in the Antarctic region is operated under the regulations by the IWC's arrangements, by the recommendations under the Antarctic
Teaty, and by the measures ofJapanese Government. Under these circumstances,
four species of baleen whales, i.e., fin, sei, minke and sperm whales are hunted.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

37

The whaling place is found throughout the whole circumpolar seas but it is divided
longitudinally into six whaling areas, from I to VI(Table 4-1). These whaling
areas were defined being based on the catch statistics of the blue whale for the
purpose of clarifying its sub-populations or stock units by the sea areas (Hjort andRuud, 1932; Mackintosh, 1942). The whaling areas defined thus seem to well
represent the stock unit of whales especially in blue and humpback whales whereas
they seem rather less convenient for sei and fin whales. It is, however, still con
venient to compare the catch statistics of sei whale with those other whale species.
Fig. 4-1 shows the distribution of whaling ground along with the whaling
areas where the Japanese floating factories have operated through past five seasons,
1967/68 to 1971/72. In recent whaling with quite larger number of sei whale
catch, most of whaling ground is found in the zone along the Subtropical Convergence between 40°S and 50°S, and little number of sei whale is caught alongwith larger number of fin whale in more higher latitudes, south of 50°S in theareas IV and VI. In the area VI a southerly expansion of the Subtropical Convergence toward higher latitude is very distinct particularly in the east of New
Zealand but sea conditions in this area as whaling ground of sei whale do not differ
much on the whole from other areas of mid-latitudes. In contrast to those situations the whaling ground close to the pack-ice in the south of 60°S as whaling in
1940's to 1950's when a larger number of blue, fin and humpback whales werecaught (e.g. Nishiwaki and Hayashi, 1950; Nishiwaki and Oye, 1951; Mackintosh,
1965; Nasu, 1966). Comparing with those distribution pattern in the whaling
grounds, it is clear that the sei whale is caught in the far northern waters north of
the Antarctic Convergence, and its whaling ground could be considered as the" Subantarctic" whaling ground instead of the historical "Antarctic " whaling
ground.
The whaling activity was most heavily conducted in the areas III, IV, and V
throughout the Antarctic whaling grounds, and the areas I, IV in the south ofAustralia and a part of the area V seems rather less important as sei whaling
ground due to a monotonous sea conditions there along with a prevaliling bad
weather especially in the area I. Since the bulk of the catch in the southern zone
between 45°S and 50°S is consisted of fin whale almost exclusively, and the proportion of sei whale catch among them is so little that the distribution of sei whaling
ground in the southern zone does not always represent well the status of sei whale
distribution. The center of sei whaling ground can be seen in general close to thenorthern boundary of the subantarctic region being related to a hydrodynamical
upwelling (Nasu, 1966) and the bottom topography.

Catch distribution
Fig. 4-2 shows the number of sei whale caught during 1966/67 to 1968/69
seasons by the Japanese floating factories being arranged in five degree square ofboth latitude and longitude. The region where the floating factories operated
usually correspond to the whaling ground itself, and the latter represents theregion where large number of sei whale have been spotted by the scouting boats
4-2.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

38

belonging to the fleet. Accordingly, the whaling ground itself could be consideredto agree with the distributions ofsei whale with high population density. However,it must be also taken into consideration that there are the case of small catch by
artificial reasons due to weather condition, geographical position, and less catch
effort on sei whale due to balancing relation with the period of fin whaling. It isclear in Fig. 4-2 and Table 4-2 that the sei whale is caught chiefly in the Subantarctic waters between 40°S and 50°S as a whole, and the 5 degree squares
with catch more than 100 animals distribute in the narrow zone along both northand south sides of the Subtropical Convergence. The sei whale catch in the southof the Antarctic Convergence is seen in the areas IV to VI but the actual numberof catch was relatively small except those in the area IV. The bulk of catch in
the area IV is found in 52°-53°S, the northern waters of Heard Island, and a 5
TABLE 4-2. NUMBER OF SEI WHALES CAUGHT AND ITS PERCENTAGE FIGURES(ITALICS) BETWEEN EACH 10 DEGREES OF LATITUDE IN THE ANTARCTIC.
Latitude (S)

66/67

67/68

68/69

69/70

70/71

71/72

72/73

40°-50°50°-60°60°-70°8775 71.0
2270 18.4
1315 10.6

5114 49.0
3373 33.0
1863 18.0

4178 72.4
870 15.1·
722 12.55112 87.4
212 3.6
528 9.05644 91.8
186 3.0
321 5.25329 97.7
105 1.9
21 0.43633 94.0
151 3.9
79 2.1TABLE 4-3.

CATCH OF SEI WHALE BY MONTH.
Whaling season

63/64

64/65

65/66

66/67

67/68

68/69

69/70

70/71

71/72

72/73

Dec.
Jan.
Feb.
Mar.
Apr.

552
3204
2451
1867
182

1001
7116
7690
3870
161

3202
5701
5579
2923
153

3168
3615
3673
1880
24

1812
3351
3055
2113
19

2358
1779
1484
694

2435
2055
792
570

2760
1958
875
520
38

2348
2098
540
441
28

1347
1681
488
346

Total

8256

19838

17558

12360

10350

5770

5852

6151

5445

3864

degree squares between 80°-85°E seem to be a particular case. The most distinct
trends of northerly shift in the distribution of whaling ground is found in the areas
II to IV between Folkland Islands and the Indian Ocean sectors, and the locationof these whaling grounds well agree with the mean position of the Subtropical
Convergence (Deacon, 1937). The catch of sei whale in such waters as to be considered warm temperate region greatly owe to the formation of feeding place by
the dense swarms of Calanus tonsus in the surface during mid-December to January,on which the sei whale feed voraciously. The fin whale, on the other hand, usually
preferably feeds on euphausiids and supposedly enters into the more higher latitudes
in early austral summer. It could, therefore, be considered a habitat segrigation inboth fin and sei whales. In those days when blue and fin whales were hunted exclusively, the catch of sei whale in February and March was quite sporadic (Hinton,
1925). However, by examining a long-term catch distribution, it is noticed that
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

...

60

w

PALMER PEN.

~

403 230 244

30

SOUTH GEORGIA IS.

66 138 3

60°

7 15
10

0

60°

120°

.

El80°

...

48 14460

7 972 633 422 76

44 130 1135 1703

3790

338 1017 106

28 284 11 49 103.,, KERGUELEN IS.
35 1859 213 87 44

52 264 291 764 390 172 203 106

Fig. 4-2. Number of sei whales caught by the Japanese floating factories in every 5 degree squares of
the Antarctic whaling ground during the seasons from 1966/67 to 1968/69.

30

22

102 312 1012 193 503 609 223

Fig. 4-1. The Antarctic whaling ground occupied by the Japanese floating factories during the seasons
from 1967/68 to 1971/72.

o·~\60"1-------'--~~~-+-~~-~---L__:_:~~-+-~~~~~ll~I~~~~~-!--~"'-""'~

50°1------~

"•

\

180°

120°

FEEDING HABITS OF SEI WHALE

39

there are some catch of sei whale in higher latitude south of 70°S in the Ross Sea
region of the areas V and VI (Kashida, 1972; Omura, 1973).
As season goes by the feeding place formed by C. tonsus will fade out due totheir wintering migration down to some deeper waters, and the sei whale moves
into more colder region where several kind of cold water food organisms of copepods,
amphipods and euphausiids occur in swarms. The representative species of thosewaters are Calanus simillimus, Euphausia vallentini, E. superba, and Parathemisto gaudichaudii. In recent whaling operations, however, the whale species of being hunted
shifts almost to fin whale in these cold waters after catching such vast number ofsei whale in the northern waters as to fill the catch quota for the season. By
considering both the number of monthly catch as shown in Table 4-3 and thesurface temperature of the whaling ground under about 3° -4°C in March, it is suggested that the region in higher latitudes as above mentioned would be far outsidefrom the usuall distribution range of the sei whale.
The catch of sei whale by pelagic whaling during the seasons of 1931 /32 to1968 /69 is shown in Table 4--4 by separating each whaling areas. A fairly little catch
TABLE 4-4. TOTAL CATCH OF SEI WHALE IN SIX WHALINGAREAS DURING THE SEASONS FROM 1931/32 TO 1969/70.
Area

Number of animal

Percent

I
II
III
IV
VI

3708
42109
20776
12774
15066
9207

3.6
40.6
20.0
12.4
14.5
8.9

Total

103640100.0v

in the areas I and VI would be due to the treatments of total banning of the catchup to 1962, and relatively small in stock size there (IWC, 1969; 1970), and also due
to some difficulties in an actual operation by ill weather condition. The most distinct
catch is seen in the areas II where one of the most productive regions in the Antarctic, say, South Georgia and Folkland Islands waters, are included. In 1964/65
season, a total of about 2000 sei whale was caught in this area. It has been reported that the cold Weddell Current extends northward distinctly, and this would
be responsible along with a complexity in bottom topography for quite highstanding stocks of zooplankton which causes high potentials in availability offood organisms for both whales and pinnipeds (Hardy and Gunther, 1935;
Ommanney, 1936; Mackintosh, 1934, 1937; Marr, 1962). However, a quite heavy
catch in this whaling area might possibly caused a considerable local decline in
the population of sei whale, and the catch was actually very poor when two
Japanese floating factories operated again in 1969 /70 season. This fact suggests
a little rcovery in sei whale stock up to today (IWC, 1970), andt herefore the sei
whale is caught principally in the whaling areas IV and V in recent operations.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

40

KAWAMURA

The schematic distribution of sei whale based on the position and the numberof catches indicate that there are trends of efficient catch particularly in the regions
under some complexity in bottom topography which responsible for arrousing the
upwelling under the distinct meandering of the Subtropical Convergence. These
general features in distribution patern of sei whale are comparable to those foodorganisms, that is, the dense swarms or aggregations into the patches in C. tonsus(see section 11). Mentioning on the formation of whaling ground in the North
Pacific, Nemoto (1957, 1959) and Nasu (1963, 1966) pointed out the close relationships between the distribution of food organisms and the upwelling by various
origins and the situation in the Antarctic region seems to be essentially analogous
in the structure of whaling ground formation.

Suiface sea temperature in sei whaling groundThe distribution of surface sea temperature of sei whaling ground in theareas II-VI during 1969/70 season is demonstrated in Figs. 4-3a-4-3f, but the

4-3.

40°

45°S

Fig. 4-3a. Surface temperature in the Area II, December, 1969. Solid line indicates the isotherm in the first decade of a month, and dotted and chain lines
indicate those in the middle and last respectively. Both MPSTC and MPAC are
the mean positions of the Subtropical Convergence and those of the Antarctic
Convergence.

general sea conditions in the areas V and VI of the same season has already beenreported by Nasu and Masaki (1970). The surface sea temperature treated here
was observed at the ship's noon position during the period of whaling operation being measured by electric thermometer which has its sensor on the ship's bottom
about 10 meter depth. There are so much measurements of sea temperature in
actual operation, but most of them are hardly obtainable for outside use except one
as mentioned above.
Fig. 4-3a-Fig. 4-3f cover the region between Folkland Islands and western
Australia, that is, 55°W-120°E, and this region represents the principal sei whaling
groud in recent operations. An outline of the monthly temperature distribution
is as follows:
December (Figs. 4-3a-4-3b): Since the whaling ground of baleen whales is
opened in nearly mid December the material treated here covers late half of theSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

41

month. The sei whaling ground spreads over in latitudinally narrow zones along
40°S, and the temperature of 15° -18°C occupies exclusively even in the areas
II (Fig. 4-3a) and IV (Fig. 4-3b). The whaling ground in Folkland region may
be formed by an oceanic front which originates from both the cold Folkland Current and the warm Brazil Current. Accordingly, the coeanic front distinctly
extends northward at 50° -55°W longitudes, and its position agrees in generalwith the mean position of the Subtropical Convergence in summer (Deacon, 1937).
In the whaling ground off southwest Australia the sea temperature principally
showed between l4°C and 16°C. The distribution pattern of isotherms does notdiffer much in general within the same month exept a slight southerly shift
about I 0 latitude on the whole in late December. This indicates the gradualgrow of warming conditions toward south during this month. Discussing on thesea conditions of sei whaling ground in the areas II and VI in 1969 /70 season,
Nasu and Masaki (1970) concluded that the sei whaling ground in the area II wasformed in the waters north of the Antarctic Convergence with temperature higher
110°E

40°

15.0

45°

Fig. 4-3b. Surface temperature in the Area IV, December, 1969.
Fig. 4-3a.

40°

45°S

Legends as inthan 4.5°C, but it was found also between 8°C and 10°C in the area VI. A slighttemperature differences in the whaling areas also found in the Folkland region andin the southwestern Australia region where the difference was 1°-2°C in the centerof the whaling ground. These suggest that there are some different hydrologicalconditions for the formation of whaling ground among those areas.
January (Figs. 4-3c-4-3d): Following to December the whaling was operated through the areas II to IV. In accordance with northerly extension of the
Subtropical Convergence beyond 40°S between 10° W and l5°E, the principal
whaling ground was covered with 15° - l 7°C waters during first half of January in
the Folkland region whereas it was 11° - l 4°C in the area III. In the area IV there
found l4°-l6°Cwaterscloseto the Subtropical Convergence zone in January. In
the regions south of South Africa between 10°E and 30°E, however, the distribution of sea temperature showed rather complicated features supposedly due to theformation of local fronts, some of which must have been formed by a confronting
both the warm Agulhas Current and the cold Benguela Current. These hydroSci. Rep. Whales Res. Inst.,
No. 26, 1974

~

~ S<
~.;~

·°'""'~~::> "'

?: ~

~

~

::,_,

Fig. 4-3c.

Surface temperature in the Areas II and III, January, 1970.

Legends as in Fig. 4-3a.

/ , , '->. > >, 0 ' , '"''°',_\,,,,\\\\\\\\\\\\\\\\\1\)\111i\)\l\\111il111i/111r/u,,/,,11/1111b111/1u1/11u/ru·rUrrr/,.ccc/r / / / / , . , ... ,.,IS

40'

~

~

~

~

FEEDING HABITS OF SEI WHALE

43

logical condition would be responsible for high productivity due to upwelling in theregion where forms another fertile whaling ground far outside from those in the
Antarctic region especially in the western coast of South Africa (e.g. Best, 1967).
By refering the Fig. 4-2 it is noticed that the regions of high density in the catch
number of whales in early January are found in relatively warmer waters in thenorth of the Antarctic Convergence up to 40°S., and the center of heavy catch liesin the zone of temperature above 14°0. The expansion of warmer conditions
towards the south is weakened gradually toward late January although there arestill slight trends getting warmer as being seen in the behavior of 8°0 isotherms
at 10°W. However, the temperature of the main whaling ground during January
showed 8°,..., 13°0 on the whole.
80°E

90°£

110·£

MPSTC

11. 0
10. o-.....
45°

Fig. 4-3d.

9.0-· ....
8. 0-· ....

Surface temperature, January, 1970.

Legends as in Fig. 4-3a.

February (Fig. 4-3e): There was no whaling activity in the area II in February, but the area II and a part of the area IV where involve Marion, Crozet
and Kerguelen Islands has been formed a well staple whaling ground through past
several seasons. In contrast to northern positioning of whaling ground close to 40°S
during December to January, it is found rather mid to higher latitudes with 45°460S as its center in February. The 7°0 isotherms along 35°E was found near to
45°S in late January, and it still continued shifting down to about 47°-48°S in
early February. The southerly shift of the isotherms, however, decreased graduallyduring mid to late February as observed in the feature of 7-8°0 isotherms at 40°500E and at 60°E, and the sea condition seems as a whole to be in the state of thosetypical one of mid summer. The whaling ground is found in the temperatureof 7°,..., 10°0, which was slightly colder than that prevailed in preceding month but
agreed with the result obtained by Nasu and Masaki (1970).
In view of the kind of food organisms which is responsible for forming principal sei whaling ground, the areas II and VI during December to January would
strongly be due to the mass occurrence of copepod, C. tonsus whereas more cold
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

44

water species such as E. vallentini, C. simillimus and P. gaudichaudii are responsible in
the areas III and IV. The whaling ground in the area IV along 80°E is usually
under very cold sea conditions with temperatures of 4° -6°C, where much fin whales
are caught than the sei whale. The principal food organisms are consisted of E.
superba and P. gaudichaudii, but they were replaced to D. pectinatus in the slightly
northern waters under the temperature of about 7° ....... 9°c.

Fig. 4-3e.

Surface temperature, February, 1970.

Legends as in Fig. 4-3a.

Fig. 4-3f.

Surface temperature, March, 1970.

Legends as in Fig. 4-3a.

March (Fig. 4-3f): The catch of sei whale distinctly decreases in March in the
recent operations. In the regions of Marion, Crozet and Kerguelen Islands there
was a sign of retreating the warm water toward north particularly in the region between 70°,.., 75°E. The sei whaling ground was formed under relatively colder sea
conditions of 4°,.., 7°C but the catch of sei whale was very little in contrast to thatof fin whale (Table 4--3).
Studying on the food habits of sei and fin whales of the North Pacific Ocean,
Nemoto (1957) reported a distinct preference to copepod food in sei whale. In the
Antarctic region, on the other hand, the large standing stocks of copepod species isSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

45

usually found in relatively warmer waters in the north of the Antarctkc Convergence (Kawamura and Hoshiai, 1969; Kawamura l 969b; l 970a, b; Kawamura andKureha, 1970). Accordingly, the principal sei whaling ground during December
to March is found usually in the northern zones of the Subantarctic region, andthen it would be hardly expected a bulk of feeding concentrations of sei whale
population in the regions south of the Antarctic Convergence even in the midst of
austral summer.
5.

FOOD OF SEI WHALE

In order to know the kind and distribution of food organisms of sei whale a totalof 462 food samples was collected from the first stomach of 462 animals. The 462
food sample is comprised of both the 117 samples collected in 1967 /68 season andthe 345 in 1969/70. These row material geographically covers for the most Antarctic
TABLE 5-1. FOOD ORGANISMS OF SOUTHERN SEI WHALE
CAUGHT DURING 1967/68 TO 1971/72 SEASONS.
COPEPODAAMPHIPODA

EUPHAUSIACEAFISHES

DECAPODAOTHERS**

Calanus simillimus GIESBRECHT
Calanus tonsus BRADY
Clausocalanus laticeps FARRAN
Drepanopus pectinatus BRADY
Parathemisto gaudichaudii GUERIN
f. compressa
f. bispinosaf. intermediate
Euphausia superba DANA
Euphausia vallentini STEBBING
Euphausia lucens HANSEN
Euphausia diomedeae ORTMANN
Euphausia similis G. 0. SARS
Thysanoessa vicina HANSEN
Thysanoessa gregaria G. 0. SARS
Thysanopoda actifrons HOLT & TATTERSALL*
Gymnospelus nicholsi (GILBERT)**
Myctophum subasperum (GUNTHER)**
Scomberesox saurus (WALBAUM)
Notolepis castsi DOLLO**
Vinciguerria attenuata (COCCO)
Gonostomatid fishes**
Penaeus sp.***Cleodora sulcata (PFEFFER)
Clione antarctica E. A. SMITH
Eukrohnia hamata (MOBIUS)* Although the bipolarity of this species is reported, the occurrence isdoubtful.
** The species gathered to feed on amphipods or copepods, and are notimportant as a staple food of sei whale.
*** Doubtful in identification.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

46

KAWAMURA

whaling areas, say, the areas II-VI, and gives us well the general knowledge concerning to the distribution of food organisms within the circumpolar seas exept a
sector which involves the eastern Pacific and the Folkland Islands region.

Kind of food organisms
Although the food habits ofsei whale in 1967/68 season has been reported previously (Kawamura, 1969b, 1970a, 197lb), some of which were also treated in thisreport to make the scheme more clear in detail. In the previous repeort (e.g. Kawamura, 1969b), it was pointed out that the sei whale principally feeds on copepods
such as C. tonsus more preferably than euphausiacea in accordance with the distinct
northerly shift in the center of sei whaling ground through recent operations. The
kind of food organisms along with those admixed by a chance are enlisted by species
in Table 5-1. As having been mentioned above the food organisms in Table 5-1
can be considered to cover the almost all foodstuff of the sei whale which distributes
within the circumpolar seas in the south of 40°S. By comparing with the foodstuff known in the northern North Pacific and Bering Sea, (Nemoto, 1957, 1959),
Kawamura (1973a) showed that those in the Antarctic region seem to be quite monotonous as a dietary environments for baleen whales, i.e., the Antarctic environment
lacks some micronectonic organisms such as the fishes molluscs, and crustaceans like
Sergestid shrimp (Omori et al., 1972). Although Nemoto (1962a, b) early reported
the newly known food organisms as found in sei whale stomachs, the principal foodstuff of sei whale in the Subantarctic waters is consisted of both copepods and small
sized euphausiids, and this fact is far different from the previous recognitions ofdepending solely on euphausiids. The following species could be considered as
newly known food orgnaisms in the Subantarctic and Antarctic regions in addition to those reported previously. They are: C. tonsus*, Clausocalanus laticeps,
Thysanopoda actifrons, Euphausia diomedeae, E. lucens, Penaeus sp. and Scomberesox saurus.
Among those food organisms given in Table 5-1, the following four species
might be the most important throughout the Subantarctic waters from their distinct occurrence in the sei whale stomach: C. tonsus, C. simillimus, E. vallentini, andParathemisto gaudichaudii. In addition to these species both Clausocalanus laticeps andDrepanopus pectinatus could be considered to be particularly important in a local sea
reg10ns.
Nemoto (1962a) early suggested that E. similis which had not been reported
as whales food would be found in the whale stomach when the whaling groundshifted to another place in the future, since E. similis forms dense swarms in thesurface water. By examining the foodstuff of minke whale, Balaenoptera bonaerensis
caught in 1967 /68 season, I found the occurrence of E. similis as food in several
number of animals (Ohsumi et al, 1970) and again in sei whale caught in 1969/70
5-1.

* Salinikov (1953, Table 4) described C. tonsus as a food of baleen whales in the North Pacific waters.

However, it is obvious in the studies by Tanaka (1954, 1956) that Salinikov's C. tonsusmust be separated from theso-called C. tonsus, the endemic species in the southern hemisphere. C. tonsus was also reported as sei whale
food from the Cape Province, South Africa (Best, 1967) but I enlisted the species again in this report since
Cape Province is located at about 33°8 being far outside from the Antarctic region proper.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

47

season. These facts strongly suggest that the food organisms of baleen whales
would supposedly form the dense swarms or aggregations into patchiness. These
results lead to a consideration that the southern saury, Scomberesox saurus andPenaeus sp., both were found in this study, also seem to form the swarms in the surTABLE 5-2.

COMPOSITION OF FOOD ORGANISMS IN THE STOMACHS OF THE
SOUTHERN SEI WHALE 1967/68.
No. ofKind of food organisms
whalesexamined

C. tonsus (CV & Female)
C. tonsus (CV)-P. gaudichaudii f. compressa (Female)
C. tonsus (CV)-P. gauchichaudii f. bispinosa (Female)
C. tonsus (CV & Female)-£. vallentini (Female & Juvenile)
C. tonsus (CV & Female)-£. sp. (Furcilia)
C. tonsus (CV)-E. vallentini (Female)-£. hamata
C. tonsus (CV)-P. gaudichaudii (Female)-£. vallentini (Male, Female & Juvenile)
C. tonsus (CIV & CV)-E. hamata-Clione antarctica & Cleodora sulcata
C. simillimus (CV & Female)C. simillimus (CIV & CV)-Drepanopus pectinatus (Juvenile)
C. simillimus (CV)-E. vallentini (Male, Female & Juvenile)
C. simillimus (CV)-E. sp. (Male & Furcilia)
C. simillimus (CV)-P. gaudichaudii f. compressa (Male & Female)C. simillimus (CV)-P. gaudichaudii f. bispinosa (Female)C. simillimus (CV)-G. nicholsi
C. simillimus (CV)-P. gaudichaudii f. compressa (Male & Female)-E. vallentini (Juvenile)
D. pectinatus (Male, Female & Juvenile)
D. pectinatus (Juvenile)-C. simillimus (CIV, CV & Female)
C. laticeps (Female & Juvenile)23

P. gaudichaudii f. compressa (Female)
P. gaudichaudii f. bispinosa (Female)-£. compressa (Female)
P. gaudichaudii f. bispinosa (Female)-E. vallentini (Male & Juvenile)
P. gaudichaudii f. compressa (Male & Female)-P. gaudichaudii f. bispinosa (Male & Female)E. superba (Male & Female)
P. gaudichaudii f. compressa (Female)-gaudichaudii f. bispinosa (Female)E. vallentini (Male, Female & Juvenile)
P. gaudichaudii f. bispinosa (Female)-E. vallentini (Juvenile)-C. simillimus (CV)918
1Euphausia superba (Male, Femal & Juvenile)
E. superba (Male, Female & Juvenile)-Myctophum subasperum & Gymnospelum nicholsi
E. vallentini (Male & Female)
E. vallentini (Juvenile)-D. pectinatus (Juvenile)

14

1

2
2

1
4
3
2
1
2
2

4
2
2

10

Degree of dominancy is on decrease from left to right in the left column and the dominant formsof each organisms are shown in parentheses.
Note: CIV-the fourth copepodites, CV-the fifth copepodites.

face waters. A little number of myctophid fishes, chaetognatha, and pteropods
were also found as contaminants. They must have been fed by a chance when theygathered in the surface waters in search of their prey organisms such as copepoda
and euphausiacea, and have little importance as sei whale food. This kind of
mixture not only occur in the mid latitudes but also occur in more higher latitudes.
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

KAWAMURA

48

According to Brown (1968) the sei whale caught in the South Georgian waters
had fed on Electorona subasper and E. normani along with E. superba, and he (Brown,1968) called them as " occasional occurrence ". By discussing the diel vertical
distributions of those myctophid fishes, Brown (1968) also supposed that the bulk of
sei whale feeds actively in the evening since those fishes usually come up near to thesurface in the night. It might, however, rather difficult to know the active feedingtime of sei whale like this way since there are also the fact that the sei whale does
not seem to feed at any definite time of a day but feed whenever there are a foodstuff
available (Kawamura, 1970a).

5-2.

Composition of the stomach contents
As mentioned before the most important food organisms for sei whale in the
Subantarctic and Antarctic waters are C. tonsus, C. simillimus, P. gaudichaudii, E. valTABLE 5-3.

COMPOSITION OF FOOD ORGANISMS IN THE STOMACHS OF THE
SOUTHERN SEI AND FIN WHALES, 1969/70.
ParaCalanus
Calanus Drepa- Euphausia Euphausia Euphausia Euphausia themisto Notolepis
tonsus simillimus p ntopust
lucensvallentini superba diomedeae
gaudicastsi
ec ina uschaudii,.-"-.., , . - " - . . , , . - " - . . , , . - " - . . , ~ ~~Sei Fin Sei Fin Sei Fin Sei Fin Sei Fin Sei Fin Sei Fin Sei Fin Sei Fin--------- ---------

C. tonsus
203* 3
C. simillimus
2
Th. vicina
Th. gregaria
1
Th. sp.
3
E. lucens
E. vallentini
E. superba
E. similis
E. diomedeae
Th. actifrons
1
P. gaudichaudii 19
Penaeus sp.
4
S. saurus
1
V. attenuata
N. castsi

5**

2

10 38

8

8

20

2

I & II : Order of dominancy.
* Including a mixture with Pseudochirella sp. in the Order II.
** Including a mixture with a few individuals of P. gaudichaudii, E. vallentini, Th. gregaria and E.
hamata in the Order II.

lentini and E. superba, whereas in Saldanha Bay, South Africa there has reported
more than fourty five food species; seven euphausiids, twenty of copepoda,
six of amphipoda, four of fishes, six of pteropoda and some others such as
megalopa larvae, and Vellella sp. (Best, 1967). However, there were only 8-9
species which seemed to be actually indispensable as food of sei whales there.
The most of other species might have found as "occasional occurrence", thoughSci. Rep. Whales Res. Inst.,
No. 26, 1974

9+

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52

organisms. It was also known that the number of species contaminated does notexceed more than two, and they also never exceed the major species in quantity,
except when the fishes were contaminated; that is, the food of sei whale usuallyformed with a species which occurs dominantly in the sea concerned. As far as theindividual numbers observed are concerned, the sei whale in the Antarctic waters
feed selectively with prefering orders of copepoda-euphaussiid, amphpoda, orboth of them-other crustaceans, and then fishes or squids as has been reported in
the North Pacific sei whale (Nemoto, 1957; 1959). However, the food preference
in sei whale varies to some extent by the case. For instances, they feed chiefly on
E. lucens, E. recruva in the South African waters, and copepods such as C. tonsus, C.
carianatus, Clausoclanus arcuicornis f. major follow to the former (Bannister and Baker,
1967; Best, 1967). Although the "selection" by the kind of food organisms inbaleen whales is still in need of more discussion, there seems to be no other ways tobelieve it possible by adding more facts observed in the North Atlantic baleen
whales where Howell (1930) described the selective feeding habits with the orders
TABLE 5-4.

MONOSPECIFIC DEGREE IN THE STOMACH CONTENTS OF SEI WHALE
IN FIVE MAJOR SPECIES OF FOOD ORGANISMS.
No. ofNo. ofmonospecific mixed
samplessamplesSpeciesCalanus tonsus
Calanus simillimus
Parathemisto gaudichaudii
Euphausia superba
Euphausia vallentini

1967/68
1969/70
1967/68
1969/70
1967/68
1969/70
1967/68
1969/70
1967/68
1969/70

23
203
4
4
27
20
14
8
10
10

9
34
12
2
4

No. of species
mixed
~

1

2

6
34

3

11

3

No. of Monospecificsamplessamples
examined
(%)
32
237
16
6
31
21
15
8
11
10

71.9
85.6
25.0
66.6
87.1
95.2
93.4
100.0
91.0
100.0

of small shrimps, anchovy, smelt, and then mackerel. In the Antarctic waters, on
the other hand, many sei whale actually feeds on C. tonsus at first then P. gaudichaudii or E. vallentini and finally E. superba notwithstanding the fact that there must be
exist so rich distribution of E. superba in the higher latitudes when they enteredinto the southern feeding place in early summer. According to my unpublisheddata the surface swarms of young euphaudiids and amphipods are observed often
in the Subantarctic waters of relatively higher latiturdes, but they never found in
the stomach contents of sei whales there. They feed exclusively on the adult formsof E. vallentini or P. gaudichaudii. However, it is still unknown that these are dueto the size of food organisms as particles. By examining the sei whale food andfeeding in South Georgian waters, Brown (1968) reported a opposite opinion that
there were no facts to believe the selective feeding habits in sei whale.
As shown in Table 5-1 the most important food organisms of the sei whale wascopepods but an actual feeding habit of sei whale seems slightly more complicatedSci. Rep. Whales Res. Inst.,

No. 26, 1974

FEEDING HABITS OF SEI WHALE

53

when Penaeus sp., Scomberesox saurus, and several another organisms occur locally as
being found in 1969 /70 season. The feeding habits of sei whale can be considered
as a whole that they feed on any kind of swarm forming organisms which dominantly
occur within their migratory regions. Bannister and Baker (1967) have also been
pointed out on this character by examining the feeding of baleen whale at Durban.
However, it must be noted that both chaetognatha which does not swarm into socalled patchiness, and tunicates which is not preferable organisms, are avoided atleast by sei whale notwithstanding the fact that they usually occur quite abundantlyin the feeding place of baleen whales.

Geographical 'distribution of food organisms
Since considerable large number of sei whale is caught by the floating factories, it is hardly possible to examine or collect the row materials of the somach contents on every carcasses. So the the food samples are usually collected with someintervals in time and space such as every tenth of carcasses, or every shifts in noon
position of the fleets. As mentioned in the foregoing section the kind of foodorganisms are recorded on the every carcasses regardless the collecting samples.
By comparing both row and data materials together, it is possible to figure out
the general geographical distribution of food organisms. The results are shown in
Figs. 5-1 to 5-3, each of which represents the distributions of copepods, amphipods, and euphausiids respectively.
In the overall distributions of copepod food, Calanus tonsus not only occurreddominantly in the south of 40°S covering vast circumpolar regions throughout theareas II to VI, but also was fed most frequently in the region. Its occurrence as
sei whale food well agrees with the distribution of patches in the surface waters underclose relation to the Subtropical Convergence (see also section 11 ). As has mentioned in the foregoing section, it must be noted that only swarm forming organisms
could be fed by the sei whale, i.e., the distribution of C. tonsus as described above
supposedly present a little different figures from those known as geographical distribution range of this species. The biomass usually found in its patches is maintained such high density as to be hardly comparable to that known in general, andhas been considered to be a characteristic niche in the marine ecosystem. Since thewhaling in the Antarctic region is opened in the south of 40°S the northern boundary in the distribution of C. tonsus in Fig. 5-1 does not show its natural habitat but
artificial. The center of rich distribution of C. tonsus is usually found in the vicinity of the Subtropical Convergence (e.g. Kawamura and Hoshiai, 1969) but its
northern most boundary may extends beyond upto 30°S or thereabouts.
According to Brodskii ( 1964) C. tonsus occurs under the temperature of 5 °,..,
l5°C, and its rich occurrence is observed between 8.5°C and 12.3°C off Otago
Peninsula, New Zealand (Jillett, 1968). In this study the surface sea temperaure
at which C. tonsus occurred as food of sei whale during December was between 9.5°
and 18.0°C. Judging from feeding percentages by sei whale the most preferable
temperaure for this species was supposed to be 9.0°,.., 13.0°C, and these sea condi-

5-3.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

54

KAWAMURA

tions correspond to the Deacons' mean temperature of the Subtropical Convergence during austral summer (Deacon, 1937).
A distinct southerly extension in the distributions of C. tonsus down to 55°S between 160° and 170°W longitudes would be comparable to the southerly shift of
the Subtropical Convergence. Actually, the Subtropical Convergence at 170°W
in 1968/69 season was located in the vicinity of 55°S latitude (Ocean Res. Inst.,
1970).
In the plankton communities of South Georgian waters including northeast
Weddell Sea and Scotia Sea, C. tonsus does not occur (e.g. Mackintosh, 1934;
Hardy and Gunther, 1935), since these regions are influcenced under a northward
movement by the cold Weddell Current lower than 5.0°C. In general C. tonsus distributes abundantly throughout the subantarctic regions so as to be fed by the sei
whale under relatively warmer conditions, i.e., 10°,..., l5°C, and no other kind of organisms which are comparable with C. tonsus as whales food were found. In thispoint of view C. tonsus could be considered ecologically most important species
being comparable with C. jJlumchrus or C. finmarchicus in the northern hemisphere.
Following to C. tonsus the secondly important food copepoda is C. simillimus.
However, as it is seen in the distributions, C. similliums occurred in the Indian sector
where both Crozet and Kerguelen Islands, and also a part of Atlantic sector are involved. So the occurrence of C. simillimus is considered rather local and hardly
comparable with that of C. tonsus as food of sei whale. This species distributes on
the whole in the slightly southern waters than C. tonsus with the distribution center
between 45° and 50°S. It is note worthy that the distibutions of C. simillimus and
C. tonsus is closely located geographically but never overlaps each other. In contrast
to rather earlier occurrence through December to January in C. tonsus, C. simillimus
occurred mostly around February. Accordingly the sea temperature when theyoccurred was also different: C. simillimus food was found under the temperaures of
5.0° -9.1°C in 1967 /68 season. According to the distributions of copepods obtained by the Japanese Antarcic Research Expedition (Kawamura and Hoshiai,1969), the successive occurrence of C. tonsus was replaced with C. simillumus at about
45°S in the southern waters of South Africa, when the surface sea conditions were
7.5°C with salinity of 34.07%0. These facts suggest that the lower most temperature for C. tonsus distribution may be 7.0°C or thereabout whereas it may be
the higher most temperature for C. simillimus. Although both C. tonsus and C. simillimus are the Subantarctic species, the former could be regarded as northern warmer water species while the latter as southern cold water species.
D. pectinatus occurred only in the waters around Kerguelen and Crozet Islands.
In the waters in the vicinity of Crozet Islands distribution of this species agreed
well with that of C. simillimus but not in Kerguelen waters where none of C. simillimus occurred at all. Nemoto (1962) also reported D. pectinatus as sei whales foodonly in the waters around Kerguelen Islands. Kerguelen Islands is located on the
Kerguelen-Gausberg under water ridges and forms a shallow depth zones which
are rarely found in the high seas in the Antarctic region. The characteristic occurrence of D. pectinatus as above mentioned may supposedly due to a local hydroSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

55

logical conditions caused by a kind of upwelling of deep waters in the IndianAtlantic Basin.
Clausocalanus laticeps is one of newly found food species in the Pacific sector ofaround 180° meridian. Although its distribution area agrees with that of C.
tonsus, C. laticeps occurred as monospecific population without mixing with anyother food species. Among the euphausiid species shown in Fig. 5-2, both E. vallentini and E. superba are the most important species in the feeding percentages andlarge shares as biomass in a stomach contents. In the mid-latitude zones theeuphausiid food represented almost exclusively by E. vallentini. Nemoto (1961)
added this species in the dietary list of southern baleen whales after finding its occurrence as food of pygmy blue whale, B. musculus brevicauda (Ichihara, 1961: Omura
et al, 1970) in the Kerguelen waters. Mackintosh (1960) had early suggested an ecological importance of E. vallentini when there had not been any scientific evidences on
the role of this species among marine food-chains. In the areas II and III E. vallentini is fed in a narrow zone between 45° and 47°S but in the areas II and IV, theIndian sector, this species was frequently found under admixing with P. gaudichaudii. Although the most E. vallentini as the case in copepods is usually fed during
December, its distribution centered in rather southern waters within the Subantartic zone. A completely isolated occurrence of E. vallentini was seen in thesoutheastern waters off New Zealand. Since its distribution is clearly isolated
from that of C. tonsus, E. vallentini could be considered a typical Subantarctic species, and these facts agree with the result by Nemoto (1962a), in which he suggested
that the distribution pattern of E. vallentini as whales food seems to agree to someextent with the distributions of shallow zone by continental shelf and under waterridges. In the environs of New Zealand particularty in its eastern regions the continental shelf develops well, which might be responsible both for the upwelling anda local swarming phenomena by E. vallentini.
As the whaling season proceeds the sei whaling ground also shift gradually
toward south. In accordance with this shift the food organisms of sei whale alsochanges into E. sujJerba which distributes in the waters south of the Antarctic Convergence, a representative of the whales food in the region. However, it seems
rather less important as sei whale food since relatively small number of sei whale
migrates into the higher latitudes in the south of the Anatrctic Convergence. Other
euphausiids species such as E. lucens, E. similis, E. diomedeae, Thysanoessa vicina, Th.
gregaria and Tfrysanopoda actifrons, were also found in slightly northern regions thanE. vallentini as sei whale food, but their occurrence was quantitatively small andrather sporadic. In this sence they seem less important as sei whales food. According to Nemoto (1957), Th. macrura was one of other kind of important species asa food of fin, blue and humpback whales but none of individuals of Th. macrura
was found in the sei whale stomachs in this study. Nemoto (1957) also had not
found its occurrence in sei whale. These facts are characteristic when comparedwith the case in E. superba since Th. macrura also distributes in the waters near to
ice-pack (Nemoto, and Nasu, 1958). One of a possible explanations is that Th.
macrura shows rather sporadic and local distributions than E. superba in addition
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

56

KAWAMURA

to its habitat in higher latitudes.
Parathemisto gaudichaudii, one of the most important food organisms for sei
whale which widely distributes in the Subantarctic waters with the center between
45° and 50°S, i.e., relatively southern waters near to the Antarctic Convergence(Fig. 5-3). Accordingly its general features of the distribution resemble with thatof E. vallentini. However, it would be noticed by comparing the distribution character with E. vallentini that the occurrence of P. gaudichaudii as food of sei whale ismuch sporadic, that is, P. gaudichaudii forms the patches so as to be fed by the whale
but distributes sporadically. P. gaudichaudii occurred most distinctly in the Indian
sector of the areas III and IV where the bottom topography is complicated by thepresence of many islands such as Kerguelen, Crozet, Marion and Heard Islands.
So its distribution resembles to that of E. vallentini. Although Nemoto (1962)
found P. gaudichaudii in the stomach contents of fin and pygmy blue whales, thisfood species would supposedly be preferred much in sei whale from the general
selecting feeding habits of baleen whales. The general agreements in the distribution of both E. vallentini and P. gaudichaudii suggest a possibilities of their co-existingin time and space. Actually they occurred in the food of sei whale by admixing
each other in some sea regions of the Indian sector (see Fig. 5-2).
Penaeus sp., Scomberesox saurus, Vinciguerria attenuata and some other kind of
myctophid fishes occurred in relatively larger quantities but were found to be lessimportant since they were considered as the contaminants mixed with the principal
food zooplankton populations. Among those above mentioned organisms, Genus
Vinciguerria, a kind of gonostomatid fishes, would be regarded relatively importantsince it forms a potential foodstuff of the skip Jack, Katsuwonus pelamis at times in
the Pacific Ocean (Kubota and Kawamura, 1972).

Latitudinal change in the distribution of food organisms
In 1967 /68 season two of four floating factories operated relatively widely
through latitudinal zone, and the materials obtained from those two fleets madeit possible to analyse the change in geographical distribution of food organisms
in terms of the feeding percentages. The general features of latitudinal change
in the Indian sector is demonstrated in Fig. 5-4 and those in the Pacific sector alongwith the Tasman Sea region is in Fig. 5-5. The feeding percentages are expressedby the ratio of the number of animals with food in the first stomach against the
total number of animals examined. " Calanus " in the figures does not mean the
Genus Calanus in taxonomical sence but represent the copepod although most of
the case in " Calanus" is represented by C. tonsus and C. simillimus (Tables 5-2 and
5-3).
In both figures it is noticed clearly that " Calanus " food being represented
by C. tonsus and C. simillimus occurs exclusively in the waters south of 40°S, down
to amidst of the Subantarctic region of about 47°S i.e., the northern half of the
Subantarctic waters of the main sei whaling ground of the copepod richwaters. Getting closer to southern region of the Subantarctic waters close
to 50°S., the representative food organism gradually changes to small-sized eu5-4.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

57

phausiids such as E. vallentini, but P. gaudichaudii was also fed by the whale in someplaces under both monospecific composition and mixture with E. vallentini. P.
gaudichaudii increased gradually in the share of occurred food organisms toward
the Antarctic Convergence. In the south of the Antarctic Convergence the dominancy of E. vallentini in a stomach contents replaced with P. gaudichaudii but thelatter is again replaced gradually with euphausiids of adult and adolescent forms
80

70

"'""

60

AC

STC
Total
-·- Euphausia (LJ--- Calanus-·-·- Euphausia (M)•••••• Amphipoda --·· Euphausia (S)

~ 50
2l

£ 40

:.a"" 30"'

Lf 20

LatitudeFig. 5-4. Latitudinal change in the occurrence of food organisms for sei whale in
the Indian sector of the Antarctic. STC: Subtropical Convergence, AC: Antarctic Convergence.

Fig. 5-5. Latitudinal change in the occurrence of food organisms for sei whalein the Pacific sector of the Antarctic and in the Tasman Sea region. STC: Subtropical Convergence, AC: Antarctic Convergence.

of E. superba. In the waters south of65°S I-year group of E. superba occurs sporadically in the stomach of sei whale. Thus the occurrence of the food organisms distinctly changes in latitudinal direction. It is note worthy that these successivechanges occur within the narrow zone of about 20° degrees of the latitudes.
The food organisms of sei whale in the waters of the both Subantarctic andAntarctic occurred toward higher latitudes in the order of copepod, euphausiid,
amphipod, and then again euphausiid. In the waters north of the Antarctic
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

58

Convergence E. vallentini was fed more preferably than P. gaudichaudii notwithstanding the fact that the former is less abundant than P. gaudichaudii in the region, andfeeding on P. gaudichaudii by the whale was distinctly increased in the south of the
Antarctic Convergence (Fig. 5-5). According to Kane (1966), P. gaudichaudii
distributes most abundantly in the waters around the Antarctic Convergence especially in its northern side. These facts lead to a consideration that the sei whalefeeds more preferably on euphausiids than amphipods while the former seems tobe less abundant than the latter in the region concerned as a function of selective
feeding habits, although it is still unknown whether the food selection is caused bymechanical process or some kind of biologically potential preferance. It may also
be possible that the selection of the food organisms in feeding habits would released
only when rich food organisms were available in the region. The selective feeding haibts in sei whale also corelate to the distribution pattern of the food organisms
whether they form the patchiness or not.

5-5.

Distribution offood organisms by the whaling area
As mentioned in the foregoing section, the food organisms of sei whale changes
one to another in a narrow latitudinal zone. The general circumpolar hydrologicalconditions in the surface waters of the Antarctic Ocean shows rather monotonous
TABLE 5-5. FEEDING PERCENTAGE (ITALICS) AND ACTUAL NUMBER OF WHALESIN THREE MAJOR FOOD ORGANISM GROUPS BY THE WHALING AREAS
III TO VI IN 1967/68 SEASON.
Kind of food organisms
Area & Sector

Calanus Ampdhipo a

Areas III & IV
191
207*
(Indian Ocean)
13.64 12.58
Areas IV & V
357**
73(Tasman Sea & Pacific Ocean) 28. 61
5.85
Areas V & VI
888
70
(Pacific Ocean)
34.82
2.75

E h "dUnE~known*** mpty

L

M

S

115

23

59

919

7.58

1.52

3.89

60.54

17

203

537

1.36 16.27

43.03

61

4.89

575

9

1008

22.55 0.35

39.53

Food/
Total

No. of
inds.
Total
1518

39.46
1248

56.97
2550

60.47

* Includes Drepanopus pectinatus.
** Includes Clausocalanus laticeps.
*** Not examined.

features (Deacon, 1937), that is, the hydrological conditions are less variable by
the whaling areas in contrast to those in latitudinal direction. In order to figureout the change in the occurrence of food organisms by the whaling areas, the composition figures were given in Tables 5-5 and 5-6. In the tables the occurrenceof food organisms by each whaling areas were expressed in feeding percentages
along with the number of animals examined. To make comparison with the caseof fin whale its figure was also given in Table 5-6. The kind of food organisms
expressed in the tables would be known by refering the foregoing sections. Bothlarge (L) and medium (M) sized euphausiids represents the 1- or 2-year group of
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

FEEDING HABITS OF SEI WHALE

59

E. superba and small (S) sized one is represented mostly by E. vallentini or E. lucens
in mid-latitudes and by the young E. superba in higher latitudes. Only P. gaudichaudii corresponds to amphipods (Am).
The feeding percentages in 1967 /68 season showed 52.3% on an average andthose in 1969 /70 was 48.6 % through the whaling areas II to VI whereas fin whale
showed 58.6%. Their percentage figures suggest that an approximately 50%
of sei whale caught in the whaling operations carries food contained stomachs.
The feeding percentage of fin whale was slightly higher than that of sei whale.
The feeding percentage by the whaling areas in both 1967 /68 and 1969/70 seasonsvaried 39.46-61.22%, which suggests the different feeding conditions to someextent by the whaling areas. In the feeding percentages by the kind of foodorganisms, "Calanus" (=copepods) food showed only 13.64% in the Indian
sector of the areas III and IV in 1967 /68 season while they increased gradually
toward the Pacific sector where the feeding percentage attained at 34.82% (Table
5-5 ). The " Calanus " food through the areas II to IV in 1969 /70 season showed
a quite high percentage being higher than 60% in the area II whereas they
decreased about one half in the areas III and IV.
TABLE 5-6. FEEDING PERCENTAGE (ITALICS) AND ACTUAL NUMBER OF FIN AND
SEI WHALES IN FOUR MAJOR FOOD ORGANISM GROUPS BY
THE WHALING AREAS II TO IV IN 1969/70 SEASON
AreaSpeciesofwhalesCalanus Am~hipo a

675

Euphausiid
~

L

M

s

2

8

0.18

0.71

Sei

60,32

Fin

30,00

Sei

29.68

Fin

1.10

Sei

579

47

34

35.76

2.90

2.10

Fin

9

445

16

18

1.34

66.12

2.38

2.67

II

III

IV

12

30.00
42

61
7.77
4
0.37

5.35

1

0.06

2

0.26

Food/
Total

No. of
inds.
Total

38,78

434

61.22

1119

8

40.00

60.00

20

446

56.82

43.18

785

UnEmpty
known**

6

6

233

Fish*

1

0.13

486

1

585

44.67

0.09

53.77

46.23

1088

950

58.67

41.33

1619

184

72.66

673

6

0.37

2

0.12
1

0.15

27.34

* Notolepis castsi.
** Not examined.

These results as observed in both seasons suggest that " Calanus" food might
be relatively less important in the Indian sector especially in the environs of Crozet
Islands. The relatively low feeding percentages of " Calanus " food in thesewaters well agree with the result obtained by Pervushin who studied on the materials during 1961/63 to 1964/65 seasons (Pervushin, 1968). On the other hand,a relatively high percentages in the areas II of Folkland Island sector and theareas V and VI indicate a potentential importance of " Calanus " food in thoseregions. However, the "Calanus" food can be regarded potentially important
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

60

KAWAMURA

diet for the sei whale whereas they seems less important for the fin whale throughout
the whaling areas. The general trends to feed more on euphausiids than copepod
in fin whale are clearly seen in Table 5-6.
Although the importance of "Calanus" food for sei whale may be quite
evident in general, the fullness of the first stomach with " Calanus" food proved
less than 50%, and none of fully repleted stomachs were observed. This fact indicates that the population density in terms of the biomass in " Calanus " foodwould be less than that in euphausiids and amphipods even when they aggregated densely to show the patchiness. Copepods as food of sei whale have been wellknown early (e.g. Collett, 1886; Ingebrigtsen, 1929; Nemoto, 1957; 1959; Kawamura, 1970a, b) and their more importance for southern sei whale than any otherorganisms is proved in this study. Accordingly it would be possible that the sei
whaling at least in December to January was determined primarily by the distribution of" Calanus" food.
Different from" Calanus" food, amphipods (P.gaudichaudii) was fed by 12.58%
of sei whale in 1967/68 season and by 7.8% in 1969/70 season. These figures
are rather lower when compared with the case of " Calanus " food. Among thewhaling areas concerned relatively high feeding percentages were observed in
the Indian sector of the areas III and IV in both seasons. However, the feeding
percentages in 1969/70 season leveled cosiderably lower than in 1967 /68 season as
well as having been noticed in " Calanus " food. These phenomena are still notproved yet, but supposedly due to the variation in geographical distribution alongwith year to year fluctuations in the population size of the food organisms. The
low feeding percentages in 1969/70 season also agree with those observed ineuphausiids. Since these trends are quite similar through the three kinds of foodorganisms, it is supposed that they must had not been influenced under theenvironmental control onto their developmental stages.
Large and medium sized euphausiids, say, E. superba had been fed quite little
by the sei whale in 1969/70 season. The feeding percentages showed only 2.16%
for large sized euphausiid in the area IV whereas it was 7.6% in the Indian sectorof 1967 /68 season. The latter was the highest feeding percentages through bothseasons. In 1967 /68 season euphausiids showed relatively higher feeding percentages than copepods and this relationship reversed in 1969 /70 season. A relative low feeding percentage in large and medium sized euphausiids supposedly due
to the geographical location of main sei whaling ground, and the varations between
whaling seasons would be due to the environmental factors which are responsible forthe development of young copepods. The sei whale might have fed on E. superba
when copepods such as C. tonsus was poorly available. Studying on the food ofbaleen whales in the North Pacific, Nemoto (1959) concluded that there must be
both Calanus rich year and euphausiids rich year in those region. A similar phenomena may be considered in the Antarctic whaling ground.
In small sized euphausiids, however, the general circumstances seem different
to some extent. Different from large sized euphausiids the small sized one ischiefly represented by E. vallentini and E. lucens, and their feeding percentages inSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

61

the Pacific sector of the areas V and IV showed 22.6%, a relatively high ratio
though it decreased gradually toward the west through Tasman Sea, Indian sector
and then Atlantic sector respectively. In 1969/70 season the feeding percentages
were distinctly low throughout the areas II to IV, and no areal variations wereobserved. The occurrence of small sized euphausiids may also be related to theabundance of copepods as having been suggested in the large sized euphausiids.
According to Baker (1954) the circumpolar distribution of zooplankton in the
Antarctic region shows a monotonous features on the whole through the three
oceans whereas they distinctly change in the north-south direction, and both of thesecircumstances agree well with the general hydrological conditions. The geographical changes in the feeding percentages of sei whale also agree as a whole withthe results reported by Baker (1954). A local but distinct occurrence of C. simillimus and P. gaudichaudii in the sei whale food agrees too with their general distribution patterns, that is, the distribution of food organisms does not differ fromthat of zooplankton.
6.

DISTRIBUTION OF WHALES, AND ITS RELATION TO PHYSICAL,CHEMICAL AND BIOLOGICAL ENVIRONMENT FACTORS

Studying on the food and feeding of sei whale caught in the 1967 /68 and 1969 /70
seasons, it was considered that the regions where feeding ground is formed in the
Subantarctic zone would closely related to the characteristic features in the distribution of food organisms. For instances, the feeding ground being formed by
the copepodite V stage of C. tonsus during early summer diminishes gradually as totheir seasonal vartical migration into the deeper waters (Jillett, 1968), and theseenvironmental changes make the whale move into more higher latitudes in searchof another foodstuff such as C. simillimus, E. vallentini and P. gaudichaudii. Thus,
the geographical shift in the whaling ground closely relates to the ecologicalcharacters in the distribution of food organisms.

6-1.

Distribution of whales and the environmental factors in the Pacific and Antarctic Ocean

Apart from the biological examination on whale carcasses, the oceanographical investigations along with plankton sampling and whale sighting were madeduring the KH-68-4 cruise of the R. V. "Hakuho Marn" of the Ocean ResearchInstitute, University of Tokyo, and a part of biological works made in this cruise
have already been reported (Kawamura, 1969a, 1970b, c; Kawamura and Kureha,1970). Leaving Tokyo on November 14, 1968, the research cruise ofKH-68-4 wasmade along two transections, i.e., l 70°W from 30°N down to 70°S and 155°E from
38°S to 70°S. The track along l 70°W crossed in the midst of main sei whaling
ground where one of the Japanese floating factories operated for two successive
seasons through 1967 /68 and 1968/69 (Ocean Res. Inst., Univ. Tokyo, 1970). A
total of 61 oceanographic stations were occupied during the cruise, and the plankton
collection from 150 m depth upto the surface was also conducted routinely with
the North ]Jacific standard net, and also several series of divided hauls from great
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62

KAWAMURA

depths with the Petersen type vertical closing net. Of approximately 9,000 miles
cruise 2,100 miles of the track was sighted for counting the number of whales andseabirds.
The number and the kind of whales sighted along l 70°W were demonstrated
in Fig. 6-1 along with the profile of the environmental factors. In general the dis-Fig. 6-1. Whales sighted and profile of the environmental factors along 170°W,
30°N-70°S during Nov. 22, 1968 to Jan. 19, 1969. Sp: sperm, B: blue, F: fin,
S: sei, D: small toothed, U: unidentified.

tribution of whales shows close relation to the bottom topography in the feeding
place (Nemoto, 1957; 1958), but the depth along the track covered by the R. V.
"Hakuho Maru" varied between 2,000 and 3,000 m, and possibly no influences on
the distribution of whales. It would be noticed in Fig. 6-1 that the distinct accumulations of whales were found in the south of 30°S especially in the Subantarctic region between 30°S and 50°S. A rather less number of sighted whales between 50°S
and 60°S was supposedly due to rough surface of the sea by stormy weather.
Sci. Rep. Whales Res. Inst.,

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FEEDING HABITS OF SEI Vl'HALE

63

An approximate physicochemical conditions in the Subantarctic waters along 170°W
were; 10°-20°C (temperature), 34.5-35.5%0 (salinity) and 0.25-0.5 µg-at/l(phosphate) and 5.5 ml/I (dissolved oxygen) respectively. On the charactersin chemical properties of the Subantarctic regions, Rochford (1961) demonstrated
35.4%0 of salinity and 0.2-0.4 µg-at/1 in phosphate as its indicators in the waters
off Western Australia, and he distinguished the water masses under the names of"south transitions zone". By taking these results into considerations it would benoticed that relatively denser consentration of whales are located not in the midstof the Subantarctic waters but in rather northerly located transition zone between

so·

40°

60°S

50°

40°

g
:5
c.

500

Q)

0

D. Oxygen (ml/ll

:5
c.
Q)

0

Phosphate-P(µg-at/l)

Silicate-Si(µg-at/ll

Nitrate-N(µg-at/ll

Fig. 6-2. Whales sighted and profile of the environmental factors along 155°E,
63°S-38°S during Jan. 23 to Feb. 1, 1969. Abbreviations of whale species are
as given in Fig. 6-1.

warm and cold waters. As clearly noticed in the distribution pattern of the environmental properties, those transition zone corresponds to the upwelling zone ofrich zooplankton standing stocks. In the south of 60°S, the proper of the Antarctic region, a great number of whales distribution was also found.
On the other hand in the Antarctic and the Tasman sea regions along 155°E
longitude, the whales were spotted only twice for sei and fin whales, which showeda poor distribution of whales in general (Fig. 6-2). The relationship between theSci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

64

distribution of whales and the environmental factors in this region was less clear
than those in l 70°W. However, relatively rich distribution of whales was foundin the Antarctic and Subantarctic waters, particularly in the transition zone ofnorthern waters.
Apart from the Antarctic regions it was note worthy that there were also distinct accumulations of whales in the region of equatorial divergence between 10°N
and l0°S, where a quite high concentrations of zooplanters were found (Fig. 6-3).
The zooplankton in this region would possibly be supported by the high primary
productivity due to upwelling (Nasu, 1972) which provides a quite rich accumulation of tuna (Sette, 1955 ). Although it is recognised in general that the baleen
whales do not feed much or entirely in the tropical seas (e.g. Mackintosh, 1965),
but those above mentioned results lead to a consideration that the whales would
potentially be in search of foodstuff even in those warmer waters.
-

400

:1=

:=:
----z, 300

c.

c

.8

"'"'

"''-'c

~

c

Cl

"'""
~

"' 200
g.

g

~

'-'
~

~ 100
;:

"'
:;::

0

0 ---'-~2~0°---'--;;;:30·0 N
0 ---'------;2~0°;---"~l~0~0 -~)-----;:0°0l~--;-;l0~0 -L----?::40~
0 -'----:C30~
70L0_S__,__~6~0°=----"~5~0~

1

)\~~

)I uJIU2~s1 )~

1

sp 1
)
lo30 s p1
SplU2 S2 Fl 01 020 01 S2 D2 050/ Sl 050
Spl Fl050 S4 01 Sp9
030
U2
Fl
05

1

I\

02
Bl 015 02
Sls~1 s~1

Fig. 6-3. Whales sighted and wet weight ofzooplankton in the upper 150 meter ofthe water column between 30°N and 70°S along l 70°W during Nov. 22, 1968 toJan. 19, 1969. Abbreviations of whale species are as given in Fig. 6-1.

Whales sighted at 40°,...., 50°S might have been on their migration toward south
since most of them were spotted in the early January. According to the catchrecords in the past 40 years (Omura, 1973), some individuals of the sei whale
spotted at higher latitudes in early summer are supposed to be as the stranger
from over wintered population.
In summerizing the above mentioned results both baleen and toothed whales
distribute in more favourable environments of the sea, where the feeding conditions
as having been indicated by the zooplankton abundance would be the most important role not only in the Antarctic region but also in the tropical seas.

Whaling ground of sei whale and the distribution of zooplankton
The whaling grounds of sei whale both in 1967 /68 and 1968/69 in the areas
V and VI of the Pacific sector were shown schematically in Fig. 6-4, and the dis6-2.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

65

tribution of food organisms found in the stomach of sei whale caught in the above
mentioned whaling grounds were demonstrated in Fig. 6-5. Black spots on themeridians of l 70°W and 155°E in the figures represents the oceanographic observation stations occupied by the R. V. "Hakuh6 Mam" during her 1968/69 cruise.
The track by the "Hakuh6 Mam" crossed over the whaling grounds although themain body of whaling ground in 1968 /69 season located somewhat eastward. The
mean position of both the Subantatctic and Antarctic Convergences are indicatedby the solid lines along with those observed by the R. V. "Hakuh6 Mam."

Fig. 6-4. Whaling grounds of sei whale in the Antarctic, Areas V and VI. Shadeby hatch indicates the region where the sei whale was heavily catched. Black
spot shows the position at which plankton sampling was made by the R. V. Hakuh6
Maru during her KH-68-4 Cruise in 1968/69. O.S.T.C., O.A.C.: Observed
positions of the Subtropical and Antarctic Convergence.

The whaling ground of sei whale in these regions is formed in the north of
the Subantarctic waters with its center along the Subtropical Convergence andits transition zone. These hydrological conditions do not differ much from thosein the Falkland region (area II) and the Indian sector (areas III and IV).
The food organisms which occur regularly in the whaling ground around New
Zealand are C. tonsus, Clausocalanus laticeps, E. vallentini, E. superba and P. gaudichaudii
among which E. superba is found only in the vicinity of Balleny Islands. This
species seems less important as sei whale food being compared with other foodspecies. Both E. vallentini and Clausocalanus laticeps as well as E. superba showed
local occurrence. In Fig. 6-5 it is considered that the whaling ground of theareas V and VI is largely formed solely by C. tonsus.
On the other hand the quantitative distributions of net zooplankton species
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

66

KAWAMURA

which were collected by the Norpac net at each stations were demonstrated in
Figs. 6-6 to 6-12. In Fig. 6-6 it was observed that the approximate number ofindividuals of C. tonsus population was 10 4 inds./1000 m 3 along the l 70°W whereas
it was 105 inds./1000 m 3 along 155°E. The figures of 104 - 105 inds./1000 m 3 ,can be considered quite distinct in abundance as the distribution density of copepods, and are still more higher density than that of C. tonsus in New Zealandwaters where they showed 0.52-83.5 inds./m3 (Jillett, 1968). The general features

Fig. 6-5. Distribution of the food of sei whale in the Areas V and VI, 1967/68.
O.S.T.C., O.A.C.: Observed positions of the Subtropical and Antarctic Convergences.

of the C. tonsus distribution in Fig. 6-6 correspond well to the characteristic distribution of whaling ground shifting rather northerly or southerly close to the
Antarctic Convergence at l 70°W and 155°E respectively.
C. simillimus was one of an important food plankton for sei whale in the Indian
sector of the area III, but none in the area V. A considerable number of species,
however, occurred in the net samples throughout the Subantarctic region with theindividual density of 103 - 105 inds./1000 m 3 or more (Fig. 6-7). The most C.
simillimus population occurred was represented by the copepodite stages II and III
along with adult forms while C. tonsus population was almost exclusively by the
copepodite V, that is, the food of sei whale. Accordingly, it would be possiblethat their life history differs each other. C. simillimus is generally recognized asSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

67
170'W

I s

I

35·

Fig. 6-6. Quantitative distribution of Calanus
tonsus in the whaling area V, 1968/69. MPS TC:
Mean position of the Subtropical Convergence,
MPAC: Mean position of the Antarctic Convergence, OSTC: Observed position of the
Subtropical Convergence, OAC: Observed
position of the Antarctic Convergence. Copepodites under IV are shown by the hatch.

Fig. 6-7. Quantitative distribution of Calanus
simillimus in the whaling Area V, 1968/69.
Legends as in Fig. 6-6.

Fig. 6-8. Quantitative distribution of Clausocalanus laticeps in the whaling Area V, 1968/69.
Legends as in Fig. 6-6.

Fig. 6-9. Quantitative distribution of Calanus
propinquus in the whaling Area V, 1968/69.
Legends as in Fig. 6-6.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

68

KAWAMURA

the Subantarctic species (Hardy and Gunther, 1935, Vervoot, 1951, 1957; Tanaka,
1960; Brodskii, 1964), and the center of distribution in this species were foundrelatively in the southern waters than C. tonsus and does not occur in those transition zone proposed by Rochford (1962).
As it was described by Brodskii (1964), C. simillimus distributes in the waters
around the Antarctic Convergence being centered between 50°S and 60°S. It was
the Kerguelen-Crozet regions of 43° -50°S that C. simillimus occurred in the sei
whale stomach in late January to mid February. In the environs of Crozet Islands
where the surface sea temperature during December to January was 9.0-13.4°C,
C. tonsus occurred as food of sei whale and it was replaced with C. simillimus in thewaters of 5.0° -9.1°C. These facts lead to a consideration that C. tonsus would berather northern warm temperate species in contrast to C. simillimus. According to
Kawamura and Hoshiai (1969) the boundary at which the distributions of both
C. tonsus and C. simillimus replaced one to another was found at 45°S in the area
between Cape Town and the Showa Base in Antarctica. The surface sea temperature and the salinity at 45°S were 7.6°C and 34.07%0, respectively. Since C.
simillimus does not occur under the conditions of low salinity lower than 33.88%0,
it would hardly be possible that C. simillimus occurs as food of sei whale in the higher
latitudes south of 60°S. Accordingly, C. simillimus would be fed by the whale in
the waters of 45° -50°S where relatively warmer water prevails although its distribution is positioned still southward than the case in C. tonsus. Therefore, the distribution pattern of C. simillimus is considered to be somewhat similar to that of
between C. propinquus and Calanoides acutus.
As the season goes by the copepodites II and III population of C. simillimus inJanuary develops gradually to copepodites III to IV while C. tonsus population of
copepodite V migrates into deeper waters (Jillett, 1968), and the feeding place forsei whale being formed by this species diminishes gradually. Then C. simillimus
forms the feeding place following to C. tonsus but it is somewhat local phenomenon;
none of C. simillimus was fed by sei whale in the waters around 155°E while a considerable number of C. simillimus occurred in net samples.
It was 1967 /68 season that Clausocalanus laticeps was recorded first in the foodof sei whale (Kawamura, 1970a). This species, as shown in Fig. 6-5 was fed by
the whale in the same region as C. tonsus, and therefore this species as food of sei
whale has not such importance as to C. tonsus because of its quite local occurrence. However, C. laticeps population occurred under monospecific composition
with high population density, and was considered locally important foodstuff forsei whale. C. laticeps distributes in the vincinity of the Subtropical Convergence,a slightly northern species, and they occurred abundantly in the Pacific sector
proper than in the Tasman Sea region. Although this species distributes upto
far north off Australia along the 155°E meridian, the population density showed
5x 10 2 inds./1000 m 3 at most. So the biomass of this species in the sea might berather small due to its body length. As a food of sei whale C. laticeps occurred in
the waters around Antipodes Island, 50°S, l 75°W,....., 180°, and the distribution ofthis species agreed well with that obtained on the net plankton samples. ThereSci, Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

69

fore, it is supposed that C. laticeps forms dense swarms in the surface waters as wellas other food species. In the South African waters near Cape Province, Clausocalanus
arcuicornis was fed by sei whale (Best, 1967). It was proved by this facts that atleast genus Clausocalanus would swarm into the patches as well as Calanus species,
and formation of the patchiness would supposedly be the general distribution characters throughout the genus.
Both Calanus propinquus and Calanoides acutus have not been recorded as thefood of baleen whales in the food materials collected by the Japanese floating factories (Nemoto, 1959; 1962a; Kawamura, 1970a). However, Peters (1955) reported C. acutus as the food of sei whale during the German whaling operations in1930's, and later Pervushin (1968) described both C. propinquus and C. acutus as
the food of the baleen whales. According to Ottestad (1932; 1936), C. acutus andC. propinquus are typically endemic species of the Antarctic waters, and they occurred only within the colder regions in the south of the Antarctic ConvergenceFig. 6-10. Quantitative distribution of Calanoides acutus in the whaling Area V, 1968/69.
Legends as in Fig. 6-6.

Fig. 6-11. Quantitative distribution of Parathemisto gaudichaudii in the whaling Area V,
1968/69. Legends as in Fig. 6-6.

(Figs. 6-9 and 6-10). In many locations these two species occured being accompanied with Rhincalanus gigas or dense vegetation of diatoms. Comparing withboth species C. acutus was more dominant than C. propinquus but the former was
represented by younger individuals of copepodites II to IV. Since C. acutus andC. propinquus distribute with a little numbers in the far southern waters apart fromthe main whaling ground of sei whale, it would be quite possible that none of those
two species occur in the diet of sei whale. However, Pervushin (1968) reported
their occurrence in the stomach contents of sei whale in the area III of Crozet
Islands and its environs although the case might be quite extraordinal. By summerizing the results on the distribution of those two species along with those reported
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

70

KAWAMURA

previously (Mackintosh, 1934, 193 7; Ottestad, 1932, 1936; Hardy and Gunther,
1935; Vervoot, 1951, 1957; Tanaka, 1960, 1964; Andrews, 1966; Kawamura andHoshiai, 1969), the northern most boundary in their distribution would on the whole
be somewhere between 50° and 60°S except the case in the Indian sector where it
was 47°43'S, at 76°48'E (Vervoot, 1957). It seems therefore curious that both
C. acutus and C. propinquus occurred as the food of sei whale in the Crozet Islands
regions since the feeding by whale itself essentially requires an abundant distribution so as to form the patches. In this respect both C. acutus and C. projJinquus may
possibly be fed to some extent by the baleen whales in some locally limited timeand place, but it may also be impossible them to form the staple feeding groundfor whales since those two species occur usually by accompanying the thick diatom
population of Thalassiothrix antarctica along with totally poor standing stocks
themselves.
Since P. gaudichaudii is provided the strong ability of swimming by its pleiopods(Kane, 1966; Nemoto and Yoo, 1970), many adult forms of this species would haveavoided from the net collection. P. gaudichaudii population occurred in net samplesof this study were excusively consisted of small immatured individuals ·with bodylength less than 10 mm. In the Subantarctic waters south of New Zealand Hurley(1961) and Kane (1962) made zooplankton collections, and found that amphipods
in those waters were mostly represented by the larval and immatured young individuals. Their net avoidance during net hauling would be obvious.
A relatively small number of P. gaudichaudii occurred regularly in the waters
between 50° and 60°S along 170°W, where the individual density showed about
10 2 inds./1000 m 3 whereas they showed much poorer but steady density throughout
the 155°E. It is supposed that the center of distribution would be around theAntarctic Convergence. The results obtained by Kane (1966) in the area III by
R.R.S. "Discovery II" and those by Hurley (1961) and Kane (1962) in the Pacificregion agree in general with the present study.
In the row materials of the stomach contents of sei whale the most individuals of P. gaudichaudii were represented by the large sized individuals of adultand possibly adolescent forms. This is largely different from the population obtained by the net. However, it was observed that the general characters of the
distribution in both populations do not differ much. So it could be said that noneof the habitat segrigations are possible in adult and young immature forms.
It has been reported that a little number of P. gaudichaudii occurred in thestomach contents of blue and fin whales as an admixture among E. superba population (Nemoto, 1959; 1963). However, the importance of P. gaudichaudii as foodof baleen whales is much increased along with the proportional increase of sei
whale in the total catches. In this respect P. gaudichaudii seems to be indispensable
food for sei whale especially in the Indian sector of the Antarctic (Table 5-4).
According to Nemoto (1959) the fin whale in the area V fed exclusively on E.
superba whereas the bulk of sei whale fed on P. gaudichaudii. Following to C.
tonsus, P. gaudichaudii as found in net samples distributed widely through the whaling
ground as shown in Fig. 6-5. Since the net avoidance by euphausiids as well as
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

71

P. gaudichaudii may also be possible as Fleminger and Clutter (1965) have discussed
on this matter, a quantitative treatments on the net samples in this study would
be difficult. Fig. 6-12 demonstrates the distribution of euphausiids species
occurred in the net samples. The populations of euphausiid were mostly consistedof young individuals of the furcilia stages, and a little number of adult forms. A
total of thirteen species of euphausiids was identified, but it was only two or three
species that were actually important as food of sei whale, i.e., E. superba, E. vallentini,and E. lucens. In contrast to euphausiid populations in the net samples, those fedby the whale were mostly represented by both adolescent and adult forms. The
furcilia and some corresponding individuals were consisted of many species, andtherefore, the distribution of furcilia of euphausiids does not seem to have any direct
relationship to the formation of feeding ground.

40'

45'

e Euphausiids(furcilia)~ Euphausia simillis var. armata
e E. simillis~E. lucens~E. vallentini~E. frigid a
Cl E. spinifera~E. sp.
~ E. superba@ Thysanoessa parva~Th. vicina®Th. macrura@ Th. gregaria@ Nematoscelis mega/ops

Fig. 6-12. Quantitative distribution of euphausiids in the whaling Area V, 1968/69.
Legends as in Fig. 6-6.

E. vallentini, the main euphausiid species occurred at 52°S and 54°S alongl 70°W longitude, and again at 51°S and 54°S along 155°E. Although the numberof individuals occurred was relatively small, the distribution pattern of E. vallentini
in both whales stomach and net samples agreed well in the region along l 70°W.
E. superba also shown a similar relationships. In Thysanoessa populations, Th.
macrura and Th. gregaria were distinct in their individual numbers. The former has
been reported as the food ofhumback and fin whales in the areas I and VI (Nemoto
and Nasu, 1958), and they (Nemoto and Nasu, 1958) suggested its importance as
whales food particularly in the region of higher latitudes. The occurrence of Th.
macrura in this study was recorded only in the south of 60°S, and this species may
possibly be less important for sei whale which is essentially considered to be copopoda feeder in the mid-latitudes. None of Th. macrura was fed by the sei whalein the areas V and VI.
The biomass of euphausiids in terms of the number of individuals in the area
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

72

V showed 103 -5X10 3 inds./1000 m 3 , and there were no notably distinct differenceswere found between local regions. As mentioned before the net avoidance byeuphausiids would be quite possible, and the euphausiids species as demonstrated inFig. 6-11 might have been entangled by reason of their dominancy among zooplankters under those circumstances. If so, E. vallentini could be considered tohave distributed abundantly in the southeast region off New Zealand, and in the
Tasman Sea region. Supposing these circumstances the feeding on E. vallentiniby the sei whale agrees with the general distribution patterns of E. vallentini.
In general the distribution of sei whaling ground can be seen as a function of
the characteristic distribution of food zooplankters, and these corelations weremore clearly observed in copepods. The organisms fed by the sei whale are represented by the patch forming species, and an agreement in the distributions
between zooplankton and whaling ground strongly suggests that those food organisms aggregate so as to form the patches to some extent at least in some local
reg10ns throughout their geographical distribution range.
7.

BODY SIZE OF CALANUS TONSUS

In the previous sections it has been discussed and proved that the zooplanktersand micronekton as food of baleen whales have to be maintained such high population density as to aggregate into patchiness. This conception would be well recognized by refering to some previous reports (e.g. Collett, 1886; 1911-12; Hinton,
1925; Mackintosh and Wheeler, 1929; Slepzov, 1955; Nemoto, 1957, 1959, 1962a,b; Marr, 1962; Mackintosh, 1973) although non-patch forming organisms would
be fed at times when the whales migrate into the region far outside from the socalled feeding ground such as the case in the South African waters.
In the course of microscopic observations on the food organisms some differences in the body size of C. tonsus population by the whaling areas were noticed.
C. tonsus, as has been described previously, is the most important foodstuff for thesouthern sei whale since it heavily aggregates in the surface waters during the firsthalf of whaling season. In this respect the composition of body size in C. tonsus by
the whaling areas was discussed in connection with its aggregating habits what itmeans ecologically.

7-1.

Composition of body size in C. tonsus population

The materials examined were the stomach contents of sei whales which werecaught in 1967 /68 and 1969/70 seasons. A total of 5,706 individuals of C. tonsus
was examined for this purpose, and the results were compiled by separating themonth into three decades (Table 7-1). However, the materials were only available during mid December to mid January since the whaling operation for baleen
whales is opened on 12th of December thereafter, and also the season for sei whaling
lasts for only a month and half to somewhere around mid January due to afading out of the feeding ground formed by C. tonsus.
As shown in Table 7-1 C. tonsus of copepodites IV-VI (female) occurred asSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

73

food of sei whale. Since adult male of this species is usually said as to be mostly
in deeper waters (Jillett, 1968), they hardly occur in the surface swarms and consequently, in the population of sei whale food. The body size was measured
on the individuals being preserved in good conditions, and they were selected
randomly. Then the number of measured individuals through copepodites IVVI could be considered as to represent the composition of food C. tonsus population, and they are represented by the copepodite V stage, since it occupied for81.9.-98.4%, with the average of 89.7% in the whole of C. tonsus populations
examined.
TABLE 7-1. MONTHLY OCCURRENCE OF C. TONSUS BY THE STAGES IN TERMS OFPERCENTAGE FIGURES. THE ACTUAL NUMBER OF INDIVIDUALSEXAMINED IS INDICATED BY PARENTHESES.
Stage

Mid-Dec.

Late-Dec.

Early-Jan.

Mid-Jan.

Total

C IV
1.6 ( 24)
CV
81.9 (1222)
C VI Female 16.5 ( 247)0.83( 20)
88. 5 (2135)
10.67( 258)

1.73( 17)96.4 (956)1.87( 18)0.62( 5)98.4 (797)0.98( 7)

1.16( 66)
89.7 (5110)
9 .14 ( 530)

2413

991

809

5706

Total

1493

In 1967 /68 season the whaling was mostly operated in the areas III, IV andV, but the area shifted rather westward in 1969 /70 season and the areas II, III andIV were occupied. The composition of body size of C. tonsus along with the averageand the number of individuals were demonstrated by regions in Figs. 7-1 and 7-2,
since the overall sea conditions as the habitat of C. tonsus are possibly variable by
the sea regions (Hempel, 1968). The body size of each copepodite stages was:
2.7-3.0 mm in copepodite IV, 3.2-3.6 mm in copepodite V, and 3.7-4.0 mm
in adult female respectively. The composition of body size in copepodite V showeda normal distribution pattern with highest frequency of occurrence at the midstof distribution range through the materials obtained in Falkland region to the east
Indian sector region. In the general trends of the variation by the ocean sectors,
the distribution range of the body size class widened gradually in the direction
from the Atlantic sector toward the Pacific sector, though the most distinct example
was in the Western Australian region. The body size of 3.5-3.6 mm usually
showed high frequency of occurrence, but in the regions both south of Western
Australia and South Africa as indicated by the letter of, D, H, J, K, M and N,
the body size of high frequency was found between 3.0 and 3.2 mm. These kind
TABLE 7-2. AVERAGE SIZE OF C. TONSUS IN THREE DIFFERENT AREAS IN 1967/68
SEASON. NUMBER OF INDIVIDUALS EXAMINED IS GIVEN IN PARENTHESES.

~MonthArea~
Average size (mm)Sci. Rep. Whales Res. Inst.,.No. 26, 1974

Late-Dec.

Mid-Dec.
III, IV
66°-82°E
C IV
CV
C VI (52)3.46 (294)3.85 ( 76)v
l 72°E-163°W

IV
83°-10l 0 E

IV
82°-87°E

3,23 (111)3.38 (78)1)
2.80 (
3. 38 (1335)
3. 78 ( 110)

74

KAWAMURA

of difference as a biological character of each swarming C. tonsus populations suggests that they might come from a different population as the reproductive units.
The pattern of body size composition following to that of the normal distribution
lasts for a month or so without any changes within the composition. The femaleof copepodite VI is considered to follow the similar way as copepodite V although
the former is much variable than the latter due to its poor number of materials.
The high frequency of the occurence in the adult female was 4.0--4.2 mm and nodifferences in the body size compositions between the whaling areas were observed.
This may possibly be due to their biological character of stopping the feeding
activity in the copopodite VI stage as it has been known in Calanus plumchrus andC. cristatus (Minoda, 1971 ). In the populations in the Indian sector as indicatedby the letters of C and G, the body size of high frequency of occurrence was 3. 7,..,
3.8 mm. In general the composition of the body size in the adult female showed
the range between 3.4 and 4.8 mm, a rather wider range in the distributions thanthe case in copepodite V. The variations of this range was most distinct in the
Atlantic sector.
TABLE 7-3.

AVERAGE SIZE 'OF C. TONSUS IN THREE DIFFERENT AREAS IN 1969/70
Mid-Dec.

Month~--Area·~
Average size (mm)C IV
CVC VI (2)Average by month

C IV
CV
C VI (2)IV
109°-ll8°E

II
33°-43°WII
37°-53°WIV
l 10°-l 18°E

2.72(5)
3.13(21)4.00( 5)

2. 78( 2)3.44(23)
4.11(19)

3.56(4)4.21 (4)

2. 77 ( 9)3.19(31)4.01 (15)

2.75
3.38
4.11

Discussion
Jillett (1968) studied the seasonal distribution of C. tonsus in the waters off
Otago Peninsula, New Zealand, and found that the C. tonsus population in thesurface water during the summer was represented by the copepodite V stage. He(Jillett, 1968) also shown that C. tonsus, as a biological characters through Calanoida,has two generations annually, and that the copepodites from the second generation of current year mostly occupy the summer population, which must be the foodof sei whale in the northern Subantarctic waters. The life history of C. tonsus
suggests generally that the individual of the copepodite V from two different
generations coexists at least during December. One comes from over winterd
population and the other from developing population in current year. The coexistence of two size groups of copepodite V from different reproductive sources
has been known in C. finmarchicus in the North Atlantic waters (Stormer, 1929,
Barnes and Barnes, 1953; Colebrook, 1963), and Barnes and Barnes (1953) considered that the difference in size composition would be due to different populations,although there is a possibility of potential male and female.
7-2.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

s
10

'\

_:ctfl

N=294
3.46

N=294

~

'

N=l335
3.38

3.46

30

N=l31
3.13

2.82.93.03.13.23.33.43.53.63.73.83.9

8-

20

40

F'f

No:

50

l_Z511

II

2'

..

•

3.23.33.43.53.63.73.8

60
IV

Ill

v

g-ow~~~~~~60~~~~~~~~~-B88811~~~~p,!j~~~~~~~~~~~~~~~70~~~~~~~~~~~~~13~0~~~~~~~~~~~~~LATE-DEC. ~EARLY-JAN. 2j MID-JAN.
00 10MID-DEC.
20 30 40 50 %170W

Fig. 7-1. Size composition of Ca/anus tonsus copepodite V in the whaling areas and months in 1969/70
season. Average body length is given above each figures, and an approximate longitudinal range
from which samples were collected is shown by the capital A to K ..

s
10

,..
N=l32

N=29

4.11

~

20

'

JiWL

4.04

,.{'

30

3.85

3.63.73.83.94.04.14.24.34.44.54.64.74.8

4015161.n8 C4.o4.t4.14.3.4.4'J

Bj

I~

N~.lljl~OL''
4.0

IP

~

50
0

3.43..53.63.73.83..94.04.14.1

.

"'

v

60
170W

Fig. 7-2. Size composition of Calanus tonsus copepodite VI (female) in the whaling areas and months
in 1969/70 season. Average body length is given above each figures, and an approximate longitudinalrange from which samples were collected is shown by the capital A to K.

FEEDING HABITS OF SEI WHALE

75

In this respects the differences in size composition of C. tonsus especially inthe copepodite V stage suggest that they should come from the different reproductive populations. It is also suggested as a whole that those different size compositions which are the distinct biological character not as individuals but as the population, would be the result of having been isolated each other. A quite few occurrence of adult male in the surface population of C. tonsus is similar to the results
by Jillett (1968). Those male occurred sporadically must be newly developed
individuals in the current season, which made Jillett (1968) consider a possibility
that the second generation would supposedly be due to a result by the parthenogenesis in the over wintered females. The small sized female populations found in the
Indian Ocean sector are considered to be the result of different environmental
conditions there, or of a complete isolation from the others. It is known in the
British waters that a local difference in environmental conditions influences on
the size of individuals which form a dominant population in those regions (Marshalland Orr, 1955). The geographical difference in body size in the Antarctic copepods also well known (e.g. Brodskii, 1964). Baker (1954) considered that many
SEASON.

NUMBER OF INDIVIDUALS EXAMINED IS GIVEN IN PARENTHESES.

Late-Dec.

Early-Jan.

Mid-Jan.

IIII

8°-46°W

36°-54WIV
99°-ll0°EIII
0°-l9°E

II
40°-51°WIV
90°-100°E

III
19°-27°E

II
14°.-52W

3.46(10)4.04( 8)2.60( 1)3.38(31)
4,23(16)2.50( 1)
3.20(23)
3.95( 7)2.89( 1)
3.18(11)
4.23( 3)3. 00 (2)3.48(5)4.30(2)

3.21 (13)2.91(3)3.13(6)2.80( 1)3.49(10)
4 .03( 3)

2.69
3.34
4.09

2.83
3.29
4.16

2.86
3.28
4.03

zooplankters would originate from the identical populations of species by species
since they show rather monotonous circumpolar distributions. However, as
Brodskii (1964) having been demonstrated in C. tonsus and other copepod species,
there is a distinct geographical differences in the composition of body size as the
local populations, and I have also shown these circumstances in the previous section of this report. Mackintosh in his recent report clearly shown that there should
be several number of distribution centers under quite high population density inE. superba while it shows a typical circumpolar distributions (Mackintosh, 1973).
The above mentioned facts indicate that the zooplankton in the Antarctic waters
distributes quite unevenly under the general circumstances of circumpolar distributions, and that their occurrence with different body size compositions invarious sea regions would perhaps belong to different reproductive populations
somewhat likely to a "stock unit ".
The largest and the smallest body sizes in female C. tonsus measured in thisstudy were 4.8 mm and 3.6 mm respectively, while the former by Brodskii (1964)
was 4.25 mm (female) and 3.40 mm (male). Although it is known that the bodysize both in copepodites V and VI (female) shows latitudinal variations as a whole,Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA76

the body size seems to differ to some extent between the individuals from sei whale
food which have had been formed the patchiness and those taken by the net haul
which must supposedly been in a sporadic distribution without forming the patchiness.

7-3.

Patchiness of C. tonsus and the density effects
The body size of C. tonsus in Table 7-4 was based on the row materials whichwere collected by the net haul along with many other zooplankters. On the other
hand, C. tonsus being fed by the sei whale as has given in Tables 5-2 and 5-3, occurred as mono specific composition, which suggests that C. tonsus populatfon as
whales food might possibly have been in an aggregation, to show the patchiness.
Both C. tonsus populations, mentioned above therefore, can be considered to beecologically unidentical to show some differences in the size composition and theaveraged body size. The food C. tonsus population seems rather larger if not entirely in the body size of individuals.
TABLE 7-4. BODY SIZE OF THE SOUTHERN SPECIES, CALANUS TONSUSBRADY REPORTED PREVIOUSLY BY THE DIFFERENT WORKERS.
AuthorCV

CVI (~)CVI (d')Brodskii ( 1964) *Tanaka (1956)

3.40-4.25
4.07-4.24**
3.66-3. 75***

3.30-4.15

3.27-3.47

Tanaka (1960)
Vervoot (1957)

3.39-3.69
3.69-4.05* 3. 71 mm on an average.
** Sjze at higher latjtudes.
*** Size at lower latitudes.

The patchiness of zooplankton has been observed but a little. Studying on
the food of blue and fin whales in the South Georgian waters, Mackintosh andWheeler (1929) stated that E. superba occurs by a monotonous composition ofsame year class or same developmental stages since the species forms such patchinessseparately as to be fed by the whales. Hardy and Gunther (1935), and Mackintosh(1966) reported the results of their observations on the behavior of E. superba underpatchy aggregations in Cumberland Bay, South Georgia, and described as "Thereappeared some guiding ' principle '-almost as if there was some leader in commandof the whole! " (Hardy and Gunther, 1935). Mackintosh (1966) also describedlater that the patches of E. superba moved in a cluster as if they were an amoeba
like organisms as a whole, and the patch was quickly reorganized within tens seconds
after disturbance. These characters of E. superba patches as a kind of animal
aggregations strongly suggest their something socialized structure within the patches
as the theme having been suggested also by Clutter (1969) in Mysidacea. In general
the features of the plankton patches can be comparable though not analogous withthe aggregations in the insects such as the reproductive aggregation by male mosquitoes (Kawamura, 197lc). An ecological homology in a plankton 'patchinessSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

77

could be imagined by the fact that the empty shells of E. superba were collected
abundantly in a net haul (Marr, 1962). This indicates that the development of
the individuals proceeds simultaneously among almost all members of the patches.
The patchiness of zooplankton can be considered as one of the distinct characters in the distribution of animals. Difference in generations could be responsible
for variable body size composition but on the otherhand it should be consideredthat there must supposedly be some ecological benefits for aggregating into patches
through the density effect. For example, it is well known fact in aggregatinginsects that the population density closely related to their relative growth rate
(e.g. Ishida, 1952; Uchida, 1952; Fujita and Uchida, 1952; Takahashi, 1956),
in which the density effect would role positively (Chauvin, 1962). In the population of cockroach, Blatella germanica the overall growth rate as the unit of population is more enforced quickly by keeping close the intervals between individuals(Ito, 1959; Ishii, 1970). This kind of function by the density effects have also
been indicated in Mysidacea population (Clutter, 1969). In the zooplakton population under aggregations, the population density is usually kept quite
high; E. superba shows one individual per cubic inch (Marr, 1962), and he described them as "jostling one another" under patchiness. Swarming E. pacifica
shows 1-2 cm of intervals between individuals (Komaki, 1967). These facts leads
to a supposition of tactile stimuli, that is, touching each other by the body or by theantennules would possibly occur to some extent during their swimming (Kawamura,
197lc). The differences in body size composition in C. tonsus populations could
be explained by this kind of density effect through their social behavior, but thescheme will be discussed more in section 11 where the C. tonsus patches were treatedon the basis of field observations and row materials obtained by net haul.
8.

FEEDING APPARATUS OF SEI WHALE

A quite characteristic adaptation in baleen whales is their specialized feeding apparatus to collect a bulk of zooplankton and some micronektons. The baleen plates
or whalebone as feeding apparatus has developed in various ways by species inplace of the tooth, and no similar feeding structures are found in any of other kindof animals. The baleen plates being arranged in a row on the upper mandible
are quite functional for gathering rather scattered foodstuff along with the specialization of oesophagus to form the first stomach which functions as a depositing sac
for swallowed foodstuff (Hosokawa and Kamiya, 1971). In this respects a row ofbaleen plates is indispensable apparatus for the baleen whales actually functions
whereas the teeth are not so functional in many toothed whales. When they werelost or heavily damaged by some desease or by an accident, a daily nutritional
requirements would hardly be filled. Rice (1961, 1969) reported the rudimentarybaleen plates in sei whale due to some microbial desease, and it caused the blubberof the whale quite thin from the insufficient food intake.
The amount of stomach contents under full repletion closely related to feeding habits of whales and to the population density of the food organisms, say, theSci. Rep. Whales Res. Inst.,.No. 26, 1974

78

KAWAMURA

patchiness of zooplankton. Since the patchiness of food zooplankton hardly be
studied by the net haul as has been pointed out by Mackintosh (1966), one of an
approaches to studying the plankton patchiness would be found in the stomach
contents of baleen whales. In this point of views Klumov (1961), Kawamura(1970b, 197lb), and Omori et al., (1972) reported on the population density of foodorganisms being based on the stomach contents of whales, and found a hardly believable high population density of zooplankton under the patchiness, which would
never be obtained or known by the net haul investigations. In regard to thiscircumstances it would be necessary to know the ability in filtering the water by
a row of baleen plates in sei whale.

Oral cavity
A row of baleen plates furnished on the upper mandible functions in collecting
the food organisms. This would be fairly comparable character with the sperm
whale whose lower jaw is fully functional against the upper jaw. The baleen plates
however would be functional actually with the supplemental aid by the lip on
lower mandible. After Ingebrigstsen's suggestion on the feeding habits in baleen
whales, Nemoto (1959) described on this scheme under three different feeding
types of baleen whales: swallowing or gulping type, skimming type, and the combined both swallowing and skimming. Then the shape of the lip which is characteristic by species closely related to the feeding type above mentioned. Both right andbowhead whales are furnished with so largely developed lips which are indispensablefor their feeding by skimming off the food contained water continuously, while
8-1.

160

E

~
3:

150

.~ 140-~0130
0:5 120Male
Females 12m;;::: + 12m:;:::• 13m
c, 13m
• 14m
o 14m
o
o
.,
0
0
• 15m
o 15m •"'~ 0 "0"' 16m:o;: 0 16m:o;:•o ~a rn.fo'da:O. oi:ia"" 0
& n
.-i..
121
... • c::r • <1
ocs;0 l:l:J ~ D D tl
D00•
.P~~,6ii ~ dl"~o~•o
.A.

"ji;

E

~ • .loDo,"s110·~ 100 «leE~::;;; 90"

..
260

~.<J

¥. '1' :<:'l:, ~~' ~~ '~o

,.+

<>

~,..,,OD0.a.•a•AM.:o""" M"C'""\1f•)o.oli9i>"~z. ot:..t~i ·£:..··A<i<;l,o0"

IZllZI

~

a.

"

e+c.

•t:.

~+"
280

340
320
300
Length of palatal ridge (cm)

360

380

400

Fig. 8-1. Relationship between the length of upper jaw and the maximum widthof lower jaw in sei whale.

balaenopterid whales are furnished with the poorly developed lip since they collect thethe foodstuff by swallowing the water although the lip of sei whale relatively welldeveloped among them. In California grey whale, the benthos feeder (Rice andWholman, 1971), the lip is rather poorly developed. It is worth taking into account that the sei whale follows after both feeding types according to the conditions of the distribution of food organisms.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SE! WHALE

79

In this respect the size of both upper and lower jaws were measured in connection with the potential ability in collecting the foodstuff. The relationships
between the sizes of upper and lower jaws against the body length of whales aredemonstrated in Figs. 8-1 and 8-2. The size of oral cavity varied to some extent
among the individuals examined, but it grows larger and larger in accordance withthe growth in body length. Dominacy of the female in the size oflarger than 330 cm
of upper jaw in sei whale and also 470 cm in fin whale is due to their biologicalcharacter that the female grows much larger than the male, and the former is preferably caught largely than the latter. In bryde's and minke whales the proportional length of the skull is larger in the female than the male (Omura, 1966;
Ohsumi et al., 1970). The sei whale which closely relates taxonomically to bryde's
whale showed considerable variations in the maximum width of lower jaw against
a definit length of the upper jaw, and in a definit width oflower jaw against the upper jaw length. The variations were more distinct in the latter. These variations
relating to the size of oral cavity along with the number of baleen plates indicate
a manifold character of baleen whales in their feeding mechanisms and in theability of collecting the foodstuff.

§ 220

Male

210

... 19mi:;; 200
;<

E

190

Ell

Female

18m~

• 20m
2lm

•
WI

22m~

+

18m~

6
0
D

19m
20m
2lm

0

22m~

0

...
0

"

:g
;< 170

...

E
§ 160

"'
><

360

+

"

t

::E 150

380., ~" o•o•E" .
0...... •... 6: ~:5 180

0

4 00

04

+6

0

6

8

4

0

0

0

I!

...

0

rn

00

oo 0

"'"

Jl, 0

0

D
0

'<lo

•

"'=
•

"" "

"

"
"s'"

" "'
0

"

"'

"' "
"'

•
400

480
460
440
420
Length of palatal ridge (cm)

500

520

Fig. 8-2. Relationship between the length of upper jaw and the maximun widthof lower jaw in fin whale.

8-2. Baleen plates
The number of the baleen plates in sei whale varies somewhere between 300
and 350 plates on one side, and two folds of this number of baleen plates functions
actually for collecting the food. However, the number of baleen plates varies by
the individuals and also by the observer (Williamson, 1973), since they are deformed
into a kind of hair-like bristles near at the tip ofrostrum and are randomly arranged.
The density of the baleen plates furnished in a row varies with the positions of themouth (Fig. 8-3, Table 8-1 ). The baleen plates (=bristles or hair) near the rostrum
were counted 1.4 plates/cm on average while they were 0.93 plates/cm at theirmaximum length as indicated by the arrow in Fig. 8-3, and they again showed ahigh density in the posterior part close to the angle of gape.
The relative ability in filtering the water per unit area of the inner surface
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

80of the apparatus is a function of meshes formed by the woven bristles. The meshes
are kept so as to show a definite filtering ability at any part of the apparatus,
since the overall ability for retaining the foodstuff might greately reduced if thefiltering ability was randomly variable by the part of the apparatus due to different
mesh sizes in its inner surface.
45

E

No. of baleen plates
286+18* /262 cm
14.1 m, Male

<..)

C>
M

•

>.

~ 40
Q)

Q)

0.

12 -

2"'

c.
"'
c:
Q)
Q)

E

10 .§.

35

"'
Q)

s%
6 "*

'°
..Q

c:
Q)

..Q

Q)

~
,, 30
z

4 "15

'°
22!
.5
>

i

2 5 LL-2L0-'--4k0-'-'60--1-~80--1-~10~0Ll~2~0Ll~4~0~1~60;:-'-;1~80;--'-;;2~00:-"-;;-22~0~24~0~
Distance from the anterior end of baleen row (cm)Fig. 8-3. Variation of the number of baleen plates in sei whale. Arrow shows the
spot where maximum length of baleen plates was found. (*bristles)
TABLE 8-1.

INTERVALS OF EACH BALEEN PLATES AT THEIR MAXIMUMLENGTH IN THE NOR TH PACIFIC SEI WHALES.

Body length
(m)

Sex

Interval(mm)

Thickness ofbaleen plate(mm)

Room between
baleen plate
(mm)

11.6
12.3
13.2
14.1
14.3
14.4

M
M
M
M
F
F

9.20
9.78
11.60
11.11
9.29*
9.40

2.2
2.2
2.15
2.2
1. 7*
2.5

7.0
7.56
9.45
8.91
7.59*
6.90

* Measurement on dried material.

The length of each baleen plates is usually measured along their outer margin
but it is the length along the inner margin to the base of palatal ridge that actuallyaffects on the total surface area of the filtering apparatus. As shown in Fig. 8-4
the unfolded shape of filtering apparatus varied by the animals along with their bodylength as a whole. Although the animals examined in Fig. 8-4 were caught inthe coastal waters of Japan, those unfolded shape is considered to be the same withthe animals throughout the oceans. The position of the maximum width does notcoinside with the position of the maximum length of baleen plates, but is found at
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

81

rather posteriorly of about 66.4 ....... 69.5 % in the total baleen row length, at which thehighest part of the lips oflower jaw is positioned. These general arrangement seems
as a whole to function effectively in collecting the foodstuff as much as possible
particularly when the whale feeds by skimming the food contained water with its
mouth half opened.
90~'E 80E~ 7o""'51 60~-E. 50'"'g 40
S? .c
7J 30

Minke
6.2m Male

~20
-0 0

2 t

5 ~

10

0'----2L0~4L0~6L0_·---1.80~1~0-0--'12-0~14L0~16~0-.-1~80~2~00~2~20'--'--24L0~26~0_,__28L0~3~00

Length along palatal ridge (cm}Fig. 8-4.

Unfolded figures of the filtering apparatus ofsei and minke whales.

8-3. Bristles on baleen plates
The bristles furnished on the inner margin of baleen plates form the fine meshedstructure as demonstrated in Plate I, and actually retains the foodstuff which havebeen skimmed off. The inner margin of the baleen plates is furnished with thebristles (Plate I, fig. 1 ), and the whole surface of filtering apparatus is covered bythis meshes although the number of bristles are considerably reduced at the tipof baleen plates (Plate II, fig. 1). Ruud (1940, 1945) examined the baleen plates
+-+
0%

t.-t.

0--0

--

25%

50%

75%

~ . . . . . .,.t_- -:~: . :.,i:6: .: ,:3c: : .m_ _: _ _u_

Lo: 54cm.-.._

Baleen plates

50

"'~~
0

U)

"'

40

~~

:g ;; 30
~~
-0 .c
"'

20
1:'>c: 0g~~~ 10. ___-+, ---------0
2

.4

12

14

16

18

20

Length of baleen bristles (cm)Fig. 8-5. Composition of the length of baleen bristles in 14.3 m sei whale caught
in the North Pacific in 1968.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

82

TABLE 8-2.

Location of catch
N. PacificJapan, coastal (off Kinkazan)"

Body length
(m)

14.3
13.5
12.3
11.6

MEASUREMENTS ON THE BALEEN

Sex

F
M
M
M

Length ofbaleen plates*

Lo/Li

~

Lo

Li

54.0
42.5
43.0
44.0

63.0
54.0
52.0
53.0

1.17
1.27
1.21
l. 21

* Length along outer edge of baleen plates (Lo), and those along inner edge (Li).
** Diameters of baleen bristles measured at the base (Db), middle (Dm), and at the tip (Dt).

of fin whale in detail and described that they are made of chitinous tissue which
is somewhat likely to a kind of fused bandage of many bristles. So the diameterof the birstles do not differ between wet and dried materials.
The length of the bristles varies with the part on the inner margin. Fig. 8-5
shows the frequency of occurrence in the bristles length in a sei whale of 14.3 m,
and the actual figures were given in Table 8-2. The number and the lengthcomposition of the bristles per unit length along the inner margin showed that
relatively longer lengthed bristles were poorly found. The length of the bristles
was measured at four parts, and was found that 3,....,5 cm long bristles were mostdominantly furnished. Accordingly the meshes in the surface of the filtering apparatus are chiefly formed with these length of bristles. It is clearly noticed thatthe length of the bristles did not show characteristic differences at any part of theinner margin. These suggest that the filtering ability in sei whale would be thesame at any part of the apparatus.
Table 8-2 represents relative abundance of the baleen bristles, their lengthand diameter. Although the triangle shaped baleen plates are well known, thelength along the inner margin of baleen plates is longer about 1.2 times than theoutside so as to form relatively larger surface of filtering area. The distribution
density of the bristles was 50 bristles /cm with the average of 45.5 bristles /cm, and thisdensity does not vary with the sex and the body length of whales. The shape of abristle itself seems rather slightly tapered: the diameter is larger at its base than
the tip. However the variation in diameter is such slight as to make them represent
the mean diameter at the mid point of the bristles.

Area of filtering apparatus
The total inner surface area of the filtering apparatus in five animals of different body lenth was measured from the unfolded figures as shown in Fig. 8-4,
and the results were given in Table 8-3. However, the filtering area given in
Table 8-3 is approximate figures, since an overall shape of the filtering apparatus shows a quite complexed features by the curved surface.
The surface area of the filtering apparatus was 2.37 m 2 in 11.0 m individual
and was 3.59 m 2 in 14.4 m individual. Then those areas seem to increase after ageometrical progression (Fig. 8-6). However, it is clearly observed that the surface
8-4.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

83

PLATES AND BRISTLES OF SEI WHALES.
No. of baleen bristles/cm
75%
44

Average length of bristles (cm)Average diameter ofbaleen bristles (µ)**

50%

25%

0%

75%

50%

25%

0%

Db

Dm

Dt

49

63

36

5.6

6.1

5.2

6.4

250
168
155
157

167
108
114
114

141
106
93
85

4.2
4.4
4.6

54
37
42

TABLE 8-3. AREA OF FILTERING APPARATUS OF SEI AND MINKE WHALESCAUGHT IN THE COASTAL WATERS OF JAPAN, MAY, 1971.
Species
of
whale

Body
length
(m)

Sex

Lk*
(cm)

Ld*
(cm)

Sei
Sei
Sei
Sei
Sei
Minke

11.6
12.3
13.2
14.1
14.4
6.2

M
M
M
M
F
M

268
236

256
228
271
262
300
102

Area of filtering
Length alongouter edge of apparatus combined
both sides
baleen plates
(m•)
(cm)
2.37
2.13
2.64
2.89
3.59
0.29

291
261
330
300
330
114

* Length of a row of baleen plates along the upper jaw.
** Strait length from the anterior end of baleen plates to the posterior end of them.
6.0
oEg• Kawamura (Present study)+ Williamson (1970)

5.0

0Nemoto (1970)
0

Bm
4.0

~ 3.0
"'"'

~

Bm Blue
Bp FinBe Bryde's
Bb Sei
Eg Right
Mn Humpback
Ba Minke

2.0
oBe

10

~
Ba
10

Fig. 8-6.

15
Body length (m)2025Area of filtering apparatus in baleen whales.

area of :filtering apparatus in sei whale is considerably larger m contrast to thebody length of animals. These sei whale's characteristics are quite interesting when
we compare the feeding habits of both sei and right whales with those of anotherbalaenopterid whales: the former two feed after skimming type with well developed
Sci. Rep. Whales Res. Inst.,.No. 26, 1974

84

KAWAMURA

filtering apparatus while the latter feed after swallowing type with poorly developed
apparatus. The area of filtering apparatus in sei whale showed moderate figures
between right whale and another balaenopterid whales. This fact will be reasonably
recogniged by supposing that the sei whale feed after the types of both skimmingand swallowing. In balaenopterid whales other than sei whale on the other
hand, the area of filtering apparatus changed rather linearly against the bodylength. In regard to these above mentioned facts it could possibly be consideredthat the feeding ability of the baleen whales would develop after geometrical progression since the body weight of whales is porportional to cube root of the bodylength (Nishiwaki, 1950) whereas the food requirement in terms of the daily ration
is linearly proportional to the body weight (Sergeant, 1969, Table 4).
9.

FILTERING ABILITY IN THE ROW OF BALEEN PLATESIn any types offeeding method the food contained water is pushed outside by the current pressure or by the co-operated functions of oral cavity, ventral grooves, andtongue. In sei whale as mentioned before the foodstuff is collected chiefly byskimming the food contained waters, and the filtering ability in the feeding apparatus may greatly affects on the overall ability for sieving the foodstuff.

Porosity in the meshes of filtering apparatus of sei whale

9-1.

The porosity, the mesh openings in filtering apparatus in terms of percentages
was calculated, and the figures were given in Table 9-1 along with those known
TABLE 9-1. POROSITY OF FILTERING APPARATUS OF SEI WHALE, WHICH IS
FORMED BY THE BALEEN BRISTLES. (MEASUREMENTS AT THE MAXIMUMLENGTH OF BALEEN PLATES AND THOSE AT THE MIDDLE PART OF
THE BALEEN PLATES ALONG INNER MARGIN).
Body
length(m)

Sex

Location ofcatch

11.6 M Japan coastal
12.3 M
13.5 M"
14.3** F N. Pacific pelagic

Porosity (%)
Total
Interval.Mean
length of of baleen dram.eter
bristles Possible thickness of filtering apparatus (mm)baleen1 t es ofbristlesPa
at Dm***
(mm)
(mm)
(mm)
2
3
4
5
1
1942
1111
2285
3000

9.20
9.78
11. 60*
9.29

0.114
0.114
0.108
0.167

72.6
81.97
77.0
1.0

86.3
90.98
88.5
55.0

90.87
93.99
92.33
70.00

77.5

82.0

* Measurement from 13.2 m, male.
** Measurements on the dried material.
*** Measurements at the middle part of baleen bristles.

in the bolting silk cloth for the comparison. The netting of the filtering apparatus
in baleen whales is structured by the following elements: the intervals between
each baleen plates (Fig. 8-3), the thickness of bristles in diameter, and the lengthof the bristles. As the measurements of these elements were given in previous
section, the calculation was made by using them (Table 9-1). Since the meshedfiltering apparatus in whales is formed with some thickness due to entangled bristles,
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

85

five cases of the thickness between 1.0 and 5.0 mm were supposed for the calculation. In sei whale, however, the actual thickness is considered about 1.0.-2.0mm(Plate I, fig. 4 and Plate II, fig. 2). The above mentioned thickness along with theintervals increase thicker and wider with the increase in body length although thelength of the bristles also grow longer.
The porosity which is usually examined in bolting silk cloth for plankton net(e.g. Anraku and Azeta, 1966) varied between 77.2% and 88.6% by the bodylength of sei whale concerned. Since the baleen plates and the bristles examined
were damaged to some extent due to ill treatment, the actual figures of the porosity
would be slightly more lower values. However it is note worthy that the overall
figures of the porosity do not seem to vary with the body length, say, the porosity
is nearly constant regardless the body length of whales.
By comparing the porosity in ballen plates with that in plankton net, the former
showed distinctly larger percentages than the latter (Plate II, fig. 2).
TABLE 9-2. POROSITIES OF JAPAN BOLTING SILK CLOTH
COMMONLY USED IN PLANKTON COLLECTION.
Kind of bolting cloth

Mesh aperture (mm)Open area* (%)

0.094
0.193
0.334
0.630
0.926
0.350

23.0
40.0
46.0
58.0
60.0
38.0
50.0**

xx 13
xx 7
GG 54
GG 32
GG 20
Pylen #60
Larva net (Cremona+Pylen #60)

* Data by the Nippon Bolting Cloth Co., Ltd. (S. Ueno pers. corn.).
** Anraku and Azeta (1966).

Relative filtering ability in a row of baleen plates
In order to know an actual filtering ability in a row of baleen plates, thefiltering ratio was measured experimentary after the similar ways employed inplankton net. The experiment was conducted at the offshore region in Suruga Bay
on boad of the R. V. "Tansei Maru" of the Ocean Research Institute, Universityof Tokyo. The general arrangements of the experimental instrument are shown in
Plate III. The dimensions of the instruments were 48.6x23.4x91.0 cm. A fl.owmeter was mounted on the inside of the instrument at about 1/3 distance posteriorly
from the mouth opening. A row of baleen plates was mounted on the bottom of the
instrument (Plate III, fig. 2). The slits between the mounted baleen plates andthe instruments, which may cause a leakage were filled with 5.0 cm thick of uretan
foam (Plate III, figs. 1 and 2). The surface of the filtering apparatus throughwater passed away in the experiment had an area of about 23.4 X 54.0 cm. The
two 10.0 kg of lead weights were mounted on the both outsides of the instrument,
which was towed by bridle. Before experiment a filter made 'of bolting silk cloth
GG 32 (0.63 mm mesh opening, 58% in porosity) was attached on the mouth of
the instrument so as to reduce the volume of passing water down to about 1 /3.
9-2.

Sci. Rep. Whales Res. Inst.,
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KAWAMURA

86

By this treatment the water entered into the instrument was well filtered off withoutcausing any overflowing. The experiment was conducted in Suruga Bay when thedepth was 426 m by towing the instrument vertically from 200 m to the surface
at the speed of 1.-1.2 m/second. The volume of water filtered by the row ofbaleen plates was calculated by the flow-meter's readings.
The results were given in Table 9-3. The relative filtering ability in baleen
plates of both sei and fin whales showed rather poor when compared with those in
the bolting silk cloth: the ability in sei whale was approximately 1 /8 of GG 54(0.33 mm mesh openings) while the fin whales' was 1 /5, and still more, it may beinteresting that those relative ability was poorer than the finely meshed XX 13
bolting silk cloth which can retain most of the representative diatoms in the sea.
From this experiments it can be safely stated that the filtering ability in sei and fin
whales, and possibly in any baleen whales is astonishingly poor than those in thebolting silk cloth. The filtering ability might reduced to some extent by ill treatment in mounting the row of baleen plates on the instrument, and even we takethese circumstances into account the results of 1 /5-1 /8 against the bolting silk
cloth are astonishing enough, that is, the filtering apparatus of baleen whales
TABLE 9-3. FILTERING ABILITY OF THE BALEEN PLATES IN SEI AND FIN
WHALES AND THOSE OF JAPAN BOLTING SILK CLOTH.
Exp.
No.
2
3
4
5
6

Item

No. of flow-meter
revolution/200 m

Relative
ability

Calibration
GG 32
GG 32+GG 54
GG 32+XX 13
GG 32+Sei*
GG 32+.Fin**

1842
610
893
648
105
184

17.55
5.81
8.50
6.18
1.0
I. 75

* 14.3m., female, caught in the North Pacific on July 18, 1969 at 45°-04 1 N, 171°-37 1 W.
** 16.8 m., female, caught in the North Pacific on August I, 1969 at 47°-55 1 N, 157°-10' W.

never exceeds in the ability of filtering off the water than in the plankton net.
These results give us some fundamentally important evidences concerningon the scheme of food and feeding habits in baleen whales: that is, there have been
two suppositions in connection with the population density of food organisms andthe amount of stomach contents acutually found (Klumov, 1961; Kawamura,1970b). They are: 1) the daily rations of baleen whales would be filled by their
extraordinal ability in collecting the foodstuff, and the ability have to be maintained
under a quite excellent functions, or 2) the food organisms have to distribute withvery high population density of such extent as hardly impossible to know it bymeans of an ordinal method being employed in plankton samplings by the net.
Those above mentioned results lead to a consideration that the food organisms
would distribute under a quite dense population in the sea to fill the daily food requirements of whales rather easily by those poorly developed filtering ability inthe row of baleen plates.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

10.

87

AMOUNT OF THE STOMACH CONTENTS AND THE POPULATIONDENSITY OF FOOD ORGANISMS

The amount of the stomach contents in three species of the baleen whales has beenreported previously though it was rather preliminary (Kawamura, 1968b, 1970a).
The predator-prey relationships which involve the baleen whales would be a characteristic scheme in the food chains of the sea, since the secondary producer are
directly connected with the consumers of the highest level in the ecological niche,that is, the energy flows quite effectively in the economy of the sea. Klumov (1961)
made a calculation and described theoretical population density of zooplanktonbasing on the amount of stomach contents which were actually measured, and heconcluded that the food requirement by the whale would hardly be filled fully withthe population density of zooplankton being widely found in the sea. After thescheme of Ivlev's "feeding rate" (lvlev, 1961), Sergeant (1969) confirmed on thedaily ammount of food consumed by the whales which has been reported previously
would be an appropriate figures, and Kawamura (1970b; 197lb) has discussed on
the population density of copepod, Calanus tonsus in relation to the amount of thisfood organism in the first stomach of southern sei whale. Prior to further examination on the population density of food zooplankton, the feeding habits such as
feeding cycle, feeding method, amount of stomach contents, speed in digestion, were
examined.
Most of whales usually spotted in the whaling ground are considered to beunder the movements in search of food, and such whales as being under feeding
activity in high seas are rarely sighted even by the whalers who had been engaged
in whaling for many years. These fact indicates that the whales feed intermittently within rather shorter hours. Under these circumstances the observations
by Ingebrigtsen (1929) on the feeding behavior in North Atlantic sei whale must be
an important. According to Ingebrigtsen's observations the sei whale under feeding shows rather dull and only pays a little attention to the surroundings. Farlater, Gunther (1949) reported a similar aspects in the southern fin whale, in whichhe (Gunther, 1949) described that the fin whale continued its feeding without
paying any attention to the approaching ship and was easily marked. The whales
fully repleted with food also show a similar behavior. When they are under starvation, however, they are quite shy and agile, and can hardly be approached within
the shooting distance. The sei whale quickly moves around in the surface withits mouth half opened when they are under feeding (Ingebrigtsen, 1929; Scoresby,
1820; Kawamura, 1972), and it is this occasion that bolting behavior was observed(Andrews, 1909; Gunther, 1949) though the whale sometimes feeds without bolting
its body (Millais, 1904-1906). In humpback whale it is also known that the whale
makes its food organisms aggregate into dense shoals by moving around them so as
to enable feeding himself more effectively (Howell, 1930).
On the other hand, the balaenopterid whales other than sei whale feed byswallowing the food contained water. However, the feeding type employed by
the whale is not always fixed but varies with case by case. In the North PacificSci. Rep. Whales Res. Inst.,
No. 26, 1974

88

KAWAMURA

relatively larger sized fishes such as the pacific saury, pacific cod, mackerel, sardines
and some squids are fed by the baleen whales in addition to the planktonic crustaceans (Nemoto, 1957; Kawamura, 1973a). Feeding by skimming type which
seems to be largely employed by sei whale may possibly be less effective against thesekind of well swimming food organisms. The sei whale, therefore, is considered tofeed by rather combined ways of both skimming and swallowing types, and this
supposition would be supported by the following fact: the variation or the change
in feeding method is known even in some plankton eating fishes. For example,
both biting and filtering types are employed in anchovy by the size of the prey organisms and their population density in the sea (Leong and O'connell, 1969).
The foodstuff swallowed is preserved temporalily in the first stomach, and thiskind of function is characteristic in baleen whales among the marine mammals. Studying on the anatomy of the first stomach of whales, Hosokawa and Kamiya (1971)
concluded after discussing on its ecological recognition that there might be need todeposit the food as far as the food is available since the whales in their feeding place
have been faced the competition with the others for a limited amount of foodstuff.
Under these above mention circumstances, the observations on the amount offood organisms in the first stomch would be an important materials in connection
with the feeding habits of whales and the distribution ecology in the food organisms such as their patchiness and the population density. However, Betesheva(1955), Nemoto (1957) and Bannister and Baker (1967) have pointed out that thewhales under chasing would possibly vomit their stomach contents supposedly fromfatigue after long and continuous chasing. If this occur actually, the amount of thestomach contents which is found in carcasses would be greatly decreased, andtherefore, before going further the matter was examined and discussed in the following section.

Influence of chasing time to stomach contents
Prior to further discussion on the amount of food in the first stomach, it was
examined and discussed that to what extent the chasing by catcher boast influences on the amount of stomach contents due to vomit. This however havebeen discussed fully in the previous report (Kawamura, 1970e, 1971a), then I will
redescribe here only an abstracts and conclusions by quoting my previous reports.
The duration of chasing time by catcher boats which might influence on thefulness of stomach contents of whales was examined on fin, sei, and sperm whalescaught in the northern North Pacific during the summer of 1969. Of 895 whales
observed only 28 whales (3.6%) vomited out their stomach contents while beingchased and sperm whale vomited most frequently (16.8%) whereas sei and fin
whales were 1.1 % and 0.78% respecitively (Table 10-1). The diet of the whales
concerned was consisted of boarfish, squids, copepods and euphausiids. It was often
observed that the whales likely to vomit out more larger sized foodstuff such as
boarfish than that of copepods and euphausiids. The amount of stomach contents vomited was negligibly few, and has no significant influences on the amountof stomach contents which have been obtained from carcasses. Most whales (80%)
10-1.

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89

FEEDING HABITS OF SEI WHALE

were caught within 40 minutes of chasing, and the number of whales which would
vomit out their stomach contents does not always increase in proportion to the prolongation in the duration of chasing time. There are one possibility left behhindthat whales may vomit the stomach contents in the subsurface waters during long
restless chasing by catcher boats, since it must be fairly difficult to detect any vomited foodstuff from the boats. The behavior of vomiting a bulk of foodstuff in thesubsurface waters, however, presumably causes heavy muscular activity for thosewhales, and they would not vomit any foodstuff in the subsurface waters since thewhales under diving likely to minimize the oxygen consumption as little as possible (Scholander, 1940; Slijper, 1958a).
TABLE 10-1. NUMBER OF WHALES OBSERVED BY SIX CATCHER BOATS WHICH
PARTICIPATED IN THE NORTH PACIFIC WHALING OPERATIONS IN 1969.
Floating factory

Kyokuyo Maru No. 3

Tonan Maru

Nisshin Maru

Catcher boats participated (day/month)
Kyo
Maru
No. 12(18/V25/VII)

Kyo
Maru
No. 15(6/V18/VIII)

Kanan
Maru
No. 25(19/V24/VII)

Kanan
Maru
No. 26(24/V8/VII)

VomitedNone
Unknown

22

74

55
15

VomitedNone
Unknown

4

25
8

Duration of operation

Sei whale

Fin whale

Sperm whale

Vomited
None

16

Toshi
Maru
No. 17(16/V4/VIII)

Toshi
Maru
No. 18(17/V4/VIll)

Total

102
13

175

5
174
6

6
427
209

16
7

24
28

85
43

1
23

21
104

20
81

* Sperm whale was not observed expect Kyo Maru No. 15 and Toshi Maru No. 18.
10-2. Amount of stomach contents
The amount of the stomach contents was measured on the animals whichwere appeared to be fully repleted with food. For the measurement in the field abasin of known volume was used to remove the stomach contents completely fromthe opened stomach, and the amount was calculated by multiplying the number ofbasin which was required for the complete removal. Since the stomach contents
removed by this way contain the water to some extent, the weighing of a basin-fullorganisms was made after draining away the water. I have reported on the amountof the stomach contents in three southern species of baleen whales (Kawamura,1970a). It is however, considered necessary to give the results for further discussions, I redescribed here a part of descriptions in the previous report.
In the Antarctic, as I showed in a brief note (Kawamura, l 968b) the amountof stomach contents of 14 sei whales which fed on Calanus tonsus, Parathemisto gaudichaudii, Euphausia superba and Euphausia vallentini was measured (Table 10-2). The
weighing of stomach contents were carried out on the stomachs being judged asSci. Rep. Whales Res. Inst.,
No. 26, 1974

90

KAWAMURA" full " by eyes, and full data including fin and minke whales were given in Appendix.
The highest value observed was 228.0 kg of Parathemisto gaudichaudii, which was fed
by 15.2 m of male, while it was only 68.0 kg in the Calanus tonsus food. Nemoto(1962b) mentioned that there was a quite well agreement between the weight andthe degree of replation of stomach judged by eyes. However, as shown in Appendix
there were considerable variations in the case of the Antarctic sei whale. It canbe said in general that the whales caught before 0900 hours carry somewhat freshand large amount of stomach contents, and the proportional weight of water gradually increases toward noon due to digestion.
TABLE 10-2. AN AVERAGE AND THE MAXIMUM AMOUNT OF STOMACH CONTENTS
OF SEI AND FIN WHALES CAUGHT IN THE ANTARCTIC OCEAN.

Species ofwhale
Sei whale

Body length(m)13.0-13.9
14 .0-14 .9
15.0-15.9
16.0-16.9

Fin whale

18.0-18.9
20.0-20.9
21.0-21.9
22.0-22.9

Sex

AverageNo. of whale
Total
examined
Volumeweight(1)(kg)

FemaleMale
FemaleMale
FemaleMale
Female
Male

2
0
2
5

FemaleMale
FemaleMale
FemaleMale
FemaleMale

03

MaximumTotalweight(kg)

Volume
(1)

171

202

182

204

118
116
152
159.2
196.5

149
140.4
200
208.5
255

141.3
154.6
152
228
196.5

180
200
200
312
255

301.3
567

391
700

301.3
567

391
700

510
133

700
238

885
133

1000
238

0

1
0
0
2
1
0

It has long been the obscure mystery or the matter of interest how much thebaleen whales take food in a day. According to Nemoto (1959) Collett (1912)
early estimated the amount of food taken by the Atlantic blue whale, and showedthat they had taken at least 1,000 liters of euphausiids. Betesheva (1954) showedthat the North Pacific sei whale can feed 600 kg of squids or 50-370 kg of Calanus
plumchrus, while Best (1967) given 59 kg of zooplankton from South African waters,
and 175 or 305 kg of krill for Antarctic sei whales (Brown, 1968). There are somemore data about this problem which has been accumulated on the whales of northern hemisphere though most of them were on fin whales (e.g. Nishimoto, et al.,1952; Nemoto, 1959; Klumov, 1961). Nemoto (1959) demonstrated that the fin
whales of 57 feet in the northern North Pacific fed on 759.0 kg. of Alaska pollack andfound a considerable variations in the amount of stomach contents (91.0- 759.0 kg)
notwithstanding the fact that the majority of all stomachs concerned were repleted
Sci. Rep. Whales Res. Inst.,
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FEEDING HABITS OF SEI WHALE

91

as "full" by eyes. His result also suggests that the weight of stomach contents
filled with copepods usually very little than those by fishes or euphausiids.
The highest and averaged amount of stomach contents show that the males ofboth sei and fin whales likely to feed on much food than the females though feeding
percentage seems to be high in the female (Brown, 1968). It is noticed that thestomach is hardly repleted when the whales feed on copepods than fish or euphausiides. This fact leads to a consideration that the larger the size of food organisms,
the larger amount of stomach contents is expected. Such a tendency is also shownin the North Pacific fin whales (Nemoto, 1959, Table 24). From my result I may
describe the following weights as an approximate amount of stomach contents
usually seen in the Antarctic (Table 10-3).
TABLE 10-3. AMOUNT OF FOOD USUALLY FOUND IN THE FIRSTSTOMACH OF ANTARCTIC BALEEN WHALES.
Species ofwhale

Kind of foodorganisms

Amount of foodusually foundSei whale

Copepoda
Amphipoda
Euphausiids
Euphausiids
Euphausiids

less than 100 kg
150-250 kg
150-200 kg
300-900kg
30kg

Fin whale
Minke whale

By comparing these values with those obtained by other workers (e.g. Klumov,
1961; Brown 1968), the amount of stomach contents of sei whale in the Antarctic
does not differ so much from those obtained in the northern North Pacific.

10-3. Daily variation in feeding activity
Although the daily variation in feeding activity in terms of the percentage
figures is rather well known in fin whales while not in sei whale, and in this point ofviews Kawamura (1970a) studied on sei whale. Since several facts found in his
report are considered to be suggestive on this matter, a section of the results wereredescribed: It has been known that the feeding activity of baleen whales
in a day in terms of the percentages varies in general with a bimodal curve which
has a maxima in the early morning and in the evening (Nemoto, 1957, 1959). Oneof his results (Nemoto, 1957) suggests that baleen whales somewhat actively
feed twice a day in many cases though each patterns of feeding activity are variablewith the kind of food partaining to their diurnal vertical movement, seasons, latitudinal positions, or sometimes with the bottom topography of whaling grounds.
Sometimes, however, the baleen whales show a evidence suggesting to feed only
once a day (Nishiwaki and Ohe, 1951). In any case, it is considered to be quite
important to accumulate the evidences on the feeding habit of baleen whales especially of sei whales in relation to estimating the nutritional budget among the foodwebs in their feeding grounds. However, it is also nesessary to figure out the feeding patterns of baleen whales by theoretical and experimental methods by assuming
such a feeding model as Klumov (1961) instituted for this purpose.
Sci. Rep. Whales Res. Inst.,
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KAWAMURA

92

The daily change offeeding activities in terms of the percentages of the stomach
with food against the whole number of stomach examined is supposed to be affected
chiefly by the diurnal vertical migrations of food organisms, because the most of
all food organisms listed in Table 5-1 were known as to show a quite distinct diurnalvertical migrations in the South Georgian whaling ground (Mackintosh, 1934,
1937; Hardy and Gunther, 1935; Ommanney, 1936). Accordingly, it is necessary
to examine the whales food by the species since each food organisms has their own
peculiar migratory patterns in a day. The hourly change in the feeding activityof sei whale in the whaling areas III, IV and Vis demonstrated in Fig. 10-1. It isnoticed in the figures that a quite high feeding percentages are observed in the earlymorning, and no distinct recovery occurs in the evening. It is also noticed thatthe most causative organisms to bring on the daily variation of feeding activity are
chiefly responsible in the case when amphipoda, large sized euphausiids or" Calanus" were fed. Medium or small sized euphausiids seems to be fed whenever they are available. The upper and lower extremes of feeding percentages
were 57 percent and 28 percent in the areas IV and V, and that in the areas III
90~Areas Ill & IV

80f

~Area V

70

60

~ 50
c.

~ 40
u
~ 30

P..
,-'

--'()..---o-.... -<:r.,,......a........ ,o--4 ..._._._.,,/
20 ....
1005'"Q. -calanus,'>f' ....""....X'"'"'-'().-~'~c~~.... rca1anus..................

.. ................................................ ._ ................'..

68....., ..",,'...... .......'I'.:\.

•,,
7

9

10 11

12

13 14 15 16 17 18 19 20 21

22

Hour

Fig. 10-1. Feeding percentage of sei whale in the whaling Areas III, IV, and V,
1967 /68.

and IV were 82 percent and 25 percent respectively. The general trends of feeding activity of sei whale in the Su ban tactic waters differ considerably from those of
fin and sei whales obtained in the northern North Pacific where it varied between
0 to 100 percent in most cases (Nemoto, 1957). This may be attributed partly tothe differences of the kind of food organisms taken in both whaling grounds. From
the figures, it is naturally considered that the most of sei whales in the Antarctic
heavily take a dense population of food once in the morning possibly by ' swallowing ' or ' skimming ', and take a relatively scattered poor population of food organisms by ' skimming ' during the daytime towards the evening, as the manner described by Ingebrigtsen (1929) and Nemoto (1957, 1959).
The components of the "Calanus" food are represented by two dominant
species, Calanus tonsus and Calanus simillimus. In the Indian sector of the Antarctic
Sci. Rep. Whales Res. Inst.,
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FEEDING HABITS OF SEI WHALE

93

a considerable number of both Calanus tonsus and Calanus simillimus occurred as
principal food source of sei whale while only Calanus tonsus was observed in the
Tasman Sea and in the Pacific sector of the Antarctic (area IV and V). Since there
are considerable differences in the behaviour of diurnal vertical migration between
Calanus tonsus and Calanus similimus the extent of the repletion of stomach contents
by the time was also differed between those two copepod species (Fig. 10-2).
Those differences due to diel migratory habits of food organisms have also been
known' in decapod crustacean, Sergestes similis which is locally important food ofsei and fin whales in the North Pacific (Omori et al., 1972). In both Calanus tonsus
and Calanus simillimus, the repletion of stomach was relatively small with the quantity of food less than 50 percent during the daytime. It is noticed clearly that thealmost fully repleted stomaches (" rrr " - " R ") with Calanus tonsus occurred more
frequently than Calanus simillimus, and the former was fed with a same feeding percentages during the daytime while the bulk of Calanus simillimus was exclusively
fed only in the evening. This fact would be the result from being exactly followed

Hour

Fig. 10-2. Number of whales fed on Calanus simillimus (black colum and solid line),
and C. tonsus (vacant col um and broken line) by the four different degrees in stomach
fulness. Degrees of fulness are as follows: 'r' less than 25 percent, 'rr' 25-50
percent, 'rrr' 50-75 percent, 'R' 75-100 percent.

to the differences of the migratory habits of both species. According to Hardy andGunther (1935), Calanus simillimus is regarded as one of the quantitatively important
copepods in the South Georgian waters, and this species shows a distinct diurnalvertical migration as well as Drepanopus pectinatus; the bulk of Calanus simillimus
population begins upward movement at about 1700 hours in the evening andreachs to 10-20 m depth at 1800 hours. They remain at the surface layer during
the night then begin to move into the deep down to about 100 m depth in very
early morning, at least 02-03 hours. This behavior leads to a consideration that
there would be only a chance for sei whale to feed on Calanus simillimus in the evening throughout a day. It is interesting to find that the stomach of "full" occurred very frequently in the evening so as to support the consideration of feeding'once' in a day.
Calanus tonsus, on the other hand, is a comparable species to Calanus plumchrus
Sci. Rep. Whales Res. Inst.,
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94

KAWAMURA

in the northern North Pacific, and this species of at least in the copepodite stage
V is recognized in general to show no such a distinct diel vertical migration as
Calanus simillimus. Judging from its hydrological and taxonomical affinities to
Calanus plumchrus (Tanaka, 1954, 1967; Jillett, 1968), it might presumably be considered that Calanus tonsus of 5th copepodite stage does not show any distinct dielvertical migration. The occurrence of " full " stomachs with Calanus tonsus during
the daytime suggests a quite well availability of this foodstuff in the surface waters
so as to be fed by the sei whale throughout a day.
In summarizing the above mentioned results, it would possibly be said thatthe sei whale feeds whenever the foodstuff is available within a reasonable spacial
range, and therefore its diel feeding pattern so far as it is expressed by the feeding
percentages concerned may be different by the kind of migratory habits of its
food organisms. Concerning to these food habits of sei whale, a recognition on theecological means of the first stomach by Hosokawa and Kamiya (1971) as mentioned
in the begining of this section would be well understood. Baleen whales in Patagonian waters (Matthews, 1932) and humpback whale in New Zealand waters(Dawbin, 1956) eat but little food, and later, Bannister and Baker (1967) considered that the whales under those circumstances would have to feed whenever
there some amount of accumulations of food organisms to fill their nutritional requirements.
10-4. Possible cause induces a decrease in the fulness of stomach contents
The number of animals well repleted with food decreased along with the duration of chasing extends over 40 minutes regardless the kind of food organisms (Kawamura, 197la). The whales with fully repleted stomach were foundup to 30 to 40 minutes of chasing time in the morning but they decreased gradually
down to less than 10 minutes chasing in the evening. The whales with moderately
repleted stomach, on the other hand, were found up to 70 to 80 minutes of chasingtime in the morning, then it remarkably prolonged to more than 120 minutes ofchasing during midday until it was shortened again in the evening. There observed to present two different kinds of varing patterns in relation to the dailychange of the duration of chasing time and the degree of the fulness of stomach.
One of possible explanations for those two different patterns would be found in
the proceed of digestion, that is, most whales with moderately repleted stomach in
midday might have had been fully repleted in the morning, and such whales would
shift to those moderately repleted stomach by digestion toward midday (Fig. 10-3).
The duration of chasing time which corresponds to the degree of the fulness of
stomach seems to be shortened from the morning toward the evening. There observed
more or less inversed relationship between the duration of chasing time and thedegree of the fulness of first stomach through three different time bands, i.e. themore the stomach fully repleted, the more shorter time of chasing is expected.
This fact makes us confirm the former observations (Ingebrigtsen, 1929; Nemoto,1957). So often occurrence of well repleted whales in the morning along withrelatively longer duration of chasing time and its hourly change lead to a consideraSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

95

tion that the most whales take a bulk of foodstuff once a day in the morning.
The copepod and euphausiid foods were found in the whales which werecaught after more or less longer duration of chasing than those fed on squids orboarfish. This may be the result partly due to the vomit, i.e. the large sized foodorganisms likely to be vomited much easier than the smaller one.
It is also supposed from the figures that an approximate time required for thedigestion of foods may be estimated. The whales which carry a well repleted
stomach were found most frequently in the morning, and consequently they were
found being widely scattered up to the duration of 30 to 40 minutes chasing before
0900 hour. However, these whales were greatly decreased in the late afternoon,
and they were only found in 10 to 20 minutes chasing after 1500 hour. This must
be a result of decrease in the absolute number of whales which are well repletedwith food. If it is assumed that the whales with fully repleted stomach could be
expected to be caught by the same difficulties throughout the day, the time being'

R0 .,,......
"··'Y •. ·,..

rrr~.

. .........t.

~.

r••• ....

Q........... ..

........................._ . :::·-..

'·................;··....··········......

·-":-·-...
'·_·-·-·-.-.
·······...::::.~:.~::::~'.;i."'·
';),'

)

........

--

+~·-·

/

Decrease ---Duration of Chasing---Jncrease----0---- Sunrise - 0900 ----0900 - 1500-+-1500 - Sunset
Fulness of Stomach (o/o): R 75-100, rrr 50- 75, rr 25 - 50, r<25

Fig. 10-3. Schematic curves showing regressive tendency of the fulness of stomach
contents along with the duration of chasing in three different time bands. Curves
with circle and cross by four different degrees of the fulness of stomach show ashift of those degrees toward lesser direction of the fulness with the lapse oftime from sunrise to sunset.

required to catch them would give an index for estimating the speed of digestion.
The number of whales with fully repleted stomach reduce down to about 70 percent in the first half of day and then to 50 percent in the afternoon; the relative
abundance of whale with fully repleted stomach is considered to have decreased
down to about half in a day time. In another words, 25 to 30 percent of theirstomach contents would be.digested within 5 hours or thereabouts, and these whales
should be shifted to the stomach conditions of " r " to " rrr " of degrees. The
fully repleted stomach with foods would be emptied in this way by the proceed ofdigestion in the following 14- 15 hours.
It is known in the blue white dolphin (Stenella caeruleoalba) and Gill's bottle
nosed dolphin (Tursiops gilli) which were kept alive in aquarium that their stomachs
likely to be emptied within 10 hours or more (Tobayama, personal communication).
He (Tobayama) also observed that a small instrument swallowed by mistake was
excreted after 17 hours. The digesting speed proposed above for baleen whales as
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

96

KAWAMURA

found in this study does not seem unreasonable estimations by taking account of
the fact that high feeding rates in percentage figures are usually found twice a daywith intervals of 10 to 15 hours.
10-5.

Daily ration in sei whale
It is interest to know the amount of food required by a whale per day, say,the daily ration. It has long been subjected to determine the daily ration in smallerwhales in captivity (e.g. Takahashi, 1961; Tobayama and Tamura, 1964). In
larger whales, however, the daily ration has hardly been determined with precisenessalthough there were some assessment being based on the analysis of the fulness andits daily changes (Nemoto, 1957, 1959; Kawamura, 1970a). However, after
examining on the daily rations as function of body weight in small cetaceans,
Sergeant (1969) described the following equations which might be applicable tolarger whales. Firstly, the feeding rate is defined:.
Food per diem. h
X 100 ........................... (!)
Feedmg rate= B do y we1g t
The feeding rate which was found in smaller cetaceans varied 5.26- 13.67
with 8.26 on average, and the values were somewhat constant regardless the species
and the age of animals. Then the following equation was introduced.
Heart weight x 100
1
Body weight
Feeding rate = O.ll ........................... (2)The heart weight as a factor of the equation was found since its weight wellindicates the extent of activity of animals through their metabolisms. According
to these formulations Sergeant (1969) concluded that feeding of 2.0.-2.5 tons per
day in ordinally sized blue whale would be appropriate, and therefore the whale
have to eat fully twice a day. However, the relationships between heart weight
and feeding rate in larger whales are still little known when compared with another mammals (Ridgway, 1966; Ridgway and Johnston, 1966). To make thematter more clear I examined the heart weight of sei whales which were caught
by the floating factories, "Tonan Maru No. 2" and "Nisshin Maru No. 3" in1970/71 Antarctic season and the results were given in Tables 10-4 and 10-5. The
weight of animals were calculated after the following formulae by Omura (1969),
Ohsumi (1969), and Ohsumi et al. (1970):
Sei whale
W=0.0016L2·43
Fin whale
W=0.00024L2. 9
Minke whale W =0.04655L2.a 1
where W is the weight of animals in 103 kg, and Lis the body length in feet but inmeter in minke whale.
The heart weight/body weight in connection with body length and sex did not
differ much (Fig. 10-4). The difference in the latter was only 0.16% 0 , and was
almost no difference in the former which was in such body length as to be caught
regally by whaling, but some variations by age have been known in smaller cetaceans
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

97(Kleinenberg, 1956; Slijper, 1958b ). Table 10-5 was given under combined figuresof both sexes, and the following was found in sei whale.
Heart weight/Body weight = 0.00487.............................. (3)Although Sergeant (1969) found the figure 0.00404 in this relation, the above
described figure would be considered more appropriate since it was derived fromTABLE 10-4.
Body length
(m)
12.1
12.4
12.7
13.0
13.4
13.6
14.1
14.3
14.5
14.8
15.1
15.6
15.7
16.2
8.4*
8.1 *

BODY AND HEART WEIGHTS OF THE ANTARCTIC SEI WHALE
BY THE DIFFERENT BODY SIZES.

Sex
Male
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
FemaleMale
Male

Body weight
(ton)
8.94
13.25
14.09
14.89
15. 72
16.56
17.51
18.42
19.35
20.35
21.40
22.40
23.55
24.75
5.38
4.96

Heart weight(kg)

Heart/Body(%0)

AverageNo. of whale
examined

53.2
45.2- 60.0
41.0- 65.1
58.0- 70.0

5.95
5.06-6.71
3.09-4.91
4.38-5.28

5.95
5.89
4.09.
4. 78

2
4
4

62.0- 66.8
61.2- 86.0
56.0- 75.0
62.0- 72.0
75.0- 89.0
70.0- 92.0
74.0- 89.0
90.0-106.0
76.2-106.0
68.0- 93.3
86.0-110.0
86.4- 97.0
83.0- 99.0
78.5-117 .o
90.0-126.0
97 .0-101.0
94.0-126.0
110.0-114.0
99.0-125.0

4.40-4. 74
4.11-5. 78
3.76-5.04
3.94-4.58
4. 77-5.66
4.23-5.56
4.47-5.37
5.14-6.05
4.35-6.05
3.69-5.07
4.67-5.97
4.47-5.01
4.29-5.12
3.86-5. 75
4.42-6.19
4.53-4.72
4.39-5.79
4.91-5.09
4.42-5.58

4.57
4.59
4.43
4.22
5.22
4.59
4.92
5.60
5.43
4.56
5.06
4.79
4.64
4.60
5.10
4.66
5.02
5.00
5.15

2
9
3
5
2
4
4
2
4
7
4
3
5
6
4
3
6
2
3

98.0-142.0

4 .16-6.03

4.91

10

102.5
21.3
22.2

4.14
3.96
4.47

4.14
4.21

* Minke whale.

more larger number of materials. Basing on the above described figures, the feeding per diem in sei whale is calculated by the equations (1) and (3) as follows:
Feeding per diem= 0.0443xBody weight ........................... (4)Then the daily ration in ordinally sized sei whale can be considered about
4.4% of body weight (Fig. 10-5), and that in fin and minke whales was 0.0407
and 0.384 respectively. The overall figures through baleen whales therefore would
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

98

be somewhere between 3.8% and 4.4%. These results as a whole agree withthose by Klumov (1963) whose figures were 3.0-4.0% though he expressed it by
the biomass basis, say, gr.-food/kg-muscle weight of animals. The food per diemdemonstrated in Fig. 10-5 shows relatively higher food requirement in sei whale,
and it gets lower gradually in fin and then in minke whales. A quite high food
• Maleo Female

•

'°
-'"".

<N

<N

~

N

~

""!

"?

...:..

~

M

<N

=

+ Male average- Female average... ....... ...,._- - - - - - -- -· -... ... -""?

=
~
=

a,

....;

.,.:,

....

.;.,

~

oO

'"".
.,.,

en

~
.,.,

.,;

..,,.

...:..

0

en

'"".

'°

.,;

Body length (mJFig. 10-4. Heart weight and body weight ratios in southern sei whale.
TABLE 10-5. BODY AND HEART WEIGHTS OF THE ANTARCTIC FIN WHALE
BY THE DIFFERENT BODY SIZES. BOTH MALE AND FEMALE ARE COMBINED.
Body length
(m)

17.3
17.6
17.9
18.2
18.5
18.8
19.1
19.4
19.7
20.0
20.3
20.6
20.9
21.2
21.5
21.8
22.1
22.5
22.8
23.1

Body weight
(ton)
29.33
31.05
32.45
34.20
35.85
37.54
39.30
41.25
43.25
44.99
47.25
49.20
51.40
53.60
55.7
57.9
60.8
63.2
65.4
68.1

Heart weight(kg)
105.6-156.0
102.0-193.0
134.0-150.0
132.0-205.0
138.0-185.0
144.0-200.0
136.0-179.4
158.0-224.0
165.0-220.0
158.0-239.0
185.0-245.0
196.0-280.0
160.0-258.0
195.5-278.0
206.0-350.0
235.0-258.4
180.0-330.0
203.0-379.0
240.0-375.0
276.0-343.0

Heart/Body(%0)

3.60-5.32
3.29-6.22
4.13-4.62
3.86-5.16
3.85-5.16
3.84-5.06
3.46-4.57
3.83-5.43
3.82-5.09
3.51-5.31
3.92-5.19
3.98-5.69
3. ll-5.02
3. 65-5.19
3.70-6.28
4.06-4.46
2.96-5.43
3.21-6.00
3.67-5.73
4.05-5.04

AverageNo. of whales
examined

4.35
4. 78
4.35
4.60
4.61
4.55
4.00
4.35
4.52
4.28
4.57
5.05
4.06
4.46
5.03
4.22
4.22
4.27
4.94
4.43

7
4
3
9
5
8
10
8
5
12
7
7
10
8
8
3
8
7
3
3

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

99

requirement in right whale along with similar trends in sei whale indicates that
their difference from fin and minke whales would be due to the feeding types;
skimming in the former whereas swallowing in the latter.
The amount of the first stomach contents which has been reported in sei whale
1.5

=
,g

1.0

E
Q)
'C
Q)

c.
-0
0

~ 0.5

OL-.L__.L___L__L__lLO_L___L__L_L~2~0-'---'---'---'---.:!:30:--'~~~'-;'40

Body weight (ton)Fig. 10-5.
whales.

Theoretical amount of food per diem in southern sei, fin and minke

TABLE 10-6. AMOUNT OF STOMACH CONTENTS OF SEI WHALE.
Kind of foodCalanus tonsus
Parathemisto gaudichaudii
Euphausia vallentini
Enphausia superba
Euphausia superba

Zooplankton
Calanoida
Pacific saury
Calanus plumchrus

Squid

Amount of food(kg)

Locality

Author54.0
57.6-200.0
79.1-149.6
128.7-149.6
175.0-305.0
59.0
300.0
500.0
50.0-370.0
620.0

Antarctic
Antarctic
Antarctic
Antarctic
S. Georgia
S. Africa
N. Pacific
N. Pacific
N. Pacific
N. PacificKawamura (1970)

Brown (1968)
Best (1967)
Klumov (1963)
Betesheva ( 1954)is given in Table 10-6. It is observed in the table that the amount of food in thestomach of ordinarily sized animal, i.e., 15-24 tons of sei whale is 200 kg or thereabout when they feed on planktonic crustaceans. It could also be said as a wholethat the amount in the stomach decreases with the size of food organisms, and thoseamounts actually measured are considerably low when compared with the daily
ration. If we follow to the daily ration in sei whale of 18 tons in body weight, theanimal requires about 800 kg of food daily, and the stomach of this animal haveto be fully repleted three to four times in a day. However, it seems hardly pasSci. Rep. Whales Res. Inst.,

No. 26, 1974

100

KAWAMURA

sible for the animal to feed 800 kg of Calanus within several hours because Calanus
remains in the surface only a few hours in the morning and evening. Therefore, thesei whale would have to eat even rather scattered food organisms which might haveremained in the surface being apart from the main body of the migrating population. According to Best (1967) the sei whale in South African waters fed zooplankton being consisted of many species, and a considerable number of them were
supposed to be non-aggregating species. This fact suggests that the sei whale can
TABLE 10-7. THE HIGHEST NUMBER OF FOOD COPEPODS PER 1000 M 3 OF
WATER IN THE UPPER 150 METER DEPTH OBTAINED BY THE NORTHPACIFIC STANDARD NET IN THE SOUTHERN WATERS OF
CENTRAL PACIFIC AND TASMAN SEA IN 1968/69.
Highest number ofindividuals by eachstages through all stations occupied ( x 10 3)SpeciesCalanus tonsus

Calanus simillimus

Calanoides acutus

Calanus propinquus

Clausocalanus spp. *Metridia gerlachei
Rhincalanus gigas

female
male
CV
C IV
C III
C II
female
male
CV
C IV
C III
C II
female
CV
C IV
C III
C II
C I
female
male
CV
C IV
C III
female & male
female & copepodites
copepodites & C I-C VI

4.53
0.51
118.28249.58
39.86
2.60
8.10
6.72
60.40
190.73186.69
72.30
1.64
38.07
51.84
103.63
51.84
89.94
1.17
0.97
4.27
6.95
11.45
3179.62384.1756.41

Highest numberof individuals ina haul by species( x 103)12.13
250.58
39.86
2.60
8.06
6.72
60.44
190.73
56.41
10.75
1.64
35.57
75.63
46.09
31.07
89.94
1.17
0.39
4.27
1.55

* Clausocalanus laticeps & C. paululus.
feed even those rather scattered food organisms by skimming type of feeding. It
may be considered anyhow that daily ration of about 800 kg would hardly be takenby the sei whale from its filtering ability and the population density of available
food organisms.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

101

Estimation of population density in the food organisms
One of the most important food organisms of sei whale in the southern whaling ground is copepods, especially C. tonsus and C. simillimus. An ecological importance of these two are comparable to C. cristatus and C. plumchrus in the North
Pacific region.
The amount of food organisms in the stomach would be the result of functionsof both feeding ability of whales and the population density of food organisms.
Studying on the baleen whales in the Fareast seas, Klumov (1961) stated that theamount of stomach contents could not be understood unless the food organisms
distribute such high density as to be hardly known by the net sampling in the sea.
In the southern sei whaling ground the food organisms distribute with the density
as given in Table 10-7 and 10-8. The figures in the tables were obtained by theplankton net hauls through 27 stations in the Tasman Sea and in the Pacific Ocean,

10-6.

TABLE 10-8. THE HIGHEST NUMBER OF INDIVIDUALS OF EUPHAUSIIDS
AND AMPHIPODS PER 1000 M 8 OF WATER IN THE UPPER 150 METER
DEPTH OBTAINED BY THE NORTH PACIFIC STANDARD NET IN
THE SOUTHERN WATERS OF CENTRAL PACIFIC ANDTASMAN SEA IN 1968/69.
Species

Individual number(X 10 8)Remarks" spinifera
Euphausia
Euphausia superba
Euphausia sp.
Euphausiids furcilia

0.26
0.054
0.001
2.81
1.41
1.41
1.11
0.47
0.45
59.48

Immature
Immature
femalefemaleImmature male
Immature female
femaleImmature
Immature

Thysanoessa parva
Thysanoessa vicina
Thysanoessa macrura
Thysanoessa gregaria

0.30
1.09
8.10
7.80

Immature
Immatur;>
Immature

Nematoscelis megalops

0.11

femaleParathemisto gaudichaudii

0.21
0.26
3.42

female, f. compressa
female, f. intermediate
Juvenile

Euphausia simillis var armata
Euphausia simillis
Euphausia lucens
Euphausia vallentini
Euphausia frigida""
and the population density in each copepod species does not show that of underaggregations. The wet weight of copepods were obtained from the stomach contents which were completely consisted of mono-specific population. On the other
hand, the amount of stomach contents was given in Appendix. The amount ofcopepod food was larger in the North Pacific than in the South African and theAntarctic waters. The availability of foodstuff seems in general to be high in the
North Pacific.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

102

The quantitative distribution of food zooplankton in the subantarctic waters
varied between 15.10 and 426.12 gr./1000 m 3 in wet weight (Kawamura and Kureha, 1970). Since the composition of whales food usually found is monospecific,
the highest figures in the poplation density in net haul samples was given by separating each species (Tables 10-7). A distinct occurrence of copepodites IV and V
in C. tonsus, and adult Clausocalanus laticeps agrees with the composition of the stomach
contents although C. simillimus differed in this respect. Both C. acutus and C. propinquus showed a quite slight occurrence as none of them were found in the stomach
contents. The figures in euphausiids are quantitatively less important due to net
avoidance. However, the dominancy of an important food species; E. vallentini
is indicated. The population density of food organisms was estimated as follows
under two feeding types particularly in the case of C. tonsus, one of a leading foodstuff in the southern feeding ground:

Swallowing
The 14.4 m sei whale caught in the Antarctic feeds at least 54.0 kg of C.
tonsus of copepodites IV and V (Table 10-6), and the average body weight of C.
tonsus is 1.2 mg/individual. (Table 10-9). Accordingly, a total of 54.0 kg. of food C.
TABLE 10-9. WET BODY WEIGHT OF FOOD COPEPODS COLLECTED FROM THE FIRSTSTOMACH OF SEI WHALES CAUGHT IN THE INDIAN SECTOR OF THE ANTARCTIC.
SpeciesCalanus tonsus

Calanus simillimus
Clausocalanus laticeps

Drepanopus pectinatus

Stage
CV
CV
CV
C V +VI female
CV
C V +VI female
femalefemalefemalefemalefemaleWeightNo. of individual
Body
measuredweightNormal Damaged* (mg)
(mg)
200
200
1000
500
500
500
20
20
20
200
20

155
151
595
256
541
745
96
110
186
587
139

26
33
83
105
144
226
71
75
57
358
89

1.19
1.20
1.57
1.66
0.82
0.58
0.15
0.14
0.09
0.26
0.11

State of material

Moderately digested
Moderately digested

Heavily digested
Heavily digested
Heavily digested
Moderately digested* Damaged individuals are supposedly lost their body fluids, and they were regarded as one halfof those undamaged individuals in the wet weight.

tonsus corresponds to 45X10 6 individuals. In order to obtain this number of C.
tonsus under the population density of 262. 7 X 103 inds. /1000 m 3 which was the highest
figures actually found (see Table 10-7), a total of 171.2X10 3 m 3 of water have to
be filtered off by the whale. The volume of water in a gulp in fin whale is supposed
about 6.0 m 3 /30 sec. (Klumov, 1961) whereas in average sized sei whale (14.5 m:
Intern. Whal. Statistics, 1969) the volume of engulped water would be somewhere
between 2.0 and 3.0 m 3 /30 sec.
To examine these figures more clearly the volume of air which was sent into theoral cavity to make the animal afloat was measured (Fig. 10-6), and the average
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

FEEDING HABITS OF SEI WHALE

103

volume of air used in fin and sei whales was 10.4 m 3 and 8.95 m 3 respectively. In
Fig. 10-7 the dots far above from main body would be due to a leakage of air. Since
the above mentioned air was pressured 3 to 4 times higher than natural pressure,
those amount of air in fin and sei whales could be converted into 2.6-3.5 m 3 , and2.24-2.98 m 3 respectively. It is clear that these figures agree well those above
mentioned. Therefore, a total of 54.0 kg. of C. tonsus could be obtained in 5.5 X
10 2 hours, say, 23 days! As it is noticed in Table 10-6, the amount of the stomach
contents in sei whale sometimes attains at 500-600 kg, and their theoretical daily
ration was 800 kg. Then if we assume 300 kg. of copepod food as their possible
daily ration since an availablity of copepod food would be considerably poor than
20

•

•

0

.6

15

"'.§.""'ill..
.........
_...... , •

=>
~

·o;

0

..,

• • •
•
•
• • ..,..

•• •
•

10

§
0

>

5

+•

0

06
0
0
0

0

Fin Av. 10.4
Sei Av. 8.95

0

0

0

•
11

Fig. 10-6.

12

13

14

15
16
17
Body length (m)

18

19

20

Volume of air used to make afloat the carcasses of sei and fin whales.

the others, a total of 54.0 kg. of stomach contents fills only 1/6 of the daily requirement in sei whale. Accordingly, it could be supposed that the population density of C. tonsus under an aggregation must be found somewhere between 3.4 X
10 7 and 5x 10 7 inds./1000 m 3 (Fig. 10-7).
According to Minoda (1958) the highest population density found in his study
in C. plumchrus in the North Pacific was 395X 103 inds./1000 m 3 , and later the another case revealed as 631X10 3 inds./1000 m 3 (Fae. Fish. Hokkaido Univ., 1963).
Comparing these figures with the density of 367.8X 10 3 inds./1000 m 3 in C. tonsus,
it is noticed that the population density in both cases agrees quite well while those
two are the phenomena in far separated regions each other. These facts indicatethat the upper extreme of population density in copepods would be about up to
4x 10 5 -6X 10 5 inds./1000m3 throughout the world seas though rarely found. The
Sci. Rep. Whales Res. Inst.,.No. 26, 1974

KAWAMURA

104

estimation described above seems quite high density which may almost unbelievable
but it is the patchiness of plankton that gives those high level of the population
density.
20
18

16

range
~ Reasonable
55338Xl0 - 504Xl0 inds./lOOOm'

~ 14
';;;-

""'
.5

8 12
c
z;.

-~

""'"'c:

:3

"'

10
8

"3
c.
0

c...

4

Volume of water filtered by a sei whale (m'/min.)Fig. 10-7. Theoretical estimation of the population density of Ca/anus tonsus patches
which fulfil! the 54.0 kg of C. tonsus within 4 hours.

Skimming
The feeding type actually and most frequently employed by the sei whale isnot swallowing but skimming (Scoresby, 1820; Millais, 1904-1906; Ingebrigtsen,
1929). The sei whale collects foodstuff by filtering continuously the food contained
sea water with the mouth half opened. The right and bowhead whale feed byskimming exlusively while sei whale does not always but frequently.
As it has examined previously a relative filtering ability in the row of baleen
plates in sei whale was l /2.2 against fin whale (Kawamura, 197la). A total
surface area of the inner surface of filtering apparatus in sei whale was about 3.6 m 2 ,and the filtering coefficient was 0.167. By these figures a sei whale can filter off
6.0 m 3 of water effectively by swimming for 10 meters. One of possible and acceptable parameters in regard to feeding by skimming would be; 3 knots in swimming speed, 4 hours in feeding duration and a total of 300 kg of food organisms
is fed. Then the population density of food organisms which fills enough above
mentioned parameters must be 2.13X103 gr./1000 m 3, that is, l.8X 107 inds./1000 m 3 •This figure as the order of density agrees well with the both estimations.
Sci. Rep. Whales Inst.,
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FEEDING HABITS OF SEI WHALE

105

According to Zenkevitch (1963) the biomass of zooplankton standing stockduring the summer in the Okhotsk Sea and Bering Sea rarely shows more than
1.0 gr./m 3 , and those exceeds 3.0 gr./m 3 would only be found in a limited regionof the northern Okhotsk Sea. Kurile region also rarely yields 1.0 gr./m 3 of zooplankton (Klumov, 1961). In the Pacific sector of the Antarctic the biomass ofzooplankton was usually between 0.5 and 2.0 gr/m 3 , and that more than 2.0 gr/m 3
was rarely found. One of the biomasses of C. tonsus in the subantarctic waters
ever found was 0.2.-0.5 gr./m 3 • (Dolgencov, 1970).
By comparing these figures with those estimations in C. tonsus, the biomass or
the population density in C. tonsus as food of sei whale seems considerable higher
numbers than those obtained by the net haul investigations. Those density infood C. tonsus would still under estimated since the 300 kg. of stomach contents
which was assumed in calculation should be doubled or more. However, it should
also be took in account that those extremely dense population would possibly befound only when C. tonsus aggregates into the patches. Manteufel (1939)
stated 9.0 gr/m 3 of population density in Calanus sp., then the biomass of about
21.3 gr/m 3 (l.8xl0 4 inds/m3 ) in C. tonsus population as estimated could be considered quite possible.
The population density of C. tonsus required to fill the 300 kg. of stomach contents by swallowing type feeding was 3.4x 10 7 .-5X10 7 inds./1000 m 3 whereas it was
l.8x 107 inds./1000 m 3 in skimming type. Comparing these figures it could be
supposed that the feeding by skimming type would be more effective to collect thesame amount of foodstuff. The scheme would also support the facts previously
known that the sei whale feeds almost always by employing the skimming typefeeding in the whaling ground.
11.

SWARMS OF CALANUS TONSUS BRADY

As has been mentioned in the previous sections, it was strongly suggested that the
population density in C. tonsus must be maintained at least in such the order of
103 .-10 4 individuals per cubic meter when they swarm into the so-called patches
as to be fed by the sei whale (Kawamura, 1971d). However, the population density
described above was an estimation being based on the amount of C. tonsus in thefirst stomach of whale carcasses and the filtering ability in a row of baleen plates asa function of feeding in sei whale. The theme therefore, was in need of being substantiated by the further evidence in the field, since the general features of swarming
copepods under patchiness along with their population density had been known
quite a few, while those swarm forming phenomena in copepods themselves had
been known in early 1800's in the North Atlantic waters, and have been relatively
well documented (e.g. Marshall and Orr, 1955).
In the following section I will describe some additional evidences on theswarming of C. tonsus in the surface waters. C. tonsus is a typical endemic species
in the Subantarctic waters and considered to be important in a role of marine
ecosystems of the southern waters. However, it is still unknown whether there
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106

KAWAMURA

were any socialized relationships among the swarming C. tonsus individuals. The
swarming C. tonsus populations are exclusively consisted of stage V copepodites
with a little mixture of adult females. A possible causations of surface swarming
behavior may perhaps be similar to those in mysids (Clutter, 1969) though there
must be some another pattern-forming agencies in copepods.
An uneven and spatial distribution of plankton species has been well known(e.g. Hardy, 1936a, b; Barnes and Marshall, 1951; Cassie, 1959a, b, c, 1960).
However, some recent works (Wiebe, 1970; Kuwahara et al., 1971)on this matterseem to give some confusion in the term of the plankton patchiness: Wiebe (1970),
for instance, defined the " patches " or " patchiness " when the overall concentrations of zooplankters under multispecies composition exceeded a central value
in a data set, which necessarily induce such a quite much occurrence of the" patches " as about 200 patches per a grid of about 100X100 m squares. Thesekind of" patchiness" are, however, considered rather usual phenomena as a distribution pattern widely seen in any localities of the sea. On the other hand, theplankton patches under considerations may be something different from thoseabove mentioned, and also may possibly be different from those quite large scaled
spacial distribution such as treated by Cushing (1962), but identical to those densemonospecific aggregations into swarms or rafts which by Marr's expression, scattered in a plane keeping 1 /3-1 /4 miles intervals of" krill-less" sea. As mentioned
in the previous sections, the composition of patch forming plankton population ismonospecific as known in Euphausia superba (e.g. Hardy and Gunther, 1935; Marr,
1955) or in copepod, Calanus .finmarchicus (Bainbridge, 1952; Kitou, 1956), whichmake the sea discolored by enormous number of swarming plankton species inthe very surface.
11-1. -?_ooplankton swarms as whales food
The swarming organisms as whales food have had been well recognized early
among British whalers in those sperm whaling days in the southern seas, and the"Brazil banks'', according to Beale (1839), is" only discolorations of water causedby myriads of animal cula:: which perhaps form the sustenance of the common
black whale's food; that consisted of" squillce" and other small animals", andBeale (1839) also called them as "submarine pastures". During the cruise on
boad of the H.M.S. "Beagle," Darwin (1906) sighted a fairely well discolored waters
by a huge number of copepod-like minute organisms, and considered that theymust have been the food of whales and seabirds. Apart from recognizing theswarming phenomena of plankton species, Klumov (1961) brought out a subject
under ecological considerations concerning to plankton patchiness that the daily
food requirements of baleen whales would never be filled by the standing stocksof zoplankton known in the Kurile region even by their uppermost extremes, andconcluded that the food zooplankton parhaps be maintained with a density of atleast about 1500-2000 mg per cubic meters. An approach to know the population density in whale's food zooplankton involved an important indications, andlater Omori et al (1972) made an estimation on the population density in swarming
Sci. Rep. Whales Res. Inst.,
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FEEDING HABITS OF SEI WHALE

107

Sergestes similis in the northern North Pacific.
At least the following zooplankton species except copepods have been observed directly as to form a dense swarms or so-called patches apart from those
uneven or spatial distributions of plankton species: Euphausia superba, E. crystallorphias (Marr, 1962), E. pacijica and many others (Komaki, 1967), E. nana, E. similis
Thysanoessa inermis, Th. raschii, Th. spinifera (Nemoto, et al., 1969), Mysid, Metamysidopsis elongata and Acanthomysis sp. (Clutter, 1969), Mysidium columbiae (Steven,1961), Galathea, Munida gregaria (Matthews, 1932; Tabata & Kanamaru, 1970),
Pleuroncodes planipes (Boyd, 1967), macruran, Sergestes lucens (Omori, 1969) andS. similis (Omori et al. 1972). The number of patch forming plankton species
would be doubled or more when we take into account the indirect evidences ofswarming such as the food of baleen whales, among which amphipods, Parathemisto
gaudichaudii is distinct. In these patch forming species the population density
under swarming is usually maintained quite high, say, 1 ind./inch 3 in E. superba(Marr, 1962), 100 crabs/m 2 in Galatheid shrimp, P.planipes (Boyd, 1967), or animals
swim as close as 1.-2 cm one to another in E. pacijica (Komaki, 1967). Patchinessin mysids are relatively well known as mentioned before. Recent biosociologicalstudy on the swarms of mysids, M. elongata and M. columbiae (Clutter, 1969) muchcontributed to our knowledge on the aggregations of zooplankton particularly on
swarming mechanisms along with possible but indicative biological cusations of theaggregation agencies such as density or grouping effects, role of sensory organs orunderwater vibrations and pheromones (e.g. Laverack, 1962; Horridge, 1966;
Ryan, 1966; McLeese, 1973; Dahl et al., 1970a, b).
11-2. Material and method
The freezer ship M. S. "Eiho Maru" (1153 ton) which was chartered by the
Nippon Suisan Co. Ltd., participated to the Antarctic whaling operation in 1971 /72
season, and made a exploratory fishing and scouting for whales on her way down
to the whaling ground. During her cruise in the south- to southwestern waters off
Western Australia in December 11.-26, 1971 when she was engaged in scouting
about for sei whales in advance of comming season, many patches of discoloredwaters by dense aggregations of copepods, Calanus tonsus Brady were observed in
the surface waters. Japanese whaleman calls them as" Uki-esa ",a floating whale's
food. The cruise track and the general hydrography are shown in Fig. 11-1. .
The ship cruised by following up the waters chiefly along the 12.0° .- l 4.0°C isotherms
since high concentrations of sei whales are usually expected under these sea conditions by the whaler's experience.
The surface swarms of C. tonsus which distributed within the range of about
2 miles of both sides of the ship's track were possible to distinguish by their darkbrownish, sometimes blick like reddish discoloration from the surrounding waters,
and the plankton sampling was conducted under inducement by the watchers who
were on the upper deck, since the margin or the boundary of swarms was hardly
visible in closer distance from foredeck. Marr (1962) suggested upon the decreasein detecting ability of the plankton patches when it is partly cloudy weather by
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108

KAWAMURA

Fig. 11-1. Track of the Eiho-Maru with approximate isotherms along the ships
cource during Dec.11-26, 1971andJan.2-11,1972. Blackspotrepresentsthe
ship's noon position. Numerals with arrow shows the serial number of patches
sampled as details are given in Tables 11-3 and 11-5.

confusion with the shades of cloud on the sea surface. However, it was usually
over cast or cloudy throughout this investigation, and any hard difficulties in spotting the C. tonsus patches were not noticed. The number of surface swarms which
occurred along the ship's cource were counted usually by three watchers.
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FEEDING HABITS OF SEI WHALE

109

While the ship cruised at the speed of about 10 knots, the shape and an approximate size of C. tonsus patches that passing through by the ship's course werealso determined as exact as possible by the author on the upper deck. As the eyelevel at the upper deck was relatively low (ea. 8 meters high above the sea level)
there was some difficulties to figure out exact shape of patches which spread out
far beyond in the surface waters where large swells and waves prevailed. The
shape of patches was determined by observing meandering margin of the patches
at least from three different directions, i.e. front, side and back as ship passing throughby them. The margin of far opposite side from the ship could be sometimes determined when the margin of the patches was on the top or on the slope of swells
and waves.
The swarming C. tonsus was collected by two kinds of sampler; the high speed
underway plankton catcher which was slightly modified from its initial model V(Motoda, 1959): 5.0 cm diameter in mouth opening, 72.7 cm overall length, Japanbolting silk cloth GG 54 netting (ea. 0.33 mm mesh apertures), and the RGS flowmeter was mounted inside the sampler for estimating the volume of water filteredby the sampler. The another one was Petersen type vertical closing net (Kawamura, 1968a), having 70 cm mouth opening with pylen #24 netting (ea. 1.0 mm
mesh apertures). The former was used exclusively for estimating the population
density itself, and the latter was for the relative abundance and qualitative analysis
of swarming population.
Towing of the sampling gear was made usually at the speed about 2.-..3 knots
by slowing down the ship's speed by keeping some distance before hand from theswarm in question. When the ship seemed to have crossed over the margin of theswarm and positioned inside, the sampling gear was thrown out in the sea. The
duration of towing varied from several tenth seconds to several minutes according
to the ship's speed and the size of each swarms. In most cases the gear was took inwhile the net seemed to be still amid of the swarms since the purpose of sampling
was focussed chiefly on to know the population density of swarming C. tonsus as
precicely as possible. Thus, ten samples from C. tonsus patches were obtained during
December 14 to 23 (see Fig. 11-1). Apart from these sampling on swarms surface
tows for ten minutes with under way plankton catcher were made routinely every
day, usually at 1000 and 1600 hours (LMT) as far as when there were no surface
swarms on the ship's passage. This sampling offered materials to know the standing stocks of copepods and other zooplankters during daytime as the overallplankton population of background waters.
The quantitative treatment on the samples obtained by the underway plankton catcher was made by the flow-meter readings by comparing them with thosein calibrations, and 80.-..90 percent of filtering coefficient under 2 knot towing was
adopted for the samples obtained by Petersen type vertical closing net.
The population density, however, varied considerably even within the identicalswarm judged by the pattern of discoloration, and those observed between manyother swarms. The underway plankton catcher might have been towed throughnon-swarming waters more or less, which would make the figures on population
Sci. Rep. Whales Res. Inst.,
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110

KAWAMURA

density small by dilution. So the results obtained were also variable by the patches
and the observed population density of C. tonsus must be considered as the " atleast " figures.
11-3. General hydrology in the investigated region
As shown in Fig. 11-1 the region investigated spreads over between the latitudes of 30°S and 40°S, and between the longitudes of 95°E and 122°E. The ship
had not equipped any research instruments, and the depth record of temperature
was not known. The general pattern of isotherm distribution was figured out by
refering to Rochford (1962) since the data obtained by the "Eiho Maru" was too
fragmentary and insufficient to show the overall pattern in temperature distribution.
The ship, however, took a cource mainly along the isotherms of about 12.0°,.., 14.0°C
in search for sei whale. The isotherms of these temperature are usually concerned
as a bounday between the Subtropical region and the Subantarctic region, say, theSubtropical Convergence. The most of surface swarms of C. tonsus were found in
this region. The isotherms run for west-east direction roughly parallel to the latitudinal lines although they showed rather complicated features in the waters southof Western Australia where the tongue-like warm waters intrude considerably fromthe north to make the surface temperature relatively high. According to Rochford(1962) the area in question can be regarded as the South Transision Zone where
distinct change in salinity is characteristic.
11-4. Shape, color, and size of C. tonsus swarms
As it is shown in Fig. 11-2 the shape of C. tonsus patches was manifolds.
However, they can be roughly classified into three types of the shape, i.e., meandering stream or streak bands, spindle shape, and quite irregular shape with many
tongue-like protorusions. The shape of patches can be said in general that they
are different to some extent in their complexity from those previously reported
in the patchiness of southern and northern euphausiids (Marr, 1962; Nemoto et al.
1969). Some of them were stream like shape being kept very long for several
miles as having been called the" plankton stream" (Nemoto, 1962). Most of thepatches ~ere formed in the very surface so as to perhaps be easily deformed by thewind and wave actions to show quite irregularity as some figures shown below in
Fig. 11-2. It is noticed in the figure that the C. tonsus patches keep more or lessdefinite shape when the sea surface conditions were smooth enough under slightwind. Movements of the whole body of patches as suggested in some euphausiids(e.g. Nemoto, et al. 1969) were unknown.
Orientation of patches in connection with the relative wind direction and force
were: the longer axis of patches against the wind direction kept at the angle ofabout 90°, or the longer axis kept nearly parallel to the wind direction, and or
incoherent by the mixed characters of the two formers. There seem, however,some another causes such as swells, tidal current or even the action by the Langmuir
circulations (Langmuir, 1938; Scott et al. 1970; George and Edwards, 1973) tobe present other than the wind action. The following fact is also note worthy when
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FEEDING HABITS OF SEI WHALE

111

we consider what causes the plankton patchiness, that is, C. tonsus patches kept
tightly even in the rough sea conditions such as the wind force 5 which seems roughenough to disturb the swarms. Mentioning on the patchiness of Euphausia superba,
Hardy (1936b) pointed out that their patches were kept under the sea condition
up to at least the wind force 7.

@-

..

~~
I
@

~i@

~

~

I~
I~®

®

~

~
t® .

.

:·-.

t

@

Fig. 11-2. Rough sketches of Calanus tonsus patches.
circle show the wind direction and the force.

Arrows and numeral in theSometimes a discoloration of waters by swarming was distinct, but it varied
considerably by patch to patch, or by part to part of even in the identical patch.
There were void or extremely poorly distributed spots in the midst of the patch
through which clear blue water was seen. It is also observed that there were clear
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KAWAMURA

112

water whenever the discolored water was smashed and turned by the ship. This
seems to be due to " hop and sink" movements as observed in C. finmarchicus
swarms (Bainbridge, 1952). He (Bainbridge, 1952) observed it within 12 inch
depth, and the above facts suggest that the dense swarms in Calanus must be foundwithin very thin layer of the surface. The variations in discoloration character
were seemed to be caused maily by the variable change in population density ofswarming C. tonsus, followed by the character of at what depth the center of swarms
is kept in the surface layer.
The distinct difference in the size of C. tonsus patches from those of euphausiids
was much widely spreading out in the former. Most usual size having been known
previously in euphausiids were several tenth meters in diameter (Marr, 1962;
Ozawa et al. 1967, 1968; Nemoto, et al., 1969), while those in C. tonsuswere hundred
to several hundreds meters in their longer axis. One of the longest sizes under
stream like patches showed the length of about several miles. Kitou (1956) also
reported 15 miles long patches of Calanus finmarchicus in the North Pacific. Thiskind of difference in size might be caused something by nature of organism itself
because the sizes of swarming in patchiness have been known characteristic by theplankton species concerned, i.e. Parathemisto and Salpa sp. usually show 3 miles
wide (Hardy, 1967), and Galathea shows much larger size than Euphausia superba(Ozawa et al. 1967).

i

1-2 patch (s)/h3-5 patches/h

5 patches/hFig. 11-3. Schematic map showing the distribution of patches of C. tonsus during
Dec. 11-24, 1971.

11-5. Distribution of C. tonsus swarms
The number of patches occurred within about 2 miles of both port and starboad
sides along the ship's cource were recorded. As mentioned before the number ofpatches, however, must be regarded as " at least " or round numbers since thepatches of poorly discolored had perhaps been overlooked when they were far
enough from the ship to be detected by binoculars.
Fig. 11-3 demonstrates the general features of the occurrence of C. tonsus
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FEEDING HABITS OF SEI WHALE

113

patches which distributed along the ship's track during December 11-24, 1971.
The patches occurred every day during that period though the frequency in termsof the number of patches per hour was most distinct during Dec. 16-20 when thesea temperature was in 13.0°......, 14.0°C. However, a complete absence in some definite region during Dec. 16- 17 cannot help to be said curious because any unusualsea conditions in temperature were not observed in the region.
The highest number of patches per hour was recorded between 1600 and 1700
hours on Dec. 16 when it was 21 patches/hr. On the way down to the south after
calling Fremantle, Western Australia on Dec. 31, the ship again crossed over theswarm rich area of Dec. 20 at the position between 115° and 116°E longitudes withthe time lag of 11 days and 15 hours. The sea temperature was 14.6°C when crossed the previous track. However, none of signs showing the presence of C. tonsus
swarming were observed in this time.
As it is noticed in the poor occurrence on Dec. 11 and Dec. 24, the C. tonsus
patches usually do not occur in the region where the sea temperature of high above
TABLE 11-1. SEA TEMPERATURE AND THE NUMBER
OF C. TONSUS PATCHES OCCURRED.
Sea temperature(°C)

11.0-11.9
12.0-12.9
13.0-13.9
14.0-14.9
15.0-15:9

No. of patches
4
46

117
49
4

16.0°-l7.0°C prevails (see Table 11-1). According to Brodskii (1964) andJillett(1968), C. tonsus occurs at the sea temperatures between 5.0°......, 15.0°C in the southernwaters, and both Kawamura and Hoshiai (1969) and Kawamura (1970b) also confirmed those temperature as possible conditions of their habitat. Accordingly the distribution range of C. tonsus as a function of sea temperatures concerned would beconsidered as relatively wide ranged. However, the main body of early summer
concentration of sei whale which exclusively feeds on C. tonsus patches (Kawamura,1970a) in the Subantarctic waters is found at the temperature of 10.0°......, 16.0°C
accompanying distinct temperature gradient (Nasu and Masaki, 1970). These fact
and Table 11-1 suggest that C. tonsus prefers somewhat warmer waters, and its
patch would not have been formed much in the waters south of 40°S and in thewaters under the temperature of higher than 16.0°C.
To know the relationship between the occurrence of patches and the sea
temperature more precisely, Fig. 11-4 was arranged. As mentioned before, C.
tonsus patches occurred most frequently during Dec. 14-20, and the trends of sea
temperature change by hours in connection with the occurrence of patches during
those period were distinct; a very frequent occurrence of patches can be expectedunder the small-scaled variable temperature change. The sea temperature in
the figure represents the value measured every hour, but it varied very frequently
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

114

CD

15~ Dec. 12
14 -: '-I Dec. 20

13

ttt

'

@®CD

12

c.-----

0- - - - - - - 5 4------ 5 - 13
12

o
15
14

13t

tt t tt

12' ' ~__'.___'.. ~ -------~CDCD @

3---------~

®@CD@

p

c-

4-3 - - - - 4 --3 --Dec. 15

®

-

be

5-4---3----Dec 22
• '
•
.

~

'
~ d--. o _________..be -------... c - - - - 1'>
2 3 4 5114 q>j@~13

16

Dec. 23

15

I-

Dec. 24

16 ~······
15

o

2

be

3 -2 ---.1 2 1 -c2-

1:Dec.19?J~~
13t~, ,CD;~
c

~

c
14 0
3-4-5------65 6 7 8 9 10 11121314 15 16 17 18 19 20
Ship's mean timec o-

5 5 6 7 8 9 10 1112 13 14 15 16 17 18 19 20
Ship's mean time
Fig. 11-4. Sea temperature and the occurrence of C. tonsus patches along the ship's
track during Dec. 12-24, 1971. Arrow shows the time of occurrece and the numeral encircled shows the total number of patches occurred in every one hour.

TABLE ll-2.

TEMPERATURE VARIATION WITHIN VERY SHORT TIME.

Date

Time (SMT)

Dec. 171520-1600
1620-1730
13001200-1220
1350-1640
17000700-0800

Dec. 18
Dec. 20

Dec. 21

Temperature change (°C)

14.5
13.3
14.2
12.3
Very frequent change within the range of 0.2 ..... 0.3
12.3
14.7
12.0
11.9
15 .1
11.9
14.8
11.9
11. 7
14.8
12.4
11.9
Sci. Rep. Whales Res. Inst.,

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FEEDING HABITS OF SEI WHALE

115

through hours to hours as an example given in Table 11-2. This suggests that theoccurrence of patches would be closely related to the characters of sea temperature
distribution especially to the small-scaled local hydrological structures, which something likely to micro-distribution of plankton as demonstrated by Cassie (1959). Poor
occurrence of the patches during Dec. 23-24 would be attributed to an unfavourable
high temperatures for swarm formation, and those on Dec. 12- 13 should be notedas the result of monotonous hydrological conditions possibly due to relatively poor
temperature gradient.
The conditions of sea surface in terms of wind force varied from class 1 to 5
throughout the investigation though the most was in the class 3 to 4. During Dec.
16-20, when the patches appeared frequently, the wind of class 4 or 5 prevailed.
This fact suggests that C. tonsus patches never be dispersed or scattered away atleast under these roughness of the sea surface.

Frequency distribution by hourFig. 11-5 demonstrates the frequency distribution of C. tonsus patches by every
observed hour. A total of 220 patches was counted. As far as the figure concerned, the diel variation of occurrence makes us confirm that C. tonsus patches
which were consisted of almost exclusively by the copepodite stage V likely to movevertically, and a considerable number of patches come up to the very surface in
11-6.

50
al
>

~ 40

.c
0

"'
"'

! 30
""'

2"' 20
c.i
"O
ci

2:

'-'

10

04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20
Ship's mean time

Fig. 11-5.
1971.

Frequency distribution in the number of C. tonsus patches, Dec. 11-24,

the evening between 1700 and 1800 hours. It is also shown that there are twoother peaks in frequency distribution: early morning and daytime, although they
are hardly comparable to those found in the evening. By analysing the feeding
habits of southern sei whale which feeds on C. tonsus exclusively, Kawamura (1970a)
considered that C. tonsus of copepodite stage V does not make any diel verticalmigration. So it seems disagree with the above results each other. It is, however,considered that C. tonsus patches move slightly down to such the depth, say, 1 or 2
meters under the surface during daytime as to be hardly detected by eyes but still
Sci. Rep. Whales Res. Inst.,
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KAWAMURA

116

remaining within the enough depth to be fed by the sei whale. This vertical movements in whale's food can be recognized as "Uki-esa ", a floating food and "Sokoesa," a bottom food which were introduced by the Japanese whalers. They are notscattered away but still keeping swarm during daytime since the routine samplingby underway plankton catcher at 1OOO and 1600 hours did not show any trends oftheir dispersal which may cause poor occurrence of C. tonsus patches during daytime.
The poor occurrence both in 0800......, 0900 hours and 1300 ,......1400 hours corresponds to 3,......4 hours of the time lag from astronomical twilight at 40°S in December(Japan Maritime Safety Agency, 1971) in the former and 4.7-5.9 hours for thelatter. This fact suggests that the light condition in or under the surface varies
considerably with both cases, and that the vertical movements of C. tonsus patches
may possibly not correspond to the light condition. Those small scale movementof patches would also differ from patch to patch themselves. The patches of E.
superba likely to occur more frequently both in the morning and evening, and thoseby Galathea are distinct in the daytime (Ozawa et al., 1967). The manifold charactors of movements by the kind of patch forming organisms make us confirm thatthe behavior of C. tonsus patches above mentioned may be, as Cassie (1960) suggested, by some internal behavior patterns within the species.
11-7. Population density in the C. tonsus patches
In the study on the food and feeding habits of southern sei whales, it wasestimated theoretically that the population density in C. tonsus patches have to be
maintained at least in the order of 10 4 individuals per cubic meter (Kawamura,
PATCHES OF CALANUS TONSUS AND PARATHEMISTOTABLE ll-3.
Position
No. ofpatches

Date

1
2
3
4
5
6
7
8
9
10
II
12
13
14
15
16

Dec. 14
Dec. 14
Dec. 16
Dec. 17
Dec. 18
Dec. 20
Dec. 21
Dec. 22
Dec. 22
Dec. 23
Jan. 3Jan. 4Jan. 5Jan. 5Jan. 6
Jan. 11I)2),..------.r'---

Lat.
(S)

Long.
(E)

37-14
36-26
37-56
38-37
39-31
38-59
39-21
39-36
39-54
38-19
49-40
53-09
55-40
56-22
57-17
57-14

99-36
101-21
106-39ll0-07111-56115-21
117-57120-22120-54120-48118-35
119-35l 19-57
118-50
117-07
97-03

Time
(SMT)

Sea
temp.

0801-0811
1845-1856
1730
1820
1556-1558
0715
0729-0730
0720-0722
1029-1032
1107-1108
1906-1916
1600
0740-0744
1500-1505
0341-0350
2029-2035

13.6
14.87
13.54
13.21
13.80
13.49
12.13
12.90
13.10
13.0
6.24
5.60
4.20
4.00
3.40
2.20

coq

Weather

0

c
c
0

c
0
0
0

c
0
BC
F
F
D

0
R

Wind
direct. &
force
NNW 3
NW 3
ENE 4
ENE 4
NNW 1
s4
SSE 5
SE 3
SSE 3
ESE 2
NW 4
NW 6
NNW 3
NNE 5
SSW 5
ENE 5

Petersen Type Vertical Closing Net (Kawamura, 1968a).
Simple Underway Plankton Catcher (Motoda, 1959).

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

117

FEEDING HABITS OF SEI WHALE

1971a). One of the purposes of the investigation on C. tonsus patches was to get
an evidense which directly supports those theoretical estimations.
Table 11-3 presents the details of ten C. tonsus patches which were sampledactually, and the quantitative results on the samples were given separately. In
the most of sampling in the field, the plankton catcher was towed as to cross overthe patches in their short axis. The duration of net towing varied from several
tenth seconds to several minutes since individual density varied both vertical andhorizontal plane even within the identical patch in addion to the variation of patch
sizes. As it will mentioned later, the patches were composed of almost solely by
C. tonsus, and the number of individuals in a haul was determined by sub-samplinga small fraction of materials such as 1.0 gram or so.
The individual number of C. tonsus patches per cubic meter of water varied
considerably by the patches: 330 inds./ms as the lower extreme and 23,680 inds./
ms followed by 21, 165 inds./ms as the upper extremes. So the individual
density found in this study was in the range of 102 to 104 inds./ms. Some ofthese figures would not represent an exact density because the sampling method
employed in this investigation was in danger of failure in quantitative treatment due
to the irregularity of individual distribution within the patch and also due to thosevariables in vertical section against the definite sampling level in the water. Accordingly the population density within the patches always tends to be under estimated as Fraser (1962) mentioned the difficulty on this matter. The population
density of 2Xl0 4 -3Xl0 4 mg/ms, which is equivalent to about 3.8-5.7Xl0 3 mg
dry weight/ms (after dry/wet weight ratio of C. tonsus by Bradford, 1972) would be
GAUDICHAUDII WHICH WERE SAMPLED BY THE NET.

Kind ofnet used
PVCN'

Kind of patchorganism
Calanus tonsus"

SUPC 2

"
"
"
"
"
"
PVCN
"
"
"
"

3)
4)

"
"

Approx.
size ofpatches 840x50030x12010x50010x400100x300
lOx 150
60x500150x100
6x500
IOOx 1000

Parathemisw gaudichaudii unknown

"
"
"
"

"
"
"

Wet weightper

No. of individuals
per
haul

m•

144633146545
4750
2884
16720
266
1525
5035
167
123
1688
389
565
353
2100
4273

670-761623-701 4
23680
4

21165
3446
6277
8626
330
350
8-9 4
36-41 4
7-84
3-44
8-94
3-44

haul (mg)m 3 (mg)

177900212690
5890
4710
27500

848-937 4
905-10184
29362

2990
9500

12305
16274

5800

27-31 4

32300
91900

129-1454
72-81 434810

Width X Length (m).
Upper and lower extremes of 80% and 90% fiiltering coefficiencies of the PVCN when towed
at the speed of 2 knots.

Sci. Rep. Whales l?es. Inst.,
No. 26, 1974

KAWAMURA

118

agreeable when compared with the previous estimations of between 3-6x 10 4
inds./m 3 (Kawamura, 197ld), and the result of 9x l0 3 mg/m 3 in Calanus (Manteufel,
1939). The above results also well agrees to the figure obtained in Calanus.finmarchicus patches by Kitou (1956), where 8,208 individuals per o,....., 1 meter haul(ea. 2X 10 4 inds./m 3 ) were recorded.

,(ooplankton abundance in the back ground waters
To compare the quite large zooplankton standing stocks maintained by C.
tonsus patches with those lying in the waters outside from the patches as a background, sampling by 15 minutes surface tow with underway plankton catcher was
conducted twice a day during Dec. 11-26, and the Petersen type vertical closing
net was drifted at times after sunset as supplemental sampling.
Table 11-4 gives the figure that zooplankwn standing stocks are very poor during the daytime when only 1-2 individuals per cubic meter of water was found. The
area investigated was so barren as a whole only maintaining organisms less than 10
mg/m 3 throughout the investigated period though they increased to about two folds ormore in the night; 1-60 mg/m 3 • Comparing a poor occurrence both in the numberof species and individuals during daytime, most samples in the night showed ratherrich zooplankton communities by C. tonsus of stages V and VI of female, Neocalanus
gracilis, Euphausia lucens, E. recurva, Ostracods, Doliolum sp. Salpa sp. and Sagitta sp.
11-8.

TABLE 11-4. AVERAGED ZOOPLANKTON ABUNDANCE COLLECTED BY UNDERWAY PLANKTON CATCHER IN 15
MINUTES SURFACE TOW DURING DEC. 11~26, 1971.
Organisms

Abundance

Calanus tonsus Copepodite V
Micro-calanoida
Amphipoda or euphausiacea
Doliolum or Sal,pa

0-1 ind./m 30-20 ind./m 3
0-1 ind./m 3
0-0.1 gr/m 8

However, in the region concerned to this study it has been known as relatively
poor in zooplankton population having been reported less than 25 mg/m 3 (Tranter,
1962), and euphausiids also distribute as poor as in the tropical region of the Indian
Ocean (Baker, 1965). The region under consideration can be noted quite characteristic where food rich waters float like a islands or rafts in the ocean which would
possibly give a quite different results in the estimation of plankton biomass by
sampling condition whether the patch or a part of it were collected or not. Baker(1965) stressed on this matter while he was studying on EujJhausia vallentini, one of
the another important whale's food in the Subantarctic region.
The occurrence of C. tonsus patches can be considered to make the region quitecharacteristic as an ecological environment particularly for the predators in higher
trophic levels such as Scomberesox saurus, Gonostomatid fishes (Kubota & Kawamura,1972) in the pelagic waters and basking sharks Cetorhinus maximus in offshore waters(Bradford, 1972).
Sci. Rep. Whales Res. Inst.,

No. 26, 1974

FEEDING HABITS OF SEI WHALE

119

11-9. Some biological characters in the C. tonsus patches
The size composition of C. tonsus population which formed patches is demonstrated in Fig. 11-6 which was obtained by examining 1152 individuals in all.
The size of C. tonsus in stage V copepodites through ten patches varied almost within
the range of 3.2-3.3 mm with an average 3.295mm (N: 1141). There were nostage V copepodites smaller than 2.8 mm and those larger than 3.6 mm. In contrast to dominant occurrence of stage V copepodite (99.2 percent on average) the
adult females occurred but very small numbers, 11 out of 1152 individuals. Theyvaried between 3.5 mm and 4.1 mm with an average 3. 764 mm, and their size
distribution agrees well to that of the populations usually found in 40°S latitude(Brodskii, 1964) where it seems about 3.78 mm as his figures concerned. A fairly
400

300

"'c:

·~
>

0

200

6

z

100

CVI
Female!

... .,.,;'"'"' '°.,.,; .,.,; .,.,;"' .,.,;.,.,; .,.,;
C'J

00

N=ll
0)

0

.,.,;

..;.

..;.

Range of body length (mm)Fig. 11-6.

Variability of the body length of Calanus tonsus which formed patches.

poor occurrence of adult females among patch forming population agrees to theresult on the food analysis in sei whales (Kawamura, 1970a) and it is also similar
to net samples obtained in circumpolar waters (e.g. Brodskii, 1964).
However, the size of C. tonsus by each patches as numbered 1 to 10 in Table
11-5 shows some interesting facts: the patches of Nos. 1 and 2 were consisted ofrelatively small sized individuals of 2.8-3.4 mm with an average 3.18 mm and of
2.9-3.5 mm with an average 3.28 mm respectively. In contrast to this figures,the patches of Nos. 3 to 10 were consisted of more larger individuals with the sizeof 3.1-3.6 mm with an average 3.32 mm. There seems two possible explanations
on the di:ffere nee in size distribution of stage V copepodites of C. tonsus:
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

120

During the austral summer it is known that two generations of C. tonsus instage V copepodites as summer population are present, that is, the individuals ofprevious year origin and that developed in the current season (Jillett, 1968), anddifferences in body length composition in the C. tonsus patches may be due to thisbreeding character. One of the anothers also could be considered. According toBrodskii (1964) there are latitudinal difference in body size of C. tonsus, and larger
individuals are usually found in higher latitudes. So it is possible that the patchesof Nos. 1 and 2 were relatively northern water origin accompanying with warm
water species of zooplankton (see Table 11-5 ). These facts suggests that both thepatches of Nos. 1 and 2 might have been isolated through their development fromothers of Nos. 3 to 10, which were possibly the offsprings from the same initial
population stock.
TABLE 11-5.
No. ofpatches

BIOLOGICAL CHARACTERS OF CALANUS TONSUS PATCHES.

Av. body Av. bodyweight
size
(mm)
(mg)

CVI ~/CV

1.23

1.65

3.18

(%)

Range of body size (mm)
CV

CVI~

2.8-3.4

3.7-3.8

2

3.28

1.45

3.22

2.9-3.6

3.6-3.8

3

3.34

1.24

0.77

3.1-3.6

4.0

4

3.35

1.63

0.87

3.1-3.6

3.8

5

3.35

1.64

16.23

3.0-3.6

3.5

6
7
8

3.28
3.39
3.25

1.96
1.89

0.78

3.0-3.5
3.0-3.7
3.1-3.5

4.1

9

3.30

3.1-3.6

10

3.27

3.1-3.5

Organisms occurredwith C. tonsus(No. of inds./haul)
Vellella vellella (18)Amphipoda (36)Cavolinia sp. (I)Dotiolum sp. (473)
and others*
Thysanoessa sp. ( 1)Amphipoda (2)Thysanoessa sp. (7)& juv. (9)Euphausia sp. juv. (2)Amphipoda (1)Copepoda (3)Amphipoda (1)* Diphyes antarctica (1), Cavolinia sp. (1), Combjelly (1), Cyllopus ?(l), Euphausia sp. juv. (6), Squid (1),and fish larva ( 1).

The geographical location where the patches Nos. 1 and 2 were collected wasin the waters far west from the others, and relative abundance of adult females islarge enough to distinguish them from another patches. The zooplankton species
entangled along with a bulk of C. tonsus seem also to be something different between
the patches Nos. 1-2 and Nos. 3-10. In the Nos. 1-2 patches there were foundsome warm water species such as Vellella vellella, Doliolum sp. and Cavolinia sp. whilethese species were completely absent in the others. Similar facts showing a different
biological characters by patches have been found in the population of Euphausia
sujJerba (Mackintosh and Wheeler, 1929; Marr, 1962), and these facts seem to be
essential characters widely found in zooplankton swarms.
The average body wet weights also varied between 1.23 and 1.96 mg. throughNos. 1 to 10 patches. However, they showed no parallel relationships to the body
Sci. Rep. Whale Res. Inst.,

No. 26, 1974

FEEDING HABITS OF SEI WHALE

121

length as noticed by comparing the patches Nos. 2, 4 and 8. The patch forming
individuals must be different in their physiological conditions even in the samedevelopmental stages.
It could be considered as a whole that the patches of C. tonsus keep their biological characters each other to some extent, and this means that they do not mingle
each other for some long time, and that they are controlled by some biological
functions to swarm or aggregate themselves, if not the patches and their biologicalcharacters could not be kept so distinctly under the continuous disturbance by strong
winds and smashing waves. Marr (1962) mentioned that patches of Euphausia superba
are maintained for long time through their life span, and rarely found mixed
populations with the individuals in different developmental stages among theidentical patches. Hardy (1967) also confirmed the similar evidence as a functionof strong self-swarming behavior by euphausiids. In the neritic waters, the planktonic larvae of Balanus and Mytilus sometimes show a dense patchy distribution, butthey dispersed over soon after as season proceeds (Raymont, 1963). The patchesof euphausiids, or copepods found in the pelagic waters would be considered different in their biological nature such as socialized function through swarming behavior
as Steven (1961) and Clutter (1969) confirmed it in the patches of mysid fromthose swarms by temporal zooplankton species.

11-10. Possible cause of swarming in zoojJlankton
The zooplankton does not evenly distribute throughout its habitat but distribute quite unevenly as has been known by an actual experience in the North
Atlantic cod fishing grounds. The swarms or the patches of zooplankton could
be seen as one of particularly distinct phenomena of spatial distribution, and thosesighted by C. Darwin in the Antarctic region near to Tierra del Fuego might possibly be the plankton patches (Darwin, 1906). The swarming phenomenon inzooplankton has been documented to some extent by Mackintosh and Wheeler(1929) who studied on the food habits in baleen whales, and also by Hardy (1936a,b) in his investigations by the continuous plankton recorder. Hardy (1936a, b)
also made it clear that the uneven distribution of zooplankton is not so special
phenomenon but rather general characters as a whole, which made him criticise
the recognition of even distribution of zooplankton by Hensen (1890). Through his
studies on the uneven distribution in plankton population, Hardy (1936a, b) called" patchy distribution " or " patchiness " of plankton when they swarmed densely
in a spacially limited region. For example, Hardy (1936a) found patchy distribution of SaljJa longicauda when they densely swarmed with more than 30-50% larger
in their population density to that of the back ground waters. The patchinessas one of a distribution patterns in animals, therfore could be considered a kind ofsociality in animals, and Imanishi (1941) called the theme as specific synusia an
ultimate pattern of population life as widely found in the insect communities.
lmanish (1941) considers these monospecific swarms or aggregations in animal community would be somewhat unusual in their daily life, but the patchy distribution
in organisms itself can also be considered rather usual distribution patterns sinceSci. Rep. Whales Res. Inst.,
No. 26, 1974

122

KAWAMURA

it is widely found throughout both animal and plant kingdoms.
One of the distinct examples of plankton patchiness is known in E. superba
and M. norvegica populations, and they swarm such densely as to make the sea
heavily discolored. It was this discoloration of the sea that has made whalers spot
the whales and whaling ground. The E. sujJerba population under patchiness is
exclusively monospecific composition, and it also clearly formed independently by
the individuals in the same developmental stages (Mackintosh, and Wheeler, 1929;
Mackintosh, 1973). Therefore, the zooplankton swarming into patches differs ina sence from the patchy or uneven spatial distribution. The latter would be widely
found in any kind of plankton communities. The E. sujJerba patches are quite important ecologically since they are voraciously fed by the baleen whales while
none of Sagitta spp. and pteropoda under similar patchiness are fed by the whales.
What whales feed exclusively on swarming organisms which form the patches
indicates that patch formation would be a staple distribution pattern through theevolutional ages since baleen whales must have adapted to develop specialized
apparatus, the baleen or whalebone so as to utilize those patch or aggregation forming organisms.
In the northern North Pacific and Bering Sea, Sagitta, Aglantha and Clione
sometimes show a distinctly larger biomass (Hokkaido University, 1962). In thesouthern seas Sagitta spp. also show a patchy distribution (Hardy, 1936a). However, a bulk of occurrence of these organisms is usually accompanied along with manyother plankton species to show a complicated zooplankton communities though
Sagitta occurs with patchiness. More recently, Wiebe (1970) observed on the small
scale spatial distribution of zooplankton in detail and he called "patchy distribution" against the plankton biomass of about 2.6-5.l times larger than the background, while Kuwahara et al. (1971) definited the patchy distribution as withthe biomass larger than the overall median value in a series of samplings. Thesepatchy distribution as definited above however may be similar but different fromso-called plankton patches by aggregating or swarming of organisms in view of
their community composition.
As it has been mentioned the population density in C. tonsus patches was maintained under somewhere between 10 3 and 10 4 inds./m 3 • These figures would belarger, approximately several thousands to several tenth thousands times than theback ground waters, and this somewhat similar to the case in E. sujJerba (Marr,
1962) and M. norvegica (Mackdnald, 1927) where it was larger about 200 to 300
times. Anyhow the zooplankton patches is quite important ecologically in themarine food chains. For instance, a squid, Nototodarus sloani sloani which distributes
in New Zealand waters feeds on the swarms of both Nyctiphanes australis and
P. gaudichaudii, and Todarodes pacijicus also feeds on E. pacijica (Kawakami et al.,
1973; Kawamura, 1974b). These facts suggest an importance of the plankton
patches other than the case in baleen whales since the squids supposedly distributeover the world oceans with great quantity.
For the convenience of further discussion on the patches in C. tonus, their
biologically important characters are given as follows: 1) a horizontal expansion
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

123

of the patches is at least several hundred meters, 2) the shape is usually recognizedwith the scale in horizontal plane, i.e. stream, round or oval shaped, 3) the patches
and its shape firmly maintanined under a rough sea of 8.0- 10. 7 m/sec in wind speed
and 5 class of waves, 4) patches occur most frequently under the temperature of
13.0- 14.0°C, where distinct temperature gradient prevails, especially in the regions
under varying temperature, 5) higher population density of patches is mostly foundin the surface waters shallower than 1.0 m but some are in more deeper waters, 6) the
population density under patchiness would be 10 3 - 104inds./m3 , 7) the patches arecomposed of copepodite V though sometimes a little number of copepodite IV
and adult female are mixed up, 8) biological characters of patch forming C. tonsus
population are maintained for long period, 9) occurrence of patches varies withtime in a day, 10) patches may show a slight diel vertical movements without any
relations to underwater illumination. Among these characters, 3), 7) and 8) strongly
indicate that the patches are neither formed by random variation in the distribution nor physical conditions of the water but purely biological causes of internal behavior to aggregate themselves.
Various discussions have been stated on the possible cause to make zooplanktonswarm so densely, and the discussion could be divided into three opinions: one ofthem discussed the phenomenon as being a subject of zooplankton or organism
itself, so the environment, have somewhat the secondary importance for it (e.g.
Gurney, 1924; Russell, 1927; Hardy and Gunther, 1935; Watermann, 1958;
Colebrook, 1960a, b ), and the other considers an inorganic environments first suchas current, circulation of the water, underwater illumination which directly influences on the formation of swarms (Fraser, 1936; Bogorov, 1938; Beklemishev,1958), and the third is a combinations of the both above mentioned, and Colebrook(1960a, b) considered the surface swarming in Daphnia hyalina would supposedly
caused by whirling movement of water and swarming behavior of Daphnia itself,
whereas Russell (1927) insisted the purely biological factors of zooplankton toaggregate themselves after his observations on decapod larva. " Localize didtribution" in M. norvegica in the North Atlantic must be due to a seasonal change
in an availability of foodstuff along with being chased by predator and with aneffect by tidal currents. However, when we consider the biological characteristicsof the swarming or patchiness in E. sujJerba and C. tonsus, it does not seem to becaused by environmental conditions such as the current, upwelling, pack-ice and its
melted water but caused primarily only by the subject of those organisms, that is,1) a monospecific composition of swarming population is maintained for long period(Mackintosh and Wheeler, 1929; Marr, 1962), 2) each patches are composed ofindividuals in a definite developmental stages but it would possible to occur two
kind of patches each of which composed of different developmental stages in thesame sea region, 3) orientation of E. superba individuals within the patches is almostin the same direction and the patches are quickly reorganized after disturbance(Hardy and Gunther, 1935), and 4) patches are well maintained through even roughweather.
In this study I sighted at many C. tonsus patches even when the surface wasSci. Rep. Whales Res. Inst.,
No. 26, 1974

124

KAWAMURA

very rough under the wave class 5, say, 8.0-10. 7 m/sec in wind speed. According
to Sakamoto (personal comm.) a theoretical current speed which is possible under
the wave height of 1-5 m would be 62.5-140 cm/sec. Under these circumstances
C. tonsus could not maintain those high population density unless they swim actively
againt to the current. According to Marshall and Orr (1955) the swimming speed
in Calanus was 0.42 cm/sec. in long ascent movement, and 1.32 cm/sec. in descent.
Still more most C. tonsus patches occur in the convergence region where the water
sinks to some extent, which would require to make C. tonsus move upward to keep
their position within the surface layer. However, Jillett (1968) stated that thedense population in C. tonsus would be due to an accumulations caused by somecurrent dynamics, and it is in these points that I disagree with his consideration.
If C. tonsus forms dense swarms or patches not by a envirommental conditions
but by its habits of gathering closely each other into an aggregation, there would
be found some physiological and ecological benefits for them as Allee (1938)
and Lorenz (1963) discussed beneficial effects in many kind of aggregating animals.
The population density in C. tonsus patches was approximately 6X10 3 -2X10 4
inds./m3 though the figures as plankton biomass still much less than E. superba.
However, these above mentioned population density would be enough to make theindividual under the patchiness touch each other while they jostling one another,and the explanation on the plankton swarming by some factors such as the structualfeatures in a column of water, photo taxis, sensory for polarized light and grazing
could not possibly keep the individuals so closely for long time. The antennule
in crustaceans functions as a sensory organ (e.g. Dahl et al. 1970a, b), and the roleof the frontal organ in Calanus, and the setae on the antennule themselves would
function similarly. The stimulation induced by physical contact while jostling would
release the behavior of swimming at least toward the surface (Rudjakov, 1970).
Allee (1938), however, considered the absence of any kind of physical contact in
copepod aggregations, but also suggested their social habits.
Apart from that above mentioned, one of another possible cause being proposed
might be a chemosensory receptors furnished on the antennule although any kind of
the receptor had not known yet in Calanus. However, its presence in brackish water
amphipoda, Gammarus duebeni as receptor for sex pheromone (Dahl et al, 1970a, b)
and in decapod crustacean, Portunus sanguinolentus (Ryan, 1966) must function as
chemotaxis of a gathering attractant. The swarming in crustaceans particularly
under monospecific compositions therefore might possibly benefit as the density
or grouping effects in growth speed and shortening the larval period, such as known
in a kind of cockroaches (Ishii, 1970; Chauvin, 1967) and in oligochaetes (Brinkhurst et al, 1972). According to Ishii (1970) the swarming and settling in a nauplii
of Balanus sp. are controlled by chemical factors though only settling factor has been
known to induce their aggregation and simultaneous settling on the bottom. Stimuli
by physical contact through body touch would make copepods ascent toward surface(Rudjakov, 1970), which may explain the occurrence of densely aggregated C.
tonsus population in the very surface waters. However, since the C. tonsus patches
were found not only at the surface but also at several meters depth, some anotherSci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

125

causations to explain these circumstances have to be considered in the future. Bainbridge (1953) indicated the importance of vision and reotactic sence which might
explain the animal aggregations, but it would be still in need of examinations
to appropriate his indications at least on to C. tonsus population and other copepods
since visual sence in these organisms is poorly developed. Learning and practising
behavior found in the fishes may also be difficult to appropriate on to swarmingin zooplankton.
12.

GENERAL DISCUSSION AND CONCLUSION

The behavioral patterns of animals in time and space are so complicated features
as to hardly be understood even their outlines. However, when we consider amovement of animals in view of the population ecology as a whole, they could beapproached to some extent since they were mostly controlled by instinctive behaviorof both breeding and feeding as has been pointed out by Elton (1927), and thewhales do not differ essentially from another mammals but rather simple due totheir seasonal migratory behavior between cold and warm seas. One of the current problems to which the population ecology faces would be to understand thestructure and mechanisms of varying animal populations under the natural andartificial factors which affect on their population along with inter-specific factors.
Food arid feeding habits of animals much influences on the number and size of
population through the marine food web. Elton (1927) suggested an importanceof study on the prey-predator relationships in connection with the animal populations.
In baleen whales a minute crustacean animals had been known early as themain nutritional sources for the whales (e.g. Scoresby, 1820; Collett, 1929). However, it was until recent years that those food habits were considered from the view
points of production ecology in which the secondary producer connected directlywith the last consumer, say, a typically simple but effective shift in energy flows. At
the sametime, the relationships around food would be an important role on controlling the overall population size of the predators such as, say, catch of anchovata and the population of pelican, whale stocks and the abundance of euphausiids(Kawamura, unpublished data). All these are in need of quantitative treatmentin their preypredator relationships. Since the stock of whales is importantamong all, the study on their food and feeding habits have to be proceeded along withthe whales itself.
The sei whale in the Antarctic region distributes along the Subtropical Convergence, and the food organisms in that region are consisted of somewhat simple
zooplankton community although the oceanic conditions differ to some extent between the whaling areas. The important food organisms in the southern sei whaling
ground are: Calanus tonsus, C. simillimus, Parathemisto gaudichaudii, Euphausia vallentini,and E. superba. The shift of whaling ground during a season is mostly to the featuresof the seasonal distribution patterns of food organisms such as disappearance of C.
tonsus from the surface by its depth migration for wintering. In the formation
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

126

KAWAMURA

of whaling ground the patchiness of zooplankton is important among all, since foodorganisms of whales are restricted in patch forming species. As has been mentioned
in the previous sections, the plankton patches as a whole are distinctly characteristic
population in general distribution and its movements being differed from the individuals outside the swarms. For example, patches of E. sujJerba usually shows diel vertical migrations to some extent (Marr, 1962), while many patches occur in the surface
layer throughout the night (Ivanov, 1970). The ecological importance of plankton
patches would be understood when we consider their biomass as shown in C. tonsus
patches where the biomass was 2 x 10 4 -3 x 10 4 mg/m 3 of water while it was only 25
mg/m 3 in the back ground waters. The patches of C. tonsus usually found under
the temperature of 13.0°,.., 14.0°C whereas the species itself is known to distributeunder wide temperature range of 5.0°,.., 15.0°C. (Brodskii, 1964). The whaling
ground formed by C. tonsus therefore could hardly be recongized only by the characters of its general distribution unless the knowledge on the ecological characters of thepatches was accompanied. The movements of whale within the feeding groundwould be well understood by studying the causations which make the food organisms
densely aggregate into the patches.
In the feeding types of baleen whales the filtering apparatus along with its
mechanisms and the feeding ability were considerably different in sei whale fromthose in blue and fin whales. The sei whale can recognized characteristic in thispoint though all these three belong to balaenopteridae. The baleen plates of sei
whale shows rather slim, and its bristles furnished on the inner margin of baleen
plates are fine as comparable with those of balaenid whales. The meshes which
cover the inner surface of filtering apparatus are quite fine with relatively larger
filtering area among balaenopterid whales. In regard to a selective feeding habits
in baleen whale (Nemoto, 1959; 1970), the sei whale prefers copepods at first theneuphausiids or amphipods in the Antarctic waters though the latter two are replaced one to another by the regions concerned (Best, 1967; Kawamura, 1970a). A
preference to copepods in sei whale also found in the North Pacific where C. cristatus
and C. plumchrus are the staple food (Nemoto, 1959; 1957) although they are replaced by fish and euphausiids in more southern waters (Kawamura, 1973a). The
food organisms of sei whale took over by C. finmarchicus in the North Atlantic (Collett, 1886). A boltering behavior is known well in fin and blue whales (Lillie,
1910; Gunther, 1949) but is less distinct in sei whale. These facts along with thefeeding mechanisms, apparatus, and habits lead to a consideration that the sei whale
is more closely related to right and bowhead whales. Nemoto (1959; 1970) however, thought the sei whale belongs to blue whale type feeding apart from right andgrey whales types. But when the above mentioned circumstances on sei whale
feeding are took into account, its feeding type would be rather much similar to thatof the right whale.
The over specialization such as elongated teeth in narwhal, Monodon monoceros
is usually seen to some extent throughout the order cetacea, and the developmentof the baleen plates could be considered as one of over specialization themselves (Howell, 1930). In this respect the gulping or swallowing type in feeding would be
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more general throughout many animals while the skimming type as employed in sei
and balaenid whales would be an over specialization. As I have assessed in theprevious section, the sei whale ·can collect more abundant foodstuff effectively byskimming the food contained water than swallowing, that is, the sei whale seems tohave aquired skimming type feeding as an more advanced habits for collecting thefoodstuff in marine biotope.
Marr (1962), Moiseev (1968) and several other workers assessed the stock sizeof E. superba being based on the consumption of E. superba by whales. Apart fromthe whales, Ryther (1969) also made an estimation of E. superba production fromthe view point of production ecology by supposing the energy coefficient of 10, 15,
and 20% in the shifts of the trophic level, and criticized the above mentioned method
as out of the discussion. Ryther's methodology itself is one of the subject in production ecology. However, it sometimes leads to a confusion or incorrect conclusions
unless the biological characteristics such as the dietary habits of prey and predator
were took into account. For example, E. superba, according to Barkley (1940) andMarr (1962) preferably feeds on diatoms, Fragilariopsis antarctica, and as a whole itseems to avoid larger or longer sized diatom and spinous diatomspecies. E. superba
likely to strongly avoids the water dominated by Thalssiothrix antarctica (Kawamura,
1973b; 1974). However, the biological aspects of feeding habits such as foodselection are omitted in the method by productivity which involves all of the organiccarbon produced. Hasle (1969) reported abundant distribution of Nitzschia spp.,
Fragilariopsis and Chaetoceros diatoms in the Pacific sector of the Antarctic but they
are supposedly less important as food of E. superba. This fact supports well a poor
distribution of E. superba in that region (Mackintosh, 1973). Marr (1962) considered the gaps or voids in phytoplankton abundance between the waters withrich E. superba patches and those separating the patches would be due to grazing.
The animals which distribute dominantly with the bulk of biomass and usuallyin higher trophic levels likely to prey on zooplankton species particularly thoseaggregate into the patches. These animals, for instance, are anchovata, gonostomatid fishes such as Viciguerria attenuata, Scomberesox saurus, squids, seabirds, crabeater
seal and whales. The role of those animals in the Antarctic ecosystem would be
distinct than any other kind of organisms, but none of them studied yet except
whales, and therefore the knowledge of food habits in those animals is considered
to be strongly in need of further investigations. However, it has been known thatthe southern blue, humpback, minke and fin whales feed almost exclusively on E.
superba though the fin whale utilizes copepods to some extent whereas the sei
and right whales feed copepods preferably, though the latter feeds much on euphausiids other than E. superba and amphipods, P. gaudichaudii. When we take
into account the ecological niche concerning to those prey organisms in quantitative and qualitative respects, it would be possible to suppose an outline of thestructure of the ecosystem in the southern ocean: the organic production in the
Antarctic regions proceeds as a whole under relatively lower trophic levels. Accordingly, the role of zooplankton in the southern ecosystem would be hardly comparable importance than that of any other places, and it seem to be almost imSci. Rep. Whales Res. Inst.,.No. 26, 1974

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possible to understand the ecology such as the movements and the inter specificrelationships among those important predators without studying on the swarming
habits of their prey organisms.
The swarming habits in zooplankton however have been poorly studied particularly in the biological and ecological knowledge except several observations as
an unusual distribution phenomena (e.g. Hardy and Gunther, 1935; Mackintosh,1967). As it has been stated in the previous section, E. superba and P. gaudichaudii
occur in patchiness which would be formed by the individuals of almost every developmental stages. On the other hand the patches in C. tonsus, E. vallentini, and supposedly in many another kind of copepods and euphausiids are formed by the individuals of some particular developmental stages such as the copepodite V in C.
tonsus. The swarming into patches in E. pacijica (Komaki and Matsue, 1958;
Komaki, 1967), The. inermis and 111. norvegica (Bigelow, 1924; Macdnald, 1927)
would be similar to the case in C. tonsus and E. vallentini.
Marr (1962) discussed on the possible cause which make E. superba aggregate
so densely but he did not figure out decisive conclusions after examining particularly
the effects by animal exclusion and the pack ice, and stated that the patchinessseems to spring from the lifelong habit of this species.
In the patchiness of C. tonsus, I have supposed a kind of density effects which
would benefit on C. tonsus not as an individual but a population to be kept staple,
since as mentioned before the aggregating or swarming phenomenon would be thesubject of ethological habits in organism itself. However, the surface swarms ofGalatheid shrimp, Munida gregaria seems to occur under some particular environments such as the depth or the front like structure of the waters (Tabeta and Kanamaru, 1970). Pleuroncodes planipes also swarms in the upwelling region accompanying the oxygen minimum layer (Anonymous, 1971). These facts however would
possibly be a phenomena which seem as if it were the causation for aggregationor swarms, that is, a kind of pseudo-factor, and the patchiness of zooplankton would
be induced purely by biological but still unknown agencies. Komaki (1967) and
Kawamura (197lc) have pointed out that the development or occurrence of socalled patches by swarms would hardly be forecasted by the figures of ordinal
physico-chemical environmental factors, that is, the swarming of zooplankton differs
essentially from the red tides.
Mackintosh (1967) stated on the behavior of E. superba patches that eachswarming individuals showed a synclonized swimming as if there were some leader
particularly in command of the whole. Their behavior as mentioned above
would be similar to " following reaction " in fish schools, which was proposed by
Crook (1961). In the school of scombid fishes, the distance separating each individuals and the movements of the tail flukes are the important factors to keep
the school tightly (Olst and Hunter, 1970). They (Olst and Hunter, 1970) also
found that the school was specially well kept tightly when the distance between
each fishes were one half of their body length. The population density in thepatches of E. superba and E. paci.fica show one individual per cubic inch or thereabouts(Marr, 1962; Komaki, 1967), and these facts make us suppose that the swarming
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in euphausiids would be similarly understood though the distance between theindividuals within the patches must be more important in euphausiids than the fishes
since visual sence is poorly developed in the former.
In the population of insects the density effects usually function in controlling
the population size (Ito and Kiriya, 1971), but it is also known in some insect
species that the density effects would be benefitial for promoting the relative growthrate by shortening the intervals between individuals, in which the density effects
make each individuals shorten their larval period being induced by the promoted
secretion of the moulting hormone possibly induced by tactile stimuli (Ishii, 1970).
The density effects are particularly more strongly functional in the earlier stages of
insect species (Chauvin, 1967). This fact indicates something analogy with thecase in swarming C. tonsus population composed of the copepodites IV and V, andalso the case in some temporal plankton species.
On the study of zooplankton patchiness, the followings are particularly hoped
to be known in the future: the geographical and the vertical distributions, theseasonal change, change within short time period, population density and its
structure, and the causations which trigger the behavior of swarming. In thisstudy a small underwater plankton sampler (Motoda, 1959) was used in collecting
C. tonsus patches, but the sampling gear seemed to be insufficient for the purposeof quantitative study since population density varied distinctly even within a patch.
In this respect the continuous plankton recorder which revealed clearly the uneven
distribution of plankton (Hardy, 1936a, b) must be revaluated. Wiborg (1971),
on the other hand, found the population density of 4000 inds./1000 m 3 in Th. inermis
and 5000 inds./m 3 in Th. raschii by towing the IKMT, and he (Wiborg, 1971)
considered these high population density would be due to their patchiness. Omori(1965) and his colleagues (Omori et al., 1965; Kawaguchi and Mammo, 1967;
Kawaguchi, 1973) have successively used the large sized ring net of 160 cm mouth
opening for collecting the micronektons, and proved that the net was considerably
effective for patchily distributed organisms such as Sergestid shrimps (Omori,1969; Omori et al., 1972). These facts suggest that the larger sized towing net
in its mouth opening would be more successive for collecting the fast moving orvery locally swarming organisms.
SUMMARY

1. The food and feeding habits in southern sei whale, Balaenoptera borealis
Lesson, which was caught during 1967/68 to 1971/72 seasons by the Japanese
floating factories were studied. The observations were chiefly focussed on the distributions of prey organisms by species through which the formation of whaling(feeding) ground was considered biologically. One of another focusses was to figureout a comparative structural characteristics of the feeding apparatus along withthe dietary habits in sei whale in connection with the patchiness of prey organisms.
2. The sei whaling ground in recent operations is found in the mid latitudesof the circumpolar seas, say, the Subantarctic waters of 40° ....... 50°S, where more
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than 70 %of sei whale have been caught among all. The formation of sei whaling
ground is closely related to the Subtropical Convergence, and the most important
areas are the Indian and the western part of the Pacific sectors between 0° and
170°W.
3. The general hydrological conditions of the whaling ground was as follows:
the favourable surface sea temperature was somewhere between 14.0°C and 18.0°C
during December though it was 8.0°C- 10.0°C by some whaling areas. As season
proceeds the whaling ground shifts to higher latitudes where the temperature was
14.0°-16.0°C in January, 7.0°-10.0°c in February and then 4.0°C-7.0°C in
March. The isotherm recede toward north with the speed of about l .0°Cf l 0
lat./month in the region concerned.
4. A total of 23 species or more of food organisms were found in the stomach
contents of sei whale, and the following 8 species were most important components:
Calanus tonsus, C. simillimus, Clausocalanus laticeps, Drepanopus pectinatus, Parathemisto
gaudichaudii, Euphausia vallentini, E. lucens, and E. superba. There were 7 species offood organisms, C. tonsus, Cl. laticeps, Thysanopoda actifrons, E. diomedeae, E.
lucens, Penaeus sp. and Scomberesox saurus which were newly known in this study as thesei whales food in the Subantarctic feeding ground though both E. diomedeae and E.
lucens had been known as sei whales food in the South African waters.
5. The composition of the stomach contents was monospecific in more than
72 % of the animals examined, and those of mixture with two or more species wererarely found. It was indicated from this fact that sei whale feed on the prey which
had so densely swarmed as to form the patchiness. Doliolum, Salpa, pteropoda andchaetognatha distribute abundantly in the region concerned, but none of themwere the important components of the stomach contents. A selective feeding habits
by species and size of the prey organisms were also observed in sei whale as has
been known in fin whale. The sei whale feeds preferably on crustaceans than any
other kind of foodstuff, and copepods were the most important among all. C. tonsus
of copepodite V stage was the main food species in the southern waters.
6. The distribution of the food organisms changed latitudinally: C. tonsus,
C. simillimus and Clausocalanus laticeps were the main composition of the stomach
contents throughout the zone between the Subtropical Convergence and about
4 7°S, although they were took over by E. vallentini or P. gaudichaudii by the sea regions, and then by P. gaudichaudii almost exclusively between 50°S and the Antarctic
Convergence. It was only E. superba that occurred in the stomach in the southern
most whaling ground close to the Antarctica. These geographical change in thecomposition of stomach contents was a phenomenon which occurred within a narrow zone of 20 degrees in latitudes.
7. In the feeding percentages in terms of the number of animals with foodagainst those examined, 28-59 % of animals in the Indian sector had carried thestomach contents whereas it was 25--82 % in the Pacific sector, and the availabilityof foodstuff would be considered higher in the Pacific. The scheme above mentionedby species was also varied with the whaling areas, that is, both C. tonsus and E. vallentini were dominant in the Pacific sector, while P. gaudichaudii and E. superba were
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important food in the Indian sector and in the Tasman Sea area. However,preference to the kind of foodstuff in sei whale was focussed first on copepods and,euphausiids and amphipods were secondarily important. The feeding percentage
by the species and seasons revealed that there might be both copepod rich year,
and euphausiid or/and amphipod rich year.
8. Sighting investigations revealed that both baleen and toothed whales seem
to take food even in the tropical seas, especially where upwelling along with abundant distribution of zooplankton prevails. The region of a relatively dense distribution of whales was indicated particularly by the distribution features of salinity andphosphate.
9. The sei whaling ground agrees on the whole with the abundant distribution of their food organisms particularly of C. tonsus and C. simillimus. Apart fromthis however, there was a local but abundant distribution of food organisms such
as D. pectinatus and Cl. laticeps, and the whaling ground formed by these foodstuff
hardly be spotted since there was no effective ways left behind to know very local
occurrence of the plankton patchiness by plankton net haul.
10. C. tonsus population is composed of copepodite stages IV, V, and adultfemale, but the population is represented by copepodite V since it occupied 89. 7 %on
average in overall compositions in individual numbers. There were two size
compositions of the copepodite V, say, 3.0-3.2 mm and 3.5-3.6 mm by the whaling
areas, and they were supposed to be the difference due to isolation by breeding
stock, and also the difference in generations.
11. The feeding ability in sei whale is a function of the mandible size which
grows proportionaly with the body length. However, the filtering structure infeeding mechanism is the row of baleen plates which furnished on the upper jaw withthe average density of 1.4 plates/cm near the rostrum and 0.93 plates/cm at the
distance of 66.4-69.5 % posterior from the rostrum where the baleen plates attainat their maximum length. The length of the bristles furnished on the inner marginof the baleen plates varied 3,...,5 cm with the averaged density of 45.5 bristles/cm
regardless to sex and body legth.
12. The total area of the filtering apparatus in sei whale was 2.37 m 2 in
11.0 m long animal, and 3.59 m 2 in 14.4 m animal. None of sexual variations
were observed. The area of filtering apparatus against the body length showedcomparatively larger figures in sei whale than the another balanopterid whales
though it was still smaller than that in balaenid whales.
13. The calculated porosity of the filtering apparatus varied between 77.2 %and 88.6 %. Comparing with the figure of 46 % in GG54 bolting silk cloth (0.33mm mesh apertures), the netting in sei whale was considerably coarsely structured
but its relative filtering ability was 1/8 of the GG 54 and 1/5 of the fin whales.
14. The number of animals which vomited the stomach contents duringchasing against the total animal observed were 6/642, 1/129, and 21/125 in theorder of sei, fin, and sperm whales respectively. This fact makes us confirm thatthe amount of stomach contents found in carcasses shows an actual figures at the
moment of animals being killed.
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15. By examining the fulness of stomach contents, it was known that 25-30 %of the stomach contents in volume reduces within the following 5-6 hours supposedly by digestion. This fact leads to tt consideration that the stomach of sei
whale would be repleted fully once in a day.
16. The amount of stomach contents which were actually found in carcasses
was 150-200 kg, 500-600 kg, and 30 kg in the order of sei, fin, and minke
whales respectively. These amount of food however, seems to be far insufficientas the daily rations of the whales. When C. tonsus was fed by sei whale its amountusually found in the stomach does not exceed more than 50-60 kg. This would
be due to a poorer availability as biomass in C. tonsus even when they form thedensely populated swarms than the case in larger sized prey organisms.
17. Feeding activity in sei whale as indicated by the feeding percentages
was high in the morning, but in some case it was unchanged throughout the day.
This suggests that many sei whales feed much once in a day, but they have to preyon continuously throughout the day to fill their daily rations particularly when
they feed on copepod. The habits of the diel vertical migration in prey organisms
also affect on the whale's feeding activity.
18. After the formulations by Sergeant (1969), the theoretical daily rationsin 3 species of the baleen whales were calculated as 4.4 %, 4.07 %, and 3.84 % ofbody weight in the order of sei, fin, and minke whales respectively.
19. The population density in C. tonsus patches was assessed being basedon the filtering ability, feeding habits, and the amount to stomach contents; andfound that C. tonsus swarms into patches with the population density of 103 -10 4
inds/m 3 • In these calculation it was also indicated that the sei whale can collectthe foodstuff more effectively by employing skimming type feeding than the caseby swallowing type.
20. A total of 220 patches of C. tonsus was investigated, and the following
evidences were found: 1) the patches showed spindle-, oval-, and stripe- or streamshape, of which the former two occurred most frequently, 2) the size was usuallysomewhere between hundred and several hundreds meters in their longer axis
though sometimes it was several miles long in stream-shaped patches, 3) the patchis maintained tightly for long period through rough weather, and each patches
are so completely isolated as to keep the overall biological characters as aggregatedpopulations, 4) the patches occurred most frequently along the Subtropical Convergence where the sea temperature was 13.0°-l4.0°C, 5) copepodite V of C. tonsus
population forms the main body of the patches with slight numbers of adultfemale, 6) the observed population density in collected samples varied between
330 inds./m 3 and 2.4x 104inds./m3 (=3X10 4 mg/m 3 ). These figures agreed withthose assessed theoretically, 7) possible causations of swarming behavior would
be the density effects which supposedly benefits to growing larger the population
as a functions of stimulation by physical contact of one to another, and then, 8)
the formation of patchiness would hardly be explained by the hydrodynamical
forces because what makes the organisms swarm or aggregate so densely would
be, after Cassie (1960), a internal behavior patterns between individuals of theSci. Rep. Whales Res. Inst.,.No. 26, 1974

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same species through a social habits, that is, the subject of organisms themselves.
ACKNOWLEDGMENTS

During the course of this study Prof. Teruyoshi Kawamura of the Faculty ofFisheries, Hokkaido University gave me an encouragement and valuable criticisms,
to whom I would like express my particular appreciation. I am also greatly
indebted to Prof. Tokimi Tsujita, Prof. Shun Okada, Associate Prof. Takashi Minoda, and Dr. Akira Taniguchi of the same university who have read and given
much instructive advice and comment on the original manuscript.
I wish to express my gratitude to Dr. Hideo Omura, Director of the Whales
Research Institute, who not only admitted my long research cruises down to the
Antarctic in 1967/68, 1968/69, and 1971/72 seasons, but also gave me well-timed
convenience and opportunity throughout the study. My colleagues, Mr. Saburo
Machida and Miss Yuko Satake of the same institute also assisted my work andgave the facilities.
Thanks are also due to Mr. Yoshio Takato of the Fishery Agency, Ministoryof Agriculture and Forestory at that time who admitted my participation to the
Antarctic whaling operations as a government inspector in 1967/68 season, throughwhich I started the present study, and Messers. Mitsuaki Kosaka and Kineo Kegasawa of the same agency who were on boad of the floating factories in 1967 /68 and1971/72 respectively gave me a helpful convenience for my work on the ship's deck.
One of my sources in obtaining the samples of the whales food was due to the kindarrangements of Dr. Seiji Ohsumi and his colleagues, Messers. Yasuaki Masaki
and Shiro Wada of the Far Seas Fishery Research Laboratory, Fishery Agency in
Shimizu, to whom I would like to extend my appreciation. In connection with
this, the collection of food samples in the field was made fruitfull by the kind arrangements and co-operations of the following companies and their personnel
concerned: Taiyo Gyogyo K.K., Nippon Suisan K.K., and K.K., Kyokuyo.
My participation to the R. V. "HakuhO Maru" Cruise (KH-68-4) to the
Antarctic was due to kind arrangements by Prof. Masaharu Nishiwaki and Prof.
Yoshio Horibe of the Ocean Research Institute, University of Tokyo, and Mr.
Kazuo Kureha of the same institute greatly assisted me in whale sighting andplankton sampling during the cruise. Without their helpful arrangements andco-operations I could not discuss on the population density of food zooplankton
and the formation of whaling ground. I am also greatly indebted to Mr. Ken
Ohturu, Capt. Teruhiko Furuno of the Nippon Suisan K.K., and Capt. Yuzo
Masumoto of the Hokoku Suisan K.K. who gave me an opportunity and convenience for investigating the disbribution of food organisms in the southern whaling ground in 1971/72 season.
One of my particular thanks is due to Prof. Toshiro Kuroki, Drs. Kouichi
Kawaguchi and Wataru Sakamoto of the Ocean Research Institute, University ofTokyo, who gave me many valuable advices and co-operated in constructing the apparatus for experimenting the filtering ability of a row of baleen plates. The exSci. Rep. Whales Res. Inst.,
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periment in the field on boad of the R. V. "Tansei Maru" would have been impossible without judicious arrangements by Prof. Ryuso Marumo, Assoc. prof. Takahisa Nemoto and Dr. Makoto Omori of the Ocean Research Institute, Universityof Tokyo. Discussions exchanged at times with Drs. Keiji Nasu and Seiji Ohsumiof the Far Seas Fishery Research Laboratory, particularly on the formation of
whaling ground and feeding habits of whales were much instructive throughout
this study.
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Note:

References written in Japanese are idicated by parentheses.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

FEEDING HABITS OF SEI WHALE

APPENDIX.

Species
No.
of
whale
sei
sei
sei
sei
sei
sei
sei
sei
sei
sei
sei
sei
sei
sei
fin
-fin
fin
fin
fin
minke

2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

143

AMOUNT OF FOOD FOUND IN THE FIRST STOMACH OF FIN, SEI AND
MINKE WHALES CAUGHT IN THE SOUTHERN OCEAN.

Sex

Body
length(m)

Kind ofstomachcontents

Date
of
catch

Time
of
catch
(SMT)

Net
weight
of
food
(kg)

Water
(kg)

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

14.4
14.1
14.2
14.6
15.2
15 .1
14.7
15.1
13.0
13.2
14.3
14.7
15.6
16.2
18.6
21.0
20.2
21.1
22.1
8.0

Calanus
Amphipoda
Amphipoda
Amphipoda
Amphipoda
Amphipoda
Eu-L
Eu-S
Eu-L
Eu-S
Eu-L
Eu-S
Eu-L
Eu-L,M,S
Eu-L
Eu-L
Eu-L
Eu-L
Eu-L
Eu-L

28/XII/67
6/1/6815/I/68
30/XIl/67
7/1/6815/1/68
21/Il/68
13/1/68
23/II/68
12/I/68
22/11/68
12/1/68
23/11/68
18/1/68
21/1/68
2/111/68
21/Il/68
20/Il/68
21/1/68
21/1/68

1115
0600
0800
1105
0925
1455
1050
1345
0830
1940
0925
1745
0540
0855
1015
0820
0550
0640
1115
1805

54.0
136.5
73.5
98.0
200.0
57.6
142.0
149.6
149.0
135.6
128.7
79.1
148.0
159.0
273.7
785.0
556.5
286.0
127.4
22.4

14.0
18.1
14.5
30.0
28.0
13.8
0.0
28.6
33.0
24.4
12.6
15.4
4.0
37.5
27.6
100.0
10.5
28.0
5.6
4.0

Total
weight of Total
stomach volume
contents
(1)
(kg)
68.0
154.6
88.0
128.0
228.0
71.4
142.0
178.2
182.0
160.0
141.3
94.5
152.0
196.5
301.3
885.0
567.0
314.0
133.0
26.4

66 1 >
171 2 >
109 3 >
1560
312
93.6
200
2204)
200
204 4l
1805)
119
200
255
391 5)
!OOO
700
4005)
238
31.2

Calanus : Calanus tonsus, Amphipoda : Parathemisto gaudichaudii, Eu-L or -M: Euphausia supera, Eu-S :Euphausia vallentini.

1) Approximate values, 2) Estimated by the stomach volume averaged on 13 whales, 3) Calculatedby the mean weight of a unit volume of amphipod food from four animals, 4) Calculated by the mean
weight of a unit volume of small-sized euphausiids taken by #12 animal, and 5) Calculated by themean weight of a unit volume of large-sized euphausiids taken by six fin whales and one minke
whale.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

KAWAMURA

144

EXPLANATION OF PLATES.
PLATE I
Fig. 1. Inside view of a row of baleen plates in sei whale, female, 14.3 m.
Fig. 2. Same as Fig. 1, but viewed from outside.
Fig. 3. Enlarged view of a row of baleen plates showing the intervals of the plates
neibouring with each other.
Fig. 4. Bristles furnished along the inner margin of baleen plate.
PLATE II
Fig. 1.
Fig. 2.

Bristles at the tip of baleen plate.
Enlarged view of the filtering meshes formed by the baleen bristles.
PLATE III

Fig. 1. An apparatus for measuring the filtering ability of a row of baleen platesof the sei whale shown in PLATE I.
Fig. 2. Same as Fig. 1, but small fraction of a row of baleen plates is attached on
the apparatus.
Fig. 3. Frontal view of the apparatus. Bridles and a flow-meter mounted on
the apparatus are shown.
Fig. 4. Flow-meter and the baleen plates attached on the bottom of the apparatus
are shown.

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

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