SC/J09/JR13

Relationship between body size, maturity, and feeding habit ofcommon minke whales off Kushiro in autumn season, from2002-2007 whale sampling surveys under the JARPN II coastal
components off Kushiro
TOSHIYA KISHIRO1, HIDEYOSHI YOSHIDA1, TSUTOMU TAMURA2, KENJI KONISHI2,NAOHISA KANDA2, RYOSUKE OKAMOTO3 AND HIDEHIRO KATO3
Contact e-mail: kishiro@affrc.go.jp
1

National Research Institute of Far Seas Fisheries, 2-12-4, Fukuura, Kanazawa, Yokohama Kanazawa
236-8648, Japan

2

The Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan.

3

Tokyo University of Marine Science and Technology, 1-5-7, Konan, Minato-ku, Tokyo, 108-0075 Japan

ABSTRACTThe relationship between body size, maturity and feeding habit of common minke whales in the coastal waters off
Kushiro, northeast Japan, was examined using data obtained from the whale sampling surveys under the JARPN II
coastal component off Kushiro conducted in 2002 to 2007. The surveys were carried out in the autumn season(September and October). A total of 254 common minke whales (182 males and 72 females) were sampled. Ratio of
sexually mature individuals was 0.58 for males and 0.15 for females, respectively. Immature whales were abundantly
distributed near the coast or the continental shelf region whilst mature males were distributed in a wider area
including the offshore waters. In the continental shelf and slope regions with water depths of less than 1,000m,
occurrences of prey species found in stomachs differed significantly with maturity stage of the whales (p<0.001).
Smaller and immature whales tend to take walleye pollock and krill whilst larger and mature whales tend to take
Pacific saury. Japanese flying squid was consumed by only mature whales. Japanese anchovy was evenly consumed
by both immature and mature whales. For the coastal waters off Kushiro in the autumn season, the results suggest that
migration and prey preference of common minke whales differed with maturity stage and that on the continental shelfand slope regions immature whales preferred to take walleye pollock and krill than mature whales. These results
implied that migration and feeding strategies of common minke whales might change with growth whereby immature
whales take more stable preys and mature whales seek nutritionally more valuable preys. It was thought to beimportant to take account of this point in estimating the local prey consumptions and constructing the ecosystemmodels for restricted coastal small area, especially in the coastal waters off Kushiro.

KEYWORDS: COMMON MINKE WHALE; NORTH PACIFIC; COASTAL WATERS OF JAPAN; SCIENTIFICPERMITS; PREY PREFERENCE; MATURITY; FEEDING HABIT

1

INTRODUCTION
Common minke whale Balaenoptela acutorostrata, is known to be widely distributed throughout theoffshore to the coastal waters in the western North Pacific and feed on various prey species includingcommercially important species such as Japanese anchovy Engraulis japonicus, Pacific saury
Cololabis saira, and walleye pollock Theragra chalcogramma. (Tamura and Fujise, 2002). The
coastal waters off Kushiro, southeastern part of the Pacific coast of Hokkaido, northern Japan, is oneof the major migrating grounds of common minke whales in autumn season (Hatanaka andMiyashita, 1997), and also important fishing grounds. The possible competitions between the whales
and the fisheries has been suggested in those areas (Tamura and Fujise, 2002), and development of
new management measures such as a multi stock management based on the ecosystem models has
been required (Government of Japan, 2002a).
Under these situations, the whale sampling surveys in the coastal components off Kushiro wasstarted in 2002 (Kishiro, et. al., 2003) and carried out in every year from 2004 to 2007 (Kishiro,
et.al., 2005; 2006; 2008, Yoshida, et. al., 2007), under the full JARPN II research plan (Governmentof Japan, 2002b; 2004). Background and details of the surveys were summarized in Kishiro, et. al.
2009. The objectives of the JARPN II are: i) feeding ecology and ecosystem studies, involving prey
consumption by cetaceans, prey preferences of cetaceans and ecosystem modeling, ii) monitoring
environmental pollutants in cetaceans and marine ecosystem, and iii) stock structure of whales.
Among them, prey preference of cetaceans is one of the important factors in considering the feeding
ecology and the ecosystem modeling.
Based on the results of the 2007 survey off Kushiro, Kishiro, et. al. 2008 suggested possibility of thepresence of difference in prey preference within a species of common minke whale as larger andmature whales tend to take Pacific saury whilst immature whales mainly took walleye Pollock. Such
difference might affect not only modeling but also estimating the prey consumption of the whales in
the local area, such as the coastal waters off Kushiro. However, sample size was small and further
analysis was needed. The present study examined these aspects further, using data of whole
2002-2007 whale sampling surveys off Kushiro.
MATERIAL AND METHODS
The present study used data and samples obtained from the whale sampling surveys in the
2002-2007 coastal components off Kushiro, under the JARPN II. These surveys were carried out inautumn season (September to October) in the coastal waters within a maximum of 50 nautical miles
radius from the Kushiro port (Fig.1), and a total of 254 common minke whales were sampled during
the period from 2002 to 2007. Distribution of sampled whales is shown in Fig.2. Methodology and2details of these surveys were described in Kishiro, et.al., 2009.
Body length and maturity of common minke whales
Body length of all sampled common minke whales were measured to the nearest 1cm on a straightand parallel to the body axis from the tip of upper jaw to the notch of flukes by researches at the
research land station. Each side of testis was weighed after separated from the epididymis andhistological samples were collected from the center of the right testis. Male with a single testis
weight of 290g or more was tentatively determined as sexually mature based on the histologicalobservation of the seminiferous tubules, spermatocyte, spermatid or spermatozoa using hematoxylin
and eosin stained sections. In females, both side of ovaries were collected and examined. Female had
at least one corpus luteum or albicans in their ovaries was determined as sexually mature.
Prey species in the stomach
At the research land station, stomach contents of the whales were weighted to the nearest 0.1 Kg byeach four chamber in both cases of including and excluding liquid. Prey species was tentatively
recorded using morphological characters. Then, a sub-sample (1-5Kg) of forestomach contents wascollected and frozen for the laboratory works. In the laboratory, detailed species identification andrestoring stomach contents weight were made by Tamura, et al., 2009 as follows. Prey species was
identified to the lowest taxonomic level as possible using morphological characters for undigested
preys or the otoliths and jaw plate for digested preys. The total wet weight of each prey species in
the forestomach was estimated by multiplying average weight of fresh prey specimens by the
number of preys (including undigested full bodies, undigested skulls, and half the total number offree otoliths) in the sub-sample and multiplying ratio of the weight of sub-sample for the total weightof forestomach contents. Details of the methods were described in Tamura, et.al., 2009. Occurrenceof the prey species was calculated as the ratio of the number of whales counted by the prey species
found in their forestomach for the total number of whales counted. When plural prey species were
found simultaneously in a stomach, the number of whales was counted as one for every prey species.
Mean composition of the restored wet weight by prey species was calculated as an average of theratio of restored weights by prey species in respective whales.
Individual identification of the stocks of common minke whales
Skin samples of all whales were collected and preserved in ethanol for the laboratory DNA analysis.
In this study, results of the individual identification of the stocks (J or O stocks) made by Kanda etal., 2009 by the microsatellite analysis were used for the comparison of the features of maturity andfeeding habit between J and O stock individuals. Details of the genetic identification methods were
described in Kanda, et.al., 2009.

3

RESULTS
Sex ratio, body length and maturity of common minke whales
A total of 254 sampled whales were consisted of 182 males and 72 females (sex ratio of males was0.72). Statistics and frequency of body length are shown in Table 1 and Fig. 3. Average body length
in respective year ranged from 6.60m to 6.87m for males, and 5.48m to 6.39m for females,
respectively. Males with body length of 7 to 7.5m were most frequent, and distinct yearly change
was not observed. Compositions of sex and sexual maturity of the whales is listed in Table 2. Sex
ratio (ratio of males) in respective year ranged from 0.64 to 0.80. In males, ratio of sexually matureindividuals ranged from 0.45 to 0.68 (total of all years: 0.58). Five males could not be determined
their maturity status due to the anomaly in both side of the testis. In females, only 11 individuals or
15% of total 72 females were sexually mature, which consisted of 4 pregnant and 7 resting
individuals. Fig. 4 shows geographical distribution of the whales by sex and maturity status, and Fig.
5 indicates sex ratio and maturity ratio by 10 minutes square latitude-longitude in the research area.
Both sex and maturity ratios of males were lower in near shore area compared with those in thewaters far from the shore. These figures suggested that immature whales tend to be distributed near
the coast while mature males were mainly distributed in the offshore or waters far from the shore.
Prey species in the stomach
Five prey species, walleye pollock, krill Euphasusiiids, Japanese anchovy, Pacific saury, andJapanese flying squid Todarodes pacificus (hereafter referred as common squid) were found in thestomach as the major preys in both occurrence and the restored wet weighs (Table 3 and 4). ‘Others’
consisted of Japanese pomfret, Brama japonica, unidentified Salmonidae, unidentified fish, andunidentified squid, but all of those ‘Others’ species were found only one individual simultaneouslywith major prey species such as Japanese anchovy and Krill in the stomach, thus they were thought
to be incidentally consumed by the whales. Most dominant prey species was Japanese anchovy in theoccurrence (46.2%), and walleye pollock in the weight (38.1%).
Relationship between body lengths of the whales with prey species in the stomach
Fig.6 shows relationship between body lengths of the whales with the occurrence of prey species intheir stomach. In both males and females, the occurrence of walleye pollock was high in small sizedwhales, and decreased with increasing the body size. The occurrence of krill was also decreased withbody length. On the other hand, the occurrence of Pacific saury was increased with the body length.
Common squid was only found in the whales with body length of 6.5m or more.
Relationship between whales’ maturity with prey species in the stomach
Comparison of the occurrence of prey species between immature and mature whales was made in4

Table 5. Most dominant prey species was Japanese anchovy in regardless of the maturity, but in bothmales and females, the occurrence of walleye pollock and krill in immature whales was higher thanthat of mature whales while the occurrence of Pacific saury in immature whales was lower than thatof mature whales. Common squid was only found in mature whales. Table 6 shows the comparisonof the mean compositions of prey species between immature and mature whales based on therestored wet weight of their stomach contents. Result was almost same with Table 5, i.e. ratio of theweights of walleye pollock and krill in immature whales was higher than that of mature whales, andratio of the weight of Pacific saury in immature whales was lower than that of mature whales.
Maturity ratio of the whales by their prey species is shown in Table 7. In both males and females,maturity ratio of the whales fed on walleye pollock and krill were lower than that of the whales fed
on Pacific saury, and highest value (1.0) was in the whales fed on common squid. Maturity ratio ofJapanese anchovy was located in between that of walleye pollock and Pacific saury. Fig.7 shows
frequency distributions of body length and maturity of the whales by their prey species found in thestomach. Although some variations were observed in respective year due to the small sample size, in
generally, frequency of the whales fed on walleye pollock and krill were higher in smaller andimmature whales while those of Pacific saury and common squid were higher in larger and mature
whales.
Comparison between J and O stocks
Around the coastal waters off Japan including the research area in this study, two stocks (J stock:
mainly distributed in the Sea of Japan; O stock: mainly distributed in the western North Pacific) ofcommon minke whales are known to be existed, and genetically identified by Kanda, et. al., 2009. If
the feeding habit were clearly different between stocks, results of this study were possibly affectedby the difference of stocks in the samples. Therefore, samples were divided to respective stock using
genetic identification results made by Kanda, et. al., 2009, and the comparisons between stocks were
made. The number of identified whales was 38 for J stock and 190 for O stock. Table 8 shows thecompositions of sex and sexual maturity of the whales by the stocks. Ratio of males in samples wasalmost same between J stock (0.76) and O stocks (0.70) but maturity ratio of males in J stock (0.39)
was lower than that of O stock (0.65). Comparisons of the occurrence and the ratio of restored
weight of prey species between stocks and maturity status are shown in Table 9 and 10. Maturity
ratio of the whales by their prey species is shown in Table 11. Both J and O stocks (except for
females of J stock which sample size was too small), same results were obtained i.e., ratio of walleye
pollock and krill in immature whales was higher than that of mature whales while ratio of Pacificsaury in immature whales was lower than that of mature whales. These results suggested that there
was no difference between the stocks.

5Geographic comparisons of the preys in the research area
Distributions of the whales by their prey species found in the stomach are shown in Fig. 8. Although
the positions of the whales were somewhat dispersed, the whales fed on walleye pollock andcommon squids were relatively concentrated in the continental shelf and slope regions of the water
depth shallower than 1,000m. On the other hand, distributions of the whales fed on Japanese
anchovy and Pacific saury were more spread, and widely distributed in the offshore area. Walleye
pollock and common squids are known to be demersal or mesopelagic species while Japanese
anchovy and Pacific saury are pelagic fishes. Those differences in distribution of preys also affectthe difference of stomach contents of the whales between immature and mature, because immature
whales are abundant in near shore areas while mature males are mainly distributed in the offshore
waters as revealed in the previous section. To eliminate the effect of the difference of distributions,samples were divided to three geographic groups based on the catch positions of the whales asfollows; ‘Continental shelf’: water depth shallower than 200m; ‘Continental slope’; water depth
from 200m to 1,000m; ‘Offshore’: water depth of 1,000m or more; and comparisons by maturity
status was independently made in each group. Table 12 indicates the compositions of sex and sexual
maturity of the whales by the geographic groups. Ratio of males and maturity ratio were lowest in‘Continental shelf’ region and highest in ‘Offshore’ region. Comparisons of the occurrence and theratio of restored weight of prey species by maturity status in each group are shown in Table 13 and14. In both the ‘Continental shelf’ and ‘Continental slope’ groups, ratio of walleye pollock and krill
in immature whales were higher than that of mature whales while ratio of Pacific saury in immature
whales was lower than that of mature whales. In the ‘Offshore’ group, the whales fed on walleye
pollock and common squid were not observed, and ratio of Japanese anchovy and Pacific saury weresimilar between immature and mature whales. Fig.9 shows frequency distributions of body lengthand maturity of the whales by their prey species in respective geographic groups and Fig. 10
indicates compositions of the occurrence of the preys by the groups. Compositions of the preys inthe ‘Continental shelf’ was significantly different between immature and mature whales (x square
test, p<0.001), and those of the ‘Continental slope’ was also significantly different (x square test,
p<0.001), but in the ‘Offshore’ group, significant difference was not observed between maturity
status (x square test, p>0.5). The differences by maturity status observed in at least the continental
shelf and slope regions were thought to be indicated the presence of the difference in prey preference
between immature and mature whales.
DISCUSSION
The differences observed between immature and mature whales in the coastal waters off Kushiro inautumn season can be summarized as follows.

6(1) Immature whales are more frequently distributed near the coast or the continental shelf region
while mature males are distributed in a wider area including the offshore waters.
(2) At least in the continental shelf and slope regions with water depths of less than 1,000m, smaller
and immature whales tend to take walleye pollock and krill while larger and mature whales tend
to take Pacific saury and common squid.
(3) Common squid is consumed by only mature whales. Japanese anchovy is evenly consumed byboth immature and mature whales.
These results suggested that migration and prey preference of common minke whales in the coastal
waters off Kushiro in autumn season possibly differed with their maturity stage, and immature
whales more prefer to take walleye pollock on the continental shelf and slope regions than mature
whales. One of the reasons of these differences might be explained by the availability of species as aprey for the whales (such as distribution, migration, and shoaling behavior of respective preyspecies) and the caloric value of species as a food. Walleye pollock is stably and restrictively
distributed in the continental shelf and slope regions, and it is possibly easy to find by the whales,
but the caloric value contained in this species (1,480Kcal/Kg: Tamura et al., 2009) is low comparedwith that of Pacific saury (3,140Kcal/Kg: Tamura et al., 2009). On the other hand, because Pacificsaury is a large migratory species and their distribution is not stable, the whales have to search for
wider areas to find and take this species. It is expected that mature whale has higher capability ofswimming and experience to seek the prey, but required more energy cost than immature whales.
These imply that migration and feeding strategies might change with growth whereby immature
whales take more stable preys and mature whales seek nutritionally more valuable preys. Since,
Japanese anchovy is distributed widely throughout the coastal area and sporadically shoaling, both
immature and mature whales possibly can take this species easily.
In the coastal waters off the Sanriku district, northeastern part of the Japanese main island, in spring
season, three species (krill, Japanese anchovy, and sand lance, Ammodytes personatus) were found as
major prey species of common minke whales, but change of prey species by maturity stage was notdetected (Yoshida, et al., 2009). However, this may be due to the difference of the locality andseason of the surveys. Sampling positions of the whales off Sanriku were concentrated in the Sendai
Bay on the continental shelf region less than 200m water depth. In this area, topography of sea
bottom is monotonous compared with those off Kushiro, and sand lance is dominantly distributed
and abundant especially in spring season. Thus, sand lance is thought to be fed easily by both
immature and mature whales as well as Japanese anchovy. Walleye pollack, common squid, andPacific saury are not distributed in the Bay in spring. Furthermore, compositions of sex and7reproductive status of common minke whales off Sanriku in spring season are also different fromthat off Kushiro in autumn season. Ratio of males off Sanriku (0.40) is lower than that off Kushiro(0.72), and the number of mature pregnant females in the samples is higher in Sanriku (31 out of 136
females or 22.8%) than those off Kushiro (4 out of 72 females or 5.6%). Apparent low ratio of
mature females in both areas indicates that only some part of the population of the whales migrate toeach coastal area in respective season, and the differences of those ratios between Kushiro andSanriku are seems to be indicated seasonal difference of the migration of the whales to respective
areas. Minke whale is well known to have segregation in their migration as the predominance offemales around the northern limit of their distribution in both the Southern Hemisphere (Kato, et al.,1990) and off northern Japan (Wada, 1989), and coastal waters off Japan is thought to be located in
the migration corridor of their north-south migration. The whale off Sanriku in spring is thought tobe on the way to the northward migration after the breeding season, and the whale off Kushiro inautumn is thought to be on the way to the southward migration from the northern feeding areas. In
the case of Gary whales, it is known that females generally migrate earlier than males and adults
earlier than sexually immature whales in their coastal north-south migration (Wolman, 1985).
Seasonal migration route of common minke whales is also known to be different between sex andmaturity status (Hatanaka and Miyahita, 1997). These appeared that the feeding strategy of common
minke whales might be also changed to adapt the local environments along with their respective
migration route in respective season.
From the viewpoint of the ecosystem modeling and multi stock managements, the difference in prey
preference by maturity stage may be affect to the local inshore ecosystem and coastal fisheries. Thus,
it is thought to be important to take account of this point in estimating the local prey consumptions
and constructing the models for restricted coastal small area, especially in the coastal waters off
Kushiro.
ACKNOWLEDGMENTS
The authors thank to all researchers, crew and stuff of the coastal components off Kushiro in
conducting the surveys. We are also greatly indebted to Dr. Hiroshi Hatanaka, the project leader ofthe full-scale JARPN II surveys, the Institute of Cetacean Research (ICR). Mr. Takeharu Bando(ICR) and Ms. Ryoko Zenitani (ICR) kindly cooperated in examination of testis and maturity statusof the samples.
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in the western North Pacific-Phase II (JARPN II). Paper SC/54/O17 submitted to the 54th IWC
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Hatanaka, H. and Miyashita, T. 1997. On the feeding migration of the Okhotsk Sea-West Pacific
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S., Hara, T., Hayashi, T., Miyashita, T., Fukudome, K., Kiwada, H. and Kawahara, S. 2003. Reportof the 2002 JARPN II survey in the western North Pacific. Part II: Coastal component – Coastal
survey off Kushiro, northeast Japan. Paper SC/55/O8 submitted to the 55th IWC ScientificCommittee, 26pp.
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H., Morita, Y., Saino, S., Hara, T., Ebisui, T., Kuroishi, H., Nishiwaki, S. and Kawahara, S. 2005.
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Kumagaya, K., Tokuda, D., Nakai, K., Funabashi, N., Ebiui, T., Wakatsuki, T., Sakaguchi, M.,
Houhana, T., Nishiwaki, S. and Kawahara, S. 2006. Cruise report of the Second phase of theJapanese Whale Research Program under Special Permit in the western North Pacific (JARPN II)
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Kishiro, T., Kato, H., Yoshida, H., Miyashita, T., Iwasaki, T., Kanaji, Y., Ryono, T., Tabata, S.,
Morita, Y., Okamoto, R., Maeda, H., Nagatsuka, S., Ogawa, N., Nakai, K., Ebisui, T., Matsumoto,A., Gokita, A., Kiwada, H. and Kawahara, S. 2008. Cruise report of the Second phase of the9Japanese Whale Research Program under Special Permit in the western North Pacific (JARPN II)
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under the JARPN II-coastal component of Sanriku and Kushiro-, with special emphasis on whale
sanpling prodedures. Paper SC/J09/JR3 presented to the JARPN II Review Workshop, Tokyo,
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whale in the Northwestern Pacific. ICES Journal of Marine Science, 59:516-528.
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Yoshida, H., Kato, H., Kishiro, T., Miyashita, T., Iwasaki, T. Minamikawa, S., Ryono, T., Tabata,Morita, Y., Sato, H., Okamoto, R., Toyoda, Y., Nakamura, G., Ebisui, T., Nakai, K., Matsumoto,A., Nishiwaki, S. and Kawahara, S. 2007. Cruise report of the Second phase of the Japanese
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Table 1. Body length (m) of common minke whales sampled by the 2002-2007 coastal components
off Kushiro in the JARPN II.

Table 2. Composition of sex and sexual maturity of common minke whales sampled by the
2002-2007 coastal components off Kushiro in the JARPN II.

Table 3. Occurrence of prey species in the stomach of common minke whales sampled by the
2002-2007 coastal components off Kushiro in the JARPN II.

Table 4. Restored weights of prey species in the stomach of common minke whales sampled by the
2002-2007 coastal components off Kushiro in the JARPN II.

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Table 5. Occurrence of prey species in the stomach of common minke whales by sex and maturity status.

Table 6. Mean compositions of restored weights of prey species in the stomach of common minke whales by sex and maturity status.

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Table 7. Maturity ratio of common minke whales by prey species found in their stomaches.

Table 8. Composition of sex and sexual maturity by stocks of common minke whales sampled by the
2002-2007 coastal components off Kushiro in the JARPN II.

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Table 9. Occurrence of prey species in the stomach of common minke whales by stocks, sex and maturity status.

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Table 10. Mean compositions of restored weights of prey species in the stomach of common
minke whales by stocks, sex and maturity status.

Table 11. Maturity ratio of common minke whales by sex, stocks and prey species found in theirstomaches.

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Table 12. Geographical comparison of the composition of sex and sexual maturity of common minke whales sampled by the 2002-2007
coastal components off Kushiro in the JARPN II.

Table 13. Geographical comparison of the occurrence of prey species in the stomach of common minke whales by their maturity status.

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Table 14. Geographical comparison of the mean compositions of restored weights of prey species in the stomach of common minke whales
by their maturity status.

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Fig.1. The IWC sub-area for western North Pacific minke whales (upper) and research area of thewhale sampling surveys in the coastal component off Kushiro, under the JARPNⅡ(lower)18

Fig.2. Distribution of common minke whales sampled by the coastal component off Kushiro in2002 to 2007.

Fig.3. Body length frequency of common minke whales sampled by the coastal component off
Kushiro in 2002 to 2007. ■: Immature (males); ■: immature (females); □: mature; □:unknown maturity.

19

Fig.4. Distribution of males and females common minke whales sampled by the coastal
component off Kushiro in 2002 to 2007. ●: Immature; ○: mature; ＋: unknown maturity

Fig.5. Sex and maturity ratio of common minke whales by 10 minutes square latitude-longitude.

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Occurrence of prey species in stomach

Fig.6. Relationship between body lengths of common minke whales with compositions of theoccurrence of prey species in their stomach in 2002 to 2007 off Kushiro.

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Fig.7. Yearly change in maturity and body length frequency of common minke whales with theirprey species found in the stomach in 2002 to 2007 coastal components off Kushiro.
■: Immature whales; ■: mature whales.

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Fig.8. Distribution of sampled common minke whales by prey species found in their stomach in2002 to 2007 coastal components off Kushiro.
●: Immature whales; ○: mature whales; ＋: unknown maturity

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Fig.9. Geographical comparison of maturity and body length frequency of common minke whales with their prey species found in the stomach in 2002 to2007 coastal components off Kushiro. ■: Immature whales; ■: mature whales.

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Table 10. Geographical comparison of the occurrence of prey species in the stomach of common
minke whales by their maturity status.

25