SC/J09/JR12

Relationship between body size, maturity, and feeding habit of common
minke whales off Sanriku in spring season, from 2003-2007 whale
sampling surveys under the JARPN II coastal component off Sanriku
HIDEYOSHI YOSHIDA1, TOSHIYA KISHIRO1, MUTSUO GOTO2, TAKEHARU BANDO2,TSUTOMU TAMURA2, KENJI KONISHI2, RYOSUKE OKAMOTO3, AND HIDEHIRO KATO3
1

National Research Institute of Far Seas Fisheries, 2-12-4, Fukuura, Kanazawa-ku, Yokohama,
Kanagawa 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

Contact e-mail: hideka@affrc.go.jp

ABSTRACT
Relationship between body size, maturity and feeding habit of common minke whales in coastal waters off Sanriku,
northeast Japan, in spring season, was studied from animals collected during the whale sampling surveys under the
JARPN II coastal component off Sanriku conducted in 2003 to 2007.

From these surveys, a total of 227 common minke

whales (91 males and 136 females) were collected.

The ratio of sexually mature animals was calculated as 0.42 formales, 0.24 for females, and 0.31 for all the animals.

Three species, krill (Euphausia pacifica), Japanese sand lance(Ammodytes personatus), and Japanese anchovy (Engraulis japonicus), were found in the whale stomachs, of which sandlance was the most dominant prey species, followed by anchovy. All the whales but two were sighted in waters withdepth of 20-100m: no obvious difference was observed in their sighting positions between males and females, immatureand mature animals, and the three prey species. Body length frequency of whales was not different among animals
feeding on each of the three prey species (P > 0.05).

Furthermore, frequency of sexual maturity stages of animals did

not differ among whales feeding on each of the preys (P > 0.05). From these, we considered that, in coastal waters off
Sanriku in spring season, both immature and mature common minke whales appear to feed on the three species evenly,
which resulted in no obvious difference in feeding habits between immature and mature common minke whales.

KEYWORDS: COMMON MINKE WHALE; NORTH PACIFIC; COASTAL WATERS OFF SANRIKU, JAPAN;
SCIENTIFIC PERMITS; PREY PREFERENCE; MATURITY; FEEDING HABIT

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INTRODUCTION
The common minke whale, Balaenoptela acutorostrata, is widely distributed in the world, from offshore
to coastal waters and feed on various prey species including commercially important species such as
Japanese anchovy Engraulis japonicus, Pacific saury Cololabis saira, and walleye pollock Theragra
chalcogramma (Tamura and Fujise, 2002).

In coastal waters off the Sanriku (northeast part of the

Pacific coast of the Japanese main island, Honshu), many common minke whales were taken, especiallyin spring, by the past land-based coastal whaling (Miyashita and Hatanaka, 1997). The coastal waters
are also very important fishing grounds. Thus, the waters are thought to be suitable for the study area
to evaluate competition between whales and coastal fisheries.
From these, the whale sampling survey in coastal waters off Sanriku was started in 2003 (Yoshida et al.
2004) and then conducted every year from 2005 to 2007 (Yoshida et al. 2006, Goto et al. 2007, Band etal. 2008), as the coastal component of the full JARPN II research plan (Government of Japan 2002,
2004).

Background and details of the surveys are described in Kishiro et al. (2009a).

The objectivesof 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 incetaceans 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.
From the result of the 2007 whale sampling survey conducted in coastal waters off Kushiro, differencein feeding habit between immature and mature common minke whales was suggested (Kishiro et al.
2008).

The difference has been also reported in other cetacean species, e.g., spotted dolphins (Bernard

et al. 1989) and Dall’s porpoises (Ohizumi et al. 2003). Information on the difference is very
important to reveal feeding ecology and more reliable prey consumption of common minke whales.

Inthe present study, thus, we examine relationship between body size, maturity, and feeding habit ofcommon minke whales in coastal waters off Sanriku in spring season, from data and samples collectedduring the whale sampling surveys under the JARPN II coastal component off Sanriku conducted in2003 to 2007.

MATERIAL AND METHODS
The present study was conducted from data and samples obtained from the whale sampling surveys in
the 2003-2007 coastal components off Sanriku, under the JARPN II.

The surveys were carried out inspring season (April and May) in coastal waters within 50 nautical miles from Ayukawa port (Fig.1).
Methodology and details of the surveys are detailed in Kishiro et al. (2009a).

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Body length measurement and determination of sexual mature stage of whales
All the animals collected were landed at the JARPN II research station established at the Ayukawa port
in Sanriku, and then biological examination such as measurement of body proportion was conducted.
Whale Body length was measured to the nearest 1cm on a straight and parallel to the body axis from thetip of upper jaw to the notch of fluke, by researches.

In males, each side of testis was weighed after

separated from the epididymis and histological samples were collected from center of the right testis.
An animal with a single testis weight of 290g or more was tentatively determined as sexually mature,from the histological observation of the seminiferous tubules, spermatocyte, spermatid or spermatozoa
using hematoxylin and eosin stained sections.

In females, both sides of ovaries were collected andcorpus luteum and albicans in ovaries were counted.

An animal possessed at least one corpus luteum

or albicans in ovaries was determined as sexually mature.

Prey species identification and restoring stomach contents weightAt the research land station, stomach contents were removed from whales and then weighted to thenearest 0.1 Kg by each four chamber in both cases of including and excluding liquid.

When the intact

or partially digested body of prey species was found, species identification was carried out from theirmorphological characters. When undigested fishes occurred, their standard lengths were measured to
the nearest 1 mm.

Then, a sub-sample (1-5Kg) of stomach contents was collected and frozen and/or

fixed with 10% formalin for the laboratory work.

At the laboratory, detailed species identification andrestoring stomach contents weight were made by Tamura et al. (2009a), as follows.

Prey species was

identified to the lowest taxonomic level as possible using morphological characters for undigested preysor the otoliths and jaw plate for digested preys.

The total wet weight of each prey species in theforestomach was estimated by multiplying average weight of fresh prey specimens, by the number ofpreys (including undigested full bodies, undigested skulls, and half the total number of free otoliths) in
the sub-sample and multiplying ratio of the weight of sub-sample for the total weight of forestomach
contents.

Details of the methods are described in Tamura et al. (2009a).

Occurrence of prey species

was calculated as ratio of the number of whales counted by each of prey species found in theirforestomach to total number of whales. When plural prey species were found simultaneously from awhale stomach, each of the species was counted (See, Table 3). Mean composition of restored wet
weight by each of prey species was calculated as the average of ratio of restored weights by each of preyspecies found from respective whales.

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Identification of stocks to each animal
Skin tissues of whales were collected and preserved in ethanol for the laboratory DNA analysis. In thepresent study, results of stock identification to each animal (J or O stocks) made by Kanda et al. (2009)
from microsatellite analysis were used for comparison between feeding habit of J and O stock animals.
Details of the genetic identification are described in Kanda et al. (2009).

RESULTSSex ratio, body length and maturity of common minke whales
During the 2003-2007 JARPN II coastal component off Sanriku, a total of 227 common minke whales
were collected (Fig. 2). They consisted of 91 males and 136 females (sex ratio of males was 0.40).
Statistics and frequency of body length are shown in Table 1 and Fig. 3.

Average body length in eachyear ranged from 6.15m to 6.29m for males and 5.67m to 6.30m for females.

Females with bodylength of 5 to 6m were most frequently recorded every year and obvious yearly change was not observedin frequency distribution of animal body length.
whales is shown in Table 2.

Compositions of sex and sexual maturity stage of the

Sex ratio of males in each year was from 0.37 to 0.43.

In the 91 males,

37 were identified as sexually mature and maturity stage could not be determined for two males.
Ratios of sexually mature males in each year ranged from 0.35 to 0.46 and 0.42 for all the year.
136 females, 33 were classified as sexually mature, of which 31 were pregnant.
mature females in each year were from 0.09 to 0.38 and 0.24 for all the year.
are shown in Figure 2, by sex and sexually mature stages.
were sighted at waters with depth of 20-100m.

In theRatios of sexually

Their sighting positions

All the whales but two immature females

No obvious difference was observed in their sighting

positions between sexes and sexually maturity stages.

Prey species found from whale stomach
In the present study, three species, krill (Euphausia pacifica), Japanese sand lance (Ammodytes
personatus), and Japanese anchovy (Engraulis japonicus), were found from whale stomachs (Table 3).
Their restored wet weights are shown in Table 4.

No other prey species were detected.

In the three

species, sand lance was most dominant each year in the occurrence (Table 3) and in the weight except
for the 2007 survey, in which the weight of anchovy was most heavy (Table 4). The second dominant
species was Anchovy.

Relationship between body lengths of whales and their prey species
Relationship between whale body length and occurrence of prey species in their stomach is shown in

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Figure 4.

In both sexes, occurrence frequency of sand lance was highest in all the body length classes,

except for a class of males with body length of 8.0 to 8.9m, in which a male took anchovies.
second frequently observed species was anchovy.

The

Although krill was not detected in stomachs ofanimals with body length of 8.0 to 8.9m, this species was fed by other class females.

Occurrence

frequency of prey species in each body length class did not change so obviously with body length
growth. Relationship of body length of whales to mean standard lengths of prey fishes obtained fromtheir stomach is shown in Figure 5.

Between them, positive relationship was not observed: larger

animals did not tend to take larger preys.

Relationship between whale maturity and their prey species
Occurrence frequency of prey species by sex and sexually mature stage is shown in Table 5.

The most

dominant prey species was sand lance in each sex and maturity stage, followed by anchovy.

Japaneseanchovy and krill were also fed by both males and females and both immature and mature animals,
though krill were not observed from mature male stomachs.

Table 6 shows comparison of mean

compositions of prey species between immature and mature whales based on restored wet weight.
Result was almost same with that shown in Table 5: ratio of weights of sand lance was highest in eachsex and maturity stage, followed by anchovy.
Table 7.

Maturity ratio of whales by prey species is shown in

In males, the highest maturity ratio was observed in animals feeding on sand lance (0.45).

Krill was not taken by mature animals.

In females, the highest ratio was recorded in animals taking

krill (0.29), followed by whales feeding on sand lance (0.27), though the ratios were almost same.

The

ratio calculated by pooling sexes was almost same among animals taking each of the prey species(0.20-0.34, Table 7).

Difference in frequency of sexual maturity stages was tested among animals

feding on each of the prey species, by the randomized chi-square test (Roff and Bentzen 1989). The
significance test from the 10,000 randomizations using the Program RandChi (Okamura 2001) indicatedno difference among them (P > 0.05).
is shown in Figure 6.

Body length frequency of whales taking each of the prey species

Although the frequency changed by year (Fig. 6a-d), pooled data of all the years

indicated similar frequency distribution between the three prey species (Fig. 6e).

Furthermore, thefrequency distribution of animals taking each of the prey species (Fig. 6e) was similar to it of all theanimals collected during the surveys (Fig. 6f).

From the the randomized chi-square test, body length

frequency of whales did not differ among animals feeding on each of the prey species (P > 0.05).

Comparison between J and O stocks
Common minke whales distributed in coastal waters of Japan including the present study area are5thought to belong to at least two stocks: one is J stock mainly distributed in the Sea of Japan and anotheris O stock mainly inhabiting the western North Pacific (Hatanaka and Miyashita, 1997).

Animals fromeach of the two stocks can be identified from genetic analysis (see, Kanda et al. 2009). Their analysis
indicates that animals from both stocks were collected in the 2003-2007 JARPN II coastal component
off Sanriku. Hence, the animals were assigned into each stock from genetic identification results madeby Kanda et al. (2009) and comparison of feeding habit between the two stocks was carried out. All
the 41 animals were assigned to the J stock and 164 were to the O stock.
of sex and sexual maturity of whales by stock.

Table 8 shows compositions

Ratio of males was higher in J stock animals but

maturity ratio in J stock animals (0.18-0.25) was lower than that of O stock (0.26-0.48).

Occurrence

frequency and ratio of restored weight of prey species are compared between stocks and maturity status(Table 9 and 10).

Maturity ratio of whales by stock and prey species is also shown in Table 11.

Between the two stocks, occurrence frequency and ratio of prey species were not obviously different:
occurrence frequency and ratio of sand lance was highest in each maturity stage of both sexes and stocks(Table 9 and 10).

Furthermore, maturity ratio of whales by prey species is almost same between thetwo stocks (Table 11).

These results suggest that there was no distinct difference in feeding habit

between the two stock animals, as discussed to animals off Kushiro (Kishiro et al. 2009b).

Geographic comparisons in the research area
Sighting positions of whales collected are shown in Figure 7, by their prey species and maturity stage.
All the whales but two taking krill were sighted at waters with depth of 20-100m and no obvious
difference was observed in sighting positions between the prey species:DISCUSSION
In the present study, relationship between body size, sexual maturity, and feeding habit of common
minke whales migrating into coastal waters off Sanriku in spring season was studied from animals
collected during the 2003-2007 JARPN II coastal component. Three species, krill, Japanese sand lance,
and Japanese anchovy, were found from whale stomachs.

In these, the sand lance was most dominant

prey and the second one was anchovy. Although body length frequency of whales feeding on each of
the prey species changed in each year (Fig. 6a-d), probably from yearly change of prey species
occurrence as shown in Murase et al. (2009), pooled data of all the years indicated similar frequency
distribution between the three prey species (Fig. 6e).

Indeed, the randomized chi-square test showedthat body length frequency was not different among animals feeding on each of the three prey species (P> 0.05).

Furthermore, frequency of sexual maturity stages did not differ among whales feeding on each6

of the preys (P > 0.05).

From these, we considered that, in coastal waters off Sanriku in spring season,common minke whales appear not to change prey species with changes in their maturity stage.
The analysis on feeding habit of common minke whales was also conducted from animals collected incoastal waters off Kushiro in autumn season (Kishiro et al. 2009b).

In their study, five species, walleye

Pollock, krill, Japanese anchovy, Pacific saury, and Japanese flying squid, were observed in the whale
stomachs as major prey species.

They found that smaller and immature whales tended to feed onwalleye pollock and krill, while larger and mature whales tended to take Pacific saury.

Flying squid

was observed only in mature animal stomachs and Japanese anchovy was taken by both immature andmature whales evenly.

For the species, caloric value was calculated (Tamura et al., 2009b): the value

contained in the walleye pollock and krill was calculated as 1480 and 850Kcal/Kg, respectively, which
was lower than that of the Pacific saury (3,140Kcal/Kg).

From these, they though that, possibly, off

Kushrio, common minke whales change their feeding habit with changes in their maturity stage:
immature animals mainly take lower caloric but not so migratory species, i.e., walleye pollock and krill,
while mature whales seek higher caloric but highly migratory species, Pacific saury.

Since Japanese

anchovy is widely distributed throughout the research area in shoals, they considered that both immatureand mature whales can take it easily.
Whales migrating into coastal waters off Sanriku in spring season are thought to be on the route to thenorthward feeding ground after winter breeding season.
Okhotsk Sea (Hatanaka and Miyashita 1997).

Their main destination is thought to be the

Our study indicates that, on the route, they stay incoastal waters off Sanriku and feed on krill, sand lance, and Japanese Anchovy.

The main research areain the present study is located on the continental shelf off Sanriku, in which topography of sea bottom ismonotonous.

Most of whales were sighted at waters with depth of 20-100m (Fig. 2).

Fauna of preyspecies for common minke whales is thought not to be so diverse there, in comparison with coastal
waters off Kushiro, where the sea bottom topography is very various, whales were collected not only on
the continental shelf but also at steep canyon with depth of 1000m or more, and five prey species were
found from whale stomachs (Kishiro et al. 2009b).

Indeed, in coastal waters off Sanriku, the preyspecies surveys concurrently conducted with the whale sampling surveys indicate that walleye pollack,common squid, and Pacific saury do not occur there in spring, but on the other hand, sand lance,
Japanese anchovy, and krill are widely distributed throughout the research area in large shoals(Kawahara et al. 2004, Yonezaki et al. 2006, 2007, 2008).

In the research area, probably, sand lance,

Japanese anchovy, and krill are easy preys for both immature and mature common minke whales, as

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discussed about Japanese anchovy off Kushiro (Kishiro et al. 2009b).

From these, we considered that,in coastal waters off Sanriku in spring season, both immature and mature common minke whales feed on
the 3 species evenly, which resulted in no obvious difference in feeding habits between immature andmature common minke whales.

ACKNOWLEDGMENTS
We thank to all researchers, crew and stuff of the whale sampling surveys in the 2002-2007 coastal
components off Kushiro, under the JARPN II.

We are also greatly indebted to Dr. Hiroshi Hatanaka,

the project leader of the full-scale JARPN II surveys, the Institute of Cetacean Research (ICR). Ms.
Ryoko Zenitani (ICR) kindly cooperated in examination of testis and maturity status of the samples.
Hiroshi Okamura gave us useful suggestions on statistical analysis.

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SC/58/O9 submitted to the 58 IWC scientific Committee, 30pp.

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Sanriku

Ayukawa

Figure 1.

Research area of the whale sampling surveys in the 2003-2007 coastal components off
SANRIKU, under the JARPN II.

Male

FemaleImmature
MatureUnknown

Figure 2.

Sighting positions of common minke whales collected in the 2003-2007 coastal components
off SANRIKU, under the JARPN II.

Isobaths of 20m, 100m, 200m, and 400m are also

shown.

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Figure 3.

Body length frequency of common minke whales collected in the 2003-2007 coastal
components off SANRIKU, under the JARPN II.

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Figure 4. Relationship of body length of common minke whales to composition of their prey species
occurrence in the 2003-2007 coastal components off SANRIKU, under the JARPN II.

Figure 5.

Relationship of body length of whales to mean standard lengths of fishes obtained from theirstomach.

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Figure 6.

Sexual maturity and body length frequency of common minke whales collected in the
2003-2007 JARPN II coastal component off Sanriku, shown by prey species (a-e).

19a-1) Immaturea-2) Matureb-1) Immature: Krillb-2) Mature: Krillc-1) Immature:
Sand lancec-2) Mature:
Sand lanced-1) Immature:
Japanese anchovyd-1) Mature:
Japanese anchovy

Figure 7.

Sighting position of common minke whales collected in the 2003-2007 JARPN II coastal component off Sanriku, shown by theirsexual maturity stage and prey species. Isobaths of 20m, 100m, 200m, and 400m are also shown.

20