Fish Sci (2012) 78:557–567
DOI 10.1007/s12562-012-0480-x

ORIGINAL ARTICLE

Biology

Habitat and prey selection of common minke, sei, and Bryde’s
whales in mesoscale during summer in the subarctic
and transition regions of the western North Pacific
Hikaru Watanabe • Makoto Okazaki • Tsutomu Tamura •Kenji Konishi • Denzo Inagake • Takeharu Bando •Hiroshi Kiwada • Tomio Miyashita

Received: 28 August 2011 / Accepted: 15 February 2012 / Published online: 11 March 2012Ó The Japanese Society of Fisheries Science 2012

Abstract This study represents the first quantitative
analysis of the characteristics of the distribution areas andstomach contents of common minke whale Balaenoptera
acutorostrata, sei whale B. borealis, and Bryde’s whale
B. edeni in relation to oceanographic and prey environments in mid summer in the western North Pacific. Common minke whales were distributed within subarctic
regions and the northernmost region of the transitional
domain, coinciding with the main habitat of their preferredprey, Pacific saury Cololabis saira. Sei whales were mainly
found in the northernmost part of the transition zone andshowed prey preference for Japanese anchovy Engraulisjaponica, which was significantly more abundant in themain distribution area of the whale than in its adjacentareas. ‘‘Hot spots’’ of Bryde’s whales were found in several
regions of the transition zone between the subarctic
boundary and the Kuroshio front. This whale species preferred Japanese anchovy as prey, for which the distribution
density was significantly higher in the main distribution
area of the whale than in the adjacent areas. These results
indicate that the summer distributions of Pacific saury andJapanese anchovy greatly influence the distributions ofthese whale species, suggesting that the whales’ habitat
selection is closely related to their prey selection.

H. Watanabe (&)  T. Miyashita
National Research Institute of Far Seas Fisheries, 2-12-4
Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japane-mail: hikaru1@affrc.go.jp

Keywords Common minke whale  Sei whale 
Bryde’s whale  Habitat selection  Prey selection Prey environment  Oceanographic features 
Western North PacificT. Miyashitae-mail: miyachan@affrc.go.jp
M. Okazaki  D. Inagake
National Research Institute of Fisheries Science, 2-12-4
Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japane-mail: mokazaki@affrc.go.jp
D. Inagake
e-mail: ina@affrc.go.jp
T. Tamura  K. Konishi  T. Bando  H. Kiwada
The Institute of Cetacean Research, 4-5 Toyomi-cho,
Chuo-ku, Tokyo 104-0055, Japane-mail: tamura@cetacean.jp
K. Konishi
e-mail: konoshi@cetacean.jp
T. Bando
e-mail: bando@cetacean.jp
H. Kiwadae-mail: kiwada@cetacean.jp

Introduction
The common minke whale Balaenoptera acutorostrata, sei
whale B. borealis, and Bryde’s whale B. edeni are themajor baleen whale species in the western North Pacific[1]. These species migrate into high latitudinal areas during
spring and summer from the low latitudinal wintering area[2]. Previous studies have reported that these whale species
frequently feed on small epipelagic fish species such as
Japanese anchovy Engraulis japonica, Japanese sardine
Sardinops melanostictus, mackerel Scomber spp., and/or
Pacific saury Cololabis saira, and euphausiids from spring
to summer [1, 3–6]. Subarctic copepod species such as
Neocalanus cristatus and N. plumchrus can also be
important prey for sei whales in regions east of 160–165°E,
where Japanese sardine, anchovy, and mackerels, which

123

558

Materials and methods
Oceanographic and whale prey surveys
We conducted surveys of oceanographic observations andwhale prey species from on board the R/V Shunyo Maru
(887 GT, National Research Institute of Far Seas Fisheries)
in the western North Pacific from 21–25 July 2008 and
10–22 July 2009 (Fig. 1). These surveys were conductedconcurrently with whale sighting and sampling surveys as
part of JARPN II by researchers on the Nisshin Maru (7575
GT, Kyodo-Senpaku Co., Ltd.) and two sighting/sampling
vessels (Yushin Maru No. 2 and No. 3, 742–743 GT,

123

July 25, 2008

NOAA daily OISST Ver.2 avhrr−only

45˚N
15

40˚N

20

Survey area of
Bryde’s whale
25

35˚N
140˚E

145˚E

150˚E

155˚E

160˚E

165˚E

July 15, 2009NOAA daily OISST Ver.2 avhrr−only

45˚N

Survey area ofcommon minke
whale

10
15

15

40˚N

Survey area of sei whale
20

20

20

are mainly distributed within Japanese waters, are lessabundantly distributed [1, 3, 7, 8]. Furthermore, the stomach content compositions of these whales are known tofluctuate in relation to sea surface temperature (SST), latitude, and longitude [1]. However, quantitative informationon the distribution patterns and feeding habits of thesewhale species in relation to oceanographic conditions andprey environments is extremely limited. Such knowledge isessential for estimating the feeding impact of these whales
on various prey species, including commercially important
small epipelagic fish species, for the sustainable use ofmarine resources, which is recognized worldwide as animportant management issue for fisheries and is one of the
major objectives of the second phase of Japanese Whale
Research Program under the Special Permit in the western
North Pacific (JARPN II) [1, 6, 9, 10]. Such knowledge isalso important for single species management of whales
given that the abundance of each whale species can be
accurately measured using sighting surveys if their main
distribution area is known. Furthermore, both extra- andinterpolation methods of estimating whale abundance inareas where sighting surveys are not conducted could beestablished based on data of the habitat selections of eachwhale species.
Here, we aimed to quantitatively examine the habitatand prey selections of common minke, sei, and Bryde’s
whales in relation to oceanographic and prey environmentsby conducting concurrent oceanographic, prey species,
whale sighting, and whale sampling surveys in the same
area within 24 h in the subarctic, transition, and subtropical
Kuroshio regions of the western North Pacific during midsummer. Prey selection of sei whale is firstly reported inthis paper. Survey areas for each whale species were
established based on previous knowledge of the latitudinal
and SST ranges of their habitats in summer, i.e., 44–47°N(SST \15°C), 40–45°N (13–17°C), and 35–40°N(20–26°C) for common minke, sei, and Bryde’s whales,
respectively [1, 11, 12].

Fish Sci (2012) 78:557–567

35˚N
140˚E

25

145˚E

150˚E

155˚E

160˚E

165˚E

Fig. 1 Survey areas, survey track lines, and distributions of seasurface isotherms in the western North Pacific in July 2008 and 2009

Kyodo-Senpaku Co., Ltd.) on the same survey track lines
during daylight periods from 1 h after sunrise to 1 h before
sunset. In 2008, we conducted a survey of Bryde’s whales
within a survey area bounded by 35–39°300 N and
146–148°E, where the SST ranged from 17.5–27.6°C(Fig. 1). In 2009, surveys of common minke and sei whales
were conducted within an area bounded by 43–45°N and
154°300 –157°300 E (SST 10.2–14.7°C), and 39–41°N and
156–160°E (SST 14.9–18.8°C), respectively (Fig. 1;
Table 1). According to previous reports [1, 11, 12], theseareas covered the southern part of the main distribution
area of common minke and sei whales, as well as the main
distribution area of Bryde’s whales, in terms of both latitudinal and SST ranges.
The distribution and abundance of the whale prey species were investigated using a midwater trawl, Multiple
Opening/Closing Net and Environment Sampling System(MOCNESS, Bess Co., Ltd.), North Pacific Standard net(NORPAC net), and quantitative echosounder with operating frequency of 38, 70, and 120 kHz (EK60, SIMRAD)

Fish Sci (2012) 78:557–567

559

Table 1 Results of target trawlings in the survey area of Bryde’s and sei whales and of predetermined trawlings in the survey area of common
minke whales by midwater trawls
Stn.

Date

SST
(°C)

Latitude(N)

Longitude(E)

Sampling
depth (m)Sampling
duration(min)

CPUE (kg/h)
Japaneseanchovy

JapanesesardineCommonmackerel

Pacific
saury

Commonsquid

Survey area of Bryde’s whale3

22 July 2008

25.6

36°12.40

146°25.50

0–30

60

1.9a

0

?

0

0

8

24 July 2008

23.2

38°35.50

147°12.70

0–30

60

0.8

0

0

0

0

16.7

40°11.70

156°22.70

0–30

60

62.5a

0.2

?

0

0

18.8

0

159°00.2

0

0–30

60

28.2a

0.1

?

0

0

12.0

44°06.80

156°45.50

0–30

60

0

0

0

163.9

0

0

156°34.7

0Survey area of sei whale
1
5

10 July 200912 July 2009

40°00.0

Survey area of common minke whale
8

18 July 200910

19 July 200912.5

43°17.4

0–30

60

0.1

0

0

0

?

11

19 July 2009

13.0

43°12.30

155°43.80

0–90

60

0

0

0

156.9

0

12

20 July 200914.7

43°37.30

155°10.70

0–30

60

?

0

0

0

0.7

0

155°34.00

0–30

60

0.1

0

0

268.3

0.4

0–30

60

4.6a

0

0

210.8

?

60

a

0

0

244.5

0.3

13

20 July 2009

12.7

44°09.2

14

21 July 200910.2

44°27.20

155°56.40

13.5

0

0

15

22 July 2009

43°39.1

156°01.4

0–30

1.5

?, 0.1 kg \ catch per unit of effort (CPUE)a

Backscatters of Japanese anchovy were detected on the echogram during trawlingson the track lines (Fig. 1). The midwater trawl used in thisstudy had a mouth opening of 20 9 30 m with a 17.5-mm
liner codend. The sampling depths and the height of the net
mouth were monitored using a net monitor system (PI32,SIMRAD). Towing speed was 3–4 knots. Trawls wereconducted to identify species and size compositions of
acoustic backscatters in the echogram as well as to collectdata on species such as Pacific saury, which are difficult to
detect with the echosounder due to their distributions close
to sea surface, generally shallower than the bottom of theship. Within the survey area of the common minke whale,
the SST was less than 15°C and Pacific saury were possibly
distributed [13, 14]. Hence, all trawling stations of this area
were predetermined, generally 20–30 nautical miles aparton the track lines, and acoustic backscatters for other small
epipelagic fish species were identified based on predetermined trawling data (Fig. 1; Table 1). At each station, a
trawl net was towed at depths of 0–100 m (oblique tow) or
0–30 m for 60 min (Table 1). All samples were identified
to the lowest taxonomic level possible and the wet bodyweight of each species was measured. For the major species, individual body length was measured from 100 randomly selected samples. When the sample size for a
species was less than 100 individuals, the body length wasmeasured for all individuals collected.
We conducted MOCNESS samplings to confirm thetaxonomic composition of euphausiid-like patchinessdetected on the echogram as well as to confirm the distribution of euphausiids in each deep scattering layer in the

Table 2 Sampling dates and positions of the MOCNESS survey
Stn.

Date

SST
(°C)

Latitude(N)

Longitude(E)

Sampling
depth (m)26.9

35°52.50

146°55.30

0–250

0

146°46.80

0–250

Survey area of Bryde’s whale2

21 July 2008

523 July 2008

24.7

37°05.1

1025 July 2008

19.2

39°31.70

146°01.80

0–250

39°18.90

157°22.50

0–250

Survey area of common minke whale
14
21 July 200910.2
44°27.20

155°56.40

0–250

Survey area of sei whale
7

13 July 2009

18.7

epipelagic zone. The mouth opening and mesh size of
MOCNESS were 1 m2 and 0.33 9 0.33 mm, respectively.
The net was towed at 2 knots at eight target depths (0–20,
20–40, 40–60, 60–80, 80–100, 100–150, 150–200, and
200–250 m) (Table 2). The volume of seawater filtered byeach net was measured with a flow meter mounted at the
net mouth. Samples were preserved in 5% formalin-buffered seawater, and size and abundances were analyzed in
the laboratory. NORPAC net samplings were conducted in
the 0–150-m layer at all sampling stations to estimate thebiomasses of the copepod species N. cristatus andN. plumchrus, which are occasional prey of the sei whales[1]. The mouth opening and mesh size of the net were0.16 m2 and 0.33 9 0.33 mm, respectively. A flow meter
was attached to the net to measure the volume of seawater

123

560

Fish Sci (2012) 78:557–567

filtered. Samples were preserved in 5% formalin-bufferedseawater.
A conductivity-temperature-depth (CTD, Sea-Bird Co.,
Ltd.) profiler cast was made to 500 m depth at each sampling station to determine the position of oceanographic
fronts on the track lines. The distribution of oceanographic
fronts around the track lines were estimated by 100-m and
200-m temperature and salinity maps by the North-East
Asian Regional-Global Ocean Observing System (NEARGOOS) database and Fisheries Research Agency-Japan
Coastal Ocean Predictability Experiment (FRA-JCOPE)
system (Fisheries Research Agency of Japan: http://fj.dc.
affrc.go.jp/; accessed 27 July 2011, Japan Oceanographic Data Center: http://near-goosl.jodc.go.jp/index.html;
accessed 27 July 2011). The distribution of the SST isotherm
was estimated using the NOAA Optimum Interpolation 1/4
Degree Daily Sea Surface Temperature Analysis database,version 2 (OISSTWeb: http://www.ncdc.noaa.gov/oa/climate/research/sst/papers/whats-new-v2.pdf; accessed 27 July 2011)[15].
Species identifications of backscatterings for euphausiids and epipelagic fish species except for Pacific saury wereconducted based on MOCNESS and midwater trawl catches, respectively. The school shapes of euphausiids andJapanese anchovy recorded on the echogram were also
taken into account, with reference to previous reports[16, 17]. For the epipelagic fish species except for Pacificsaury, acoustic data collected at 38 kHz were used with thethreshold set at -60 dB within 0–250 m. For euphausiids,
data collected at -120 kHz were used with the threshold
set at -80 dB within 0–250 m. The difference in the volume backscattering strength (SV) was also considered to
identify euphausiids, because echoes could be identified as
euphausiids when difference of SV between 120 and
38 kHz was between 10 and 15 dB [18]. When the difference in the SV was calculated, a threshold of 38 kHz
was set at -80 dB. A biomass estimation for each species
was conducted according to the method of Jolly andHampson [19].
The mean backscattering area per square nautical mileof sea surface (SA) by species for every one nautical mile ofsurvey transect over defined depth interval was calculatedusing the following formula:
 2 
Zr2
m
2
2
SA ¼ 4pr0 1852
sV dr
;
n.mi2
r1

where r is the depth from the sea surface, and r0 = 1 m,
representing the reference range for backscatteringstrength. The following length–target strength (TS)relationship for Japanese anchovy [20], which was themain component in the epipelagic fish community (seebelow), was used:123

TS ¼ 20logSL  64:0;
where SL is the scale length in centimeters. We estimatedbody wet weight of each individual from the SL values
using the following equation [21]:W ¼ 0:010SL3:00 ;where W is weight in grams.
Because no length–TS relationship for euphausiids wasavailable in this study area, we assumed that the TS ofeuphausiids was -83.3 dB, which was the value reported
for Euphausia pacifica of 16.4 mm body length [22]. The
average body length of euphausiids selected randomly
from all samples was 16.0 mm (n = 301), suggesting that
this assumption is suitable for this study. The average
weight of euphausiids was calculated using a formula
described by Odate [23]. The acoustic cross-section wasconverted from TS as follows:r ¼ 4p100:1TS :The average area biomass density (q) for each species
was calculated as
X SA
fi Wi ;
q¼
rwhere fi is the frequency distribution of the ith length class.
The frequency distribution of each class (fi), which
represents the acoustical contribution to the area
backscattering for each length class, was calculated as
fi ¼

1
X

nj L2j ;j¼1

where nj is the number of individuals in size class j and L islength (Ona E, unpubl. data, 1993).
The biomass of Pacific saury, which was one of the mostabundant epipelagic fish species in the habitat of thecommon minke whale (Table 1), was estimated frommidwater trawl sampling data. We assumed that the distribution depth of this species is 0–10 m, the area of mouth
opening for the trawl net is 600 m2, and the catchingefficiency of the trawl we used is 14.4%, based on similarsized trawl data [14]. To estimate the water volume filteredin each sampling, the average towing speed of midwater
trawl in each sampling (approximately 4–5 knots) was
adopted.
Whale sighting and sampling surveys
Whale sighting surveys were conducted mainly in a‘‘closing’’ approach mode. In this mode, the vessels
diverted from the track line by accelerating from thestandard vessel speed to about 15 knots, and approached
the detected animals in an attempt to identify the species

Fish Sci (2012) 78:557–567

and to count all animals in the group [24]. Because thesurvey track lines were established systematically in eachstudy area, each sampling position for the whales could beconsidered to reflect their actual distribution patterns.
Furthermore, because almost all target whale species foundwithin a 3-nautical-mile radius from the track line were
captured in this study, we deemed that the distribution
density of the sampling position of each whale species
generally reflected their relative abundance on the trackline. However, some individuals of common minke whale
could not be captured due to the dense distribution ofcommercial fishing vessels for Pacific saury in the northwestern part of the study area (Bando T, unpubl. data,
2009, see Fig. 2a). For this area, we also used whalesighting data obtained by sighting/sampling vessels toestimate the distribution patterns of the whales (Bando T,
unpubl. data, 2009). We defined the main distribution areaof sei and Bryde’s whales as the area where whale individuals were continuously distributed within an interval of
5 nautical miles on the track line. Because all individuals ofcommon minke whale were distributed continuously within
an interval of 5 nautical miles in several parts of the study
area, we divided the distribution area and other areas toestimate the distribution pattern of this whale (Bando T,
unpubl. data, 2009, Fig. 2a).
In total, seven common minke, 23 sei, and 24 Bryde’s
whales were sampled in each sampling area. Body length,
body wet weight, and wet weight of stomach contents inthe first and second compartments of the stomachs, whichwere regarded as newly consumed prey [1], were measured. The prey species were identified by examining thewhales’ stomach contents. We collected subsamples byrandomly selecting 2–4 and 0.5 kg of stomach contentsfrom the first and second stomachs, respectively. The total
number of each fish species in a subsample were determined by summing the number of undigested fish and half
the total number of free otoliths. The total weight of eachprey species before ingestion in the subsample was estimated by multiplying the average weight of fresh specimens by the number of individuals. Then, the total wet
weight of each prey species in the first and second stomachs was estimated by applying the weight ratio in thesubsamples to the total wet weight of the first and second
stomach contents.

561(a) Common minke whale
Subarctic
region 4
44˚N

Subarctic
front

5
6
7

8

Transitional domain

42
154

158˚E

156

(b) Sei whale
42˚N

Transitional domain

34.0
40

34.2

Transition zone

34.4

38
156

158

160˚E

(c) Bryde’s whale
40˚N

Transi- 6
tionaldomainSubarctic
boundary(34.0 PSU)

12

38

Transition zone

14
16

36

18
20

Kuroshio
region145

148˚E

Main distribution area of the whale
Other area
Sampling position of the whale
Sampling position of midwater trawl

Analysis of prey preferences
We used Manly’s selection index to determine the prey
preference of the whales in their main habitats [25], detailsof which was also described in Murase et al. [26]. The
standardized form of the selection index, Manly’s a [27],was applied to the North Atlantic stock of minke whales toreveal prey selectivity [28, 29]. Specifically, assuming that

Sampling position of MOCNESS

Fig. 2 a Distribution area of common minke whale and the
distributions of isotherms at 100 m deep, b distribution area of sei
whale and the distributions of the isohaline at 100 m deep, andc distribution area of Bryde’s whale and the distributions of isotherms
at 100 m deep as well as the subarctic boundary, on the survey tracklines. The sampling positions of the midwater trawls and MOCNESS
in each area are also indicated

123

562

Fish Sci (2012) 78:557–567

the jth stomach contents with the ith prey species areavailable, the total weight of prey species i used by all
animals (ui?) (kg) is
uiþ ¼

J
X

uij ;

j¼1

where uij is the weight of prey species i used by animal j.
The total weight of prey species used by animal j (u?j) is
I
P
uij .
uþj ¼
i¼1

The total weight of all prey species used by all animals
I P
J
P
uij :
(u??) is uþþ ¼
i¼1 j¼1

The sample proportion of prey species i by weight usedby all animals is oi ¼ uiþ =uþþ :The sample proportion of available units of prey species
i is
,
I
X
^ i ¼ mip
mii¼1

where mi is the amount of available units of prey species
i in a sample of available resource units. Manly’s selection^i ¼ oi =^indices are described as wpi .
Confidence intervals of the selection index were estimated as follows to determine the statistical significancelevel of whether whales randomly fed on prey species: The^i is given by
Bonferroni confidence interval of w^i Þ:^i  za=ð2IÞ seðww
If the confidence interval contains the value 1, whales^i is greater or lessfeed on prey species randomly [25]. If w
than 1, species i is actively or negatively selected,
respectively [25]. The value of a was set at 0.05.

Results
Oceanographic conditions
Two oceanographic fronts are distributed in the study area
north of 38°N: the subarctic front between the subarctic
region and the transitional domain, and the subarctic
boundary between the transitional domain and the transition zone [30, 31]. According to Kawai [32] and Murakami[33], the subarctic front is defined by a water temperatureof 5°C at 100 m. The subarctic boundary is defined by avertical 34.0 PSU salinity front in the epipelagic zone [30].
In the survey area of common minke whales, the watertemperature at 100 m was 4–7°C, indicating that this area
was located in the subarctic region and the northern transitional domain close to the subarctic front (Fig. 2a). In the123

survey area of sei whale, surface salinity front of 34.0 PSU
generally extended to 100 m depth, indicating that position ofsubarctic boundary could be estimated by the salinity at
100 m. The salinity at 100 m in the survey area of sei whales
ranged from 33.9 to 34.2 PSU, indicating that this area was
located in and around the subarctic boundary (Fig. 2b).
In the area west of approximately 152°E, anotheroceanographic front (the Kuroshio front) is defined as thearea south of the 14.0°C isotherm at 200 m deep [32]. The
temperature at 200 m at the survey area of Bryde’s whales
ranged from 15.1°C to 17.5°C in the area south of 36°N,
indicating that this area was located in the subtropical
Kuroshio region south of the Kuroshio front, whichapproximately corresponded to the 15°C isotherm at 100 min this study (Fig. 2c). In the area north of 36°N within thesurvey area of Bryde’s whales, the water temperatures at
100 and 200 m deep were[8°C and\14.0°C, respectively,
and the salinities at 100 m deep were [34 PSU south of
39°N and \34 PSU north of 39°N (Fig. 2c). These data
indicate that the southern area between 36°N and 39°N andthe northern area north of 39°N were located in the transition zone and transitional domain, respectively.
Species composition of the small epipelagic fish
community
The results of the target trawling showed that most of thesmall epipelagic fish-like acoustic backscatters were Japanese anchovy in the survey areas of both sei and Bryde’s
whales, indicating that Japanese anchovy was the mostabundant species in the small epipelagic fish community inthese regions (Table 1). In the survey area of common minke
whales, acoustic backscatters of small epipelagic fish species
were sometimes observed during predetermined trawl samplings, and we confirmed that these backscatters mostly
represented Japanese anchovy (Table 1). In this area, however, Pacific saury was frequently detected and most abundantly captured by the predetermined trawl samplings(Table 1). In this study period, main distribution areas ofJapanese anchovy and Pacific saury corresponded well to thenorthernmost region of the transition zone and the subarctic
region, respectively (see below).
Distribution and diet composition of whales
Common minke whale
This species was distributed in the subarctic region andnorthernmost region of the transitional domain, where thetemperature at 100 m deep was\6°C (Fig. 2a). Among theprey species, the abundances (mean ± standard deviation)of Pacific saury and Japanese anchovy were significantly
higher in the habitat of the whales than in the adjacent area toFish Sci (2012) 78:557–567

563

the south (12.5 ± 3.2 versus 0 mg m-2 and 1.6 ± 0.5 versus
0.4 ± 0.2 mg m-2, respectively, Mann–Whitney U test,
P \ 0.05, Fig. 3a). Conversely, euphausiids in the 0–250 m
layer were less abundant in the distribution area of the whales
than in the adjacent area (54.0 ± 8.8 versus 91.5 ± 10.3
mg m-2, Mann–Whitney U test, P \ 0.05, Fig. 3a). These
data indicate that the distribution centers of the common
minke whale, Pacific saury, and Japanese anchovy greatly
overlapped. The results of stomach content analysis of thewhales indicated that the most common prey item was Pacificsaury, followed by Japanese anchovy, which reflected the preyspecies composition within the whales’ habitat (Table 3).
Sei whale
Sei whales were mainly distributed in areas where thesalinity at 100 m deep was 34.0–34.1 PSU, corresponding

Fig. 3 Mean abundance of theprey species of common minke,sei, and Bryde’s whales in theirmain distribution area andadjacent areas. Bars indicate
standard deviation. Neocalanus
spp. includes N. cristatus andN. plumchrus. DA distribution
area of the whale, MDA main
distribution area of the whale,
OA other areas, SR subarctic
region, TD1 transitional domain
close to the subarctic front
where the temperature at 100 m
was \6°C, TD2 transitional
domain where the temperature
at 100 m was C6°C, TZ
transition zone, KR Kuroshio
regionto the transition zone within or close to the subarctic
boundary (Fig. 2b). Among the prey species examined,
Japanese anchovy was significantly more abundant in themain distribution area of the whale (8.1 ± 3.5 mg m-2)
than in other areas both in the transition zone (0.4 ± 2.2
mg m-2) and the transitional domain (0.3 ± 1.6 mg m-2,
Kruskal–Wallis test with Bonferroni-type adjustments,
P \ 0.05, Fig. 3b). However, euphausiids and two copepod
species, N. cristatus and N. plumchrus, in the epipelagiczone were more abundant in the transitional domain than in
the transition zone, including the main habitat of the whale(Kruskal–Wallis test with Bonferroni-type adjustments,
P \ 0.05, Fig. 3b). These results suggest that the main
habitats of sei whale and Japanese anchovy greatly overlapped in the survey area. This whale species mostly fed onJapanese anchovy, and mean wet weight values of this preyin the whale stomachs (mean ± standard deviation) were(a) Common minke whale
Pacific saury

Japanese anchovy

20

Euphausiids
120

2
80
10
1

40

Zero
0

0

0

(b) Sei whale

Copepods

Japanese anchovy
12

Euphausiids(Neocalanus spp.)

120
1200

8

80
800

4

40

400

0

0

0

(c) Bryde’s whale
Japanese anchovy2

Euphausiids
30
20

1
10
0

0

123

564

Fish Sci (2012) 78:557–567

significantly higher in its main distribution area than in theother two areas (Mann–Whitney U test, P \ 0.05, Table 3;
Fig. 4a).
Bryde’s whale
This species was frequently found in several regions within
the transition zone, whereas no and only a few individuals
were distributed in the transitional domain and the Kuroshio region, respectively (Fig. 2c). Among the prey species
examined, Japanese anchovy was significantly moreabundant in the main distribution area of the whale than inother areas in the transition zone and in the Kuroshio
region (Kruskal–Wallis test with Bonferroni-type adjustments, P \ 0.05, Fig. 3c). Euphausiids at 0–250 m werealso significantly more abundant in the main distribution
area of the whale than in the other two areas (Kruskal–
Wallis test with Bonferroni-type adjustments, P \ 0.05,
Fig. 3c). These data indicate that Bryde’s whales mainly
inhabit the distribution centers of Japanese anchovy andeuphausiids. In the stomachs of the whales, Japanese
anchovy and euphausiids ranked as the first and second
most common preys (Table 3). The mean wet weight values of Japanese anchovy in the whale stomachs were significantly higher in the main distribution area than in the
Table 3 Mean wet weight (kg) (mean ± standard deviation) of eachprey species found in the stomachs of common minke, sei, andBride’s whales
Prey species

Common minke(n = 7)Sei (n = 23)

Bryde’s(n = 24)

Euphausiids--

7.3 ± 25.2

Pacific saury25.2 ± 7.4--

Japanese anchovy

10.3 ± 7.7

255.8 ± 382.9

42.5 ± 106.0

Mackerels-

4.2 ± 14.4?

Japanese sardine-

1.1 ± 3.7?

n no. of whale stomachs examined, ? average value was less than
0.1 kgother two areas (80.6 ± 118.1 versus 7.4 ± 15.9 and3.7 ± 5.0 kg, Kruskal–Wallis test with Bonferroni-typeadjustments, P \ 0.05, Fig. 4b). Furthermore, the mean
wet weight of euphausiids in the stomachs of the whales
was also significantly higher in the main Bryde’s whale
distribution area than in other areas in the transition zone
(14.3 ± 35.0 versus 0.6 ± 1.4 kg, Mann–Whitney U test,
P \ 0.05, Fig. 4b). No euphausiids were found within thestomach contents of this whale in the Kuroshio region.
Prey preference^i ) [25] and analysis of the BonManly’s selection index (w^i indicated that the common
ferroni confidence interval of w
minke whale showed positive selection for Pacific saury^i = 4.791) and Japanese anchovy preys (w^i = 5.228), and(w
sei and Bryde’s whales positively selected Japanese anchovy^i = 6.177 and 11.368) (Table 4). The results of theprey (wselection index also indicated that Bryde’s whales selected^i = 0.162), although the whales
against euphausiid prey (wfrequently consumed this prey type. Furthermore, althougheuphausiids were abundantly distributed in the habitats ofcommon minke and sei whales, these whales fed on any
euphausiid prey (Table 3; Fig. 3). These results might indicate that it is difficult for these whale species to use euphausiid
prey as their biomass.

Discussion
The distribution of common minke whales generally corresponded to the location of the subarctic front. Because
common minke whale is mainly distributed in the subarctic
region in summer [1, 6, 12], the survey area of this study
was thought to be located at the southern limit of the main
distribution area of the whale. The subarctic region andnorthern transitional domain close to the subarctic front
constitute the main habitat of Pacific saury in the summer(b) Bryde’s whale(a) Sei whale

Japanese anchovy

Japanese anchovy

Euphausiids

200

800

40

600
100

400

20
200
0

0.8

0.3 + 0.8

0Fig. 4 Mean wet weight (kg) of prey species found in the stomachs
of sei and Bryde’s whales in their main distribution area and adjacentareas. Bars indicate standard deviation. MDA main distribution area

123

Zero
0

of the whale, OA other areas, TD transitional domain, TZ transition
zone, KR Kuroshio region

Fish Sci (2012) 78:557–567

565^i ) and the results of the statistical test for
Table 4 Selection index (wcommon minke, sei, and Bryde’s whales in their main habitat in thewestern North Pacific during mid summer
Euphausiids

Japaneseanchovy

Pacific
saury

Common minke whale
^i
w

0.000

5.228

4.791

Bonferroni CI (low)

–

2.447

4.443

Bonferroni CI (high)

–

8.010

5.138

Sei whale
^i
w

0.000

6.177

–

Bonferroni CI (low)

–

6.148

–

Bonferroni CI (high)

–

6.207

–

Bryde’s whale
^i
w

0.162

11.368–

Bonferroni CI (low)0.132

11.000–Bonferroni CI (high)

0.191

11.736–[14], which was also demonstrated by our sampling data. In
this study, Japanese anchovy was also more abundantly
distributed in the minke whale habitat than in the transitional domain, probably due to high productivity of theformer region in summer [34]. Common minke whales fed
mainly on Pacific saury and secondarily on Japanese
anchovy, suggesting that this whale species preferred themain distribution area of these fish prey for feeding.
However, given that the main distribution area of Japanese
anchovy in the summer is located in and around the transition zone chlorophyll front (TZCF) and south of thetransitional domain, as discussed below [35, 36], Pacificsaury appear to affect the distribution of common minke
whales more significantly than do Japanese anchovy. Thishypothesis is supported by the present result that common
minke whales preferred Pacific saury prey, in combination
with previous results that this whale species feeds mainly
on Pacific saury in their main habitat in summer and early
autumn in or close to the subarctic region [1]. According toMurase et al. [26], common minke whales positively selectJapanese anchovy prey in the Kuroshio–Oyashio transition
region south of the habitat of Pacific saury in August,suggesting that this whale species prefers small epipelagic
fish species rather than zooplanktons, both in its main
distribution area and in its southernmost distribution area
during summer.
With respect to sei whales, the main distribution area
was located in and around the subarctic boundary.
According to Konishi et al. [1] and Fujise et al. [12], sei
whale was mainly found in the transition zone south of
40°N in May and June and in the transitional domain of
40–45°N from July to September, suggesting that thisspecies migrates into the transitional domain from thetransition zone in July. Therefore, the present result that sei
whale was mostly found in the northernmost part of transition zone likely indicates the distribution pattern of thewhale in early July, when this survey was conducted. The
subarctic boundary also corresponds to the summertime
location of the TZCF, which migrates to the subarctic
boundary from the subtropical region during spring andsummer [35, 37–39]. The TZCF is a zone of surface convergence, where cool surface waters from the north, containing high levels of chlorophyll a, sink beneath warm,
oligotrophic waters to the south. This indicates that theregion of the TZCF and north of the TZCF is a productive
area [35, 37, 38]. The present result that sei whales preferred and heavily fed on Japanese anchovy prey, which aremainly distributed in and around the TZCF in early summer [36, 37], suggests an enhanced feeding regime for the
predator in the TZCF in this period. Conversely, euphausiids and two copepod species, N. cristatus andN. plumchrus, were less abundant in the transition zone,
including the main habitat of the whale, than in the transitional domain, suggesting that these prey do not significantly affect the distribution of the whales in this study
area. This whale species showed no prey preference for
euphausiid prey and fed on any copepod prey, thus supporting this view. Such south–north distribution patterns ofzooplankton are also shown in previous reports [34].
However, because we could not obtain adequate data in thetransitional domain in this study, future surveys in thisregion are required to estimate distribution and migration
pattern of sei whale in summer.
Main distribution area of Bryde’s whale is locatedwithin an area bounded by 35–40°N in summer in thewestern North Pacific [1, 12], showing that survey area of
this study was covered latitudinal distribution range of thewhale. In this study, the main habitat of Bryde’s whales
was within the less productive transition zone south of the
TZCF. However, from a trophic standpoint, the transitionzone may be favorable over the wintering area of thesubtropical region for the whale during the summer
because the former area is an oligotrophic region in thisseason, whereas the latter area is more productive [40–42].
Given that Japanese anchovy and euphausiids greatly
overlapped within the main habitat of Bryde’s whale, thiswhale species appears to select the habitats where theseprey are abundantly distributed. The present results that
Bryde’s whale feed mainly on Japanese anchovy and secondly on euphausiids, and that the feeding activity of thewhale was significantly higher in its main habitat than inother areas, might support this view. However, positive
prey selection of the whale was only shown for Japanese
anchovy, suggesting that Japanese anchovy affects the
distribution of the whale more significantly than euphausiids. Previous reports that Bryde’s whale generally

123

566

heavily feed on Japanese anchovy prey between the area of
35–40°N in the western North Pacific in summer might
support this view [1, 12].
The present results indicated that the distributions ofPacific saury or Japanese anchovy preferred by common
minke, sei, and Bryde’s whales greatly affect the distributions of these whale species, which strongly suggeststhat the habitat selection of the whale species is closely
related to their prey selection in summer in the western
North Pacific. These prey species are known to undertake
northward migration and migrate into subarctic regions or
southern transitional domains from subtropical regions
during spring and summer as the TZCF moves northward[8, 13, 37]. In spring in the western North Pacific, the main
prey species of sei and Bryde’s whales is also Japanese
anchovy in the lower latitudinal areas between 38°N and
40°N, and 35°N and 38°N, respectively, which represent
the main habitats of these whales in this season [1]. In
summer, these whale species also mainly feed on Japanese
anchovy in the higher latitudinal area as was shown in thepresent and previous studies [1, 12]. These observations
suggest that the predator–prey relationship is almost
maintained from spring to summer, since their seasonal
migration patterns are spatiotemporally similar to one
another. Therefore, these whales might exploit the distribution area of Japanese anchovy as a migration route andas a forage habitat from spring to summer. In case ofcommon minke whales, the main prey species shifts fromJapanese anchovy to Pacific saury from spring to summer
during its northward migration period from subtropical tosubarctic regions [1, 6]. This whale species continues todepend on Pacific saury during late summer and early
autumn in or close to the subarctic region [1, 6]. Considering that Pacific saury migrate northward prior to Japanese
anchovy from spring to summer, and that the summer
distribution area of Pacific saury is the adjacent area northof the habitat of Japanese anchovy [43], common minke
whales might migrate northward in the prey-rich area fromspring to summer. Therefore, the summertime northwardmigrations of the three whale species are believed to befeeding migrations, like in many other predators such as
swordfish Xiphias gladius, albacore Thunnus alalunga, andneon flying squid Ommastrephes bartramii, which also
feed on seasonal south–north migratory prey like Japanese
anchovy [35, 36, 42, 44–46].
The distribution of these whale species was also generally explained by the locations of the subarctic andKuroshio fronts as well as the subarctic boundary, likely
because the summer distributions of the preferred preyspecies are closely related to these oceanographic features[8, 23, 43, 44]. Although the present results were obtained
from mesoscale data (i.e., a scale of several hundred
kilometers), these fronts and boundaries are widely

123

Fish Sci (2012) 78:557–567

distributed in the western North Pacific [31, 32]. Because
the whale species and their preferred prey species arewidely distributed in the western North Pacific [1, 8, 23,
43, 44, 47], the present results could reflect the general mid
summer distribution patterns of these whale species inrelation to both physical oceanographic and prey environments in the western North Pacific. Therefore, informationof the locations of these three fronts or boundaries isimportant for estimating the main distribution areas ofthese whales in this period. In this study area, SST has
generally been used to estimate the distributions of baleen
whale species [1, 11]. However, because these oceanographic fronts and boundaries could be recognized by thetemperature at depths of 100 or 200 m or vertical salinity
profiles in the epipelagic zone [31, 32], the mid summer
distributions of these whale species might be more accurately estimated using prediction models of oceanographic
conditions, including the positions of these fronts andboundaries.
Acknowledgments Special thanks are given to Captains Y. Terada
and S. Sawadaishi and crews of Shunyo Maru, Captain T. Ogawa andcrews of Nisshin Maru, and late G. Fujiwara of National ResearchInstitute of Far Seas Fisheries (NRIFSF) for their skillful assistance incollecting data. K. Sawada of National Research Institute of Fisheries
Engineering, H. Hatanaka of the Institute of Cetacean Research,
T. Ichii of Japanese Fisheries Agency, K. Taki, and Y. Kanaji of
NRIFSF gave valuable information and comments during the courseof this study. This research was supported by a grant from JapaneseFisheries Agency.

References
1. Konishi K, Tamura T, Isoda T, Okamoto R, Matsuoka K,
Hakamada T (2009) Feeding strategies and prey consumption of
three baleen whale species within the Kuroshio-Current extension. J Northwest Atl Fish Sci 42:27–40
2. Kasamatsu F, Miyashita T, Yoshioka M (2009) Field guide towhales, dolphins, and porpoises in the western North Pacific andadjacent waters. Tokyo University Press, Tokyo (in Japanese)
3. Nemoto T (1962) Prey items of baleen whale. Rep Inst Cetacean
Res 4:1–136 (in Japanese)
4. Kawamura A (1973) Food and feeding of sei whale caught in thewaters south of 40°N in the North Pacific. Sci Rep Whales Res
Inst Tokyo 32:155–197
5. Tamura T, Fujisese Y, Shimazaki K (1998) Diet of minke whales
Balaenoptera acutorostrata in the northwestern part of the North
Pacific in summer 1994 and 1995. Fish Sci 64:71–76
6. Tamura T, Fujise Y (2002) Geographical and seasonal changes ofthe prey species of minke whale in the Northwestern Pacific.
ICES J Mar Sci 59:516–528
7. Kuroda K (1991) Studies on the recruitment process focusing on
the early life history of the Japanese sardine, Saldinops melanostictus (Schlegel). Bull Natl Res Inst Fish Sci 3:25–278 (inJapanese with English abstract)
8. Takahashi M, Watanabe Y, Kinoshita T, Watanabe C (2001)
Growth of larval and early juvenile Japanese anchovy, Engraulisjaponicus, in the Kuroshio–Oyashio transition region. Fish Oceanogr 10:235–247

Fish Sci (2012) 78:557–567
9. Garcia SM, Zerbi A (2003) Ecosystem approach to fisheries:
issue, terminology, principles, institutional foundations, implementation and outlook. FAO Fish Tech Paper 443:1–71
10. Word Summit on Sustainable Development (2002) Plan of
implementation. International Union for Conservation of Nature,
Johanesburg
11. Nemoto T (1959) Food of baleen whales with reference to whale
movements. Sci Rep Whales Res Inst Tokyo 14:149–291
12. Fujise Y, Tamura T, Bando T, Konishi K, Yasunaga G (2004) Sei
and Bryde’s whales. Geiken Sousyo No. 11 (in Japanese)
13. Odate S, Hayashi K (1977) Ecological studies on the larvae andjuveniles of the Pacific saury Cololabis saira (Brevoort). Bull Tohoku
Reg Fish Res Lab 38:89–101 (in Japanese with English abstract)
14. Ueno Y, Suyama S, Kurita Y, Kumazawa T (2004) Design andoperation methods of a mid-water trawl for quantitative sampling of asurface pelagic fish, Pacific saury (Cololabis saira). Fish Res 66:3–17
15. Reynolds RW, Rayner NA, Smith TM, Stokes DC, Wang W(2002) An improved in situ and satellite SST analysis for climate.
J Clim 15:1609–1625
16. Ohshimo S, Hamatsu T (1996) Vertical distribution and acoustic
estimation of biomass of walleye pollock Theragra chalcogramma, and anchovy Engraulis japonicus in the Pacific coast ofeastern Hokkaido. Bull Hokkaido Natl Fish Res Inst 60:225–237
17. Miyashita K, Aoki I, Asami T, Mori H, Taki K (1998) Study on
acoustical estimation of distribution and abundance of Isada krill,
Euphausia pacifica, Hansen, off the Sanriku and off the Jhoban,
northern Japan. J Korean Soc Fish Res 1:128–135
18. Miyashita K, Aoki I, Seno K, Taki K, Ogishima T (1997) Acoustic
identification of isada krill, Euphausia pacifica Hansen, off theSanriku coast, north-eastern Japan. Fish Oceanogr 6:266–271
19. Jolly GM, Hampton I (1990) A stratified random transect design for
acoustic surveys of fish stocks. Can J Fish Aquat Sci 47:1282–1291
20. Amakasu K, Sadayasu K, Abe K, Takao Y, Sawada K, Ishii K (2010)
Swimbladder shape and relationship between target strength andbody length of Japanese anchovy (Engraulis japonicus). J Mar
Acoust Soc Jpn 37:46–59 (in Japanese with English abstract)
21. Kubota H, Sakachi H, Takasuka A, Kawabata J, Akamine T,
Shimizu A (2010) Japanese anchovy in the Pacific Ocean. In: The
current status of fishery stocks around Japanese waters. Fisheries
Agency of Japan, Tokyo, pp 725–752 (in Japanese)
22. Miyashita K, Aoki I, Inagaki T (1996) Swimming behavior andtarget strength of isada krill (Euphausia pacifica). ICES J Mar Sci
53:303–308
23. Odate K (1991) Fishery biology of the krill, Euphausia pacifica,
in the northeastern coast of Japan, vol 40. Japan Fisheries
Resource Conservation Association, Tokyo, pp 1–100 (in Japanese with English abstract)
24. Ksamatsu F, Ensor P, Joyce GG, Kimura N (2000) Distribution ofminke whales in the Bellingshausen and Amundsen Seas (60°W–
120°W), with special reference to environmental/physiographic
variables. Fish Oceanogr 9:214–223
25. Manly BFJ, McDonald LL, Thomas DL, McDonald TL, Erickson
WP (2002) Resource selection by animals: statistical design andanalysis of field studies. Kluwer Academic, The Netherlands
26. Murase H, Tamura T, Kiwada H, Fujise Y, Watanabe H, Ohizumi
H, Yonezaki S, Okamura H, Kawahara S (2007) Prey selection ofcommon minke (Balaenoptera acutorostrata) and Bryde’s (Balaenoptera edeni) whales in the western North Pacific in 2000 and
2001. Fish Oceanogr 16:186–201
27. Manly BFJ, Miller P, Cook LM (1972) Analysis of a selective
predation experiment. Am Nat 106:719–736
28. Lindstrøm U, Haug T, Nilssen KT (1997) Diet studies based oncontents from two separate stomach components of northeast
Atlantic minke whales Balaenoptera acutorostrata. Sarsia 82:63–68
29. Lindstrøm U, Haug T (2001) Feeding strategy and prey selectivity in common minke whale (Balaenoptera acutorostrata)

567
foraging in the southern Barents Sea during early summer.
J Cetacean Res Manag 3:239–249
30. Favorite F, Dodimead AJ, Nasu K (1976) Oceanography of thesubarctic Pacific region, 1960–71. Bull Int Natl North Pac FishComm 33:1–187
31. Pearcy WG (1991) Biology of the transition region. NOAA Tech
Rep NMFS 105:39–55
32. Kawai H (1972) Hydrography of the Kuroshio and the Oyashio.
In: Masuzawa J (ed) Physical oceanography II. Fundamental
lectures of oceanography. Tokai University Press, Tokyo, pp 129–321(in Japanese)
33. Murakami M (1994) On long-term variations in hydrographic
conditions in the Tohoku area. Bull Tohoku Natl Fish Res Inst
56:47–56 (in Japanese with English abstract)
34. Taniguchi A (1981) Plankton productivities in the Pacific subarctic boundary zone. Spec Vol Res Inst North Pac Fish Hokkaido Univ Fac Fish 23–35 (in Japanese with English abstract)
35. Polovina JJ, Howell E, Kobayashi DR, Seki MP (2001) The
transition zone chlorophyll front, a dynamic, global featuredefining migration and forage habitat for marine resources. Prog
Oceanogr 9:71–82
36. Watanabe H, Kubodera T, Masuda S, Kawahara S (2004) Feeding
habits of albacore Thunnus alalunga in the transition region of
the central North Pacific. Fish Sci 70:573–579
37. Polovina JJ, Kobayashi DR, Parker DM, Seki MP, Balazs GH(2000) Turtles on the edge: movement of loggerhead turtles(Caretta caretta) along oceanic fronts spanning longline fishinggrounds in the central North Pacific, 1997–1998. Fish Oceanogr
9:71–82
38. Bogard SJ, Foley DG, Schwing FB, Wilson C, Laurs RM, Polovina JJ, Howell EA, Brainard RE (2004) On the seasonal andinterannual migration of the transition zone chlorophyll front.
Geophys Res Lett. doi:10.1029/2004GL020637
39. Baker JD, Polovina JJ, Howell EA (2007) Effect of variable
oceanic productivity on the survival of an upper trophic predator,
the Hawaiian monk seal Monachus schauinslandi. Mar Ecol Prog
Ser 346:277–283
40. Cullen JJ (1982) The deep chlorophyll maximum: comparing vertical profiles of chlorophyll a. Can J Fish Aquat Sci 39:791–803
41. Sassa C, Kawaguchi K, Kinoshita T, Watanabe C (2002) Assemblages of vertical migratory mesopelagic fish in the transitional
region of the western North Pacific. Fish Oceanogr 11:193–204
42. Ichii T, Mahapatra K, Sakai M, Okada Y (2009) Life history
differences between seasonal cohorts of the neon flying squid
associated with the oceanographic regime in the North Pacific.
Mar Ecol Prog Ser 378:1–11
43. Pearcy WG, Fisher JP, Anma G, Meguro T (1996) Species
associations of epipelagic nekton of the North Pacific Ocean,
1978–1993. Fish Oceanogr 5:1–20
44. Ichii T, Mahapatra K, Sakai M, Inagake D, Okada Y (2004)
Differing body size between the autumn and the winter-springcohorts of neon flying squid (Ommastrephes bartramii) related tothe oceanographic regime in the North Pacific: a hypothesis. Fish
Oceanogr 13:295–309
45. Watanabe H, Kubodera T, Ichii T, Kawahara S (2004) Feeding
habits of neon flying squid Ommastrephes bartramii in thetransitional region of the western North Pacific. Mar Ecol Prog
Ser 266:173–184
46. Watanabe H, Kubodera T, Yokawa K (2009) Feeding ecology ofthe swordfish Xiphias gladius in the subtropical region andtransition zone of the western North Pacific. Mar Ecol Prog Ser
396:111–122
47. Tsuruta Y, Takahashi S (1997) Reproductive ecology of theJapanese anchovy (Engraulis japonicus H.) in the Kuroshio
extension and the mixed water region. Bull Hokkaido Natl Fish
Res Inst 61:9–15 (in Japanese with English abstract)123