SC/J09/JR11
Distribution and abundance of prey species and prey preference ofcommon minke whale Balaenoptera acutorostrata in the coastal
component of JARPN II off Kushiro from 2002 to 2007
HIKARU WATANABE1*, SHIROHKUMAGAI2, TOSHIYA KISHIRO1,
KAWAHARA3YONEZAKI2, HIROSHI KIWADA2, SAEKOHIDEYOSHI YOSHIDA1, AND SHIGEYUKI

1National Research Institute of Far Seas Fisheries (Yokohama), 2-12-4 Fukuura Kanazawa
Yokohama Kanagawa, 236-8648, Japan
2
The Institute of Cetacean Research, 4-5 Toyomi-cho Chuo-ku Tokyo, 104-0055, Japan
3
National Research Institute of Far Seas Fisheries (Shimizu), 5-7-1 Orido Shimizu Shizuoka
Shizuoka, 424-8633, Japan*Author in correspondence, e-mail: hikaru1@affrc.go.jp

ABSTRACT: We studied prey preference of common minke whale Balaenoptera acutorostrata
based on the data from midwater trawl, IKMT, and acoustic surveys in their habitat as well as
the stomach content data of the whales off Kushiro, northern Japan. All surveys were conductedin the autumn of 2002-2007 as a part of the coastal component of the Research Program underSpecial Permit in the Western North Pacific (JARPN II). Among the potential prey species,
Pacific saury Cololabis saira and euphausiids were mainly distributed in or close to the Oyashio
region where sea surface temperature (SST) was 11-16˚C. Japanese anchovy Engraulisjaponicus and Japanese common squid Todarodes pacificus were mainly found in the northerntransition region of 14-18˚C SST, although distributions of the squid were almost restricted tothe slope water region. Walleye pollock Theragra chalcogramma were widely distributed at100-250 m depth in the continental shelf region where SST was 12-22˚C. The total abundanceof these species was generally higher in the continental shelf region than in the offshore region
due to high SST field (> 19˚C) in the latter region. These results suggest that slope water regionof less than 18˚C SST is a rich prey environment in the both epi- and mesopelagic zones. The
result of whale sighting surveys indicated that the common minke whale was generally
distributed in the slope water region of less than 16˚C SST and that its distributions expanded tothe offshore region in 2002 when SST of both inshore and offshore region was less than 15˚C
due to the strong influence of the Oyashio. Therefore the common minke whale might preferrich prey environment affected by the Oyashio not only in the continental shelf region where
walleye pollock, Pacific saury, and euphausiids could be distributed but also in the offshore
region where Pacific saury and euphausiids could be distributed. Based on the data of oursurveys and whale sampling surveys, it was suggested that immature common minke whales
prefer walleye pollock while mature animals prefer Pacific saury, although both frequently fed
on Japanese anchovy in some years in the area within 50 nautical miles from Kushiro. These
differences in prey preference might be explained by the trade-offs of cost of foraging activity
for prey and/or energy demands between immature and mature whales.
KEY WORDS: common minke whale, distribution, abundance, prey species, prey preference,
autumn, Kushiro, Oyashio, JARPN II

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BACKGROUND ON PREY SPECIES SURVEY FOR CETACEANS
Generally, fisheries resources especially small epipelagic fish species such as
sardine and anchovy largely fluctuate in decadal scales as seen in the remarkablerebound and collapse of the Japanese sardine (Sardinops melanostictus) stock from the1970s to 1990s (Kuroda 1991, Nakata et al. 1994). The fluctuation of Japanese sardine
stock greatly affected the total catch of Japanese fisheries, which decreased drasticallyfrom 13 to 7 million tons during 1988 and 1998. Historically, fishery management was
conducted based on the ecological and biological information of a single target species
and physical oceanographic conditions in their habitat. This approach was, however,inadequate to understand the primary factors of fluctuations and future status of theresource. Since the single species management focuses on maximizing the catch of thetarget species, this often ignores predators and prey of the target species, although suchinformation is essential to understand the dynamics of these species. Therefore, theFisheries Agency of Japan adopted the principle which first priority is given to
ecosystem-based management within Japanese EEZ for the fundamental policy on
fisheries in the future.
Ecosystem-based fisheries management based on ecosystem model is a new
direction for fishery management. Building the ecosystem model is the worldwide
recognition of the important approach to future fisheries (WSSD 2002, Pikitch et al.
2004). This is one of the major goals of the second phase of Japanese Whale Research
Program under Special Permit in the western North Pacific (JARPN II, Government ofJapan, 2002). The JARPN II ecosystem model is expected to establish the new systemof sustainable utilization and management of whole marine ecosystem including marine
mammals in the western North Pacific. In 2000, JARPN II started to construct Ecopath
with Ecosim (EwE) model, which is one of the major ecosystem models (Okamura et al.
2001, Mori et al. 2008). In relation to the development of this model, the factor of prey
preference of each cetacean species is essential to determine their ecological role. To
obtain the information of prey preference, comparison between prey species
composition in the cetacean stomachs and that in the cetacean habitat is necessary.
Furthermore, International Whaling Commission (IWC) recommended a morequantitative estimation of the temporal and geographical variation in diet of cetaceans(IWC 2000). Therefore, JARPN II has conducted cooperative surveys of prey species
and whale sampling.
The common minke whale Balaenoptera acutorostrata is one of the major
baleen whale off Kushiro, eastern Hokkaido, where is one of the major fishing groundsof Japan, during summer and autumn. Results from previous program, JARPN(conducted from 1994 to 1999), indicated that this species feed mainly on commercially
important fish and squids like Japanese anchovy Engraulis japonicus and Pacific saury
Cololabis saira, suggesting the competition between the whale and fisheries (Tamura
and Fujise 2000). In the JARPN II coastal component off Kushiro, prey species survey
started in the autumn of 2002 during the sampling survey period of common minke
whales to estimate abundance of potential prey in the habitat of the whale and prey
preference of the whale. In this area, abundance and fishing area of these prey species isknown to largely fluctuate by year due to annual changes of interaction between
subarctic Oyashio waters and subtropical waters detached from the Kuroshio (Kawai
1972). These changes are thought to greatly affect the prey environment of common

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minke whale. Therefore, Kushiro area is thought to be suitable for both prey preference
analysis and studies on feeding strategy of the whale.

OBJECTIVE
Based on the backgrounds above mentioned, we report the results of prey species
survey in the JARPN II coastal component off Kushiro from 2002 to 2007 by focusing
on the following four goals.
1. To clarify the geographical distribution patterns of prey species of common minke
whale in relation to oceanographic features.
2. To estimate annual changes in biomass of the major prey species of common minke
whale.
3. To clarify the geographical distribution pattern of common minke whale in relation toprey environment and oceanographic features.
4. To estimate the prey preference and feeding strategy of common minke whale. (Inthis paper, prey preference is defined as the animal choosing one particular resource
over others irrespective of amount of resources.)

MATERIALS AND METHODS
Prey species survey
Prey species survey was conducted off the eastern Hokkaido, from 142º30’E to
147º00’E, and north of 41ºN except for Russian EEZ in autumn from 2002 to 2007 (Fig.
1). During the survey, the sampling survey of common minke whale was also conductedin the area of within 50 nautical miles from Kushiro (Kishiro et al. 2009a). For thequantitative analyses of prey environment, the survey area was divided into coastal andoffshore blocks, and the coastal part was further divided into eastern, central, westernand off-Hidaka blocks (Fig. 1). Zigzag track lines were set to cover each small block of
the study area. On these lines, the distribution and abundance of prey species were
investigated with the quantitative echosounder (38 and 120 kHz), midwater trawl, andIsaacs-Kidd midwater trawl (IKMT) during the daytime. We conducted two kinds of net
samplings. One is target trawling to identify fish species and size compositions of
acoustic backscatters. The other is so-called ‘predetermined trawling’ that was towed
generally every 20 to 30 nautical miles on the track lines to examine the distribution andabundance of squids and neustonic organisms like Pacific saury that are difficult to
detect by the echosounder. A Conductivity-Temperature-Depth (CTD) profiler cast wasmade down to 500 m depth or near the bottom of shallower than 500 m depth at eachsampling station to depict the oceanographic conditions (see Okazaki et al. (2009).
All samples were identified to the lowest taxonomic level possible and wet bodyweight of each species was measured. For the major species, body length of eachindividual was measured from randomly selected 100 individuals. However, we did notmeasured body length of salmons in 2005. We also obtained the total wet weight ofthese samples to estimate the total catch number for each sampling.

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Geographical distribution of major prey species
We estimated the geographical distribution patterns of each potential prey speciesof the common minke whale generally based on our acoustic and/or trawling data. Since
our midwater trawl surveys near the bottom of the continental shelf were greatly
restricted, we also used the data of bottom trawl samplings conducted by Hokkaido
National Fisheries Research Institute in August and/or September from 2002 to 2006 to
determine the distribution of walleye pollock Theragra chalcogramma and Japanesecommon squid Todarodes pacificus (Honda unpubl. data 2002, 2004, 2005, and 2006).

Estimation of biomass for potential prey species
We estimated biomass of Japanese anchovy, walleye pollock, and euphausiids
based on the acoustic data. Biomass estimation of these species was conductedaccording to the method of Jolly and Hampson (1990) as follows:
Mean backscattering area per square nautical mile of sea surface (SA) by species forevery 1 nautical mile of survey transect over defined depth interval is calculated byfollowing formula;
S A = 4πr021852 2 ∫rr2 sV dr (1

m2
n.mi 2)where r is depth from the sea surface, r0 = 1m representing the reference range forbackscattering strength. A length-target-strength (TS) relationship for Japanese anchovy(Anonymous 1990) is used;
TS = 20 LogTL − 72.5

where TL is total length in cm. A length-weight relationship for Japanese anchovy(Anonymous 1990) is used;
W = 0.004TL3.09

where W is weight in gram.
A length-target-strength (TS) relationship for walleye pollock (Foote and Traynor 1988)
is used;TS = 20 LogFL − 66

where FL is folk length in cm.
A length-weight relationship for walleye pollock (Pereyra et al. 1981) is used;
W = 0.0077FL2.906

where W is weight in gram.
Because no length-TS relationship for euphausiid was available in this area, we
assumed that all euphausiids observed during two surveys was Euphphausia pacifica
and length and TS were 16.4mm and –83.3 dB (Miyashita et al. 1996), respectively. The
average weight was 30.6 mg calculated using formula described by Odate (1987).
Average area biomass density ( ρ ) for each species is calculated as follows;
ρ =∑

SA

σ

f iWi

where fi is frequency distribution of ith length class. The acoustic cross section isconverted from TS as follows;4

SC/J09/JR11
σ = 4π 100.1TS

Frequency distribution of each class (fi) that is the acoustical contribution to the area
backscattering for each length class (Ona 1993);∞

fi = ∑ n j L2j
j =1

where nj is the number of individuals in size class j and the length is L.
Following procedures are adopted from Jolly and Hampton (1990). Weighted
mean of SA of each block is;Nk∑ S Aki (nk i )S Ak = i =1 Nk∑ nk ii =1where S Ak = mean SA in kth block, N k = number of transects in kth block, S Aki =mean SA on the ith transect in kth block and nk i = number of 1 n.mile averaging
intervals on the ith transect in kth block. In this formula, each transect is regarded as asingle biomass density sample. Then variance of S Ak is calculated with the formula(Jolly and Hampton 1990);
Nk− S Ak ) 2 nk i 2∑ (SN k i =1 Ak i
Var ( S Ak ) =2
Nk −1⎛ Nk ⎞⎜ ∑ nk i ⎟⎜ i =1 ⎟⎠⎝is converted to ρ using above motioned formula.
Abundances of Pacific saury, less abundant small pelagic fishes, Japanese sardine
and mackerel Scomber spp., and larger epipelagic nektons, Pacific pomfret Bramajaponica and chum salmon Onchorhyncus keta, were estimated from the midwater trawl
sampling data. We assumed that the area of mouth opening for the trawl net is 600 m2,
average towing speed of the net is 4 knots, and the catchability of the trawl is 14.4% for
Pacific saury (Ueno et al. 2003), 18% for Japanese anchovy and mackerel, based on thesimilar sized Japanese anchovy (Japan Marine Fishery Resource Research Center 1991),
and 10% for Pacific pomfret and chum salmons. The main distribution depth was
assumed at 0-30 m for Pacific saury, Japanese sardine, and mackerel and at 0-100 m for
Pacific pomfret and chum salmons. Using the data of midwater trawl, we also estimatedthe biomass of Japanese common squid above 100 m assuming that the catchingefficiency of the trawl is 14.4%. We also used the data of the bottom trawl samplings
conducted in the 100-250 m layer during the daytime to estimate the biomass of thissquid assuming that mouth opening of the trawl is 10 m2, average towing speed is 3.5
knots, and catchability of the trawls is 100% (Honda unpubl. data 2002, 2004, 2005,
and 2006).
SA

Analysis of geographical distribution of common minke whale
The data of cetacean sighting survey of JARPN II by the line transect method,and our trawl and CTD data were used for this analysis (Fujise et al. 2003, Kishiro et al.
2003, Tamura et al. 2005, Kishiro et al. 2005, Kishiro et al. 2006, Yoshida et al. 2007,
Kiwada et al. 2008). Cetacean sighting survey was conducted concurrently with preyspecies and whale sampling surveys in 2002 and 2004-2007, and also conducted as apart of offshore component of JARPN II in 2003 when prey species and whaling
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surveys were not conducted in the coastal area off Kushiro (Tamura et al. 2004). All
sighting surveys were conducted basically in passing mode, but abeam closing mode
was also adopted for sighting within two nautical miles perpendicular to track line when
the sighting was likely a baleen whale but species was not identified. During the survey,
primary observers were allocated at the top barrel (three observers) and the upper bridge(two observers). Sea surface temperature (SST) was also monitored during the survey.

Analysis on prey preference
For prey preference analysis, we used prey species survey data conducted within
50 nautical miles from Kushiro and stomach content data of the common minke whale(Kishiro et al. 2009b). Temporal difference between prey species data and stomach
content data was within 10 days before or after. We adopted Manly’s α (Manly et al.
1972, also known as Chesson’s index, Chesson 1978) to reveal prey preference of thewhale. This index was successfully applied to the North Atlantic stock of minke whale
to show prey preference (Lindstrøm and Haug 2001). Standardized Manly’s α waswritten as;
αi = ri / ni {1/Σ(rj / nj)}
where ri is percentage of ith prey species in wet weight in the whale stomachs, ni ispercentage of ith prey species in wet weight in the environment, rj is percentage of jth
prey species in wet weight in the whale stomachs, and nj is percentage of jth preyspecies in wet weight in the environment. If α is equal to 1/m (m indicates total numberof species in the environment), species i is randomly selected. If α is greater than 1/m,
species i is actively selected. If α is less than 1/m, species i is avoided.

RESULTS
1. Geographical distribution and habitat temperature range of potential preyspecies
1.1. Oceanographic conditions
Our results of SST and oceanographic features reported by Okazaki et al. (2009)indicated that the subarctic Oyashio region, which is defined as a salinity of less than
33.6 PSU in the 50-200 m layer and a water temperature lower than 6˚C at 100 m(Kawai 1972), was approximately corresponded to the area of less than 14-15˚C SST in
autumn off eastern Hokkaido. In the sampling survey area of common minke whale
within 50 nautical miles from Kushiro, the area of less than 15˚C SST was accounted
for more than 90% in 2002, 2004, and 2007 (Fig. 2). On the other hand, SST in that
region was warmer in 2005 (48%) and 2006 (3%). These results show that Kushiro
water shifted warmer from 2004 to 2006 then colder from 2006 to 2007 (Fig. 2). Amongthe small sampling blocks, this tendency was also shown in coast east and coast central
blocks, which were located in the upstream region of the Oyashio (Fig. 3). In 2002,
Oyashio influence was the greatest among all prey species survey years bringing cold

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water less than 15˚C SST to the offshore blocks and blocks of coast west and off Hidaka
which were located in the downstream region of the Oyashio. However, among the six
years from 2002 to 2007, Oyashio influence was the strongest in 2003 when SST areaof less than 14-15˚C greatly expanded to the offshore region (Japanese Coast Guard:
http://www1.kaiho.mlit.go.jp/KAN1/kaisyou/dwn/2003.html). Unfortunately, only
sighting survey was conducted this year.

1.2 Prey species composition in the environment
The result of our predetermined trawl survey indicated that Japanese anchovy andPacific saury were abundant component in the epipelagic smalll fish community (Table
1). According to our acoustic and IKMT data, and data from bottom trawl samplings(Honda, unpubl. data 2002, 2004, 2005, and 2006), other abundant potential preyspecies were walleye pollock, Japanese common squid, and euphausiids (most of whichwere Euphausia pacifica, see below).

1.3. Geographical distribution and habitat temperature range of potential prey species
1.3.1. Abundant five species
Pacific saury: The result of predetermined trawling survey indicated that this species
was mainly distributed in the 11-15˚C SST areas with a strong influence of the Oyashio
Current (Figs. 4 and 5). Although distribution area of this species in 2002 expanded tothe offshore region due to relatively low SST, its main distribution was the coastal
region in all years (Figs. 3 and 5). The southern limit distribution area of this species
was corresponded to the 17 or 18˚C SST isotherms (Figs. 4 and 5).
Japanese anchovy: This species was mainly distributed in the Oyashio-Kuroshiotransition region south of the Oyashio front where SST was 14-19˚C (Figs. 4 and 6). Its
geographical distribution pattern was greatly changed in each year because distributionsof 14˚C and 19˚C isotherms at 0 m depth were largely fluctuated by year. In 2002, thisspecies was widely distributed from coastal to offshore region except for northern mostarea (< 14˚C SST) where was strongly affected by the Oyashio (Fig. 6). Its main
distributions were restricted to the coastal region from 2004 to 2007 due to hightemperature field in the offshore waters (> 18-19˚C SST).
Walleye pollock: This species was abundant around continental shelf area at 100-250 m
depth in the daylight period (Watanabe and Kiwada unpubl. data, see Fig. 24) with the
SST was 12-22˚C (Figs. 4, 7 and 8).
Japanese common squid: This squid was seemed to prefer slope water region where
SST was 14-18˚C (Figs. 4, 8, and 9).
Euphausiids: The main habitat of euphausiids was in the slope water region affected by
the Oyashio waters (Figs. 10a and 11a). In 2002 when Oyashio influence was

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remarkable in both inshore and offshore regions, its distribution expanded to theoffshore region. Vertically, euphausiids were mainly distributed in the 100-200 m layer(Watanabe and Kiwada unpubl. data).

1.3.2. Less abundant four species
Japanese sardine: This species was captured in the area of 14-18˚C SST south of the
Oyashio region in 2002, 2005, 2006, and 2007 (Figs. 4 and 12).
Mackerel: Mackerel were distributed mainly in the area of 14-18˚C SST, which was
located in the Oyashio-Kuroshio transition region (Fig. 4). Its abundance was very small
during 2002 and 2006 (Fig. 13). However, relatively high catches were recorded in theoffshore region in 2007 where SST was 17-18˚C (Fig. 13).
Pacific pomfret: This large pelagic nekton was frequently captured in 2007 in the
Oyashio and its adjacent regions where SST were 12-14˚C, including whale samplingarea off Kushiro (Fig. 14).
Chum salmon: This species was collected in 2002, 2005, and 2007 in the Oyashio andits downstream regions where SST were 11-16˚C (Fig. 15).
These results indicated that geographical distribution patterns of each potential preyspecies could be categorized in relation to oceanographic conditions and/or continental
shelf as follows; (1) Distributions were almost restricted to the Oyashio region: Pacificsaury, euphausiids, Pacific pomfret, and chum salmon; (2) Distributions were mainly inthe northern transition region: Japanese anchovy, Japanese sardine, and mackerel; (3)Distributions were mainly in the continental shelf region where was located in thenorthern transition region: Japanese common squid; (4) Distributions were in almost
entire part of the continental shelf region: walleye pollock.

2. Annual changes in biomass of major potential prey species
Biomass of euphausiids largely fluctuated year-to-year within three orders andshowed prominent peaks in 2002 and 2007 in the cold period (ANOVA, p < 0.05,
Fig.10). Among the potential fish and squid prey, biomass of Pacific saury, Japanese
anchovy, and Japanese common squid fluctuated annually within two or three orders ineach block (ANOVA, p < 0.05, Fig. 10). This large fluctuation was also shown among
blocks in the same year (ANOVA, p < 0.05). Contrasting with these species, biomass ofwalleye pollock in the coastal blocks was relatively stable in each year. It was
remarkable that biomass of each species was extremely low in the offshore region
except for 2002 (Fig. 10). In 2002, euphausiids and Japanese anchovy were relatively
abundant in the offshore region although biomasses of these species were several times
higher than those in the coastal blocks. These results indicate that coastal blocks weremore favorable than offshore blocks in terms of prey environment of the whale during
the five years studied.

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In the sampling survey region of common minke whale, euphausids wereabundant in the cold period in 2002 and 2007 and less abundant in the warm period
from 2004 to 2006 (Fig.11). A total biomass of major fish and squid prey species
showed prominent peak in 2002 due to densely distribution of Japanese anchovy andJapanese common squid. In other four years, biomass of fish and squids was relatively
low but stable due to small annual fluctuation in abundance of walleye pollock. These
results indicate that annual change in prey environment of common minke whale in themesopelagic zone is relatively small comparing with epipelagic zone.

3. Geographical distribution of the common minke whale
The result of cetacean sighting survey indicated that common minke whale wasmainly found in the area of < 16˚C SST (Fig. 16). This strongly suggests that thisspecies was mainly distributed in or close to the Oyashio region in autumn off Kushiro.
Geographically, hot spot of the whale seemed to be located in or close to the slope water
region in each year. However, its distribution expanded to the offshore region in 2003,
when SST of offshore region was extremely low (< 15˚C, Fig. 17). In 2006, this species
was often found in the area of 16-20˚C SST, but its abundance was low (Fig. 17).

4. Prey preference of common minke whale
Size compositions of the potential fish and squid prey of the common minke
whale were depicted in Figs. 18-22, indicating that these species could be categorized as
small (10 to 20 cm in body length: Japanese anchovy, mackerel, and Japanese sardine),
middle (20-35 cm: Pacific saury and Japanese common squid), and large (35-70 cm:
walleye pollock, Pacific pomfret, and chum salmon) size classes.
Prey species compositions by wet weight in the stomachs of immature(approximately < 7 m in body length) and mature (> 7 m) common minke whale wereshown in Table 2 (after Kishiro et al. 2009b). Immature whale fed mainly on walleye
pollock in almost all years although euphausiids and Japanese anchovy were also
popular preys in some years. The main prey of mature whale was Japanese anchovy inall years. Pacific saury (in 2002, 2003, and 2007) and Japanese common squid (in 2002,
2005, and 2006) were also one of the main preys of mature whale. In 2005, walleye
pollock was the most popular prey of mature whale, but this prey was minor component
in other four years (Table 2). These results indicated that there was a little overlap ofmain prey category or species between immature and mature whales except forJapanese anchovy.
According to Manly’s α, both immature and mature whales avoid euphausiids
which were the most abundant in all years in the whale sampling survey area (Table 3,
Fig. 11). This indicates that the occurrences of the potential prey species in theenvironment were not reflected in stomach contents of the whale. Manly’s α also
indicated that immature whale selected walleye pollock regardless of the year (Table 3).
In addition, immature whale’s pick for other species was varied annually, i.e. Pacificsaury in 2002 and Japanese anchovy in 2004, 2006, and 2007. Contrasting withimmature whales, mature whales unlikely pick walleye pollock in almost all years

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except for 2005 and Pacific saury and Japanese anchovy were the most favoured bymature whales in the four of five years (Table 3).

DISCUSSION
1. Geographical distribution of potential prey species
The occurrence rates of each prey species in relation to SST at the samplingstations reflected the geographical distribution pattern of these species, as SST generally
decreases from eastern or northern upstream region to the western or southerndownstream region of the Oyashio. Among the potential prey species, occurrence rates
of Pacific saury, Pacific pomfret, and chum salmon decreased with increasing SST,
indicating that these species occurred off eastern Hokkaido with southward intrusion of
Oyashio in autumn. Japanese anchovy, Japanese common squid, mackerel, and Japanesesardine were almost restricted to the mid SST areas. This reflects their summer
northward migration and stay at the productive northern Oyashio-Kuroshio transition
region north of the transition zone chlorophyll front, which is corresponded to theisotherm of 18-19˚C SST (Polovina et al. 2004). Furthermore, euphausiids were mainly
distributed in the Oyashio region of less than 16˚C SST. These facts indicate that the
SST is a good indicator to predict not only distributions of these prey species but also
prey environment of common minke whale in autumn of the study area. SST, however,
could not explain the habitat of walleye pollock. Considering that walleye pollock is
distributed in the Oyashio Undercurrent below mixing zone all day, the distribution ofthis species should not be affected by the oceanographic conditions in the epipelagiczone. The distributions of these potential prey species greatly overlapped in thecontinental shelf area in or close to the Oyashio front where SST was 14-16˚C,suggesting that this area was rich prey environment for common minke whale in terms
of prey species diversity and also prey biomass as we discuss below.

2. Annual change in prey environment
The decrease in abundance of euphausiids, most of which were E. pacifica, from2002 to 2004 and increase in its abundance from 2006 to 2007 coincided with the shiftof SST regime from cold to warm and from warm to cold, respectively. Biomass ofPacific saury was the highest in 2002 and the lowest in 2006. This fluctuation could bealso explained by annual change in Oyashio influence, because SST field was thecoldest in 2002 and warmest in 2006. In 2006, southward migration of Pacific saury
was greatly delayed (Hokkaido Kushiro Fisheries Experiment Station,
http://www.fishexp.pref.hokkaido.jp/exp/kushiro/). Hence this species was rarely
collected in this study area.
Biomass of Japanese anchovy is known to have been at high level since mid
1990s (Takahashi et al. 2001). However, its biomass was extremely low in 2005 due torecruitment failure in 2004 (Takasuka et al. 2006). Therefore, prey environment forcommon minke whale in the epipelagic zone is thought to greatly change by year due toboth fluctuation of Oyashio influence and recruitment status of each species.

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Contrasting with epipelagic fishes, biomass of walleye pollock was relatively stablethroughout the five years studied. This probably indicates that mesopelagic zone alongcontinental shelf is favorable foraging area for common minke whale in terms ofstability of prey density.

3. Distribution of common minke whale related to physical and prey environment
The present results indicated that the common minke whale was mainly
distributed in the slope water region affected by Oyashio in each year and their main
distributions expanded to the high sea area in 2003 when Oyashio influence was thegreatest. In the slope water region, rich prey environment might be assured by the
distribution of walleye pollock, which biomass showed small annual fluctuation.
Furthermore, distribution center of other potential prey species were also located in thisregion although their biomass were largely fluctuated annually. These facts suggest that
common minke whale prefer the productive slope water region in both warm and cold
periods (Fig. 23).
In 2003 when common minke whale was abundant both in the slope water andoffshore regions, Pacific saury might be also plenty in the coastal to offshore regions
because most part of these regions were less than 14-15˚C in SST (see Figs. 2 and 4).
Expansion of Oyashio followed by Pacific saury may have led common minke whales
to swim around from coastal to the offshore regions in 2003 (Fig. 23). This might
indicate that common minke whale has so-called area preference, which is related to the
distribution of Pacific saury in autumn off eastern Hokkaido. Pacific saury migrates
from subtropical to highly productive subarctic regions to feed in summer (Odate 1977,
Taniguchi 1981). According to Tamura et al. (2009) and Konishi et al. (2009), common
minke whale also carries out a northward migration and feed mainly on Pacific saury
during summer in the subarctic region. Considering that Pacific saury is the mostabundant small epipelagic fish in the subarctic region in summer (Odate 1977, Ueno etal. 2003, Murase et al. 2009), their distribution area might be a favorable feeding
environment for common minke whale. Therefore, common minke whale possibly
migrate following the southward migration of Pacific saury to maintain rich preyenvironment during summer and autumn. This view is probably adopted at least for
mature common minke whale, because they showed a strong prey selection for Pacificsaury (see below).

4. Prey preference
According to Kishiro et al. (2009b), two small epipelagic fish, Japanese sardine
and mackerel, were not found in the stomachs of common minke whale, which probably
due to extremely low abundance of these species in the whale sampling area. Althoughsize ranges of walleye pollock and Pacific pomfret were similar, Pacific pomfret is moremuscular than walleye pollock. Furthermore, chum salmon also has muscular body.
Therefore, the present result that common minke whale rarely fed on these species
probably due to their swimming ability (Kishiro et al. 2009b). Generally, common
minke whale prefer small pelagic fishes like anchovy and herring and slope water11

SC/J09/JR11
demersal fish like pollock, which are distributed above 100-200 m depth (Harbitz and
Lindstrøm 2001, Haug et al. 1996, Lindstrøm and Haug, 2001, Skaug et al. 1997,
Murase et al. 2007). These prey species is known to form dense school and theirswimming ability is relatively low among the pelagic nektons (Yamamura et al. 2002,
Takahashi et al. 2001, Ueno et al. 2003). Hence common minke whale seems to have afeeding strategy to utilize abundant and dense schooling resources effectively withlower foraging energy cost. Euphausia pacifica are usually abundant and making dense
school in the subarctic region especially in the slope water region (Taki et al. 1996), as
was also shown in this study. However, both immature and mature whales did not prefer
this prey, which is reasonable in terms of their lower caloric value comparing with otherpotential fish and squid preys (3500 kJ/kg, vs. 4200-11000 kJ/kg, Watanabe unpubl.
data).
In the whale sampling survey region off Kushiro, immature and mature whales
were generally distributed in the shallower slope water region (< 500 m depth) and fromthe deeper slope water to the offshore regions, respectively (Kishiro et al. 2009b). The
present results indicate a preference of immature and mature whales for mesopelagicand epipelagic fishes, respectively, suggesting that habitat segregation of immature andmature whales reflect their different feeding strategies. Immature whales preferredwalleye pollock in all years studied. Generally, small epipelagic fish and squids like
Japanese anchovy are unevenly distributed with dense school, suggesting that predators
require more energy to seek and capture them (Fig. 24). In contrast, walleye pollock
was almost uniformly distributed in the slope water region of 100-250 m depth (Fig. 24).
Furthermore, its biomass was relatively high and stable among the year. Therefore,
walleye pollock may be a favorable prey for common minke whales that have low preysearching ability, although caloric value of this demersal fish is much lower than thoseof small epipelagic fish preys like Japanese anchovy and Pacific saury (4200 kJ/kg vs.
6300-11000 kJ/kg, Watanabe unpubl. data) and diving behavior entails a high energetic
cost for cetaceans (Goldbogen et al. 2007). Immature whales may therefore trade off theenergy cost of in and out associated with feeding behavior. The present result also
showed that immature whales also preferred Pacific saury or Japanese anchovy in someyears, suggesting that they could utilize these epipelagic fish prey when they abundantly
and/or frequently migrate into slope water region.
Contrasting with immature whale, mature whales preferred Pacific saury in all
years except for 2006 when abundance of this species was extremely low due to high
SST field off Kushiro. Furthermore, mature whales also preferred Japanese anchovy for
four of five years. Among the potential prey species found in this study, caloric value ofPacific saury is the highest (11000 kJ/kg), followed by Japanese anchovy (6300 kJ/kg,
Watanabe unpubl. data). Because foraging ability and/or energy demands thought toincrease as they grow, mature whales may prefer epipelagic fish prey with higher
caloric value. However, mature whales frequently fed on walleye pollock in 2005. Thismight relate to the low abundance of Pacific saury and Japanese anchovy off Kushiro
and suggests that the main prey of large whale switch to walleye pollock when small
epipelagic fish prey is not sufficiently available. Similar prey switching from epipelagic
to mesopelagic prey species is also reported for Dall’s porpoise Phocoenoides dalli in
the Sea of Japan (Ohizumi et al. 2000). However, this tendency was not recognized in
2007, when abundance of epipelagic fishes was also extremely low. Hence further
cooperative survey of prey species and whale sampling is needed to clarify this prey

12

SC/J09/JR11
switch hypothesis.

5. Conclusion and future direction
In autumn in the Kushiro area, difference in feeding habits between immature andmature common minke whales could be explained by difference in their habitat andprey selection procedures (Fig. 25). These differences are probably reflected thedifferences in seeking or catching ability for prey species, or a trade-off between low
and high-energy demands of their lifestyles. In the future, data on feeding behavior of
the immature and mature whales might be accumulated by using depth and temperature
data loggers to clarify their feeding strategies. Such surveys should be conductedconcurrently with prey species and whale sampling surveys because prey environmentis thought to greatly affect feeding behavior of the whale and lethal survey is also
important to obtain actual stomach content data of the whale. Prey species survey is alsoneeded to construct the ecosystem models off eastern Hokkaido in the future. Thismodel would contribute to aid the recovery of the resource of walleye pollock around
Hokkaido, which showed low levels of abundance in recent years (Hokkaido National
Fisheries
Research
Institute,2007:
http://cse.fra.affrc.go.jp/anishimu/hnfweb/doko1000/Summary1000R.pdf).

ACKNOWLEDGEMENTS: We thank Drs H. Hatanaka, S. Ohsumi, and K. Konishi of
the Institute of Cetacean Research (ICR) for their helpful suggestions and criticism on
the manuscript. K. Matsuoka of ICR provided us cetacean sighting data off Kushiro.

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Ecol. Prog. Ser. 238: 187-198.

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Table 1. Species composition (CPUE, kg /h) by predetermined trawling survey. A crossindicates a catch of less than 0.1 kg. A bar indicates no catch.
Species / Year
Japanese anchovy
Pacific saury
Mackerels
Japanese sardine
Pacific pomfret
Chum salmon
Others

2002
83.2
16.6
0.1
0.1
+

2004
87.6
8.1
1.8
2.5
-

2005
56.3
22.2
0.5
2.5
18.3
0.1

2006
99.2
+
0.7
+
-

2007
50.7
1.9
23.6
0.2
21.7
1.9
0.1

Table 2. Stomach contents of common minke whale during and within 10 days before
and after prey species survey period in the whale sampling survey area off Kushiro(after Kishiro et al. 2009b).
(A) Immature whale
No.
whalesYear sampled
2002
19
2004
18
2005
22
2006
12
2007
18

Euphau- Japanesesiids
anchovy
36.4
29.4
0
85.0
16.0
2.0
1.4
30.9
0
9.5

Stomach content composition (%WW)
Pacific
WalleyeJapanese
Pacific Chum Common
saury
pollock
sardine Mackerel pomfret salmon squid
2.1
32.1
0
0
0
0
0
3.8
11.1
0
0
0
0
0
0
82.0
0
0
0
0
0
0
67.7
0
0
0
0
0
2.0
88.6
0
0
0
0
0

Total wtof stomach
content (kg)
415.1
305.2
1696.2231.3
331.2(B) Mature whale
No.
whalesYear sampled
2002
14
2004
23
2005
22
2006
10
2007
10

Euphau- Japanesesiids
anchovy
0
23.4
0
72.0
7.2
17.7
0
79.9
4.5
22.4

Stomach content composition (%WW)
Pacific
WalleyeJapanese
Pacific Chum Common
saury
pollock
sardine Mackerel pomfret salmon squid
36.7
2.4
0
0
0
0
37.5
27.3
0
0
0
0
0.4
0.3
2.9
59.1
0
0
0
0
13.1
0
0.7
0
0
0
0
19.4
71.6
1.2
0
0
0.4
0
0

Total wtof stomach
content (kg)
439.9
597.9
994.6
188.1
283.8

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SC/J09/JR11
Table 3. Manly’s α of common minke whale for each prey species in the whale
sampling survey region off Kushiro in the autumn of 2002-2007. -: Species was notfound in the environment. m: total number of species in the environment. If Manly’s α
is greater (smaller) than 1/m, prey species i is actively (negatively) selected. Shaded
area indicates positive prey preference.
(A) Immature whales
Prey species / Year 2002
Euphausiids
0.002
Pacific saury
0.501
Japanese anchovy
0.156
Japanese sardine
Mackerel
Pacific pomfret
Chum salmon
Walleye pollock
0.340
Common squid
0.000

2004
0.000
0.026
0.746
0.000
0.228
0.000

2005 2006 2007
0.002 0.000 0.000
0.000- 0.155
0.103 0.663 0.670- 0.000 0.000
0.000 0.000
0.000 0.000 0.000
0.000 0.000 0.000
0.896 0.347 0.175
0.000 0.000-

1/m 0.200 0.167 0.125 0.125 0.167(B) Mature whales
Prey species / Year
Euphausiids
Pacific saury
Japanese anchovy
Japanese sardine
Mackerel
Pacific pomfret
Chum salmon
Walleye pollock
Common squid

2002
0.000
0.934
0.013
0.003
0.005

1/m 0.200

2004
0.000
0.207
0.703
0.000
0.000
0.090

2005 2006
0.000 0.000
0.299
0.308 0.680- 0.000
0.000 0.000
0.000 0.000
0.000 0.000
0.221 0.002
0.172 0.318

2007
0.000
0.780
0.220
0.000
0.000
0.000
0.000
-

0.167 0.125 0.125 0.167

Fig. 1. Sampling area and distribution of each small block in the JARPN II coastal
survey off Kushiro in 2007. Dotted area indicates the whaling survey area of common
minke whale. Thin lines indicates the prey species survey line in 2007.

Fig. 2. Percent composition of each SST area in the sampling survey region of common
minke whale.
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Fig. 3. Percent composition of each SST area in each small block.

Fig. 4. Percent occurrence of total number of predetermined midwater trawl samplingsin relation to SST for eight potential prey of minke whale during autumn off eastern
Hokkaido. Five years of data were pooled.

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SC/J09/JR11

Fig. 5. Geographical distributions of Pacific saury based on the result of midwater trawl
survey off eastern Hokkaido during 2002-2007. Isotherms of 200, 1000, and 3000 m
and SST during the survey period was also indicated. Dotted area indicates the whaling
survey area of common minke whale. Thin lines indicate the track line of prey species
survey.

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SC/J09/JR11

Fig. 6. Geographical distributions of Japanese anchovy based on the result of midwater
trawl survey off eastern Hokkaido during 2002-2007.

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SC/J09/JR11

Fig. 7. Geographical distributions of walleye pollock based on the result of midwater
trawl survey off eastern Hokkaido during 2002-2007.

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SC/J09/JR11

Fig. 8. Geographical distributions of (a) walleye pollock and (b) Japanese common
squid based on the result of bottom trawl survey conducted by Hokkaido National
Fisheries Research Institute off eastern Hokkaido during 2002-2006.

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SC/J09/JR11

Fig. 9. Geographical distributions of Japanese common squid based on the result of
midwater trawl survey off eastern Hokkaido during 2002-2007.

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SC/J09/JR11

Fig. 10. Annual change in biomass (g/m2) of the major potential prey species ofcommon minke whale in each sampling block. ND: survey was not conducted; CE:coast east block; CC: coast central block; CW: coast west block; H: off Hidaka block;O: offshore block.

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SC/J09/JR11

Fig. 11. Annual change in biomass (g/m2) of the major potential prey species ofcommon minke whale in the whale sampling survey region off Kushiro. ND: survey
was not conducted.

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SC/J09/JR11

Fig. 12. Geographical distributions of Japanese sardine based on the result of midwater
trawl survey off eastern Hokkaido during 2002-2007.

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SC/J09/JR11

Fig. 13. Geographical distributions of mackerel based on the result of midwater trawl
survey off eastern Hokkaido during 2002-2007.

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Fig. 14. Geographical distributions of Pacific pomfret based on the result of midwater
trawl survey off eastern Hokkaido during 2002-2007.

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Fig. 15. Geographical distributions of chum salmon based on the result of midwater
trawl survey off eastern Hokkaido during 2002-2007.

Fig. 16. Occurrence rate of common minke whales in each SST area. Six years from2002 to 2007 of data were pooled.

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Fig. 17. Geographical distribution of common minke whales off eastern Hokkaido
during 2002-2007. Isotherms of 200, 1000, and 3000 m and SST during the survey
period was also indicated. Thin lines indicate sighting survey line for cetaceans.

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Fig. 18. Size frequency of Pacific saury in the environment of the area within 50
nautical miles from Kushiro.

Fig. 19. Size frequency of Japanese anchovy in the environment of the area within 50
nautical miles from Kushiro.

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Fig. 20. Size frequency of walleye pollock in the environment of the area within 50
nautical miles from Kushiro.

Fig. 21. Size frequency of (a) Japanese common squid and (b) Japanese sardine in theenvironment of the area within 50 nautical miles from Kushiro.

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Fig. 22. Size frequency of (a) mackerel and (b) Pacific pomfret in the environment of
the area within 50 nautical miles from Kushiro.

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Fig. 23. Distribution patterns of common minke whale and its main prey species in the(a) warm and (b) cold periods off eastern Hokkaido in autumn. WP: walleye Pollock;
PS: Pacific saury; JA: Japanese anchovy.

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Fig. 24. Echogram (38 kHz) in the slope water region off Kushiro in September 15,
2002, showing that walleye pollock was distributed more uniformly than Japanese
anchovy.

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Fig. 25. Alternative interpretation in prey selection procedure for immature and maturecommon minke whales off Kushiro in autumn.

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