FISHERIES OCEANOGRAPHY

Fish. Oceanogr. 16:2, 186–201, 2007

Prey selection of common minke (Balaenoptera acutorostrata)
and Bryde’s (Balaenoptera edeni) whales in the western North
Pacific in 2000 and 2001

HIROTO MURASE,1,* TSUTOMU TAMURA,1
HIROSHI KIWADA,1 YOSHIHIRO FUJISE,1
HIKARU WATANABE,2 HIROSHI OHIZUMI,2SHIROH YONEZAKI,3 HIROSHI OKAMURA2
AND SHIGEYUKI KAWAHARA2

Key words: Bryde’s whale, common minke whale,
Japanese anchovy, krill, prey selection, walleye
pollock

1

INTRODUCTION

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku,
Tokyo 104-0055, Japan
2
National Research Institute of Far Seas Fisheries, 2-12-4
Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan
3
National Research Institute of Far Seas Fisheries, Fisheries
Research Agency, 5-7-1, Orido, Shimizu-ku, Shizuoka 4248633, Japan

ABSTRACT
A study of common minke and Bryde’s whales was
conducted in the western North Pacific in the 2000
and 2001 summer seasons to estimate prey selectionof cetaceans as this is an important parameter inecosystem models. Whale sighting and sampling surveys and prey surveys using quantitative echosounderand mid-water trawl were carried out concurrently inthe study. Biomasses of Japanese anchovy, walleye
pollock and krill, which were major prey species ofcommon minke and Bryde’s whales, were estimatedusing an echosounder. The results suggested that
common minke whale showed prey selection for Japanese anchovy while they seemed to avoid krill inboth the offshore and coastal regions and walleye
pollock in the continental shelf region. Selection forshoaling pelagic fish was similar to that in the eastern
North Atlantic. Bryde’s whale showed selection forJapanese anchovy in August 2000 and July 2001, while
it showed prey selection for krill in May and June in
2001.
*Correspondence. e-mail: murase@cetacean.jp
Present address. Hiroshi Ohizumi, Department of Marine
Biology, School of Marine Science and Technology, Tokai
University, 3-20-1 Orido, Shimizu-ku, Shizuoka 424-8610,
Japan.
Received 6 January 2004
Revised version accepted 28 February 2006
186

A wide variety of prey species of common minke
whales (Balaenoptera acutorostrata) in the western
North Pacific was qualitatively described using commercial whaling data (Omura and Sakiura, 1956;
Kasamatsu and Hata, 1985; Kasamatsu and Tanaka,
1992). These included Japanese anchovy (Engraulisjaponicus), Japanese sardine (Sardinops melanostictus),sand lance (Ammodytes personatus), chub mackerel(Scomber japonicus), walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus), Pacificherring (Clupea pallasi), squid, krill and copepods.
A quantitative feeding ecology study of minke whale
was conducted from 1994 to 1999 in the Japanese
Whale Research Program under Special Permit in thewestern North Pacific (JARPN) (Tamura and Fujise,2002). The study showed a large variation of main
prey species with area, season and year. The mostimportant prey species in May and June was Japanese
anchovy, and it changed to Pacific saury in Julyand August. Krill was the most important prey in
September. Walleye pollock was also one of theimportant prey species in June and September incontinental and shelf-edge waters off Hokkaido. Although Bryde’s whale (Balaenoptera edeni)1 fed onJapanese anchovy, Japanese sardine, horse mackerel(Trachurus japonicus), chub mackerel, lantern fish andkrill (Nemoto, 1959), those data came from commercial whaling data and no quantitative analysis has
been conducted. Because fishing effort was concentrated on the high-density area of the cetaceans,samples from commercial whaling had an inherent

1

Nomenclature adopted by the International Whaling
Commission (IWC) (Anonymous, 2001) was used. It contains more than one species but it is still unsettled in IWC.

doi:10.1111/j.1365-2419.2006.00426.x

Ó 2007 The Authors.

Prey selection of common minke and Bryde’s whales

bias. Random sampling of cetaceans should be required to remove the bias.
Prey species of cetaceans have shown decadal
fluctuations around Japanese waters. In the western
North Pacific, commercial catch histories of pelagic
fishes have shown drastic fluctuation and quasidecadal species alterations, so-called species replacement since the 1950s (Yatsu et al., 2001). Dominant
species, were chub mackerel in the 1970s, Japanesesardine in the 1980s, and Pacific saury, Japanese anchovy and Japanese common squid in the 1990s. It issuggested that ocean-climate regime shifts have substantial impacts on the species replacement together
with species interactions, although the mechanism of
the replacement has not been clearly understood (PICES, 2004). Commercial catches of Pacific stock ofwalleye pollock were around 0.25 million tons from1975 to 1985 but gradually declined to 0.15 million
tons in 1996. It returned to 0.26 million tons in 1998
because of the presence of a strong cohort in 1995 but
the catches declined again from 1999 to 0.11 million
tons in 2002 (Fisheries Agency of Japan, 2004).
Fisheries scientists have been compelled to conclude that models that aim to contribute to the sustainable marine resources management have to takethe ecosystem approach because of the recognition
that exploited stocks are parts of ecosystems and thatspecies usually interact (e.g. predator–prey relationship) (Sumaila et al., 2000). Because common minke
and Bryde’s whales fed on target species of commercial fisheries and those target species of commercial
fisheries showed year-to-year fluctuations, development of ecological models including cetacean in thewestern North Pacific is desirable from the marine
living resources management and conservation perspectives. In such models, like the multispecies virtual
population-based model, MULTISPEC (Bogstad
et al., 1997) and the mass balance model, ECOPATH(Pauly et al., 2000), prey selection data are very
important. To estimate prey selection using both food
habit data of predator and prey availability data, thewhale and the prey surveys must be conducted concurrently.
This paper presents the results of the first attempt
to conduct concurrent whale and prey surveys in thewestern North Pacific in 2000 and 2001 as a part theJapanese Whale Research Program under Special
Permit in the western North Pacific Phase II (JARPN
II). The underlying question is whether common
minke and Bryde’s whales show any prey selection.
Figure 1. Research area, small blocks and planed tracklines: (a) 2000, (b) 2001, and (c) 2001 (additional blocks).
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

(a)

(b)

(c)

187

188

H. Murase et al.

In past discussions of food use patterns, the terms‘selection’ and ‘preference’ have been used interchangeably, raising some confusion (Litvaitis, 2000).
In this paper, preference is defined as the likelihoodthat an animal selects a particular resource given equalamounts of others, whereas selection is defined as theanimal choosing a resource irrespective of amount of
resources according to Johnson (1980).

(a)

MATERIALS AND METHODS
Survey area, period and vessels
The research area of the concurrent whale and preysurveys was off the Pacific coast of northern Japan(Fig. 1). The northern part of the research area wasunder the influence of Oyashio (cold low-salinity
water) whereas the southern part of the area was under
the influence of Kuroshio (warm high-salinity water).
The area between Oyashio and Kuroshio is called theKuroshio–Oyashio inter-frontal zone or transitionalzone. As the first attempt to conduct concurrent
whale and prey surveys in the western North Pacific,
we mainly focused on a wide area coverage given the
short survey periods to identify the conditions of cetacean feeding habitat in different water masses as well
as bottom topography. To achieve our goal, small
blocks were established within the survey area and thelength of trackline in each small block was set to allow
the survey to be completed in each small block within
2–5 days. Degree of coverage (d) defined by Aglen(1989) is commonly used to measure the sampling
intensity of an echo sounderDegree of coverpﬃﬃﬃsurvey.
ﬃ
age is written as d ¼ D= A where D is the total
length of the trackline and A is the size of the survey
area. Although it is pointed out that a value of d morethan 6 is an indication of sufficient coverage, priority
was given to wide area coverage instead of degree ofcoverage. Seven small blocks (Blocks 1–7) were set
within the area in 2000 but Block 5 was not surveyed
because of logistical reasons. Five small blocks (Blocks
1–5) with an additional three blocks (Blocks A–C)
were set in 2001. The small blocks were predetermined
mainly based on the bottom topography and theinformation on the sea surface temperature just before
the survey. Because sample sizes of Bryde’s whales in
the predetermined area in 2001 were small in number,the additional blocks were set adoptively to acquire
more samples. Blocks 5 and A in 2001 were not

(b)

(c)

Figure 2. Sighting positions of sampled minke and Bryde’s
whales: (a) 2000, (b) 2001, and (c) 2001 (additional blocks).
Thin lines showed 200 and 1000 m isobath.
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Prey selection of common minke and Bryde’s whales

189

Table 1. Summary of stomach contents of minke whales (a) and Bryde’s whales (b) used in this analysis.
Survey period

Year
(a)
2000

2001

(b)
2000
2001

Stomach content

Prey

Cetacean

Block

Sampledanimals (n)5–7 August
2–4 August
19–24 June
17–21 May

1
2
1
2

22–26 May
15–18 June

7 August
3–5 August
19–23 June
14–18 May
21–22 May
22–27 May
13–18 June

22–24 August
22–26 May
11–13 June
15–18 June
7–15 July

22–26 August
22–27 May
10–13 June
13–18 June
11–13 July

JA

WP

n

wt (kg)

1
5
10
18

1
5
5
18

26.3
205.3
110.0
435.7

3
B

4
6

1
2

162.6
85.8

4
3
4
B
C

10
1
3
12
6

8

4288.7
–
–
–
19.1

3

n

8

Kr
wt (kg)
–
–
403.8
–
–
–

n

wt (kg)Total wt (kg)

–
–
35.2
–

26.3
205.3
549.1
435.7

3
4

13.9
236.2

176.5
322.0

3
1
3
12
4

191.1
215.4
93.8
1021.714.3

4479.8215.4
93.8
1021.733.4

2

JA, Japanese anchovy; WP, Walleye Pollock; Kr, Krill. n, number of stomachs that contained given species; wt (kg), total weightof each prey species that was found in all sampled animals given survey block. Because some animals fed on two species
simultaneously, number of sampled animals was not always equal to total number of n.

considered in the analysis because of insufficient
survey coverage. The concurrent survey was conductedin August 2000 and from mid-May to mid-July in
2001. Three vessels, Yushin-Maru (YS1: 720GT), KyoMaru No. 1 (K01: 812GT) and Toshi-Maru No. 25(T25: 739GT), were engaged in the whale survey
consisting of sighting and sampling of whales. Stomach contents of sampled minke and Bryde’s whales
were examined on the research base ship, NisshinMaru (NM: 7575GT). Two vessels, Kyoshin-Maru No.
2 (KS2: 368GT) and Shunyo-Maru (SYO: 396GT),
were engaged in the prey survey in 2000. KS2 conducted the acoustic survey. SYO conducted the midwater trawling to identify species compositions of
acoustic backscattering. During the daytime, KS2
steamed at 10–11 knots along the track line. SYO
followed KS2 at the distance of 1–2 nautical miles sothat SYO could cast a mid-water trawl to identify the
species of marks on the echosounder. Torishima (TOR:
426 GT) took over SYO in 2001.
Stomach content analysis
Common minke and Bryde’s whales have a fourchambered stomach system. Because the forestomach
contents only gives information about the last feeding
event (e.g. Lindstrøm et al., 1997), contents from only
the forestomach were used in the analysis to avoid
including prey species of whales which might havebeen consumed outside the survey area. Each foreÓ 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

stomach content was weighed to the nearest 0.1 kg
then subsamples were taken and frozen for laboratory
analysis on the research mother ship. In the laboratory, prey species were identified to the lowest taxonomic level if possible. Ideally only undigested ormoderately digested stomach contents should be used
to reduce some of the main sources of uncertainty inreconstruction of stomach contents such as different
passage and degradation rates by prey species and theirsizes and accumulation of hard parts such as otolishs(Lindstrøm and Haug, 2001), but digested stomach
contents were also used in this study to achieve sufficient sample sizes. The total weight of each prey species in the forestomach was estimated applying theweight ratio in the subsamples to the total weight offorestomach contents. Animals with empty stomachs
and stomachs destroyed by the harpoon were not usedin the analysis. Prey species less than 1% of total
stomach content weights were not included in thisanalysis. Details of stomach content analysis were
described in Tamura and Fujise (2002).
Estimation of prey biomass
A quantitative echo sounder (Simrad EK500 with
software version 5.30) was operated during daytime on
board KS2 to acquire acoustic data with operating frequency at 38 and 120 kHz. Calibrations were carriedout in each year using the copper sphere techniquedescribed in EK 500 operation manual (Simrad, 1997).

190

(a)

(b)

(c)

H. Murase et al.

Figure 3. Distribution patterns and densities (SA) of krill in2000 (a), 2001 (b), and 2001 with additional blocks (c),were overlaid on water temperature maps. Maps published by
Tohoku National Fisheries Research Institute (TNFRI) weremodified. Water temperature maps at 200 m were used.
August monthly map was used in 2000. June monthly map
was used in 2001.

The targeting trawlings were made using a midwater trawl net to identify fish species compositions of
acoustical backscatter. The trawl net was towed at thedepth of the backscatters. Towing speed was 3–4
knots. Trawl hauls were continued until target schools
were captured though maximum trawling duration was
set as 30 min. Catches were sorted into species andweighed. For the major species, 100 animals were
sampled at random and the length was measured.
Acoustic data were analyzed with the aid ofSonarData Echoview (version 2.10.51) software. In
principle, species identifications of backscatterings
were conducted based on the targeting trawling catches. In addition, reported school shapes of krill(Miyashita et al., 1998), Japanese anchovy and walleye pollock (Ohshimo and Hamatsu, 1996) recordedon the echogram were also taken into account. For
Japanese anchovy and walleye pollock, data collected
at 38 kHz were used with the threshold set at )60 dB,
and the depth range from 7 to 100 m in 2000 and from10 to 250 m in 2001. For krill, data collected at
120 kHz were used with the threshold set at )80 dB.
The analyzed depth range was from 12 to 250 m. It
was reported that echoes were identified as krill if thedifference of SV between 120 and 38 kHz fell between
10 and 15 dB (Miyashita et al., 1997). The differenceof SV was also taken into account in this study toidentify krill. When the difference of SV was calculated, threshold of 38 kHz was set at )80 dB. Biomass
estimation was conducted according to the method of
Jolly and Hampton (1990). Details of the methods ofthe estimation of the resource abundance are describedin Appendix A.
Prey selection analysis
Although there is no general agreement in the literature about which of several existing indices is thebest measure of selection, the selection index of Manly
et al. (2002) would appear to be one of the best indicesof selection for most situations (Krebs, 1999). The
standardized form of the selection index called Manly’sa (Manly et al., 1972), also known as Chesson’s
index (Chesson, 1978), was successfully applied to theNorth Atlantic stock of minke whale to reveal prey
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Prey selection of common minke and Bryde’s whales

(a)

191

Figure 4. Distribution patterns and densities (SA) of Japanese anchovy 2000 (a), 2001 (b), and 2001(additional
blocks, c) were overlaid on water temperature maps. Maps
published by Tohoku National Fisheries Research Institute(TNFRI) were modified. Water temperature maps at 50 m
were used. August monthly map was used in 2000. Junemonthly map was used in 2001.

selectivity (Lindstrøm and Haug, 2001). Confidence
intervals of the selection index were estimated as
described below to test statistically whether whales
randomly fed on prey species.
Assuming jth stomach contents with ith prey species are available, a total weight of prey species i usedby all animals (ui+) (kg) is
uiþ ¼
(b)

J
X

uij

ð1Þ

j¼1

where uij is weight of prey species i used by animal j.
The total weight of prey species used by animalj(u+j) is
I
X
uij :
ð2Þ
uþj ¼
i¼1

The total weight of all prey species used by all
animals (u++) is
Figure 5. Distribution pattern and density (SA) of walleye
pollock in 2001 was overlaid on water temperature map. A
map published by Tohoku National Fisheries ResearchInstitute (TNFRI) was modified. Water temperature map at
200 m in June was used.

(c)Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Other squid
species

0.0
0.0
+
0.0
0.0
0.0
0.0
+
0.0
0.0
0.0
0.0
0.0
0.0
0.0

Todarodes
pacificus

29.0
+
0.2
0.0
3.2
0.0
0.0
+
2.0
+
0.0
0.0
3.4
0.0
0.0

H. Murase et al.

uþþ ¼

2001

The sign ‘+’ represents catch weight less than 0.1 kg. Trawling stations which failed to capture the target backscattering were not included in the table.

0.2
+
0.0
0.2
0.0
+
+
0.0
+
+
+
+
9.6
0.0
+
0.0
0.0
0.0
0.0
0.0
0.0
0.0
94.6
12.4
0.0
0.0
0.0
0.0
0.0
0.0
64.6
367.0
41.0
9.0
2.1
5.2
43.8
0.0
0.0
0.0
400.8
50.0
749.2
1.0
0.9
0:30
0:30
0:30
0:30
0:30
1:30
1:00
0:30
0:30
0:30
0:33
0:30
0:30
0:30
0:30
1
1
2
2
3
6
6
1
1
1
2
2
2
2
C
2000

5 August
5 August
3 August
4 August
8 August
27 August
29 August
20 June20 June21 June
16 May17 May17 May20 May8 July41–56.9N, 143–59.8E
42–20.9N, 144–00.0E
40–44.9N, 142–24.3E
41–22.5N, 141–43.2E
40–22.9N, 144–46.4E
36–28.7N, 145–02.2E
37–38.2N, 145–16.7E
42–38.3N, 144–01.1E
42–29.4N, 144–08.7E
42–00.8N, 144–35.1E
37–45.1N, 141–59.7E
38–21.6N, 142–56.5E
38–28.2N, 143–22.0E
40–12.8N, 143–07.9E
39–07.8N, 148–14.9E

0–30
0–30
0–30
0–30
0–30
0–30
0–30
70–100
0–30
0–30
0–30
0–30
0–30
30–60
0–30

Other fish
species
Theragrachalcogramma
Engraulis
japonocus
Sampling
depth (m)Trawling
duration
Block

Date

Position

I X
J
X

uij :

ð3Þ

i¼1 j¼1

Year

Table 2. Results of target trawlings. Catch weight (kg) at each trawling stations was shown.

192

Sample proportion of prey species i by weight usedby all animals is
uiþ
oi ¼
:
ð4Þ
uþþ
Sample proportion of available units in prey i ismið5Þp^i ¼ PIi¼1 miwhere mi is an amount of available units in prey i in asample of available resource units. Manly’s selection
indices areoi^i ¼ :w
ð6Þ
p^i^ i is given by
Bonferroni confidence interval of w
^ i  za=ð2iÞ seð^
wi Þ:
w

ð7Þ

If the confidence interval contains the value 1,
whales feed on prey species randomly (Manly et al.,2002). The value of a was set at 0.05.
Bonferroni confidence interval of the difference^ k , is written as:^ i and w
between two selection indices, w^ k Þ  za=f2iði1Þg seð^^ k Þ:^i  wðw
wi  w

ð8Þ

If the confidence interval contains the value 0,^ k is not statistically sig^ i and w
difference between w
nificant (Manly et al., 2002). The value of a was set at
0.05. Details of statistical calculations are described inAppendix B.
Standardized Manly’s selection index, Manly’s a
was written as:^i^ i ¼ Pw
:
B
^i
iw

ð9Þ

^ i ¼ 1/I, species i is randomly selected; if B
^ i > 1/
If B
^ i < 1/I, species i isI, species i is actively selected; if B
avoided.
RESULTS
Stomach contents
A total of 44 minke and 32 Bryde’s whales were usedin the analysis. If only undigested stomach contents
were selected, the numbers of available sample sizes
were reduced to 26 individuals of minke and 16 individuals of Bryde’s whales. Sighting positions of sampled animals are shown in Fig. 2. Sightings of minke
whales were made in the northerly cold water region
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Prey selection of common minke and Bryde’s whales

193

Table 3. Vertical distributions of krill (Kr), Japanese anchovy (JA) and walleye pollock (WP) in each block in 2000 (a), 2001(b) and 2001 (additional blocks) (c).
Block 1

Block 2Block 3Block 4Block 6Block 7

Depth

Kr

JA

Kr

JA

Kr

JA

Kr

JA

Kr

JA

Kr

JA

(a) 2000
7–50 m50–100 m
100–150 m
150–200 m
200–250 m

0.00
0.00
0.52
34.10
28.18

4.85
0.00
0.00
0.00
0.00

0.00
0.01
5.13
19.36
16.06

13.22
0.00
0.00
0.00
0.00

0.00
0.12
7.23
27.34
16.23

0.47
0.00
0.00
0.00
0.00

0.00
0.00
1.56
2.72
3.11

0.97
0.00
0.00
0.00
0.00

0.00
0.00
0.46
3.42
15.69

3.76
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00

Block 1

Block 2Block 3Block 4

Depth

Kr

JA

WP

Kr

JA

Kr

JA

Kr

JA

(b) 2001
7–50 m50–100 m
100–150 m
150–200 m
200–250 m

4.98
2.97
3.55
0.62
0.22

0.62
0.00
0.00
0.00
0.00

0.05
2.03
1.60
0.07
0.06

1.25
1.43
1.13
5.06
4.07

11.38
1.47
0.03
0.00
0.00

3.01
0.03
0.33
4.49
7.74

1.28
0.00
0.00
0.00
0.00

0.94
0.17
0.12
1.27
6.06

0.19
0.97
0.05
0.00
0.00

Block ADepthKr(c) 2001 (additional blocks)
7–50 m1.36
50–100 m
1.20
100–150 m
1.25
150–200 m
5.41
200–250 m
10.67

Block B

Block C

JA

Kr

JA

Kr

JA

0.00
0.00
0.00
0.00
0.00

1.95
3.90
2.36
5.23
6.82

0.49
0.00
0.00
0.00
0.00

0.14
0.87
0.20
1.07
4.55

7.25
0.17
0.00
0.00
0.00

Nominal mean densities (t n.mile)2) in each 50 m depth bin were shown in the tables.

whereas those of Bryde’s were in the southerly Kuroshio–Oyashio transition zone. Summary of stomach
contents is shown in Table 1. Ten minke whales and
17 Bryde’s whales could not be used in the analysis
because of either empty stomachs or because thestomachs had been damaged by the harpoon. Japanese
anchovy was found in the stomachs of minke whales inall small blocks from late May to early August in eachyear. Thirty-two out of 44 animals fed on Japanese
anchovy. Walleye pollock was found only in Block 1
in 2001 which is located off the coast of eastern Hokkaido. Eight animals fed on walleye pollock. Krill was
found in three small blocks from May to June in 2001.
Seasonal comparison can be made only in Block 2 in2000 and 2001. Stomach contents were Japanese anchovy in May and August. The result suggested thatJapanese anchovy was the most important prey inBlock 2 although coverage was slightly different from2000 to 2001. Five stomachs contained two prey species. One individual in Block 1 in 2001 fed on bothÓ 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

walleye pollock and krill. The ratio was 3:7. Fourindividuals in Block 1 in 2001 fed on Japanese anchovy and walleye pollock. Ratios varied from 8:2 to
2:8.
Japanese anchovy was found in the stomachs of
Bryde’s whales in Block 4 in August 2000 and in Block
C in July 2001. Krill was found in all small blocks from
May to August in each year. Eleven stomachs contained Japanese anchovy while 23 stomachs contained
krill. Two stomachs contained both Japanese anchovy
and krill. Proportion of Japanese anchovy to total
stomach contents was less than 1% in one animal
while it was 30% in another.
Distribution and biomass of prey species
Horizontal distributions of krill, Japanese anchovy andwalleye pollock are shown in Figs 3–5, respectively.
Results of the target trawlings are summarized inTable 2. Most of the krill was observed in the cold
water area. Their distribution was well correlated with

0.00
0.00
0.00
0.00
0.00
0.00
0.42
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
3.81
0.00
0.00
0.00
0.00
0.00
3.4
3.1
2.0
2.3
2.5
1.7
3.7
3.7
2.1
1.8
5.8
4.4

63.83
43.33
51.20
7.39
19.57
0.00
12.34
12.93
15.61
8.56
19.28
6.82

348.97221.33486.41
57.14
193.430.00
101.94161.30172.7994.61
102.03155.820.09 4.93
0.63 10.55
0.13 0.60
0.57 0.97
0.32 3.76
0.00 0.00
0.17 0.43
0.25 12.87
0.37 1.28
0.32 1.21
0.26 0.18
0.32 7.42

26.94
53.88
5.66
7.47
37.16
0.00
3.53
160.59
14.19
13.41
0.94
169.490.70 0.00
0.56 0.00
0.48 0.00
0.21 0.00
0.61 0.00
0.00 0.00
0.63 18.44
0.48 0.00
0.28 0.00
0.52 0.00
0.71 0.00
0.57 0.00

Figure 6. Relative frequency of prey species by weight inthe minke whale stomach contents (oi) and in the environment (pi). JA, Japanese anchovy; WP, Walleye Pollock;
Kr, Krill. Surveyed blocks were showed as ‘surveyed year –block name’. For example Block 2 in 2001 was showed as‘2001–2’.

5,467
5,108
9,501
7,736
9,883
9,883
8,260
12,473
11,06911,057
5,292
22,849
249
219
192
206
251
167
334
414
224
193
425
668
6
3
1
2
2
1
4
4
1
1
0
1
5–7 August4
2–4 August
4
8 and 30 August 2
22–24 August2
27–29 August2
25–26 August2
19–24 June4
17–21 May5
22–26 May2
11–13 June2
15–18 June11
7–12 July
4

Density
Biomass(t n.mile)2) (103 t) CV
Density
Biomass(t n.mile)2) (103 t) CV
Prey survey
Year Block period

2000 12
3
4
6
7
2001 1
2
3
4
B
C

Walleye pollock

Krill
Distance
coveredby echo Survey
Degree ofsounder area
coverage Density
Biomass(n.mile) (n.mile2) (d)(t n.mile)2) (103 t) CV

Japanese anchovy

H. Murase et al.

No. oftargetNo. oftrawl
trackline point

Table 4. Summary of prey survey and biomass estimations of krill, Japanese anchovy and walleye pollock in each block.

194

water temperature at 250 m depth. Most of observed
krill was considered as Euphausia pacifica. Krill was
sporadically observed in the southern part of the survey area where krill consisted of several species otherthan E. pacifica. The distribution pattern of Japanese
anchovy well reflected water temperature at 50 m
depth. Japanese anchovy was scarce in the southernpart of the survey area where the influence of Kuroshio
was strong. It was abundant in Block 2 in August 2000
and May 2001. Japanese anchovy in Block 1 wasabundant in 2000 but scarce in 2001. The difference
might be explained by the seasonal migration to thenorth in August. Distribution of walleye pollock was
only found in the continental slope-shelf zone in
Block 1 in 2001.
Vertical distribution patterns of krill, Japanese anchovy and walleye pollock are summarized in Table 3.
Most of the krill was concentrated at water depth
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Prey selection of common minke and Bryde’s whales

Figure 7. Relative frequency of prey species by weight in
the Bryde’s whale stomach contents (oi) and in the environment (pi). JA, Japanese anchovy; Kr, Krill. Surveyedblocks were showed as ‘surveyed year – block name’. For
example Block 4 in 2001 was showed as ‘2001–4’.

195

Prey selection
Proportions of occurrence of prey species in stomachs
of minke and Bryde’s whales in each block are shownin Figs 6 and 7, respectively, with proportion of
occurrence of prey species in the sea. Proportions ofoccurrence of prey species were not reflected in minke
whale stomach contents. Manly’s a indicated thatminke whales showed selection for Japanese anchovyin all survey blocks regardless of season while they
avoided krill although statistical analysis suggested
that there was no prey selection for Japanese anchovyin Blocks 1, 3 and B in 2001 (Tables 5 and 6). In
Block 1 in 2001, pairwise statistical comparison suggested that minke whales showed selection for walleye
pollock and against krill. Bryde’s whales showed contradictory results. Manly’s a indicated that they
showed selection for Japanese anchovy in August 2000
(Block 4) and July 2001 (Block C) although statisticalanalysis suggested that there was no selection forJapanese anchovy in Block C. In contrast, Bryde’s
whales selected krill in May and June in 2001 (Blocks
3, 4, and B).
DISCUSSION

deeper than 150 m. An exception was found in Block
1 in 2001 where most of krill occurred shallower than
150 m water depth. In 2000, all Japanese anchovy
schools were found shallower than 40 m depth. In
2001, most of Japanese anchovy schools were observed
shallower than 50 m water depth, but some schools
were observed from 50 to 150 m water depth especiallyin Blocks 2 and 4. Walleye pollock was abundant at
depths deeper than 50 m and the peak was at depths
between 100 and 150 m.
Estimated biomasses of Japanese anchovy, walleye
pollock and krill are shown in Table 4. Degree ofcoverage in each block was less than 6 which wasrecommended by Aglen (1989). Krill was the mostabundant in most of blocks except in the southern partof the survey area. Coefficient of variation (CV) of thebiomass estimates for each species ranged between 0.1
and 0.7 in each small block.
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Manly’s a suggests that minke whale seem to selectfor Japanese anchovy. Selection for pelagic shoaling
fish such as herring (Clupea harengus) and capelin(Mallotus vilosus) was observed in the northeast
Atlantic (Haug et al., 1996; Skaug et al., 1997;
Harbitz and Lindstrøm, 2001; Lindstrøm and Haug,
2001). Foraging success is measured by maximizationof energy intake rate and minimization of timenecessary to obtain nutrient (Schoener, 1971). Caloric values of Japanese anchovy, walleye pollock,and krill were 6402, 6192, and 3556 kJ kg)1,
respectively (Tamura and Fujise, unpublished data).2
Japanese anchovy was concentrated shallower than
50 m water depth, while walleye pollock and krill

2

The results were reported in the unpublished paper as follows: Tamura, T. and Fujise, Y. (2002). Daily and seasonal
prey consumption by common minke whale and Bryde’s
whale in the western North Pacific. Appendix 4, pp. 85–96.
In: Government of Japan (Y. Fujise, S. Kawahara, L.A.
Pastene and H. Hatanaka, eds), Report of 2000 and 2001
feasibility study of the Japanese Whale Research Program
under Special Permit in the western North Pacific-Phase II(JARPN II). Paper SC/54/O17 presented to the IWC Scientific meeting April 2002 [Available from the International
Whaling Commission].

196

H. Murase et al.

Table 5. Selection Index (wi) and the results of statistical test, and Manly’s a of minke and Bryde’s whales in each small blockin 2000 and 2001 surveys.
Block 1

Block 2(a) Minke whale in 2000

Kr

JA

Kr

JA

wi
CV(wi)
Bonferroni CI (low)Bonferroni CI (high)
Manly’s a
1/I

0.000
–
–
–
0
0.5

13.954
–
–
–
1
0.5

0.000
–
–
–
0
0.5

5.108
–
–
–
1
0.5

Block 1

Block 2Block 3Block B(b) Minke whale in 2001

Kr

JA

WP

Kr

JA

Kr

JA

Kr

JA

wi
CV(wi)
Bonferroni CI (low)Bonferroni CI (high)
Manly’s a
1/I

0.086
1.049
)0.130
0.302
0.008
0.3

7.771
0.525
)1.99817.540
0.703
0.3

3.202
0.212
1.574
4.830
0.290
0.3

0.000
–
–
–
0.000
0.5

2.004
–
–
–
1.000
0.5

0.085
1.309
)0.165
0.335
0.007
0.5

12.1360.411
0.946
23.326
0.993
0.5

0.740
0.269
0.294
1.187
0.025
0.5

29.241
0.815)24.187
82.6700.975
0.5

Block 4(c) Bryde’s whale in 2000

Kr

JA

wi
CV(wi)
Bonferroni CI (low)Bonferroni CI (high)
Manly’s a
1/I

0.049
0.761
)0.035
0.131
0.006
0.5

8.281
0.224
4.123
12.4460.994
0.5

Block 3Block 4Block B

Block C(d) Bryde’s whale in 2001

Kr

JA

Kr

JA

Kr

JA

Kr

JA

wi
CV(wi)
Bonferroni CI (low)Bonferroni CI (high)
Manly’s a
1/I

1.082
–
–
–
1
0.5

0.000
–
–
–
0
0.5

1.142
–
–
–
1
0.5

0.000
–
–
–
0
0.5

1.009
–
–
–
1
0.5

0.000
–
–
–
0
0.5

0.895
0.604
)0.317
2.107
0.449
0.5

1.096
0.466
)0.049
2.242
0.551
0.5

If the Bonferroni confidence interval of wi contained the value 1, whales fed on prey species randomly. If Manly’s a was equal to
1/I, species i is randomly selected. If Manly’s a was greater than 1/I, species i is actively selected. If Manly’s a was less than 1/I,
species i is avoided.

were distributed deeper than 100 and 150 m water
depth, respectively. Considering that usual foraging
depth of minke whale was upper 100 m (Blix and
Folkow, 1995), Japanese anchovy will be the first
choice of prey to gain maximum energy intake withminimum dive time. The same kind of selection for
epipelagic prey species was also observed in Dall’s
porpoises (Phocoenoides dalli) around Hokkaido, Japan(Ohizumi et al., 2000). It should be noted that caloric values of prey species could be varied with theirbody sizes, season and year.
Although Manly’s a showed prey selection ofminke whales for Japanese anchovy, statistical test
could not detect it in three Blocks (Blocks 1, 3 and B
in 2001). High CVs of biomass estimates of Japanese
anchovy in those blocks could be one of the reasons
Ó 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Prey selection of common minke and Bryde’s whales

197

Table 6. Pairwise comparisons of selection indices between prey species of minke and Bryde’s whales. If the confidence interval^ k is not statistically significant.
^ i and w
contained the value 0, difference between w
Block 1(a) Minke whale in 2001
Bonferroni CI (low)Bonferroni CI (high)Kr versus JA

Kr versus WP

JA versus WP

Block 3
Kr versus JA

Block BKr versus JA)18.4623.092)5.019)1.213)7.19816.336)23.313)0.788)82.334
25.332Block 4
Kr versus JA(b) Bryde’s whale in 2000
Bonferroni CI (low)Bonferroni CI (high))12.413)4.059Block CKr versus JA(c) Bryde’s whale in 2001
Bonferroni CI (low)Bonferroni CI (high)of the result of the statistical test. The survey inBlock 1 was conducted off the coast of southeastern
Hokkaido in June 2001. It was reported that Japanese
anchovy was concentrated in the continental shelfzone of off southeastern Hokkaido in June 1991
(Mihara, 1998). Limited survey effort in the continental shelf zone in Block 1 could result in high CV ofbiomass estimate of Japanese anchovy. Blocks 3 andB in 2001 were located in the Kuroshio–Oyashiotransition zone, where oceanographic conditions werecomplex. Those heterogeneities of oceanographic
conditions could affect the distribution pattern ofJapanese anchovy and could result in high CVs.
Walleye pollock was mainly distributed in the continental slope zone (150–250 m water depth) andcontinental shelf zone (70–150 m water depth) off
southeastern Hokkaido (Shida, 2002). The lower
survey effort in the continental shelf-slope region
could increase the CV of walleye pollock in Block 1.
Finer scale consideration of oceanographic conditions
and bottom topography will be required in future
surveys to reduce CVs.
Krill was selected, avoided, or randomly selectedin the eastern North Atlantic (Haug et al., 1996;
Skaug et al., 1997; Harbitz and Lindstrøm, 2001;
Lindstrøm and Haug, 2001). In the eastern North
Atlantic, krill was well distributed in the upper
100 m water depth whereas it was distributed mainly
deeper than 150 m water depth in this study.
Because E. pacifica was mainly found at waterÓ 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

)2.4312.028

temperature range 7–8 °C, it was mainly distributed
in 200–300 m water depth range in daytime from
June to February (Taki et al., 1996). Availability ofkrill to minke whales may be varied because thevertical distribution pattern of krill depends on water temperatures.
Bryde’s whales selected for krill in the early season(May and June) and then they selected for Japanese
anchovy in the late season (July and August). Prey
selection of Bryde’s whale could change as the season
progresses but because the survey coverage of eachmonth was slightly different, it is difficult to distinguish between seasonal and geographical change ofprey selection at this stage.
Lindstrøm and Haug (2001) pointed out four
underlying assumptions of prey selectivity studies: (1)whale samples are randomly selected in the given
survey area, (2) sampled whale feed in the survey
area, (3) forestomach contents are reliably reconstructed, and (4) estimated prey biomass is reliable
and constant during the survey period. Assumptions(1) and (2) were already considered in past surveys(Lindstrøm et al., 1998; Tamura and Fujise, 2002).
Ideally, only undigested stomach contents should be
used to satisfy the assumption (3) but we have totrade reduced sample size with the accuracy of theweight of stomach contents. Because of the difficultyof obtaining the stomach contents of cetaceans,digested stomach contents were used to make gooduse of available samples in this study. Regarding

198

H. Murase et al.

assumption (4), considerations of the vertical distributions and the diurnal behavior of preys are necessary. Maximum depth of acoustic survey was set at
250 m because prey species of minke and Bryde’s
whales mainly occurred shallower than 250 m. It
seemed that minke whales feed on prey shallower
than 100 m (Blix and Folkow, 1995). No diving
depth of Bryde’s whale has been measured. It waspointed out that prey species such as Japanese anchovy (Ohshimo, 1996), walleye pollock (Abe et al.,1999) and Euphausia pacifica (Taki et al., 1996)
showed diurnal vertical migration. Those species
formed dense schools in daytime whereas they disperse at night. Because there was little diurnalchange in the mean stomach content weight ofminke whales during daytime, minke whales would
feed on pelagic schooling prey species at the surface
throughout the daytime (Tamura et al., 1998). Given
information on diurnal distribution patterns of preyspecies and the mean stomach content weight ofminke whale in the daytime, prey survey conducting
only during daytime is adequate. Because diurnalchange of stomach content weight of Bryde’s whale
has not been reported, this point should be investigated in future surveys.
In the 2000 and 2001 surveys, we could not cover
the autumn season when Pacific saury was abundantin the survey area. Because it was consumed by minke
whales (Tamura and Fujise, 2002), whale and theconcurrent prey surveys must be conducted in September and October to assess whether minke whale
have preference for it. In the western North Pacific,
biomass of pelagic fish such as Japanese sardine,
Pacific saury, Japanese anchovy and chub mackerel
has shown drastic fluctuations, so-called species
replacement (Yatsu et al., 2001). Stomach contents ofminke whales reflected the historical change indominant species in the survey area (Kasamatsu andTanaka, 1992). Long-term concurrent monitoring ofboth whale stomach contents and the prey species
abundance is critically important to develop ecosystem models that are suitable to the western North
Pacific.
ACKNOWLEDGEMENTS
The authors would like to thank Drs Hiroshi Hatanaka and Seiji Ohsumi of the Institute of Cetacean
Research, Japan for their critical comments on the
manuscript. Dr Kazushi Miyashita (Hokkaido University) kindly provided critical comments on
acoustic data analysis methods. Comments from two
anonymous referees greatly improved the manuscript.

We also thank all of the researchers and crews who
were engaged in the 2000 and the 2001 JARPNII
surveys.
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APPENDIX A
Details of formulas used in acoustic data analysis
Mean backscattering area per square nautical mile ofsea surface (SA) by species for every 1 n.mile of survey
transect over defined depth interval is calculated by
the following formula:SA ¼ 4pr20 ð1852Þ2

Z r2
r1



m2
sV dr
n.mile2ðA  1Þ

where r is depth from the sea surface, r0 ¼ 1 m representing the reference range for backscatteringstrength. A length–target-strength (TS) relationshipfor Japanese anchovy (Anonymous, 1990) is used;

200

H. Murase et al.

TS ¼ 20 log TL  72:5

ðA  2Þ

where TL is total length in cm. A length–weight
relationship for Japanese anchovy (Anonymous, 1990)
is used:
W ¼ 0:004 TL3:09

ðA  3Þ

where W is weight in gram.
A length–target-strength (TS) relationship forwalleye pollock (Foote and Traynor, 1988) is used:
TS ¼ 20 log FL  66

ðA  4Þ

where FL is folk length in cm.
A length–weight relationship for walleye pollock(Pereyra et al., 1981) is used:
W ¼ 0:0077FL2:906

ðA  5Þ

where W is weight in gram.
We assumed that all krill observed during two
surveys was Euphausia pacifica and length and TS were
16.4 mm and )83.3 dB (Miyashita et al., 1996),
respectively, because no length–TS relationship for
krill was available in this area. The average weight was
30.6 mg calculated using formula described by Odate(1987).
Average area biomass density (q) for each species iscalculated as follows:
X SA
fi Wi
q¼
ðA  6Þr
where fi is the frequency distribution of ith lengthclass. The acoustic cross section (r) is converted fromTS as follows:ðA  7Þr ¼ 4pð10Þ0:1TS
Frequency distribution of each class (fi) that is theacoustical contribution to the area backscattering foreach length class (Ona, 1993):
fi ¼

1
X

nj L2jðA  8Þtransect in kth block and nki¼ number of 1 n.mile
averaging intervals on the ith transect in kth block. In
this formula, each transect is regarded as a single
biomass density sample. Then variance of SAk is calculated with the formula (Jolly and Hampton, 1990):
NkVarðSAk Þ ¼
Nk  1PNk

2 2i¼1 ðSAki  SAk Þ nki
P2
Nki¼1 nkiðA  10ÞSA is converted to q using the above-mentioned formula. Biomass is estimated as;
Bk ¼ Ak q^kðA  11Þ

where Bk is density biomass in kth block and Ak is areaof kth block. Variance of Bk is calculated with thefollowing formula:varðBk Þ ¼ A2k varðqk ÞðA  12Þ

Coefficient of variation of Bk is calculated as:pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃvarðBk ÞCVðBk Þ ¼
BkðA  13Þ

APPENDIX B
Details of statistical analysis procedures of prey selection
analysis^ i s are calculated based on a formula of a
Variances of w
ratio estimator
(
2
J 
1
J X
uij
^ i uþjwVð^wi Þ  2uþþ ðJ  1Þ j¼1 p^i)ðB  1Þ
J
X
2
2
2
þ^
wi CV ð^
pi Þ
uþj ;
j¼1

(Cochran, 1977; Manly et al., 2002). CVð^pi Þ is estimated using variance of mi based on a delta method(Seber, 1982) fromj¼1

where nj is the number of individuals in size class j andthe length is L.
Following procedures are adopted from Jolly and
Hampton (1990). Weighted mean of SA of each blockis:
PNk
SAki ðnki Þ
SAk ¼ i¼1
ðA  9ÞPNk
i¼1 nki
where SAk¼ mean SA in kth block, Nk¼ number oftransects in kth block, SAki¼ mean SA on the ithVarðpi Þ ¼ Varðmi Þm2ðiÞ

þ4m2i

fVarðm1 ÞðRmk Þ
ðRmk Þ4þ    þ Varðmi1 Þ þ Varðmiþ1 Þ þ    þ VarðmI ÞgðB  2Þ

þ1where mðiÞ ¼ m1 þ    þ m1
i þ mi þ    þ mI .

Variance of the difference between two selection
ratios can be estimated as^ k Þ ¼ Vð^^ kÞVð^wi  wwi Þ þ Vð^
wk Þ  2Covð^wi ; wðB  3ÞÓ 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

Prey selection of common minke and Bryde’s whales^ i and w^ k can be estimated as
Covariance of w
(
J
J
1 X
^ kÞ 
Covð^
wi ; w2p^i p^k uþþ ðJ  1Þ j¼1^ i pi uþj ukj  w^ k uij pk uþj þ w^ iw^ k pi pk u2þj Þðuij ukj  w
)
J
X
^ iw
^k
w
Covð^
pi ; p^k Þu2þj :
þJ
j¼1
ðB  4ÞÓ 2007 The Authors, Fish. Oceanogr., 16:2, 186–201.

201

where covariance of p^i and p^k can be estimated asmðiÞ m2 mk^k Þ ¼ CV2 ðmi Þ P i 4
Covð^pi ; pð ml Þ
2Xmmm
mi mk m2iðkÞ^l Þ P l4
 CV2 ðmk Þ P k 4 þ    þ
CV2 ðmð ml Þ
ð ml Þ
l6¼i;kðB  5Þ