SC/J09/JR10

Estimation of prey preference of common minke whales(Balaenoptera acutorostrata) in a coastal component (offSanriku) of JARPNII in 2005 and 2006
HIROTO MURASE1, SHIGEYUKI KAWAHARA2, HIROSHI NAGASHIMA3, KEIICHI ONODERA3, TSUTOMU TAMURA1,
RYOSUKE OKAMOTO4, SHIROH, YONEZAKI1, RYUICHI MATSUKURA5, KENJI MINAMI5, KAZUSHI MIYASHITA6,HIDEYOSHI YOSHIDA7, MUTSUO GOTO1, TAKEHARU BANDO1, DENZO INAGAKE8, MAKOTO OKAZAKI8, HIROSHIOKAMURA7 AND HIDEHIRO KATO4
1

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan
National Research Institute of Far Seas Fisheries, 5-7-1, Orido, Shimizu, Shizuoka, Shizuoka, 424-8633, Japan
3
Miyagi Prefecture Fisheries Technology Institute, 97-6 Sodenohama, Watanoha, Ishinomaki, Miyagi 986-2135,
Japan
4
Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo, 108-8477, Japan
5
Graduate School of Environmental Science, Hokkaido University, 3-1-1 Minato-cho, Hakodata, Hokkaido 0418611, Japan.
6
Field Science Center for the Northern Biosphere, Hokkaido University, 3-1-1 Minato, Hakodate, Hokkaido,
041-8611, Japan
7
National Research Institute of Far Seas Fisheries, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 2368648, Japan
8
National Research Institute of Fisheries Science, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648,
Japan
2

ABSTRACTPrey preference of common minke whales in the coastal water off Sanriku, Japan, was studied to
examine how they interacted with target species of local commercial fisheries. This was the first reportof prey preference of minke whales in this region. To estimate the prey preference, sampling surveys ofminke whales and their prey surveys were conducted in the same area at the same timing (April). The
surveys were conducted as a part of a coastal component of JARPN II in 2005 and 2006. A local
fisheries science institute, Miyagi Prefecture Fisheries Technology Institute, took part in the surveys. A
prey preference index, Manly’s α, was used in the analysis. Sum of Manly’s α for all prey species is 1
and prey species with large values of Manly’s α indicates preference for it. Minke whales fed on North
Pacific krill, Japanese anchovy and Japanese sand lance (adult) during the surveys. These are important
species of local commercial fisheries. Estimated average values of Manly’s α for krill, anchovy andadult sand lance in 2005 and 2006 were 0.02 (se=0.02), 0.09 (se=0.08) and 0.89 (se=0.09), respectively.
Minke whales showed preference for adult sand lance. As previously reported in other regions, krill was
not preferred prey of minke whales. Ecosystem modelling work (Okamura et al., 2009) suggested thatchange in a functional response form had substantial effect on predation impact on sand lance by minke
whales. Functional response form can be estimated if long term prey preference data are available.
Continuation of JARPN II will provide data to develop reliable ecosystem models for fisheries
management.

INTRODUCTION
Removal of large whales from marine ecosystem as the results of commercial harvesting
demonstrates that these whales play important role in marine trophic dynamics. Krill surplus,
prey release due to removal of large whales by harvesting in the Antarctic, led increase in
number of other krill predators such as Antarctic minke whales (Balaenoptera bonaerensis)
and crabeater seal (Lobodon carcinophaga) (Laws, 1985), is well known example. In the
North Pacific, it was postulated that decline of populations of seals, sea lions and sea otters
was derived from prey switching of killer whales from large whales after post world war II
commercial harvesting (Springer et al., 2003) though opposite views were also presented(DeMaster et al., 2006; Mizroch and Rice, 2006; Trites et al., 2007; Wade et al., 2007). Afterthe ban of commercial harvesting of large whales, these populations show increasing trends
worldwide in recent years. In tern, potential of competition between whales and commercial
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fisheries has been postulated (Tamura, 2003). However, those studies on trophic role ofwhales in ecosystem were conducted at large scale (e.g. ocean basin scale) and effect ofwhales on local scale (e.g. coastal fisheries area) is rarely studied. Study of effect of whales
on local fisheries is important for fisheries management because it is expected that interaction
between fisheries and whales is intense at local level and it would directly affect the
livelihoods of fishermen.
Sendai Bay is located in the north-eastern region of Japan (Fig. 1). The bay isbelonging to Sanriku region and it is an important fishing ground of local communities. A
wide variety of species are harvested by local fishermen in the bay. The seasonal effect ofchanges in the location of the subarctic western boundary current with cold low-salinity water(Oyashio) and the subtropical western boundary current with warm high-salinity water(Kuroshio) is strong in the bay. Distribution patterns of biological organisms in Sendai Bay
drastically change as the season progresses as the result of the changes in the oceanographic
conditions. In springtime, North Pacific krill (Euphausia pacifica) and Japanese sand lance(Ammodytes personatus) are major target species of commercial fishing (Nagashima, 2000;
Taki, 2002) Japanese anchovy (Engraulis japonicus) starts migration from south to north inspringtime and some earlier migrants are observed in the bay in spring. Anchovy is also targetof commercial fisheries (Nagashima, 2007). The bay is a part of the migration corridor ofcommon minke whales (B. acutorostrata) and they are distributed in there in spring.
Commercial whaling of minke whales by small-type whaling boats had been also one of theimportant local fisheries but it has been banned by the International Whaling Commission
since 1986 without scientific justification. Past records suggested that minke whales mainly
fed these commercially important species in this region (Kasamatsu and Tanaka, 1992).
Though the impact of predation by minke whales on these preys could be large at the local
scale, no systematic survey was conducted to assess the magnitude in the past.
The second phase of the Japanese Whale Research Program under Special Permit in
the North Pacific (JARPN II) has been conducted since 2000. The research activity of JARPN
II is legal according to Japanese national regulation and the International Convention for the
Regulation of Whaling (ICRW). The overall goal of JARPN II is to contribute to theconservation and sustainable use of marine living resources including whales in the western
North Pacific, especially within Japan’s EEZ. One of the main objectives of JARPN II isestimation of prey preference of cetaceans. To estimates prey preference of cetaceans,cetacean sampling and prey surveys have been conducted simultaneously as a part of JARPN
II. The surveys were conducted as feasibility studies in the first two years (2000 and 2001).
Prey preferences of common minke and Bryde’s (B. edeni) whales were estimated using data
sets collected in the feasibility studies and results were reported to IWC/SC (Government ofJapan, 2002) and published in peer reviewed scientific journal (Murase et al., 2007). Based on
the success of feasibility studies, JARPN II was expanded to full scale in 2002. In the fullscale survey, JARPN II is divided into offshore and coastal components. Coastal componentof JARPN II has been conducted off Kushiro and off Sanriku. Coastal component of JARPN
II was conducted by the National Research Institute of Far Seas Fisheries and the Institute of
Cetacean Research in cooperation with the Tokyo University of Marine Science andTechnology. In addition, a local fisheries science institute, Miyagi Prefecture Fisheries
Technology Institute, took part in the survey because of interests in feeding impact of minke
whales on local fisheries.
It was planned that the results of full scale JARPNII will be review every six years.
First six years period was 2002-2007. This paper presented the results of estimation of prey
preference of minke whales off Sanriku in 2005 and 2006 using data collected bysimultaneous cetacean sampling and prey surveys. Though survey was also conduced in 2007,

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the results were not presented because analysis was not completed. This was the first attempt
to estimate prey preference of minke whales off Sanriku especially in Sendai Bay. In thispaper, preference is defined as the animal choosing a resource irrespective of amount of
resources.
MATERIALS AND METHODS
Survey area, timing of survey and vessels
Prey survey
The simultaneous cetacean sampling and prey surveys were conducted around Sendai Bay in
April in 2005 and 2006 as a part of the coastal component of the JARPN II. For prey species
surveys, the survey area was stratified into 7 blocks (A-G block) based on bottom depth andadministrative boundary (Fig. 1). A zigzag trackline was set to cover each block. According
to Aglen (1989), accurate biomass estimation will be expected if the index of survey distance(surveyed distance divided by square root of survey area) is larger than 6. The distance index
in each survey block was at least 6 in this survey. Acoustic and oceanographic surveys wereconducted using trawler-type RV, “Takuyo maru” (Miyagi prefecture, 120 GT). The RV
steamed at 9 knots on tracklines. A quantitative echosounder, Simrad EK500 (Norway), with
operating frequencies at 38 and 120 kHz was used for the acoustic survey. The data wererecorded with the aid of Echoview (Myriax,, Australia). Calibrations were carried out prior to
the survey every year using the copper sphere technique described in the EK 500 manual
which was based on Foote (1982). A trawl net (mouth opening of 7 m (width) * 3.5 m(height) with 3mm liner cod end) was used to examine species compositions of representativeechosigns in the survey area. Targeted trawls were conducted in the blocks where densities of
species of interest were high. Representative echosigns were selected based on our previous
knowledge. Because acoustic surveys using EK500 have been conducted in the survey area
since 1999, we have qualitative information on the echosigns in the survey area. Details of
prey survey methods were also described by Yonezaki et al. (2009).
Cetacean survey
Four small-type whaling catcher boats were engaged in sampling of minke whales in April
and May from 2005 to 2006. Stomach contents of sampled minke whales were initially
examined at the land station. The land station was established at Ayukawa port area in Miyagi
prefecture. Details of cetacean surveys were described by Kishiro et al. (2009).
Oceanographic observation
Oceanographic conditions from sea surface to 1m above bottom were recorded by CTD. Sea
surface temperature was recorded every 1 minute while RV steamed on the trackline. Contour
maps of water temperature at surface and bottom were drawn based the ordinary Kriging
method.
Stomach content analysis
Baleen whales have a four-chambered stomach system. Among them, the stomach contents
remain in the forestomach (1st. stomach) and fundus (2nd. stomach) were used in this study.
Details of stomach contents analysis were described in Tamura et al. (2009). Thoughsampling survey of minke whales conducted in April and May, samples collected in April
were used in the analysis because prey surveys were conducted only in April.

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Biomass estimation of prey species
Species identifications of echosigns were conducted based on the differences of mean
volume backscattering strength between 120 kHz and 38 kHz (ΔMVBS120-38) and the meanof volume backscattering strength ( SV ) at 38 kHz and at 120 kHz. SV and ΔMVBS120-38
were calculated based on the results of targeting trawl hauls. Ranges of S V and ΔMVBS12038 for three species were as follows; krill: -70 - -45 dB and 10 - 20 dB, anchovy (adult): -50 -35 dB and -10 - 5 dB, sand lance (adult): -70 - -50 dB and -10 - 10 dB. We verified theresults of the species identifications by visual inspection of echogram based on our previous
knowledge. Nautical area scattering coefficient (sA) by species for every 0.1 n.mile of survey
transect from sea surface to bottom was calculated with a aid of Echoview. Theoretical target
strength (TS) of krill and sand lance at 120 kHz were calculated based on the Distorted Wave
Born Approximation (DWBA) model (Stanton and Chu, 2000 for review). Calculated lengthTS (dB) relationships of krill and sand lance at 120 kHz based on DWBA were
TS=50.7log(TL, mm)-150.4 and TS=49.3log(SL, mm)-160.2, respectively (Matsukura
unpublished data). Length-TS relationship of anchovy was TS=20log(TL, cm)-72.5 (Iversen,
et al., 1993). Biomass densities ( ρ , t/n.mile2) of three species were estimated asρ = ∑ (s A / σ bs ) f iWi where σ is acoustical scattering cross section (4π10^(0.1*TS)), fi was thefrequency distribution of ith length class and Wi was weight of ith length class. Biomasses andthe variances of three species were estimated based on a stratified random sampling method
as described by Jolly and Hampton (1990). Because sand lance is capable to move verticallybetween pelagic water and bottom substrates where it buries itself during daytime (Robards
and Piatt, 1999), sand lance detected by echosounder considered as not absolute biomass but
as available biomass for minke whales for feeding.
Estimation of prey preference
The standardized form of Manly’s selection index called Manly’s α (Manly et al., 1972), also
known as Chesson’s index (Chesson, 1978), was used in the study as in the cases ofLindstrøm and Haug (2001) and Murase et al. (2007). Selection index is calculated as follows.
Sample proportion of number of individuals with dominant prey species i in their stomach (ni)in survey year j isIoi = nij / ∑ nij .
i =1

If more than two prey species is found in the stomachs, we only considered dominant species(e.g. if 100 kg of anchovy and 10 kg of krill are found in a stomach, the individual is treatedthat it consume only anchovy). It was assumed that each individual consumes the average
daily prey consumption weight of dominant prey species i in the stomach. Thus, oi is equal tosample proportion of prey species i by weight used by all animals. Sample proportion ofavailable units (biomasses in survey block) in prey i in survey year j isI

π ij = mij / ∑ mij
i =1

where mij is an amount of available units in prey i in survey year j in a sample of available
resource units. Manly’s selection index is
wij = oij / π ij .

Standardized Manly’s selection index, Manly’s α is written as;

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Bi =

wi
∑ wi
i

If B̂i is equal to 1/I (I is total number of prey species utilized by a predator species), species i
is randomly selected. If Bi greater than 1/I, species i is actively selected. If Bi is less than 1/I,
species i is avoided.
The log-likelihood function based on a multinomial distribution is given by
 Bi aij I J∑ ∑ nij log ∑ wk a kj 
i =1 j =1where aij is the density of prey i in the survey year j. Our objective is to estimate the Bi
parameters by maximizing the above equation. Data in 2005 and 2006 were used to estimate
average Bi in two years. Each density, aij, has its uncertainty in the form of CV. To accountfor the variation in estimation of selection indices, we used a Monte Carlo simulation
technique with 1000 permutations. When aij* ~ LN(log(aij), CVij2), we calculate the Bi*s foreach aij*. Then the variance of Bi is given by var(Bi) = E(var(Bi*)) + var(E(Bi*)) wherevar(Bi*) is calculated by a Hessian matrix. Variance of Bi in each year is also estimated usingsame methods.
RESULTS
Oceanographic conditions
Contour maps of surface and bottom temperature were shown in Fig. 2. These maps suggested
that oceanographic condition in the survey area was different in each year.
Distribution patterns and stomach contents of whales
Sighting effort and sighting positions by small-type whaling catcher boats were shown in Fig
3. Sighting positions and stomach contents of sampled minke whales were also shown in Fig.
3. Minke whales were sampled in B, C, E and F blocks. Summary of stomach contents inthese blocks were shown in Table 1. Krill, anchovy and adult sand lance (≥ 10 cm) were only
species found in the stomachs.
Distribution patterns and biomasses of prey species
Distribution patterns of krill, anchovy and adult sand lance were shown in Fig. 4. Krill was
mainly distributed in offshore and northern region (A, D and G blocks). Distribution patternsof anchovy were varied year to year according to change in sea surface temperature. Adult
sand lance was distributed in nearshore region (B, C, E and F blocks). Biomasses of these
species were summarized in Table 2.
Prey preference of minke whales
Prey preference of minke whales in each year and average values in two years (2005 and2006) was summarized in Table 3. Because minke whales were mainly sampled in B, C, E
and F blocks, data in these blocks were pooled to estimate the prey preference. Minke whales
showed preference for sand lance in each year. Krill and anchovy were avoided. Averagevalue of Manly’s α in 2005 to 2006 indicated that minke whales preferred firstly sand lance,
secondary anchovy and thirdly krill in the survey area in April.

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DISCUSSION
This was the first report of prey preference of minke in Sendai Bay. As in the cases of studies
in the Norwegian water (Harbitz and Lindstrøm, 2001; Haug et al., 1996; Lindstrøm andHaug, 2001; Skaug et al., 1997; Sivertsen et al., 2006) and in the western North Pacific(Murase et al., 2007; Muraase et al. 2009), minke whales showed preference for pelagic
shoaling fishes while they avoided krill in this study. Previous studies suggested that minke
whales showed preference for anchovy in the western North Pacific in summer (Murase et al.,2007; Muraase et al. 2009). In contrast, they showed preference for sand lance in Sendai Bay
in spring. Biomass estimates of sand lance were higher than anchovy in all years in this study.
The results indicated that minke whales could show preference for abundant pelagic shoaling
fishes if more than two pelagic shoaling fish are presented. Krill was mainly distributed in
offshore and northern region (A, D and G blocks). Few minke whales were sighted in theseblocks (Hakamada et al., 2009). Though prey preference in these blocks could not be
estimated in this analysis, few minke whales sightings in these blocks indicated that krill was
not preferred prey for them.
Sand lance is distributed in the central region of Sendai Bay where the water depth
was 40-70m with medium to large gravel bottom substrate (Kobayashi et al., 1995). Adult
sand lance in the bay bury themselves in the substrate and estivate without feeding during
summer while their feeding occur in pelagic water from late winter to early summer(Kobayashi et al., 1995; Kodama 1980). Their spawning occur rest of season. Because of theirunique life history, sand lance is resident species in the bay in contrast to other migrantpelagic fish such as anchovy. Sand lance in Sendai bay in spring would be highly predictable
prey for minke whales. This could be one of the reasons why minke whales are distributed in
the bay in spring. It was reported that lesser sandeel (A. marinus) was an important prey in a
local region (the eastern North Sea) of the northeastern Atlantic (Windsland et al., 2007). A
small scale habitat study in the coastal water of Scotland suggested that seasonal movement ofminke whales could be related to availability of lesser sandeel (Macleod et al., 2004).
Together with the results of this study, it can be concluded that Ammodytes spp. are locallyand temporally important species for minke whales. Windsland et al. (2007) reported thatminke whales could switch their prey from sandeel to haddock in the eastern North Sea when
the recruitment of sandeel was poor. It is expected that such prey switching might occur in
Sendai Bay when biomass of sand lance is low.
Commercial fishing of sand lance is operated in Sendai Bay in spring. Traditionally,juvenile (≤ 10 cm) and adult sand lance are captured by lift net with light and large dip net,respectively. Bottom trawl fishing boats participated in adult sand lance fishing from 1984 to
1989 (Nagashima, 2004). Because effect of bottom trawling on abundance of sand lance wasdevastated, local fishing associations abstained trawl fisheries and set fishing quota for lift net
fishing in 1990 (Nagashima, 2004). The abundance of sand lance increased after 1990. Sandlance in Sedai Bay is considered as a distinctive stock based on biological and morphologicalcharacteristics (Kodama, 1980) though they are distributed in coastal waters around Japan.
Because both commercial fishing and minke whales target on the single stock of sand lance inthe small area, effect of commercial catch and feeding by minke whales is expected to be high.
Results of preliminary ecosystem modelling work suggested that feeding of minke whales had
substantial impact on MSY of sand lance but the magnitude was depended on shape offunctional response (relationship between consumption by predator and prey availability)(Okamura et al., 2009). It was strongly recommended that effort be focused on appropriate
data collection and/or experiments to develop the most appropriate functional response form
to represent feeding behaviour (Pláganyi, 2007). Multispecies functional response of common
minke whales in the southern Barents Sea was estimated at micro scale (Smout and Lindstrøm,

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SC/J09/JR102007). For fisheries management purpose, ecosystem model at meso to macro scale isrequired. Functional response form at different scales could be different. Functional response
form required by ecosystem model for fisheries model needs long term collection of prey
preference data. It was suggested that biomass of lesser sandeel could be controlled by both
top-down and bottom-up processes in the North Sea (Frederiksen et al., 2007). To modelinteractions between marine mammal and fisheries, it is important to be able to investigate thelong term dynamics of a system (Matthiopolos et al., 2008). Continuation of JARPN II isimportant to develop appropriate functional response form for minke whales.
ACKNOWLEDGMENTS

Authors express thank you to the crews and the researchers who dedicated to collect data in
the field. Special thanks are given to Ms. Tomoko Hasegawa who supported the handling of
huge amount of data set. This survey was supported by Fisheries Agency of Japan, Fisheries
Research Agency of Japan, Miyagi Prefecture and the Institute of Cetacean Research. We
thank these institutions for their support.
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Table 1. Summary of stomach contents of minke whales in B, C, E and F blocks. Number ofindividuals with dominant prey species in their stomach was summarized. If two prey species
were found in the stomach, only a dominant species in wet weight was counted.

Year
2005
2006

Krill

Japaneseanchovy

Sand lance# of ind.
20# of ind.
0
1# of ind.
25
17

Table 2. Biomasses of preys in B, C, E and F blocks.
Krill
Year

Biomass(103t)

2005
2006

22.78
3.03

Japanese anchovyCV

Biomass(103t)

0.36
0.11

1.32
9.06

Sand lanceCV

Biomass(103t)

CV

0.33
0.19

7.61
28.34

0.19
0.07

Table 3. Values of Manly’s α as a means of indications of prey preferences of minke whales
in B, C, E and F blocks. Standard error (se) is also shown. If Manly’s α is equal to 1/I, species
i is randomly selected. If Manly’s α is greater than 1/I, species i is actively selected. If
Manly’s α is less than 1/I, species i is avoided. I is total number of species consumed byminke whales.
Japanese
Anchovy

Krill
Year

2005
2006
Average

Sand
Lance

Manly's αse

Manly's αse

Manly's α

0.03

-

0.00

-

0.97

-

0.84
0.89

0.14

0.00
0.16
0.14
0.02
0.02
0.09
0.08
*Standard error in 2005 couldn’t be estimated.

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se

0.09

SC/J09/JR10

Fig. 1. Map of the survey area in Sendai Bay, Japan. The survey area was stratified into 7
blocks (A-G) for the purpose of a stratified random sampling to estimate biomass of krill,
Japanese anchovy and sand lance.

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Fig. 2. Contour maps of surface (left) and bottom (right) temperature in the survey area.

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Fig 3. Cruise tracks and sighting positions of minke whales (left) and sighting positions ofanimals and their dominant stomachs contents of sampled minke whales (right). Blue line:
cruise tracks; pink circle: sighting positions; red circle: stomach with krill; sky blue circle:
stomach with Japanese anchovy; green circle: stomach with adult sand lance.

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SC/J09/JR10

Fig. 4. Distribution patterns of krill (left), Japanese anchovy (middle) and adult sand lance(right) in the survey area detected by the echosounder.

15