DOI

https://doi.org/10.34331/cpops.2022F001

J-STAGE Advance published date: October 25, 2022

DISTRIBUTION OF MINKE WHALE OFF HOKKAIDOEFFECT OF SEA SURFACE TEMPERATURE
ON THE DISTRIBUTION OF COMMON MINKEWHALES OFF SOUTHEASTERN HOKKAIDO, JAPAN,
BETWEEN 2002 AND 2006, WITH NOTES ON THE
FORMATION OF PACIFIC SAURY FISHING GROUNDS
Hiroto MURASE1*, Koji MATSUOKA2 and Kazuyoshi WATANABE3
Laboratory of Cetacean Biology, Tokyo University of Marine Science and Technology,
4–7–1 Konan, Minato-ku, Tokyo 108–8477, Japan
2
Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan
3
Japan Fisheries Information Center, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan*Corresponding author: hmuras0@kaiyodai.ac.jp
1

Abstract
The relationships between oceanographic conditions and the distribution of common minke
whales oﬀ southeastern Hokkaido was investigated in this study. Sighting surveys of common
minke whales in this region were conducted in September, in 2002 and from 2004 to 2006.
The density index (DI, the number of schools per 100 n.miles) of the whales decreased from
5.6 schools in 2002 to 1.2 schools in 2006. During the 4 years in which surveys were conducted, the monthly mean sea surface temperature (SST) in the survey area increased from 16.6°C
in 2002 to 19.5°C in 2006, while the mean SST recorded at sighting locations was 15.5°C. The
proportions of surveyed areas with a monthly mean SST ≤16°C in 2002 and 2004 were 46.5%
and 17.0% respectively, whereas in 2005 and 2006, none of these areas had a monthly mean
SST of less than 16°C. Paciﬁc saury are among the major prey items of the minke whales inthis region, and the number of the ﬁshing boats was used as an indicator of the presence of thespecies in the surveyed area. The 4 year mean SST at the locations of the boats was 15.1°C. A
reduction in the local abundance of the saury from 2002 to 2006 was inferred from the numberof the boats operating in the region. These ﬁndings suggested that the apparent decline in theabundance of whales in the study area was associated with changes in SST and the availabilityof Paciﬁc saury.
Key words:

Balaenoptera acutorostrata, Cololabis saira, habitat, North Paciﬁc, Oyashio,
spatial distribution.

Introduction
The common minke whale (Balaenoptera acutorostrata, hereafter minke whale) is a small species of baleen whale with body length of approximately 8 m at physical maturity (Kato, 1992), andhas been reported to live to ages of up to 49 years (Maeda et al., 2017). Records of past commercial
catches indicate that the waters oﬀ southeastern Hokkaido (also known as the Doto region), Japan, areamong the most important summer feeding areas for the Okhotsk Sea–West Paciﬁc Stock (O Stock) of
the species, particularly for immature individuals and mature males (Hatanaka and Miyashita, 1997;
Kasamatsu and Tanaka, 1992). The results of the Japanese Whale Research Program under Special
Permit in the western North Paciﬁc (JARPN) and its Phase II (JARPN II) between 1994 and 2016
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MURASE, MATSUOKA AND WATANABE

showed that they fed on a variety of prey species, such as krill, Japanese common squid (Todarodes
paciﬁcus), Japanese anchovy (Engraulis japonicus), Paciﬁc saury (Cololabis saira, hereafter saury) and walleye pollock (Gadus chalcogrammus) in the western North Paciﬁc (Konishi et al., 2009;
Murase et al., 2007; Tamura et al., 2009). It has been suggested that the spatial distribution of minke
whales oﬀ southwestern Hokkaido during September could be associated with the distribution of saury (Tamura and Fujise, 2002). Nevertheless, despite this conjecture, it has yet to be suﬃciently established whether changes in the minke whale spatial distribution are associated with the distribution ofsaury and/or oceanographic conditions (e.g., sea surface temperature, SST).
The saury, along with other small pelagic ﬁshes, such as Japanese sardine (Sardinops melanostictus), Japanese anchovy, and chub mackerel (Scomber japonicus), is regarded as an economicallyimportant species in Japan (Watari et al., 2019). Since the 1900s, the commercial catches and biomassof these small pelagic ﬁsh have ﬂuctuated, reﬂecting decadal changes in species composition (i.e., species replacement) (Yatsu, 2019). Fluctuations in the populations of small pelagic ﬁsh detected during
the 20th century have been linked to cold and warm water temperature regimes, as indicated by the
Paciﬁc Decadal Oscillation (PDO) index. However, the regime around Japan from the 2000s to 2010s
was now deﬁned as an unconventional regime as the relationship did not follow previous ones (Kuroda et al., 2020). It should be noted that the biomass of saury was high in the 2000s in the time series
between 1980 and 2018 (4th Meeting of the Technical Working Group on Paciﬁc Saury Stock Assessment, 2019).
Saury generally (1) spawn during the winter months in the Kuroshio (the subtropical westernboundary current with warm high-salinity water) waters oﬀ the southern coastline of Japan, (2) migrate to the North Paciﬁc subarctic–subtropical transition zone during summer to feed and (3) commence their southbound return migration to the spawning grounds (Fuji et al., 2020; Fukushima,
1979; Kosaka, 2000; Watanabe and Lo, 1989). It is in September during this southbound migrationthat the saury is commercially ﬁshed in grounds oﬀ the southeastern coast of Hokkaido. With thetransition of the seasons from summer to autumn, the ﬁrst branch of the Oyashio (the subarctic western boundary current with cold low-salinity water) extends southward along the coast of Hokkaido
and Honshu, showing annual variation in both pattern and extent. The location of these saury ﬁshinggrounds oﬀ Hokkaido is determined by the location of the Oyashio front (the 5°C water temperature
isotherm at a depth of 100 m), and accordingly shifts from year to year, tracking the variable pattern of
the Oyashio front (Yasuda and Kitagawa, 1996). The primary indicator determining the location of thesaury ﬁshing grounds is SST, and it has been reported that the mode of SST at the ﬁshing grounds inthis area is 15°C (Tseng et al., 2011; Watanabe et al., 2006). It can thus be speculated that the spatial
distribution of minke whales oﬀ southeastern Hokkaido in September is associated with the seasonal
extent of the Oyashio current and/or the southbound saury migration.
The ﬁndings of previous studies have provided evidence indicating that the spatial distribution ofminke whales worldwide is associated with a diverse range of biotic and abiotic factors, including
bottom depth, topography, seaﬂoor substrate, oceanographic conditions (e.g., SST); and the distribution of prey species (Chavez-Rosales et al., 2019; Doniol-Valcroze et al., 2007; Hamazaki, 2002;
Hooker et al., 1999; Ingram et al., 2007; Macleod et al., 2004; Naud et al., 2003; Skern-Mauritzen
et al., 2011; Tetley et al., 2008; Waggitt et al., 2020; Zerbini et al., 2016). However, given that theseassociations tend to diﬀer depending on location and season, it has hitherto not been possible to draw
any general conclusions. Consequently, to gain a better understanding of minke whale ecology, thelocation and season-speciﬁc eﬀects of biotic and abiotic factors on the spatial distribution of thesewhales warrant further investigation.
In this study, it was sought to determine the associations among oceanographic conditions, and the
distribution of saury and minke whales oﬀ southeastern Hokkaido in September. Speciﬁcally, the aims
were to investigate (1) whether the distribution of minke whales in this area in September is related to
SST, and (2) whether the spatial distribution of minke whales is related to the presence of saury. An

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earlier version of this paper was submitted to the International Whaling Commission’s Scientiﬁc Committee as Appendix 4 of paper SC/59/O7 in 2006.

Materials and Methods
Sighting surveys of minke whales were conducted oﬀ the coast of southeastern Hokkaido, Japan, in
2002, 2004, 2005, and 2006, as a part of the coastal component of JARPN II (Hakamada et al., 2009).
The longitudinal boundaries of the survey area were set at 143°15′E and 146°00′E, whereas the latitudinal boundaries were set at the 50 m isobath and 41°N (Fig. 1). Surveys were conducted during
the month of September in the aforementioned years (Fig. 2). The survey area between 143°15′E and
146°00′E was stratiﬁed into the following four strata: an Oﬀshore (O) stratum and three coastal strata[coastal-Center (C), coastal-East (E), and coastal-West (W)], with the boundary between the oﬀshore
and coastal strata being set at a distance of 60 n.miles from the Hokkaido coastline. In 2005 and 2006,
an additional stratum (H) was surveyed oﬀ Hidaka sub-prefecture (between 142°30′E and 143°15′E).
Sighting surveys were conducted from onboard the survey vessel (SV) Kyoshin-Maru No. 2 (372
GT; Kyodo Senpaku Co., Ltd). Primary observers were allocated to the top barrel (3 observers) andthe upper bridge (2 observers). Zigzag tracklines with randomly selected starting points were constructed in the survey area. The sighting survey (i.e., on-eﬀort) was conducted during daylight hours
from 1 hour after sunrise to 1 hour before sunset under acceptable weather conditions (i.e., the visibility was 2.0 nautical miles or more, the wind speed was 20 knots or less, and the Beaufort wind force
scale was less than 6), and the nominal steaming speed along the tracklines was 10 knots. Sightings
during the on-eﬀort were treated as primary sightings.
There were two survey modes: Closing Mode and Passing Mode. Closing Mode was applied in
2002 and 2004 while Passing Mode was applied in 2005 and 2006 to increase survey coverages. Whenthe sightings during Closing Mode (primary sightings) were likely to be minke whales, the SV approached them to conﬁrm species and individuals within the schools. Sightings during the approaches
to primary sightings were secondary sightings and were not included in the analysis. During Passing
Mode, no approach was made to sightings. All sightings made in the Passing Mode were treated as

Fig. 1. Surveyed strata oﬀ the coast of southeastern Hokkaido, Japan [coastal-Center (C), coastal-East (E),coastal-West (W), Oﬀshore (O)] and oﬀ Hidaka sub-prefecture (H). Black lines represent the boundaries between strata, and thin gray lines indicate isobaths.
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Fig. 2. Surveyed tracklines (black lines) and positions of common minke whale sightings (pink circles) oﬀ
the coast of southeastern Hokkaido, Japan, in September 2002, 2004, 2005, and 2006. The monthly mean sea
surface temperatures (SSTs) derived from GHRSST are also shown. Dashed black lines represent the 16°C
isotherms, and white lines indicate the boundaries between strata.

primary sightings. However, if the identity of the species sighted was uncertain, abeam closing was
conducted during Passing Mode. Given that the minke whales observed during the survey tended tooccur individually rather than in schools, application of the Passing Mode is deemed adequate.
Density indices of schools and individuals (DI, the number of schools and individuals sighted per
100 n.miles) were calculated for each stratum in each survey year to show relative abundance of minke
whales. Mean values of the SSTs recorded at the time of each sighting were obtained for each stratumin each of the survey years. Estimated abundance of minke whales based on the line transect sampling(Hakamada et al., 2009) and amount of saury consumed by the whales (Tamura et al., 2009) at thetime of surveys were also used in this study.
As an indicator of the presence of saury, numbers of commercial saury ﬁshing boats were summarized by each stratum in each survey year. Mean of SST recorded by the ﬁshing boats was also summarized by each stratum in each survey year. The data of ﬁshing positions of the boats and SST were
provided by the Japan Fisheries Information Service Center (JAFIC).
SST data, A Group for High Resolution Sea Surface Temperature (GHRSST) Level 4 sea surface
temperature analysis (JPL MUR MEaSUREs Project, 2010), were used to characterize oceanographic
conditions at the time of the surveys. Monthly mean September SST data corresponding to the survey periods with a global 0.01 degrees grid spatial resolution were extracted using Marine Geospatial Ecology Tools 0.8a73 (Roberts et al., 2010). Data pertaining to sightings, ﬁshing boat locations,and SST were overlaid on maps using a geographic information system (GIS), ArcGIS Desktop 10.7
(ESRI, US). The area of sea surface classiﬁed by 1°C SST increments was also calculated using the
GIS. ETOPO1 global relief model (Amante and Eakins, 2009) and A Global Self-consistent, Hierarchical, High-resolution Geography Database (GSHHG) (Wessel and Smith, 1996) data were also used10

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DISTRIBUTION OF MINKE WHALE OFF HOKKAIDO

in the maps. The UTM zone 55N projection was used for the mapping.

Results
All strata
The surveyed tracklines and the sighting positions of minke whales were overlaid on the SST maps
(Fig. 2), as were the positions of the saury ﬁshing boats (Fig. 3). These maps indicated an increase in
the overall SST in the survey area during the course of the survey period from 2002 to 2006. Mean
SST was found to have risen from 16.6°C in 2002 to 19.5°C in 2006 (Table 1). The frequency of thenumber of minke whale sightings with respect to SST (1°C increments) is shown in Fig. 4. During thefour survey years, the mean SST at sightings positions was 15.5°C (CV=12.4%, n=87), whereas thatin the years 2002, 2004, 2005, and 2006 was 14.3°C (CV=8.0%, n=38), 15.4°C (CV=4.5%, n=17),15.6°C (CV=8.4%, n=20), 19.1°C (CV=6.0%, n=12), respectively (Table 1). Correspondingly reductions in the DIs, abundance, and saury consumption of minke whales were detected from 2002
to 2006. The frequency of the number of saury ﬁshing boats with respect to SST (1°C increments) isshown in Fig. 5. Mean SST at the ﬁshing positions of saury ﬁshing boats over the four survey years
was 15.1°C (CV=10.2%, n=2,646), whereas that in 2006 was 16.4°C (CV=10.3%, n=450) (Table 1). Notably, there were fewer boats ﬁshing for saury in 2006. The frequency of area (n.mile2) of
SST ≤16°C and >16°C in all survey strata in September 2002, 2004, 2005, and 2006 is summarizedin Table 1. Given that over the four survey years, the mean SST at the sighing positions of minke
whales was 15.5°C, a temperature of 16°C was set as a cutoﬀ point. From 2002 to 2006, a contraction

Fig. 3. The ﬁshing positions of commercial Paciﬁc saury ﬁshing boats (open squares) oﬀ the coast of southeastern Hokkaido, Japan, in September 2002, 2004, 2005, and 2006. The monthly mean sea surface temperatures (SSTs) derived from GHRSST are also shown. Dashed black lines represent the 16°C isotherms, andwhite lines indicate the boundaries between strata.
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Table 1. Summary of the September sightings and Paciﬁc saury consumption of common minke whales, sea
surface temperatures (SST) derived from GHRSST, and commercial Paciﬁc saury ﬁshing boats in the survey
area. The density index (DI) indicates the number of schools/individuals per 100 n.miles. Values for the abundance and Paciﬁc saury consumption of minke whales were obtained from Hakamada et al. (2009) and Tamura et al. (2009). Note that data for Paciﬁc saury consumption are only available for “All strata”.
Common minke whale sightings & saury consumption
Year

Eﬀort
Mean
DI
DI
Sch. Ind.
(n.miles)SST (°C) (Sch.) (Ind.)SST in survey blocks (GHRSST)

Commercial Paciﬁcsaury boats

Abundance

Saury
Mean
Area ≤16°C Area 16°C< Number Mean SST
consumption(%)(%)
of boats(°C)
Ind. CV(mt)°C CV

All strata
2002 681.5
2004 809.4
2005 827.1
2006 994.0

38
17
20
12

40
18
21
12

14.3
15.4
15.6
19.1

5.6
2.1
2.4
1.2

5.9
2.2
2.5
1.2

0.4
0.4
0.4
0.4

861
1,075
39
153

16.6 9.8
17.1 6.6
19.1 4.6
19.5 5.8

46.5
17.0
0.0
0.0

53.5
83.0
100.0
100.0

1,815
256
125
450

14.8
14.9
15.5
16.4

C stratum
2002 272.6
2004 238.7
2005 365.3
2006 247.4

33
12
10
5

34
13
11
5

14.4
15.5
15.4
18.7

12.1
5.0
2.7
2.0

12.5 323 0.2
5.4 175 0.4
3.0 81 0.7
2.0 75 0.4

—
—
—
—

15.1 2.3
16.2 2.6
18.5 2.0
18.6 4.1

100.0
30.2
0.0
0.0

0.0
69.8
100.0
100.0

403
36
47
25

14.6
15.0
15.0
15.7

E stratum
2002 73.5
2004 140.6
2005 127.7
2006 145.0

5
1
9
5

6
1
9
5

13.8
15.1
15.3
18.9

6.8
0.7
7.0
3.4

8.2 117 1.1
0.7 16 1.1
7.0 135 0.3
3.4 65 0.7

—
—
—
—

15.0 3.4
15.4 3.9
17.8 4.3
17.8 2.7

100.0
83.9
0.0
0.0

0.0
16.1
100.0
100.0

431
56
81
324

14.2
15.0
15.5
16.5

W stratum
2002 48.2
2004 164.9
2005 180.3
2006 161.9

0
3
1
1

0
3
1
1

—
14.9
20.7
20.0

0.0
1.8
0.6
0.6

0.0
1.8
0.6
0.6

0
52
15
19

0.0
0.4
1.1
0.9

—
—
—
—

17.1 7.1
17.1 2.2
19.1 1.5
19.7 1.5

28.2
0.0
0.0
0.0

71.8
100.0
100.0
100.0

305
59
0
4

15.2
15.6
—
16.7

O stratum
2002 287.2
2004 265.2
2005 41.7
2006 285.3

0
1
0
0

0
1
0
0

—
17.4
—
—

0.0
0.4
0.0
0.0

0.0
0.4
0.0
0.0

0
27
0
0

0.0
1.0
0.0
0.0

—
—
—
—

16.5 4.7
17.2 3.1
19.3 1.1
20.1 4.4

29.2
3.9
0.0
0.0

70.8
96.1
100.0
100.0

664
105
7
98

15.0
14.5
18.1
16.2

H stratum
2002 —
2004 —
2005 112.1
2006 154.4

—
—
0
1

—
—
0
1

—
—
—
21.3

—
—
0.0
0.6

—
—
0.0
0.6

— —
— —
0 0.0
18 1.0————19.6 3.2
19.1 2.1
20.7 2.1
20.4 2.0

0.0
0.0
0.0
0.0

100.0
100.0
100.0
100.0

17
0
0
0

16.9
—
—
—

601
368
316
241

in the size of the area with an SST ≤16°C was recorded, with none of the areas having an SST of less
16°C in 2005 and 2006.
Coastal-Center (C) stratum
Overall patterns of SST distribution, sightings of minke whales, and the positions of saury ﬁshingboats in the C stratum were similar to those characterizing the entire survey area (Table 1). Most ofsightings during surveys were found in this stratum. Over the four survey years, the mean SST recorded at the positions of minke whale sightings in this stratum was 15.1°C (CV=10.1%, n=60), whereas
in 2006 it was 18.7°C (CV=3.9%, n=5). Comparatively, the mean SST at the ﬁshing positions ofcommercial saury boats during these 4 years was 14.7°C (CV=7.4%, n=511), and that in 2006 was
15.7°C (CV=0.8%, n=25). The location of the 16.0°C isotherm SST front indicated that SST in theentire stratum was lower than 16.0°C in 2002, whereas in 2004, the temperature in only a small portion of the eastern part of the stratum was lower than this value (Fig. 2). Contrastingly, in 2005 and12

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DISTRIBUTION OF MINKE WHALE OFF HOKKAIDO

Fig. 4. Frequency of the number of common minke whale sightings according to sea surface temperature(SST: 1°C increments) in all surveyed strata oﬀ the coast of southeastern Hokkaido, Japan, in September
2002, 2004, 2005, and 2006. Data from all the years are pooled to calculate the frequency. SSTs recorded atsighting positions were used.

Fig. 5. Frequency of the number of commercial Paciﬁc saury boats according to sea surface temperature (SST:1°C increments) in all surveyed strata oﬀ the coast of southeastern Hokkaido, Japan, in September 2002,
2004, 2005, and 2006. Data from all the years are pooled to calculate the frequency. SSTs recorded at the positions of boats were used.

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MURASE, MATSUOKA AND WATANABE

2006, no evidence of a 16.0°C isotherm SST front in this stratum was detected. Corresponding to anincrease in the mean SST of this stratum from 2002 to 2006, a reduction in DI values was detected,
and similarly a reduction in the frequency of SST ≤16°C during this period was recorded (Table 1).
Strata other than the C stratum
Compared with the C stratum, fewer sightings were made in the O and H strata, where mean SSTs
were high regardless of the year of survey (Table 1). In the W stratum, the DIs and abundance werehighest in 2004, coinciding with a period during which an area of relatively low SST extended overthe stratum (Fig. 2), giving rise to the lowest (Table 1) and spatially homogeneous (Fig. 2) SST. Contrastingly, in the E stratum, DIs and abundance were the lowest in 2004. With respect to other survey
years, the W stratum was found to be characterized by homogeneously high SSTs in 2005 and 2006
and a marked 16.0°C isotherm SST front in 2002, whereas in 2005 and 2006, DIs and abundance inthe E stratum were higher than those in the C stratum.

Discussion
The mean SST (15.1°C) recorded at the positions of minke whale sightings in the C stratum lies
within the range of 7 to 17°C that has previously been reported for sightings of this species in thewestern North Paciﬁc (Matsuoka et al., 2000). Notably, relatively few minke whales were sighted in
the C stratum when the SST was higher than 16.0°C, and thus it is conceivable that the distribution ofminke whales is limited to within the SST range reported by Matsuoka et al. (2000). Consistent withﬁndings in the present study, Watanabe et al. (2006) have reported that mode SST at the saury ﬁshingboat positions was 15.0°C. Consequently, it would appear that minke and saury have a similar suitable
range of SSTs. In this regard, it has been reported that the timing and route of the saury migrationfrom northern waters to those oﬀ the southeastern coast of Hokkaido show considerable changes coinciding with annual ﬂuctuation in oceanographic conditions (Fukushima, 1979; Yasuda and Kitagawa,
1996; Yasuda and Watanabe, 1994).
Historical commercial catch data indicate that the migration and spatial distribution of minke
whales in the western North Paciﬁc diﬀer with respect to sex and maturation stage (Hatanaka andMiyashita, 1997; Ohsumi, 1983; Omura and Sakiura, 1956; Wada, 1989). The ﬁndings of JARPN II
in the C stratum have revealed that immature individuals are generally distributed in coastal waters
whereas mature males tend to be found at both coastal and oﬀshore waters (Kishiro et al., 2009). In
this regard, it has been suggested that mature minke whale males showed prey preference for sauryover other species, such as krill and walleye pollock (Kishiro et al., 2009). The number of mature
males in the C stratum during the period of the JARPN II survey declined from 2002 to 2006 (Kishiro
et al., 2009), coinciding with the decline of DIs and abundance reported in this study.
At least two diﬀerent stocks of minke whales have been identiﬁed in the waters around Japan,
namely the J and O stocks (Goto and Pastene, 1997; Kato, 1992; Ohsumi, 1977; Omura and Sakiura,
1956; Pastene et al., 2007; Wada and Numachi, 1991). Of these, O stock is mainly distributed in thewaters of the western North Paciﬁc, whereas J stock is mainly distributed in the Sea of Japan, although it has been reported that a small proportion of this stock mixed with O stock oﬀ the southeastern coast of Hokkaido (Pastene et al., 2016). Although the exact route whereby mature O stock males
migrate is still under investigation, it is evident that during the summer months, at least a proportionof these individuals migrates northward in oﬀshore regions of the western North Paciﬁc (e.g., east of
150°E) (Zenitani et al., 2000). Moreover, it seems probable that during autumn, they migrate southward along the coastal region of Japan, including waters oﬀ the southeastern coast of Hokkaido in autumn. The migration timing and route of mature male minke whales would thus appear to be similar tothat of the saury, and indeed, these whales are known to feed on saury in both oﬀshore regions and the14

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waters oﬀ southeastern coast of Hokkaido (Lindstrøm et al., 1998; Tamura et al., 1998).
Findings in the present study provide evidence in support of the hypothesis that the migration routeof mature male minke whales coincides with a suitable SST (approximately 15°C) and the migrationof their primary prey species, the saury. In the C stratum, these mature males could make a high contribution to the observed changes in DIs and abundance. For example, in response to a suitable SST
and an abundance of saury (as in 2002), many mature males could migrate into the stratum, whereas
in contrast, when SSTs exceed a suitable level, and the availability of saury is low (as in 2006), there
would be relatively few migrants in this stratum.
Collectively, the ﬁndings of this study indicate that the abundance of minke whales in a particulararea is inﬂuenced to varying extents by changes in oceanographic conditions and the availability of
prey. In this regard, abrupt changes in the abundance of minke whales have been reported in other regions of the world. For example, changes in abundance of this species in Icelandic waters have been
linked to changes in oceanographic conditions and the patterns of prey species distribution (Víkingsson et al., 2015). Consequently, to gain a reasonably clear understanding of the factors underlying
such changes, analyses of whale sighting data should simultaneously take into consideration data obtained for a range of environmental variables, thereby enabling us to distinguish between apparent andabsolute changes in abundance.
A stock assessment for a time series between 1980 and 2018, revealed that the overall abundanceof saury was generally high during the period covered by the present study (2002–2006) (4th Meetingof the Technical Working Group on Paciﬁc Saury Stock Assessment, 2019). Furthermore, the ﬁndingsof a population dynamics study appear to indicate no abrupt change in abundance of the O stock ofminke whales during the period between the 1990s and 2010s (Kitakado and Maeda, 2016). On thebasis of these ﬁndings, it can be concluded that observed changes in the abundance of minke whales
in waters oﬀ the southeastern coast of Hokkaido between 2002 and 2006 were apparent rather thanactual changes, attributable to annual variations in oceanographic conditions. These variations similarly inﬂuence the route and timing of saury migrations. It seems probable that the apparent change
in the abundance of minke whales coincided with changes in local oceanographic conditions and theavailability of saury. It was reported that contractions in the size of potential saury ﬁshing grounds
oﬀ southeastern Hokkaido were associated with increases of SST and sea level anomaly, a reduction
in chlorophyll concentration, an increase in the frequency of clockwise eddies, and a reduction in
Oyashio transport (Kuroda and Yokouchi, 2017). It should be noted that the abundance index of sauryin the present study area in August and September was high in 2002, although low between 2004 and2006 in response to the environmental conditions (Kuroda and Yokouchi, 2017).
Nevertheless, given that minke whales also consume other prey items, such as Japanese anchovy,
walleye pollock, and Japanese common squid (Tamura et al., 2009), the interrelationships among minke whales, their prey, and oceanographic conditions in this region might be more complex than hitherto assumed. In this context, minke whales in the surveyed region have in the past fed on Japanesesardine and mackerel when the abundances of these prey were high (Kasamatsu and Tanaka, 1992).
However, it has yet to be ascertained how the minke whale distribution changes in response to thereplacement of pelagic ﬁsh species, along with changes in oceanographic conditions. Accordingly,
long-term monitoring of both the whales and their prey, together with oceanographic conditions, will
be necessary to gain a realistic insight into the factors underlying changes in the distribution of minke
whales in the waters oﬀ southeastern Hokkaido.
Acknowledgements

The authors express their thanks to all the members engaged in the coastal component of JARPN II oﬀ Kushiro. The Japan
Fisheries Information Service Center kindly provided data regarding commercial Paciﬁc saury ﬁsheries boats. This research
was supported by the Fisheries Agency of Japan, the Fisheries Research Agency of Japan, and the Institute of Cetacean Research. We thank these institutions for their support. The authors express their sincere thanks to the anonymous reviewers.

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Received: March 1, 2021
Accepted: July 26, 2022
Published online: October 25, 2022

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