SC/62/NPM2

Revised abundance estimate of western North Pacific common
minke whales based on JARPN II sighting data.
Takashi Hakamada1, Koji Matsuoka1 and Tomio Miyashita2
1

The Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-Ku, Tokyo, 104-0055, Japan.

2

National Research Institute of Far Seas Fisheries (Yokohama), 2-12-4, Fukuura, Kanagawa,Yokohama, 236-8648, Japan.
Contact e-mail: hakamada@cetacenan.jp
ABSTRACT
In order to access the status of common minke whale (Balaenoptera acutorostrata) stocks in thewestern North Pacific, it was required to estimate the number of these whales in the JARPN II
survey area as a part of the whale stocks. Considering the migration pattern of these whales in thearea revealed by our sighting survey data, abundance was estimated separately for the early and lateseasons. The estimates for the common minke whales were 6,395 (CV=0.717) in the early and 2,872(CV=0.458) in the late seasons assuming that g(0)=0.798 (SE=0.134) (Okamura et al., 2010). It is
important to note that these estimates should not be used for assessment because the estimatedfigures represent only a part of the whole population considered.
INTRODUCTION
JARPN II survey area is Sub-areas 7, 8 and 9 excluding foreign EEZ Because the common minke
whales migrate to north for feeding in spring and summer, the number of whales distributed in
JARPN II survey area could be differ among the month. It is considered that more common minke
whales pass through these areas in spring than in summer. But previous abundance estimates in thesearea was not taken migration into account. In this paper, migration/staying of the common minke
whales was investigated and estimate abundance taking migration into account.
MATERIALS AND METHODS
Dedicated sighting survey was conducted in JARPN II survey from 2006 and 2007. Timing ofsurvey and surveying order of strata was changed substantially year by year in order to know how
the number of distributed whales changed and how whales moved seasonally (for more details, see
Kiwada et al., 2009). During 2006-2007 the whole survey area was planned to be covered at once. In2006, the survey was conducted from south to north. In 2007, the survey was conducted from northto south. More details of the sighting survey procedure are explained in Kiwada et al., (2009).
Stratification of the JARPN II survey area is shown in Fig. 1. Sub-areas 8 and 9 are stratified at 41oN
latitudinal line. Distributions of trackline surveyed and primary sightings of the common minke
whales are shown in Fig. 2.

1

Based on sighting data obtained during these surveys, the number of the whales distributed in theJARPN II survey area and their CVs was estimated from the following formulae;

N=AnE ( s )2 wl(1)2⎧ ⎛ n ⎞⎫22CV ( N ) = ⎨CV ⎜ ⎟⎬ + {CV (w)} + {CV (E ( s ) )}⎩ ⎝ l ⎠⎭(2),

where N was the number of the whales distributed, A was area size of the surveyed area, E(s) wasestimated mean school size, w was effective strip half width and l was searching distance. In casethat data were pooled over the two surveys to estimate w and E(s), CV of the estimated N wasestimated from⎛ E ( s ) n Ai ni ⎞⎞⎛ nCV ⎜ ∑ N i ⎟ = CV ⎜⎜∑i l ⎟⎟2w⎠⎝ ii⎠⎝2⎧ ⎛ n A n ⎞⎫22= ⎨CV ⎜⎜ ∑ i i ⎟⎟⎬ + {CV (w)} + {CV (E (s ))}⎩ ⎝ i l i ⎠⎭(3),⎛ ni ⎞n∑ A var⎜⎜ l ⎟⎟i=i⎝ i ⎠ + {CV (w)}2 + {CV (E (s ))}2Ai ni∑i linwhere i was an index of strata. n was the number of strata whose data were pooled. In thiscalculation, it was assumed that⎛ n nj ⎞cov⎜ i , ⎟ = 0⎜l l ⎟⎝ i j ⎠(i ≠ j)

(4).

The whale numbers were estimated using the program DISTANCE ver. 5.0 (Thomas et al., 2006)
g(0) was assumed to be 0.798 with SE=0.134 (Okamura et al, 2010) for Top barrel and Upper bridge
for the minke whales.
RESULTS
Distribution and movement of the common minke whales
As discussed later, the common minke whales mainly distribute in southern part of the survey area
(south of 41oN) in early season (May – June) and migrate to north of 41oN in late season (July –September). In 2006, the southern part of the area was surveyed in early season and the northern partof the area was surveyed in later season. Therefore, our survey vessels likely followed migrating
common minke whales. This survey process should have caused the highest estimate observed forthe common minke in 2006. In 2007, northern part of the area was surveyed when the whales had
not migrated there yet and southern part was surveyed in late season after the whales migrated out to2

north.
Fig. 3 shows monthly searching effort by latitude. Because there was not enough searchingeffort in April and October, we used data from May to September. Fig 4 shows Density Index (DI:the number of the sightings per 100 n.minles) by latitude during the 1994-2007 JARPN and JARPN
II (Matsuoka et al., 2009). Fig 4 suggested that the minke whales were distributed south of 41oN in
May and June and north of 41oN in July and August.
Estimation of the numbers of whales distributed
Table 1 shows the number of minke whales distributed in survey areas in 2006 and 2007,
respectively. Because sighting surveys were conducted from May to August, these figures could beaffected by the migration pattern suggested by Fig. 4. Considering above changes in distribution andthe migration of the minke whales, data set were selected so as to include sighting data in the peakmigration area in the early (May and June) and late (from July and August) seasons, respectively, but
not to combine the both data set to avoid double counting due to migration. The area of low
coverage was not selected. The selected data set were obtained and listed in Table 2. The estimatednumber of whales distributed in JARPN II survey area in the early and late season can be obtainedusing these data. Table 3 shows the estimated number of whales distributed in the JARPN II survey
area in the early and late season. Coverage of the survey area for each of the sub-areas 7, 8 and 9
was 85.3%, 100% and 100%, respectively. The estimated numbers divided by these coverage. Theestimates for the common minke whales were 6,395 (CV=0.717) in the early and 2,872 (CV=0.458)
in the late seasons. Fig. 5 shows distributions of primary sightings used for the estimation of thecommon minke whales in the early and late seasons, respectively. The main distribution area of thecommon minke whales moves to north of JARPN II survey area. This figure likely well reflects thedistribution pattern suggested by Fig. 4. Fig. 6 shows the detection probability functions for thecommon minke whales. The detection probability functions seemed to fit the relative frequency ofthe distribution of perpendicular distance of the detections.
DISCUSSIONSThe estimated numbers of the common minke whales distributed in the survey area in the late season
was less than that in the early season. This can be interpreted that the minke whales migrate tofurther north of the JARPN II survey area. In fact, the common minke whales mainly distribute in
the Sea of Okhotsk and in the waters east of the Kamchatka Peninsula and the Kuril Islands inAugust and September (Buckland et al, 1992; Miyashita, 2010).
As discussed above paragraph, the distributed whales in the JARPN II survey area were a partof the whole stock. In order to obtain abundance estimates appropriate for assessment of whalestocks, we also need the information of the common minke abundance outside of the JARPN II
survey area.
ACKNOWLEGEMENT
Authors would like to express gratitude for all the researchers and crew involved the JARPN II3

offshore component. We also thank Saeko Kumagai, Hiroshi Kiwada and Tomoko Hasegawa for
their assistance in this study. We also thank Hiroshi Hatanaka (The Institute of Cetacean Research)
for their valuable comments and suggestions on this study.
REFERENCE
Buckland, S. T., Cattanach, K. L. and Miyashita, T. 1992. Minke whale abundance in the northwest
Pacific and the Okhotsk Sea estimated from 1989 and 1990 sighting surveys. Rep. Int. Whal.
Commn. 42: 387-92.
Kiwada, H., Kumagai, S. and Matsuoka, K. 2009. Methodology and procedure of the dedicatedsighting surveys in JARPN II –Offshore and coastal component of Sanriku and Kushiro-.
Paper SC/J09JR2 presented to the JARPN II Review Workshop, Tokyo, January 2009
(unpublished).16pp.
Matsuoka, K., Kiwada, H., Fujise, Y. and Miyashita, T. 2009. Distribution of blue (Balaenopteramusculus), fin (B. physalus), humpback (Megaptera novaeangliae) and north pacific right(Eubalaena japonica) whales in the western North Pacific based on JARPN and JARPN II
sighting surveys (1994 to 2007) . Paper SC/J09/JR35 presented to the JARPN II Review
Workshop, Tokyo, January 2009 (unpublished). 12pp.
Miyashita, T. 2010. Abundance of common minke whales in the Russian EEZ in the Okhotsk Sea
and east of the Kurile archipelago- the Kamchatka, estimated from 2003 and 2005 sighting
survey Paper SC/62/NPM6 presented to this meeting (unpublished).
Okamura, H., Miyashita, T. and Kitakado, T. 2010. Revised estimate of g(0) for the North Pacificminke whale. Paper SC/62/NMP9 presented to this meeting (unpublished). [Available from
IWC Secretariat]. 7pp.
Thomas, L., Laake, J.L., Strindberg, S., Marques, F.F.C., Buckland, S.T., Borchers, D.L., Anderson,D.R., Burnham, K.P., Hedley, S.L., Pollard, J.H., Bishop, J.R.B. and Marques, T.A. 2006.
Distance 5.0. Release 2. Research Unit for Wildlife Population Assessment, University of St.
Andrews, UK. [Available: from http://www.ruwpa.st-and.ac.uk/distance/ ]4

Table 1. The estimated number of the common minke whales during JARPN II surveys in 2006 and2007 assuming g(0)=1.
2006
straum
SA7
SA8S
SA8N
SA9S
SA9N

latitude

AreanLn/L

CVESW

CVE(s)CV

DPCV

35-41N

period5/17early
5/19,35-38N, 5/19early
38-41N 5/25,
41N+ 7/11-7/17 late
35-38N, 5/27-6/5,
early
38-41N 6/7-6/18
41N+ 7/17-8/26 late173,679

23.2

1135.800.0205

0.821

0.605

0.266

1.16

0.046

0.020

3,416

0.864

119,2512.0

771.580.0026

0.642

0.605

0.266

1.16

0.046

0.002

297

0.697
0.657

43,4131.0

267.460.0037

0.599

0.605

0.266

1.16

0.046

0.004

156

221,4667.7

1497.100.0052

0.367

0.605

0.266

1.16

0.046

0.005

1,101

0.456

277,7118.0

1741.30

0.0046

0.511

0.605

0.266

1.16

0.046

0.004

1,227

0.578

2007
straum
SA7
SA8S
SA8N
SA9S
SA9N

latitudeperiod
38-41N, 6/22-6/27,
late
35-41N 7/25-7/30
38-41N, 6/27-7/2,late35-38N 7/20-7/24
41N+
6/10-6/15 early
38-41N, 7/2-7/10,
late
35-38N 7/10-7/20
41N+
5/16-6/10 early

Arean

L

n/L

CV

ESW

CV

E(s)

CVD

P

CV

173,6793.0

795.46

0.004

0.489

0.605

0.266

1.16

0.046

0.004

630

0.559

119,2510.0

625.84

0.000

0.000

0.605

0.266

1.16

0.046

0.000

00.27043,4132.0

288.05

0.007

0.657

0.605

0.266

1.16

0.046

0.007

290

0.710221,466

1.0

761.21

0.001

0.963

0.605

0.266

1.16

0.046

0.001

280

1.000

277,7110.0

1305.90

0.000

0.000

0.605

0.266

1.16

0.046

0.000

00.270

Table 2. Sighting data used for the estimated number of whales distributed in early and late season.
periodearly（May-June）
late（July-August）

SA 7
2006 *12007 *2

SA 8South of 41oN in 2006 andNorth of 41oN in 2007
North of 41oN in 2006 andSouth of 41oN in 2007

SA 9
South of 41oN in 2006 and Northof 41oN in 2007
North of 41oN in 2006 and Southof 41oN in 2007

*1：Survey period in SA7 included several days in early July in 2006.
*2：Survey period in SA7 included several days in late June in 2007.

Table 3.The estimated number of the whales distributed in JARPN II survey area in early and lateseason. g(0) is assumed to be 0.798 with its SE=0.134 (Okamura et al,2010). The figure in SA7 wasdivided by coverage for sub-area 7 (85.4%).

periodearlylateSA7PCV
4,280 0.880
789 0.584

SA8N
PCV
363 0.730
196 0.678

SA8S
SA9N
SA9SPCVPCVPCV
372 0.717
0 0.000 1,379 0.485
0 0.000 1,537 0.602 350 1.014

5total
PCV
6,395 0.717
2,872 0.458

Fig. 1. JARPN II survey area and its stratification.

Fig. 2. Distribution of the effort and primary sightings of western North Pacific common minke
whales by dedicated sighting vessel during JARPN II 2006 and 2007.

6

10000ALL AREAEFFORT

9000

AprEffort [n.m.]

8000
7000

May

6000

Jun

5000

Jul
4000

Aug

3000
2000

Sep

1000

Oct035

36

37

38

3940

41

42

43

4445

46

47

48

49

50

51

Latitude [deg N]

DI (sighting per Effort 100n.m.)

Fig. 3. Effort by latitude from April to October during 1994-2007.
2.5

ALL AREA
Minke

2May

1.5

Jun1Jul
0.5

Aug035363738394041424344454647

48495051

Latitude [deg N]

Fig.4. DI (whale/100n.mile) by latitude from May to September during 1994-2007 for the commonminke whales.
earlylate

Fig.5. Distribution of primary sightings of the common minke whales in early and late seasons. Leftpanels are figures in early season and right ones are in late season.

7

Detection probability

1n =4800
1.5Perpendicular distance (n.miles)
Fig. 6. Plot of detection probability functions for the common minke whales. n is the number of the
primary sightings during JARPN II 2006 and 2007 to estimate the detection probability functions.

8