SC/F16/JR11

The number of the western North Pacific common
minke whales (Balaenoptera acutorostrata) distributedin JARPNII coastal survey areas
TAKASHI HAKAMADA1, KOJI MATSUOKA1, KISHIRO TOSHIYA2 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, Fisheries Research Agency (FRA), 2-12-4 Fukuura,
Kanazawa, Yokohama, Kanagawa 236-8648, Japan
Contact E-mail: hakamada@cetacean.jp
ABSTRACT
In order to examine the impact of common minke whales on Japanese fisheries in Kushiro andSanriku regions through estimating the amount of prey consumed by minke whales or using an
ecosystem model, it was required to estimate the number of the common minke whales
distributed in each of the survey areas during the JARPNII survey periods. Because it wasconsidered that the impact of the minke was important in the operation area of the coastal fishery,
the number of the common minke was estimated. The estimated numbers off Kushiro were 461
and 433 in early (May-June) and late seasons (July-September) in 2012, respectively. The
estimated number off Sanriku in the early season was 124 in 2012. Note that these numbers are
not abundance estimates of the minke whale stock in the areas because the sighting data we used
for the estimation covered only a part of the stock distribution.
INTRODUCTION
Elucidation of feeding ecology and ecosystem studies is one of the main objectives of the JARPNII.
Common minke whales (Balaenoptera acutorostrata) consume some of the main Japanese fisheryresources off Kushiro and off Sanriku. The impact of whales on Japanese fisheries in the JARPNII survey
area can be assessed from estimating the amount of prey consumed by the whales in survey areas (Tamura
et al., 2009; 2016: SC/F16/JR17). Hakamada et al. (2009) estimated abundance of common minke whales
in Sanriku and Kushiro coastal survey areas for prey consumption estimates by common minke whales inthese regions.
MATERIALS AND METHOD
In 2012, two dedicated sighting surveys are conducted which covered sub-areas 7CS, 7CN, 7WR and 7E
by the dedicated sighting survey vessel Yushin-Maru No.3 (YS3) (Matsuoka et al., 2013). Because
sufficient searching effort were allocated in sub-areas 7CS and 7CN, which includes Sanriku and Kushiro
coastal areas, sighting data obtained during these sighting surveys can be used to estimate the number ofcommon minke whales distributed in these coastal areas. The first survey was conducted in sub-areas 7CS,
7CN and a northern part of 7WR (North of 41oN, hereafter 7WRN) in May and June (Figure 1). Hereafter,
we refer this period as the “early” season. The second survey was conducted in sub-areas 7CN and the partof sub-area 7WR in September (Figure 1). We refer period from July to September as “late” season. In thisanalysis, the number of the common minke whales distributed in Sankiku in the early season and those inKushiro area in early and late seasons are based on sighting data collected during the dedicated sighting
survey conducted in 2012.
Dedicated sighting surveys were conducted in Kushiro area in 2002, 2003, 2004, 2005, 2006 and 2007 andin Sanriku area in 2003, 2004, 2005 and 2006 (Kiwada et al., 2009; Miyashita, 2009). The generic survey
area in those surveys in Sanriku and Kushiro area are shown in Figure 2a. Sanriku area is in sub-area 7CS.
The coastal part of Kushiro area is in sub-area 7CN and the offshore part of Kushiro area is in sub-area
7WRN (Figure 2b).
Abundance estimation

1

First, common minke whale density/abundances were estimated for each sub-area. Secondly, density
estimates for each sub-area were multiplied by area size of Sanriku/Kushiro area, where the dedicatedsighting surveys were conducted. In the first step, analytical procedures used are similar to Hakamada andMatsuoka (2015) as follows.
For this analysis it is assumed that g(0)=0.798 with CV=0.134 (Okamura et al., 2010). Detections aretruncated at 1.5 n.miles. Abundance and its CV were estimated based on a Horvitz-Thompson like estimatorof abundance expressed by formula (1) and (2), respectively.

P==

A n si∑
2WL i =1 p i ( z i )A n ˆ∑ s i f (0 | z i )2 L i =1
(1)

where P is abundance estimate, A is area size of the surveyed area, W is truncation distance (1.5 n.miles),
L is searching effort, n is the number of schools detected within perpendicular distance of W, si is school
size of ith detection, pi(zi) is the probability that school i is detected given that it is within the perpendiculardistance W and given the covariate zi. f(0|zi) is conditional probability density function of distance 0 given
covariates zi
2
2
K
r
r
 PCk PC 
∂PC ∂PC −1 1 A  
var(P ) = lk +
H jm (θ ) (2)
−
 
∑
∑∑
L 
 2WL   L( K − 1) k =1  l k
j =1 m =1 ∂θ j ∂θ mwhere K is the number of transect, lk is searching distance in kth transect, PCk is abundance estimate incovered region (within 3 n.miles from track line surveyed) in kth transect, PC is abundance estimate in the
covered region, Hjm-1(θ) is the jmth element of inverse of Hessian matrix of detection function for covariateθ.
Multiple Covariate Distance Sampling (MCDS) Engine in DISTANCE program was used (Thomas et al.,2010). Given previous discussions at the IA sub-committee on detection function (IWC, 2015), Half Normaland Hazard Rate models were considered as candidate models for the detection function. Full model of thedetection function was provided by−b  (3)g ( x) = 1 − exp−  x
a exp(Size + Beaufort + Year ) 
 

 2

(4)
g ( x ) = exp − x
22a exp{2( Size + Beaufort + Year )}


where x is perpendicular distance, a and b (b≧1) are parameter, Size is observed school size, Beaufort iscategorical variable for Beaufort sea state (good: 0-3, bad: 4-5) and Year is categorical variable for year. To
estimate detection function, all primary sightings occurred during 2008-2014 dedicated sighting surveys
were used.
AIC was used to select the best model to estimate detection probability of 1/Wf(0|zi).
Smearing was not conducted on running MCDS because MCDS doesn’t deal with smearing. Perpendiculardistance was not binned on fitting detection function because selection of cut point could affect results ofmodel selection and coefficient estimates of detection function different from previous analysis.
In the second step, multiply density estimate in each sub-area by area size of Sanriku/Kushiro survey area
shown in Figure 2a.

2

RESULTS
Table 1 shows abundance estimate for each sub-area. The estimated number in 7CN and 7WRN combined
are similar to each other in early and late seasons. Figure 3 shows plot of selected detection function by
AIC and its QQ-plot. The fit seems good. Using density estimation in Table 1, the estimated numbers off
Kushiro were 461 and 433 in early (May-June) and late season (July-September) in 2012, respectively. The
estimated number off Sanriku in the early season was 124 in 2012 (Table 2).
DISCUSSION
It is important to note that the numbers of the minke whales estimated in this paper do not represent the
number of the whale in a whole stock because the sighting data we used covered only a limited area of thestock distribution. For this reason, the estimated number of the common minke whales in Sanriku andKushiro area themselves should not be used for assessment of this whale stocks.
In this study, it was assumed that density of the common minke in Sanriku area is the same as that in 7CS
and that the density in coastal part of Kushiro area is same as that in 7CN and that in offshore part ofKushiro is same as that in 7WRN. If there is a tendency that the common minke density increases as thecoast is approached, these assumptions on density in Sanriku and Kushiro area would cause anunderestimate of the number of whales.
Table 3 shows abundance estimate in Hakamada et al. (2009) with assuming g(0) estimate of 0.798 with
CV=0.134 in Okamura et al. (2010). Comparing previous estimates,, there were no significant differencesfrom the estimates in 2012 and the abundance estimates fluctuate among years. One of possible reason forthis fluctuation could be fluctuation in environmental variables such as sea surface temperature. Murase etal. (2007) showed that high surface temperature caused low influx of common minke whales to the Kushiro
water in September 2006. Mean water temperature at 10m depth is low in 2002-2004 and high in 20062007 (Okazaki et al., 2016; SC/F16/J06)). Comparing abundance estimates in Kushiro (Table 3), it wassuggested water temperature is negatively correlated to the number of the minke whale off Kushiro. Further
sighting surveys is necessary to investigate relation between sea water temperature and the numbers of thecommon minke whales in the coastal areas.
In previous analysis, the number in sub-area 7CS was estimated 890 animals (CV=0.393) while abundance
estimates in sub-area 7CN in early (May to June) and late season (September) were 302 animals(CV=0.454) and 398 animals (CV=0.507), respectively, assuming g(0)=1 based on the dedicated sighting
surveys in 2012 (Hakamada et al., 2013). This difference in abundance estimate is probably due todifference in estimation of detection function. The difference in estimation of detection function is a resultof the data set and covariates used. In the previous analysis, primary sightings of minke whales in 2012 wasused whereas primary sightings for 2008-2014 were used in this analysis. As for covariates, sub-area andBeaufort sea state were used in the previous analysis whereas Beaufort sea state, school size and year wereused in present analysis. Reconsideration of stratification of data and covariates to estimate detection
function may improve precision of detectability of the sightings.
ACKNOWLEDGEMENT
Authors thank all researchers and crews involved to the JARPNII dedicated sighting surveys. Doug
Butterworth is thanked for his comments to improve the manuscript of an earlier version of this paper.
Hajime Nakamichi is thanked for his help in data preparation, running DISTANCE program and preparing
tables and figures.
REFERENCES
Hakamada、T., Matsuoka, K. NIshiwaki, S. and Miyashita, T. 2009. The number of western North Pacificcommon minke whales (Balaenoptera acutorostrata) distributed in JARPNII coastal survey areas.
Paper SC/J09/JR8 presented to workshop to review JARPNII survey, January 2009. (unpublished).
12pp.
Hakamada, T., Matsuoka, K. and Miyashita, T. 2013. Abundance estimate of western North Pacific minke
whales using JARPNII dedicated sighting survey data obtained in 2012. Paper SC/M13/NPM3rev

3

presented to IWC Scientific Committee Second Interssessional Workshop on the Implementation
Review for western North Pacific minke whales, 19-23 March 2013, La Jolla, USA (unpublished).
7pp.
Hakamada, T. and Matsuoka, K. 2015. Abundance estimate for sei whales in the North Pacific based onsighting data obtained during IWC-POWER surveys in 2010-2012. Paper SC/66a/IA12 presented to
IWC Scientific Committee, May 2015. (unpublished). 11pp.
International Whaling Commission, 2015. Report of IA sub-committee. Annex G of 66a IWC ScentificCommttee Report.
Kiwada, H., Kumagai, S. and Matsuoka, K. 2009. Methodology and procedure of the dedicated sighting
surveys in JARPNII - Offshore and coastal component of Sanriku and Kushiro -. Paper SC/J09/JR2
presented to workshop to review JARPNII survey, January 2009. (unpublished). 16pp.
Matsuoka, K., Yoshimura, I. and Miyashita, T. 2013. Cruise report of the Japanese cetacean sighting survey
in the western North Pacific in 2012. Paper SC/65a/O4 presented to IWC Scientific Committee, June
2013. (unpublished). 8pp.
Miyashita, T. 2009. Abundance estimate of common minke whales in the Sendai Bay, estimated from 2004
sighting survey. Appendix of SC/J09/JR8 presented to workshop to review JARPNII survey, January
2009. (unpublished). 3pp.
Murase, H., Kiwada, H., Matsuoka, K. and Nishiwaki, S. 2007. High sea surface temprature caused low
influx of common minke whales to the Kushiro water in September 2006. Appendix 4 of SC/59/O7
presented to IWC Scientific Committee, June 2007. (unpublished). 14pp.
Okamura, H., Miyashita, T. and Kitakado, T. 2010. g(0) estimates for western North Pacific common minke
whales. Paper SC/62/NPM9 presented to IWC Scientific Committee meeting, 2010 June,(unpublished). 7pp.
Okazaki, M., Masujima, M., Murase, H. and Morinaga, K. 2016. Oceanografic conditions in the survey
area of JARPNII coastal component off Kushiro in September from 2000 to 2013 using FRA-ROMS
data.Paper SC/F16/JR5 presented to the JARPNII special permit expert panel review workshop,
Tokyo, February 2016 (unpublished) .**pp.
Tamura et al., 2009. Prey consumption and feeding habits of common minke whales in coastal areas off
Sanriku and Kushiro. Paper SC/J09/JR9 presented to workshop to review JARPNII survey, January
2009. (unpublished). 12pp.
Tamura, T., Kishiro, T., Yoshida, H., Konishi, K., Yasunaga, G., Bando, T., Saeki, M., Onodera, K.,
Mitsuhashi, M., Yamashita, Y., Funamoto, T. and Kato, H. Updated prey consumption by common
minke whales and interaction with fisheries in coastal areas. Paper SC/F16/JR17 presented to the
JARPNII special permit expert panel review workshop, Tokyo, February 2016 (unpublished) .**pp.
Thomas, L., Buckland, S.T., Rexstad, E.A., Laake. J.L., Strindberg, S, Hedle, S. L., Bishop, J. R.B., T. A.
Marques, T.A., and K. P. Burnham. 2010. Distance software: design and analysis of distance sampling
surveys for estimating population size. Journal of Applied Ecology 47: 5-14.

4

Table 1. Abundance estimate in sub-areas 7CS, 7CN and 7WRN based on JARPNII dedicated sighting
surveys in 2012 assuming g(0)=1. A is area size of the surveyed area, ns and nw are the number of schools
detected and the number of individuals detected within perpendicular distance of 1.5 n.miles, L issearching distance, P is abundance estimate and CI is abbreviation for confidence interval.
Year

Stratum

PeriodSeasonns

nw

n w /L

CV(n w /L )

P

CV(P)

95%LL

2012

7CS

18 - 24 May, 24 - 29 Jun

early

26,826 850.9

A

L

17

19

0.022

0.287

537

0.346

269

95%UL
1,070

2012

7CN

24May - 2 Jun

early

16,171 649.2

17

23

0.035

0.501

542

0.601

164

1,790

2012

7WRN

2 - 3 Jun

early

6,874

175.7

2

2

0.011

0.913

64

0.935

3

1,471

2012

7CN

15 - 25 Sep

late

16,171 550.2

19

23

0.042

0.472

599

0.525

205

1,757

2012

7WRN

26 - 27 Sep

late

6,874

-

-

-

-

-

-

-

-

177.4

Table 2. Abundance estimate in Sanriku and Kushiro using density estimates in Table 1 and assumingg(0)=0.789 in Okamura et al. (2010). Early refers to May-June and late refers to July – September.
Year

Area

Season

P

CV(P)

95%LL

95%UL

2012

Sanriku

early

124

0.371

61

251

2012

Kushiro

early

461

0.593

157

1,352

2012

Kushiro

late

433

0.542

160

1,172

Table 3. The number of the minke whales distributed off Sanriku in early season and those off Kushiro inlate season during the survey period assuming g(0)=0.789 in Okamura et al. (2010) for comparison.
Estimate is slightly changed from previous analysis (Hakamada et al., 2009) due to update of g(0) estimate
assumed. Early refers to May-June and late refers to July – September.
Sanriku
Year
2004
2005
2006

Area

SeasonP

CV(P)Sanriku

early

260

0.557

95%LL

95%UL

94

719

Sanriku

early

401

0.321

217

741

Sanriku

early

216

0.407

101

466

Area

Season

P

CV(P)

95%LL

95%UL

0.350

KushiroYear
2002

Kushiro

late

551

283

1,073

2003

Kushiro

late

888

0.406

413

1,908

0.352

2004

Kushiro

late

338

173

660

2005

Kushiro

late

290

0.350

149

564

2006

Kushiro

late

221

0.351

113

431

2007

Kushiro

late

130

0.553

47

358

5

7CN

2012
early

7CN

2012
late

7WR

7WR

7CS

Figure 1. Plot of primary sightings of the minke whales and track lines actually surveyed in sub-areas 7CS,
7CN and a northern part of sub-area 7WR. Left panel is the plot for the first survey (May-June) in 2012
and right panel is plot for the second survey (September) in 2012.

Sanriku

KushiroFigure 2a. Survey area off Sanriku (left panel) and off Kushiro (right panel) used to estimate abundance inTable 2. O: Offshore, E: coastal-East, C: coastal-Central, W: coastal-West, and H: off Hidaka sub-prefecture.
Offshore area in Kushiro is indicated by O. Rest of the Kushiro areas (Ares H, W, C and E) are coastal area.
Figures are referred from Hakamada et al. (2009)6

Figure 2b. Dedicated sighting survey area off Sanriku and off Kushiro conducted during 2002-2007 in Subareas 7CS, 7CN and northern part of 7WR. Yellow zone is survey area off Sanriku and Purple zone is survey
area off Kushiro.

Figure 3. Plot of the estimated detection function fitted to the number of schools as a function of
perpendicular distance (n. miles) from the track line for selected model (Left panel) and QQ-plot of thedetection function (right panel).

7