SC/63/IA13

Preliminary estimation of North Pacific sei whale
abundance derived from 2010 IWC/Japan Joint Cetacean
Sighting Survey data
Takashi Hakamada1, Hiroshi Kiwada1, Koji Matsuoka1 and Toshihide Kitakado2
1: The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055
2: Tokyo University of Marine Science and Technology, 5-7, Konan 4, Minato-ku, Tokyo 108-8477, Japan
Contact e-mail:hakamada@cetacean.jp
ABSTRACT
Abundance of North Pacific sei whale (Balaenoptera borealis) was preliminary estimated using 2010 IWC/Japan
Joint Cetacean Sighting Survey Cruise. Sensitivity analyses were also conducted. Abundance estimate in thecentral North Pacific (north of 40oN, south of Aleutian Islands, between 170oE and 170oW) was 9,286(CV=0.35) for reference case. Abundance estimate ranges between 8,528 and 9,188 in sensitivity analyses.
Abundance estimates based on IWC/Japan Joint Cetacean Sighting Survey Cruise could be used for in-depth
assessment of this species.
INTRODUCTION
In-depth assessment for North Pacific sei whale (Balaenoptera borealis) is planned to start at this meeting (IWC,2011). In 2010, the plan of the cruise was agreed at the SC62 and IWC/Japan Joint Cetacean Sighting Cruise wasconducted in North Pacific (Matsuoka et al., 2011). The plan of the survey had been drawn up following
guidelines agreed at the North Pacific programme intersessional meeting (IWC, 2011). This survey provided
valuable data to estimate North Pacific sei whale abundance. This paper preliminary estimates abundance of thesei whale derived from this survey using the standard methodology (Branch and Butterworth, 2001a).
MATERIALS AND METHODS
Survey area and period
Sighting survey was conducted by dedicated sighting vessel Kaiko-Maru (KK1) in the period of 8 July – 25August. Surveyed area was in the central North Pacific (north of 40oN, south of Aleutian Islands, between 170oEand 170oW) and the survey area was divided into two strata by latitudinal line of 47oN (Figures 1a and 1b). Areasizes of northern and southern strata were 238,637n.miles2 and 365,244n.miles2, respectively. They werecalculated by Arc GIS (ver 9.31).
Searching effort
The searching effort was 490.46n.miles and 1325.74n.miles in northern and southern strata, respectively (Table1). Plot for searching effort and the primary sightings of the sei whales are shown in Figure 2. Survey coverage
is higher in southern stratum than in northern stratum.
Sighting records
Primary sightings from top (30 schools/56 individuals) and those from upper bridge (15 schools/33 schools)
were used for this analysis. Because it is assumed that g(0)=1, those from IO platform was not used. Observed
distance and angles were corrected using the Angle and Distance Experiment data (Branch and Butterworth
2001a).
Survey modeClosing mode (36 schools/77 individuals) and passing mode (9 schools/12 individuals) were conducted in thesurvey. Sighting and effort data in both survey modes were pooled for abundance estimation because of limited
sample size as in the case of IDCR/SOWER based abundance estimation for large baleen whales (Branch andButterworth, 2001b; Branch 2008).
Abundance estimation
It was assumed that g(0)=1 to estimate abundance. The observed data of radial distance and angle are smearedusing the method II of Buckland and Anganuzzi (1988). Detections are truncated at 3.0 n.miles. Abundance and1

its CV were estimated by formula (1) and (2), respectively. DISTANCE ver 6.0 (Thomas et al., 2010) was used
for abundance estimation.

N=AnE ( s )2wL(1)

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

where N is abundance estimate, A is area size of the surveyed area, n is the smeared number of schools detected
within perpendicular distance of 3.0 n.miles, E(s) is estimated mean school size, w is effective strip half width(ESW) and l is searching distance.
On estimation of CV for total abundance, we took into account that estimated ESW and mean school size fornorthern and southern strata are in common assuming that encounter rate in northern and southern strata aren’tcorrelated. The CV was estimated by⎧⎛ n ⎞⎫⎪ ∑ Ai2 var⎜⎜ i ⎟⎟ ⎪⎪ i
⎝ li ⎠ ⎪ + {CV (w)}2 + {CV (E ( s ) )}2CV ( N ) = ⎨2 ⎬⎛⎞n⎪⎪2i⎪ ∑ Ai ⎜⎜ l ⎟⎟ ⎪⎝ i ⎠ ⎭⎩ i(3)where i is index for strata.
The smeared and truncated sightings data for schools were grouped into intervals of 0.3 n. miles to estimate thedetection function. We consider the model Half-normal and Hazard rate models as a candidate model fordetection function. Model was selected by AIC. We combined data in northern and southern strata to estimate
ESW and mean school size because there were not sufficient detections in the northern stratum. Mean school
size was estimated from the primary sightings whose school size was confirmed only in closing mode.
Regression method in Buckland et al. (1993) was conducted to estimate mean school size.
Some sensitivity analyses were conducted as following.
1. Mean school size could differ between northern and southern strata.
2. Northern stratum was ignored because survey coverage in northern stratum is not high.
3. Width of perpendicular distance intervals was changed from 0.3 to 0.6.
Because school size is one for all the sightings in northern stratum, pooling data in northern and southern strata
to estimate mean school size could cause over estimate of abundance. This is reason for conducting sensitivity 1.
Sensitivity 3 was conducted to improve fit of the detection function.
RESULTS AND DISCUSSIONS
Sei whales were mainly distributed in southern stratum with some concentration areas but they also weredistributed in northern stratum, too. Effect of school size on detection was not significant at 15% level (i.e.
p-value was 0.30) therefore observed mean school size was used to estimate abundance (Table 2). Table 1 shows
estimate of encounter rate, ESW, mean school size and their CVs. Abundance estimate in northern and southern
strata were 1,394 (CV=0.53) and 7,893 (CV=0.34), respectively and total abundance estimate was 9,286(CV=0.35) for reference case. Coverage is low in northern stratum due to bad weather condition. This could
cause worse precision of abundance estimate in northern stratum. For sensitivity 1, abundance estimate innorthern and southern strata were 707 (CV=0.52) and 8,128 (CV=0.38), respectively and total abundance
estimate was 8,835 (CV=0.35). For sensitivity 2, abundance estimate was 8,528 (CV=0.35) in southern stratum.
Abundance estimate would not change substantially among the sensitivities.
AIC for Half Normal model is smaller than that for Hazard rate model (Table 3) and therefore Half-Normal
model was selected by AIC. Plot of the detection function was shown in Figure 3. The fit of the model seems
good except interval of [0.3, 0.6]. For sensitivity, width of the interval for perpendicular distance was changed
from 0.3 to 0.6. Abundance estimate were 1,380 (CV=0.53) and 7,808 (CV=0.38) in northern and southern strata,2respectively. Total abundance estimate was 9,188 (CV=0.38). This sensitivity improved in fit of the detection
function (Figure 4). Half Normal model was selected by AIC for the sensitivities, too. Detection function
considering covariates such as Beaufort state and so on could be examined for further investigation by increasing
the data collected in 2011 Cruise.
Kanda et al. (2009; 2011) indicated that the open water of the North Pacific was mainly occupied by theindividuals from a single stock of the sei whales. The abundance estimate in western North Pacific (north of35oN and west of 170oE, excluding foreign EEZ) was 5,406 (CV=0.300) in July-early September (Hakamada etal., 2009). Abundance estimate for North Pacific stock of sei whales could be estimated at least 14,000,
assuming that there is a single stock of the sei whale in the North Pacific. The IWC/Japan joint cetaceansighting surveys will be conducted further eastern area in 2011, and better abundance estimate could beobtained in near future. These data and estimates could be used in the in-depth assessment of this species.
ACKNOWLEDGEMENT
We thank Captain, crews and international researchers of the Kaiko-Maru for their effort. We also thank Drs.
Cherry Allison and Greg Donovan for their advice on permission of using the sighting data in this analysis. We
also thank Steering Group of the Cruise (Hidehiro Kato (Convener), Yong-Rock An, Robert Brownell, Phillip
Clapham, Paul Ensor, Tomio Miyashita, Hiroto Murase, Luis Pastene and Lars Walloe) for their assistance onplanning and execution of the cruise.
REFERENCES
Branch, T. A. and Butterworth, D. S. 2001a. Southern Hemisphere minke whales: standardised abundance
estimates from the 1978/79 to 1997/98 IDCR-SOWER surveys. J. Cetacean Res. Manage. 3:143-174.
Branch, T. A. and Butterworth, D. S. 2001b. Estmates of abundance south of 60oS for cetacean species sighted
frequently on the 1978/79 to 1997/98 IDCR-SOWER surveys. J. Cetacean Res. Manage. 3:251-270.
Branch, T. A. 2008. Humpback abundance south of 60°S from three complete circumpolar sets of surveys. J.
Cetacean Res. Manage. special issue (in press)
Buckland S. T. and Anganuzzi A. A. 1988. Comparison of smearing methods in the analysis of minke sightings
data from IWC/IDCR Antarctic cruises. Rep. Int. Whal. Commn 38: 257-63.
Buckland, S.T., Anderson, D.R., Burnham, K.P. and Laake, J.L. 1993. Distance Sampling: Estimating
Abundance of Biological Populations. Chapman and Hall, reprinted 1999 by RUWPA, University of St
Andrews. 446pp.
Hakamada, T., Matsuoka, K. and Miyashita, T. 2009. Distribution and the number of western North Pacificcommon minke, Bryde’s, sei and sperm whales distributed in JARPN II Offshore component survey area.
Paper SC/J09/JR15 presented to the Expert Workshop to review results of JARPN II. (unpublished) 18pp.
International Whaling Commission. 2010. Report of the intersessional meeting on the North Pacific Sighting
Survey Programme. Paper SC/62/Rep3 presented to IWC Scientific Committee meeting, May, 2010
(unpublished) 9pp.
International Whaling Commission. 2011. Report of the Scientific Committee. J. Cetacean Res. Manage.
(suppl.). xx-xx.
Kanda, N., Goto, M., Yoshida, H. and Pastene, L. A. 2009. Stock structure of sei whales in the North Pacific as
revealed by microsatellite and mitochondrial DNA analyses. Paper SC/J09/JR32 presented to the Expert
Workshop to review results of JARPN II. (unpublished) 14pp.
Kanda, N., Goto, M., Matsuoka, K., Yoshida, H. and Pastene, L. A. 2011. Preliminary microsatellite analysis ofsei whales obtained from the North Pacific in 2010. Paper SC/63/IA12 presented to this meeting.
Matsuoka, K., Hakala, S., Kim, H. W., Aki, M. and Shinyashiki, Y. 2011. Cruise Report for 2010 IWC/Japan
Joint Cetacean Sighting Survey Cruise in the North Pacific. Paper SC/63/O5 presented to this meeting(unpublished) 43pp.
Thomas, L., S.T. Buckland, E.A. Rexstad, J. L. Laake, S. Strindberg, S. L. Hedley, J. R.B. Bishop, T. A. Marques,and K. P. Burnham. 2010. Distance software: design and analysis of distance sampling surveys forestimating population size. Journal of Applied Ecology 47: 5-14.

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Table 1. Abundance estimates for the sei whales and their CV by strata. A was area size of the surveyed area, n is
the smeared number of schools detected within perpendicular distance of 3.0 n.miles, l is searching distance,
ESW is effective strip half width, E(s) is estimated mean school size, D is density (individual/n.miles2), P isabundance estimate and CI is abbreviation for confidence interval.
a. Mean school size was estimated with pooling northern and southern strata (reference case).
Stratum

Arean2n /l *100l

CV(n.miles)(n.miles )

CVESW

E(s )

CV

(n.miles)D(ind./n.
miles 2)

PCV

(ind.)

95% CI 95% CI
LL

UL

Northern

238,6274.0

490.46

0.82

0.50

1.38

0.12

1.97

0.10

0.006

1,394

0.53

528

3,677

Southern

365,24440.0 1,325.74

3.02

0.35

1.38

0.12

1.97

0.10

0.022

7,893

0.38

3,863

16,127b. Mean school size was estimated separately for northern and southern strata (sensitivity 1).
Stratum

Arean2n /l *100lCV(n.miles)(n.miles )

CVESW

E(s )

CV(n.miles)

D(ind./n.
miles 2)

PCV

(ind.)

95% CI 95% CI
LL

UL

Northern

238,6274.0

490.46

0.82

0.50

1.38

0.12

1.00

0.00

0.003

707

0.52

272

1,837

Southern

365,24440.0 1,325.74

3.02

0.35

1.38

0.12

2.03

0.10

0.022

8,128

0.38

3,977

16,614

CV

E(s )

CVD
(ind./n.
miles 2)PCV

95% CI 95% CI

c. Using data in southern stratum only (sensitivity 2).
StratumAreann /l *100

l2(n.miles )(n.miles)365,24440.0 1,325.74

CVESW(n.miles)(ind.)

LL

UL

16,495

Northern
Southern

3.02

0.31

1.31

0.12

2.03

0.10

0.023

8,528

0.35

4,409

D
(ind./n.
miles 2)PCV

95% CI 95% CI

d. Width of intervals for perpendicular distance was doubled (sensitivity 3).
StratumArean2n /l *100lCV(n.miles)(n.miles )

CVESW

E(s )

CV(n.miles)(ind.)LL

UL

Northern

238,627

4.0490.46

0.82

0.50

1.39

0.12

1.970.10

0.006

1,380

0.53

522

3,644

Southern

365,24440.0 1,325.74

3.02

0.35

1.39

0.12

1.97

0.10

0.021

7,808

0.38

3,815

15,981

Table 2. Results for regression of log of observed school size and detection probability for reference case.

slope-0.063

SE
0.287

student's-t-0.219p -value
0.414

Table 3. AIC estimate for each model of detection functions for reference case.
modelAIC
Hazard rate
181.13Half-normal
177.904

Figure 1a. Survey area of 2010 IWC/Japan Joint Cetacean Sighting Survey Cruise in the North Pacific.

Figure 1b. Cruise tracklines design and strata for the survey. The survey area was divided into northern andsouthern strata by 47oN latitudinal line. Broken lines indicate cruise track. Bold arrows indicate survey order.
bold dotted lines indicate transit in the research area.

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Figure 2. Plot of searching effort and primary sightings of the sei whales in the northern and southern strata.
Open circle represents the primary sighting position.

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1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0
0.0

0.5

1.0

1.52.02.5

3.0

Perpendicular distance in nautical miles

Fig. 3. Plot of the estimated detection function fitted to the number of schools as a function of perpendiculardistance (n. miles) from the track line for reference case.

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

Perpendicular distance in nautical miles

Figure 4. Plot of the estimated detection function fitted to the number of schools as a function of perpendiculardistance (n. miles) from the track line for sensitivity 3.

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