SC/65a/IA09
SC/65a/IAxx

Preliminary abundance estimation of North Pacific sei whalebased on the 2012 IWC-POWER sighting survey data
TAKASHI HAKAMADA1 and KOJI MATSUOKA1
1

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055
Contact e-mail:hakamada@cetacean.jp

KEYWORD: ABUNDANCE ESTIMATE, NORTH PACIFIC, SEI WHALE, SURVEY-VESSEL,
IWC-POWER.
ABSTRACT
Abundance of North Pacific sei whale (Balaenoptera borealis) was preliminary estimated using sighting
data obtained during the 2012 International Whaling Commission-Pacific Ocean Whale and Ecosystem
Research (IWC-POWER) Cruise. Sensitivity analyses were also conducted to investigate robustness of
the abundance estimate to alternative assumptions on stratification and detection functions. Abundance inthe eastern North Pacific (north of 40°N, south of the Alaskan coast including both the US and Canadian
EEZ between 150°W-135°W), from July to August was estimated as 12,180 (CV=0.327) for the base casescenario. The abundance estimate ranged between 12,062 (CV=0.327) and 13,446 (CV=0.388) for the
scenarios considered in the sensitivity analyses. In the near future abundance estimate of sei whale in the
North Pacific will be refined by considering all IWC-POWER sighting data obtained in the period 2010
to 2012. Abundance information will be useful for the in-depth assessment of this species in the North
Pacific being planned by the IWC Scientific Committee (IWC/SC).
INTRODUCTION
Commercial whaling of the sei whale (Balaenoptera borealis) was conducted in the North Pacific until
1976 when the commercial take of this species was banned by the IWC. The population size of the North
Pacific sei whale after the exploitation period was estimated at 9,000 whales (or 21% of the initial size)(IWC, 1977). Information on abundance estimate and abundance estimate trend of the sei whales in the
North Pacific in recent years is required.
In recent years sighting data of sei whales have been collected in the western North Pacific throughdedicated Japanese whale sighting surveys, and the sighting component of the Japanese Whale Research
Program under Special Permit in the western North Pacific (JARPN and JARPN II). Distribution of sei
whales based on Japanese Scouting Vessels (JSV), JARPN and JARPN II sighting data was summarized
by Miyashita et al., (1995), Matsuoka et al. (2000; 2009). Sei whale abundance in the western North
Pacific (east of Japanese coast, west of 170oE, north of 35oN, south of Russian EEZ) based on JARPN II
sighting data was estimated at 5,406 whales (CV=0.300) in July and August (Hakamada et al., 2009).
The IWC-POWER surveys started in 2010. These surveys were planned to cover the central and eastern
North Pacific (Figure 1). Preliminary abundance estimates of sei whales were made based on the sighting
data collected during the 2010 and 2011 IWC-POWER surveys. Abundance estimates of 9,286 whales(CV=0.35) and 6,587 whales (CV=0.42) were obtained in the central North Pacific (north of 40oN, south1

of Aleutian Islands, and between 170oE and 170oW), and in the eastern North Pacific (north of 40oN,
south of Alaskan Peninsula, between 170oW and 150oW), respectively (Hakamada et al, 2011; 2012).
The plan for the 2012 IWC-POWER sighting survey was agreed at the 2011 IWC-POWER planningmeeting. The plan was made following the guidelines agreed at the IWC-POWER planning meeting in2010 (IWC, 2012a). The 2012 survey was carried out successfully and results are presented in Matsuoka
et al. (2013).
An in-depth assessment of this species in the North Pacific is being planned by the IWC/SC to start in2013 (IWC, 2012b). It is expected that new abundance estimates for sei whales derived from the
IWC-POWER and other surveys will contribute for this assessment in conjunction with new information
on stock structure (IWC, 2012b).
This paper reports preliminary abundance estimates of the sei whale based on the 2012 IWC-POWER
sighting data and standard line transect methodology (e.g. Branch and Butterworth, 2001).
MATERIALS AND METHODS
Survey area
The surveyed area comprised the eastern North Pacific north of 40oN, south of the Alaskan coast
including both the US and Canadian EEZ, and between 150oW and 135oW. The survey area was divided
into northern and southern strata by the EEZ line of the USA and Canada (Figure 2). The area sizes of thenorthern and southern strata were 142,427n.miles2 and 529,362n.miles2, respectively. They were
calculated using Arc GIS (ver. 9.31).
Brief narrative
The 2012 IWC-POWER survey was conducted in the period 24 July – 30 August 2012. The ship departed
Shiogama on 13 July and started the sighting surveys on 24 July at 150º00’W. It completed the sighting
surveys in the northern stratum on 3 August at 135º00’W and started the sighting survey in the southern
stratum on 3 August. It completed the sighting surveys on 30 August at 150º00’W and arrived at
Shiogama on 5 September (Matsuoka et al., 2013).
Research vessel
The sighting survey was conducted by the research vessel Yushin-Maru No.3 (YS3). Specifications of thevessel are provided in Matsuoka et al. (2013).
Survey design
Cruise tracks were designed using the program DISTANCE (Thomas et al., 2010) following theprinciples outlined in the IWC/SC’s Requirements and Guidelines for Surveys (IWC, 2005). Planned
cruise tracks and survey order were shown in Figure 3. Observed distance and angles were correctedusing data obtained from the Angle and Distance Experiment. All sighting data obtained were validated.
Survey mode
Because use of passing mode in the POWER would result in very high proportions of unidentified
cetaceans, the technical advisory group (TAG) recommended that Passing with abeam closing mode
(NSP) is the most appropriate survey mode, both with respect to confirming species identity and school2

size (IWC, 2013). Passing with abeam closing mode was used during the survey. Two topmen wereobserving from the barrel at all times. There was open communication between the upper bridge and thebarrel. The observers on the upper bridge should communicate with the topmen only to clarify
information and should not direct the topmen to disrupt their normal search procedure unless directed to
do so by the Cruise Leader (Matsuoka et al., 2013).
Analytical procedure
For the analysis it is assumed that g(0)=1. 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 and its CV were estimated by formula (1) and (2), respectively. DISTANCE ver 6.0 (Thomas
et al., 2010) is used for abundance estimation.

N=

AnE ( s )
(1)
2 wl

2

  n 2
2
CV ( N ) = CV   + {CV (w)} + {CV (E ( s ) )} (2)  l 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.
The smeared and truncated sighting data for schools are grouped into intervals of 0.3 n. miles to estimate
the detection function. We consider half-normal and hazard-rate models as candidate models for thedetection function. Model is selected by AIC. Mean school size is estimated from the primary sightings
whose school size was confirmed. Regression method in Buckland et al. (1993) is applied to estimate
mean school size.
Sensitivity Analysis
In order to examine the robustness of the abundance estimate to alternative assumption on stratificationfor estimation of mean school size and detection functions, sensitivity is conducted as following.
1. Mean school size was estimated by stratum.
2. Alternative detection function is applied.
3. Width of perpendicular distance intervals is changed from 0.3 to 0.6 n.miles.
RESULTS
Sighting effort
Survey order in the research area is shown in Figure 3. The searching effort was 767.5 n. miles and1,358.6 n.miles in the northern and southern strata, respectively (Table 1). Survey coverage was relatively
high in the southern (85%) and the northern (80%) strata (Matsuoka et al., 2013).
Sightings
Plots for searching effort and primary sightings of the sei whales are shown in Figure 4. There were 79
primary sightings (148 individuals) in the research area. Figure 5 shows composition of observed school
size for primary sightings during the surveys. 51 schools are solitary schools out of 79 sightings (64.5%).

3

Sei whales were distributed mainly in the southern stratum between 135oW and 145oW.
Abundance estimate
Table 1 shows the abundance estimates for the base case scenario (1a) and for the scenarios of thesensitivity test (1b-1d). The effect of school size on detection was not significant at the 15% level.
Therefore the observed mean school size is used to estimate abundance (Table 2). The AIC for half
normal model is smaller than that for hazard-rate model (Table 3), and half-normal model is selected by
AIC. Figure 6 shows the plot of the detection function for the base case scenario (using AIC-selected
detection function). Abundance estimate of sei whale is 12,180 (CV=0.327) for the base case scenario(Table 1a).
Sensitivity Analysis
Estimating mean school size by stratum
Table 1b shows the abundance estimates for the sensitivity 1. For this sensitivity, mean school size and its
CV were calculated by strata. Mean school size estimates were not different substantially but CV innorthern stratum is higher than the base case. Encounter rate and ESW and their CVs are common withthe base case scenario. Abundance estimate is 12,163 (CV=0.327) for the sensitivity 1.
Alternative detection function
Table 1c shows the abundance estimates for the sensitivity 2. The CV of ESW derived from thehazard-rate model (0.237) was higher than the base case (0.110), therefore the use of half-normal model
was preferable to estimate ESW. Figure 7 shows the plot of the detection function for sensitivity 2. The
fitting of the model is worse than the base case. Abundance estimate is 13,446 (CV=0.388) for thesensitivity 2.
Width of perpendicular changes from 0.3 to 0.6 n.miles
Table 1d shows the abundance estimates for the sensitivity 3. ESW estimate and its CV would not change
substantially from the base case. Figure 8 shows the plot of the detection function for sensitivity 3. Shapeof the detection function is not substantially changed from that for the base case. Abundance estimate is
12,062 (CV=0.327) for the sensitivity 3.
DISCUSSION
Distribution of the sei whales
Figure 4 shows that the sei whales mainly distributed in the eastern part of the survey area
(145oW-135oW). This may suggest that sei whales distributes to the east of 135oW longitudinal line.
Sighting survey in this area could provide useful sighting data to estimate sei whale abundance there.
Robustness of abundance estimates
Abundance estimates in the sensitivity analyses did not differ substantially from the base case. Sensitivity
analysis suggested that abundance estimate is robust to alternative assumptions on stratification toestimate mean school size and detection functions examined in this study. Investigating detection function
using covariates such as Beaufort state and year and pooling data with sighting data collected in otheryear, could refine abundances estimate of the sei whales.
Abundance estimate for North Pacific sei whales in previous studies

4There were some sei whale abundance estimates in previous studies as mentioned earlier. Sei whale
abundance estimate in the western North Pacific (east of Japanese coast, west of 170oE, north of 35oN,
south of Russian EEZ) based on JARPN II data was 5,406 (CV=0.300) in July and August (Hakamada etal., 2009). Abundance estimate in the 2010POWER was 9,286 (CV=0.35) (Hakamada et al., 2011) andthat in the 2011POWER was 6,587 (CV=0.420). Kanda et al. (2009; 2011; 2013) suggested that the open
water of the North Pacific was mainly occupied by the individuals from a single stock of the sei whales.
Murase et al., (2009; 2013) suggested that distribution of the sei whales predicted by a generalised
additive model is continuous in southern part of survey area during 2010-2012 IWC-POWER. Under thisassumption and the assumption that there is no correlations among the estimates, the combined abundance
estimates was 33,458 (CV=0.195) in the parts of North Pacific surveyed by those surveys in July August.
Future work
Abundance estimate for the sei whale will be refined using all IWC-POWER sighting data for the period
2010-2012. Detection function (Half normal and Hazard rate) with covariate such as Beaufort state andyear, will be examined using MCDS engine in the program DISTANCE. Also an examination ofstratification of data to estimate mean school size will be carried out.
ACKNOWLEDGEMENT
We thank the captain, crews and international researchers onboard the Yushin-Maru No.3 for their hard
work during the survey. We thank Luis Pastene for their valuable comment to improve this manuscript.
We also thank Greg Donovan of the Head of Science of the IWC, the steering group (Robert Brownell,
Phillip Clapham, Paul Ensor, Tomio Miyashita, Luis Pastene and Lars Walløe) and the technical advisorygroup (Sharon Hedley, Mark Bravington, and Natalie Kelly) for their scientific contributions to organize
the POWER cruise.
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6

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7

Table 1. Abundance estimates for the sei whales and their CV’s by stratum from July to August. A is area
size of the surveyed area, n is the smeared number of schools detected within perpendicular distanceof 3.0 n.miles, l is searching distance, ESW is effective strip half width, E(s) is estimated meanschool size, D is density (individual/n.miles2), P is abundance estimate and CI is abbreviation for
confidence interval.
a. Encounter rate was estimated separately by stratum (base case).
Stratum

Area

n
2

l

n /l *100

CV

0.629
0.297

(n.miles)(n.miles )

ESW

CV

E(s )

CV

D
(ind./n.
miles 2)

LL

UL

0.110
0.110

1.842
1.842

0.093
0.093

0.001
0.023

136
12,044

0.645
0.330

43
6,416

433
22,6080.018

12,180

0.327

6,527

22,729PCV

95% CI 95% CI(n.miles)

Northern
Southern

142,427
529,362

1.7
73.7

767.5
1,358.6

0.228
5.424

Total

671,789

75.4

2,126.1

3.548

2.196
2.196

P

CV

(ind.)

95% CI 95% CI

b. Encounter rate and mean school size were estimated separately by stratum.
StratumArean(n.miles 2)l

n /l *100

CV(n.miles)

ESW

CV

E(s )

CV

(n.miles)

Northern
Southern

142,427
529,362

1.7
73.7

767.5
1,358.6

0.228
5.424

Total

671,789

75.4

2,126.1

3.548

0.629
0.297

2.196
2.196

0.110
0.110

2.000
1.838

0.500
0.095

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

LL

UL

0.001
0.023

148
12,015

0.811
0.330

37
6,394

595
22,581

0.018

12,163

0.327

6,515

22,708PCV

95% CI 95% CI

c. Alternative detection function (Hazard rate model) was applied.
Stratum

Area

n
2

l

n /l *100

CV

0.629
0.297

(n.miles)(n.miles )

ESW

CV

E(s )

CV

D
(ind./n.
miles 2)

LL

UL

0.237
0.237

1.842
1.842

0.093
0.093

0.001
0.025

150
13,296

0.679
0.391

45
6,348

502
27,8490.020

13,446

0.388

6,458

27,997CV

95% CI 95% CI(n.miles)

Northern
Southern

142,427
529,362

1.7
73.7

767.5
1,358.6

0.228
5.424

Total

671,789

75.4

2,126.1

3.548

1.989
1.989

(ind.)

d. Width of perpendicular distance intervals was changed from 0.3 to 0.6 (n.miles).
Stratum

Area

n
2

l

n /l *100

CV(n.miles)(n.miles )

ESW

CV

E(s )

CV

(n.miles)

Northern
Southern

142,427
529,362

1.7
73.7

767.5
1,358.6

0.228
5.424

Total

671,789

75.4

2,126.1

3.548

0.629
0.297

2.217
2.217

D
(ind./n.
miles2)

0.113
0.113

1.842
1.842

0.093
0.093

P
(ind.)

LL

UL

0.001
0.023

135
11,927

0.646
0.331

42
6,344

429
22,4250.018

12,062

0.327

6,453

22,545

Table 2. Results for regression of log of observed school size and detection probability for base case. SE
is standard error of the estimate slope.
slope-0.319SE
0.366

Student's-t p -value-0.871
0.193

Table 3. AIC estimate for each model of detection functions for base case. Selected model is indicated byan asterisk.
Model
AIC
Half Normal*
347.4
Hazard Rate
349.2

8

2010

2011

2012

POWER

POWER

POWER

40oN
170oE

170oW

150oW

135oW

Figure 1. Research area of the 2010-2012 POWER cruises.

Figure 2. Research area of the 2012 IWC-POWER cruise. The area was divided into northern andsouthern strata by the US and Canadian EEZ boundary line (Matsuoka et al., 2013).

Figure 3. Planned cruise track (line) and survey order (arrows). Bold black line represents boundary forsurvey area. Grey colored area represents for the US and Canadian EEZ. Survey started at 150oW andcompleted at 150oW.

9

60N

55N

50N

45N

40N
210W
150W

220W
140W

135W

Figure 4. Plot of actual survey track (red line) and positions of primary sightings of the sei whales (bluecircle) during the 2012 IWC-POWER survey. Bold black line represents boundary for survey area.
Green line represents boundary for the US and Canadian EEZ.

60

50Frequency

40

30

20

10

0
1

2

3

4
5
6
Observed school size

7

8

Figure 5. Composition of observed school size for primary sightings of the sei whales during the survey.

10

1.4

1.2

1

0.8

0.6

0.4

0.2

0
0

0.5

1

1.5

2

2.5

3

Figure 6. Plot of the estimated detection function (red line) fitted to the number of schools as a function of
perpendicular distance (n. miles) from the track line for base case scenario (Half normal model wasused).

1.4

1.2

1

0.8

0.6

0.4

0.2

0
0

0.5

1

1.5

2

2.5

3

Figure 7. Plot of the estimated detection function (red line) fitted to the number of schools as a function of
perpendicular distance (n. miles) from the track line, for sensitivity 2 (Hazard rate model was used).

1.4

1.2

1

0.8

0.6

0.4

0.2

0
0

0.5

1

1.5

2

2.5

3

Figure 8. Plot of the estimated detection function (red line) fitted to the number of schools as a function of
perpendicular distance (n.miles) from the track line, for sensitivity 3 (Width of interval was doubled).

11