SC/64/IA11

Preliminary estimation of North Pacific sei whale abundance
based on the 2011 IWC-POWER sighting survey data
TAKASHI HAKAMADA1, KOJI MATSUOKA1 and TOSHIHIDE KITAKADO2
1

The Institute of Cetacean Research, 4-5 Toyomi-cho, Chuo-ku, Tokyo 104-0055
Tokyo University of Marine Science and Technology, 5-7, Konan 4, Minato-ku, Tokyo 108-8477, Japan
Contact e-mail:hakamada@cetacean.jp
2

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 2011 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
estimate in the eastern North Pacific (north of 40oN, south of Alaskan Peninsula, between 170oW and
150oW), from July to August was 6,587 (CV=0.420) for the base case scenario. Abundance estimate
ranged between 6,162 (CV=0.417) and 7,403 (CV=0.550) for the scenarios considered in the sensitivity
analyses. In the near future abundance estimate of sei whale in the North Pacific will be refined byconsidering 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 170 oE, 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).

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The IWC-POWER surveys started in 2010. These surveys were planned to cover the central and eastern
North Pacific. Preliminary abundance estimates of sei whales were made based on the sighting datacollected during the 2010 IWC-POWER survey. An abundance estimate of 9,286 whales (CV=0.35) was
obtained for an area in the central North Pacific comprised north of 40oN, south of Aleutian Islands, andbetween 170oE and 170oW) (Hakamada et al, 2011).
The plan for the 2011 IWC-POWER survey was agreed at the 2011 IWC/SC meeting. The plan was madefollowing guidelines agreed at the IWC-POWER planning meeting in 2010 (IWC, 2011a). The 2011
survey was carried out successfully and results are presented in Matsuoka et al. (2012).
An in-depth assessment of this species in the North Pacific is being planned by the IWC/SC to start in2013 (IWC, 2011b). 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, 2011b).
This paper reports preliminary abundance estimates of the sei whale based on the 2011 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 Alaskan Peninsula, andbetween 170oW and 150oW. The survey area was divided into northern and southern strata by the EEZline of the USA (Figure 1). The area sizes of the northern and southern strata were 193,560n.miles2 and569,167n.miles2, respectively. They were calculated using Arc GIS (ver. 9.31).
Brief narrative
The 2011 IWC-POWER survey was conducted in the period 11 July – 8 September 2011. The ship
departed Shimonoseki on 11 July and started the sighting surveys on 21 July at 170º00’W. It completed
the sighting surveys in the northern stratum on 31 July at 150º00’W and started the sighting survey in thesouthern stratum on 1 August. It completed the sighting surveys on 31 August at 170º00’W and arrived at
Hakodate on 8 September (Matsuoka et al., 2012).
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. (2012).
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 2. Observed distance and angles were correctedusing data obtained from the Angle and Distance Experiment. All sighting data obtained were validated
by the authors.
Survey mode2

Passing with abeam closing mode was used during the survey. Two topmen were observing from thebarrel at all times. There was open communication between the upper bridge and the barrel. 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., 2012).
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)
2wl

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 stratificationand detection functions, sensitivity is conducted as following.
1. Abundance is estimated without stratification.
2. Alternative detection function is applied.
3. Width of perpendicular distance intervals is changed from 0.3 to 0.6.
RESULTS
Sighting effort
Survey order in the research area is shown in Figure 2. The searching effort was 723.8 n. miles and1,674.0 n.miles in the northern and southern strata, respectively (Table 1). Survey coverage was higher inthe southern stratum (78%) than in the northern stratum (58%) because poor weather conditions were
found in the northern stratum (Matsuoka et al., 2012).
Sightings
Plots for searching effort and primary sightings of the sei whales are shown in Figure 3. There were 37

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primary sightings (72 individuals) in the research area. Sei whales were distributed in the southern
stratum mainly between 160oW and 170oW.
Abundance estimate
Table 1a shows the abundance estimates for the base case scenario and for the scenarios of the sensitivity
test. 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 thanthat for hazard-rate model (Table 3), and half-normal model is selected by AIC. Figure 4 shows the theplot of the detection function for the base case scenario. Abundance estimate of sei whale is 6,587(CV=0.420) for the base case scenario (Table 1).
Sensitivity Analysis
Without stratification
Table 1b shows the abundance estimates for the sensitivity 1. For this sensitivity, encounter rate and its
CV is calculated. Encounter rate is slightly smaller than the base case scenario. ESW and estimated meanschool size are common with the base case scenario. Abundance estimate is 6,162 (CV=0.417) for thesensitivity 1.
Alternative detection function
Table 1c shows the abundance estimates for the sensitivity 2. The CV of ESW derived from thehazard-rate model was higher than the base case, therefore the use of half-normal model was preferable toestimate ESW. Figure 5 shows the plot of the detection function for sensitivity 2. Abundance estimate is7,403 (CV=0.550) for the sensitivity 2.
Width of perpendicular changes from 0.3 to 0.6
Table 1d shows the abundance estimates for the sensitivity 3. ESW estimate and its CV would not change
substantially from the base case. Figure 6 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 is6,572 (CV=0.421) for the sensitivity 3.
DISCUSSION
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 assumption on stratification anddetection functions examined in this study. As far as using data used in this analysis, further sensitivity
test would not be expected to change abundance estimate substantially. As mentioned later, investigating
detection function using covariates such as Beaufort and pooling data with sighting data collected in otheryear, can refine abundances estimate of the sei whales.
Sightings in northern stratum
No primary sightings were made in the northern stratum. It is possible that sei whales did not migrate into
the northern stratum during the survey period because of the low Sea Surface Temperature (SST)
observed there. The range of the SST in the sighting positions of sei whales was from 11.4 oC to 22.4 oC(Matsuoka et al.,2012). The range of SST in the northern stratum was different (6.9oC-13.9oC) (Matsuoka

4pers. commn.). A survey in a later period in the northern stratum probably had found sei whales might. It
is suggested the use of spatial modeling (e.g. Konishi et al, 2009; Murase et al., 2009) to investigate
relationship between environmental, geographical and seasonal covariates and the density of this species.
In addition smaller survey coverage in the northern stratum could have reduced the possibility of primarysighting of the sei whales in that stratum.
Abundance estimate in previous studies
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 170 oE, 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 central North Pacific (north of 40oN, south of Aleutian Islands,
between 170oE and 170oW) was 9,286 (CV=0.35) derived from 2010 IWC-POWER survey data
(Hakamada et al., 2011). Kanda et al. (2009; 2011) suggested that the open water of the North Pacific wasmainly occupied by the individuals from a single stock of the sei whales. Under this assumption and theassumption that there is no correlations among the estimates, the combined abundance estimates was
21,278 (CV=0.243) 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 with covariate such as Beaufort state, year, strata (north/south) will be
examined. Also an examination of stratification of data to estimate mean school size will be carried out.
ACKNOWLEDGEMENT
We thank Hiroshi Hatanaka and Luis A. Pastene for their valuable comment to improve this manuscript.
We also thank Steering Group of the IWC-POWER (Hidehiro Kato (Convener), Yong-Rock An, Robert
Brownell, Phillip Clapham, Paul Ensor, Tomio Miyashita, Hiroto Murase, Luis Pastene and Lars Walløe)
for their assistance on planning and execution of the cruise. We also thank the captain, crews andinternational researchers onboard the Yushin-Maru No.3 for their hard work during the survey.
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Matsuoka, K., Hakamada, T., Y., and Miyashita, T., 2000. Distribution pattern of minke whales based onsighting data during the JARPN 1994-1999. Paper SC/F2K/J16 presented to the IWC ScientificCommittee Workshop to Review the Japanese Whale Research Programme under Special Permit for
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Table 1. Abundance estimates for the sei whales and their CV 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).
StratumArean2

193,560
569,167

n /l *100

CV

0.000
2.191

0.000
0.378

ESW

(n.miles)(n.miles )Northern
Southern

l

0.0
36.7

723.8
1,674.0

CV

E(s )

CV

D
(ind./n.
miles2)

LL

UL

0.144

1.946

0.114

0.000
0.012

0
6,587

0.420

2,987

14,522

CV

E(s )

CV

D
(ind./n.
miles2)PCV

95% CI 95% CI
LL

UL

0.144

1.946

0.114

0.008

6,162

0.417

2,810

13,513PCV

95% CI 95% CI(n.miles)

1.842

P

CV

(ind.)

95% CI 95% CI

b. Abundance was estimated without stratification.
StratumAn2n /l *100

CV

1.529

0.375

ESW

(n.miles)(n.miles )Survey area 762,727

l

36.7

2,397.8(n.miles)

1.842(ind.)

c. Alternative detection function (Hazard rate model) was applied.
StratumArean2

193,560
569,167

n /l *100

CV

0.000
2.191

0.000
0.378

ESW

(n.miles)(n.miles )Northern
Southern

l

0.0
36.7

723.8
1,674.0

CV

E(s )

CV

D
(ind./n.
miles2)

LL

UL

0.387

1.663

0.099

0.000
0.013

0
7,403

0.550

2,702

20,278PCV

95% CI 95% CI(n.miles)

1.401(ind.)

d. Width of perpendicular distance intervals was changed from 0.3 to 0.6.
StratumAn2

193,560
569,167

n /l *100

CV

0.000
2.191

0.000
0.378

ESW

(n.miles)(n.miles )Northern
Southern

l

0.0
36.7

723.8
1,674.0

CV

E(s )

CV

D
(ind./n.
miles2)

0.147

1.946

0.114

0.000
0.012

(n.miles)

1.846(ind.)

0
6,572

0.421

LL

UL

2,975

14,514

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-0.285SE
0.344

student's-t-0.828p -value
0.207

Table 3. AIC estimate for each model of detection functions for base case.
Model

AIC

Half Normal

162.6

Hazard Rate

163.4

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Figure 1. Research area of the 2011 IWC-POWER cruise. The area was divided into northern andsouthern strata by the US EEZ boundary line.

Figure 2. Planned cruise track (red line) and survey order (blue arrows). Survey strared at 170oW andcompleted at 170oW. The survey area was divided into northern and southern strata by the US EEZ
boundary (green line).

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Figure 3. Plot of actual survey track (red line) and positions of primary sightings of the sei whales(triangle) during the 2011 IWC-POWER survey.

9

Figure 4. 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 base case scenario.

Figure 5. 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 sensitivity 2 (hazard rate model was used).

Figure 6. 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 sensitivity 3 (width of interval was doubled).

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