Technical Reports of the Ins�tute of Cetacean Research (2022)

pp. 11–18

Technical Report

What do we know about whales and ecosystem in the western North
Paciﬁc Ocean? Part 2: summary of results on abundance es�mates inbaleen whale species
Megumi Takahashi*, Takashi Hakamada and Koji Matsuoka
Institute of Cetacean Research, 4–5 Toyomi-cho, Chuo-ku, Tokyo 104–0055, Japan*Contact e-mail: takahashi@cetacean.jp

ABSTRACTThe Ins�tute of Cetacean Research (ICR) has been conduc�ng whale sigh�ng surveys in the western North
Paciﬁc Ocean for more than 25 years. The main purpose of these surveys is the collec�on of sigh�ng datarequired for abundance es�ma�on of baleen whale species in this oceanic basin. Results of the analyses ofthe data collected have revealed the pa�ern of distribu�on and abundance es�mates of each whale species in
diﬀerent years and in diﬀerent periods within a year. Abundance es�mates have been used for management
purposes (e.g. calcula�on of catch limits for sustainable commercial whaling) in the case of some species. This
paper presents a brief outline of the survey design and analy�cal procedures used, and of the main results
obtained so far.

INTRODUCTION
Informa�on regarding the distribu�on and abundance ofwhale species is essen�al in understanding the role andimpact of whales in the ecosystem. Abundance is also an
important piece of informa�on required for establishingpolicy of management of whales. For example, a �meseries of es�mates of absolute abundance derived fromsigh�ng surveys is required under the Revised Management Procedure (RMP) for se�ng safe catch limitsof commercial whaling. The RMP is the management
procedure developed by the Interna�onal Whaling Commissionʼs Scien�ﬁc Commi�ee (IWC SC) for baleen whale
species.
One of the most common approaches for collec�ngsigh�ng data for abundance es�ma�on purposes is thevessel-based survey using the Line Transect (LT) method.
Japan through the Ins�tute of Cetacean Research (ICR)
has conducted systema�c research on whales in the western North Paciﬁc for more than 25 years. The ﬁrst one
was the Japanese Whale Research Program under Special
Permit in the western North Paciﬁc (JARPN, 1994–1999),
which was followed by the second phase of this program(JARPNII, 2000–2016). Subsequently, the New Scien�ﬁcWhale Research Program in the western North Paciﬁc(NEWREP-NP) was designed and implemented between
2017–2019. All these three research programs (JARPN,
JARPNII, NEWREP-NP) had an important sigh�ng survey

component based on the LT method. Abundance es�mates based on sigh�ng data collected by this survey
component have been crucial in the context of some ofthe objec�ves of those programs, e.g. the es�ma�on of
prey consump�on by whale species. As noted above,abundance es�mates are key informa�on for calcula�ng
catch limit for commercial whaling on some species.
The objec�ve of this paper is to presents a brief outlineof the survey design and analy�cal procedures used by
the ICR sigh�ng surveys in the western North Paciﬁc, andto present the main results obtained so far.

RESEARCH AREA
The research area for the sigh�ng surveys comprises the
Paciﬁc side of Japan, north of 35°N, the Sea of Japan, andOkhotsk Sea (see Figure 1). Each of these research areas
were divided into smaller strata, and the surveys wereconducted in spring (mainly May to June) and summer(mainly August to September) seasons to inves�gate
whale distribu�on and abundance in each season.

VESSEL USED
Specialized vessels which have three observa�on platforms were used in the surveys (Figure 2). Observers collected whale sigh�ng data from the pla�orms using binoculars in the �me period between sunrise and sunset.

11Technical Reports of the Ins�tute of Cetacean Research (2022)

SURVEY METHODOLOGY
Sigh�ng surveys were conducted by the LT method. The
survey protocols followed the IWC SCʼs Requirements andGuidelines for Conduc�ng Surveys and Analyzing Data
within the Revised Management Scheme (IWC, 2012a).
Under the LT method, vessels move on pre-determinedzigzag track lines, which cover the en�re or a part of amanagement area (Figure 3A). The start point is randomly
selected for each survey. The design of track lines does

not follow physical features such as isobaths which may
be correlated with whale distribu�on and their migra�on.
Observers search for whales from each pla�orm on the
research vessels. When a whale school is detected, thedistance between observers and whales and the angle
from the track line to whales were recorded (Figure 3B)
together with the posi�on of the research vessel and �meof the sigh�ng.

ANALYTICAL PROCEDURES
Abundance es�ma�on using sigh�ng data is made using
distance sampling calcula�on (Buckland et al., 2001).
The methodology involves two main components. The
ﬁrst component involves ﬁ�ng the detec�on func�on tosigh�ng data. Ini�ally it is assumed that whales are uniformly distributed and that they have the same probability to be detected. However, searching ac�vity is aﬀectedby environmental factors (e.g. glare, fog, sea state), andwhales occurring far from the vessel are more diﬃcult
to detect (Figure 4). Therefore, detec�on func�on (e.g.
Half-normal func�on and Hazard-rate func�on) should
be examined, and the best model is selected using the
Akaike Informa�on Criterion (AIC) values.

Figure 1.

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Research area on the Paciﬁc side of Japan, southern
Okhotsk Sea and Sea of Japan, covered by Japanesesigh�ng surveys.

Figure 2.

Specialized research vessels used for sigh�ng surveys (le�: Kaiyo-Maru No.7, right: Yushin-Maru No.2). The
ﬁgures show the observa�on pla�orms (taken from Takahashi [2020] and Isoda et al. [2021]).

Figure 3.

Diagrams showing the zigzag track lines for a research area under the LT survey (A); and the whale observa�on from diﬀerent pla�orms (B).

What do we know about whales and ecosystem in the western North Pacific Ocean? Part 2

Figure 4.

Diagram showing the detec�on probability assuming g(0)=1. The probability of detec�on decreases
with distance from the vessel.

 x2 
Half-normal: f ( x, z ) = exp  − 2  2σ 
 − x  − b 
Hazard-rate: f ( x, z ) = 1 − exp  ,
 aσ  
where x is perpendicular distance, z is vector of several
covariates; and σ represents scale parameter for incorpora�ng the covariates in the detec�on func�on and a andb are parameters of each key func�on.
The second component is for es�ma�ng abundance
using the detec�on probability derived from the detec�on func�on. The Horvitz–Thompson-like es�mator is
one of the most frequently used methods. The principleof the es�mator is that the total number of whales present are es�mated from the propor�on of the abundancein the area surveyed (the covered area) to the en�re survey area. To do this the abundance in the area surveyed is
calculated using the detec�on probability. The detec�onfunc�on models are denoted f(x; _) where x is perpendicular distance (from the track line to whales) and θ is avector of parameters of the key func�on to be es�mated.
Nˆ covered =

n


i=1

w 1

 w
0

si
, zi ; θ )dx
f ( x

,where w is trunca�on distance from the track line, si issize of ith detected cluster (I=1, …, n).
Then, it is scaled up to the full study area by mul�plying it by the ra�o of covered area to study area. Here,
A/2wL represents propor�on of en�re research area toarea searched:

A 
Nˆ =
N
2wL covered
In reality, because of diving behavior, not all animals
are available for detec�on even when they are near the

Figure 5.

The searching eﬀort by each Lat. 1°×Long. 1° gridsquare covered by JARPN and JARPNII surveys during
1994 to 2014 (Matsuoka and Hakamada, 2019). The
gray scale shows the degree of searching eﬀort.

vessel and/or on the track line. Therefore, this bias should
also be considered in the abundance es�ma�on process.
This bias is known as availability bias and is diﬀerent frompercep�on bias caused by human error under diﬀerent
environmental condi�ons.

RESULTS ON DISTRIBUTION AND ABUNDANCE
Figure 5 shows the main research area and the total primary searching eﬀort by each Lat. 1°×Long. 1° grid square
during 1994 to 2014 under JARPN and JARPNII surveys.
Table 1 shows the abundance es�mates for each whale
species. The numbers of whales distributed in the JARPNII
survey area were es�mated in the early and late seasons(see Hakamada and Matsuoka, 2016a; 2016b).
Blue whales
The main distribu�on area for blue whales moved northward from 35°N–40°N to north of 40°N during the spring(May to June) to summer (August to September) seasons(Matsuoka et al., 2016) (Figure 6). In the late 20th century, they had never been observed in this area in June,
but this could have been caused by the diﬀerence inregula�ons of the whaling opera�ons for coastal and oﬀshore whaling. In the late season, blue whales were more
frequently sighted north of 40°N (Figure 6), and the abundance es�mate was 958 animals (CV= 0.461) (Table 1).
Fin whales
Fin whales were observed mainly in the Paciﬁc side ofJapan but also occurred in the Okhotsk Sea and Sea ofJapan (Matsuoka et al., 2016). The main distribu�onarea was north of 37°N, and the highest density area wasnorth of 45°N. Fin whales were widely distributed acrossthe western North Paciﬁc in the spring season, but they13

Technical Reports of the Ins�tute of Cetacean Research (2022)
Table 1Abundance es�mates for several large baleen whale species. All es�mates assumed g(0)=1, except for common minke whales inwhich g(0)=0.798 (CV=0.134).
Spring season

SpeciesBlue whale
Fin whale
Sei whale
Bryde's whale
Humpback whale
North Paciﬁc right whale
Common minke whale

Summer season

20092011+20122008

38 (0.977)
413 (0.569)
4,734 (0.177)
2,957 (0.394)
1,136 (0.438)
0 (—)
3,629 (0.586)

161 (0.474)
1,369 (0.295)
2,988 (0.304)
1,851 (0.413)
1,921 (0.318)
1,147 (0.434)
2,122 (0.371)

958 (0.461)
3,958 (0.425)
5,086 (0.378)
13,306 (0.251)
392 (0.877)
416 (0.653)
3,080 (0.677)

References
Hakamada and Matsuoka (2016b)
Hakamada and Matsuoka (2016b)
Hakamada and Matsuoka (2016a)
Hakamada and Matsuoka (2016a)
Hakamada and Matsuoka (2016b)
Hakamada and Matsuoka (2016b)
Hakamada and Matsuoka (2016a)

Spring season: May to June, Summer season: August to September.

Figure 6.

Blue whales observed during the surveys (top). Sigh�ng posi�ons of blue whales (blue circles) on the tracklines, by survey period (bo�om).

were more frequently sighted north of 40°N in the summer season (Figure 7), and the abundance was es�matedat 3,958 animals (CV= 0.425) in this season (Table 1).
Humpback whales
The main distribu�on area of humpback whales was
north of 37°N from May to September (Matsuoka
et al., 2016). The highest density area was observed northof 35°N (west of 158°E) and north of 45°N (east of 158°E).
They moved northward from 37°N to 43°N in the inshore
area of Japan during May to June, and to further northof 45°N during July to August (Figure 8). The abundancees�mates were 1,136 (CV= 0.438) to 1,921 (CV= 0.318) inthe early seasons. Despite the surveys covering the en�rewestern North Paciﬁc in the summer season, sigh�ngs of
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this species were not frequent in this season (Table 1).
North Paciﬁc right whales
During the survey programs, 48 schools (68 individuals)
were observed north of 37°N from May to September(Matsuoka et al., 2016) (Figure 9). The total abundance
was es�mated to be 1,147 animals (CV= 0.434) basedon the JARPNII data in the spring season of 2011/12(Table 1).
Sei whales
The main distribu�on area of sei whales shi�ed northward during spring to summer seasons (Murase et al.,2016; Figure 10). The abundance es�mate in the summer season was higher than that in the spring season

What do we know about whales and ecosystem in the western North Pacific Ocean? Part 2

Figure 7.

Fin whales observed during the surveys (top). Sigh�ng posi�ons of ﬁn whales (red circles) on the tracklines, by survey period (bo�om).

Figure 8.

Humpback whales observed during the surveys (top). Sigh�ng posi�ons of humpback whales (light blue
circles) on the track lines, by survey period (bo�om).

(Table 1). The es�mates should be considered as a part of
the total stock number of this species (see Taguchi et al.,this issue).
Brydeʼs whales
The main distribu�on of Brydeʼs whale shi�ed northward during the spring to summer season (Hakamada
et al., 2009). Their distribu�on was in lower la�tudes
compared to other large baleen whale species (Figure 9). Table 1 shows the abundance es�mates for
Brydeʼs whales in the spring and summer (July to September) seasons in this JARPNII area.

Common minke whales
Common minke whales are known to migrate further
north of the JARPNII survey area through early season tolate season (Hakamada et al., 2009) (Figure 12). Common
minke whales distribute primarily in the Sea of Okhotsk
and in the waters east of the Kamchatka Peninsula andthe Kuril Islands in August and September (Buckland et al,
1992; Miyashita, 2019). The es�mated numbers distributed in the survey area were 3,629 (CV= 0.586) and 2,122(CV= 0.371) in the spring season and 3,080 (CV= 0.677) inthe summer season (Table 1).

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Technical Reports of the Ins�tute of Cetacean Research (2022)

Figure 9.

North Paciﬁc right whales observed during the surveys (top). Sigh�ng posi�ons of North Paciﬁc rightwhales (purple circles) on the track lines, by survey period (bo�om).

Figure 10.

Sei whales observed during the surveys (top). Sigh�ng posi�ons of sei whales (orange circles) on the tracklines, by survey period (bo�om).

USE OF THE ABUNDANCE ESTIMATES FOR MANAGEMENT PURPOSES
Japan resumed commercial whaling of North Paciﬁc sei,
Brydeʼs and western North Paciﬁc common minke whales
in July 2019. Catch limits for each species were set inline with the RMP (IWC, 2012b). The core component of
the RMP is the Catch Limit Algorithm (CLA) (Aldrin andHuseby, 2007; Aldrin et al., 2008), which is a feedback
16

control algorithm that sets baleen whale harvest levels
based on catch histories and a �me series of es�matesof absolute abundance derived from sigh�ng surveys.
Implementation Simulation Trials (ISTs) are conducted toassess uncertain�es for several parameters in the case ofmul�-stock scenarios.
Abundance es�mates from ICR sigh�ng surveys havebeen fundamental in the process of applying the CLA
and ISTs to the three North Paciﬁc baleen whale species.

What do we know about whales and ecosystem in the western North Pacific Ocean? Part 2

Figure 11.

Brydeʼs whales observed during the surveys (top). Sigh�ng posi�ons of Brydeʼs whales (green circles) on
the track lines, by survey period (bo�om).

Figure 12.

Common minke whales observed during the surveys (top). Sigh�ng posi�ons of common minke whales(purple circles) on the track lines, by survey period (bo�om).

For species like the sei and Brydeʼs whales, total stock
abundances have been obtained by combining the ICR es�mates in the western North Paciﬁc with es�mates fromother surveys in the central and eastern North Paciﬁc.
See details of the process for catch limit calcula�on in
Japanʼs RMP Team (2019).

FINAL REMARKSThe ICR has conducted sigh�ng surveys for more than
25 years, in collabora�on with other ins�tu�ons, usinga systema�c survey procedure. The surveys revealed the
distribu�on and abundance of several large baleen whale

species in the western North Paciﬁc Ocean. Moreover,abundance es�mates from the ICR surveys contributed
to the process for calcula�ng catch limits for sustainable
commercial whaling of sei, Brydeʼs and common minke
whales. There are several aspects that could improve theabundance es�mates in the future. Among them are thees�mates of g(0) for all species, evalua�on of addi�onal
variance for mul�-year surveys and the use of modelbased approaches.

ACKNOWLEDGEMENTS
We thank all researchers and crew members who par17

Technical Reports of the Ins�tute of Cetacean Research (2022)

�cipated in the ICR sigh�ng surveys for their hard work incollec�ng data in the ﬁeld. Our thanks are also extended
to all researchers who assisted in the analyses. We are
grateful to the Fisheries Agency of Japan for providing
funds for the surveys. Our gra�tude to Luis A. Pastene(ICR) for his assistance in preparing this manuscript. We
also thank the Editorial Team of TEREP-ICR for the editorial work on this manuscript.

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