SC/56/O19

INCREASING TREND AND ABUNDANCE ESTIMATE OF
SEI WHALES IN THE WESTERN NORTH PACIFIC
TAKASHI HAKAMADA, KOJI MATSUOKA AND SHIGETOSHI NISHIWAKIThe Institute of Cetacean Research: 4-5, Toyomi-cho, Chuo-ku, Tokyo104-0055, Japan
contact email: hakamada@cetacean.jp
ABSTRACTDuring the 2002 and 2003 JARPNII surveys, dedicated sighting surveys were conducted in sub-areas
8 and 9 (excluding foreign EEZ) in August and September. The analytical program DISTANCE wasused to estimate abundance of sei whales based on data collected in those surveys, resulting in the
figure of 4,100 animals (CV=0.281). Japanese scouting vessel (JSV) data were used to estimate an
extrapolation factor and then abundance estimate in the western North Pacific (north of 35oN andwest of 180o) in recent years, was possible. Result of the estimation was 68,000 animals (CV=0.418,
95% CI: 31,000-148,000). There is a possibility that the extrapolation factor estimate has a low
precision due to small number of JSV sightings. The present estimate is more than double of theformer estimate of 28,400 and several explanations are given for this difference. Density index was
higher in recent JARPN II surveys than in previous JSV surveys, which support the view ofincreasing trend in abundance for this population.
KEY WORDS: ABUNDANCE ESTIMATE, WESTERN NORTH PACIFIC, SEI WHALE

INTRODUCTION
From the sighting positions of sei whales during 1994-2001 JARPN and JARPNII feasibility surveys,
it was suggested that sei whales distributes uniformly in sub-areas 7, 8 and 9. JSV data also suggestthat sei whales distribute uniformly in the western North Pacific. Twenty-six years have passed since
the ban of catch of this stock, and the recovering trend appears remarkable in recent years by theJapanese whale sighting surveys and the JARPN sighting surveys (Government of Japan, 2002). In
2002, the abundance in the western North Pacific (west of 180o) was estimated as 28,400 animals(CV=0.414) by extrapolating from abundance estimate 4,905 (CV=0.405) from JARPN surveys(1997-2001) (Government of Japan, 2002).
To investigate abundance estimate in the western North Pacific further, dedicated sighting surveys in2002 and 2003 were planned. Because sighting survey conducted during 1997 – 2001 did not cover
the survey area fully, further sighting surveys were conducted in 2002 and 2003 in order to get amore accurate abundance estimate. These sighting surveys were conducted in sub-areas 8 and 9 bydedicated sighting vessels. Abundance was estimated from these surveys and extrapolated using thesame method as Government of Japan (2002).
Figs. 2 and 3 suggest that sei whales distribute to the east of 170 oE as well as in sub-areas 8 and 9.

JSV data were used to extrapolate abundance by JARPNII because a) JSV surveys collected sighting
data consistently for a long period and b) wide areas were covered by the surveys. JSV data cannot
be used to estimate absolute abundance because there is no record of sighting distance data (e.g.
radial distance and angle), but JSV data are useful to estimate relative abundance index (Miyashita etal., 1994). There were some previous studies using JSV data for extrapolation purpose (Butterworth
et al., 1994; Miyashita et al., 2001; Minamikawa et al., 2002).
MATERIALS AND METHODS
Data
JARPNII sighting data
Fig. 1 shows the survey area covered by a dedicated sighting vessel in 2002 and 2003. The figure
shows that effort was allocated uniformly in sub-areas 8 and 9, but there was insufficient effort in
sub-area 7 due to time restriction. These surveys were conducted in August and September.
JSV data
JSV data in the waters comprised between 30oN and 60oN and east of 180o obtained from 1972 to
1988, were used. We did not use JSV data in the Okhotsk Sea and Bering Sea. We used only the data
in August and September because differences of distribution pattern are suggested from JSV data, as
shown in Figs. 2 and 3. Table 1 shows searching distance, number of sighted whales, area in
JARPNII survey and whole of the western North Pacific excluding JARPNII survey area used in thisstudy. The number of sighted whales and searching distance by JSV in 5o square by month and by
year were also used.
Abundance estimation
Abundance was estimated by using DISTANCE (Buckland et al., 1993). The perpendicular distance
distribution is truncated at 2.4 n. miles. Detection function was fitted to hazard-rate function with noadjustment terms;

  x  − b f ( x ) = 1 − exp  −     a  

(1)

where, x is perpendicular distance and a and b are parameters. Abundance estimator isfollowing formula;P=

AE ( s )n2wL(2)where, P is abundance estimate, A is area size of survey area, E(s) is estimated mean school size, n is
the number of primary sightings, w is effective searching half-width and L is searching distance.
Only the sightings for which school size is confirmed are used for estimation. We used the method of
estimation of mean school size described Buckland et al. (1993). Coefficient of Variance (CV) of theabundance is estimated byn
CV ( P) = {CV ( )}2 + {CV ( E ( s ))}2 + {CV ( w) }2
L
2

(3).

Assuming P is log-normally distributed, 95% confidential interval is (P/C, CP), whereC = exp( Z 0.025 log e [1 + {CV ( P)}2 ] )(4)and Z0.025 is 2.5 percentile of standard normal distribution. For further details, see Buckland et al.
(1993), for example.
Extrapolation to western North Pacific
Extrapolated abundance (PE) for western North Pacific of this species, which weighted by theJapanese Scouting Vessel (JSV) data (sighting rate of 5o×5osquare) collected in August and
September during 1972 to 1988 (Fig. 3) were analyzed by the following formula;
PE = P × Σ/ Σ*
where,(5),Σ = ∑ (n / L) i , j Ai , j (sum over all squares in the west of 180oand the north of 30oN)i, jΣ = ∑ (n / L) i , j pi , j Ai , j (sum over only squares for which abundance estimate applied)*i, jn : number of sei whales, L: searching distance n.miles,(n / L) i , j : sighting rate of 5o×5osquare of latitudinal band i and longitudinal band j.
Ai , j : Area size of 5o×5osquare of latitudinal band i and longitudinal band jpi , j : Proportion of area size of surveyed area during JARPNII survey within 5o×5osquare oflatitudinal band i and longitudinal band j
We estimate extrapolation factor (=Σ/Σ*) from JSV data in August and September because sighting
survey was conducted in August and September. CV of extrapolation factor is estimated by
Jackknife method (Gray and Schucany, 1972; Miller, 1974) taking year as a sampling unit.

RESULTS
Abundance estimate in sub-areas 8 and 9
Abundance estimate in sub-areas 8 and 9 is 4,085 (CV=0.281) from 2002-2003 JARPNII dedicatedsurvey (Table 2). Fig. 4 shows detection function for each survey. Comparing the former estimate(Table 4), CV is smaller by about 0.1 and therefore precision is increased. One of the reasons for thisis because effort was designed to be uniformly allocated in the survey area.
Extrapolated abundance
Extrapolated abundance using JSV data in August and September is 67,642 (CV=0.418) (Table 3).
The present estimate for CV of extrapolation factor (0.310) is much higher than the first estimate(0.071), because CV of extrapolation factor is high probably due to less effort in August (22,000 n.
miles) and September (18,000 n. miles) than in June (70,000 n. miles) and July (52,000 n. miles).

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DISCUSSION
Abundance estimate from JARPNII data
Though it may not be able to compare directly two estimates due to the difference of survey period,
the present estimate of 4,085 (CV=0.281) is not significantly different from the first estimate of
4,909 (CV=0.405). The present abundance estimate might be underestimated due to lack of effort in
sub-area 7.
It may be possible that summing abundance estimates in latitudinally different area surveyed indifferent year caused double counting of the population. CV of the abundance estimate may beunderestimated, because additional variance is not taken into account.
Extrapolated abundance
From comparison between Figs. 2 and 3, it was suggested that density of sei whale is lower in
sub-areas 8 and 9 than in to the east of 170oE in August and September, whereas there were similarlevel of sightings of sei whales in sub-areas 8 and 9 as well as to the east of 170oE in June and July.
This may suggest seasonal change in distribution pattern of sei whales.
From Table 1, there were only 2 sightings in sub-areas 8 and 9 according to JSV data, though thesearching distance amounted to 8,000 n. miles. There are two possible interpretations of this. One isthat (1) main distribution area of sei whales shift north or east of sub-areas 8 and 9 and density in
sub-areas 8 and 9 is low in August and September. Another is that (2) when the JSV data wascollected, abundance estimate of 9,000 (IWC, 1975) was lower than that in recent years and thedensity in sub-areas 8 and 9 was low. In contrast, the population has been increasing and density inthis area became higher, recently. In this survey, 103 whales were detected per 5,600 n. miles, but
only 43 whales were detected per 40,000 n.miles according to 1972-88 JSV data. In other words, thepresent density index is 17 times as much as that of JSV. It’s reasonable to think that this change isdue to multiple effects of seasonal migration and increase of the population. Sighting surveys toinvestigate seasonal migration patterns of sei whales and distribution outside of sub-areas 8 and 9 arenecessary. Data from these sighting surveys could improve both the estimate and CV ofextrapolation factor.
The present estimate is more than double of the former estimate of 28,400. There will be threepossible reasons for this result. First, the extrapolation factor is biased due to small size of sightingsin sub-area 8 and 9. Second, seasonal migration affects on the abundance estimate. JSV data support
seasonal migration. Third, the population has been increasing up to now. This is supported bycomparison of JSV data and JARPN data mentioned in above paragraph.
Estimated CV of extrapolation factor using Jackknife method using year as sampling unit may beunderestimated, because the searching effort is more in 1970’s than in 1980’s. In fact, applying
Jackknife method to only the data in 1970’s, estimated CV is 0.441, which is more than theestimated CV of 0.310 using all years. It is necessary to consider analysis method other than
Jackknife method to estimate CV of extrapolation factor more precisely.

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ACKNOWLEGEMENT
We would like to thank all researchers and crew involved in JARPNII surveys. We are very gratefulto Tomio Miyashita (NRIFSF), for kindly allowing the use of JSV data and useful comments toimprove manuscript. We also would like to express appreciations to Hiroshi Hatanaka (ICR),
Hidehiro Kato (NRIFSF), and Seiji Ohsumi (ICR) for valuable comments. We also thank Shoko
Okawa for her assistance.
REFFERENCE
Buckland, S.T., Anderson, D. R., Burham., K. P. and Lakke, J. L. 1993. DISTANCE sampling. Chapman and Hall,
London. 446pp.
Butterworth, D. S., Borchers,.D. L., Chalis, S., De Decker J. B., and Kasamatsu, F. 1994. Estimates of abundance forsouthern hemisphere blue, fin, sei, humpback, sperm, killer and pilot whales from the 1978/79 to 1990/91
IWC/IDCR sighting survey cruise, with extrapolation to the area south of 30oS for the first five species based onJapanese scouting vessel data. Paper SC/46/SH24 presented to the IWC Scientific Committee, 1994 (unpublished).
129pp.
Gray, H. L. and Schucany, W. R. 1972. The generalized Jackkife Statistic. Marcel Dekker. New York. 308pp.
Government of Japan, 2002. Research Plan for Cetacean Studies in the Western North Pacific under Special Permit(JARPN II). Paper SC/54/O2 presented to the IWC Scientific Committee, April 2002 (unpublished). 45pp.
International Whaling Commission (IWC). 1975. 27th Report of the Commission. Report of the Scientific Committee.
52pp
Matsuoka, K., Hakamada, T.,Fujise 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 Scientific Committee
Workshop to Review the Japanese Whale Research Programme under Special Permit for North Pacific Minke
Whales (JARPN), Tokyo, 7-10 February 2000 (unpublished). 17pp.
Miller, R. G. 1974. The Jackkife – a review. Biometrika. 61:1-15.
Minamikawa, S., Miyashita, T. and Okamura, H. 2002. Further analysis on the JSV data of the Antarctic minke whale
using GLMs. Paper SC/54/IA12 presented to the IWC Scientific Committee, April 2002. 11pp.
Miyashita, T. and Shigemune, H. and Kato, H. 1994. Outline of the sighting strategy of scouting vessels attached toJapanese whaling fleets. Rep. int. Whal. Commn. 44:273-6.
Miyashita, T., Kato, H. and Kasuya, T. (eds.). 1995. Worldwide Map of Cetacean Distribution Based on Japanese
Sighting Data. National Research Institute of Far Seas Fisheries, Shimizu, Japan. 140pp.
Miyashita, T., Shono, H. and Okamura,.H. 2001. GLM analysis of the JSV data for the Antarctic minke whale. Paper
SC/53/IA11 presented to IWC Scientific Committee, June 2001. 6pp.

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Table 1. Searching distance and the numbers of sighted whales from JARPNII area and JSV data
used in this study.

the number of searching the number of area of survey
region5ox5osquare*1 distance sighted whales

survey(n.miles2)(n. miles)
JARPNII
JSV (JARPNII survey
area)
JSV (outside of
JARPNII survey area)

13

5,587

103*2

671,429*3

13

8,003

2

671,429*3

23

31,58641

1,790,350*4*1 If only a part of the square was surveyed, we counted it.
*2 the number of sighted schools multiplied by estimated mean school size
*3 excluding Russian EEZ*4 excluding Okhotsk Sea and Bering Sea

Table 2. Sei whale abundance in sub-areas 8 and 9 from JARPNⅡ2002-2003 (in August and
September)sub-area
8 + 9S
9N
Total

season
2002
2003

Area
534,449
136,980
671,429nL
36 3054.5
24 2532.9
60 5587.4

n/L
0.012
0.009
0.011

CV
ESW
0.279 1.563
0.575 1.352

CV E(s)0.101 1.72
0.390 1.28

CVD
0.075 0.0065
0.086 0.0045
0.0061

P
3,470
616
4,085

CV
0.306
0.700
0.281

Table 3. Estimate of extrapolated abundance using JSV data to western North Pacific (WNP)(north of 35oN and west of 180o)

sub-areas 8 and 9
western North PacificEstimate
4,085
67,642

CV
0.281
0.418

Table 4. Sei whale abundance estimate in Government of Japan (2002）
Year

Month sub-area

1997+ June+
2001 July

7,8,9

area size sub-area(n.miles2) coverageL(n.miles)

431,157

12,530

49.8%

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n

n/L

esw
mean
abundanceExtrapolated(n.miles) school size
abundanceby JSV data
122 0.010
0.913
1.38
4,909
28,400(0.317) (0.159)(0.045)(0.405)(0.414)

Fig.1. Research area, searching effort and primary sighting positions of sei whales in August and
September of 2002 and 2003 JARPNII surveys.

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Fig. 2. Density indices (whales per 10,000 n. miles) of sei whales from JSV data in 1972-1988 inJune and July. Bold lines indicate boundary of the 2002-2003 JARPNII survey area

Fig. 3. Density indices (whales per 10,000 n. miles) of sei whales from JSV data in 1972-1988 inAugust and September. Bold lines indicate boundary of the 2002-2003 JARPNII survey area.

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Fig. 4. Detection function of sei whales estimated from JARPNII 2002 and2003（upper panel：2002,
lower panel：2003）.

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