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SC/O05/BWI5

Estimation of pregnancy rate of the western North PacificBryde’s whale
TAKEHARU BANDO, TAKASHI HAKAMADA AND SEIJI OHSUMIThe Institute of Cetacean Research, 4-5, Toyomi-cho, Chuo-ku, Tokyo, 104-0055, Japan.
Contact e-mail: bando@cetacean.jp
ABSTRACTThe pregnancy rate and some behaviors related to the reproduction are examined for the western North
Pacific Bryde’s whale stock using biological data collected by the Japanese Whale Research Program
under Special Permit in the North Pacific (JARPN II) in 2000-2005. The Bryde’s whales appear to have along breeding season. The body length at birth is estimated at 384 cm, and the body length at weaning at
about 860 cm. The pregnancy rate of this whale stock is estimated at 55-59 %, and lactation rate of
mature females at 29-32 % in the feeding ground. The breeding cycle of this stock is estimated at 1.7-1.8
years.
KEYWORDS: BODY LENGTH AT WEANING, BREEDING CYCLE, BRYDE’S WHALE,LACTATION RATE IN FEEDING GROUND, PREGNANCY RATE, WESTERN NORTH PACIFICINTRODUCTION
IWC/SC is now developing the IST of the RMP for the western North Pacific Bryde’s whale, and aworkshop was held for completing the pre-implementation assessment in Tokyo in March, 2005 (IWC,2005). At that workshop it was suggested to prepare the data on the breeding cycle and range of MSYR
for the future work (IWC, 2005). For this objective it is necessary first to obtain the value of pregnancyrate.
One of the methods to estimate the breeding cycle and birth rate is by mean of the long time observationof identified individuals. However this kind of non-lethal method is not practical because many baleen
whales have large population size, are pelagic, and exhibit large scale migration. Then lethal method
involving the examinations of internal tissues are more adequate to estimate those biological parameters.
It is practically difficult to estimate a true pregnancy rate for above kinds of baleen whales, without the
conduct of research for all the year round. As pointed out by Omura（1962）、Ohsumi (1977)、Kato andYoshioka (1993; 1995), the long breeding season of the Bryde’s whale makes difficult to estimate the true
pregnancy rate for this species.
There are several papers on the pregnancy rate of the Bryde’s whale based on the analyses ofcommercial whaling data （Omura, 1962; Ohsumi, 1977; Yoshioka, 1989; Kato and Yoshioka, 1993；
1995）. However, these data come from the whaling operation in which various regulation measures wereforced. It has passed about 20 years after the moratorium of commercial whaling, and it will be needed toestimate recent figure on the pregnancy, as the population condition may have changed during the years.
Japan has been conducting the Japanese Whale Research Program under Special Permit in the North
Pacific (JARPN) under Article VIII of the International Convention for Regulation of Whaling since 1994,
and the collection of samples of the Bryde’s whale started in 2000 as the second stage of the research
program （JARPN II）.
The objective of this paper is the estimation of the pregnancy rate of the western North Pacific Bryde’s

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whale by using data collected by JARPN II.
MATERIALS AND METHODS
Materials
Among 293 Bryde’s whales collected by JARPN II during years 2000-2005, 113 sexually mature females
and 9 suckling calves are used for the analysis. Samples were collected during seasons from late May to
middle September, in the area comprised between 35°- 42°N, 143°-170°E.
Fifty Bryde’s whales have been sampled annually in the JARPN II. Cow and calf pairs were collected
in 2000 and 2001, but since 2002 cow calf pairs have not been targeted for sampling. Therefore, it will beneeded to separate the period 2000 and 2001 from 2002 onward for the examination of the pregnancy
rate.
Various kinds of measurements and collection of samples from whales caught by the sighting/sampling
vessels (SSVs) are carried out on board of a research base ship. In this paper the following data were used
for the analysis: date of catch, sex, corpora lutea and albicantia, lactation, sex and body length of fetus.
Methods
The sexual maturity of females is determined by the existence of corpus luteum or corpus albicans ineither side of the ovaries. An individual which has a corpus luteum in the ovaries but no fetus is found in
the uterus is classified as an ovulating animal. However, this animal is regarded as an early pregnant
whale, considering the breeding behavior of this whale species. A pregnant whale is a female with a fetus
in its uterus. A lactating whale is a female in which milk is found in the mammary gland. Pregnancy rateis defined as ratio of pregnant or ovulating whales in mature female.
RESULTS
Growth of fetuses and suckling calvesFig. 1 shows the relationship between dates and body length of fetuses and suckling calves. In this figure,the dates of suckling calves are set at one year later, because they are regarded to be pregnant in theprevious year.
The maximum body length of fetuses is 376cm, and the minimum body length of sucking was 670cm.
The present samples are collected from the feeding ground, and a new born calf has not been reported yet,but the body length at birth will be between these two lengths. On this regard, Ohsumi (1966) found theinterspecies relationship between the body length at sexual maturity of the female and body length at birth.
As Kato and Yoshioka (1995) estimated the body length at sexual maturity of female Bryde’s whale fromthe western North Pacific at 11.6-11.8 m, the body length at birth of this whale is calculated at 384cm by
the formula of Ohsumi (1966). This figure is consistent with the actual figures above.
Furthermore, Fig. 1 indicates that the pregnant whales migrate to feeding ground just after the mating in
the breeding ground and stay in the feeding ground until just before the calves are born.
Considering the fact that it is difficult to find embryo just after the mating, Fig. 1 shows ovulating
whales as a whale which has o cm long fetus. As some ovulating females are found in middle
September, it can be regarded that the breeding season of the western North Pacific Bryde’s whale is long,and it lasts from winter to the beginning of autumn.
It is estimated from Fig. 1 that the delivery season occurs from August of a year to May of the next year,
and its peak is in December. And, from this figure it is estimated that there are some whales which
fertilized in the last year in the research season. Although it is difficult to estimate pregnant period fromthis figure, it is estimated to be 11-12 months by use of estimated body length at birth and pattern ofseasonal distribution of fetal length.
There are lactating whales which are simultaneously pregnant. Comparing these whales with usuallypregnant whales, there is no difference in the fertilization date each other. This indicates that there are

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some whales which become pregnant just after the delivery in the breeding season.
The smallest calf with its cow was 676 cm, and there is relatively large gap from the body length atdelivery, although breeding season is long. This may indicate that cow and calf pairs stay in thebreeding ground for a while after delivery before moving to the feeding ground.
It has been recognized that the weaning body length of calves is about two times of the body length atbirth in cetaceans. However, the body lengths of two suckling calves were 854 and 857 cm, respectively,
among 9 cow and calf pairs. This result indicates that the weaning body length of the Bryde’s whale islarger than two times (770 cm) of the body length at birth, and it has a possibility to be around 860 cm.
Furthermore, from Fig. 1, the lactation period of the Bryde’s whale is estimated to be at least 7 months
which is longer than 6 month as considered previously.
Compositions of reproductive conditions of adult females collected
Lactating females (L) which were collected by JARPN II are classified into four categories by theexistence of calves (C) and pregnancy (P). They are lactating simultaneously pregnant with calf
(L+C+P), lactating simultaneously pregnant (L-C+P), lactating with calf (L+C-P), and lactating without
calf (L-C-P).
As mentioned in the section of Materials, cow and calf pairs were sampled in 2000 and 2001, and suchkind of sampling has not carried out since 2002. Therefore, it will be needed to separate two periods forthe examination in this paper. Table 1 shows the composition of adult females by sexual conditions. In
this Table, P indicates pregnant, O indicates ovulating, L indicates lactating, C indicates calf, and R means
resting.
The apparent pregnancy rate in the later period (70%) is higher than that in the former period (59 %).
It can be estimated that the pregnancy rate in former period indicates the un-biased value, because norestriction was applied to sampling of cow and calf pairs. On the other hand, the higher figure in later
period is caused by the exclusion of catch of cow and calf pairs, and then it is needed to correct thefigures to estimate un-biased pregnancy rate.
For this purpose two kinds of correction method are used here. One (method 1) is the value of P in
2000 and 2001 is set as the standard of pregnancy rate, and the figures of each reproductive condition in
2002-2005 is corrected by proportional distribution (51 %/63 %), then the figure of (L+C-P) is obtained
as the difference of accumulation of each condition from 100 %. The result is shown in Table 1.
Another method (method 2) is the correction of pregnancy rate in 2002-05 based on the figure of (L+C-P)
to total figure in 2000-01. The corrected figures of each reproductive condition are shown also in Table
1. Thus, comparing the pregnancy rates（P+O+(L+C+P)+(L-C+P)）after correction, it is 59% in the former
period and 57 % or 55 % in the later period. They are almost the same each other. Furthermore, when
the lactation ratios（（L+C+P）+(L-C+P)+(L+C-P)＋(L-C-P)）are calculated, the former is 30% and thelatter is 29 % or 32 %, respectively. They are also close each other.
Pregnancy rate and breeding cycle
Therefore, it will be concluded that the pregnancy rate in the research area is 55-59% recently. However,as it can be considered that the pregnant whales which are just after fertilization or just before the delivery
may leave the feeding ground, the true pregnancy rate may be higher than these figures. On the contrary,some cow and calf pairs may remain around the breeding ground, and this may be a factor to reduce theapparent figure of pregnancy rate. Considering these phenomena which are possible, the above figures(55-59 %) will be close to the true pregnancy rate compensating both factors.
As the breeding cycle is a reciprocal number of the annual pregnancy rate, it is calculated to be 1.7-1.8
years for the western North Pacific Bryde’s whale.
DISCUSSION

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A biological characteristic of the western North Pacific Bryde’s whale in the estimation of pregnancy rateis that the breeding lasts for long season. As shown in Fig. 1, this whale stock has a possibility to breed
though the year, although the peak is estimated to be in winter. Therefore, it will be ideal to conduct
research in all life ranges and seasons. However, the research period in this paper has been from late
May to middle September and the research area has been north of 35 N and west of 170 E.
On the other hand, we were able to collect valuable research data, which could not be obtained by thecommercial whaling, in 2000 and 2001. It was the sampling of cow and calf pairs. As the sampling ofsuch pairs has not carried out after 2002, the composition of sexual conditions has been changed. As theresult, the apparent pregnancy rate increased to 70 % after 2002 from 59 % in 2000 and 2001. A figure ofpregnancy rate of 76.5 % which was reported by Yoshioka (1989) was the result that the cow and calf
pairs were prohibited to be caught in the commercial whaling by the IWC, and the apparent pregnancy
rate which is obtained by commercial whaling has a possibility to be higher than true value is shown in
the present study.
Some papers report on the pregnancy rate of the western North Pacific Bryde’s whale. Omura (1962)’s
figure of 42.9 % comes from small number (28 whales). Ohsumi (1977a) reports a figure of 41.7 % is
excluded the ovulating animals, and when the ovulation rate (5.1 %) is included in the pregnancy rate, therevised pregnancy rate becomes 46.8 %. Yoshioka (1989) reported that apparent pregnancy rate of
76.5 %. Furthermore, Kato and Yoshioka (1993) describe that the pregnancy rate is most likely about
60 % for the western North Pacific Bryde’ whale. This figure is close to the present result.
ACKNOWLEDGEMENTS
We would like to express our sincere thanks to Dr. Hiroshi Hatanaka of the Director General of ourInstitute to his kind comments on our draft paper.
We express also our thanks to researchers and crew who were engaged in the research of JARPN II.
The data which were used for the present study were collected and analyzed by their endeavor.
LITERATURES CITED
IWC.2005. Report of the Workshop on the pre-implementation assessment of western North PacificBryde’s whales, Tokyo, Japan, 21-24, March 2005. Paper SC/57/REP3 presented to the IWC
Scientific Committee, May, 2005 (unpublished). 30ｐｐ.
Kato, H. and Yoshioka, M. 1993. Biological parameters and their real and temporal variations of Bryde's
whales in the North Pacific, with special reference to stock identification. Paper SC/45/O8 presented
to the IWC Scientific Committee, May, 1993 (unpublished). 30ｐｐ.
Kato, H. and Yoshioka, M. 1995．Biological parameters and morphology of Bryde’s whales in thewestern North Pacific, with reference to stock identification. Paper SC/47/NP11 presented to the
IWC Scientific Committee, May, 1995 (unpublished). 19pp.
Omura, H. 1962. Further information on Bryde’s whale from the coast of Japan. Sci. Rep. Whales Res.
Inst. 16: 7-18.
Ohsumi, S. 1966. Allomorphosis between body length at sexual maturity and body length at birth incetaceans. J. Mamm. Soc. Japan 3(1): 3-7.
Ohsumi, S. 1977. Bryde’s whales in the pelagic whaling ground of the North Pacific. Rep. int. Whale.
Commn (Spec. Issue 1): 140-150.
Yoshioka, M. 1989. Bryde’s whales taken by the Japanese land based whaling in the Bonin Island waters.
Rep. Int. Whal. Commn 39: 462.

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Table 1. Comparison of reproductive conditions of mature female Bryde's whales which were collected by
JARPN II.
Sample size
2000-2001 2002-2005
Preg.
Ovu.
Preg. & Lact. & calf
Preg. & Lact. & no calf
Lact. & calf
Lact. & no calf
Rest.
Total
Preg. Rate (% )

19
2
0
1
8
2
5
37

48
4
0
1
0
9
14
76

Ratio (%)
2000-2001 2002-2005 Correction of B Correction of B
(A)(B)
by method 1
by method 2
51.4
63.2
51.4
49.5
5.4
5.3
4.3
4.1
0.0
0.0
0.0
0.0
2.7
1.3
1.1
1.0
21.6
0.0
18.7
21.6
5.4
11.8
9.6
9.3
13.5
18.4
15.0
14.4
100.0
100.0
100.0
100.0
59.46
69.74
56.70
54.66

1000.0Length of fetus/calf (cm)

Fetus
800.0

Calf
Ovulating

600.0

400.0

200.0

0.0
0

30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570
Catch date (days from 3/31)

Fig. 1. Growth of fetuses and suckling calves of the western North Pacific Bryde’s whale. Open circle:
fetus, Closed circle: Calf, Cross: Ovulating, Dotted line: Length at birth (384cm).

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