EFFECT OF EXPLOITATION ON REPRODUCTIVE
PARAMETERS OF THE SPOTTED AND STRIPEDDOLPHINS OFF THE PACIFIC COAST OF JAPAN*

TOSHIO KASUYA**ABSTRACTAge composition and reproductive status of samples from two speciesof dolphins taken in the Japanese drive fishery are analyzed. A slightdecline in the minimum age at attainment of sexual maturity was noted infemales of the lightly exploited spotted dolphin. Females of the more
intensely exploited striped dolphin underwent a decline in the mean age
at sexual maturation from 9.7 years (1956 cohort) to 7.2 years (1970
cohort), accompanied by a decline of the minimum age at maturity.
A more tentative finding on the species is the shortening of the mean
reproductive cycle from 4.00 years (1955) to 2.76 years (1977). This is adirect result primarily of the probable change in the lactation period but
also of the change in the resting period and in the proportion of lactating
females simultaneously pregnant. Exploitation and response of the population has probably changed the age composition in the striped dolphin
more than that of the spotted dolphin.
INTRODUCTION

The Japanese dolphin fishery was widely distributed along the coast of Japanin the last century but has been limited in recent years to Taiji (driving), lzu
coast (driving, Fig. 1), Choshi in Chiba Prefecture (hand harpoon), Okinawa(driving and hand harpoon), Tsushima and Goto Islands off northern
Kyushu (driving) and Sanriku coast (hand harpoon) (Bureau of Fisheries,
1911; Wilke, Taniwaki and Kuroda, 1953; Ohsumi, 1972; Miyazaki, Kasuya
and Nishiwaki, 1974; Kasuya, 1978; 1982; in press; Miyazaki, 1980; 1983).
Although the hand harpoon fisheries can take dolphins of most species, large
catches of Stenella are limited to the first two locations. Details of the recent
geographical distribution of the catch were reviewed by Miyazaki (1983).
Driving or netting of the striped dolphin, Stenella coeruleoalba (Meyen,* This paper was presented at the symposium on reproduction of cetaceans held at La Jolla in 1981.
** Ocean Research Institute, University of Tokyo, Minamidai, Nakanoku, Tokyo 164, Japan.
Present address: Far Seas Fisheries Research Laboratory, 5-7-1, Orido, Shimizu-shi, Shizuoka 424,
Japan.

Sci. Rep. Whales Res. Inst.,

No. 36, 1985, 107-138

108

KASUYA

1833), has been carried out along the lzu coast since the late 19th century(Fig. 1) and at Taiji (33°36'N, 135°56'E) on the Kii Peninsula only in recent
years. The stock of the species has declined since World War II (Kasuya,
l 976a; Kasuya and Miyazaki, 1982).
The drive fishery for spotted dolphins, S. attenuata (Gray, 1846), wasstarted at Arari on the Izu coast in 1959 (Nishiwaki, Nakajima and Kamiya,1965). Although small number of this species must have been taken withhand harpoon elsewhere (e.g. Taiji), the catches have been relatively small
compared with that of the striped dolphin (Miyazaki et al., 1974; Miyazaki,1983) (Catch of this species at Arari given by Miyazaki et al. (1974) erroneously
included "Hasunaga" which is now known to be the bottlenose dolphin,
Tursiops truncatus (Kasuya, l 976b).)This study analyzes recent changes in life history parameters of the
striped dolphin off the Pacific coast of Japan in comparison with those of theless intensely exploited population of spotted dolphins in the same area.
MATERIALS AND METHODS

Materials
The data are based on schools driven at Kawana and Futo on the lzu coast
and Taiji on the Kii Peninsula in the period 1952 to 1980 inclusive. Until
recently all the dolphin schools driven by the fishery and examined by any
biologist were numbered serially by species. Among these schools only thoselisted in Table 2 were examined by myself; these constituted the main body ofthe present material (other schools were not directly accessible to me). These
were 13 schools of striped dolphins driven on the Izu coast and 13 schools of
TABLE 1.
Species

MATERIALS USED IN THIS STUDY
Striped dolphin

A.

Main data, school
analyses

3,000 individuals in
13 schools caught in
1967-1980

B.

Ancillary age data

110 individuals in
6 schools caught in
1960-1967

C.

Ancillary data on
neonatal sex ratio(not included in item A)

287 individuals in
16 schools caught in
1952-1970

D.

Published data (Kasuya
and Miyazaki, 1982, someincluded in item A)

Data on pregnancy
rates 1952 to 1972

Spotted dolphin
1,319 individuals in
13 schools caught in
1970-1980

22 individuals ina school caught in1975 (school no. 8)

Sci. Rep. Whales Res. Inst.,
No. 36, 1985

109

REPRODUCTION IN STENELLA

spotted dolphins driven on the Izu coast or at Taiji (A in Table 1; Table 2).
With the exceptions of the schools examined by Dr N. Miyazaki in the 1970 to1973 seasons (included in Din Table 1, used in analysis of annual change ofpregnancy rate), schools examined by biologists tended to be relatively larger
schools for which processing started or continued on the day following thedrive. The processing and examination of dolphins in these schools weredone as follows. In the fishing harbour, a group of fishermen netted a part ofthe dolphin school and landed them (10 to 30 individuals at one time, alive or
TABLE 2.
School
no.

Place
of
driving

DETAILS OF SCHOOL CLASSIFIED IN ITEM A IN TABLE I
Dateof
driving

Dateof datacollection

Number of individuals
caught examined aged

7
8
9
10
12
22
38
43
44
45
46
47
48
49

Striped dolphin
26, Nov., '67
Kawana15, Nov., '68
Fu toKawana16, Nov., '68
18, Nov., '68
Kawana12, Oct., '70
Fu toKawana14, Dec., '71
17, Nov., '72
Kawana15, Nov., '73
KawanaKawana20, Nov., '73
Kawana22, Nov., '75
Kawana23, Nov., '75
3, Nov., '76
Fu to24, Nov., '77
Kawana27, Nov., '80
Kawana28, Nov.
17-18, Nov.
16-17, Nov.
18, Nov.
14, Oct.
15, Dec.
20, Nov.
19, Nov.
22, Nov.
24-25, Nov.
24-25, Nov.
4,Nov.
25, Nov.
28, Nov.

3300
1680
322
600
330
903
411
414
1724

150
372
322
21
84
306
201
249
466

153
225
222

1000

399

362

735
795
431

161
250
140

147
248

I
2
3
4
5
6
7
8
9
10
12
13
14
15

Spotted dolphin
Kawana23, Oct., '70
10, Nov., '70
Kawana4, Nov., '72
Kawana13, Nov., '72
KawanaFu to
16, Nov., '72
Taiji
12, Feb., '73
Taiji
2,Jul., '73
Taiji
24, Jan., '75
16, Jan., '76
Taiji
Kawana19, Oct., '76
25, Nov., '77
Fu to13, Dec., '78
Fu to18, Nov., '80
Kawana20, Nov., '80
Kawana25, Oct.
15, Nov.
4,Nov.
13, Nov.
18, Nov.
12, Feb.
2,Jul.
24-25,Jan.
17, Jan.
19, Oct.
26-27, Oct.
14-15, Dec.
18, Nov.
21, Nov.

264
1381
189
192
67
146
60
102
91
377
344
756
308
750

131
272
46
117
23
122
35
102
43
45
119
166
100
100

52
52
38
109
23
115
21
0
6
44
118
151

1) Aged using dentine layers: individuals over 11 layers excluded.
2) Age determination not completed.

Sci. Rep. Whales Res. Inst.,
No. 36, 1985

Remarks

QI)

24 1)
104 1)
041)
280 1)

Q2)

Q2)
Q2)

Item C in Table I

Two schools mixed
after driving

Half of the school
was driven.

Item C in Table I

110

KASUYAdead). Another group of fishermen killed the landed animals if necessary andremoved the viscera. Biologists (usually a group of three or four persons)working among the latter group of fishermen collected the required data andsamples. As soon as that group of dolphins was handled, the next group waslanded. From 500 to 1,000 individuals were processed in a day. We tried not
to bias our sample by choosing animals of any particular growth or reproductive stages; this was helped by the need to process animals very quickly. The
dolphins in the harbour were completely mixed by the repeated operation,and no difference of composition was detected between landings from one
school (confirmed in two days of operations on School no. 8). Therefore I
believe that the sample was representative of the schools examined.
Another small body of data on the striped dolphin was collected on theIzu coast by other biologists in the period not covered by the above data;
these were aged by myself and used. in an analysis of recent change in age atattainment of sexual maturity (B in Table 1).
The estimation of neonatal sex ratio is based on 287 striped and 22
spotted dolphins Uuveniles) examined on the Izu coast by various biologists in
1952 to 197 5 (C in Table 1) in addition to those mentioned above (A in Table1). Pregnancy rate data on the striped dolphin in Table 3 of Kasuya andMiyazaki (1982) were combined with those collected more recently and used
for the analysis of year-to-year change in pregnancy rate.
Methods
Age was determined by myself using growth layers in dentine and/or cementum. Cemental counts were used only for individuals with closed pulp
cavities. The method of preparation and reading is described in Kasuya(l 976a) and Perrin and Myrick (1980). The deposition rate of these layers wasshown to be annual for spotted dolphin and assumed to be annual for the
striped dolphin based on the similarity of life history in the two species(Kasuya, l 976a). Ages betwen n and n + 1 years are represented by n +0.5
years (n being an integer).
Sexual maturity of females was determined by the presence of a corpus
luteum or albicans in the ovaries.
Reproductive status of female dolphins was classified into four stages:
resting, pregnant, pregnant and simultaneously lactating, and lactating. The
mean length of the reproductive cycle was calculated from the proportion of
females at each stage, with necessary corrections for sample bias (see REPRODUCTIVE CYCLE) and assuming a gestation time of 12 months for the striped
dolphin (Kasuya, 1972. Although Miyazaki (1984) gave estimation of 13.4
months, the difference is not important because the present study analyzes
the trend rather than the absolute values of the reproductive cycle) and 11.24
months for the spotted dolphin (Kasuya, Miyazaki and Dawbin, 1974). The
simultaneously pregnant and lactating class was treated as representing an independent phase, and its mean period was estimated as for the other phases.
Sci. Rep. Whales Res. Inst.,
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REPRODUCTION IN STENELLA

111

Pregnancy was determined in 10 schools (nos 7, 8, 9, 43 through 49) by
the presence of fetus. However, nonpregnant females of the striped dolphin
having a corpus luteum were recorded as pregnant in other schools, forwhich ovaries were not examined by myself. This overestimates the pregnancy
rate. Therefore these data were adjusted, using the proportion of nonpregnant females having a corpus luteum of ovulation in the former schools.
Females having more than one corpus luteum of ovulation were rare, andthey were dealt as those having one.
Change of fishing operation and scope of samples
Kasuya and Miyazaki (1982) briefly described historical changes in the dolphin fishery along the Izu coast, where most of the present samples wereobtained.
Enoshima was once known by the dolphin product (Matsuse, ea 1645,
cited by Hawley, 1960, p.181). This is the oldest record of the possible dolphin fishery on the coast of Sagami Bay available to me (Fig. 1).
Kawana village, one of the two places now operating a dolphin fisheryon the Izu coast, is known to have established a hunting team in 1888 (recorded on a stone monument at Kawana built in 1922). Another record(Bureau of Fisheries, 1911) shows, however, that the fishery was common in
the area in those days and the dolphin species hunted were the Pacific whitesided dolphin, Lagenorhynchus obliquidens Gill, 1865, (which is not now hunted
because of difficulties in driving it), the short-finned pilot whale, Globicephalamacrorhynchus Gray, 1846, and the striped dolphin. The main changes in thefisheries have been expansion of the maximum operational distance, throughimprovement of fishing vessels, from coastal waters (a few nautical miles
offshore), to Oshima Island (20 nm), and then to occasional searching up to
50 nm offshore, and a decrease in the number of driving groups (teams offishermen) on the coast (Table 3). The effect of such changes in the operationon the present study was ignored, because (1) the operation had expanded
to the waters adjacent to Oshima Island (20 nm) before the collection of thepresent materials started in 1952, (2) drives from such a far distance as 50 nm
seemed have not been recorded, and (3) all the present samples were obtained
from schools taken in Sagami Bay or within a few nautical miles of OshimaIsland (25 nm from port, Fig. 1).
At Taiji, the catch of these species was usually below 1,000 individuals
per year (Miyazaki et al., 1974). It increased after the establishment of a driveing team in 1973 (Miyazaki, 1980; 1983) and a second team in 1980. Only
four schools of spotted dolphins were examined at Taiji for the present study(Table 2). Although there is no direct evidence, the spotted dolphins caught
in the two locations are assumed to belong to one population, following theconclusion of Miyazaki et al. ( 197 4), deduced from comparison of seasonalchanges in oceanographical conditions and the distribution of the dolphins.
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

112

KASUYA

Sagami Bay

Fig. 1. Map of the area for the dolphin fishery off lzu coast and adjacent area.
Thick solid line indicates coastline, and thin solid line the 200 m depth. Ten locations of previous operation (open circle) and two places of present operation(closed circle) are indicated. Types of the fishery were driving (Arari, Futo,
Heda, Inatori, Ito, Kawana, Nishiura, Uchiura, Ugusu and Tago), dragnet(Hiratsuka), and unrecorded (Enoshima). (Bureau of Fish., 1911; Hawley, 1960;
Kasuya, unpub.)Sci. Rep. Whales Res. Inst.,
No. 36, 1985

113

REPRODUCTION IN STENELLATABLE 3.

CHRONOLOGY OF THE BIOLOGICAL DATA AND DETAILS OF THE FISHERIESFOR STRIPED AND SPOTTED DOLPHINS IN JAPANESE WATERS

year
1911
1920's1945-'50

Dolphin fishery
Common on the Izu coast.
Expansion of fishing groundto Oshima I. (20 nm) withintroduction of motor vessels.
Postwar expansion of catchat Arari, Futo, Inatori* and Kawana.

1952

1962

1968

First driving of spotted dolphin.
Last significant catch (> 100/yr)
at Arari on Izu coast. Fisherycontinues at Kawana and Futo.
Expansion of searching area close
to Miyake I. (50 nm) with theintroduction of high-speed boats.
Last two driving teams on Izu
coast joined in operation.

1970
1973
1980
1982

Births of oldest dolphins in the present
sample.

Start of monitoring of pregnancy rate,
striped dolphin.
First cohort examined for annualchange of maturation age.

1956
1959
1961

Biological data

Start of driving fishery for
Stenella at Taiji.
Establish of 2nd driving teamatTaiji.
Two teams at Taiji joined and set
catch limits by species, which were
not reached by following operations.

Last cohort examined for annualchange of maturation age.
Start of monitoring of pregnancy rate,
spotted dolphin.
Last data of present study.

* Inatori ceased operation between 1945 and 1961.

SCHOOL VARIATION

Fig. 2 shows the age frequency distribution for the eight striped dolphinschools analyzed in this study. Four schools (nos 7, 8, 9 and 12) were excluded
from Fig. 2. These were breeding schools containing old individuals, but age
for these was determined using only dentinal layers and individual ages over
11 growth layers (presumably years) are often unreliable (Kasuya, l 976a).
Data from School no. 22 were for 62 immature females below eight years and
218 males below 13 years (mostly immature); this is considered to be an immature school as described by Miyazaki and Nishiwaki (1978), who listed
more examples of this types of school. Six other schools contained immature
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

10
School 43
90 females

135 mates

~

10

"'

::J
"O 20

·;;:

"O 30

c

0 10

School 44

~

QI

..0

E
::J

z

10

10

10

20
School 47
31 females
116 males

10

20
School 48

10

180 female

68 males

10
20

10

20

30

Age

in years

40

50

60

Fig. 2. Age composition of some of the striped dolphin schools used in this study.
Only schools with large sample size and listed in item A in Table 1 are indicated.

Sci. Rep. Whales Res. Inst.,
No. 36, 1985

115

REPRODUCTION IN STENELLA

and mature individuals of both sexes and were considered to be breeding
schools. These latter schools show peaks and troughs at various ages below 10
years. This, together with the presence of immature schools, suggests that agroup of immature individuals in a breeding school may occasionally segregate to form an immature school (Kasuya, 1972; Miyazaki and Nishiwaki,1978). This can cause an important bias in the estimates of age frequency andsex ratio of immature individuals.
Fig. 3 shows the age composition of 10 spotted dolphin schools (excluding School 6 that is represented by a small sample). Although they do not include immature schools as observed in the striped dolphin, the presence oftroughs at ages below 10 years and the lack of dolphins at around five years ofage (ages between weaning and attainment of sexual maturity, see AGE ATSEXUAL MATURITY) in the total age frequency (Fig. 9) suggest the possibility
that juveniles of both sexes are segregating from these breeding schools(Kasuya et al., 1974).
Postnatal sex ratios in the schools of striped and spotted dolphins areshown in Table 4. The percentage of females in the sample varies from 22 to73% (x = 50.5%, SD = 16.4) in the striped dolphin, and from 44 to 66% (x =57.5%, SD = 6.9) in the spotted dolphin, with smaller between-school variation in the latter species.
Similar analyses were carried out on proportions of sexually mature

TABLE 4.

Schoolno.

SCHOOL VARIATION IN POSTNATAL SEX RATIO

Striped dolphin (1967-'80)
Males
Femalesno.
no.
%Schoolno.

Spotted dolphin (1970-'80)
Males
Females
no.
no.
%

7
8
9
12
22
38
43
44
45/46
47
48
49

51
134
182
35
235
73
146
256
206
126
68
57

99
238
140
49
71
128
103
210
193
35
182
83

66.0
63.9
43.4
58.3
23.2
63.7
41.3
45.0
48.3
21.7
72.8
59.2

1
2
3
4
5
6
7
9
10
12
13
14
15

50
108
23
58
8
50
17
15
19
40
63
45
56

81
164
23
59
15
72
18
28
26
79
103
55
44

61.8
60.2
50.0
50.4
65.2
59.0
51.4
65.l
57.7
66.3
62.0
55.0
44.0

Total
Mean
SD
CV

1,569

1,531

49.4
50.5
16.4
0.3

Total
Mean
SD
CV

552

767

58.2
57.5
6.9
0.1

Sci. Rep. Whales Res. Inst.,
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116

KASUYA!;;! g.~"':~hool :, 33 females

0

, ,
0

°':19 males

I.

School 2 30 females
22 males

(/)

Ill 5
::J
0

"O

> 5

"O

c

-

io females
11 males0

School 10
n
n

•

I...

Q.I

..0

E
::J

z

25 females
19 males78 females

5040 males5
10

95 females

056 males

10

0

10

20

Age in

30

40

so

years

Fig. 3. Age compositions of some of the spotted dolphin schools used in this study.
Only schools with large sample size and listed in item A in Table 1 are indicated.

females and reproductive status of adult females (Tables 5 and 6). The
between-school variation of these parameters was consistently greater in the
striped dolphin, even accounting for differences in school-size frequencies.
This means that the between-school variation of these parameters is greaterin the striped dolphin schools and suggests that larger sample size may berequired for the striped dolphins in order to estimate structure of the population migrating to the fishing ground.
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

117

REPRODUCTION IN STENELLATABLE 5.

Schoolno.

SCHOOL VARIATION IN FEMALE MATURITY RATE

Striped dolphin (1967-'80)
Immature
Matureno.
no.
%

Schoolno.

Spotted dolphin (1970-'80)
Immature
Mature
no.
no.
%

7
8
9
12
22
38
43
44
45/46
47
48
49

46
41
113
15
71
23
38
66
69
22
53
31

51
195
20
33
0
103
63
143
115
10
129
52

52.5
82.6
15.0
68.7
0.0
81.7
62.3
68.4
62.5
31.2
70.8
62.6

1
2
3
4
5
6
7
9
10
12
13
14
15

24
67
5
15
5
13
3
10
8
22
40
21
13

48
87
18
42
10
56
15
18
18
56
62
31
30

66.6
56.4
78.2
73.6
66.6
81.1
83.3
64.2
69.2
71.7
60.7
59.6
69.7

Total
Mean
SD
CV

588

914

60.8
54.8
26.0
0.5

Total
Mean
SD
CV

246

491

66.6
69.3
8.2
0.1

TABLE 6.

School no.

SCHOOL VARIATION IN FEMALE REPRODUCTIVE STATUS
Spotted dolphin (1970-'80)
PL
Lact
Preg

Striped dolphin (1967-'80)
Preg
PL
Lact

Rest

School no.
1
2
3
4
5
6
7
9
10
12
13
14
15

15
17
2
3
4
4
2
1
3
11
13
9

1
5
2

Total, no.
Total,%
Mean,%
SD,%
CV,%

85
19.1
19.5
13.7
0.7

16
3.6
3.0
3.2
1.1

7
8
9
12
22
38
43
44
45/46
47
48
49

2
78
0
7
0
79
20
58
34
2
29
19

0
2
0
0
0
1
0
21
0
2
9
12

40
86
18
20
0
7
22
47
45
81
15

9
22
2
4
0
2
5
6
19
4
6
5

Total, no.
Total,%
Mean,%
SD,%
CV,%

328
39.0
32.8
23.8
0.7

47
5.6
6.5
9.5
1.5

382
45.4
47.3
25.9
0.5

84
10.0
13.4
11.5
0.9

3
2
0
1
0
0
0
0

18
47
15
28
4
21
4
15
13
39
36
16
21

0
17
1
7
0
13
7

277
62.2
62.5
15.6
0.2

67
15.1
15.0
15.1
1.0

Preg: pregnant, PL: pregnant and simultaneously lactating, Lact: lactating, Rest: resting

Sci. Rep. Whales Res. Inst.,
No. 36, 1985

Rest

1
5
5
4
6

118

KASUYASEX RATIO

Neonatal sex ratio
Neonatal sex ratio was estimated from the sex ratio of fetuses and postnatal
individuals below one year of age. Fetuses below 5 cm were excluded, because
sex identification is unreliable. All other postnatal individuals below 164 cm(striped dolphin) or below 142 cm (spotted dolphin) were included. These
correspond to the age of one year on the respective mean growth curves(Kasuya et al., 1974; Kasuya, 1976a). One of the reasons for the inclusion of
calves is to increase the sample size (for further discussions see Kasuya andMarsh, 1984). The results are shown in Table 7.
In the striped dolphin, the sex ratio was not significantly different from1 : 1 in all years, in both fetuses and calves (Chi-square test, p > 0. 9). The 841
sexed fetuses and calves yield an estimation of proportion of neonate femalesof 50.4%.
TABLE 7.
Species and Season
Striped dolphin
1950-1959 females
males
1960-1969 females
males
1970-1980 females
males
Total females
males
Spotted dolphin
1970-1980 females
males

NEONATAL SEX RATIO
Total

Fetusesno.

Calves
no.

no.

%

5
6
55
53
103
93
163
152

57
52
120
124
84
89
261
265

62
58
175
177
187
182
424
417

51.66

48
62

83
99

131
161

44.86

49.71
50.67
50.41

In the sample of spotted dolphins, there were fewer females than males
in both fetal and postnatal stages below one year. The neonatal sex ratio estimated from 292 fetuses and calves is 44.9% (females). This is not significantly
different from parity (0.05 < p < 0.1).
No change in neonatal sex ratio was detected in either species during thestudy period.
Postnatal sex ratio
Fig. 4 shows age-related changes in the postnatal sex ratio expressed by the
proportion of females. In the striped dolphin, the ratio was low between 2
and 10 years of age. This is an effect of the data from an immature school
Sci. Rep. Whales Res. Inst.,
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119

REPRODUCTION IN STENELLA(no. 22), where males predominated. After this age, change in the sex ratio isslight, the ratio being approximately 1 : 1.
The spotted dolphin showed different changes. The proportion of
females steadily increased till the age of 40 years. This reflects either a higher
mortality rate or segregation of adult males (see AGE COMPOSITION AND
MORTALITY).

No temporal trend in the postnatal sex ratio was detected in either of thetwo species.
AGE AT SEXUAL MATURITY

MethodFemale age at the attainment of sexual maturity was analyzed for each cohort(defined as the individuals born in the same year, determined from date ofdeath and age at death).
The process is explained below using the following hypothetical agematurit_y composition of three years' samples.
3.5

Total

10

0

90

15

20

45

60

20

0

180

0

20

30

40

90

200

120

40

0

360

0

40

60

80

180

Age at death (yr)

0.5

1.5

2.5

1980 catch, immature

50

30

0

10

100

mature1981 catch, •immature
mature1982 catch, immature
mature

The above data yield the ratios of immature mature individuals in eachcohort at various ages as follows (catch year in parentheses):Age at death (yr)0.51979 cohort
1980 cohort

50: 0 ('80)

1.5

2.5

3.5

30: 10 ('80)

20: 30 ('81)

0: 80 ('82)

60:20 ('81)

40: 60 ('82)

Then the proportion of sexually mature individuals of a cohort at different
ages are calculated from these figures. This method assumes that there is no
selection in the fishery between mature and immature individuals of the sameage caught in a given fishing season. This is certainly not the case for the
striped dolphin and possibly also the spotted dolphin (see SCHOOL VARIATION).
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

120

KASUYA100-'O' 80
O"'
I ll
~ 60

9

l1S

,1
I

E

?\

q

~ 40
d

\,'
tS

z 20

,

~
,.'
b .......0 /

,/''

,"

,'~\
\

\

,'

\

,'

\

,'
I
I

I
I

I

,'

'' ,,
I
I

,,

I'

0

II

0

10

20

30

40

50

Age in years
Fig. 4. Age specific sex ratio of striped (open circle and dotted line) and spotted
dolphin (closed circle and solid line). Eight striped dolphin schools (nos. 22, 38
and 43 through 48) and 11 spotted dolphin schools (nos. 1 through 13, excluding
8 and 11) are used.

However, the method has the advantage of being free from the sort of biasthat occurs, for example, when mean age at sexual maturation is estimatedfrom the transition phase in the earplug of baleen whales (Lockyer, 1972;
Cook and de la Mare, 1983; Kato, 1983; Sakuramoto, Kato and Tanaka,
1984).
The age ranges in samples for the earliest'cohort and latest cohort wereover four years (1950 cohort) and under eight years (1970 cohort), respectively, in the striped dolphin, and over nine years (1961 cohort) and underseven years (1972 cohort) in the spotted dolphin. These earliest and latest
cohorts were specified so as to include both mature and immature individuals.
I analyze below the relationships between year of birth and the average
age at the attainment of sexual maturity, age of the youngest sexually mature
female in the sample, and age of the oldest immature female in the sample.
These indices (especially minimum age at the attainment of sexual maturity)
seem to be sensitive in some mammals to environmental changes (Laws,
Parker and Johnstone, 1975; Ohsumi, in press).
There are several methods for calculating "average" age at attainment ofsexual maturity; each can yield different results (DeMaster, 1984). In the present study, it was estimated as the age at 50%-mature on a linear regression
weighted by sample size. Even though the proportion of sexually mature individuals may not increase strictly linearly with increasing age, the error can be
minimized by using in the regression only the points in the 50%-mature
region. Because of the limitation of samples, the analysis was done combining
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

121

REPRODUCTION IN STENELLA

...c
Cl.I

u

100 ·ss.:sa ·s9.:51
80
60
40

•

00
0

'65-'67•••--- --- - -----------• -----------

'

:•
'

L...

20
0. 0

8 10

...

8 10 126

8 10 12•>. 100
L...
80
:::J
60
~ 40

6 8 10 12•"'

10 12 6 8 10

6

0

7(
8

8 10

6

'70-'72...

200Striped G i n

'
'

Cl.I

c

------~-·- -__:"f''_

8 10

6

8

Spotted dolphin

68 10 12Age in years
Fig. 5. Estimation of age when half of the females are sexually mature. Cohorts ofthree successive birth years are grouped. The least-squares regressions are calculated for points indicated by closed circle and weighted by sample size.

data for three contiguous cohorts, yielding five estimates for the striped
dolphin and four for the spotted dolphin (Fig. 5).
Striped dolphin, age at sexual maturity
The average age at sexual maturation was estimated at 9. 7 years for the 1956' 58 cohorts. It steadily declined to 7.4 years for the 1968-'70 cohorts (Fig. 6).
The least-squares regression between the mean age at attainment of sexual
maturity (Y, year) and the calendar year of birth (X, using the central year foreach cohort group) is expressed by
Y = -0.183X + 367.70,
1957::SX::Sl969, r=0.99
Equation 1
The slope is significantly different from zero (t-test, 0.001 <p<0.01).
In any of the cohorts before 1962, the observed minimum age of mature
females and the maximum age of immature females do not overlap. This is a
result of small sample size, as is the apparent increase in maximum age ofimmature females.
However, in the recent cohorts where sample is large, the age of theyoungest mature female seems to be declining (Fig. 6). It was between sevenand eight years in the cohorts of 1963 to 1966, but in the more recent cohorts
some females matured before six years of age. Also, the age of the oldest imSci. Rep. Whales Res. Inst.,
No. 36, 1985

122

KASUYA

11:·---,-\

''

I

9
~

I
I
I
I
I
I
I
I

\

7

'I
'

en
.... 5''I'I '''

I
'I

"!I

.:
"b

tO

QI

>- 13
c
11
QI

~

9
I

7

'

I
I

''
,'

5
1956

'58

'60

'62

'64

'

9

'66

'68

'70

'72

Year of birthFig. 6. Annual changes of the age where 50% of females are sexually mature (thicksolid line), of the age of the youngest mature female (closed circle and solid line),and of the age of the oldest immature female (open circle and dotted line).
Shaded area indicate the ages not included in the analysis.

mature female appears to have declined after the 1963 cohort, but the change
is less clear than that of the maturation age.
Spotted dolphin, age at sexual maturity
The average age at sexual maturity for females ranged between 8.6 years(1964-'66 cohorts) and 10.1 years (1961-'63 cohorts) (Fig. 5). The regression
coefficient of average age at sexual maturity on year of birth was not significantly different from zero (p>0.1) (Fig. 6.)
A least-squares regression between age of females (X, year) and maturity(Y, %) for all data from 1961 to 1972 yields
Y = 27.455X - 194.70,
6.5:SX:Sll.5, r=0.95
Equation 2
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

REPRODUCTION IN STENELLA123

Y = 50 when X = 8.9 (Fig. 5). This is not different from the value of 9.0 years
obtained in earlier analyses (Kasuya, 1976a).
The age of the youngest mature female was over eight years for thecohorts before 1965, but in more recent cohorts there were some females thatmatured at an age below seven years
REPRODUCTIVE CYCLE

Mean reproductive cycle
Two kinds of corrections were made before estimating the length of theaverage reproductive cycle. The first was to separate true pregnant females
from nonpregnant females having corpus luteum of ovulation.
For the striped dolphin, only the data for 10 schools (nos 7, 8, 9 and 43
through 49) distinguish between the two cases. The numbers of pregnant,pregnant & lactating, lactating and resting females in these schools were 242,
47, 354 and 78, respectively. Among these, the individuals with corpus
luteum of ovulation but no fetus number 15 (14 resting and one lactating).
I therefore estimate that (242+47)/(242+47+ 15) or 0.951 of the "pregnant
females" from other schools were probably actually pregnant.
For the spotted dolphin, all the pregnant females were distinguishedfrom nonpregnant females having a corpus luteum of ovulation. The true
pregnancies were 0. 971 of the number with corpus luteum, close to the result
for the striped dolphin.
The second correction is for possible bias in the apparent pregnancy
rate. Table 8 shows the number of pregnant females in the sample (corrected
as explained above) and the number of calves below a body length corresponding to the age of one year. Since the length of gestation is about one year,
the nursing period lasts more than one year, and breeding in these species isseasonally diffuse (Kasuya, 1972; Kasuya et al., 1974; Miyazaki, 1977; 1984),
the relative abundances of these two states are expected to be similar Guvenile
mortality is ignored). However, they are rather dissimilar, and the differences
are opposite in the two species, i.e. pregnant females exceed calves in the
striped dolphin sample, but calves exceed pregnant females in the spotted
dolphin sample.
I consider the main cause of this discrepancy to be sample bias resulting
from the difference in catchability of schools at the two reproductive states(i.e. difference in timing of migration to the fishing ground, geographicalsegregation, or timing of the breeding peaks). The small number of schools
examined could also be a problem. Inaccurate ageing of juvenile is not a likely
cause, because the ageing techniques are the same for the two species (for discussion of juvenile growth rates in these species, see Kasuya, 1972; Kasuya etal., 1974; Perrin, Coe and Zweifell, 1976). A similar, but more pronounced,
discrepancy was found in a sample of the short-finned pilot whale, Globicephalamacrorhynchus, caught by the drive fishery (Kasuya and Marsh, 1984).
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

124

KASUYATABLE 8.

COMPARISON OF NUMBER OF PREGNANT FEMALES AND CALVESBELOW AGE OF ONE YEAR, FOR SCHOOLS IN TABLE 2
Confirmed

Extrapolated*

Total

Striped dolphin (1967-'80)
Number of pregnant females**Females pregnant and lactating**
Calves at or below 164 cm
Difference

328
47
262

34.8
2.8

} 412.6

Spotted dolphin (1970-'80)
Number of pregnant females**Females pregnant and lactating**
Calves at or below 142 cm
Difference

85
16
172

10.0
1.5

262.0
+150.6} 112.5
172.0
-59.5

*: Extrapolated for the mature females examined but without record of reproductive status.
**: Non pregnant females having corpus luteum of ovulation excluded.

TABLE 9.

ESTIMATION OF TRUE REPRODUCTIVE CYCLE, FOR SCHOOLS IN TABLE 2.
No. individuals
apparent*
corrected

Striped dolphin (1967-'80)
Pregnant
Preg. and lactating
LactatingResting
Total
Spotted dolphin (1970-'80)
Pregnant
Preg. and lactating
LactatingResting
Total

Proportion(%)
apparent*
corrected

Length (year)
apparent* corrected

362.8
49.8
409.8
91.6
914.0

212.2- 362.8
49.8- 49.8
409.8- 560.4
91.6- 91.6
763.4-1064.6

39.0
5.6
45.6
10.0
100.0

29.8- 34.1
6.5- 4.7
53.7- 52.6
12.0- 8.6
100.0-100.0

0.87
0.13
1.02
0.22
2.24

0.82-0.88
0.18-0.12
1.48-1.36
0.33-0.22
2.81-2.58

95.0
17.5
302.3
76.2
491.0

95.0- 154.5
17.5- 17.5
242.8- 302.3
76.2- 76.2
431.5- 550.5

19.l
3.6
62.2
15.l
100.0

22.0- 28.1
4.1- 3.2
56.3- 54.9
17 .6- 13.8
100.0-100.0

0.79
0.15
2.57
0.62
4.13

0.79-0.84
0.15-0.09
2.02-1.65
0.63-0.42
3.59-3.00*Corrected for small number of mature females of unknown reproductive status.

The proportions of the reproductive states were adjusted for the striped
dolphin in two alternative ways to yield a range of estimates: by assuming that
pregnant females were overrepresented or that calves (i.e. number of lactating females) were underrepresented (Table 9). For example, the corrected
number of the pregnant striped dolphins in Table 9 was obtained using
figures in Table 8 as 362.8-150.6=212.2 (assuming overrepresentation ofpregnant females), and the lactaing females as 49.8+150.6=560.4 (assuming
the underrepresentation of lactating females). The latter assumes thatfemales which have been lactating less than one year and those lactating for
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

REPRODUCTION IN STENELLA

125

more than one year are biased in the same way in the present sample. The
reverse was assumed for the spotted dolphin. The resting females are thus
assumed to be schooling with lactating females or with pregnant females. Thismay cover the probable range of bias. Table 9 also shows the mean length ofeach reproductive stage.
With the above correction, the length of lactation (including lactationaccompanied by pregnancy) in the striped dolphin increases about 29 to 44%
over the uncorrected value, and the resting period is also slightly extended.
On the other hand, in the spotted dolphin, the length of lactation is shortenedby about 20 to 36% from the uncorrected figure, and the resting period isslightly shortened.
Thus the mean breeding cycle is extended in the striped dolphin from
2.24 years (uncorrected) to 2.58 or 2.81 years. Since there is no reason to pref er one of these two figures, I tentatively consider that the true calving interval will be between the two, or close to their mean (2.70 years). This is 20.3%
longer than the uncorrected figure. The corrected calving interval for the
spotted dolphin is about 3.30 years, which is about 20.2% less than the uncorrected figure.
This correction for sample bias in apparent pregnancy rate should beconsidered rather tentative because it does not take into account uncertainties
about mortality and growth of juveniles.
Age-specific reproductive cycle
Fig. 7 shows the age specific reproductive cycles for female striped andspotted dolphins. The mean length of each reporoductive stage was claculated using the above described method, but no correction of sample bias inreproductive states rate was done. Five or more year-classes were grouped foreach estimate.
Striped dolphin. The oldest female was 57.5 years old and was resting.
The two next oldest females were 49.5 years old (one lactating and the otherresting), and the oldest (confirmed) pregnant female was 48.5 years old. The
oldest simultaneously pregnant and lactating (PL) female was 42.5 years old,but the next oldest female of this status was only 30.5 years old.
The correlation between the length of lactation (Y, year) and age (X,year) is not high, but the data for animals less than 45 years old give the following least-squares regression,
Y = 0.0328X + 0.40, X<45, r=0.76,
Equation 3
and the slope is significantly different from zero (t-test, 0.02 < p < 0.05).
The resting period (Y, year) is very short in young females being lessthan 0.2 years, but increases after 30 years of age. The correlation is shown by
Y = 0.0293X - 0.49, X>30, r=0.91
Equation 4
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

126

KASUYA

Striped dolphin
54

~6

127

0

49

2 ;_.---o--~--"Oo--Calving interval

0

19

37

10

0

5o

---- -+- _ !-~c~~t ion0-- -+- - --- -+-------+- ____ -~.::___.=_ ____ _ _ _ _ _ _:_-:_ :_----<>~:_ting __~ 0~-~-~·~-~--=-~-~-~·-~-~-~-~-:!;::-~-=-~-~~o:=::&::::-~-I--=::-:::::J.!:'.P~L~--L~~.....L~~.L---+--_-~----_-_-_-_J­
0

CI>

I

>- 8

23

Spotted dolphin
16

6

-4

68

83

•

'}',,
,,.-'

2

,'
'

0

--+------+'//

I

',

0

I

1
1

',

'',',,//

\

I

0

\

\

-

'

/

\

I

0

-

'

/Lactation

0-- __

O PL

I

t.----~1 ',,

,'\
--o

I

4
Calving interval

38

O'I

c:
~

I

9

0

c:
..c:

0

16

\

'/
_.o-----~----=_!.-_..--- Resting-0------0-----0,....., .. / .. /

10

20

40

30

Age

in

50

years

Fig. 7. Age specific reproductive cycle of female striped and spotted dolphins.
Contiguous 5, 10 or 15 age classes are grouped for the analysis. Data used arefrom eight striped dolphin schools (nos 22, 38 and 43 through 48), and 11 spotted dolphin schools (nos 1 to 13, excluding 8 and 11). Figures are calculated fromthe ratio to pregnant females and are not corrected for sample bias of reproductive stages. Females with corpus luteum of ovulation are not dealt as pregnant.
Numerals at the top indicate sample size.

and the regression coefficient is significantly different from zero
(0.02<p<0.05).
The mean length of calving interval (Y, year) shows high correlation for
all age range (X, year), giving a least-squares regression,
Y = 0.0273X + 1.60, 5<X<55, r=0.73,
Equation 5
and the slope is significantly different from zero (0.02 < p < 0.05).
The age above which there are no pregnant and simultaneously lactating
females coincides with an increase of the resting period from below 0.2 year
to about one year. This is an indication that the reproductive potential of
female striped dolphins declines after about 30 years.
Spotted dolphin. The oldest female spotted dolphin was 45.5 years old andwas lactating. The next oldest female (44.5 years) was resting, and the oldest
pregnant female was 40.5 years old. PL females were 20.5 years old or less.
Although the length of lactation seems to be positively correlated withage, the slope of a least-squares regression is not significantly different fromzero (0.8<p<0.9).
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

REPRODUCTION IN STENELLA

127

The resting stage (Y, year) is short ( < 0.6 year) below the age (X, year) of
25 years, after which it increases considerably with age. The least-squares
regression fitted for points above 20 years is expressed by
Y = 0.886X - 13.18, X>20,
r=0.92,Equation 6
and the slope is significantly different from zero (t-test, 0.02 < p < 0.05).
The mean calving interval also appears to show positive correlation withage but the linear-regression is insignificant (0.3 < p < 0.5).
As described above, the only significant age-related changes in reproductive parameters in female spotted dolphins are in the abundance of PL
females and resting females. These two parameters show a negative correlation with each other, as observed in the striped dolphin. Therefore, it isreasonable to conclude that reproductive potential in the species declines withage. The absence of significant correlation between age and calving intervalor lactation length may be an artifact of small sample size.
Conclusion. The longevity of the female striped dolphin is probably about
10 years greater than that in the spotted dolphin, and the stage of low reproductive potential (indicated by rapid increase of resting females and disappearance of PL females) starts later in the striped dolphin. The absolute valueof reproductive potential of the striped dolphin remain higher even withgreat age (indicated by the shorter lengths of resting period and calving
interval). These differences are at odds with general tendency in mammals
for a longer-lived species to have lower annual productivity. Thus I suspectthat the difference in the reproductive parameters between the two species
may not be entirely due to species-specific differences but relate in somedegree to differences in history of exploitation.
Historical change in reproductive cycle
Fig. 8 shows the relationship between mean length of calving cycle (and its
components) and the date of capture. Because variation among schools in the
proportions for the reproductive stages was large, as mentioned above, years
represented by only one school were combined with successive seasons inmost cases to reduce year-to-year fluctuation. No correction for sample bias inreproductive states was made.
Striped dolphin. As mentioned above (see MATERIALS AND METHODS), most of
the data for the striped dolphin do not distinguish pregnant females from
nonpregnant females with corpus luteum of ovulation. Therefore in all theanalyses of the striped dolphin in this section all females having corpus
luteum are dealt as pregnant. The true number of pregnant females is about
0.951 of the females having a corpus luteum (see Mean reproductive cycle ofthis section).
Although there is an apparent downward trend for the striped dolphin,
only one component of the cycle, proportion of PL females, exhibits a statistically significant increasing trend, and that only in recent years. Correlation of
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

128

KASUYA_

Striped dolphin 032

32

259

192

0

0

98

134

0

0

57

51

2

Spotted-Calving interval

dolphin

35

6

~

,

Ol

119

~ 4
__J

+,
I\

/
/

/

2

4

/~I

\~6

/
/

60
0

'\

¥Lactation

,'
0

0

~
IR~sting

I \

o-_)

PL \

'70

'75

5Jl

'i..
",

,,,;

\/'

-o- -·-

0

OL..L...L...J-1.....L.J......1.......JL....L...L..J.--1-.L...L...L...l-1.....L..l:::::t::;~d:::::t:::!:::t:::::I..::&::l:.....J

1955

'60

'65

'80

Calender year
Fig. 8. Annual change in length of reproductive cycle of female striped and spotted dolphins. The data for striped dolphins in the years 1952, '53, '57, '58, '61,
'70, '71 and '72 are based on Kasuya and Miyazaki (1982). Figures are calculated
from the proportion of pregnant females and are not corrected for sample biasof proportion of reproductive stages. For the striped dolphin, all the females
having a corpus luteum of ovulation are dealt as pregnant. Numerals at the top
indicate sample size.
'

the mean length of the PL stage (Y in years, uncorrected for the bias due tosegregation) with year (X) for the eight seasons after 1966) is expressed by
Y = 2.7590X - 50.25, X2::1967, r=0.75,
Equation 7
and the slope is significantly different from zero (t-test, 0:02 < p < 0.05). This
result should be confirmed by collecting more data, because it is not based ona very long period of time.
The mean length of the resting period seems to have a downward trend
in recent years, but the regression coefficient is not significantly different
from zero (0.1 < p < 0.2).
The mean length of lactation also shows an apparent downward trend,
but the slope is not significantly different from zero (0.1<p<0.2).
The mean calving interval (Y in year, uncorrected for bias of corpus
luteum of ovulation and segregation) in relation to season (X, after 1952) isshown by the least-squares regression
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

REPRODUCTION IN STENELLA

Y = -0.04440X + 89.97,

X ~ 1952,

129

r=0.55,Equation 8
Although the regression coefficient is not significantly different from zero
(0.05<p<O.l), the general downward trend in many of the reproductive
parameters mentioned above suggest that more data will prove it significant.
This equation and the correction factors for the underrepresentation of
lactating female (1.203) and for nonpregnant females having a corpus luteumof ovulation (0.951) give the following estimates of the possible calving interval for the species.
4.00 years
1955 season: 3.160 · 1.203 I 0.951
1960 season: 2.938 · 1.203 I 0.951
3.72 years
1977 season: 2.183 · 1.203 I 0.951
2.76 years
The mean calving interval could have decreased about 1.24 years in the past
22 years between 1955 and 1977.
Spotted dolphin. There are no statistically significant trends over theperiod sampled.
AGE COMPOSITION AND MORTALITY

Catch data can be used for estimation of mortality rates only when a fisherydoes not select certain age classes and when the annual recruitment andpopulation size are constant (Caughley, 1966). These conditions are not met
for the striped and spotted dolphins. Therefore, the present effort was
directed to comparing the age composition of the striped dolphin with that of
the less intensively exploited spotted dolphin to attempt to elucidate thepossible effect of the longer-term fishery on the age composition of the
striped dolphin.

Spotted dolphin
The frequency of juveniles in the present sample is unreasonably low (Fig. 9).
This is presumably due to segregation of weaned juveniles from the breeding
schools that formed the present sample. Males of adult age classes are consistently fewer than the females of the same age. This could be due to segregation and/or higher mortality rates in males. The two oldest females were aged
at 45.5 and 44.5 years, respectively, and the oldest male at 42.5 years. Thissuggests a slight longer longevity in females.
Caughley (1966; 1977) reviewed the age composition of wild-mammal
populations and concluded that mortality rate is dependent on age, i.e., ahigh juvenile mortality rate that decreases with age, and a lower adult mortality
rate followed by a higher rate in older age classes. Males of some polygynous
species have a W-shaped mortality curve (Ralls, Brownell and Ballou, 1980).
The age composition of spotted dolphin shows the pattern indicated by
Caughly (1966; 1977), except for the juvenile bias, but the data are insufficient to allow detection of details of age-related changes. However the leastSci. Rep. Whales Res. Inst.,
No. 36, 1985

KASUYA

130

so.--~~~~~~~~~~~~~~~~~~--.

21 30

"'

0

:J 20

0

....

"'O

0

..

00 ...... ,

>

:0 10
c
-0

0

o',,~

•

7

•

0

''t).

,

..... ......

••

0

0

'"O..

i

',
•

5

·Q; 3

0

0

' ' 00
' '·
',

0

0

''

..0

E
:J

z

0

o,po

'

''

'

''

1 '--~'--~'--~'--~'------''------''--""'-'~•-o~o-=-oo~~~'~o~o~~

0

10

20

30

40

50

Age in years
Fig. 9. Age composition of the spotted dolphin. Open circle and dotted line indicate females, and closed circle and solid line males. The thick straight lines (dotted and solid) are least-squares regressions for the indicated age ranges. Elevenschools (nos 1 to 13, except 8 and 11) are included.

squares regressions fitted to the intermediate and older age classes (Fig. 9)
suggest higher apparent mortality rate for the older age class (Table 10). The
calculations include only data below the age where zero frequency starts toappear in order to avoid bias due to ignoring ~ero frequency in the calculation. These results are similar to the estimation using the geometric methodof Robson and Chapman (1961) (Table 10).
The difference of the apparent mortality rate between medium (approximately from 10 to 22 years of age) and the older age classes is significant(t-test, p < 0.01 ). Although the male mortality appeared slightly higher thanthat of the females of the same age, as suggested by the female sex ratio
increasing with age, this difference is not significant in the present small
sample.
Assuming a stationary population, a rough estimation of juvenile mortality
rate was done as follows. First, the annual production of 43.49 female calves
was calculated by (Number of aged females, 431 individuals) X (Proportion of
adult females, 0.666) X (Neonatal sex ratio, 0.5) I (Average calving interval,
3.30 years). This annual production is close to the observed frequency of 42
females between 0 and 1 year of age. Then a hypothetical uniform mortality
rate was calculated for ages between year 0 (43.49 females) and year 11 (20.01
females calculated from the regression in Fig. 9). This gives 6.9% as the meanannual mortality rate of females between 0 and 11 years of age.
Sci. Rep. Whales Res. Inst.,
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131

REPRODUCTION IN STENELLATABLE 10.

APPARENT ANNUAL MORTALITY RATE CALCULATED FROM AGE
COMPOSITION OF SPOTTED AND STRIPED DOLPHINS

MethodSpotted dolphin
Female, 11-26 yr
24-43 yr
>24yr
Male,
7-22 yr
20-34 yr
>20yr
Striped dolphin
Female, 11-22 yr
20-34 yr
32-41 yr
>32 yrMale,
11-22 yr
20-34 yr
32-46 yr
>32 yrA

B

0.06090.11040.0563 ±0.0310
0.1026 ±0.0418
0.1348 ±0.0282
0.0590 ±0.0361
0.1494 ±0.0734
0.1618 ±0.0439

0.05550.14370.11410.07730.01920.13400.05360.13280.1074 ±0.0376
0.0622 ±0.0442
0.2132 ±0.1453
0.1408 ±0.0468
0.1486 ±0.0366
0.0534 ±0.0471
0.1489 ±0.0771
0.1419 ±0.0398

A: From least-squares regression in Figs. 9 and 10.
B: After Robson and Chapman (1961), best point estimate with 95% confidence interval.

Striped dolphin
The age frequency below 10 years is largely dependent on the catch of theuncommon immature schools and is not reliable. The oldest individuals wereaged at 57.5 years (one male and one female) and the next oldest at 53.5
years. Five males and five females were older than the oldest spotted dolphin(45.5 years old), suggesting a greater longevity in striped dolphins. A sexes
difference in longevity is not suggested by the present sample.
The apparent mortality rate was calculated in the same way as for the
spotted dolphin (Table 10). Since the population has been exploited probably
for more than one century and there are symptoms suggesting decline of the
population size (see DISCUSSION), it is not possible to estimate the true mortality
rate from these figures. However the general pattern of the steeper slope ofthe oldest age class (Fig. 10) is similar to that of the spotted dolphin. This
will reflect the age specific change of the natural mortality rate.
The slope of the age frequency below 33 years is less steep, but it seems
to change at an age of about 21 years (slightly steeper in the younger age).
Kasuya and Miyazaki (1982) found a similar change in age frequency data
calculated from the ovarian corpora frequency. Although the present sample
was insufficient for further analysis, it is probable that it reflects one or bothof "the fishing mortality increase" and "increase in the annual recruitment"that might have occurred about 20 years before the commencement of samplSci. Rep. Whales Res. Inst.,
No. 36, 1985

132

KASUYA90
70-(/) 50"'::J 30
:2 20

>

"'O

c: 10

0

•

00
0

7

.... 5

<l>
.0

E

\

•• \

'

•

0

\

3

0

\.

0

\
\

::J 2

\

z

0

0

\

••

\

0

\
\

'

\

ooe

1
0

10

20

30

\

Ii•

40

••

50

Age in years
Fig. 10 . .Age composition of the striped dolphin. Closed circle and solid line indicate males, and open circle and dotted line females. The thick straight lines (dotted and solid) are least-squares regressions for the indicated age ranges. Eightschools (nos 22, 38 and 43 through 48) are included.

ing in 1972 to 1977 as the result of high catches after World War II (Kasuya
and Miyazaki, 1982; Miyazaki, 1983).
DISCUSSION

Kasuya and Miyazaki ( 1982) suggested for the striped dolphin population off
the Pacific coast of Japan that the population could not sustain the catch levelof the early l 970's, because it exceeded the maximum sustainable yield. Theirconclusion was based on the catch curve and changes in the operation patternsuch as the number and quality of vessels used, expansion of the fishingground, and an increased proportion of other delphinids in the catch (theiranalysis of catch curve was later refined by Kasuya ( l 976a) using improved
age determination, but the conclusion remained unchanged). Although thesecatch curve analyses may need further examination (Anon., 1981 ), thechanges in the fishing operation will still remain valid as an indication ofchange of the population level.
Later, Miyazaki (1983) analyzed the catch per unit of effort (CPUE) in
the fishery, expressed by the number of captured individuals or the numberof schools caught per searching day. Although he was not certain if theSci. Rep. Whales Res. Inst.,
No. 36, 1985

REPRODUCTION IN STENELLA

133

•

Taiji

10,000
8,000
6,000
(/)

•

4,000

c
..c 2,000
a.
0

"O

0
0

0

0 10,000
0 8,000

'•

0

0

•

0

0

0

0

0

lzu

0

•

••• • •
•

•

•

•

z

6,000
4,000
2,000
0

'65

•70

1

75

·so

Fishing season
Fig. 11. Landing of the striped dolphin at Taiji and off Izu peninsula (Kawana andFuto) by hand-held harpoon (open circle) and drive fishery (closed circle) for theperiod where a complete statistics are available. Both equipment and number ofhunting teams remained unchanged at Izu coast for the period indicated, butthey had changed considerably at Taiji. Straight line represent a linear regressionof the annual catch on the fishing season Y = -348.0X + 691,800). The fishingseason off Izu coast is indicated by the starting date of the season. The shot-andlost individuals in the hand-held harpoon fishery at Taiji are not included(see text).

population was stational (Miyazaki, pers. comm.) he could not find a significant
decline in the annual catch or in the CPUE serieses.
However, in fisheries such as the drive fishery, where the scouting vessels stop their searching operation for the day and start driving as soon as
they find a dolphin school, the change in CPUE underrepresents that of thestock level (Cook, in press). Furthermore, the recent catch of the striped dolphin
off the Izu coast has further declined without any change in the equipmentused (246 individuals in 1982 and 40 in 1983. Anon., 1984; 1985). The declineof catch after 1961, where complete statistics are available for the two villages(Futo and Kawana) and the equipment remained the same, is significant(p<0.001, t-test of the gradient of the regression coefficient of annual catchon fishing season, see Fig. 11). Using only data from the period of thecooperative operation by the two teams (since 1968) gives the same result
Sci. Rep. Whales Res. Inst.,
No. 36, 1985

134

KASUYA(0.005<p<O.Ol). The decline of dolphin meat demand was not indicated,because the catch of the Dall's porpoise in the northern Japan increased inrecent years (Anon., 1983; 1984).
The fishing season off lzu coast, where they drive southbound schools,
has also changed over the period sampled, from October-January (1960's), to
October-December (1969 to 1979), to September-December (1980), andfinally to September-November since 1981 (most of these data are available in
Miyazaki, 1983), although the higher demand for dolphin meat in colder
months remained the same.
These changes as well as those pointed out by Kasuya and Miyazaki( 1982), suggest the decline of the stock level. However·, the catch of thespecies off Taiji has not shown a significant decline since the start of the drive
fishery in 1973. This could be explained if the density change occurredearlier at the periphery of the population's distribution (off the Izu coast) before it happened near the core area (Taiji) (Cook, in press). This could also
explain the change in the fishing season off the Izu coast mentioned above.
The possible density dependent changes in the life history parameters of
the striped dolphin population may appear too small to account for drastic
decline in the catch off the Izu coast. This could be due to either the incomplete mixing of population members or a population decline accompanied by
reduction of the range size. The first explanation assumes that the population
declined only in the area close to the fishing ground and that the fishery continued to take schools migrating in from the surrounding area that was damaged less by the fishery. The second assumes that, due to the reduction of
population range, even with rapid mixing, the density of the remaining stockchanged less than would be expected from the apparent decline of the population level in the fishing ground. In both cases, the apparent decline of the
population size may result in neither a parallel change of density in theremaining population nor in the expected improvement of its reproductive
capacity. It is also true in these cases that the decline of the population level inthe periphery of the range may exaggerate the decline of the entire population.
The above discussion, suggesting the decline of the striped dolphin
population, is consistent with the density dependent changes of the life history parameters suggested in the present study. Therefore, I consider that thestock of the striped dolphin exploited off the Pacific coast of Japan could notsustain the annual catch of 3,600 to 16,000 individuals (X.=7,915), off Izu andTaiji in 1961 to 1981 (Miyazaki, 1983; Anon., 1984; 1985). This figure does
not include the individuals shot and lost by the past harpoon fishery off Taiji,
which used hand-held harpoon and shot gun and seemed to have landed only
about half of the total kill).
Kasuya and Marsh (1984) suggested for G. macrorhynchus that, even when
the total catch was the same, the population might respond more effectively
to cropping from many schools rather than to the capture of whole schools.
Sci. Rep. Whales Res. Inst.,
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REPRODUCTION IN STENELLA

135

However, because the schools of the striped dolphin seem to be more fluid
than those of the pilot whale (Miyazaki and Nishiwaki, 1978) it is questionable
if this is strictly applicable to the striped dolphin.
The magnitude of the contribution of the changes in life history
parameters to the improvement of population productivity can be roughly
estimated as follows.
The average age of female striped dolphins at the attainment of sexual
maturity declined from 9.7 years at around 1966 (for the 1957 cohort, using
Equation 1) to 7.4 years in 1976 (for the 1969 cohort), i.e. a decline of about
2.3 years during about 10 years. In the 1636 striped dolphins for which ages
were estimated (both sexes combined to smooth age frequency), there were
787 individuals over 10 years old and 949 over 8 years old. Thus the observed
decline in the age at maturation could have increased population productivity
by about 20.6% in this period. However, this figure has to be considered as
tentative, because it is affected by the biased juvenile age composition andassumes that the age composition was unchanged for 10 years.
On the other hand, if the calving interval of the striped dolphin reallydeclined in the same period (10 years) to 83.6% of the initial level (Equation8), it could have contributed to the 19.6% increase in the annual pregnancy
rate. If this change was accompanied by the decline in age at maturation mentioned above, the gross reproductive rate could have increased by about 44%( 1.206 X 1.196 = 1.44) of the initial level. However, this figure is less certain
than that above (20.6% increase), because the change in the pregnancy rate ismore uncertain.
The only detectable change in the life history parameters of f~male spotted
dolphins off the Pacific coast of Japan was the possible decline of theminimum age at the attainment of sexual maturity. Ohsumi (in press)
analyzed the age of female fin whales at maturation for the exploited population in the North Pacific, using individuals with only one corpus in theovaries. He found that the density dependent change appeared first as an increase Df early-maturing individuals, which had little effect on the age where
50% of the females were sexually mature. This change was followed by a decline in the age of 50 % maturation, but some females still attained maturity ata higher age as before. The last phase of the change was the disappearance of
these relatively old immature females. Thus females that matured at between
8 and 17 years of age (x = 12.4) in the 1957-'58 seasons matured in 1974-'75
seasons at about 4 to 11 (x = 7.0) years of age (Fig. 3 in Ohsumi, in press).
Laws et al. ( 197 5) also showed for populations of the African elephant,
Loxodonta africana, in different habitats, that sexual maturation occurredamong females at ages of 10 to 34 years in unfavorable habitat (50% maturation at about 22 years), but at ages of 10 to 22 years in more favorable habitat(50% maturation at about 17 years). Both studies suggest that a change in theage at sexual maturation may not occur equally in all age classes.
I believe that this kind of change could have started recently in theSci. Rep. Whales Res. Inst.,
No. 36, 1985

136

KASUYA

population of spotted dolphins. The drive fishery for the spotted dolphin
may have started in 1959 (Table 3), but annual catches were small (less than
1,000) in most of the seasons before 1973 (Miyazaki et al., 1974), and increased only after 1976 to an average of 1,251 (0 to 4, 184) concurrent withthe decline of the striped dolphin catch (Kasuya and Miyazaki, 1982;
Miyazaki, 1983). However, the catch was still low, even in recent seasons(1,441 in 1982 and 629 in 1,983, Anon., 1984; 1985) compared with that of
the striped dolphins in the area (Miyazaki, 1983). This suggests that if achange of the minimum age at sexual maturity has really occurred in thespotted dolphin population, it could be the result of not only the decline ofthe population of spotted dolphins but also that of the striped dolphin population in the same geographical area, due to reduced competition for food.
The limited catches and almost indetectable changes in life history
parameters suggest that the apparent mortality rate of the spotted dolphins
calculated in this study might be close to the natural mortality rate of thespecies, even though it may be an overestimate due to the calculation
technique (de la Mare, 1984).
ACKNOWLEDGEMENT

I wish to express my sincere thanks to all who helped me in the present study.
Biological study of dolphins caught by the drive fishery on the Izu coast wasstarted in the l 950's by Dr M. Nishiwaki and his colleagues. His biological
data for striped dolphins caught before 1966 were used in this study. Dr S.
Ohsumi provided me with tooth samples and the biological data collected before 1967. These data were extremely valuable in the analysis of historicalchange of life history parameters.
Collection of my own data, most of which have already been publishedelsewhere was done with cooperation of many biologists, including Dr N.
Miyazaki, Dr T. Kajihara and Dr W. H. Dawbin, and many volunteers. Dr
W. F. Perrin, Dr]. Barlow, Dr T. Smith, Dr H. Marsh, Dr R.L. Brownell and
Dr K. Ralls critically read the manuscript and corrected the English. Fishery
Cooperative Unions at Kawana, Futo and Taiji allowed my investigation of
the catch. Miss T. Shirai helped me in the data analyses, typewriting anddrafting of figures.
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