THE LIFE HISTORY OF DALL'S PORPOISE WITH
SPECIAL REFERENCE TO THE STOCK OFF THE
PACIFIC COAST OF JAPANTOSHIO KASUYA
Ocean Research Institute, University ef Tokyo, Tokyo
ABSTRACTThe geographical variation of the colour types of the species indicates
three local stocks in the northwestern North Pacific and western Bering Sea(WNP/BS), off the Pacific coast of Japan (PJ), and in the Sea of Japan andOkhotsk Sea (SJ/OS).
The catches by the salmon gillnet fishery in WNP/BS and the present
and past harpoon fishing in PJ and in SJ/OS, and the age structure basedon the cemental growth layers give the following estimation of the life history
parameters. The calves are born in August to September at the length of
100 cm after the gestation of 11.4 months, and nursed for about 2 years. Sexual maturity is attained in males at the age of 7.9 years and the length of
195.7 cm, and in females at 6.8 years and 186.5 cm. The mean calving interval is about 3 years, and the annual natural mortality rate of the adultfemale will be below 0.1. Data are insufficient to indicate the degree of between stocks difference of the parameters.
The weaned immature individuals are distributed in the south in thewinter (PJ, WNP/BS, and SJ/OS) and possibly in the summer too (WNP/
BS). The segregation of pregnant females by the size of the fetus is expected(WNP/BS). The harpoon fishing mainly exploits the weaned immature andnewly matured individuals (PJ and SJ/OS), however the salmon gillnet fisherykills the calves of suckling age and newly matured individuals (WNP/BS).
The lactating females are underrepresented in the both fisheries.
INTRODUCTION

The Dall's porpoise, Phocoenoides dalli (True 1885 ), is widely distributed in thenorthern part of the North Pacific. It ranges from the Sea' of Japan and Okhotsk
Sea to the west coast of North America as far south as 33°N (Norris and Prescott
1961, Nishiwaki 1967a), and the northern limit will be in the Chukchi Sea (Sleptsov1961). On the other hand the True's porpoise, P. truei Andrews 1911, has a limited
distribution off the Pacific coast of northern Japan and of the Kuril Islands. Its
southern limit is at about 35°N in the winter season. In the summer it seems to
be distributed from 42°N to 54°N, and from the east coast of the Kuril Islands tol68°E (Sleptsov 1961, Ohsumi 1975, Kasuya 1976b).
On the systematic relationship of the two kinds of porpoises, Kuroda ( 1954)
considered, based on the probable incomplete genetic isolation suspected from
Sci. Rep. Whales Res. Inst.,
No. 30, 1978, 1-63

KASUYA

2

the observation of the school structure and the pigmentation of the fetus, that theTrue's porpoise is a geographical race and a subspecies of P. dalli. Houck (1976),
however, through the study of the osteology and external morphology of the porpoises found that only the colour variation attained the plausible level of differentiation in the genus. Finally, considering the fact of coexistence of the two types
in the western North Pacific, he concluded that Phocoenoides dalli seems to be polymorphic in colour pattern with two major colourmorphs i.e. the dalli-type andtruei-type. In the same year Kasuya (1976b) analysed the geographical variationof the ratio of the two types in the northwestern North Pacific, and found that theratio of truei-type in the total Phocoenoides caught in the winter season off the Pacificcoast of northern Japan (southern part of Sanriku region which covers the coastal
part from 38°N to 41 °30'N) is about 96 % and its ratio in the sightings and in theincidental catch by Japanese salmon gillnet fishery decreases in the offshore waters.
This led him to conclude that there can be one coastal population of Phocoenoides
X1000

.,.

160

p

Ul

~120

I

Ul
<!.I

>

I

0 80
,,

0

z 40
,,
0 1900 '10

/

I

,~

,

,,..~~

I

I

50
40

I

~4

30
20

/

'20 '30 '40 '50 '60

10
0

Vear
Fig. 1. Number of the Japanese fishing vessels equipped with engine (closed circle
and solid line), and its ratio in the total number of fishing vessels (open circle anddotted line), statistics cited from Takagi (1965).

off the Pacific coast of northern Ja pan and off the east coast of Kuril Islands, whichis entirely or mostly composed of the truei-type and intermingles at its northeasternboundary with another offshore population composed of only dalli-type. From
the fact that the Kuril Islands seems to block the migration of truei-type into the
Okhotsk Sea, he considered that the dalli-type found in the Okhotsk Sea will not
pass the Kuril Islands and there will be a third population of Phocoenoides in the
Okhotsk Sea and the Sea of Japan. However, in the study, he retained the conclusion if the dalli-type in the catch off Sanriku region is original in the coastal
population or the migrant from other population.
A brief history of the recent exploitation of Phocoenoides in the Japanese coastal
waters was described by Kasuya (1976b) based on Japanese published records.
Some statistics not cited in Kasuya (1976b) and the harpoon fishing gears arereported in Wilke et al. (1953). The exploitation of Phocoenoides, as well as that
Sci. Rep. Whales Res. Inst.,

No. !JO, 1978.

DALL'S PORPOISE

3

of other delphinids, seems to have had a peak in late 1930's to 1940's affected by
the social factors during and after the war, and was operated at various part ofJapan using the method or fleets introduced from Sanriku coast (Yoshida 1939,
Matsui and Uchihashi 1943, Hirashima and Ono 1944, Wilke et al. 1953). However, in the recent years, the fishing at most of the places ceased the operation
except for the Sanriku coast. The most uncertain is the history of the exploitationin earlier period. It is known that there was operated in the past a driving fisheryof dolphins at Oura of Yamada Town on the Sanriku Coast. The operation wasmade only when the dolphins came close to the entrance of the bay. The species
ofa catch of 2,385 porpoises made in February 1882 was identified on a photographas Lagenorhynchus obliquidens. However, since the school of Phocoenoides is small andusually stays far from the coast, the driving fishery will not have exploited thespecies. The alternative method taking the species seems to be harpooning theindividuals come to the bow wave of the boat. The fishing is most efficient witha vessel with engine. Since the number of the Japanese fishing vessels equippedwith engine was negligible before 1920 (Fig. 1), the exploitation of Phocoenoides atsignificant level would have started after late 1920's. This date is almost samewith the introduction of motor vessels into the driving fishery of Stenella coeruleoalba
off Izu coast (Kasuya and Miyazaki 1976a).
The season for the recent porpoise hunting off Sanriku coast is from the last
decade of January to the beginning of April when no other fishing is profitable
and the market price of the porpoise is high (Kasuya 1976b), and it is operated
with hand harpoon and small fishing vessels of about 20 to 30 gross tons. The
fishing area ranges approximately between 38°N and 40°N, and between about 10
nautical miles (18 km) and 40 to 50 miles (80 km) off the coast. Accordingly
this fishing is mainly harvesting the Japanese east coast population of Kasuya(1976b ). The annual catch is about 6,000 individuals. Almost nothing is known
about the life history of the individuals of this population.
The offshore population, inhabiting the northwestern North Pacific and western
Bering Sea, has been captured incidentally by the Japanese salmon gillnet fishery,
which has continued since 1930 with an interruption from 1946 to 1951. The
statistics of the incidental catch is not available. Mizue et al. (1966) estimated it
as about 20,000 individuals in 1964 season (data based on person. comm. of K.
Mizue ). Though the analysis of the species caught by the salmon gillnet fishery
was made by Mizue and Yoshida (1965) and Mizue et al. (1966), the absence of
the adequate way of age determination limited the accuracy of the studies.
The present study intends to analyse the life history of the porpoise caught
by the harpoon fishing off Sanriku coast, together with the supplemental considerations of that of the two other populations distributed in the offshore north-western
North Pacific and western Bering Sea and in the Okhotsk Sea and the Sea of Japan.
MATERIALS AND METHOD
The analyses of the school structure are based on the same data used in Kasuya
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

4

KASUYA(1976b). However, few records where the estimation of the school size is lessreliable are excluded, and a school observed by Dr Kajihara at the east side of
the Soya Straight was added.
TABLE 1.

MATERIALS USED IN THIS STUDYMonth*

Jan.

Feb.

Year

B
'74

A
'72

A
'74

B
'74

B
'75

B
'76

A
'74

B
'72

B Total'74

No. examined, males
femalessex unknown
Body length, males
females
No. aged, males
females
Testis, histology
weight only
Females, maturity determined

147
75
0
96
56
68
42
55
0
45

68
14
14
53
12
33
9
0
29
4

52
47
0
52
47
4
4
4
0
17

53
50
0
38
33
32
26
27
0
30

28
19
0
28
19
17
16
14
0
18

127
111
4
94
93
87
87
85
0
78

66
69
0
62
63
20
35
14
0
57

58
88

9
0
0
9
0
8
0
9
0
0

Mar.

53
76
25
29
0
31
54

Apr.

608
473
19
485
399
294
248
208
60
303

* A: From the lst to 15th, B: From 16th to the end of month.

The biological information and samples were collected in the years from 1972
to 1976 covering the season from January to April, at the seven landing ports listed
below.
Miyako:39°39'N, 141°57'E
Yamada:39°28'N, 141°57'E
Funakoshi: 39°27'N, 141°58'E
Otsuchi:
39°19'N, 141°55'E
Kamaishi: 39°14'N, 141°53'E
Ofunato:39°04'N, 141°43'E
Rosaura:39°00'N, 141°44'E
Though total of 1,100 individuals were examined, the available informations werevariable between the individuals because the internal organs were usually removed
before landing.
The body length was measured at nearest 1 cm interval with steel tape on astraight line connecting the tip of snout and bottom of the tail notch. In analysing
the body length data, they are usually grouped into 4 cm intervals expressed by
4N ;;:;;X;;:;;4N +3, where N indicates a positive integer and X the body length.
The pigmentation was, in the field, classified into the three major categoriesof dalli-type, truei-type, and black-type. The pigmentation of randomly selected
537 individuals was recorded on black and white photographs for the further analyses of the individual variation.
The reproductive stage of the female was determined through the observationof mammary gland, ovaries, and the reproductive tract, and recorded as " Immature", "Pregnant", "Lactating", and "Resting". Since the ovaries and uterusof the immature females are always left on the carcass, the identification of theSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

5

immature stage is easiest. However, as the ovaries and large portion of uterus of
the adult females are often removed by the fishermen, the distinction of the pregnant
and resting females is often impossible. In such a case only the maturity wasconfirmed from the fragments of reproductive tract left in the body cavity.
The ovaries were fixed in 10 % formalin solution. The weight and the mean
diameter of the corpora and mean diameter of the largest follicle were measured in
the laboratory. The mean diameter was calculated as the cube root of the multiple of the three diameters.
One testis and the epididymis were collected from each male and fixed withthe same way as ovaries. In the laboratory the smear was taken from the testis
tissue at its midlength and from the epididymis at the corresponding position, andthe presence and the relative density of spermatozoa were observed after staining
with toluidine blue. The histological examination was made on the epididymis,
and on the peripheral and central testicular tissues taken from the midlength of
the testis. The preparation was made after ordinary way.
The age was determined based on the growth layers in the cementum of the
tooth. Several teeth taken from the upper or lower jaw were fixed in 10% formalin. In the laboratory, the tooth was separated from the jaw bone and soft
tissue was removed with fingers. Then they were dried and embeded in poliester
resin for the convenience of handling. The embeded tooth was ground with whetstones until the center of the pulp cavity was exposed, then it was glued on a clear
plastic plate with synthetic resin and the other side was ground to get a thin longitudinal ground section of 10 to 15 µm thickness. The ground section was decalcified in 5 % formic acid for several hours, stained with Mayer's haematoxylin
for 30 minites, and finally mounted with Canada balsam. The growth layers werecounted with a microscope ( X 150 to X 300).
As mentioned in the next section, at least some of the dalli-type individuals
caught off the Sanriku region are considered to belong to the offshore populationin the northwestern North Pacific and western Bering Sea. However, since there
is no way of distinguishing these individuals and the ratio of the total dalli-type isso low as 4%, the samples of the two types collected off Sanriku coast were dealt
together in the analyses of the growth and reproduction.
Some ancillary materials collected by myself in the southern Okhotsk Sea or
by S. Nagahora of the Iwate Prefectural Fisheries Research Laboratory in the northwestern North Pacific are used in this study.
After the analyses of the biological data were nearly completed, Dr A. Takemura of the Nagasaki University provided for me the tooth samples and the biological data of the 107 Dall's porpoises taken by the North Pacific salmon gillnet
fishery. Since these materials represent the offshore population in the northwestern North Pacific and western Bering Sea and the age structure of the catchhas not been published, they are briefly analysed in the Addendum.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

6

KASUYA

DISTRIBUTION AND SCHOOL STRUCTURE

Distribution of the populations
There is no new information to add the range of the distribution of the three
populations of Phocoenoides distinguished by Kasuya (1976b). Though Sleptsov(1961) mentions that the truei-type is found even in the southern part of the Okhotsk
Sea and in the Sea of Japan, it is not supported by other data available to me(Kuzin and Perlov 1975, Kasuya 1976b). However, as it is not impossible to expect few truei-types to migrate into the Okhotsk Sea or into the Sea ofJapan throughthe Kuril Islands or the Tsugaru Strait, further study will be needed before arriving
at the definite conclusion.
TABLE 2.

ANALYSIS OF SCHOOL OF PHOCOENOIDES DALL! OFF SANRIKU COAST
Observed no. and %School size
T
1

2
3
4
5
6
7
Total

2
8
5
4
3
3
3
28

TD

100
61.5
100
100
100
75.0
100
82.4

0
0
0
0
0
l3l
0

Probability in %
T

D

0
0
0
0
0
25.0
0
2.9

0
5
0
0
0
0
0
5

0
38.5
0
0
0
0
0
14.7

90.8
82.4
74.9
68.0
61.7
56.0
50.9
72.4

TD1>

8.4
22.7
29.6
37.6
43.3
48.9
22.7

TD2l

D

8.4
2.3
2.4
0.7
0.7
0.2
4.0

9.2
0.8
0.1
0.0
0.0
0.0
0.0
0.9

1): Mixed school truei-type exceeding dalli-type, 2): Mixed school dalli-type exceeding truei-type, 3): Fivetruei-types and one dalli-type, T: School oftruei-type, TD: Mixed school, D: School of dalli-type.

Wilke et al. (1953) stated that the school of Phocoenoides containing both dallitype and truei-type is not frequent off the southern coast of Hokkaido. Table 2
shows a detailed analysis of the frequency of the two colour types in the schools
observed in January, February, May, and June. The location covers the areasurrounded by 37°30'N, 41°N, 143°E, and the coast of Japan, which is nearly samewith the harpoon fishing ground of the species in the winter season. Only theschools not exceeding 7 individuals are used in the analysis, because the correct
estimation of the school size and the identification of the colour types of all theindividuals are difficult on the larger schools. Of 34 schools dealt here 5 schools
contained only dalli-type, I school was composed of 5 truei-types and I dalli-type.
Other 28 schools classified into the school of truei-type includes the two cases, thefirst is that all the individuals in a school were confirmed as truei-type, and thesecond is the case of 4 schools where only the part of the individuals were confirmed as truei-type but the presence of dalli-type was not confirmed. The ratio of
the truei-type in the 34 schools is estimated as 90.8% and that of dalli-type 9.2%.
The probability of the ratio of the two types in a school was calculated based on
the above ratio and on the assumption that the combination of the two types isSci. Rep. Whales Res. Inst.,
No. 30, 1978.

7

DALL'S PORPOISE

random. As shown in Table 2, the actual frequency of the mixed schools is much
lower than the probability calculated on the assumption of random combination.
Especially the discrepancy found for the schools of 3 or less individuals must be
significant, because the identification of the individuals is most reliable. If theabove calcultion is based on the ratio of dalli-type 3.6% observed in the catch off
the Sanriku coast, the disagreement with the actual sightings is more exaggerated.
This indicates that the school formation of Phocoenoides off Sanriku coast is, at leastin some degree, dependent on the colour pattern.
Though it is not used in the present analysis, a school of about 20 dalli-types
sighted in September at 38°30'N, 142°30'E (Nishiwaki 1967b) must be consideredby the same way. Mr Y. Jinno, the captain of the research vessel Tanseimaru, sighted and tagged in November and December 1972 three schools of thespecies approximately at 42°05'N, 141°05'E. These schools were composed of 5,
7, and 20 to 25 individuals of the dalli-type. The correctness of the record of thepigmentation was confirmed on the photographs. And in May 1974 anotherschool of 3 dalli-types was sighted by him at the same position mentioned above

EEr-\!.'J3
SEA

"\

'

SEA

+ •

.+

+ +'"ff>..

,,+,,/'" +

~

+

T

Z

----~~-+~~-,-~~-r+'-,<-;'~~~~t~·-1--~~~--1~

,:1

!t+

°+/

LI')

+ +

.....

+ +
+
•

·i

....

::

z

'df---,."'-~~-+~~~~~~+--~~~~~·-+~~~~-j~~Fig. 2. Position of sighting or of sporadic catch of Phocoenoides. Open circle orbar: truei-type, closed circle or cross: dalli-type, half closed circle: school with two
types. Circle indicates the school used for the analyses of the school size, larger
circle 10 schools, the smaller one, solid lines the spring-summer migration of thestock off the Pacific coast of Japan or that of the stock in the Sea of Japan andOkhotsk Sea, and dotted line the autumn-winter migration of the stock in thenorthwestern North Pacific and western Bering Sea.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

8

KASUYA(Fig. 2). As already indicated by Kasuya (1976b), it is difficult to explain theseschools as caused by the random assemblage of the less frequent dalli-type.
It will be reasonable to conclude from the above discussions that all the dallitype individuals found off the Pacific coast of Japan are not the members of theJapanese east coast population, but at least some of them, especially those in theschools not containing the truei-type, are migrant from the offshore population in
the northwestern North Pacific and western Bering Sea or from the Sea of JapanOkhotsk Sea population. This is consistent with the informations on the ratio of
the three colour types analysed in page 10. The interbreeding between the populations will be more scarce than expected from the apparent coexistence of thetwo types.
Kuroda (1954) stated that a fetus of truei-type pigmentation was obtained
from a female of dalli-type, and suspected the presence of the interbreeding between
the two types. His data is based on one of the two fetuses reported by Wilke
et al. (1953), where more precise description of the fetuses is available. Wilke
TABLE 3.

SCHOOL SIZE OF PHOCOENOIDES DALL! IN THE WESTERNNORTH PACIFIC AND ADJACENT SEAS
School size

Area & month
2

3

4

5

6

7

Coastal Pacific (39°-54°N), truei-type
Jan.
2
Feb.
2
2
May
1
21)
3
6
3
Jun.
2
2
2
Jul.
Aug.
Total
6
5
5
3
2
8
4
Coastal Pacific, dalli-type
Feb.
May
2
Jun.
2
Nov.
Dec.
Total
0
0
0
5
Offshore Pacific (42°-44°N, 162°-168°E), dalli-type
0
May
0
1 0
0
1 3
Sea ofJapan2l, dalli-type
May
0
5
0
0
0
2
Okhotsk Sea3 ', dalli-type
May
1
Jun.
2
5
1
6
Jul.
Sep.
8
3
3
7 4
Total
4
4
0 16 10 10

8

10

13

15

20

22

35

2
1
2
2
0

0

7

0

Total
schools

Mean
size

2
9
6
20
5
1
43

6.0
6.4
10.0
3.9
6.6
4.0
5.7

9

2.0
2.5
2.0
6.0
22.0
5.2

7

8.4

0

8

I.9

0

2
3
16
25
46

3.0
2.3
4.3
3.8
3.4

3
2
2
0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1) One mixed school of 5 truei-types and 1 dalli-type included, 2) Type unidentified 4 schools (10 individuals) included. 3) Type unidentified 2 schools (6 individuals) included.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

9et al. (1953) state that a [near term] fetus of 89.5 cm obtained from dalli-type
female on 16 May was "marked, although in indistinct gray tones, like truei" but"the pattern of 10 pound fetus [77.8 cm in body length] taken 24 May [from atruei-type female] was indistinct". The feature of the former fetus seems to becontinuous to the pigmentation of the newborn calf of the dalli-type, where theanterior lateral region is pigmented in dark gray and changes later into black(Kasuya 1976b). Kasuya (1976b) states on the young truei-type that the corresponding part is pigmented in light gray tone but its posterior border is indistinct.
Accordingly, the example of the two fetuses described by Wilke et al. (1953), can
not be a proof of the presence of the interbreeding, but can be a suggestion thatthe two types are distinguishable at the stage of about 80 to 90 cm in body length.
Table 3 shows the school size frequencies of Phocoenoides observed mostly by myself and few by other trained observers. The positions of these sightings are indicatedby circles in Fig. 2. Other sporadic records of the catch or of sightings indicatedby cross or bar are cited from Kasuya (1976b) and Ohsumi (1975), but are not included in Table 3. Part of the latter data could be overlapping since some of theirdata sources are same. The schools of Phocoenoides are usually composed of lessthan 7 individuals (87.6% of total number of schools). Since the mode exist at 2
individuals, it is suspected that the basic unit of the school of this species would
probably be 2 or 3 individuals and that the schools with 4 to 7 individuals will beformed by the aggregation of the basic units. It is also suspected that the uncommon large schools exceeding 10 individuals is formed by the repeated aggregation of the schools. In the present scanty materials, the seasonal variation of theschool size is not indicated.
Between the colour types there is observed a slight difference of the school
size. If the 32 schools of the truei-type sighted from May to September is compared with 54 schools of dalli-type sighted in the same season in the Sea of Japanand Okhotsk Sea, the former gives the mean school size of 5.4 individuals and thelatter 3.5. The ratio of the schools composed of 4 or more individuals is 59.4%
in the former, and 37.0% in the latter. This is an indication of the presence ofthe difference of the schooling behavior between the two populations. It is notclear, at present, if this difference is caused by the geographical difference of foodavailability and of the density of porpoise population, or by the difference of thehereditarily decided schooling behavior. However, the fact that the school sizeof the dalli-type in the area where the truei-type is dominantly distributed is also
sligtly small may suggest the latter possibility.
VARIATION OF PIGMENTATION

Ratio of the three colour types
Table 4 shows the ratio of the three colour types in the catch off Sanriku
coast. If all the available data are combined, the ratio of truei-, dalli-, and blacktypes in the catch is given as 96.3 %, 3.4%, and 0.3 % respectively. The ratio oftruei-type fluctuates between 90.9% and 98.6% in half month groups. And there
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

10TABLE 4.

SEASONAL FLUCTUATION OF THE RATIO OF THREE COLOUR TYPESOF PHOCOENOIDES IN THE CATCH OFF SANRIKU COAST,IN NUMBER AND PERCENT
Feb.

Jan.

Apr.

Mar.

Total

Colour types
B
Males
truei-type
dalli-type
black-type
total
Femalestruei-type
dalli-type
black-type
total

A

B

94.4 203
5
4.8
0
0.8
208
100

97.6
2.4
0.0
100

145
2
0
147

98.6
1.4
0.0
100

113
6

72

96.0
2.7
1.3
100

96.7 174 96.7
59
3.3
6
3.3
2
0.0
0
0.0
0
180 100
61 100

2
75

120

A
60
6
0
66

B

90.9 54 93.1
5.2
9.1
3
1.7
0.0
100
58 10097.7
66 95.7 86
3
2.3
4.3
2
0
0
0
0
69 JOO88 100B
9 100.0
0.0
0
0.0
0
9 100

584 96.1
22
3.6
2
0.3
608 100

0
0
0
0

457
15
473

96.6
3.2
0.2
100

A: lst to 15th, B: 16th to the end of month.
TABLE 5.
Body length
(cm)
Males
truei-type
dalli-type
black-type
total
Femalestruei-type
dalli-type
black-type
total

RATIO OF THREE COLOUR TYPES OF PHOCOENOIDES IN THE CATCH
OFF SANRIKU COAST, IN NUMBER AND PERCENT
156-175

176-195 (d')
176-187 (~)

196-217 (d')
188-195 ( ~)

104
4
0
108

96.3
3.7
0.0
100

164
4
0
168

97.6
2.4
0.0
100

118
4
0
122

96.7
3.3
0.0
100

81
6
0
87

93.1
6.9
0.0
100

310
9
1
320

96.9
2.8
0.3
100

80
2

96.4
2.4
1.2
100

83

is detected a slight increase of the dalli-type from 1.8% in January to 3.3% mFebruary, and to 5.0% in March.
The ratios of the three colour types in the growth stages are shown in Table
5, where the body length are grouped from the smallest to 175 cm or the mean
length at 2 years of age, from 176 cm to the approximate lengths at sexual maturity,
and above them. The ratio of the dalli-type is highest in the smallest group.
However the seasonal change of the ratio is too large to be explained by the higher
ratio of dalli-type in the juveniles and the seasonal increase of the ratio of thejuveniles (Table 14). It is more reasonable, as already suggested by the analysisof the school structure, to consider that there are dalli-type porpoises of otherpopulation(s) migrating to the Sanriku coast and intermingling with the stock off
the Pacific coast of Japan.

Individual variation of the pigmentation
The detailed analysis of the individual variation of the pigmentation is madeSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

11

based on the black and white photographs of 537 individuals caught by the harpoon
fishing off Sanriku coast. The pigmentation of the lateral region is classified into
the following 14 minor groups.
·
1. Truei-type, spots in the lateral region absent or scarce (Pl. I, Figs 1-2).
2. T ruei-type, spots abundant in the anterior lateral region (Pl. I, Figs 3-4 ).
3. Truei-type, spots distributed in the entire white area (Pl. I, Figs 5-6).
4. Truei-type, spots restricted to the posterior lateral region, in some individuals a vertical belt of spots may exist at mid-lateral (Pl. I, Figs 7-8).
5. Truei-type, a vertical belt of spots at anterior lateral region (Pl. I, Fig. 9).
6. Truei-type, two vertical belts of spots at anterior lateral region (Pl. I,
Fig. 10).
7. Truei-type, three vertical belts of spots at anterior lateral region (Pl. II,
Fig. 1).
8. Truei-type, four or more vertical belts of spots, or belts unseparable (Pl.
II, Fig. 2).
9. Truei-type, same as type-8 except for the dark background and fusedmottling (Pl. II, Fig. 3).
10. Truei-type, same as type-8 except for a belt of spots at center of lateralregion (Pl. II, Fig. 4).
11. Truei-type, all the lateral area grayish (Pl. II, Fig. 5 ).
12. Black-type, entire body pigmented almost same darkness (Pl. II, Figs
6, 8).
13. Dalli-type, without spot (Pl. II, Fig. 7).
14. Dalli-type, spots in the posterior lateral region (Pl. II, Fig. 9).
The above classification is somewhat arbitrary. The distinction of type-I
and type-2 depends on the density of the spots in the anterior lateral region andthe variation of the two types is continuous. However, they are clearly distinguished from type-4 with spots in the posterior region or from type-3 with spots
in the entire lateral region. All the 6 types, from type-5 through type-10, arecharacterized by the presence of the dense spots forming one or more verticalbelts, and the presence of some common factors is suspected. The type-11 andtype-12 (black-type) are separated only by the darkness of the lateral region.
Since the density of pigmentation of lateral region, especially that of inguinal
region, is variable between the individuals classified into black-type, I suspect thatthe two types could be found as continuous when more data are accumulated.
The presence of the vertical belt of spots at mid-lateral region, type-10 and someof type-4, will perhaps be a variety of the spotting in the anterior or posterior
lateral region.
As mentioned in the former section, the young truei-type porpoise has palegrayish pigmentation at the chest region. This character is not analysed in thepresent study because such feature is not correctly recorded on the photograph.
There is, except for dalli-type, expected no significant difference of the mean bodylengths between the various types, and it is considered that the pigmentation patterns dealt here are not the growth characteristics. This is same with the case of
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

12TABLE 6.

FREQUENCY OF MINOR PIGMENTATION TYPESOF PHOCOENOIDES OFF SANRIKU COAST

Males

FemalesUnknown

Types

No.

%

Body length (cm)
range

I.

2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.

trueitype

"
"
"
"
"
"
"
blacktypedallitype14.
Total "

meanSE

157 53.2 149-214 186.1
78 26.4 164-217 185.4
18
6.1 168-201 188.6
0.7 168
168.0
2
3.1 180-198 191.1
9
1 0.3 182
182.0
0.7 177
177.0
2
0.7 168-192 180.0
24
1.4 178-190 183.03
1.0 199-204 201.55
1.7 176-202 186.0

1.13
1.42
3.47
2.58

12.0
3.00
2.50
8.08

0.7

179-207200.0

12 4.100295 100

163-196

175.4 4.122

149-217

14.0

No.

%

No.
range

102 47.7
60 28.0
19 8.9
0.9
2
2 0.9
2 0.9
0 0
4
1.9
9 4.2
0.5
4
1.9
0

Total

Body length (cm)
mean

SE

160-204
163-205
171-194
159-185
171-178
180-186

182.1 1.12
181.5 1.55
181.8 2.20
172.0 13.0
174.5 3.50
183.0 3.00

166-193
176-194190
175-185

179.0 7.81
183.6 3.04
190.0
180.0 2.89

0

7 3.3 168-190
2
0.9 175
159-205
214 100

No.

17 60.7
9 32.1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7.1
2
0

175.8 4.10
175.0

%

0

0
0
0
0
28 100%276 51.4
147 27.4
37
6.9
4 0.7
11
2.0
3 0.6
2 0.4
6
1.1
13
2.4
4
0.7
11
2.0
2

0.4

19
3.5
2
0.4
537 100

1) For the number and the percentage the length unknown individuals included.

the three major colour types.
Though it is left for the future study, several factors will have to be assumed
theexplanation of the presence of the various minor colour types. One of theforpossible example is shown below, where each factor is represented by a group ofgene or genes.
Factor A: promotes the melanization of the anterior lateral region
Factor B : promotes the melanization of the posterior lateral region
Factor C : inhibits the melanization of the anterior lateral region
Factor D : inhibits the melanization of the posterior lateral region
The coexistence of a promoter and inhibitor will result in the various density ofspotting in the controlled region. When the capitals indicate the presence of thevariable number of genes grouped into one of the 4 factors, and the small letterthe absence of the genes, most of the pigmentation types could be explained by thefollowing combination.
Truei-type (no spot, or spots in the anterior region, type-I, 2, and 5 through10, 86.0%) : abcd, abcD, abCd, abCD, AbCd, AbCD
Truei-type (spots in the posterior region, type-4, 0.7%): aBcD, aBCD
Truei-type (spots in the entire lateral region, type-3, 6.9%): ABCD
Black-type and type-I I of truei-type (2.4%): ABcd
Dalli-type (without spot, 3.5%): Abed, AbcD
Dalli-type (with spots, 0.4%): ABcD
Sci. Rep. Whales Res. Inst.,
No. JO, 1978.

DALL'S PORPOISE

13

However the absence of the individuals expressed by ABCd, aBCd, or aBcd,
which are expected to have the black posterior area and white or, spotted anterior
flank, will need further investigation.
PRENATAL GROWTH

Neonatal length
Because of the restricted fishing season, no full term fetus nor newborn calf
was obtained from the catch off Sanriku coast. Mizue et al. (1966) analysed the
24 full term fetuses and 19 newborn calves caught incidentally by the salmon
gillnet fishery in the northwestern North Pacific and western Bering Sea. The
body lengths of the smallest postnatal individual and of the largest fetus were
92 cm and 109 cm respectively, and they concluded that the mean neonatal lengthof the species will be about 100 cm. Table 7 shows the length frequencies of
TABLE 7. RATIO OF FETUS AND NEWBORN CALF OF PHOCOENOIDESIN THE NORTHWESTERN NORTH PACIFIC AND WESTERN BERING SEA
Published recordsll

Present data

Total in%

Body Length
(cm)

fetus

postnatal

fetus

postnatal

fetus

postnatal

85
90
95
100
105
110
115
Total

4
5
3
5
1
1
0
19

0
1
2
7
2
2
4
18

0

0
0
0

100.0
85.7
60.0
38.5
33.3
25.0
0.0
50.0

0
14.3
40.0
61.5
66.7
75.0
100.0
50.0

0
0
0
0
0

0
0
2

1) Cited from Mizue et al. (1966) and Koga (1969).

fetuses and newborn calves in Mizue et al. (1966), in Koga (1969), and few dataobtained by the research vessel of the salmon fishery and provided by Mr. S. Nagahora. The least squares regression is shown by
y = 3.201x-269.0

where x indicates the body length in cm, and y the ratio of postnatal individuals
in %. The body length where the ratio reaches 50% is 99.7 cm, which is rounded
and the mean body length at birth 100 cm is obtained for the population of Phocoenoides in the northwestern North Pacific and western Bering Sea.

Feta! growth and length of gestation
The fetal growth was estimated from the seasonal change of the fetal length
off Sanriku coast. The body lengths of 39 fetuses plotted against the date ofcatch show a linear increase (Fig. 3). The straight line calculated by the leastsquares is shown by the following equation,
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

14

KASUYAy = 0.3333x+ 19.4

where x indicates the days from the first of January, and y the fetal length in cm.
The fetal growth rate of the Japanese east coast population is about 3.3 mm/day.
On 28 August, the extended straight line reaches 100 cm or the mean neonatal
length of the population in the northwestern North Pacific and western Bering
Sea. This is an approximate estimation of the mean date of birth of Phocoenoides
off the Pacific coast of Japan. Some inaccuracy must exist in this estimation,
because the neonatal length of the offshore population is tentatively used for theJapanese east coast population.

00
0

10 0

E

u

c 80

-

..c

Ol

c

60

,•·~~r
.

>. 40

J/:.
!

lJ

0

CD

20
0

•

•

Q)

--- ---.. _... 0

N

D

J

F

M

A

M

J

J

A

s

Month
Fig. 3. Seasonal change of the body lengths of fetuses and infants of Phocoenoides.
Small closed circle and solid line: fetus off Sanriku coast, large closed circle: fetus
in the northwestern North Pacific and western Bering Sea (Mizue et al. 1966),
large closed circle with bar: that in Koga (1969), cross mark: that provided by A.
Takemura, open circle: infant in the same area in Mizue et al. (1966). Opensquare indicate the range of two fetuses in the southern Okhotsk Sea (present materials), and dotted square the range of 7 fetuses in the same area in Okada andHayashi (1951).

It is known that the growth of mammalian fetus is expressed by the earlier
slow growth stage and later linear growth stage (Huggett and Widdas 1951, Laws
1959). The backward extraporation of the above fetal growth line crosses withthe axis of time on 3 November of the preceeding year. The length of time fromthis date to the mean date of birth is 300.1 days. If the value of 0.135 estimatedby Perrin et al. (1976) for the fetal growth of Stenella attenuata, which has the gestation slightly shorter than 1 year, is used, the equation of Laws (1959) is shown as
follows.
Sci. Rep. Whales Res. Inst.,

No. 30, 1978.

DALL'S PORPOISE

15

tg-to = 300.1
t 0 = 0.135tg

Where t 0 indicates the length of time from the start of conception to the date whenthe extended straight line of the growth cuts the axis of time, and tg the total gestation length. These equations give the total gestation of 346.9 days or 11.4 months(using months of 30.4 days). The mean date of conception is, in the Japaneseeast coast population, approximately on 17 September.
AGE DETERMINATION

Tooth structure and growth layers
Following observations were made on the thin tooth sections decalcified andstained with haematoxylin. The tooth of the species is constituted of enamel,
dentin, and cementum. The enamel does not dissolve in the formic acid, which
is different from other delphinids. The decalcified enamel is stained blue withhaematoxylin. This is an indication of high content of organic matrix. There areobserved many fine growth layers in the decalcified and stained section of enamel.
The neonatal line in dentine is clear as a thin unstainable layer. There
are nearly 10 faint minute layers in the prenatal dentine. The growth layers are
irregular and unclear in the postnatal dentine laid down after the first layer.
Though the pulp cavity is wide on newborn individuals, it is soon filled with postnatal dentine leaving a narrow canal, and the base of the root is finally covered
with thick cemental layers. This narrow pulp cavity is connected with the surfaceof the tooth through canals penetrating the cementum.
The cemental growth layers are, on the longitudinal section, most clearlyobserved between the level of the proximal end of the fetal dentine and that of theproximal end of the first postnatal dentinal layer. Though the cementum of older
individuals is thick at the proximal end of the tooth, the growth layers are not welldetected in it.
The individual variation of cementum thickness is large. And there are often
observed accessory layers in the thick cementum, which makes the age determination difficult. The accessory layers are usually distinguished through the carefulobservation of the entire cemental layers of the tooth. The accuracy of age determination of the species by the cemental growth layer is so bat thCl.t two independent
counts made carefully on a tooth section often give the discrepancy of 20%.

Accumulation rate of the layers
The comparison of the independent counts of the dentinal and cementa! layers
is shown in Fig. 4, where only the individuals with readable dentinal layers are
selected. The number of growth layers in the two tissues is same on the individuals
younger than 4 cemental layers. At the older age, however, the accumulation of
the dentinal layer seems to cease as in the case of the striped and spotted dolphins
suspected by Kasuya (1976a) or the layer becomes indistinguishable.
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

16

KASUYA

Mizue et al. (1966) estimated the postnatal growth of theo ffshore populationin the northwestern North Pacific and western Bering Sea, based on the analysisof the body length and the body weight, that the body length of one year old individual is from 145 to 169 cm and that of the 2 years old is from 170 to 187 cm.
Since the breeding season of the species is unimodal and restricted to the summer
season, the reliable separation of some young age groups can be done based on
the length frequency. An estimation of the accumulation rate of the growth layers
is made in comparison with their study. As shown in Table 8, no trace of cemental
layer is observed on the teeth of two newborn individuals, I 00.5 cm female and110.0 cm female, caught by the salmon gillnet fishery. The postnatal dentine is
not formed in the former individual, but there is observed a very thin postnatal
dentine (5 µm in thickness) on the tooth of the latter individual (Pl. III, Figs I and
2). On the other hand, the teeth of slightly older individuals, 132 to 159 cm,
caught in winter off Sanriku coast have one stainable layer in cementum and thesame number of layers in dentine. Most of the 45 aged individuals of the length

/
/

8
Ul

.....

(!)

7

>.!!! 6
0

c 5
c

0

:;:;

<II

-

/

4

0

3

0 2

i

z

0

:·

!I

&I

o•

::

>'+·

:j

o\J•

o!

~

•

2

5 6
3 4
7 8 9
No. of cementa! layers/00//
ill! 0

"'O

0/:
••

•:

:/
0

10

11

12

Fig. 4. Scatter diagram of the relationship between the number of dentinal layers
and that of cementa! layers. Large circle indicates 5 individuals, the small one,
open circle female, and closed male. All the available data included.

TABLE 8. COMPARISON OF DENTINAL AND CEMENTAL GROWTH
LAYERS OF NEWBORN AND JUVENILE PHOCOENOIDES
Body length
(cm)
100.5
110.0
132.0
149.0
156.0
159.0

Sex

No. cementa!
layersNo. dentinal
layers

S2
S2

0
0

0
0

6
6
S2
S2

Date of catch

Remarks

21, VII, '73
26, VII, '72'74
30, I,
3, III, '74
27, II,'74
19, II,'76

dalli-type, N. Pacific"truei-type, Sanriku"
black-type,"Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

17

from 141 cm to 168 cm caught in late May to early July and dealt in the Addendumof this study are considered, though there might be the inclusion of few older individuals, to have been born in the preceeding summer. Among these 45 juveniles,
38 individuals or 84.4% of the total have only one stainable cementa! layer in thetooth, and only 7 or 15.6% have two cementa! layers. These informations indicatethat the cementum is absent on the tooth of newborn individual but the first stainable cementa! layer is completed or being deposited in the first winter, and that thedeposition of the next layer is not started in the season from late May to early July.
This is a strong suggestion that the stainable cementa! layer is deposited in autumn
and winter season as in the cases of the striped and spotted dolphins (Kasuya
1976a) or of the Baird's beaked whale (Kasuya 1977). Accordingly, the age ofthe individual with N cementa! layers and caught off Sanriku coast can be approximately N-0.5 years, because the birth in the species occurs in summer andthe fishing season is from late January to early April.

Females

80

c 60
<!J

~

<!J

D... 40R

20?, ,' \''tl..0"0.

0

140

160

180

200

160

180

'

'

',

'I

200

Body length in cm

Fig. 5. Phocoenoides off Sanriku coast, ratio of the individuals aged (closed circle andsolid line), and the ratio of the age indeterminable individuals in the tooth sampledindividuals (open circle and dotted line).

The ratio of the individuals aged by the above method is 60.0% of the length
known males and 62.4% of the females. As shown in Fig. 5, the ratio is slightly
low in the larger individuals, and especially in females. This seems to have beencaused by the failure of the preparation, higher frequency of the falling off of the
tooth in older individuals, or by the unreadability of tooth layers. However, thebias will not be so large to have significant effect on the analyses in the later sectionsof this study.
POSTNATAL GROWTH IN THE MALE

Length frequency
The body lengths of 485 males taken off the Pacific coast of Japan ranged
from 132 cm to 219 cm, with the mean length of 185.2 cm. Single mode wasSci. Rep. Whales Res. Inst.,

No. 30, 1978.

18

KASUYA

present at the length group of 180 to 183 cm. As shown in Fig. 6, the ratio of
the larger individuals and the mean body length decrease, in both sexes, with theprogress of the fishing season. Though the decrease in the half month interval is
not significantly large, the change is considered to be significant because the trend
continues from late January to late March.
Another important feature of the length frequency is, in both sexes, the scarcityof the individuals below 170 cm and above 200 cm. This will be related, as mentioned in the later sections, with the segregation and the bias caused by the fishingmethod.
10

males

Ja

96

5
15

F,
Fe

10

<!-

c 15
>- 5

MA

~ 20 Me

~o

~

~

QI

:J

a-

10

QI

.....

LL. 20

JL

Ae

10

~

145

+

62

~63.

53

~76

9

I
I

0
15

~

105

+

5
20

fem~
56
--+--

Total

399

485

5
0

140

160

180

200

160

180

200

Body length in cm
Fig. 6. Body length frequencies of Phocoenoides caught by the harpoon fishing off
Sanriku coast. Mean (vertical line) and range of two standard errors (horizontalline) are indicated. Suffix A attached to month indicates from the lst to 15thof the month, B from 16th to the end, and open circle and dotted line the ratioof dalli- and black-type in the total Phocoenoides.

Growth curve
The relationship between the body length and the age is shown in Fig. 7. It
is known in some delphinids that the mean growth curve of the male at the age
between 3 and 10 years is expressed approximately by a straight line (Kasuya
et al. 1974, Kasuya 1976a). In the present species, the mean body lengths at theSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

19

age from 1.5 to 6.5 years seems to come on a straight line.
gression of these points gives the following equation,y = 3.30x+ 169.58,

The least squares re-l.5~x~6.5

where x indicates the age in years, and y the mean body length in cm. The
gradient 3.30 indicates the mean annual growth rate. The corresponding figuresof Stenella coeruleoalba and S. attenuata are 4.40 cm/year and 4.20 cm/year respectively(Kasuya et al. 1974). Accordingly it is suggested that in spite of the similarity of
the neonatal length and length at 1 year of age, the growth of the later part of thelife is much slower in Phocoenoides than S. coeruleoalba, and that the asymptotic
i

i
0

;.

2.0

.. -r-.,,

..
..

..•

c 1.6
(!)

....
§ ,:>"

.I

E
c

•

°'!'

0

~

6

0

_._ •

___;:_--<>-0

•

_i_
0

.

--<>-0

I

I

•

• •

•• •

..

I

---

/

/

... -.- i.
:- • Lt ..• = • • ••

1.8

Ol

• •

l f.

•

.c.

.00
0

---------

8

i-

'?

-g>

00

0

0
0

>.
"O

0

co
1:4

1.2

2

4

6

8

10

12

14

16

Age in years
Fig. 7. Male Phocoenoides off Sanriku coast, scatter plot of body length on age (N =294). Large closed circle indicates the immature, closed with bar the maturing,
open circle the mature, and small closed circle the unidentified individual. Horizontal bar indicates the mean body length.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

20

length of the former species could be much smaller than that of the latter species.
Because of the scarcity of the adult individuals, the asymptotic length can not be
estimated in the present study. In Fig. 7, it was tentatively assumed to be between
200 and 210 cm and to be attained at or after the age of 15 years as in the case of
the female.

Male growth stages
The materials are collected from the catch of harpoon fishing. As the season
is limited to January through April, or 5 to 8 months prior to the peak of the mating
season estimated in this study, the activity of the male organ will be at the lowest.
For the preliminary examination, the maturity of the male was examined by
the following four methods.
1. Epididymal smear: classified into the three stages, "spermatozoon absent ", " scanty ", " copious ".
2. Testicular smear: classified as above
3. Epididymal tissue: classified into the following three stages.
" Immature", ductus epididymidis is simple and narrow, and the epithelial
cell is low.
"Maturing", folds of the ductus epididymidis are developed, but the lowepithelial cells are still present at some part.
" Mature'', folds of the ductus epididymidis are extremely developed,
and all the epithelial cells are tall.
4. Testicular tissue: classified into the following three stages by the examination of the peripheral and central testicular tissue.
" Immature ", both spermatid and spermatocyte absent in the sections
examined.
" Maturing", central tissue is mature and peripheral tissue immature.
" Mature ", both central and peripheral tissues are mature [ =spermatidor spermatocyte presentl
In the above criteria of the maturity of epididymal or testicular tissue, the presenceor absence of the spermatozoon is not taken into cosideration, because even in someundoubtedly mature testis the spermatozoon is absent or scanty in the season.
The spermatozoon in the testis tissue was, if present, always found with spermatid
TABLE 9. COMPARISON OF THE MATURITY INDICESOF MALE PHOCOENOIDES OFF SANRIKU COAST
Testis(histology)

Epididymal smear

Testicular smear

+

+t

+

+t

Im.

M'ing

Mat.

Immature
Maturing
MatureTotal, no.

157
10
23
190
94.5

0
1
7
8
4.0

0
0
3
3
1.5

5

0
0

71
0
0
71
73.2

11
3
1
15
15.5

0
5
6
11
11.3

%

152
13
18
183
89.7

14
20
9.8

0.5

Ductus epididymidis- : no spermatozoon, +: scanty spermatozoa, +t: spermatozoa copious.
Sci. Rep. ·whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

21

or spermatocyte, but the latter two not always with spermatozoon.
Among the 208 individuals examined of the testis tissue, there was no individuals where the peripheral testis is mature in spite of the immaturity of thecentral testicular tissue. Few individuals had no trace of spermatid nor spermatocyte but showed the presence of scanty spermatozoa in the testicular smear.
Though these individuals could have been producing the spermatozoa at some
restricted part of the testis, they will safely be considered as not acting as an adultmale because the ductus epididymidis is not fully developed and no spermatozoon
was found in the epididymal smear (Table 9).
On the other hand the individuals with the " mature " testis can be considered as really attained the sexual maturity, because about one third of theindividuals of this category showed the presence of spermatozoa in the epididymal
smear, and because the spermatocytes or spermatids in the winter season will growto the spermatozoa by the next mating season.
The most uncertain is the individuals with "maturing" testis. At least someof the males of this category are suspected to act as adult male, because there
is a case of the "maturing" male where the spermatozoa are present in the epididymal smear and more than half of the individuals of this category had the"mature " epididymis. However, it is impossible to consider that all of the malesof this category are sexually mature, since many individuals of this category still
have the " maturing " epididymis. Accordingly the males with the testis of thiscategory are dealt as the maturing, and the males with the testis of two othercategories as the immature and mature.

Attainment of sexual maturity in males
Table 10 shows the relationship between the age and the development of theepididymis. The ductus epididymidis start to develop at various age between 1.5
and 9.5 years. The fully completed stage appears at about 4.5 years. Since asmentioned in the later section, there is an underrepresentation of the matureindividuals, Table 10 can not be used for the estimation of the mean age at theTABLE 10. AGE AND DEVELOPMENT OF DUCTUS EPIDIDYMIDIS
OF PHOCOENOIDES OFF SANRIKU COAST
Age(yrs)

Immature

Maturing

Mature

0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5

0
7
10
13
12
9
4
5
5
2
0

0
1
2
3
2
2
0
2

0
0
0
0
2

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

5
1
0

0
0

KASUYA

22

completion of the development of the epididymis.
If the weight of testis and mean diameter of the seminiferous tubules areplotted on the logarithmic scale, there is observed a linear relationship (Fig. 8).
The least squares regression of 201 individuals is shown by
y = 33. 79xo.22a4

where y indicates the mean diameter of the tubules in µm at the center of testis,and x the weight of testis in grams. Both the immature and mature individuals
are expressed by the single equation. This feature is different from that observedon S. attenuata (Kasuya et al. 1974), which is possibly related with the length oftime from the mating season. The absence of the observable seasonal change inthe diameter of tubules will also have direct relationship with the restricted seasonof the samples. In Fig. 8, the points are clearly separated into two groups at the
150
130
Cll

o,p II':;. o
0.g.... 100

08 0
0

•

-

0

0

0 0

0

L.

Cll

a; 70
E

•

0

i5
50

.
...

....-:..: .....:, . . ..
.. ., . ,.. .·.
• •• 1· ••• ..·i., .
• -,•..:.?.,.. ;:.
.,.-····'
. ...
•

I

0

9

eofl'

• •

~

40"'---'--'--'-'--'-'--~-'=--:"-:,----',----'~-'--'--'---'--'--'---'-~~

5

7

10

15

20

30

50

70

100 150Weight of testis in g
Fig. 8. Phocoenoides off Sanriku coast, scatter plot of the mean diameter of seminiferous tubules (µm) on weight of testis (N=201). Closed circle indicates the immature, closed circle with bar the maturing, and open circle the mature.

tubule diameter of about 70 µm, or at the testis weight of 25 g. The group with
developed testis is mainly composed of the mature individuals, and the opposite
the immature. The individuals with the maturing testis, which is smaller innumber, situate at the intermediate position. This feature is shown graphicallyin Figs 9 and 10. The ratio of sexually immature individuals is 50 % at the tubule
diameter of about 70 µm, and that of sexually mature male at about 80 µm. As
the maturing stage must be passed by all the males before attaining the sexual
maturity, the former figure corresponds to the mean tubule diameter at the attainment of the maturing stage, and the latter to that at the attainment of sexual
maturity.
For the identification of the maturity of the male, the weight of testis is more
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

23

DALL'S PORPOISE
ioo.-..=--=--~~~~~~~~~~~~p~

I
I

.-..

~ 80

I

I

>u 60
c

I

I

I

\

\

\

I

I

I

\I

'd

I

~

I
I
I
'

(!)

',II

:J
O" 4 0....(!)

LL

20
0

52

--

62

72

82

92

102

112

Diameter of tubule
Fig. 9. Phocoenoides off Sanriku coast, relationship between the sexual maturity andmean diameter of seminiferous tubules in µm. Closed circle and thick solid line
indicate the immature, open circle and dotted line the mature, and thin solidline the maturing.
7

3

2

3

,,

4

3

4

I I

I

I

80

I
I
I

c
60
>u
c
~ 40

O"
OJ

....

LL

20

10

20

30

40

50

60

Weight of testis in g
Fig. 10. Phocoenoides off Sanriku coast, relationship between sexual maturity andthe weight of testis. 'The numerals at the top indicate the sample size. For
symbols see Fig. 9.

practical than the histological examination. The relationship between the maturity and weight of testis is shown in Fig. 10. The ratio of immature individuals
decreases in accordance with the increase of the testis weight from 10 g to 45 g.
The least squares regression is given by the equation,y = -5.382x+ 188. 73,
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

IO.O~x~34.9

24

KASUYA

where x indicates the weight of one testis, and y the ratio of immature individuals.
The value of y becomes 50% at the testis weight of 25.77 g, which is the mean
weight of testis at the attainment of the maturing stage. The relationship between
the ratio of mature individuals (y in %) and the weight of testis (x in g) is shownby the following equation.
y = 2.560x-34.10,

20.0~x~54.9

The mean weight of one testis at the attainment of sexual maturity is obtained fromthis equation as 32.85 g. This estimation is based on the assumption that thereis neither segregation nor difference of the catchability between the individuals of
the same testis weight and belonging to the different growth stages. Though thisquestion is not solved in this study, if it exists, the above estimation does not indicate the true figure of the population, but only can be applied in estimating thematurity of the harpoon fishing catch.
Mizue et al. (1966) examined the testis of 41 Dall's porpoises caught in theoffshore waters of the northwestern North Pacific and western Bering Sea in theseason from May to July. Though they did not give the weight of testis at theonset of sexual maturity, it is suspected from the relationship between body length
and weight of testes (Fig. 3, Mizue et al. 1966, and Fig. 27 of this study) that thesexual maturity will be attained between 100 and 200 gin the weight of combined
testes. The maximum weight of the combined testes in the summer catch is about
700 g. These values are more than twice larger than those observed in the wintercatch off Sanriku coast. This difference is too large to be attributed to the difference of the population, but more reasonably be considered in connection withthe annual reproductive cycle of the male. Mizue et al. (1966) state based on
the histological examination of the testes that there was found no " fully mature "individuals in the season, and considered that the weight of testes will increase in
the coming mating season. Though they did not give the definition of the criterion
nor explain if the " fully mature " stage indicates one of the growth stages or oneof the stages of the annual reproductive cycle of the adult male, I suspect that they
indicated the latter. Then more increase of the testis weight is expected in themating season in August and September.

Age at the attainment of sexual maturity
In calculating the age at the attainment of sexual maturity, the sexual maturity of some individuals was decided by the weight of testis. Namely the testis
weighing 25. 7 g or less was classified into the immature, that 32.9 g or more themature, and the intermediate into the maturing.
As shown in Fig. 11, the age of the youngest sexually mature individual is
3.5 years and that of the oldest immature 15.5 years. The maturing individuals
are found in the age of 4.5 to 7.5 years. These figures suggest that the mean ageof the male at the attainment of sexual maturity is approximately at 5 to 10 years.
The facts that the ratio of the mature males exceeds 50% at the age above 13
years and the number of individuals older than this age is extremely scarce, suggest
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

25

DALL'S PORPOISE
150~r~~~~~~~~~~~~~~~~~~~~~~~~~~

(Jl

c

10(
0
0
0

60

tll

.....tll 40

-....
0

.-- _":. _________.>.•- - - - - - - - - - - - - - - - - - - - - -"- - - - - - - - - __<:!_ ___ -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --....Cl.I 30 -----

20

.r:.

Ol 10 •

- - - --

0- - -- - ----- - --- -------------- --- ----- -•...

.. .f! =~~ t } :!i ·r. ..;...... i•r.c I'.. .r ,,.

>

:~

6
4

6
8
10
12
Age in years

4

2

0

14

16

Fig. 11. Phocoenoides off Sanriku coast, scatter plot of the weight of testis (logarithmic
scale) on age. Dotted lines indicate the mean weights at the attainment of maturing stage and of sexual maturity. For other symbols see Fig. 7.

40
30
Ul

<ii:::; 20
-0

> 10

"'O

c

......
0

0

z

8
6

•

4
3

0

0

0

•

0

•

•

2
1
0

0

2

4

6

8

10

12

14

16

Age in years
Fig. 12. Age frequency of male Phocoenoides in the catch of harpoon fishing off
Sanriku coast. Thin solid line indicates all the aged males, thick solid line thematurity known individuals, large open circle the actual number of aduit individuals, closed circle that of the maturing, and small open circle number of theadult males corrected for the fishing bias. For further explanations see text.

that the present sample underrepresents the older individuals. Possibly the older
males, and especially the adults, will be less attracted by the bow wave of the ship
and captured less intensely as in the case of the adult females and suckling calves
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

26

KASUYATABLE 11.

PHOCOENOIDES OFF SANRIKU COAST CLASSIFIED BY AGE
AND MATURITY, MALES
Calculated no. & %Actual no.

Age
(yrs)
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5

Im.

M'ing

Mat.

25
35
32
27
11
14
9
10
2
1
2
0
0

0
0
4
3
5
2
0
0
0
0
0
0
0
0
0

0

0

100.0
97.2
82.0
77.1
42.3
53.8
38.3
47.2
10.5
5.8
12.8
0
0
8.7
0

25
35
32
27
11
14
9
10
2
1
2
0
0

3
5
10
5
4
4
2

0

0

Male

o

I

I I
I

o'"' "',..! /:-...!!
0

M'ing

Im.

80

I I
I I
I I

c

•

.,,

~ 60

I

I

0
0
4
3
5
2
0
0
0
0
0
0
0
0
0

\
\
\I

Mat.

0
0
10.3
8.6
19.2
7.7
0
0
0
0
0
0
0
0
0

0
3
5
10
10.0
14.5
11.2
17.1
16.3
13.4
14.1
12.7
10.5
10.4

'I

Female

I

"

I<{

0

0

'o

I

I\

I

I\
10\
I
I I
I I
Q.... ~,

I

c

I

,,

Cl.I
:::J

I

I

,,\1 ,/

I

I

•

I

"
I

I

,'
0

15

I
I

0

1'
I

0- 40

0
2.8
7.7
14.3
38.5
38.5
61.7
52.8
89.5
94.2
85.9
100
100
91.3
100

,'•

<IJ

....

I

0

I

lL

I

I

,fy~O

,,

II '

20

'O

I

,,"

10

I

co
(!)

~,

02

4

6

8

10

12

14 16 2 4 6Age in years

8

10

12

Fig. 13. Phocoenoides off Sanriku coast, the relationship between the sexual maturity
and age. Closed circle and thick solid line indicate the frequency of immatureindividuals, open circle and dotted line that of the adults, and thin solid line thatof the maturing invididuals. The larger circle indicates the actual frequency andthe smaller the frequency corrected for the fishing bias.

mentioned later.
As the increase of the number of the mature males is almost linear at the ages
from 3.5 to 6.5 years, the fishing selection by the maturity seems to be less intenseat these ages (Fig. 12). Furthermore, as mentioned in the other section, most of
the calves seem to be weaned before 3.5 years and the fishing bias related with theSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

27

nursing can be negligible in the above range. Then the age frequency of the malein these age classes will indicate the approximate survival rate. The least squares
regression for the age range gives the total annual mortality rate of 9. 7 % or 0.102
in the instantaneous rate. The true ratio of the mature individuals in the ageclasses above 6.5 years is calculated based on the assumption that the mortality
rate is same in the older individuals and the shortage of the frequency above theage is caused by the underrepresentation of the adults (Table 11 and Fig. 12).
The corrected ratio of the mature individuals (y in %) is shown by the following least squares equation, when x indicates the age in years (Fig. 13).
y = 10.124x-32.17,

3.5~x~l3.5

The half of the males are sexually mature at the age of 8.1 years, which is an estimation of the mean age at the onset of sexual maturity of the males. The similar
regression calculated for the ratio of immature individuals is shown by the following equation.
y = -9.534x+ 123.52,

2.5~x~ 13.5

The ratio of immature males reaches 50% at the age of 7.7 years. This is themean age when the male attains the maturing stage. The fact that the difference
between the two ages is only 0.4 year suggests that the maturing stage is passed
within short period.
Another estimation of the age at the attainment of sexual maturity is obtained
as the mean of the age of the youngest mature individual and that of the oldest
immature. Though this methods gives a more direct estimation than the former,
the accuracy is lower because it is based on the small number of samples at theextremities of the ordinary distribution and because the estimation of the age ofindividuals is less reliable. This method, excluding the two textreme cases, gives(4.5+12.5)/2=8.5 (years), which is sufficiently close to the estimation obtained by
the first method.
Body length at the attainment of sexual maturity

The unbiased estimate of the mean body length at the onset of maturing
stage and that at the onset of sexual maturity are obtained from the mean growth
curve and the corresponding ages estimated above. They are 195.0 cm and
196.3 cm respectively.
As shown in Fig. 14, the rapid increase of the testis weight starts at the bodylength of 184 cm. This feature is almost identical with the species in the offshore
waters of the north western North Pacific and western Bering Sea studied by Mizueet al. (1966), or dealt in the Addendum of this study. The relationship between
the body length and the ratio of sexually mature individuals is shown in Fig. 15.
The largest immature male is found in the length group of 212 cm to 215 cm,
and the smallest mature male in that of 180 cm to 183 cm. The males in thematuring stage are found at the body length between 176 cm and 215 cm.
The least squares regression between the body length (x in cm) and the ratio
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

28

KASUYA150
oi 100.i:: 60

Ill

I ll
111

.......
0

30
20

_.
.i::.
Ol
111

10

:s:

6
4

160

180

220

200

Body length in cm
Fig. 14. Phocoenoides off Sanriku coast, relationship between weight of testis (logarithmic scale) and body length. Thin solid line indicates the mean in January(n= 55 ), thick solid line that in February (n= 157), and dotted line that in March(n= 39). For other symbols see Fig. 7. Nine data in April are plotted.

100

....

25

39

46

36

24 26
Male

22

11

,,4

6

'

~I

,',~

,,II\

80

"
''

.£

I

I
I
I

I

I

I

1

5

''''
' '

28

43

I

I

'

43

39

41

'

I

I

\,'

"

0.

'

"

;>. 60

0

u

24

24

9

' ,. . ~
'
'
''
'I
,,
''
I'
I
'
,,
,
I
I
,""
'
,,,o
"~

'' '

0

30

Female

'

I

'
''

1;1'

c

CIJ

I

5- 40

p-: "

,,,
"
,,

QI

....

lL.

20

,,

£,0

,,''

.,..IJ-,-/:.d
0

/

177.5

185.5

193.5

201.5

209.5

217.5 169.5

177.5

185.5

193.5

201.5

Body length in cm
Fig. 15. Phocoenoides off Sanriku coast, relationship between sexual maturity andbody length. Numerals at the top indicate the sample size. For other symbols
see Fig. 13.

of the immature males (y1 in %) and that between body length and the ratio of
mature males (y 2 in %) are as follows.
Y1 = -5.8225x+ 1181.37

y2 = 6.4850x-1220.37

l88~x~203
188~x~203

These equations give 194.3 cm for the body length where half of the individuals
are immature, and 195.9 cm for the length where half of the males are mature.
The discrepancy between the two lengths is caused by the presence of the males
at maturing stage. Since these calculations are not corrected for the fishing bias,
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

29

the lengths are able to be used only for the purpose to get the unbiased estimateof the maturity of the harpoon fishing catch, but are not applicable to the samples
obtained by other method.
Generally speaking, it is not expected even for the Dall's porpoise that thetwo sets of the body lengths estimated above by the different methods coinside,
because the growth of the length seems to be very slow after the attainment ofsexual maturity. Similar feature is indicated on S. coeruleoalba by Kasuya (1972).
The coincidence of the two sets of figures are considered to have been caused by
the underrepresentation of older individuals in the present materials.

Conclusion
In the present study the maturity of the male is classified into the three categories of the immature, maturing, and mature. However, this is too complicatedfor the practical use in the analyses of the life history of the species. For theunbiased classification of the males into two categories of the immature and mature,
it will be practical to take the mean of the values at the attainment of the maturing
stage and of the mature stage. They are as follows ;Weight of testis: (25.77+32.85)/2=29.31 g
Age(7.7+8.1)/2=7.9 years
Body length(195.0+196.3)/2=195.7 cm
The last figure is practically same with another estimate (194.3+195.9)/2=195.l
cm.
POSTNATAL GROWTH IN THE FEMALE

Length frequency
The body lengths of 399 females ranged from 156 cm to 209 cm with the mean
length of 183.5 cm (Fig. 6). Single mode will exist at the length group of 184 to
187 cm. The mean length decreases, as in the case of the male, with the progressof the fishing season. The large adult individuals and the juveniles of the sucklingage are scarce in the catch.
Growth curve
The relationship between body length and the age in the female is shown in
Fig. 16. The mean growth curve seems to be almost identical with that of the male
until the age of 3 years, then the growth becomes slower than the male (Fig. 31).
Because of the scarcity of the adult females, the growth after the age of 7 years is
not directly estimated. Possibly the asymptotic length of between 195 and 205
cm will be attained at about 15 years of age as in the case of S. coeruleoalba (Kasuya
1976a). The mean growth curve in Fig. 16 was drawn by eye based on the above
assumptions.
Age at the attainment of sexual maturity
As shown in Fig. 17, the combined weight of ovaries is usually smaller thanSci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

30

g

2.0

0

..
c

§

0

~

00

0

.,

0

8o

- - - o.0

g ()

~.

..

Ol
Cl.I

eeo

:.

E
c
.c

-

r

•

1.8

•

•

•8o

•

1.6 •

>-

""O

0

co
1.4

1.2

1.0'--~---'"-~~~-~~~~~~-'--~---'"-~~~-~~

0

2

4

6

8

10

12

14

16

Age in years
Fig. 16. Female Phocoenoides off Sanriku coast, scatter plot of body length on age.
Growth curve is drawn by eye. For marks see Fig. 7.

3.5 g in the immature individuals having no follicle measuring 1 mm or more in
the mean diameter. However, that of the immature females with the follicle
exceeding the size shows heavier weight. The weight of 8 pairs of ovaries with
corpus luteum ranged from 6.8 to 14.5 g.
The age of the youngest sexually mature female is 2.5 years, and that of theoldest immature is 11.5 years (Table 12). However, considering the fact that theage determination of this species is less accurate than that of other delphinids andthat the ovulation occurs about 0.5 year before or after the fishing season, it will
be better to say, excluding the samples at the both limits of the range, that thefemale attains the sexual maturity at the age between 3 and 11 years. The centerof the range situates at 7 years of age, which can be a rough estimate of the mean
age of the female at the onset of sexual maturity.
Another estimation of the age at the onset of sexual maturity is calculated
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

I
)

en 14

c
Vl
GI

12

.... 10

0

>
0

--

8

0

6

CIJ

2

31

---.........

.c

en 4

3=

0

'

' '

I.

"--k~
I

0

. . +-'

I
I

I'---1 .....
--"--..l
I
I

' ...
..-+

l

_.. /

I

...... --{.........
I

10

8

6

4

2

:

Age in years
Fig. 17. Phocoenoides off Sanriku coast, relationship between combined weight ofovaries and age. Thin dotted line indicates 86 immature females without measurable follicle (~ 1 mm), thin solid line 34 immature females with measurable follicle, thick dotted line 2 adult females without corpus luteum, and thick solidline 7 adult females with corpus luteum. Mean and range are indicated.

40
30
Ul

0

20

:J
"O

10
8
"O
c 6

>

0

....0 4
0

z

0

0
0

3

2
0

1

0

2

4

6

8

10

12

14

16

Age in years
Fig. 18. Age frequency of female Phocoenoides in the catch of harpoon fishing off
Sanriku coast. For symbols see Fig. 12.

from the ratios of adult females in each age groups. The underrepresentation of
the adult females was corrected, using the total annual mortality rate of 9. 7 %,in the same way as used for the males (Table 12 and Fig. 18). The least squares
regression gives the equationy = 9.483x-14.39,

l.5;:;:;x;:;:; 12.5

where y indicates the corrected ratio of the adult females m %, and x the age in
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

32
TABLE 12.

Age
(yrs)
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5

PHOCOENOIDES OFF SANRIKU COAST CLASSIFIED BY
AGE AND MATURITY, FEMALES
Calculated no. & %Actual no.
Im.

Mat.

23
17
25
21
18
13
8
2
5
3

0
5
7
7
20
8
8
4
3

Im.

I

0
0

I

2

Mat.
100.0
94.4
83.3
75.0
72.0
39.4
41.5
11.5
31.8
21.1
7.8
0
0

23
17
25
21
18
13
8
2
5
3
0
0

0
5
7
7
20
11.3
15.4
10.7
11.2
11.8
11.6
10.4

0
5.6
16.7
25.9
28.0
60.6
58.5
88.5
68.2
78.9
92.9
100.0
100.0

years (Fig. 13). This equation gives 6.8 years for the age when 50% of the females
are sexually mature, which is sufficiently close to the figure obtained by the first
method.

Body length at the attainment of sexual maturity
The body length at the age of 6.8 years, the mean age at the attainment ofsexual maturity, is read on the mean growth curve as about 186.5 cm.
As shown in Fig. 16, the individuals oflarge body size attain the sexual maturity at an age younger than the smaller. The smallest mature female is found in
the length group of 172 to 175 cm, and the largest immature in that of 200 to
203 cm. This range is not significantly different from the values of the stock in
the western Bering Sea and northwestern North Pacific (Mizue et al. 1966, andAddendum of this study). The least squares regression between the percentageof the adult individuals, y, and the body length, x in cm, is expressed by
y = 3.8807x-674.72,

176~x~l99

and shown in Fig. 15. This equation gives 186.7 cm for the body length where
50% of the females are sexually mature.
REPRODUCTION

Breeding season
Two different opinions have been published on the gestation length of thisspecies. Okada and Hayashi (1951) considered on the Sea of Japan-Okhotsk Sea
stock that mating will take place in about one month from the end of June to thebeginning of July. However, in the same paragraph, they tell that the copulationmust occur in August. The gestation was considered to last for 7 to 9 months.
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

33

One of the reasons for this conclusion is their observation that " Most of the females... collected [by them] in August in the Okhotsk Sea were found to be carrying
small embryos of about 5.5 mm in length ". Another reason is, though the data
source is not clear, that "the adults with embryos of 5 mm in body length werecaught commonly in the Okhotsk Sea during the summer from June to August,and the adults caught off Tajima, [southern] Sea of Ja pan during March and April
had either already given birth or had a large fetus of one meter in body length."
In their study, no comment was made on the neonatal length.
Mizue et al. (1966), on the other hand, estimated the mean neonatal length
as about 100 cm from the largest fetus and smallest infant caught by the Japanesesalmon gillnet fishery in the western Bering Sea and northwestern North Pacific.
They suggested the parturition season in late July to early August, the mating
season after the end of salmon fishing season in early August, and consequently thegestation of less than 1 year.
In the present study, the gestation of 11.4 months and the parturition peakin late August were calculated for the population off the Pacific coast of Japan.
These conclusions are similar to those estimated by Mizue et al. (1966).
The most important difference between the three studies concerns if the peakof parturition of the population in the Okhotsk Sea and Sea of Japan really exists
in April as suggested by Okada and Hayashi (1951). If this is correct the lengthof gestation can be about 7 to 9 months (Okada and Hayashi 1951) and the daily
growth rate of the fetus at the linear growth stage can be approximately between
100/30.4(7-7x0.2)=0.587 cm/day and 100/30.4(9-9x0.2)=0.457 cm/day. On
the other hand, Kasuya (1977) showed that there is a linear relationship between
the neonatal length of delphinids (x, cm) and the daily fetal growth rate at thelinear part of the fetal growth (y, cm/day). The equation was shown as follows.
y = 0.001462x+0.1622

This equation and the neonatal length of 100 cm give the daily growth rate of
Phocoenoides as 0.308 cm/day, which is much smaller than the rate calculated above
but closer to the value 0.333 cm/day obtained in the former section. This suggeststhat the parturition season and the gestation length estimated by Okada andHayashi (1951) is wrong.
The extended line of the fetal growth off the Sanriku coast shows good coincidence with the fetal lengths in May and early June in the western Bering Sea andnorthwestern North Pacific. However, most of the points in late June and Julycome above this line. There could be two interpretations for it. One is to consider that the growth of the near term fetus is faster than the previous stage. Another possibility is to expect the segregation of pregnant females in relation to thesize of the fetus. Though the precise locations of the samples are not known, thepresence of segregation is suggested by the fact that the fetuses in 1968 season(Koga 1969) come below the growth curve and those caught in 1964 and 1965
(Mizue et al. 1966) and in 1967 (provided by Dr A. Takemura) come above theline. Possibly the pregnant females of the stock with nearterm fetus will stay, in
Sci. Rep. Whales Res. Inst.,.No. 30, 1978.

34

KASUYAlate June and July, in the waters from the western Aleutian Islands to the east
coast of the Kamchatka Peninsula, and the cows with smaller fetus will migrate tofurther north.
Figure 19 shows the frequency distribution of the estimated date of parturitionin the two stocks of the species. The dates were calculated, using the fetal growth
curve of the Japanese Pacific coast stock, as the date when calves smaller than
110 cm in body length and all the fetuses might have attained the mean neonatal
length of 100 cm. Since the dates and the lengths offetuses in Mizue et al. (1966)
were read on their Fig. 2, the accuracy might be worse. An apparent parturition
peak exists in late July to early August, or slightly before that of the Japanese
Pacific coast population. However, as the larger fetuses are overrepresented in
the sample, the real parturition peak will be in late August to early September asin the case of the population off the Pacific coast of Japan.

(/)

15

d:J 10"O.'.".'.

5

"O

c:

5
A

Month

Fig. 19. Frequency of the date of birth calculated from the mean fetal growth curveand feta! length in the catch by salmon gillnet fishery in the western Bering Sea
and northwestern North Pacific (Top), and that in the catch of harpoon fishing
off Sanriku coast (bottom, based on fetuses). For the offshore stock, white squareindicate 12 infants in Mizue et al. (1966), square with lines 24 fetuses in the same
study, dotted square 20 fetuses provided by A. Takemura, and black square 8
fetuses in Koga (1969).

The mating season is not precisely known for the stock off the Pacific coast ofJapan, because of the restricted fishing season and of the inaccuracy of the estimation of the fetal growth in the early pregnancy. However, rough estimation of therange of the season is made by moving the fetal growth curve back and forth.
Then, the mating season from middle August to late October with a peak in September is estimated. Possibly this is consistent with the fact that the development of
the follicles in the ovary of immature female starts in March (Fig. 20). On the
Sea of Japan-Okhotsk Sea population, the occurrence of small embryos in August
was confirmed by Okada and Hayashi (1951) and that in September by myself.
Accordingly it is reasonable to expect that the mating season and probably theSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

35

parturition season of the population is not different from those of the stock off the
Pacific coast of Japan. At present there is no data to consider that the mating
season of the stock in the western Bering Sea and northwestern North Pacific isdifferent from that of two other populations dealt in this study. The concentrated
unimodal breeding season of the species is different from the pattern found in thetropical or subtropical species, but seems to be common in all the boreal delphinids(Kasuya 1972).
11

11, __________ 26

68
I
I

5

40

/

E
E
c: 4

',

,/

.............

........ _- ---

°lo

50

I

I

30

/

20

....

10

<II

a; 3
E

0

0 2

0 '-----'~~~-'-~~~--'-~~~..__~~--'-~-'
JA

FA

Fa

MA

Me

Month
Fig. 20. Seasonal change of the diameters of the largest follicle in the ovaries ofimmature females off Sanriku coast. Dotted line and right scale indicate theratio of the females with measurable follicle (;s 1 mm), thick solid line the mean
diameter and the range of the largest follicles in those individuals, and thin solidline the mean of all the immature females calculated assuming 0.5 mm for theunmeasurable follicles. The suffix A attached to the month indicates from thelst to 15th of the month, B from 16th to the end, and numerals the sample size.

Sex ratio
Of the 39 fetuses from the Sanriku coast, 21 fetuses or 53.8% of the total wererepresented by the female. The records of 51 fetuses of the stock in the western
Bering Sea and northwestern North Pacific were compiled from 24 fetuses in Mizueet al. (1966), 8 fetuses in Koga (1969), and 19 fetuses provided by A. Takemura.
They contained 33 females or 64. 7% of the total. Though these data suggest theslight excess of the female fetus in the two populations of the Dall's porpoise, further
study is needed for the conclusion.
The overall sex ratio of the postnatal individuals seems to be even in the 290
samples captured by the salmon gillnet fishery in the western Bering Sea andnorthwestern North Pacific (Table 13). However, as shown in Table 14, thefeature is slightly complicated in the catch by harpoon fishing off Sanriku coast,
if analysed by the season and maturity. The maturity of most of the males wasSci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

36

TABLE 13. SEX RATIO AND MATURITY RATE OF PHOCOENOIDES CAUGHT
BY SALMON GILLNET IN THE N.W.N. PACIFIC AND W. BERING SEA
Author2

Sex ratio4>
Adult

N

%
Immature

N

%
Total

N

%
Maturity•>
Male

N

%
Female

N

%

34
58.8
73
42.9
107
49.5

148
54.l

54
25.9
53
37.7

60
43.3

3

Total

35
40.0

34
58.8
73
42.9
290
50.7

14
57.l

54
16.7
127
42.5

1) Provided by Takemura, 2) Mizue et al. 1966, 3) Koga 1969, 4) Ratio offemales, 5) Testes exceeding
150 g in combined weight are dealt as mature.

TABLE 14. SEX RATIO AND MATURITY RATE OF PHOCOENOIDESCAUGHT BY THE HARPOON FISHING OFF SANRIKU COAST1 >
Jan.
B

Month
Sex ratio 2 >Adult
Immature

B

44

63.6
108
25.9
222
33.8

53
49.l
111
29.7
186
32.8

69
62.3
237
43.0
488
46.5

35
74.3
90
41.1
135
51.l

24
66.7
105
57.l
145
60.7

4
0.0
5
0.0
9
0.0

229
60.7
656
39.6
1,081
43.8

96
16.7
56
50.0

105
25.7
59
44.1

161
16.1
145
29.7

62
14.5
63
41.3

53
15.1
76
21.l

9
44.4
0

486
18.5
399
34.8

N
N

N

%
FemaleA%%
Maturity
Male

B

Apr.
B

Total

A

N

%
Total

Mar.

Feb.

N

%

A, lst to 15th. B, 16th to the end of month.
2) Ratio offemales.

1) Based on body length when genital organ is not studied.

identified by the histological examination of the testis, however the males with atestis weighing 29.4 g or more were considered as sexually mature when the testis
was "maturing" or testis was not examined histologically. The maturity of someother males was classified by the body length (196 cm or more into the mature).
The females measuring 187 cm or more were considered as sexually mature, when
the reproductive tract was not examined.
The ratio of females in the immature individuals shows an increase from 26%
in late January to 57% in late March. The age classes producing such a seasonal
fluctuation is from 4.5 to 7.5 years of age, namely the individuals after the cornSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

~ 70

/
,"

,'
'

"

'\
\

'

\

\

37

\Mar.

c

'

I

(/) 60

I

''I

i

I
I

(!)

ni
E 50

I
I

I
I

I
I

I
I
I

OJ

0 40

·;·. ~
I:

I

I------\!

0iii 30

I
I

0:::

I

.,,
I

~

20
/

-1.5

3.5

5.5

7.5

9.5 10.5-

Age in years
Fig. 21.

Phocoenoides off Sanriku coast, seasonal change of the sex ratio.

-+-

i!-

c: 20

p--o'

,~
I

I

I
I

>.
u

c:

<I>
:J 10
0-

....(i)u..
OL.L>-L__L_14L0...LJ..L.....l'-""'~16~0J_JL-=..L--'-1~8-01-.1.--'--'-.1...l-L--'-'--U..~'--J

Body length in cm
Fig. 22. Body length frequency of Phocoenoides caught by the harpoon fishery off
Tajima coast compiled from Noguchi (1946). Closed circle and solid line indicate male (n= 140), and open circle and dotted line female (n=57). Mean bodylength and the range of two standard errors are indicated.

pletion of weaning and before the attainment of sexual maturity (Fig. 21). The
decline of the female ratio at the ages of 7.5 and 8.5 years will have a relationshipwith the fact that the females attain the sexual maturity about one year earlier
than the male and the sexually mature individuals are underrepresented in thecatch of harpoon fishing. In the adult individuals the number of the females
exceeds that of the males in all the months studied. Though the trend of seasonalchange of the sex ratio is same with the immature individuals, the seasonal difference is much smaller.
Noguchi (1946) provided the body length measurements of 197 Dall's parSci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

38

poises caught by the harpoon fishing off Tajima coast (southern Sea of Japan).
There is some question if the length was measured by the same way used in thisstudy because both the modal and mean lengths are about 5 cm larger than thepresent materials collected off the Pacific coast of Japan (Fig. 22). The date of
the catch is somewhere between the middle of March and middle of May. The
overall sex ratio is so low as 28.9%, and if the maturity is estimated based on the
body length criteria obtained in this study, the ratio of the females in the immatureindividuals is 26.5% (n=ll7) and that in the mature 32.5% (n=80).

Maturity rate
The ratio of sexually mature individuals in the catch off Sanriku coast is
shown in Table 14. The maturity of some individuals is identified by the bodylength. The ratio shows, in both sexes, a gradual decrease from late January tolate March. The change is more conspicuous in the female, and the maturity
rate of the sex in late March is about a half of that in late January.
This result, together with the seasonal change of the sex ratio discussed above,
indicates the presence of the segregation by the sex and maturity. The immatureindividuals seem to have tendency to arrive fishing ground off Sanriku coast after
the adult individuals. This tendency is much clear in females than in males.
Reproductive cycle
Since the length of gestation is estimated in the former section as 11.4 months,
the lengths of lactation and resting period are estimated in this section. The
number of females at each reproductive stage in the catch off Sanriku coast is shownin Table 15. Though the stages were decided through the observation of the
reproductive tracts, there are many adult females which reproductive stages were
not decided obstacled by the removal of the internal organs. Since they, 34
individuals, were not lactating, they should have been classified into pregnant or
resting females. Then the more correct estimates of the pregnant and restingfemales can be from 58 to 92 and 2 to 36 individuals respectively. If the sample
correctly represents the ratio in the population and the length of gestation is 11.4
TABLE 15.

RERPODUCTIVE STAGES OF ADULT FEMALES OF PHOCOENOIDES 1 l

Locality
Sanriku

N.W.N. Pacific &W. Bering Sea

SeasonJan. B
Feb. A
Feb. B
Mar. A
Mar. B
Total
1
2
3
Total

Preg.

P. & L.

Lact.

Rest.

Unknown

22
3
9
15
9
58
20
24
8
52

1
0
0
4
0
5
0
0
0
0

0
0
3
3
0
6
0
1
0

0
0
2
0
0
2
0
1
0

2
6
22
3
34
0
0
0
0

1) For abbreviations see Tables 13 and 14.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

39

months, the mean length of a reproductive stage in months is given by
X-11.4/P

where P indicates the number of pregnant females, and X the number of females
at one of the reproductive stages.
Above calculation gives the following estimation for the stock off the Pacificcoast of Japan.
Simple pregnancy
10.50-10.81 months
Overlapping of pregnancy and lactation
0.90- 0.59 months
Simple lactation
1.09- 0. 71 months
Resting
6.51- 0.24 months
Total
19.00-12.35 months
The mean length of calving interval is from 1.0 year to 1.6 years. Though this isextremely short, the cycle is not impossible, if most of the females are conceived
in the summer of the parturition. However, the most improbable is the shortnessof the lactating period. The overall period of lactation obtained above is only
from 1.3 months to 2.0 months. Generally speaking the lactation of the odontoceti
last at least for about one year (Brodie 1969, Kasuya 1976a, Perrin et al. 1976).
It is reasonable to consider, apart from the underrepresentation of the adult individuals, that the lactating females are underrepresented in comparison with thepregnant and resting females.
This is indirectly indicated by the body length frequencies (Fig. 6), where
the individuals smaller than 172 cm is scarce and those smaller than 156 cm isalmost entirely lacking. The former length corresponds to the age of about 1.5
years on the mean growth curve, and the latter to that of about 0.5 year. Similar

TABLE 16. ESTIMATION OF NURSING PERIOD OF PHOCOENOIDESOFF SANRIKU COAST
Age
(yrs)

0.
0.25
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Mean weaning age
Standard error
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

Annual mortality rate

0

0.1

0.15

No. weaned individuals

Weaning/year

Weaning/year

Weaning
/year

5

5

5

39

39.50

39.75

13

17.40

19.60

21

26.70

29.55

8.80

12.70

1.96
0.11

2.06
0.11

~

~

Total

0

0

0

3

2

5

18

26

44

34

23

57

44

34

78

45

34

79
1.69
0.11

40

KASUYA

feature is seen in the catch by harpoon fishing in the southern Sea ofJapan studied
by Noguchi (1946). In his data the number of individuals under 172 cm is only
14 or 7.1 % of the total. The absence of the juveniles does not seem, as mentioned in the latter section, to be a result of the size selection of harpoon fishermen. It is most reasonable to consider that suckling calf and nursing cow are
not effectively captured by the harpoon fishing. In the other words, the gradualincrease of the catch of calves at the age from 0.5 to 4.5 years will be relatedwith the completion of weaning. As shown in Table 16, such probably weaned
individuals start to appear before the age of 0.5 years and cease at 4.0 years ofage, and when the sexes are combined the number is hightest between the ages of
0.5 and 1.5 years. Though another interpretation is discussed at the end of thissection, this is an estimation of the range of the weaning age. For the correct
estimation of the mean age at weaning, the mortality of weaned calves needs tobe considered. When nx indicates the apparent number of calves weaned at theage x, Nx the number of calves weaned at the age x and corrected for the mortality, and M the annual total mortality rate of the weaned calves, there can be
the following approximation.
Nx = llx+o.s-(1-M)·nx-0.5

Defining the age x as the mean age of the calves which have weaned between
x -0.5 and x +0.5 years of age, and assuming the total mortality rate of 0, 0.1,
and 0.15. The estimates of the mean age at weaning 1.69, 1.96, and 2.06 are
obtained. Though the estimation of the mortality rate at these age classes isdifficult, the first assumption is without doubt an underestimate and the last possibly an overestimate.
The above calculation is affected by two kinds of biases. The first is thepossible inclusion of some sucklings as indicated by the presence in the catch, of
11 females lactating or lactating and simultaneously pregnant. Possibly at least
some of the five individuals dealt, in the above calculation, as weaned by the ageof 0.5 year could still being nursed. This bias causes the underestimation of theweaning age. If the youngest 11 calves, the number of lactating females, areexcluded, the above calculation gives the mean age of 2.07 years (M=0.10). The
other bias is caused by the time lag between the end of lactation and the start ofthe independent life of the calf. As reasonably expected, if the weaned calves stayby the mother for a considerable length of time and protected from the fishery,
the above estimation can be higher than the true weaning age. In case of Stenella
coeruleoalba off the Pacific coast of Japan, the weaned calves are considered to stayin the nursing school for about 1.5 years in the average (Miyazaki l 977b ). However, this will not necessarily mean that the weaned calves of S. coeruleoalba accompany the mothers. Since the school size of Phocoenoides is usually so small
as 2 or 3 individuals, there is not expected such a developed social behavior as
observed in S. coeruleoalba, and the seasonally restricted breeding season of the present species will help the rapid recombination of the school member. Therefore
the time lag in the present species will not be so large as in S. coeruleoalba. As theSci. Rep. Whales Res. Inst.,

No. 30, 1978.

DALL'S PORPOISE

41

conclusion, because the two kinds of biases must exist at the unknown degrees, itwill be safe to say that the length of lactation of Phocoenoides off the Pacific coastof Japan have the variation from 0.5 year to 3.5 years with the possible average ofabout 2 years. Further study is needed for the better estimation.
Though the mean length of the resting period, 0.24 to 6.51 months estimatedfrom the ratio of the number of resting females to that of pregnant females, is within
the reasonable range, the accuracy is insufficient. The fact that the lactating
females and females pregnant and simultaneously lactating are caught in the laterpart of the fishing season suggests that these females would have been near theend of lactation and weaning peak will exist in between February and the peakof the next mating season in September (Table 15).
The mean calving interval of the species off the Pacific coast of Japan is notprecisely estimated. If the above estimation of the lengths of lactation and ofresting period is correct, the mean calving interval will be between (11.4+24+
0.24)/12=3.0 years and (ll.4+24+6.51)/12=3.5 years. Another estimation ismade by assuming that the calves are usually separated from the mother when themother starts the next gestation. This means, in other words, that the length oflactation estimated above is the sum of the lactating and resting periods. Thenthe claving interval can be (11.4+24)/12=3.0 years. However, since such a
detailed discussions are impractical at present, it will be possible only to say thatthe mean calving interval is approximately about 3 years.
The frequency of reproductive stages of the females caught by the salmon
gillnet fishery in the western Bering Sea and northwestern North Pacific was calculated from the published and unpublished data (Table 15 ). The scarcity oflactating or resting females is similar to the catch by harpoon fishing off Sanriku
coast. However, the capture of many calves smaller than 130 cm presumablyborn within 1 year (one in Takemura's sample and 19 in Mizue et al. 1966) indicates the presence of many lactating females in the population. Possibly thelactating females are underrepresented in the catch compared with the catch of
the pregnant females. At present there is no exact way of estimating the reproductive cycle of the females in the stock, however an attempt is made in Addendum.

Accumulation rate of corpora in the ovaries
Table 17 shows the relationship between the age and the reproductive stagesof the females off Sanriku coast. The age compositions of the lactating and restingfemales, which are few in number, are same with that of pregnant individuals.
The frequency of the corpora number shown in Table 18 indicates, together with
Table 17, that most of the adult females caught by the fishery are very young with
only 1 or 2 ovulations. Possibly, most of the catch of the adult individuals are
before the first conception, in their first gestation, or in the lactation followed by
the first parturition. If all the 19 pregnant females which have 1 to 3 corpora areconsidered as being in their first gestation, the mean number of ovulations experienced before the first conception is 1.3. This is close to the figure 1.67 estimatedby the same method for an exploited population of Globicephala melaena (SergeantSci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

42

TABLE 17. RELATIONSHIP BETWEEN AGE AND REPRODUCTIVE
STAGES OF FEMALE PHOCOENOIDES OFF SANRIKU COAST
MatureAge(yrs)

Immat.

2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5

17
25
21
18
11
8
2
5
3
2
0
0
0
0
0

Preg.

P. &L.

Lact.

Rest.

0
1
2
4
16

0
0
0
0
0
2
0
0
0
0
0

0
0

0
0
0
0
0
1
0
0
0
0
0
0
0
0
0

5
4

0
0
0

0
0
1

2
0
0
0
0
0
0
0
0

0
0

Total1l
5
7
7
20
8
8
6

1
2
3

1) Adults of unknown reproductive stage included.

TABLE 18.

RELATIONSHIP BETWEEN NO. OF OVULATIONS AND REPRODUCTIVE
STAGES IN ADULT PHOCOENOIDES OFF SANRIKU COAST

No. corpora
12
3
9
Totalll

Preg.
14
4

Lact.
0
1
0
0
6

P. &L.
0
0
0
0
5

58

Rest.
0
0
0
2

1) Individuals of unknown corpora number included.

d 8
.....
0

a. 6
.....
0
u

-

4

0

2

0

z

!

•

0 ::.. 1$ I·
46.,.,, : • I
8

10

I

12

14

16

Age in years
Fig. 23. Scatter plot of the number of copora in the ovaries on age, Phocoenoides
off Sanriku coast. Large circle indicates 5 individuals, and the small one.
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

43

DALL'S PORPOISE

1962), but significantly smaller than the figure 2.08 of an almost unexploited population of Stenella attenuata (Kasuya et al. 1974).
The scatter plot of the ovulation number on age is shown in Fig. 23. The
data is insufficient to estimate the mean annual ovulation rate. The rough estimate of the range is given as from (9-1)/(16.5-5.5)=0.73 to (9-1)/(16.511.5)=1.6 per year.

Regression of corpora diameter
The diameters of 20 corpora lutea of pregnancy ranged from 17.2 mm to24.6 mm, and the mean was 21.7 mm. No accessory corpus luteum was observed.
The size distribution of the corpora albicantia of 10 adult females is shown in Fig.

<0.5

R, Feb.( s)

< 0.5

2

R, Jui. (OS)15
· L, Feb.( s)

0.5

3
4
5
6
7
8
9

0.5

P,Feb.(S)0.5

P, Jan.( s)
0.5
1-1.

0.5

P,Jan.(s)P, Jan.( s)P, Mar.( s)P,Jul.(NP)
P,Jan{S)10
24 222018 10

9

8

7

6

5

4

3

Diameter of corpus in mm
Fig. 24. Distribution of the diameter of corpus luteum (open circle) and of corpus
albicans (closed circle). Reproductive stages (R: resting, L: lactating, P: pregnant), Date of catch and location (OS: southern Okhotsk Sea, NP: northwestern
North Pacific, S: off Sanriku coast) are indicated. The numerals by the circle
indicate the approximate time after the start of regression.

24. There is a group in the size of corpus at the diameter of 6 mm to 8 mm. It isrepresented by the female nos. 3, 4, 5, 6, 8, caught in the winter season off the
Pacific coast ofJapan. Since the pregnant individuals are considered to be in theirfirst gestation, these corpora are thought as the corpora of the ovulations in thelast summer, namely about 0.5 year after the ovulation or the parturition (no. 3).
The larger corpora albicantia of the no. 2 resting female caught in July in thesouthern Okhotsk Sea are suspected as those of the ovulation in the summer of thecapture. The corpus albicans of the no. 1 resting female is considered as derived
from the ovulation occurred after the last mating season. Other corpora ablicantia possibly regressed for I or more years.
DISCUSSION
The assuption of the nonrandom catch is an important factor in the present
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

44

KASUYA

study of the life history parameters of the Dall's porpoise. The discussion is madeat first step on this problem, then on the segregation of the species and the rates ofmortality and recruitment.

The underrepresentation of suckling calves
The scarcity of the juvenile calves at the suckling age is a peculiar feature of
the catch by the harpoon fishing off Sanriku coast and in the southern Sea ofJapanstudied by Noguchi (1946). This is not observed in the incidental catch by thesalmon gillnet fishery in the western Bering Sea and northwestern North Pacific.
As the cause of this age structure, there can be the following three possibilities.
1. The selection by the fishermen, who generally hopes to get larger individuals.
2. The recent decrease of the recruitment.
3. The scarcity of the cows and suckling calves in the fishing ground, whichis caused by the segregation of those individuals.
4. The technical inability of taking the suckling calves.
The first may exist in some degree, because the catch is sold by the weight of the
carcass weighed after removing the viscera. However, the effect seems to be almost
negligible when the condition of the fishery is considered. Usually the fishingvessels on Sanriku coast arrive at 7 to 8 o'clock at the fishing ground, 10 to 40 or
50 nautical miles (18 to 80 km) off the coast, and search and harpoon the porpoises
until 14 to 15 o'clock to return to the port by the evening. Two or less number of
porpoises are usually caught from one school. The mean daily catch is only 4
to 6 individuals per ship even when the boats which come back without catch are
excluded (Kasuya 1976b). Furthermore, since the porpoises quickly swim aroundthe bow of the boat, there is little chance for 1 to 3 harpooners on the bow to selectlarge individual. If the first hypothesis is correct and the other 3 hypotheses arewrong, there must be more catch of lactating females, because the mean lengthsof lactation in odontoceti are at least about 1 year (page 39) and the number of
lactating females in a population is expected to be nearly equal to or more thanthat of pregnant females.
The 2nd hypothesis will be safely denied. Since the exploitation of the Japanese east coast population has continued for more than 30 years and there is detected no indication of the rapid decrease of the population density (Kasuya l 976b ),it is difficult to expect a rapid decrease of the recruitment in recent few years.
The 3rd hypothesis will not be denied at present, when it is considered that
there is expected a segregation of pregnant females with near term fetus in thesummering ground of the offshore stock in the western Bering Sea and north western
North Pacific, and that the individuals of the Japanese east coast population showsa segregation by the maturity and sex. Further study is needed for the confirmation of segregation of lactating females.
The 4th hypothesis implies that the suckling calves accompanying the mothers
migrate to the fishing ground, but are not captured by some ecological or technicalreasons. Because the fishermen harpoon only the porpoises riding the bow wave,
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

45

the above age composition will occur if the mother and calf are less attracted by theboat compared to the weaned juveniles. Actually it is well known that there exist
many schools which do not approach the bow of the boat even when fisherwen try
to let them do so. According to the personal communication of Mr K. Sasaki,
a porpoise hunter, and to my own observations on board of the porpoise hunting
boat and on the research vessels, the number of such schools is slightly morethan half of the total number of Phocoenoides schools sighted. Mr Sasaki tells thateven when a school comes to the bow of the ship the presumable mother accompanied by a calf and the very large individuals stay only for a short length oftime or stay far from the ship, however, the individuals of the medium size playon the bow wave longer and are the easiest to catch. This is the reason why I
consider the 4th hypothesis most probable as the explanation of the scarcity of
calves at the suckling age. Therefore the underrepresentation of the calves at thesuckling age will be fundamentally ascribed to the behavior of the lactating females
discussed in the next section.

The underrepresentation of adult individuals
In the former section, it is shown that the number of adult individuals is
scarce, in both sexes, in the catch by harpoon fishing. The analyses of age andof the corpora number in the ovaries of the catch off Sanriku coast suggest that
only 6 females have presumably experienced the 2nd gestation and 42 adult females
are probably before the 2nd gestation. This could be explained by one of thefollowing two hypotheses.
5. The mortality is high in the population.
6. The older individuals are not captured by the harpoon fishing.
The instantaneous annual total mortality rates of the Dall's porpoise off
Sanriku coast tentatively calculated from the right side slope in the age frequencies
are as follows.
Male0.2185, between 4.5 and 9.5 years of age.
Female: 0.3356, between 6.5 and 10.5 years of age.
As the lactating females are underrepresented in comparison with the pregnant
females (page 39), the apparent mortality rate of the females can be expressedslightly higher. If the total mortality rate of the female, M, is same for all the ageclasses, there can be the following equations, where S 1
S1 =

lof:

e-MxdxS2 = lof~ e-Mxdx

A=

S2
S1+S2

indicates the number of immature females, S 2 that of adult females, 10 numberof females at the age 0, and A the ratio of adult females in the total female population. When 0.2185 the lower figure is taken for M, A is 21.7%. Assuming
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

46

KASUYAthe sex ratio at birth as 1 : 1, and the length of mean calving interval as 2 years
the possible shortest, or 3 years the more reasonable estimate obtained in thisstudy, the gross annual recruitment rate of 5.43% or 3.62% is obtained. Whereas
the total annual mortality rate is (1-e- 0 • 21 8 5 )-100=19.6%. Accordingly, ifthere
is no bias in the catch, the population must decrease at the rate of about 15 % per
year. However, the exploitation of the population have continued at least for 13
years at the level of about 6,000 individuals per year (Addendum Table of Kasuya
1976b ), and there is detected no indication of rapid decrease of the stock. If the
population really decreased at the rate of 15% per year, the stock should havedecreased in the 13 years to 12% of the level in 1963. Such a large decrease musthave resulted in the decrease of the annual catch and that of the mean daily catchper boat. This is the reason why I consider the 5th hypothesis incorrect.
The indication suggesting the correctness of the 6th hypothesis is obtained by
the comparison of the age compositions of Stenella coeruleoalba caught by the driving
method and by harpoon fishing. The ages were determined based on the dentinal
growth layers in case of the individuals younger than 10 to 15 layers, but based on
the cementa! layers in older individuals (Kasuya 1976a). The age frequencies of
29 males and 35 females in the catch by harpoon fishing were obtained off Taiji(33°35'N, 135°57'E) in the winter of 1971, where the harpoon fishing was operated
only for two or three years until the driving fishery of the species started. All the1388 (564 females and 824 males) samples by driving fishery were obtained off
the east coast of lzu Peninsula (34°56'N, 139°09'E) and randomly collected bymyself or by Kasuya and N. Miyazaki. They are the catch of following 7 drivings,
where only the numbers of the aged individuals are listed. The maturity of thefemale was decided by the presence of corpus in the ovary and that of the maleby the weight of testis (smaller than 15 g immature).
Driving 22: 14, Dec., 1971. Immature school. 62 females (2 to 8 layers,
adult absent) and 218 males (2 to 12 layers, adults 2.8%).
Driving 38: 17, Nov., 1972. Nursing school. 101 females (1 to 49 layers,
adults 77.0%) and 52 males (1 to 20 layers, adults 53.8%).
Driving 43: 15, Nov., 1973. Nursing school. 89 females (1 to 40 layers,
adults 58.1 %) and 135 males (1 to 31 layers, adults 62.4%).
Driving 44: 20, Nov., 1973. Nursing school. 111 females (0 to 43 layers,
adults 66.7%) and 111 males (1 to 58 layers, adults 68.5%).
Driving 45 and 46: 23 and 24, Nov., 1975. Probably nursing schools, mixed
in the harbor. 170 females (0 to 58 layers, adults 59.9%) and
192 males (0 to 58 layers, adults 60.4%).
Driving 47: 3, Nov., 1976. Males dominant. 31 females (2 to 26 layers,
adults 29.0%) and 116 males (3 to 46 layers, adults 69.8%).
The fishing grounds of the two fisheries are separated only about 330 km, andconsidered as being migrated by a single population (Miyazaki et al. 1974, Kasuya
1976c). Though the between schools differences of the age composition and of
the sex ratio are not small, the population structure will be roughly estimatedby combining the 7 schools. The ratio of sexually mature individudals is 55. 7 %Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

47

of the 553 maturity and age known females caught by the driving, and 47.4% of
the 816 males. On the other hand, when all the maturity known individuals
are included (40 females and 42 males), the corresponding figures of the harpoon
fishing are so low as 17.5% and 9.5% respectively, These figures are similar to
those of Phocoenoides caught by the harpoon fishing both in the amplitude and in
the between sexes difference. The bias of the fishing methods is demonstrated
more tipically in the age frequencies of Fig. 25. If the instantaneous total mortality rate is calculated from right side slope of the age frequencies of the catch by
the driving fishery, the following figures are obtained.

70
50
40
30

.!!1 20
0

:::J
"'O

>

'6 10
£ 8

0

d

z

6
5
4
3
2

1 ........................_._......................................................._._~._._'-'-'.............................._._,,_-'--"'~Jl.:.2............."-""'L..!l...,,WC:

0

5

10

15

20

25

30

35

40

45

No. of tooth layers
Fig. 25. Stenella coeruleoalba off the Pacific coast of Japan, age frequencies of thecatch by driving fishery (small circles and thin lines) and that by harpoon fishery(large circles and thick lines). Closed circle and solid line indicate male, andopen circle and dotted line female.

Female:0.0537, 3 to 14 layers
0.0944, 15 to 35 layers
Male0.0529, 4 to 13 layers
0.0726, 14 to 38 layers
When the instantaneous natural mortality rate of the adult female is assumed as
0.06 or 0.07 (Kasuya and Miyazaki 1976), the fishing mortality rate of from 0.0244
to 0.0344 is estimated, and then the natural mortality rate of the females before thematuration 0.0193 to 0.0293. The left side slope of the age frequency of the catchby driving fishery seems to be caused by the loss of juvenile calves (Kasuya 1972)
and by the segregation of the immature individuals. Especially the inclusion of
Sci. Rep. Whales Res. Inst.,

No. 30, 1978.

48

KASUYAthe data of the immature school, driving no. 22, causes the high frequency of thejuvenile females of the ages between 3 and 6 layers and males between 3 and 10
layers.
Though the above age frequency of the catch by the driving are extremely
different from the age structure of Phocoenoides off Sanriku coast, that of the Stenella
coeruleoalba caught by the harpoon fishing is similar to the latter in the predominanceof the individuals below 9 years of age, the mean age at the attainment of sexual
maturity (Kasuya 1972, Miyazaki 1977a).
From these reasons I consider that the 6th hypothesis will be close to thetruth. Though the possibility of the segregation is not perfectly denied, it seems
to be more reasonable to consider that the Dall's porpoise, as well as the striped
dolphin, becomes less attracted by the bow wave of the ship or becomes morecautious of danger soon after the onset of sexual maturity. This change of the behavior has a effect to lower the fishing mortality rates of the adults and suckling
calves accompanying the mother. The underrepresentation of the lactating females(page 39) will be explained by the enhancement of the behavior during the lactation in relation to the limited swimming ability of infant, or by a simple fact that
lactating females are generally older than the pregnant. The possibility of thesegregation of the lactating females is still left to be studied.
The discussions on the age structure of the Dall's porpoise incidentally captured by the salmon gillnet fishery is made in Addendum.

Segregation by age and maturity
In the former sections, the following phenomena are indicated through theanalyses of the catch by harpoon fishing off Sanriku coast.
1. In the adult, the female ratio is high and stable throughout the season.
2. In the immature, the ratio offemales is low, but increases from late January
to late March.
3. In the male, the ratio of the adult is low, and changes little.
4. In the female, the ratio of the adult is high, but decreases from late January
to late March.
The different analysis of the same material is shown in Table 19, where the seasonalincrease of the ratio of the immature females is clearly demonstrated. These seasonal fluctuations of the composition are difficult to be explained by the catch biasof the fishing, but is reasonable to be considered that the migration of the immatureindividuals, especially of the female, increases from January to March.
Kasuya ( l 976b) showed on the catch of the species off Sanriku coast that thecatch per day's work of the harpoon fishing boat has a peak in April when thevalue is about twice of that in January, and concluded that the fishing season closes
in April, by the decrease of the demand of the porpoise meat and the migrationof other profitable fishing objects, before the peak of the north bound migration of
the species. The seasonal change of the catch per fishing boat described by
Kasuya ( 1976b) will indicate, in a strict sence, that of the abundance of the young
exploitable age classes, and coincides well with the above feature of the seasonalSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE
TABLE 19.

COMPOSITION OF PHOCOENOIDES CATCH OFF SANRIKU COAST,SHOWN BY SEX AND SEASONSex andMaturity1 >
Males
Adult

no.

%
Immature

no.

Total

no.

%
0/
,o

Females
Adult

49

no.

%
Immature

no.

Total

no.

Grand total

no.

%
%
%

Feb.

Mar.

Jan.
B

A

A

B

Apr.
B

Total

B

16
10.5
80
52.7
96
63.2

27
16.5
78
47.5
105
64.0

26
8.5
135
44.1
161 2 '
52.6

9
7.2
53
42.4
62
49.6

8
6.2
45
39.4
53
45.6

4
44.4
5
55.6
9
100

90
10.2
396
44.8
486
55.0

28
18.4
28
18.4
56
36.8
152
100

26
15.9
33
20.1
59
36.0
164
100

43
14.1
102
33.1
145
47.4
306
100

26
20.8
37
29.6
63
50.4
125
100

16
12.4
60
46.5
76
58.9
129
100

0
0
0
0
0
0
9
100

139
15.7
260
29.4
399
45.1
885
100

1) Based on body length when genital organ is not studied.
2) One length unknown individual included. For abbreviations see Table 1.

change of the catch composition.
In the Japanese coastal waters the Dall's porpoise is distributed mainly in thewaters north of the l 7°C isothermal line, and in the waters above this surface watertemperature the temperate and subtropical dolphins are dominant (Kasuya 1976b).
The l 7°C isothermal line moves seasonally from southernmost point at 35°N in
February to the northernmost point of 42°N in August along the Pacific coast ofJapan (Fig. 26). The surface water temperature at the fishing ground is lowest in
February, and it becomes gradually higher in March and April. They will offer asuggestion on the seasonal change of the distribution of the species off the Pacificcoast of Japan.
Other indication of the seasonal migration of the population is found in thesighting records of dolphins (Kasuya and Miyazaki 1976b, Kasuya unpub.). The
Dall's porpoise arrives off Sanriku coast (38°N to 41°N) in September, but it does not
arrive south of 37°N in November. The observation outside of the fishing groundin winter is lacking. In May there migrate many Lagenorhynchus obliquidens andfew Grampus griseus and Orcinus area off Sanriku coast, but there are still found many
Phocoenoides. The southern limit of Phocoenoides in this month is approximately at
37°N on the Pacific coast of Japan, and Lagenorhynchus is recorded between 36°N
and 42°N. In June Phocoenoides retreat to 38°30' and Orcinus, Grampus, and Globicephala are recorded south of 39°30'N. However, no Dall's porpoise is recorded
south of 42°N in July in spite of the presence of Lagenorhynchus, Orcinus, Grampus,
Delphinus, and Tursiops in the area. The facts that the southernmost sightings of
Phocoenoides were made in May at about 37°N and that the catch of the species
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

50

could not be comfirmed at Choshi fish market situating at 35°45'N (Kasuya 1976b)
suggest that the southern limit of the ordinary distribution of the species in winterseason will be about 36°N in the Pacific coast of Japan.
Possibly the species will arrive at the southernmost range in early February, andbe followed by the retreat to the north in February through August. Accordingly
the start of the fishing season in the last decade of January coincides nearly withthe end of the southbound migration and the begining of northbound migration.
The seasonal change of the ratio of the immature females suggests that these individuals migrate to the Pacific coast of Japan earlier than the adults and possibly

4
2
J

FMAMJ

JASOND

Month
Fig. 26. Surface water temperature off the Pacific coast of Japan. Closed circle
and solid line the range and mean at 39°30'N, 142°30'E off Sanriku coast in thefive years from 1972 to 1976, and the dotted area the range of the latitude of thel 7°C isothermal line at 20 to 30 nautical miles off the coast in the same period(data based on Anon. 1972-1976).

spend the winter in the southern waters. The adult females, on the other hand,will arrive later and stay in the northern region. The males, possibly the adulttoo, seems to winter in the further south near the southern limit of the distribution.
This is suggested from the catch off Sanriku coast in April (Table 14) and from thecatch off Tajima coast studied by Noguchi (1946). The porpoise hunting off
Tajima coast (southern Sea of Japan) was operated from late March to early June,
and the catch of Phocoenoides was replaced in late April by Lagenorhynchus the temperate species. Namely the hunting was operated near the southern limit of the disSci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

51

tribution of the stock in the season. If the maturity is determined by the bodylength there are 140 males of 54 adults and 86 immatures, and 57 females of 26
adults and 31 immatures. The ratio of the males is 71.1 % and the maturity rate
is 38.6 %, and both of these figures are higher than those off Sanriku coast. The
maturity rate of the female is similar to the catch off Sanriku coast.
The tendency of the immature individuals being distributed near the southern
periphery of the wintering ground seems to exist even in the stock in the northwestern North Pacific and western Bering Sea. This is suspected from the factthat the ratio of dalli-type is higher in the juveniles in the catch off Sanriku coast(page 10).

Rates of mortality and reproduction
The total mortality rate of the male at the pubertal ages was estimated as0.102 in instantaneous rate or 9. 7 % per year (page 27). However the accuracyof the estimation is doubtful, because it is based on the age frequency of the restricted age classes and limited number of samples. The annual changes of the
population structure and of the timing of migration and sampling will cause the biasof unknown degree. The following mathematical approach is made in order tohave a suggestion on the possible range of these parameters.
When the abbreviations are defined as follows,
M 1, M 2, Ma: Instantaneous natural mortality rate at the ages between 0
and 2 years, 2 and 7 years, and above 7 years respectively.
F 1 , F 2, Fa : Instantaneous fishing mortality rates of the above age groups.
S 1 , S 2 , Sa: Number of females of the above three age groups.
R: Ratio of females at birth, assumed as 0.5.
C : Calving interval, assumed as 2 or 3 years.
the number of females at each age groups in a stational population is shown by thefollowing approximations.
S1 = loJ: e-cM,+F,>xdx
S2 = lo·e-2CM,+F,>s:e-M,+F,>xdx

Sa= lo·e-2CM 1 +F 1 >.e-5CM2+F2 >J~ e-cM,+F,>xdx
lo= Sa·R/C

Then the ratio of the adults in the population is calculated by Sa/ (S 1 +s 2 +Sa),
and the net production rate by S(l-e-F)/(S 1 +s 2 +Sa), where S indicates thestock size of the exploited age classes and F the sustainable fishing mortality rateof the same age classes. F is calculated from the 3rd and 4th equations byassuming the natural mortality rate. The calculation was made based on thefollowing three hypotheses.
Hypothesis 1: M 1 =M 2 =Ma>0, F 1 =F 2 =F 3 >0, nonselective catch and constant mortality rate.
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

52

KASUYA

Hypothesis 2: M 1 =M 2 =M 3 , F 1 =F 3 =0, F 2 >0, selective catch and constant natural mortality.
Hypothesis 3: M 1 =2·M 3 up to 1 year, then 0.25·M 3, M 2 =0.25·M 3 , M 3 >0,
F 1 =F 3 =0, F 2 >0, selective catch, natural mortality rate variable.
The first hypothesis is not supported by the results of the present study. The 3rd
hypothesis, the most optimistic, is based on the result of the present study indicating
the selective exploitation of the juveniles between the weaning and sexual maturity,and on the natural mortality rate of Stenella attenuata analysed by Kasuya (1976a).
Kasuya (1976a) showed that the mean natural mortality rate offemale S. attenuata
in the entire immature period could be less than a half of that of the adult female,
but he suggested that the mortality of the newborn claves could be higher. The
low mortality rate before the attainment of sexual maturity is also suggested in theage frequency of S. coeruleoalba shown in Fig. 25. This is in the good resemblancewith the mortality of the human race, where the mortality rates of the women at
the ages between 4 years and 19 years is shown to be at the minimum in the life
(Anon. 1976). Possibly this feature will be common in some of the high trophic
k-selective mammal species, and is expected for the Dall's porpoise. Accordingly,
the natural mortality rates in the 3rd hypothesis are so assumed that the meanannual natural mortality rate in the entire immature stage is a half of that of theadult period and the rate is highest in the early suckling age of 0 to 1 year. The
2nd hypothesis is the intermediate of the first and the 3rd. The natural mortality
rate of the adult female, M 3 , is assumed as 0.05, 0.075, and 0.10, and the calving
interval as 2 years and 3 years. The former cycle is attained when the gestation
occures, in most of the adult females, with the interval of one year, and seems to
be shorter than the cycle of the Dall's porpoise off the Pacific coast of Japan.
The latter cycle, 3 years, is estimated in the present study and seems to be morereasonable.
The results of the calculations are shown in Table 20. It indicates that theTABLE 20. POPULATION PARAMETERS IN HYPOTHETICALSTATIONAL POPULATION OF PHOCOENOIDES
M 3 , assumed
C, calving interval

0.050
3 years

Nonselective catch, hypothesis-I
0.039
Fi=F2=Fa
Net production
3.8%
Ratio of adult53.6%
Selective catch, hypothesis-2
0.171
Fz
Net production
4.0%
Ratio of adult
56.4%
Slective catch, hypothesis-3
0.206
Fz
Net production
4.8%
Ratio of adult
56.7%

0.075

0.100

2 years

3 years

2 years

3 years

2 years

0.063
6.1%
45.3%

0.014
1.4%
53.6%

0.038
3.7%
45.3%

<0
<0

0.013
1.3%
45.3%

0.252
6.3%
48.5%

0.055
1.5%
54.4%

0.136
4.0%
46.9%

<0
<0

0.043
1.4%
45.8%

0.287
7.1%
49.0%

0.107
2.9%
55.0%

0.188
5.4%
47.6%

0.032
1.0%
53.6%

0.113
3.6%
46.5%

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

53

DALL'S PORPOISE

selective catch of the immature individuals gives slightly higher net production.
The fact that the net production is negligible under the assumption of M 3 =0.10
and C=3, suggests that the actual natural mortality rate of the adult female will
be below 0.10. If the population off the Pacific coast of Japan is stational with theannual catch of 6,000 individuals, the population need to be between 6,000/0.048=
125 thousands and 6,000/0.015=400 thousands (2nd and 3rd hypotheses with M=
0.05 or 0.075, and C=3).
ADDENDUM

Dr A. Takemura of the Faculty of Fisheries, Nagasaki University provided for thepresent study the biological data and tooth samples of Phocoenoides of dalli-typecaught incidentally by the mother ship salmon gillnet fishery in the western Bering
Sea and northwestern North Pacific. Since these samples belong to a stock different from Japanese Pacific coast population, and its biological informations arestill very limited, some brief analyses are made in comparison with the results
obtained on the latter population.
700
Ol

•

500

c 300

•

:a 200

•

.....

Ill

Cl

.......

1000......

.!:

Ol
(!)

3:

30
20

•
10

8

-

6

er
z 4
2
0

140

160

180

200

220

Body length in cm
Fig. 27. Male Phocoenoides in the western Bering Sea and northwestern North
Pacific. Body length frequency (bottom) and scatter plot of the weight of combined testes (logarithmic scale) on body length (top).

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

54

The samples were collected in the period from May 23, 1967 to July 9, 1967
on board of a mother ship. The ordinary biological observations were made andrecorded by A. Takemura.

Age determination
The age was determined by myself with the same method as used in the former
sections. Since the accumulation of the cementa! layers is _considered as annual
and the parturition peak as from late August to early September, the number ofstainable cementa! layers will approximately coincide with the age of the porpoisein years. However, the length of time from the mean date of catch of the aged
individuals, 15th of June, and the expected peak of the parturition in early September will cause some signigicant bias in the estimation of the growth of juveniles.
This is the reason why the body length of the porpoise with N stainable layers isplotted in Fig. 27 at the age of (N -1 /4) years.
Length and age frequencies
The body lengths ranged from 139 cm to 220 cm in males, and from 144 cm
to 212 cm in females. The calf smaller than 130 cm, which is considered as bornin the year (Mizue et al. 1966), is only one individual in spite of the same sampling
111

c
0

.....

B
6

d
::J

4

0

2

>

=I I : • :0zllfi::+•n:i••0.......
~
080.......

.....>. 60....
.....::J 40
0

~

20
6

4
0

z

2
0

140

160

180

Body length in cm
Fig. 28. Female Plwcoenoides in the western Bering Sea and northwestern North
Pacific. Body length frequency (bottom), relationship between body length andthe ratio of the sexually mature individuals (middle), and the scatter plot of thenumber of corpora in the ovaries on l;iody length (top).
Sci. Rep. Whales Res. Inst.,

No. 30, 1978.

DALL'S PORPOISE
TABLE 21.

LIFE HISTORY PARAMETERS OF PHOCOENOIDESESTIMATED IN THE PRESENT STUDYParameters
I.
2.
3.

Length at birth (cm)Length at 1 year (cm)Length at sexual matirity (cm)Japanese Pac. Coast

(f
~

4.
5.

55

Testis weight at maturity (g)Age at sexaul maturity (years)(f~

6. Parturition season
7. Length of gestation (months)
8. Length oflactation (years)
9. Calving interval (years)
10. Prenatal sex ratio (female %)

165
195.7
186.5
29.31
7.9
6.8
Aug.-Sept.
11.4
2
3
53.8 (n=39)

N. W. N. Pac. / B. S.
99.7
165
189.1 *
177.9*
50-1004-5*
4-5*Aug.-Sept.
1-2
2-3
64.7 (n=51)*: Considered as an underestimation of unknown degree caused by the sample bias.

season with Mizue et al. (1966), where total of 19 juveniles of the size are recorded.
It also need to be noted that the frequency is low, in both sexes, at the lengths
about 170 cm to 180 cm. This corresponds with the low frequency at the age of
3 years. The larger individuals are more abundant in the present sample thanthat from the harpoon fishing.
The presence of a dominant mode at one and two years of age is a feature
different from that of the catch by harpoon fishing, where the calves of sucklingages are extremely scarce. The other mode at the older age is nearly same withthat of the catch by harpoon fishing.
Sexual maturity

On the males caught by the salmon gillnet fishery, the mean weight of thetestis at the attainment of sexual maturity is not estimated. However, the scatterplots of the weight of testes against the body length suggest that the weight would
be from 100 to 200 gin the combined weight. Then the sexual maturity seems to
be attained at the length from 180 to 200 cm (Fig. 27). This feature is almost
same with the data in Mizue et al. (1966) and Koga (1969). When the weightof 150 g is tentatively used as the mean weight of testes at sexual maturity, theleast squares regression between the ratio of sexually mature individuals and the
body length gives 189.1 cm as the length where half of the males are mature (thepresent data, and those in Mizue et al. 1966 and in Koga 1969 included). The
half of the males in the present materials are sexually mature at 4 to 5 years ofage (Fig. 29).
The present scanty materials suggests that the sexual maturity of the females
is attained at a length between 180 and 187 cm (Fig. 27). The corresponding
figure read on Fig. 5 of Mizue et al. (1966) is from 170 to 190 cm. The least squares
regression calculated for the present data and those in Mizue et al. (1966) gives
177.9 cm as the length where half of the females are sexually mature. The mean
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

56

KASUYA

700
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6

8

10

12

14

16

18

Age in years
Fig. 29. Male Plwcoenoides in the western Bering Sea and northwestern North Pacific.
Age frequency (bottom) and the scatter plot of the weight of combined testes onage (top). Both are plotted on semi-logarithmic scale.

I

0

I

No. of corpora

• I

z

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20
~ 15
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4

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8

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18

Age in years
Fig. 30. Female Phocoenoides in the western Bering Sea and northwestern North
Pacific. Age frequency (bottom) and the scatter plot of the number of corpora
in the ovaries on age (top).

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

57

age at the attainment of sexual maturity is between 4 and 5 years in the present
materials (Fig. 30).
As mentioned later, the mean ages and lengths at sexual maturity estimatedabove will in some degree be affected by the bias of the sample.

Reproductive stages
The sex ratio and ratio of the adults in the catch by the salmon gillnet fisheryare within the ranges of the corresponding figures of the catch by harpoon fishing
off Sanriku coast (Tables 13 and 14).
The adult females are almost exclusively composed of the pregnant females(Table 15). The number of corpora in the ovaries is only one in most of the adultfemales in the present materials (Fig. 27). However, this feature is different fromthe catch studied by Mizue et al. (1966) in 1964 and 1965.
Growth curve
The age-length relationship of the present materials at the ages below 2 years
does not seem to be significantly different from the growth curve fitted for the
population off the Pacific coast of Japan. However, the body lengths of the individuals from 3 to 7 years of age situates above it (Fig. 31). Although this will indicate the faster growth in the northwestern North Pacific and western Bering Sea
population, the following factor can not be neglected. Some premature individuals
attain the sexual maturity, in the two stocks, at the age of 3 or 4 years, and these
individuals are usually larger than the immature individuals of the same age (Figs7, 16, and 31). Since the underrepresentation of the weaned juveniles is indicatedin the age frequency, it is reasonably expected that at the age of 3 to 7 years thesexually immature individuals, which are smaller in body length, are underre~presented than the adults of the same age. This bias causes the mean growth curve
to be higher than truth.
Discussion and conclusion
The Dall's porpoise caught incidentally by the salmon gillnet fishery are constituted mainly of the possible suckling calves and the young adults of both sexes.
The incidental death of many calves and few lactating females are of contrast.
Mr S. Nagahora of the Iwate Prefectural Fisheries Research Laboratory informed
me several observations where mother stays by a dead or alive calf entangled in
the gillnet. The calf is less experienced and will easily be entangled in the gillnet,
and perhaps the accident of the calf will indicate for the mother the presence ofthe gillnet. This can be an explanation of the low mortality of the lactating
females. The decrease of the catch of the calves from 1 year through 3 years will
be a reflection of the completion of weaning. Some juveniles will already be
weaned by the age of 1 year and will be underrepresented in the present material.
This age structure suggests the mean nursing period of from 1 year to 2 years, andconsequently the mean calving interval of 2 to 3 years.
The reason of the low mortality of the weaned immature individuals is not
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

KASUYA

58

..

2.2

..

0-

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00-

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-&.

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6

8

10

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14

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18

Age in years
Fig. 31. Plwcoenoides in the western Bering Sea and northwestern North Pacific.
Scatter plot of the body length on age. Closed circle indicates male, and open
circle female, and the circle with bar the adult. The solid line (male) and dottedline (female) are growth curves fitted for the species off Sanriku coast.

clear. Possibly one of the following two reasons is working for it.
I. Presence of segregation, the weaned immature individuals are distributed
outside of the sampling area in the season.
2. Change of the behavior at the weaning and at the attainment of sexual
maturity.
The difference of the food preference between the growth or reproductive stages isreported by Mizue et al. (1966). The nonpregnant females were feeding almost
exclusively on squid, but the pregnant females on more variety of food organisms
including fish and shrimp. It is not clear in their study if the difference of fooditems is related to the difference of the location or to that of the real preference.
Sci. Rep. Whales Res. Inst.,

No. 30, 1978.

DALL'S PORPOISE

59

However, since the segregation by maturity and sex is suggested in the wintering
ground off the Japanese coast (pages 10 and 50), it will be more reasonable tosupport the first hypothesis.
The scarcity of the individuals over 7 years of age will indicate the underrepresentation of the older individuals. Possibly the older and more experienced
individuals are not usually entangled in the gillnet.
Since the immature, and probably smaller, individuals at the ages above 4
years are underrepresented compared with the adult larger individuals of the sameages, the mean body lengths and mean ages at the attainment of sexual maturity
obtained in the present study can be the underestimations. Therefore it is unreasonable, at present, to conclude that the difference of the growth between thestock off the Pacific coast of Japan and that in the north western North Pacific andwestern Bering Sea is so large as observed in the present materials.
ACKNOWLEDGMENTS

Dr A. Takemura of the Faculty of Fisheries, Nagasaki University, kindly provided
the tooth specimens and the biological data of the porpoise caught incidentallyby the salmon gillnet fishery. Mr S. Nagahora and Mr J. Iwagiri of the Iwate
Prefectural Fisheries Research Laboratory, Mr T. Okazaki of the Far Seas Fisheries
Research Laboratory kindly recorded the colour types of the porpoise caught incidentally by the research vessels of the salmon gillnet fishery. Dr Y. Naito of the
National Institute of Polar Research, Dr N. Miyazaki of the National Science
Museum, and Dr T. Kajihara and Dr M. Iwata of the Ocean Research Instituteprovided the sporadic records of the porpoise in the Japanese coastal waters. This
study is indebted also to the crews of the research vessels Hayachinemaru, Enoshimamaru, Hakuhomaru and Tanseimaru, Mr S. Miyoshi the owner and crews of thewhaling boat Ginseimaru no. 2, and to the staffs of various fish markets in the
Sanriku region for their cooperation. This study was technically assisted byMisses Y. Oka, M. Oya and S. Wada. I would like to convey my deep appreciation to all who assisted and cooperated in this study.
REFERENCES
ANON., 1972-1976. Monthfy report of the outline ofthejishing ground and oceanographical conditions off the northeastern
coast of Japan. ed. Tohoku Regional Fisheries Res. Lab., Sendai, Japan (in Japanese)
ANON., 1976. Simplified life table, 50th year of Showa (ed. Ministry of Welfare of Japan), Koseitokei Kyokai,
Tokyo. (in Japanese)
BRODIE, P. F., 1969. Duration of lactation in cetacea: an indicator of required learning? Amer. Midland
Naturalist, 82(1): 312-314.
HIRASHIMA, Y. and S. ONO, 1944. The porpoise fishing off the coast of Abashiri region. Mon. News Hokkaido Prefect. Fish. Res. Lab., 1 (2): 82-90. (in Japanese)
HoucK, W. J., 1976. The taxonomic status of the porpoise genus Phocoenoides. Paper ACMRR/MM/SC/114,
FAO Scientific Consultation on Marine Mammals, Bergen.
HUGGETT, A. ST. G., and W. F. WmnAS, 1951. The relationship between mamalian foetal weight andconception age. J. Physiol. 114: 306-317.
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

60

KASUYAKASUYA, T., 1972. Growth and reproduction of Stenella caeruleoalba based on the age determination by meansof dentinal growth layers. Sci. Rep. Whales Res. Inst., 24: 57-79.
KASUYA, T., 1976 a. Reconsideration oflife history parameters of the spotted and striped dolphins based on
cementa! layers. Sci. Rep. Whales Res. Inst., 28: 73-106.
KASUYA, T., 1976 b. Preliminary report of the biology, catch and populations of Phocoenoides in the western
North Pacific. Paper ACMRR/MM/SC/21, FAO Scientific Consultation on Marine Mammals, Bergen.
KASUYA, T., 1976 c. The stock of striped dolphin, parts 1 and 2. Geiken Tsushin, 295: 19-23, 296: 29-35.
(inJapanese)
KASUYA, T., 1977. Age determination and growth of the Baird's beaked whale with a comment on the
fetal growth rate. Sci. Rep. Whales Res. Inst., 29: 1-20.
KASUYA, T. and N. MIYAZAKI, 1976a. The stock of Stenella coeruleoalba off the Pacific coast of Japan. Paper
ACMRR/MM/SC/25, FAO Scientific Consultation on Marine Mammals, Bergen.
KASUYA, T. and N. MIYAZAKI, 1976b. An observation of epimelatic behavior of Lagenorhynchus obliquidens.
Sci. Rep. Whales Res. Inst., 28: 141-143.
KASUYA, T., N. MIYAZAKI, and W. H. DAWBIN, 1974. Growth and reproduction of Stenella attenuata in the
Pacific coast of Japan. Sci. Rep. Whales Res. Inst., 26: 157-226.
KOGA, S., 1969. On the Dall's propoise, Phocoenoides dalli (True), caught by the Japanese salmon gillnet in
the northern waters of the Asian side. Bull. Shimonoseki Univ. Fish., 18(1): 53-63. (in Japanese, withEnglish summary)
KURODA, N., 1954. On the affinity of the Dall's and True's porpoises. Bull. Yamashina Ornithol. Res. Inst.,
5: 44-46. (in Japanese, with English summary)
KuzIN, A. E. and A. C. PERLOV, 1975. Distribution and some external morphology of Dall's porpoise (Phocoenoides dalli True, 1885). Izb. TINRO, 96: 259-268. (in Russian)LAws, R. M., 1959. The fetal growth rates of whales with special reference to the fin whale, Balaenoptera
physalus Linn. Discovery Rep. 29: 281-307.
MATSUI, K. and K. UcHIHASHI, 1943. On the Dall's porpoise off Tajima coast. Hyogoken Chutokyoiku Hakubutsugaku :(asshi, 8 (9): 35-39. (in Japanese)
MIYAZAKI, N., 1977 a. Growth and reproduction of Stenella coeruleoalba off the Pacific coast of Japan. Sci.
Rep. Whales Res. Inst. 29: 21-48.
MIYAZAKI, N., 1977 b. School structure of Stenella coeruleoalba. Rep. int. Whal. Commn. 27, 498-499.
MIYAZAKI, N., T. KASUYA, and M. NrsmWAKI, 1974. Distribution and migration of two species of Stenellain the Pacific coast of Japan. Sci. Rep. Whales Res. Inst., 26: 227-243.
MIZUE, K. and K. YosHIDA, 1965. On the porpoise caught by the salmon gill-net in Bering Sea and theNorth Pacific Ocean. Bull. Faculty Fish. Nagasaki Univ., 19: 1-36. (in Japanese, with English summary)
MIZUE, K., K. YOSHIDA, and A. TAKEMURA, 1966. On the ecology of the Dall's porpoise in Bering Sea andthe North Pacific Ocean. Bull. Faculty Fish. Nagasaki Univ., 21: 1-21. (in Japanese, with English summary)
MOREJOHN, G. V., V. LoE, and D. M. BALTZ, 1973. Coloration and sexual dimorphism in the Dall porpoise.
J. Mamm. 54 (4): 977-982.
NrsmwAKI, M., 1967 a. Distribution and migration of marine mammals in the North Pacific area. Bull.
Ocean Res. Inst. Univ. Tokyo, 1: 1-64.
NrsmWAKI, M., 1967 b. Observation and tagging of whales by TANSEIMARU in 1966. Geiken Tsushin,
187: 1-4 (in Japanese).
NoGucm, E., 1946. Dolphins and the utilization, In: Noguchi, E. and A. Nakamura, Utilization of dolphinsand scomber fishery, Kasumigaseki Shobo, Tokyo, 70 p (in Japanese).
NORRIS, K. S. and J. H. PRESCOTT, 1961. Observations on Pacific cetaceans of Californian and Mexican
Waters. Univ. Calif. Puhl. ,:Cool., 63 (4): 291-402.
0HSUMI, S., 1975. Incidental catch of cetaceans with salmon gillnet. J. Fish. Res. Board. Can., 32 (7): 12291235.
OKADA, Y. and R. Hayashi, 1951. Studies on the Delphinidae ofJapan (IV). Notes on the migration andfoetus of Phocoenoides dalli (True) in Japan. Rep. Fae. Fish. Prifectural Univ. Mie, 1 (1): 4--8.
PERRIN, W. F., J.M. CoE, and J. R. ZWEIFEL, 1976. Growth and reproduction of the spotted porpoise,
Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE

61

Stenella attenuata, in the offshore eastern tropical Pacific. Fish. Bull. 74 (2): 229-269.
SERGEANT, D. E., 1962. The biology of the pilot or pothead whale Globicephala melaena (Traill) in Newfoundland waters. Bull. Fish. Res. Board. Can., 132: 1-84.
SLEPTsov, M. M., 1961. Observation of small cetaceans in the far east seas and north-west part of the
Pacific Ocean. Trud. Inst. Morfol. ,Zivotnix, 34: 136-143. (in Russian)
TAKAGI, J., 1965. Gyosen [=Fishing vessels], In: World Encyclopedia 6: 229-236, Heibonshya, Tokyo.
(in Japanese)
WILKE, F., T. TANIWAKI, and N. KuRODA, 1953. Phocoenoides and Lagenorhynchus in Japan, with notes on
hunting. J. Mamm., 34 (4): 488-497.
YOSHIDA, S., 1939. The new rising of the harpoon fishing of porpoises and dolphins. Suisan Kenkyu-shi,
34 (1): 39-43. (in Japanese)

Sci. Rep. Whales Res. Inst.,

No. 30, 1978.

62

KASUYA

EXPLANATION OF PLATESPLATE IThe individual variation of the pigmentation pattern of Phocoenoides dalli caught by
the harpoon fishing off Sanriku coast.
Fig. 1. Type-I, male.
Fig. 2. Type-I, male.
Fig. 3. Type-2, 195 cm, male.
Fig. 4. Type-2, 177 cm, male.
Fig. 5. Type-3, female.
Fig. 6. Type-3, male.
Fig. 7. Type-4, 168 cm, male.
Fig. 8. Type-4, male.
Fig. 9. Type-5, 178 cm, female.
Fig. 10. Type-6, 182 cm, male.
PLATE II
The individual variation of the pigmentation pattern of Phocoenoides dalli caught by
the harpoon fishing off Sanriku coast.
Fig. 1. Type-7, male.
Fig. 2. Type-8, 193 cm, female.
Fig. 3. Type-9, 181 cm, female.
Fig. 4. Type-10, 204 cm, male.
Fig. 5. Type-11, female.
Fig. 6. Type-12, 207 cm, male. White patches on dorsal fin and chest region are
artifact of the reflection of light.
Fig. 7. Type-13, 168 cm, female.
Fig. 8. Type-12, ventral view of the individual shown in Fig. 6.
Fig. 9. Type-14, 175 cm, female.
Fig. 10. Type-3, male. An individual different from that in Fig. 6, Plate I.
PLATE III
Tooth sections of Phocoenoides dalli, decalcified and stained with haematoxmylin. Abbreviations are as follows.
C: Cementum, D: Dentine, E: Enamel, M: Periodontal membrane, N:
Neonatal line in dentine, Open circle: Dentino-cementa! boundary, Closed
circle: Cemental growth layer.
Figs l and 2. 110 cm, female, dalli-type, 26, July, 1972, 46°43'N, l62°24'E. Bornin the year of catch, no cemental layer is deposited.
Figs 3 and 4. 149 cm, male truei-type, 3, March 1974, off Sanriku coast. Presumably born in the preceding summer. One unstainable and a stainable cementa! layers are deposited. Cementum of this individual is extraordinary thick, andan accessory layer is present.
Figs 5 and 6. 182 cm, male, truei-type, 21, Feb., 1976, off Sanriku coast. Sexually
immature. 9 stainable layers in cementum.
PLATE IV
Figs l and 2. 196 cm, male, truei-type, 27, Feb., 1974, off Sanriku coast. 12
stainable layers in cementum. For abbreviations see Plate III.
Fig. 3. Immature testis tissue. 185 cm, truei-type, 20, Feb., 1976, off Sanriku coast.
Weight of testis 10.8 g.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

DALL'S PORPOISE
Fig. 4. Immature testis tissue. 174 cm, truei-type, 20, Feb., 1976, off Sanriku coast.
Weight of testis 7.2 g.
Fig. 5. Mature testis tissue. 194 cm, truei-type, 19, Feb., 1976, off Sanriku coast.
Weight of testis 43.9 g.
Fig. 6. Mature testis tissue. 190 cm, truei-type, 20, Feb., 1976, off Sanriku coast.
Weight of testis 41.4 g.
Tissues in Figs 3 to 6 are taken from the center of the testis.

PLATE VThe growth stages of the epididymal tissue of Phocoenoides dalli caught off Sanriku
coast. Scale indicates 0.1 mm.
Fig. 1. Immature epididymis. 184 cm, truei-type, 18, Feb., 1976. Weight of testis

6.4g.
Fig. 2. Immature epididymis. 166 cm, truei-type, 20, Feb., 1976. Weight -of testis
7.3 g.
Fig. 3. Maturing epididymis. 191 cm, truei-type, 18, Feb., 1976. Weight of testis
24.7 g.
Fig. 4. Maturing epididymis. 206 cm, truei-type, 17, Feb., 1976. Weight of testis
66.2 g.
Fig. 5. Mature epididymis. 194 cm, truei-type, 19, Feb., 1976. Weight of testis
43.9 g.
Fig. 6. Mature epididymis. 212 cm, truei-type, 19, Feb., 1976. Weight of testis
81. 7 g.

Sci. Rep. Whales Res. Inst.,
No. 30, 1978.

63

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