DISTRIBUTION AND ABUNDANCE OF
DALL'S PORPOISES OFF JAPAN*

TOMIO MIYASHITA** AND TOSHIO KASUYA**ABSTRACT

Results of nine whale sighting cruises in the Japanese coastal waters in1983 through 1986 and some additional cruises provided following
conclusion. Dall's porpoises inhabit waters below 24°C, but the upper
temperature limit is lower in winter: Dalli-type individuals occur throughout the range of the species, but the majority of them winter in the Sea ofJapan, and summer in the Okhotsk Sea and Pacific coast of Japan. Trueitype individuals inhabit only waters off the Pacific coast of northern Japanand southern Kuril Islands. They winter off the Pacific coast of Japan, andsummer mainly in the waters between 40° and 45°N and west of I 55°E. A
minimum estimate of the number of dalli-type that winter in the eastern
Sea of Japan is about 46,000 individuals, of these about 15,000 migrate
through the Tsugaru Strait to the Pacific coast of Japan with the major
portion of the stock summering in the Okhotsk Sea and northern Sea ofJapan. The number of truei-types in the nearshore surveyed area is estimated at about 26,000, and the extraporation of the density to the offshore
range of the .type provides an estimation of total truei-type population of
about 58,000 individuals. The continuation of current Japanese harpoon
fishery will further deplete the stock(s) of Dall's porpoises.
INTRODUCTION

The Dall's porpoise, Phocoenoides dalli (True, 1885), includes two major color
morphs designated as dalli-type and truei-type (Houck, 1976; Kasuya, 1978),the former being mainly distributed in the cold waters of the North Pacificand the latter in a limited area of the northwestern North Pacific (Nishiwaki,
1972; Ohsumi, 1975; Kasuya, 1978, 1982). Considering distribution of thesecolor types and their seasonal movement, Kasuya (1978) suggested thepresence of at least three local stocks of the Dall's porpoise in the north western
North Pacific and the adjacent seas, i.e. I) off the Pacific coast of northern
Japan and southern Kuril Islands, 2) offshore northwestern North Pacific and
the Bering Sea, and 3) in the Sea ofJapan and the Okhotsk Sea. He considered* An earlier version of this manuscript was submitted to the Scientific Committee of the International Whaling
Commission, Bournemouth, June 1987 (Document no. SC/39/SMI 1).

** Far Seas Fisherifs Research Laboratory, 5-7-1, Orido, Shimizu-ski, Shizuoka-ken, 424 Japan
Sci. Rep. Whales Res. Inst.,
No.39,1988,121-150

122

MIY AS HITA AND KASUY A

that all the dalli-type individuals in the truei-type range off the Pacific coast ofJapan would not be the members of Japanese east coast stock, but that at least
some of them, especially those in the schools containing no truei-types, would
be migrants from the offshore stock in the northwestern North Pacific and
the Bering Sea or from the Sea of Japan-Okhotsk Sea stock. A similar view
was presented by Miyazaki and Fujise (1985).
Recent data on the distribution of mother-calf pairs, parasite load, electrophoretic analysis of isoenzymes and pollutant levels in Dall's porpoisesuggest that the hypothetical stock of Kasuya ( 1978) in the north western
North Pacific and the Bering Sea probably includes two or more stocks, each
breeding in the Bering Sea, south of the western Aleutian Islands, and perhaps
off the east coast of northern Kuril Islands, respectively (Kasuya and Ogi,
1987; Subramanian, Tanabe, Fujise and Tatsukawa, 1986; Walker, 1987;
Winans and Jones, in press). Additionally, Yoshioka, Ogura and Shikano( 1987) presented data suggesting another apparently isolated breeding
ground in the Gulf of Alaska.
Subramanian, Tanabe, Fujise and Tatsukawa (1986) found that the PCB/
DDE ratio in the dalli-types taken in summer off the Pacific coast of Hokkaido
was closer to those in the Sea of Japan than the truei-types in the same region,and considered that the dalli-types in the two locations could have shared acommon feeding area. This suggests that the dalli-types migrate seasonally
between the Sea of Japan and the Pacific coast of northern Japan.
The number of Dall's porpoises in the principal area of truei-type distribution has been estimated, using sightings data pooled for the months
April to August since 1980, as about 64,000 (Kato, 1986) or about 125,000
individuals (Kato, 1987).
The present study analyzes the distribution and segregation of two typesof Dall's porpoises in the Japanese waters using data obtained through nine
whale sighting cruises in the western North Pacific, Sea of Japan and the
Okhotsk Sea. The population of the two types of Dall's porpoises in the area
is estimated using line transect methodology.
MATERIALS AND METHODS

The main body of data was obtained from the nine whale sighting cruises
conducted using two whale catcherboats (Toshimaru No. 15, 647.3 gross tons,
3,480 HP; Toshimaru No. 25, 739.9 gross tons, 3,600HP) chartered by theJapan Fisheries Agency in 1983 through 1986 (Table 1). Other sighting
cruises in which no Dall's porpoises were observed were excluded. The
seasons covered were nine months of the year, excluding April, November
and December.
In these cruises, survey was conducted along predetermined track line
placed systematically perpendicular to the coast line. Two mast-top observers
and three to six upper wheel-deck observers conducted sightings during
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

123

DALL'S PORPOISES OFF JAPANTABLE 1. WHALES SIGHTING CRUISES USED IN THE PRESENT STUDY
Cruiseno.

Period*Year

Name of vessel

2
3
4
5
6
7

12 Jan. - 24 Mar.
11 Jan. - 10 Mar.
10 Jan. - 10 Mar.
2 May - 30 June
9 June - 4 Sep.
10 June - 5Aug.
8June- 5 Sep.

1983
1984
1985
1986
1984
1983
1985

8
9

I Sep. - 30 Oct.
1 Sep. - 30 Oct.

1985 Toshimaru No. 15
1986 Toshimaru No. 15

Toshimaru No. 15
Toshimaru No. 15
Toshimaru No. 15
Toshimaru No. 25
Toshimaru No. 25
Toshimaru No. 25
Toshimaru No. 25

Biologists
on board
S. Shiraga
S. Nishiwaki
H. Shimada
T. Kasuya (7 July-6 Aug.)Y. Fujise (88 June-8 July),
F. Kasamatsu (6-20 Aug.),
H. Kishino (6-20 Aug.)
T. Miyashita (1-30 Sep.)
H. Shimada,
M. Ichihara (2 Sep.-8 Oct.)

Number of schools sighted
D

T

M

u

0
24

0
0
0

85
9
9
47

0
0
0
71
47
38
24

2
0
I

3
3
0
0
5
0
9

24
34

5
16

0
4

0
6

D: dalli-type Dall's porpoise. T: truei-type Dall's porpoise. M: mixed school of the two color types.
U: school of unknown color type. *: Total period of the cruise including the period that cruised out side theranges of Figs 2 to 7 and had no Dall's porpoise sightings.

daytime from 15 minutes after sunrise to 15 minutes before sunset except
during poor weather condition (Kasuya, 1986). The method of sighting wasbasically the same with that used for the Southern Hemisphere minke whale
assessment cruises and described by Best and Butterworth (1980).
Sighting effort has usually been discontinued when visibility is less than
1.0 nautical mile (n. m.) or the sea state is Beaufort 5 or over. The vessel
remained at the termination position until the weather changed, as far as time
permitted. Only sighting effort during searching mode under good weather
conditions and with mast-top observers was used as the primary sighting
effort, and sightings made under these condition are designated as primarysightings. We used only primary sightings and accompanying effort data forthe estimation of abundance. Any sightings other than primary sightings weredealt as secondary sightings and used only for the analyses of distributional
ranges.
All the marine mammals sighted were attempted to approach for
identification of species, color types of Dall's porpoise and school composition.
The following data were collected by the biologist and the captain for eachsighting; sighting time, sighting cue, angle from the bow of the vessel to theschool, radial distance, position and swimming direction of the school when
first seen, and estimated size composition. Surface water temperature at theposition of school was recorded, except for the 1983 and part of the 1984
cruises of Toshimaru No. 25. Radial distance between the vessel and Dall's
porpoise school was estimated usually when school was within about 0.5 n. m.,
but when school was apart for over the distance it was estimated from thevessel speed (15 knot for closure) and the time required for closure to a certain
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

124

MIYASHITA AND KASUYATABLE 2. NUMBER OF DALL'S PORPOISE SIGHTINGS IN THE JAPANESE
COASTAL WATERS BY AREA, MONTH AND COLOR TYPE
MADE DURING THE NINE CRUISES IN TABLE I
dalli-type

u

mixed

truei-type

Total*

Area & Month
no.

%

no.

%

no.

%

Pacific
March
May
July
AugustSeptember
October

4
18
21
18
35

5.3
15.3
63.3
64.3
77.7

70
98
II
6
10

93.3
83.I
33.3
21.4
22.2

l
2

1.3
1.7
3.0
14.3

Sea ofJapan
March
May
June
July
October

25
22
56
8
2

100.0
100.0
100.0
100.0
100.0

Okhotsk Sea
May
July
October

3
18
3

75.0
100.0
37.5

Total

233

no.

no.

%

75
Jl8
33
28
45

100
100
100
100
JOO

25
22
56
8
2

100
100
100
100
100

6

4
18
8

100
100
JOO

26

442

3

4

5
I
2
4
3

2

25.0
5
20J

62.5
8

U: school of unidentified color type. mixed: school of the two color types.
*: excluding school of unidentified color type.

position from the school where the rest of distance could be estimated visually.
However, the majority of the Dall's porpoise sightings occurred within theformer range.
School size was estimated by the two mast-top observers. Calves wereidentified by their small size and close association with a larger individual(presumably the mother). Color type of the porpoise was identified by the
biologist on board or by the captain. When color type was not recorded for
all the members of a Dall's porpoise school, we considered that all individuals
in the school were of the same color type. This could have underestimated the
proportion of mixed schools composed of both col or types. Since 1983 summer
cruise, an attempt was made to photograph all the marine mammal sightings
for later confirmation of the species identificaiton.
The surface isotherm was drawn based on surface water temperaturehourly recorded. Although this does not reflect the actual isotherm at any
particular moment, it is useful to indicate the relationship between the distribution of Dall's porpoise and surface water temperature.
Additional data recorded by the officers of research vessels include
position of the vessel and hourly meteorological records from 4 a.m. to 8 p.m.
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

125

DALL'S PORPOISES OFF JAPANTABLE 3. OCCURRENCE OF DALL'S PORPOISE SCHOOLS OFF THE PACIFIC COAST OF
JAPAN IN RELATION TO THE SURFACE WATER TEMPERATURE AT THE POSITION
OF SIGHTING. NOON POSITIONS INCLUDE ALL RECORDS FOR DAYS WITH
ORDINARY SIGHTING EFFORT DURING THE CRUISES IN TABLE I*
Surfacewatertemperature(OC)

3.0- 3.9
4.0- 4.9
5.0- 5.9
6.0- 6.9
7.0- 7.9
8.0- 8.9
9.0- 9.9
10.0-10.9
11.0-11.9
12.0-12.9
13.0-13.9
14.0-14.9
15.0-15.9
16.0-16.9
17.0-17.9
18.0-18.9
19.0-19.9
20.0-20.9
21.0-21.9
22.0-22.9
23.0-23.9
24.0-24.9
25.0-25.9
26.0-26.9
27.0-27.9
Total

May
N

August

July

D

T

2

3

N

D

T

N

D

September
T

N

D

T

October
N

D

T

1

2

10
8
12
9

2
5

2

7
7
2
5
5

2

3
2

10

2

2

7

1
3
5

2

1
2

2

2
7
7

3
3
3
3
3
4

4

3
9
3

22

5

71

36

4 31

1
5
5
3
3
3
2
6
29

3
4

9

1
2

2
1

4
1

3

2

2
3

8
7

11
13
2

l
2

7
10
10
4

2
2

3

55

35

9

11

6
6
1
3
4

2
20

2

54

22

10

N: number of noon positions, D: number of dalli-type schools, T: number of truei-type schools.
*: mixed schools are included into schools of each color type.

in local time during steaming, and when the activity of the vessel changed.
The northeastern range of truei-type porpoise was determined using
information recorded by biologists on board several Dall's porpoise research
vessels operated by the Japan Fisheries Agency during transits to the northwestern North Pacific and Bering Sea. These cruises and the referenceswhere details of the cruise including track lines and sightings are available areas follows;
May to June, 1984: Miyazaki, Fujise, Komuro and Taketomi (1984)
June to August, 1978: Miyazaki, Jones and Beach (1984), Kasuya andOgi (1987)
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

126

MIYASHITA AND KASUYATABLE 4. OCCURRENCE OF DALL'S PORPOISE SCHOOLS IN THE SEA OF JAPAN IN
RELATION TO THE SURFACE WATER TEMPERATURE AT THE POSITION OF
SIGHTING. NOON POSITIONS INCLUDE ALL RECORDS FOR DAYS WITH
ORDINARY SIGHTING EFFORT DURING THE CRUISES IN TABLE I
Surfacewatertemperature(oC)

2.0- 2.9
3.0- 3.9
4.0- 4.9
5.0- 5.9
6.0- 6.9
7.0- 7.9
8.0- 8.9
9.0- 9.9
10.0-10.9
11.0-11.9
12.0-12.9
13.0-13.9
14.0-14.9
15.0-15.9
16.0-16.9
17.0-17.9
18.0-18.9
19.0-19.9
20.0-20.9
21.0-21.9
22.0-22.9
23.0-23.9
24.0-24.9
Total

March

N

May

D

N

D

N

2

2

15

3

7
3
1
3

June

7

October

July
D

N

D

1

3
3

N

D

6
12
10
18

14

7
2

2

2

2

1
4

5
2

2

4
6
4

9

25

4

23

29

3
1
3
4
56

5

8

22

2

N: number of noon positions, D: number of dalli-type schools.

August to September, 1982: Kasuya and Jones (1984), Kasuya and Ogi(1987)
August to September, 1983: Ogi and Fujise (1984), Kasuya and Ogi(1987)
August to September, 1985: Ogi, Tanaka, Kuramochi and Yamamoto(1986), Kasuya and Ogi (1987).
SURFACE WATER TEMPERATURE

Sea of Japan-Okhotsk Sea dalli-types
Only dalli-type individuals were sighted in the Sea of Japan. Althoughtruei-types were sighted in summer months near the southeastern border ofSci. Rep. Whales Res. Inst.,
No. 39, 1988

127

DALL'S PORPOISES OFF JAPANTABLE 5. OCCURRENCE OF DALL'S PORPOISE SCHOOLS IN THE OKHOTSK SEA
IN RELATION TO THE SURFACE WATER TEMPERATURE AT THE POSITION
OF SIGHTING. NOON POSITIONS INCLUDE ALL DAYS WITH ORDINARY
SIGHTING EFFORT DURING THE CRUISES IN TABLE I*
Surfacewatertemperature

May

(oC)

N

2.0- 2.9
3.0- 3.9
4.0- 4.9
5.0- 5.9
6.0- 6.92

7.0- 7.9

2

D

T

N

D

T

N

D

T

3

8.0- 8.9
9.0- 9.9
10.0-10.9
11.0-I l.9
I2.0-I2.9
I3.0-I3.9
Total

October

July

3

5

3

I

I

2

II
5

3

3

I8

3

3

5

N: number of noon positions, D: number of dalli-type schools, T: number of truei-type schools.
*: mixed school of the two color types are included into schools of each color type.

the Okhotsk Sea (Figs 2 and 6), the primary color morphs of the Dall's
porpoise inhabiting the sea was considered to be dalli-type. In these twowaters, the survey was conducted in surface water temperature between 2°(May) and 25°C (October), and the Dall's porpoise occurred in the temperature
range of 6° (May) to 20°C (October) (Tables 4 and 5).
Sighting effort below 6°C was about 6 hours (one day) in the Sea ofJapan and 18 hours (three days) in the Okhotsk Sea resulting in a survey
distance of about 290 n. m. The absence of Dall's porpoise during the period
suggests that dalli-type Dall's porpoises in the Sea of Japan and Okhotsk Sea
may be uncommon in the waters below 6°C. Further definition of the lower
temperature bound of the species is not possible due to the limited survey
effort near the northern limits of the species.
The comparison between distribution of surface water temperature at
the noon position (indication of effort distribution) and the temperature atthe position of the Dall's porpoise sightings suggests that dalli-type Dall's
porpoises are rare in waters over l6°C in the Sea of Japan and in the Okhotsk
Sea (Tables 4 and 5). If one exceptional October record between 19° and·
19.9°C is excluded, the upper bound of surface water temperature for the
distribuion of the species in the Sea of Japan varied from about l3°C in
March, l4°C in June, and to l6°C in July and October (Table 4 and 5, andFig. 1). This change will be more enhanced if an exceptional record in OctoberSci. Rep. Whales Res. Inst.,
No. 39, 1988

128

MIYASHITA AND KASUYA

is taken into consideration. Thus we consider that Dall's porpoises do notchange the geographical distribution seasonally so much as expected from thechange in the surface water temperature, but they tend to live in higher watertemperature in the summer season than in the winter.
The number of dalli-type sightings relative to the quantity of survey
effort in certain water temperature range (represented by the number ofnoon position at certain surface temperature) was high in the temperature
ranges of 7° -11°C in March, 8°-l 3°C in June, and 12°-l 6°C in July, indicatinga temperature increase of 3°-5°C during the three months. Although data
were scanty in May and October, the feature was probably close to March andJuly, respectively.
Pacific coast truei-types
Although six schools of truei-types were sighted in May and October inthe southern Okhotsk Sea (Figs 2 and 6, and Table 5), they were very close
to the boundary to the Pacific Ocean. Therefore we consider they have notaltered our general concept that the southern Okhotsk Sea is primarily
inhabited by dalli-types (Kasuya, 1978).
Off the Pacific coast survey was conducted within the surface watertemperature of 3° to 28°C (May to October), and truei-type individuals weresighted at temperature below 25°C (Tables 3 and 5). The lower bound of
distribution was not detected, because the surveyed area was limited well tothe south of the northern range of distribution of truei-types indicated inFig. 8.
The upper bound of the temperature range in May ( l 9°C) increased to
25°C in July, and this was followed by a gradual decline to 24°C in August,
20°C in September, and to l9°C in October (Table 3). Three schools of Dall's
porpoise of unknown color type were sighted in March at about 35°N off the
Pacific coast of Japan (Fig. 1). Surface water temperatures for these sightings
were between 12° and l3°C. Thus it is suggested that seasonal migration oftruei-type Dall's porpoises is less than the magnitude of seasonal change of
surface water temperature, and that they inhabit waters of relatively hightemperature in summer season. A similar trend was indicated on dalli-types
in the Sea of Japan-Okhotsk Sea (see above), but the absolute temperature
ranges were several degrees higher in the Pacific truei-types.
The water temperature range of high sighting frequency was between 5°
and l 4°C in May and did not seem to fluctuate seasonally, which appeared tobe different from feature in the Sea of Japan-Okhotsk Sea stock.
Pacific coast dalli-types
Although, upper bounds of the temperature range of Pacific dalli-typesightings in July through October did not significantly differ from that oftruei-types in the same area, it was considerably lower in May and the rangeof the most preferred temperature seemed to be lower than that of truei-type
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

DALL'S PORPOISES OFF JAPAN129

in August through October (Table 3).
This will reflect the distribution of dalli-types in the northern part of thesurveyed area. The factors controlling this geographical segregation were
unclear.
GEOGRAPHICAL RANGE

Sea of Japan-Okhotsk Sea dalli-types
Data are available for February/March (Fig. 1), May (Fig. 2), June (Figs
2 and 5), July (Fig. 5), and October (Fig. 6) to indicate seasonal change of
Dall's porpoise distribution in the Sea of Japan and Okhotsk Sea.
The currently confirmed southern limit of the distribution in the Sea ofJapan has been between 35° and 36°N. A small number of individuals wasrecorded in winter from northern Kyushu at latitude of about 34°N during
the extensive sighting survey of dolphins in the Iki Island area between
southern Sea of Japan and the East China Sea (Kirishima, 1986). Thus, weconsider that the southern limit of Dall's porpoise in the Sea of Japan is in the
Tsushima Strait between Korea and Kyushu in western Japan, and that they
do not migrate to the East China Sea. Dall's porpoises have not been recorded
from China (Zhou, 1986).
The southern limit moved with the progress of season from the Tsushima
Strait to 40°N in early June (Fig. 2), and to 43°N in early July (Fig. 5). In
October some sporadic sightings of the dalli-type were recorded in thenortheastern part of the Sea of Japan between 41° and 43°N (Fig. 6). Noguchi(1946) reported past records of dolphin fishery off Tajima (35°30'N) in thesouthern Sea of Japan. The records indicated that the season of Dall's porpoise
hunting ended while the Pacific white-sided dolphin started in early May. No
data are available to indicate southward movement in the autumn/winterseason.
Although the majority of Dall's porpoise in the Sea of Japan leave there
in summer months and enter the Okhotsk Sea (Kasuya, 1978; 1982), someindividuals may summer in the Tatarskii Strait region where the surface watertemperature remains below 17°C.
Pacific coast truei-types
Data on the seasonal change of Dall's porpoise distribution off the Pacificcoast of Japan were available in March (Fig. 1), May (Fig. 2), July (Figs 3, 4
and 5), August (Figs 3, 4 and 5), September (Figs 6 and 7), and October(Fig. 7).
Truei-types were not observed in the Sea of Japan (Table 2), and thewestern limit observed in the present study was about 30 n. m. northeast of
the eastern entrance of the Tsugaru Strait (Fig. 3). Kawamura, Nakano,Tanaka, Sato, Fujise and Nishida (1983) reported the occurrence of truei-types
in the Tsugaru Strait area at 41°30'N, 141°00'E in June and December. This
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

130

MIYASHITA AND KASUY A
Cruise No.1
0

12 Jan-2~MarJ3°
1983, ",: -150

7

130°E

·._~ ___'./ ~
,

~

360 N

34°N

140°E

Fig. 1. Southern limits of Dall's porpoises in winter season observed during threewhale sighting cruises of the Toshimaru No. 15 Qanuary, February and March in
1983, 1984 and 1985). Solid line indicates track line cruised with ordinary sighting
effort, dotted line the surface water isotherm in centigrade, open circle sighting of
dalli-types, and star that of type unidentified Dall's porpoises. Dall's porpoiseschools of secondary sighting included.

Fig. 2. Dall's porpoise sightings, surface water isotherms (dotted lines, in centigrade),and the track lines of the Toshimaru No. 25 cruised with ordinary sighting effort in
May through June, 1986 (solid line and arrow head). Open circle indicates schoolof dalli-types, closed circle that of truei-types, open triangle that containing twocolor types, and star that of unidentified color types. Secondary sightings of Dall's
porpoise schools included.
Sci. Rep. Whales Res. Inst.,

No. 39, 1988

131

DALL'S PORPOISES OFF JAPANPacific OceanJ)

1

14 Aug.

20 Aug.(350N

Fig. 3. Dall's porpoise sightings, surface water isotherms (dotted lines in centigrade),and the track lines of the Toshimaru No. 25 cruised with ordinary sighting effort inJuly through August, 1984 (solid line and arrow head). Open circle indicates schoolof dalli-types, closed circle that of truei-types, open triangle that containing twocolor types, and star that of unidentified color types. Secondary sightings of Dall's
porpoise schools included.

019Jul~'

jFig. 4. Dall's porpoise sightings, surface water isotherms (dotted lines in centigrade),and the track lines of the Toshimaru No. 25 cruised with ordinary sighting effort inJuly through August, 1983 (solid line and arrow head). Open circle indicates schoolof dalli-types, closed circle that of truei-types, open triangle that containing twocolor types, and star that of unidentified color types. Secondary sightings of Dall's
porpoise schools included.
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

MIYASHITA AND KASUYA

132
Cruise No.7···16

Fig. 5. Dall's porpoise sightings, surface water isotherms (dotted lines in centigrade),and the track lines of the Toshirnaru No. 25 cruised with ordinary sighting effort inJuly through August, 1985 (solid line and arrow head). Open circle indicates schoolof dalli-types, closed circle that of truei-types, open triangle that containing twocolor types, and star that of unidentified color types. Secondary sightings of Dall's
porpoise schools included.

130°E
140°E
Fig. 6. Dall's porpoise sightings, surface water isotherms (dotted lines, in centigrade),and the track lines of the Toshimaru No. 15 cruised with ordinary sighting effort in
September through October, 1985 (solid line and arrow head). Open circle indicatesschool of dalli-types, closed circle that of truei-types, open triangle that containing
two color types, and star that of unidentified color types. Secondary sightings of
Dall's porpoise schools included.

Sci. Rep. Whales Res. Inst.,
No. 39, 1988

DALL'S PORPOISES OFF JAPAN133

Fig. 7. Dall's porpoise sightings, surface water isotherm (dotted lines in centigrade),and the track lines of the Toshimaru No. 15 cruised with ordinary sighting effort in
September through October, 1986 (solid line and arrow head). Open circle indicatesschool of dalli-types, closed circle that of truei-types, open triangle that containing
two color types, and star that of unidentified color types. Secondary sightings of
Dall's porpoise schools included.

is the confirmed western limit of the color type in the Tsugaru Strait area.
The southernmost sighting of truei-types was recorded in May at about
34°N and at the water temperature of about 24°C (Fig. 2 and Table 3). This
was an isolated southern record for the season and we do not consider it torepresent the ordinary range of the color type in the season (Fig. 2). The next
southern records (three schools of unknown ·color type) were obtained in
March in between 35° and 36°N (Fig. 1) and this agreed with the previously
reported range of the species off the Pacific coast of Japan (Kasuya, 1978).
The sourthern range of truei-types in the Japanese coastal Pacific waters
moved from around 35°N in March to around 36°N in early May and earlySci. Rep. Whales Res. Inst.,
No. 39, 1988

134

MIYASHITA AND KASUYA
"\)

i I

-

\

'
lr~

-

_!
I

I

Tl

~

[/

"""-- -~ I? -t1,,,_!:12-" Ul , ,

,

~

Tl'

T1

T3

D3Ul

f_.19'<
7' 1,
UlO ::.- ',/TG=
TS T3 T2 ' T4 T1
.D3Dl
Dl
~
U2'- u:iT 12~-::: =\- - Ul
'J
D3
T3- Ul TG- J4', Tri
Dl
D3
Dl
\
I~
,
; Ul
I 1i
D2 / /
I--,,-""-...
~

LJ

jY

Ul

/

~~

-

-/

,OJ

Ul,, T17 Ul D4'/ / D2
U9
U2/
Dl J>3 D21'y7/£_ ,,. .... ~1/

~ ~/
'.fl'
-:) Dly ~(DB.
--- l1/

DB', D8
UG

45°N

~

Ul

-- '

~

/

//

--- --T3_ --- -- - T2_ >----- J2- --TL T4- --U6
Ul~+- Dl D3

J-

D2;;P~ U4

U2, U2Dl_
D1- Tl-- _!].;:' TS"- Tly DG:.. ~5Ul ~Ul
D3 B~ lJ 1,
U4,.. 6'
I
T4
D4 y.2"- Dl...._
T2'
Tl,~
Ul
D)/ D2u//lJ4 U3 , I
J4/
T1
Tl~ 'Tll ,
B~/
~ Ul
/

sv
-y.;y- -- 7- ----....... 6f~
A- -- -- /
,-

/,/

/
//

D2,, .()2

D}/ DV Dl

,

U2

//,

/

tr
T4

~

.

Dl
U2

A

/'~

]

.

,

T1

,,

DJ'

v

U2

Fig. 8. Number of Dall's porpoise sightings off the Pacific coast of northern Japanand southern Kuril Islands in May through September appeared in the publishedrecords of the Dall's porpoise research cruises (Kasuya and Jones, 1984; Miyazaki
et al. l 984a; Miyazaki et al., l 984b; Ogi and Fujise, 1984; Ogi et al., 1986). Secondarysightings included. Color type code: T, truei-type school; D, dalli-type school; U,
school of unidentified type. Mixed schools of two color types are included into eachtype.

July (Figs 2 and 4), to 40°N in early August (Fig. 4). The southern limit ofdalli-type in the coastal waters tended to be to the south of their range in theoffshore waters (Figs 5 and 7). This reflects the presence of cold water in the
coastal area.
Kasuya (1978) indicated that although the proportion of dalli-type catch
increased with month the majority (about 96%) of Dall's porpoise catches off
Sanriku Region between 37°30'N and 40°00'N were truei-types in January to
March. The feature is similar in principle to our sighting data during May
through July (Figs 2 to 4), but the predominance probably changes in August
as seen in Figs 4 and 5.
In August, truei-types were absent in the coastal Pacific waters during thecruise no. 7 in 1985 (Fig. 5), but present during the cruise no. 6 in 1983 (Fig.
4) suggesting some annual fluctuation in the timing of seasonal movements
and predominance change. Truei-types were almost totally absent fromJapanese coastal waters in September of 1985 (cruise no. 8) and 1986 (cruise
no. 9) (Figs 6 and 7). However, a survey in September and October 1986
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

DALL'S PORPOISES OFF JAPAN

135(cruise no. 9) suggested that the porpoise had probably moved in summer tothe offshore waters between 144° and 14 7°E and returned to coastal waters in
October (Fig. 7). Some truei-types were sighted in late October in the boundary
area between the southern Okhotsk Sea and Pacific Ocean at latitudes around
44°N (Fig. 6). We conclude therefore that truei-types predominates over dallitypes in the coastal waters along the Pacific coast of northern Japan during
winter and spring seasons (probably, from November to July) but the
predominance alters during a short summer/autumn period (September toearly October) with possible annual fluctuation.
Information on the northern range of truei-types has been reported bybiologists on board of five Dall's porpoise research cruises in the north western
North Pacific and the Bering Sea (Fig. 8). Although Dall's porpoises weresighted almost continuously during the cruises in the area, the dominantcolor type altered rather sharply. The approximate positions of the change inthe predominant color types of the Dall's porpoise are as follows (truei-type
predominating on the southwestern side of the designated points);
1. May38°30'N, 152°00'E (Miyazaki, Fujise, Komuro, Taketomi,1984).
42°30'N, 153°00'E (Miyazaki, Fujise, Komuro, Taketomi,
2. June1984).
45°00'N, 150°00'E (Miyazaki, Jones and Beach, 1984).
3. June42°00'N, 143°30'E (Ogi, Tanaka, Kuramochi and
4. AugustYamamoto, 1986).
43°00'N, 154°00'E (Miyazaki, et al., 1984).
5. August
6. August
43°30'N, 152°00'E (Ogi and Fujise, 1984).
7. August
44°00'N, 157°00'E (Kasuya and Jones, 1984).
8. September 43°00'N, 145°00'E (Kasuya and Jones, 1984).
9. September 43°00'N, 151°00'E (Ogi and Fujise, 1984).
10. September 43°00'N, 153°30'E (Ogi et al., 1986).
Two coastal points (nos 4 and 8 in the above list) reflect the local concentrationof dalli-types mentioned below and may not be a good indicator of the eastern
limit of the range of truei-types. The point no. 1 will possibly represent theeastern range of truei-types in May.
In other months (June to September), the observed northern boundaryof distribution of truei-types was located between 42° and 45°N, and showedno north/south movement. This is in good contrast with the disappearance ofmost of truei-types from the Japanese coastal waters in September andOctober (see above).
The above analysis indicates that truei-type Dall's porpoise has seasonal
northeast to southwest movement within their ·range between 35° and 45°N,
and in late summer they almost disappear from a narrow Japanese coastal
waters south of 43°N to concentrate in the northern and presumably slightly
offshore waters. Northern limit of truei-types in the winter season is still to be
examined.
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

136

MIY AS HITA AND KASUY A

Pacific coast dalli-types
Data on the distribution of dalli-type Dall's porpoise off the Pacific coastof northern Japan were obtained during cruises in March (no color typeidentification available, Fig. 1), May (Fig. 2), July (Figs 3, 4 and 5), August(Figs 4 and 5), September (Figs 6 and 7), and in October (Fig. 7).
About 4% of the catch of the winter operation of harpoon fishery off
Sanriku were dalli-types (Kasuya, 1978), and there was no difference detectedin the temperature preference between the two color types off the Pacificcoast of Japan (see above). Therefore, we consider that the southern limit of
the normal distribution of dalli-types would be at about 35°N as in the case forthe truei-types.
Our data recorded small local concentrations of dalli-types to the east of
the Tsugaru Strait as already indicated by Kasuya ( 1978) and Miyazaki andFujise (1985). This concentration occurred in May along the southern coastof Hokkaido from the east of the Tsugaru Strait to 41°N, l 45°E (Fig. 2). The
feature was similar in July 1984, when we observed a concentration in 42°N,
l 43°30'E close to the southern coast of Hokkaido (Fig. 3). Such concentration
possibly existed in August 1983 in the southeastern area of the Tsugaru Strait(Fig. 4).
The abundance and distribution of dalli-type individuals off the Pacificcoast of Japan was high during the months when truei-types were becoming
scarce in August (Fig. 5), almost absent (September, Figs 6 and 7), or probably
just starting to reappear (October, Fig. 7). Among Dall's porpoise sightings in
Fig. 5, all the 24 truei-type sightings occurred in July, 2 dalli-type sightings in
the Pacific area in July, and 19 dalli-type sightings in August.
Our data confirmed that the distribution of dalli-types in the Pacificwaters was continuous through the Tsugaru Strait to the Sea of Japan at leastin May (Fig. 2), September (Fig. 7) and October (Fig. 6). However the density
in the strait area appeared to be low. Similar observations have been made by
Kawamura et al. (1983) in the Tsugaru Strait area. They recorded moredalli-types in the strait area in April through July than truei-types, but they
considered that the frequency was low compared with that of other dolphin
species.
We consider therefore that the distribution of dalli-type Dall's porpoises
is continuous between the Sea of Japan and Pacific waters during spring andautumn season, and that most of the dalli-types in the latter area are probably
migrants from the former waters. Thus, we presume that a portion of theputative Sea of Japan-Okhotsk Sea stock of Kasuya (1978) passes the Tsugaru
Strait in spring to migrate to the Pacific coast of Japan and summer there
while the majority of truei-types summer further offshore in the northernarea. Dalli-types will return to the Sea of Japan in autumn preceding thearrival of truei-types. The bases for the above conclusion are summerized asfollows;( 1) A small local concentration of dalli-types occurred in the waters east
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

DALL'S PORPOISES OFF JAPAN137

of the Tsugaru Strait in May when the distribution of dalli-types wascontinuous through the strait between the Sea of Japan and Pacificcoast of Japan (Fig. 2).
(2) The dalli-type range extended east/south before summer when the
distribution of the species became discontinuous between the twoseas (Fig. 5).
(3) The distribution of dalli-types was continuous through the Tsugaru
Strait in September and October, when the migration of dalli-types
in the southern Okhotsk Sea did not apparently reach to the west
coast of Hokkaido (Fig. 6).
This migration pattern of the Dall's porpoise is similar to schematic drawing
by Noguchi (1946), which has not received the attention of biologists, presumably due to the lack of presentation of basic data for the hypothesis.
Data are not available to determine whether dalli-types summering off
the Pacific coast of northern Japan and in the southern Okho.tsk Sea (they are
considered to belong to Sea of Japan/Okhotsk Sea stock) intermingle in thewaters adjacent to the southern Kuril Islands and eastern Hokkaido.

ABUNDANCE
Mathematical methodology
Population size (N) was estimated for the two color types and sub-areas
defined below using following equation,
N=n"f(O)·A·s

2·L

where n represents number of primary sightings, A size of surveyed area
(n. m. 2), L length of transect line (n. m.),j(O) probability density function of
the perpendicular distances evaluated at the origin, and s mean school size.
The coefficient of variation (c. v.) for n, s and f(O) were estimated by theequations of Anon. (1985a), then that of N by following equation,c.v.(N)2 = c.v.(n)2 + c.v.(f(0)) 2 + c.v.(s) 2
assuming each factor was independent statistics.
In this study we firstly fitted both Fourier series (FS) and negative
exponential models (NE) to estimate j(O) value, then chose the model with
better fit for each data using the Chi-square goodness of fit test (Table 7). The
probability of seeing a Dall's porpoise school on the track line was assumed
as one. No smearing was done to eliminate the rounding error of sighting
angle and radial distance estimate.
Sub-area definition
We used only data obtained through six spring/summer cruises (cruisenos 2, 4, 6, 8 and 9) which had relatively good coverage of the Dall's porpoise
range of distribution and had a large number of sightings. Five sub-areas (twoSci. Rep. Whales Res. Inst.,
No. 39, 1988

MIYASHITA AND KASUYA

138

45°N

T3

40°N
/

I

i

,-T2--/---·

/

'

I

/

'

I

I

/

I

I

I
I

'

Tl

140°E

35°N

145°E

150°E

1ss E
0

Fig. 9. Sub-areas defined to estimate the population of Dall's porpoises off Japan.
DI, observed range of dalli-types in the Sea of Japan; D2, observed range of dallitypes off the Pacific coast of Japan; TI and T2, observed ranges of truei-types off
the Pacific coast of Japan; T3, known range of truei-type predominance but
unsurveyed in the present study.

for dalli-type range and three for truei-type range) (Fig. 9 and Table 6) were
defined to cover the whole range of Dall's porpoise sighting. Of these fivesub-areas, four (DI, D2, TI and T2) are areas covered by our sightings effort,
but the remaining one (T3) is the unsurveyed area known to be inhabited bytruei-types (see above). A minor modification is done for T3 to meet theboundary difference of TI and T2. We have extrapolated the truei-type
densities in the two surveyed sub-areas (TI and T2) into the unsurveyed
sub-area (T3) in order to obtain a rough estimate of the total population oftruei-types. The methodological problem that surrounds this procedure will
be discussed later.
School size
We used primary sightings of I 43 dalli-type schools and I 36 truei-type
schools for the population estimate, but school size estimate was not available
for one dalli-type school. There were three mixed schools of the two color
types, and we regarded them as separate school for each color type.
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

139

DALL'S PORPOISES OFF JAPANTABLE 6. PARAMETERS FOR THE ESTIMATION OF DALL'S PORPOISE
POPULATION IN THE SURVEYED AREA
Area(n. mile) 2

Transectlength(n. mile)

no.ofsightings**

Meanschoolsize (c.v.)

Sighting
rate***(c.v.)

1986
1985
1985
1986

48,80225,36525,36525,36525,365

1,296
1,003
1,398
1,756
3,154

75
14
17
32
49

6.7 (0.07)
13.3 (0.19)
9.6 (0.19)
7.5 (0.16)
8.4 (0.13)

3.75 (0.19)
1.40 (0.40)
1.36 (0.65)
1.65 (0.33)
1.48 (0.30)

2 May - 23 May 1986
8 July - 3 Aug. 1984
3 Aug. 1983
14 July 8 July - 3 Aug.

93,71874,31 I74,3 I I
74,31 I

2,296
1,124
836
1,960

68
35
33
68

6.4 (0.08)
3.5 (0.10)
9.7(0.15)
6.5 (0.13)

2.92 (0.22)
2.94 (0.32)
3.95 (0.34)
3.42 (0.24)

Periodofdata

Cruise Subnumber* area
dalli-type
4
7
8
9
8+9
truei-type
4
5
6
5+6

DI

D2
D2
D2
D2
TI

T2
T2
T2

26 May 7 July 16 Sep. 11 Sep. I I Sep. -11 June26 Aug.
30 Oct.
30 Oct.
30 Oct.

*: see Table 1. **: number of primary sightings; ***: number of primary sightings per JOO n. miles.

The size of 142 dalli-type schools ranged from one to 34 individuals, withthe modal school size at four to six individuals in the three sub-area/cruises(Fig. 10). Mode was unclear in the Pacific sightings in sub-area D2. Mean
school sizes of dalli-type were from 7.5 to 13.3 individuals in the Pacificsub-area D2 and 6.7 in the Sea of Japan sub-area Dl. The first and last
figures were significantly smaller than the largest figure for the Pacific subarea (Mann-Whitney U-test, p<0.04), but other means were not significantly
different each other (p>0.19).
The 136 primary sightings of truei-types ranged from one to 70 individuals(the second largest school was composed of 34 individuals). We excluded
from the following analysis this largest school of 70 individuals recorded
during the cruise no. 6 in sub-area T2, because we were uncertain if it was areal large school or included some nearby schools that should have beenclassified as secondary sightings. This exclusion decreased the mean school
size from 11.5 (with the largest school) to 9.7 (used here). It could have also
caused small degree of underbias in the population estimate (Tables 7 and 8)
through the under representation of school density. However, we do notconsider this effect very important, because the population estimate thus
calculated still had extraordinarily large coefficient of variation (see below).
The modal figures of the truei-type school sizes varied between three to
10 individuals in the three sub-area/cruises (Fig. 10), and the mean values
between 3.5 and 9. 7 individuals (Table 6). Only the smallest mean school size
(3.5 individuals) for cruise no. 5 in sub-area T2 was significantly smaller thanany other values (Mann-Whitney U-test, p<0.01).

Sci. Rep. Whales Res. Inst.,
No. 39, 1988

140

MIYASHITA AND KASUYA
A. dalli - typesub-area 01
cruise no. 4

15

E.

truei-typesub-area T 1
cruise no.4

G.

truei-typesub-area T 2
cruise no. 6

15

10

10

5
Ul

0

0

-5Ul 0+------.-----'--"-r----'-'-T"-----'-r'--..... 20~

~ 0 +---'---'-rL-1......__....,...__.___'-T"--__......._

E

5

i. 2

lJO
34

5

0
5

5

-0-D

10

15

20 30

nD-0

0
0

5

10

15

20 26 30 70

School size
Fig. 10. School size composition of primary sightings of Dall's porpoises.

Sighting rate
The sighting rate (number of primary sightings per I 00 n. m. of transectline) of the dalli-type schools was between 1.36 and 1.65 in the Pacific sub-area
D2 (cruise nos 7, 8 and 9) and 3.75 in the Sea of Japan sub-area DI (cruise
no. 4) (Table 6). The figure for the Sea of Japan sub-area was significantly
larger than any of the three corresponding figures of the dalli-type in the
Pacific (Mann-Whitney U-test, p<0.0 I).
The three sighting rates for truei-type in the Pacific sub-area TI and T2
ranged from 2.92 to 3.95 (Table 6), and the difference was not statisticallysignificant (Mann-Whitney U-test, p>0.14).
Perpendicular distanceThe distribution of perpendicular distances of dalli-type (Sea of Japan)
and truei-type (Pacific) sightings during the cruise no. 4 (Toshimaru No. 25)
had clear shoulder at 0.2 to 0.3 n. m. (Fig. 11 A and E), but different cruises
by one vessel often had different distribution pattern as seen in the contrast
between A or E and For Gin Fig. 11 (Toshimaru No. 25 cruise). This suggeststhat the distribution pattern of the perpendicular distances is not dependent
on the vessel but varies between cruises (presumably affected by observer orweather).
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

141

DALL'S PORPOISES OFF JAPAN

25 •

~

20

A. dalli-typesub- area 01
cruise no.4

E.

t ruei-typesub-area T 1
cruise no.4

15

''

10

\

''

5

-E 0

+---~,.........__._-~_.___ _...,

o
..C

u

'

'

dalli-typesub-area 02
cruise no.7

5

I/)

F.
,,

truei-typesub-area T2
cruise no.5

.....
0

.... 0 +-~"-'"--+---'-'"-T---.....
CIJ

~ 10

C.

::J

~

z

I

\

dalli-typesub-area 02
cruise no.8+--~-'-----'--+'---==~

3.2
G.

5

D.
~

10

'

_q

truei-typesub-area T2
cruise no.6

dalli- typesub-area 02
cruise no.9
I

~\

'

\

5

~\

O+----~-'-~T-=---~-~''--.-,

0

0.5

1.0

1.5

+-~--l-.........._~~...._~.Q
0
0.5
1.0
2.8

Perpendicular distance(n. m.)
Fig. 11. Perpendicular distances of primary sightings of Dall's porpoises (whitesquares) and Fourier or negative exponential model fitted to them (closed circles anddotted line). For models used see Table 7.

Sci. Rep. Whales Res. Inst.,
No. 39, 1988

142

MIYASHITA AND KASUY A

20\ dalli-type, sub- area 02, cruises 8+9
'

10

0
l/l

0
0

.c.
u
l/l

0.5

0
30

1.0

~[b
0

5

y

10

15

20

10

15

20

truei-type, sub-area T2, cruises 5+6

....0

~

Q.I

.a
E

20

~\

::J

z

''

~'
'

10

\

0 +---....-----'----l..::\'"'~o.L-~
0

0.5

1.0

Perpendicular d ist. (n. m.)

05School size

Fig. 12. Distribution of school size and perpendicular distance for d.alli-types (sum of
the cruise nos 8 and 9) and truei-types (sum of the cruise nos 5 and 6) off the Pacificcoast of Japan. For marks see Fig. 11 and Table 7.

Population estimate
Either Fourier series or negative exponential model was selected forindividual data set based on Chi-square goodness of fit test (Table 7). The
Fourier series model showed better fit for dalli-type sightings in all sub-area/
cruises and one truei-type data set (cruise no. 4, sub-area TI). Anothertruei-type data set (cruise no. 5, sub-area T2) showed better fit to the negative
exponential model. And only negative exponential model could be successfully
fitted to a truei-type data set in the Pacific sub-area T2 (cruise no. 6). The
Fourier series model could not be fitted to this data set, because the terms of
the Fourier series fit exceeded the number to satisfy the 'stopping rule' of
Burnham, Anderson and Laake. (I980).
Population of dalli-types in the Sea of Japan (for sub-area DI) was
estimated at 3 I,789 individuals (with 95% confidence interval of I 7,770 to
45,808) using data in May to June, I986 (cruise no. 4) (Table 7). This value
represents only a portion of the Sea of Japan-Okhotsk Sea stock of the Dall's
porpoise (dalli-type), because it does not include individuals in the whole
Sci. Rep. Whales Res. Inst.,
No. 39, 1988

143

DALL'S PORPOISES OFF JAPANTABLE 7. POPULATION ESTIMATE OF DALL'S PORPOISES IN THE SURVEYED
AREA. f(O) WAS CHOSEN BY THE CHI-SQUARE FITNESS TEST
Cruiseno. m
Table I

f(O) (c.v.
andmodel used)*

Chi-squ.
goodnessof fit**Population
size(c.v.)

Subarea

Periodofdata

dalli-type
4
7
8
9
8+9

DI
D2
D2
D2
D2

May -June 1986
July - Aug. 1985
Sep. - Oct. 1985
Sep. - Oct. 1986
Sep. - Oct.

3.36 (0.09, FS)
6.19 (0. 24, FS)
5.69 (0.25, FS)
3.31 (0.21, FS)
4.91 (0.19, FS)

0.816
0.600
0.082
0.430
0.247

31,789 (0.23)
14,579 (0.50)
8,424 (0. 72)
5,737 (0.42)
8,126 (0.38)

truei-type
4
5
6
5+6

TI
T2
T2
T2

May
July July July -1968
Aug. 1984
Aug. 1983
Aug.

2.94 (0.06, FS)
2.88 (0.24, NE)
3.87 (0.25, NE)
3.36 (0.17, NE)

0.452
0.522
0.004
0.018

26,113 (0.25)
11,662 (0.41)
55,057 (0.45)***27,844 (0.32)****: FS, Fourier series model. NE, Negative exponential model.
**: probability of obtaining a deviation as great or greater than calculated Chi-square value.
***: low reliability due to poor model fit.

range of the stock considered to extend during the survey period to the
Okhotsk Sea (Kasuya, 1978) and western part of the Sea of Japan (see Fig. 2)
or to the Pacific coast of northern Japan (see above). At the time of the cruise(early June) the southern limit of distribution has already moved to 40°N
from the southernmost limit at 35°N in winter season. The cruise (no. 4)
proceeded from north to south against the seasonal movement of the Dall's
porpoise (Fig. 2). This also could have caused some degree of underbias in
the estimate of population size.
Three population estimates of dalli-types in the Pacific sub-area (D2)ranged from 5,737 to 14,579 individuals (Table 7), but none of these figures
were statistically different (p>0.33). Since none of the parameters of dalli-typesightings during the two cruises of no. 8 and no. 9 in the Pacific sub-area D2
and in September and October, we combined the data to increase sample size
and to decrease the confidence interval. The procedure gave dalli-type
population of 8,126 with 95% confidence interval of 2,105 to 14,146 (Tables
6 and 7). This was not significantly different still from 14,579 estimated forJuly-August data of the same area. However, the September/October figure
was only half of the July/August figure, and it is suggested that the difference
may reflect in some degree the migration of the dalli-type individuals to the
Sea of Japan in autumn (see above). Therefore, we considered that about
15,000 individuals of dalli-types in the Sea. of Japan migrate through the
Tsugaru Strait and spend summer off the Pacific coast of Japan. The rest of
individuals in the stock will summer in the northern Sea of Japan or in the
Okhotsk Sea (Kasuya, 1978).
The above population estimates of dalli-types in the Sea of Japan (DI)Sci. Rep. Whales Res. Inst.,
No. 39, 1988

144

MIY AS HITA AND KASUY A

and Pacific (D2) area were made using data obtained in slightly different
seasons of the year (late May to early June in the Sea of Japan, and early Julyto late August in the Pacific). Although there is some possibility that individuals
counted in the Sea of Japan were again included in the population estimate
for the Pacific area, we consider that the effect may not be very large comparedwith the individuals distributed in the western Sea of Japan or in the Okhotsk
Sea far from the Hokkaido Qapanese) coast and excluded from the calculation.
We note that the southern boundary of the Dall's porpoise in the Sea of Japan
has almost passed the latitude of the Tsugaru Strait at the survey time in early
June (Fig. 2). We have excluded some dalli-types found in the offshore
unsurveyed truei-type area (T3), which belonging is still uncertain to us. Thus
we consider that the currently available best figure of the minimum estimateof Sea of Japan-Okhotsk Sea stock of Dall's porpoises is about 46,000(=3I,789+ I4,579) individuals with 95% confidence interval of 26,290 to
66,445.
We obtained primarily three population estimates for truei-types in thesurveyed area, one for May and two for July/August. Although their range
was wide (I I,662 to 55,057 individuals, Table 7), none of them were statisticallydifferent from others (p>O. I 4). Although the mean school sizes were different
between the two cruises (nos 5 and 6) conducted in the same season Quly/
August) and in the some sub-area T2, we combined these data as in the dallitype case (cruise nos 8 and 9) to improve the estimate of truei-type population
size. This procedure provided an estimate of 27 ,844 individuals with 95%
confidence interval of 10,380 to 45,308 (Tables 6 and 7). This figure Quly/
August, sub-area T2) is not significantly different from 26, I I 3 (with 95%
confidence interval of I3,522 to 38,704) estimated using May data in the
sub-area TI.
Above estimates of truei-type population do not include individuals
known to be distributed in their summering ground outside the present
survey area. Extrapolation of the density data corresponding to the above
three estimates to the unsurveyed area almost doubles the estimation oftruei-type population (Table 8). This procedure assumes equal Dall's porpoisedensity in any particular moment for both surveyed and unsurveyed part ofthe sea. Table 9 compares, using data listed in Fig. 12, Dall's porpoise density
between offshore and inshore Pacific areas off the northern Japan, whichroughly correspond to T3 and TI or T2 of this study, respectively. The ratiosof truei-types: dalli-types in each of the area are I 90:59 (northern offshorearea), I86:5I (northern inshore) and 52:II (southern inshore), and the trueitype proportion ranged from 76 to 82% showing no significant difference
between the three areas. The density of Dall's porpoises were almost same
between two northern areas while that in the southern area was lower than
the two.
These suggest that the Dall's porpoise density in the present surveyed
areas (TI and T2) is almost same with that in unsurveyed area (T3) if averaged
Sci. Rep. Whales Res. Inst.,
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DALL'S PORPOISES OFF JAPANTABLE 8. TRUEI-TYPE POPULATION ESTIMATES EXTRAPORATED TO UNSURVEYEDAREA OF THE NORTHEASTERN PORTION OF THE RANGE (T3*)

surveyedarea

Unsurveyed
area ofdistribution(n. mile 2)

Populationin
unsurveyed
area

Total
population

26,113
11,66255,057
27,844

113,834
112,852
112,852
112,852

31,71817,71083,612**
42,235**57,831
29,372
138,669**
69,779**Population
Cruise
no.

Month

4
5
6
5+6

May
July - Aug.
July - Aug.
July - Aug.

m*: see Fig. 9 for range.

**: low reliability due to poor model fit.

over months (May through September) and years (5 seasons). However, thisdoes not necessarily imply that the densities of truei-types in the northern
inshore and offshore areas are same in any particular month or year. Rather,
we consider that the number of truei-types in the inshore area and that in theoffshore area are negatively correlated, and that the densities are also
dependent each other. Therefore, the estimates of truei-type population in
the unsurveyed area thus extrapolated can be biased to the same direction asthat in the surveyed area, and the bias can be amplified in the total population
estimate. We feel that two extreme estimates of the total truei-type populationin Table 8 (29,372 for cruise no. 5 and 138,669 for cruise no. 6) have higher
probability of being far from the true figure than other figures. Additionally
two figures in Table 8 using the cruise no. 6, i.e. 138,669 and 69,779, havelower reliability due to the poor goodness of fit of the model (Table 7). Only
remaining estimate is 57 ,831 based on the cruise no. 4 in May 1986. As theconclusion, we consider that the population of the truei-type will be about
58,000 individuals. The confidence interval can not be estimated, but the c.v.
has to be over 0.25 the figure calculated for surveyed area.
DISCUSSION

We made two assumptions in the present population estimate. They are (1)all the Dall's porpoises on the trackline are counted, and (2) Dall's porpoiseschools do not respond to vessels. If Dall's porpoise schools respond to an
approaching research vessel by moving toward or away from the vessel before
the schools are detected by researchers the population estimate of the present
study will be seriously under or over biased, respectively (Hammond, 1986).
The response of the species to the vessel was investigated using helicopters(Bouchet, Braham and Tsunoda, 1983), and there was an attempt to calculate
the correction factor (0.22-0.41) for the attraction of porpoises to the vessel(Turnock, 1987). However, the species could be either attracted to the vessel,
neutral, or avoid the vessel depending on the area, season, growth stage, sex,
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146

MIYASHITA AND KASUYATABLE 9. GEOGRAPHICAL COMPARISON OF THE
DALL'S PORPOISE DENSITY BASED ON DATA IN FIG. 8
School density

Area

Northern
offshore*
Northerninshore**Southerninshore***Porpoise densityno.
schoolsightedtransectlengthmn.m.

density/100 n.m.

no.
individuals

transectlengthin n.m.

density/100 n.m.

30

815

3.7

116

755

15.4

84

1,111

7.6

364

976

36.9

135

1,777

7.6

389

1,435

27.1

*: 40°-45°N, 14 7°-l 55°E. **: Pacific area north of 40°N and west of I 47°E. ***: Pacific area between 38°
and 40°N, and west of 155°E.

and reproductive status (Kasuya and Jones, 1984). Therefore we felt it risky
to use the above correction factor without further observation of the porpoise
behavior. If we missed some individuals on the trackline, which we believe
very possible especially for individuals that avoid vessels, it could have caused
an underestimate. Additionally, present estimate of the Dall's porpoise
population will also include possible underbias due to rounding of the sighting
angle and radial distance (Buckland and Anganuzzi, 1987), which have notbeen considered in the present study.
Using sighting data obtained from the salmon research vessels cruisingfrom Hokkaido to northern North Pacific in April to August (mainly June
and July), Kato (1986) estimated the Dall's porpoise population of both color
types in his truei-type area (40°-44°N, east of 155°E, which is slightly smaller
than corresponding area of the present study), as about 64,000 individuals.
Although Kato ( 1987) added 1986 data to the previous data set, expanded thesouthern range to 38°N (where some sighting efforts existed as well as the
distribution of Dall's porpoises), and obtained the estimate of 125,000, thiswill be an overestimate due to the extrapolation of northern high density tosouthern low desnity area (Table 9). Our estimate of the Dall's porpoise in thesimilar area, 72,410 individuals (57,831 truei-types plus 14,579 dalli-types) is
close to Kato (1986).
.
Our sighting survey confirmed results of previous studies on the general
pattern of distribution of two color types of Dall's porpoises in the Japanese
coastal waters, i.e. Sea of Japan and southern Okhotsk Sea inhabited bydalli-types, and the Pacific coast of northern Ja pan and southern Kuril Islands
by both dalli- and truei-types.
Although the previous studies (Kasuya, 1978, 1982) have assumed
without firm data that the majority of Dall's porpoises that summer off the
Pacific coast of northern Japan would be truei-types, the present study showedthat the assumption was not always correct. Dall's porpoises which winter off
Sci. Rep. Whales Res. Inst.,
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DALL'S PORPOISES OFF JAPAN147

the Pacific coast of northern Japan south of 40°N are composed mainly oftruei-types (Kasuya, 1978, and also see below). They migrate in late summer
and autumn to waters between 40° and 45°N and east of l 45°E, and almost
disappear from the Japanese coastal waters for some short period in
September/October. We cosidered that their southbound migration probably
start in October.
The Pacific coastal waters off Sanriku (38° -41°N) and southern Hokkaido(41°-43°N) are occupied mostly by dalli-types while truei-types are summeringin an offshore northern area. This agrees with the fact that the Dall's porpoisecatches of hand harpoon fishery which was landed in October and November
at Otsuchi (39°20'N) and Yamada (39°30'N) Fish Markets on the Sanriku
coast and reportedly caught off the Pacific coast of Hokkaido were mainly(87%) dalli-types (Anon., 1985b; N. Miyazaki, per. commn in 1986), but thosetaken off the Sanriku coast in January to March and landed at the same fish
markets were mostly (96%) truei-types (Anon., 1985b; Kasuya, 1978). Suchhigh dalli-type proportion was also confirmed in early November 1986 by oneof the authors (T.M.) on the Kamaishi Fish Market (39°15'N) on the Sanriku
coast.
Hand harpoon fishery for the Dall's porpoise had been operated only in
winter season and off the Sanriku coast (Kasuya, 1978; 1982), and the annual
catch during the 1960s and 1970s fluctuated between 5,000 and 9,000
individuals with the annual mean of 6,000 individuals (Kasuya, 1982). Kasuya(I 978) considered on the truei-type stock that the population of 125 to 400
thousands will be required to sustain the mean annual take. The corresponding
net annual recruitment rates were 0.048 to 0.015. The maximum net recruitment rate (Rmax) of the offshore stock of dalli-type Dall's porpoise has been
estimated to be 0.09 (Hester, 1986), 0.08 (Ohsumi, I 986), or 0.06 (Jones,
Breiwick, Bouchet and Turnock, 1986). Although these estimates will needfurther improvement with improved biological parameters or refined model,
it will be too optimistic to consider that the truei-type stock can sustain theannual take of over 10% of the population. If the present population of
60,000 individuals is not large enough to sustain the past level of the annual
catch, then it means that the population has declined by the past catch.
Although our data are unable to prove the past decline of the stock, we
believe that such change could have occurred in the population.
Some fishermen from Iwate Prefecture (part of the Sanriku area) started
in recent years the operation during other seasons and in other areas such as
Sea of Japan off Akita Prefecture (4l 0 N), off the Pacific coast of Southern
Hokkaido, and in the southern Okhotsk Sea (Anon., 1985b; Kawamura et al.,
I 983; Kasuya, unpublished), while operating the fishery off the Sanriku coastin winter as before. This expasion of fishery was due to the increased demandof porpoise meat. The total annual catch increased to around between 10,000
and 13,000 individuals, and there has been expressed a great concern on theeffect of the catch on the Dall's porpoise populatin (IWC, 1984).
Sci. Rep. Whales Res. Inst.,
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148

MIY AS HIT A AND KASUY A

The results of the present study indicate that the recent hand harpoon
fishery in northern Japan for the Dall's porpoise hunts truei-type population
off the Pacific coast of Japan during winter and dalli-types in the Sea ofJapan-Okhotsk Sea stock in summer operation. Available statistics of thefishery do not give the catch by color type or by month. With the absence ofcatch statistics by stock or any other alternative way to estimate it, we find it
impossible to assess the effect of the current fishery on the Dall's porpoisestocks. However, it should be stressed that the current catch of Dall's porpoises
by the Japanese harpoon fishery is still over 10% of the total number of Dall's
porpoises known to migrate annually to the fishing grounds, i.e. 104, 199
individuals (46,368 dalli-types plus 57 ,831 truei-types), and that there is agreat risk of further depletion of one or more stocks of the species by thefishery.

ACKNOWLEDGEMENTS
We wish to express our thanks to Dr L. L. Jones, National Morine FisheriesService (USA), for her reviewing the manuscript and correcting English. We
also owe to Dr N. Miyazaki, National Science Museum Qapan), for his criticalreview on the earlier version of the draft, and to two anonymous reviewers
who made valuable comments on the manuscript. Mrs Y. Yamamoto and Mrs
M. Ikeya helped us in data analysis and drawings.

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