THE POST-BREEDING DISTRIBUTIONS OF ICE-BREEDING
HARBOUR SEAL (Phoca largha) AND RIBBON SEAL
(Phoca jasciata) IN THE SOUTHERN SEA
OF OKHOTSKYASUHIKO NAITO
National Institute of Polar Research, TokyoAND
SHIGEYUKI KONNOTokyo University of Agriculture and Technology, Tok)•O

ABSTRACTThe distribution of harbour seal (Pho ea largha) and ribbon seal (P. fasciata)
was studied. The field survey was performed from 1 May to 3 June of 1975
in the four pack ice areas along the eastern coast of Sakhalin. In theseareas harbour seal was dominant in the southern area and overcomed by
ribbon seal in northern areas. The pups of both species appeared in low
frequency. However in the southern area, the pups of ribbon seal appeared
frequently. The female adult harbour seals were dominant compared withthe adult males, and it was suspected that adult females stay longer on theice floes. In this connection ribbon seal showed the same sex ratio between
males and females through all age classes except pups. However, the different
sex ratio of subadult seals was observed between areas such as male dominantarea and female dominant area. While the group size of harbour seal wascomposed of small number of individuals (average 1.8 seals), ribbon seal did
not form the any group but appeared in solitary.
INTRODUCTION
The harbour seals (Family Phocidae, Genus and Subgenus Phoca 1758) were studied
from the point of view of taxonomy and systematics in recent years, and it was
supposedly concluded that there exist two harbour seal species in the North Pacific.
Those are, namely, ice breeding harbour seal (Phoca largha Pallass 1811) and land
breeding harbour seal (Phoca vitulina richardsi Gray 1864 ). Among these seals,
ice living harbour seal is still little known species. Especially the knowledges
concerning to their life in the ice region are quite scarce. As a consequence, knowledges on their distribution and migration are some times speculative. Similar toice breeding harbour seal, ribbon seal (Phoca fasciata Zimmermann 1783; Family
Phocidae, Genus Phoca, Subgenus Histriophoca) is also one of the most unknown
species.
The unconfirmed knowledges concerning to their distribution and migration
Sci. Rep. Whales Res. Inst.,
No. 31, 1979, 105-119

106

NAITO AND KONNO

145°E

140°E

',

\

\
\

I

,J

OKHOTSK SEA

50°N

50°N

45°N

45°N

145°E
Fig. 1. Sampling was performed in the above 4 areas in the southern Sea of
Okhotsk. The figures in each area show the date of sampling. Dotted line
shows 200 m isodepth line.

are due to the difficulties in the ice region survey. Especially the latter seal havepelagic habitat visiting coast seldom after the ice floes melt (Burns 1970). Therefore it is still more difficult for us to access to them.
Our objective here are generally to provide some informations on their distribution and migration in the ice floe region and especially to add the informations
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

HARBOUR SEAL AND RIBBON SEAL

107

on their social structures.
Our field studies were dependent on the commercial sealing which started
from the first of May continuing for about a month when the breeding season
was over and the ice floes were melting and retreating. Therefore, we missed theimportant breeding season for our purpose. The breeding season of harbour
seal was suggested from mid to end of March (Naito and Nishiwaki 1972), andthe ribbon seal was from mid March to first of April (Tikhomirov 1971). Therefore we studied the distributions of post breeding season (1-1.5 month later).
MATERIALS AND METHODS
From 1 May to 4 June in 1975, field studies were made in the eastern coast of
Sakhalin to analize the harbour seal and ribbon seal distribution. The sealing
were first operated from south to north following the retreating pack ice. The
pack ice was almost final melting stage in this season, and the pack ice showed
relatively low density and they were dispersing. However the sealing boat insome occasion did not penetrate into the pack ice to stop sealing. In this study
the areas where the sealing operated were separated into four areas in accordancewith the pack ice distribution. These areas were: area I, the south eastern partof Aniva Bay or the first operation area; area II, the mouth of Terpeniya Bay or
the second operation area; area III, the eastern mouth of Terpeniya Bay or thelast operation area; and area IV, the eastern coast of northern Sakhalin or thethird operation area (Fig. 1). Among these areas, except area III the sealing was
performed from south to north as season advances. The ice conditions, depth ofwater, date of sealing of these areas were shown in Table 1.
TABLE 1. THE ICE CONDITION, DEPTH OF WATER AND DATE OF OPERATIONOF EACH SEALING AREA. THE ICE CONDITION WAS INDICATED BY
THE DEGREES OF ICE COVERED AREA IN SIGHT AND THE ICE
COVERED AREA WAS CLASSIFIED INTO TEN DEGREES.
Area

Average amountof iceRange

Depth ofwater (m)

Date

II
III
IV

3.75/10
3.40/10
3 .10/10
1/10

3/10-4/10
1/10-7/10
1/10-4/10
1/10

100-200
60-100150-200
100-150

1-5 May
6-19 May
26 May-! June
3-4 June

Through this field work, the right lower jaws were collected from 390 harbour
seals and 761 ribbon seals, and preserved in 5-10% formalin. These collected
numbers reached at about 40% of total number of seals caught, and these canine
teeth sampling was made quite at random for the purpose of this study. In thisfield study we also made field observations from the upper bridge of the motherboat to collect the data for group size analysis.
Concerning the age determination of harbour seal, the same methods asreported by Naito and Nishiwaki (1972) was followed. However, the method of
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

108

NAITO AND KONNO

ribbon seal age determination was newly studied by using the tooth of known age
of dead seal which was kept at Kamogawa Sea World in Chiba Prefecture, Japan.
The age determinations of the ribbon seals were successfully performed by reading
both dentine layers and cementum layers (Naito and Tobayama; unpublished).
Concerning the catch selection in this study, there exist no strong selectivity
between each species, for sealing were made only on the ice floes by using the riffiewith the scope. However, in general harbour seal was more cautious than the other.
The species dealt in this study were only harbour seal and ribbon seal. There
distribute ringed seal (Phoca hispida) and bearded seal (Erignathus barbatus) in theseareas. However these seals, if identified by hunters, were not caught owing to thelow quality of their fur. Therefore, these species were not studied in this study.
SPECIES COMPOSITIONIn this study it was difficult to obtain data to evaluate catch effort such as
hunter's skill, catch difficulty by ice or weather conditions and etc., therefore inthis chapter only comparative species ratio was treated.
TABLE 2.

Area
II
III

IV
Total

SPECIES COMPOSITION AMONG HARBOUR SEAL AND
RIBBON SEAL IN EACH AREA

No. harbour
seal

122
216
39
13
390

%
80.8
33. l
18.2
9.8

No. ribbon
seal

29
437
175
120
761

%
19.2
66.9
81.8
90.2

Total
number

151
653
214
133
1, 151

Species composition of harbour seal and ribbon seal collected in each area
are shown in Table 2. The numbers collected in all 4 areas were 390 harbour
seals and 761 ribbon seals respectively. This result may indicate that ribbon
seal distributed 1.95 times as much as harbour seal in these areas and season. To
the contrary, however, each areas showed the different specific structure. As
shown in Table 2, area I was considered as a harbour seal dominant area, and area
II was possibly considered as both species mixed area. Ribbon seal highly dominant in areas III and IV, showing 81.8% and 90.2% respectively. Therefore it
was considered that ribbon seal appeared frequently in northern areas and later
season. Although the explanation for this phenomena is still uncertain, it may
related with difference of ice utilization by both species. Tikhomirov (1961) andNaito and Nishiwaki (1975) stated that in May and June harbour seal begin toappear in the coastal area coinciding with ice melting season, and Tikhomirov(1961) also reported that ribbon seal seldom descend into the water in May andJune when they are moulting. In this connection, it was quite natural that ribbon
seal was generally abundant in these areas, and being hunted in later season inareas III and IV where harbour seal was less dominant.
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

109

HARBOUR SEAL AND RIBBON SEALAGE AND SEX COMPOSITIONSHarbour seal
Age composition of harbour seal is shown in Table 3 and Fig. 2. Of all age
classes the pups and one year old seals of both sexes appeared most frequently, andthe appearance frequency of older seals gradually decreased. The fem ales seemedto show less gradual decrease, and the oldest seal was 30 years old in male and 33
years old in female.
In general the standing stock of pups should be most abundant. However
AGE COMPOSITION OF 390 HARBOUR SEALSCOLLECTED FROM ALL 4 AREAS

TABLE 3.

Age

No. malesin sample% inmale sample

No. females
in sample% infemale sample

40
46
19
22
17
13
8
10
5
2
0
4
5
2
6
2

18.0
20.7
8.5
9.9
7.6
5.8
3.6
4.5
2.2
0.9
1.3
1.8
2.2
0.9
2.7
0.9
0.5
0.9
0
0.9
0.9
0
0.5
0.9
0.5
0.5
0.5
0
0
0.9
0.5
0.5
0
0.5

222

0

47
47

2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33

11

0
0
0

28.0
28.0
6.5
8.9
7.7
3.0
4.1
0.6
1.8
1.8
0
1.8
0.6
1.8
0
1.2
0.6
0.6
0.6
0.6
0
0
0.6
0.6
0
0
0
0
0
0
0.6
0
0
0

Total

168

rno.o

Sci. Rep. Whales Res. Inst.,
No. 31, 1979

15
13
5
7
3
3
0
3
1
3
0
2

I

0
0

0
0
0
0
0
0
I

2
0
2
2
0
2

0
0
2

0
1

100

NAITO AND KONNO

110

in this study pups appeared in same abundance as one year old seals. According
to Natio and Nishiwaki (1975), pups are dependent on ice floe even after weaning,
and they are much dispersed by ice movements. This suggestion may explain
the above result. Concerning the sex ratio, it is very clear that both sex of pups
and 1 year old seal appeared almost same frequency and, in the most of other age
classes females were dominant. Not shown in Table 3, but the sex ratio is 52.2%
females for both pups and one year old seals and 64.8% females for other older seals.
This may suggest that the older female seals remain longer on ice floes than maleseals. To analyse more precisely, the age was classified into several classes, suchas (1) pup class, (2) immature class (1-2 years old), (3) subadult class (3-6 years

•O

30

20

10

Fig. 2. Age composition of harbour seal. Black column represents the male seal
and white column represents the female seal.

old), (4) adult class (7-10 years old) and (5) old class (11 or more years old). As
seen in Table 5, the sex ratios with 46% and 52.8% female for pup and immatureclass clearly differ from the ratio with 60%, 74.1 % and 69.8% females for subadult, adult and old classes. From these facts, it was possibly concluded that theadult female harbour seal remain longer on ice floes.

Ribbon seal
Age composition of ribbon seal is shown in Table 4 and Fig. 3. A number ofpup seals appeared in quite low level as in the case of harbour seal pup. However,after one year old, the both sexes of seal decreased gradually in almost same rate.
The oldest seal was 26 years old in male and 22 years old in female. Concerning the longevity of this seal, almost same result was obtained as reported by
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

HARBOUR SEAL AND RIBBON SEALTABLE 4.

111

AGE COMPOSITION OF 761 RIBBON SEALS COLLECTED FROM ALL 4 AREAS

Age

No. malesin sample';;~ inmale sample

No. females
in sample% infemale sample

0

18
90
71
52
49
29
14
17
6
9
7
6
2
4
3

30
96
61
47
35
31
18
16
lO
6
5
3
2
4

3
0
0
0
0
0
0
0
2
0
0
0
0
0
0
0

4.7
23.4
18.5
13.5
12.8
7.6
:3.6
4.4
l.6
2.3
1.8
1.6
0.5
1.0
0.8
0.3
0
0.3
0.8
0
0
0
0
0
0
0
0.5
0
0
0
0
0
0
0

8.0
25.5
16.2
12.5
9.3
8.2
4.8
4.2
2. 7
l.6
1.3
0.8
0.5
1.1
0.3
0.5
0.5
0.3
0.8
0
0.5
0.3
0.3
0
0
0
0
0
0
0
0
0
0
0

384

100

377

2
3
4
5
6
7
8
9
10
II
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Total

0

2
2
3
0
()

0
0
0
0
0
0
0
0
0
0
0

100

Tikhomirov (1971). He reported that the maximum age reached by ribbon seal
in natural conditions is 22-26 years.
The low appearance frequency of harbour seal pups was considered in relation to it's ice dependent life and ice movements. In ribbon seal, though uncertain, the same reason may be considered as well. Contrary to harbour seal, sex
ratio of pups seemed to be quite abnormal. However this low representation ofmale pup in Fig. 3 seemed to be caused by small sample size. As well as harbour
seal, the difference of sex ratio was compared between several age classes such as(1) pup class, (2) immature class (1-2 years old), (3) subadult class (3-6 years old),(4) adult class (7-10 years old), (5) old class (11 or more years old). These age
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

NAITO AND KONNO

112

30

20

,0

30

AGE

Fig. 3. Age composition of ribbon seal. Black column represents the male seal
and white column represents the female seal.
TABLE 5. AGE GROUP COMPOSITION OF 390 HARBOUR SEALS (UPPER) AND
761 RIBBON SEALS (LOWER) COLLECTED FROM ALL 4 AREAS
Harbour Seal
Age class

No. malesin sample% inmale sample

No. females
in sample% infemale sample

Total No.

% intotal sample

0
1-2
3-6
7-10
11-

47
58
40
7
16

54.0
47.2
40.0
25.9
30.2

40
65
60
20
37

46.0
52.8
60.0
74.1
69.8

87
123
100
27
53

22.3
31.5
25.7
6.9
13.6

Total

168

43.1

222

56.9

390

100.0

Age class

No. malesin sample~lo inmale sample

No. females
in sample% infemale sample

Total No.

% intotal sample

0
1-2
3-6
7-10
11-

18
161
144
39
22

37.5
50.6
52.4
51.3
50.0

30
157
131
37
22

62.5
49.4
47.6
48.7
50.0

48
318
275
76
44

6.3
41.8
36. l
10.0
5.8

Total

384

50.5

377

49.5

761

100.0

Ribbon Seal

classes were settled by considering the sexual maturity data by Tikhomirov (1968).
The sex ratios of each age class are shown in Table 5. The sex ratio of 62.5%of female pup is little higher than expectation. However, the ratios of 49.4%,
Sci. Rep. Whales Res. Inst.,

No. 31, 1979

HARBOUR SEAL AND RIBBON SEAL

113

47.6%, 48.6%, 50% females for immature class, subadult class, adult class and oldclass indicate that both sexes of this seal appeared in same ratios, and this result
was quite different from the result obtained from harbour seal. The ratios for maleand female of all classes in total are 50.5% and 49.5% respectional. From thesefacts it may be concluded that sexual segregation does not occur, even in adult andsubadult classes. This result, however, was different from that indicated by Tikhomirov (1961 ). He described that in some aggregations occasionally the predominance of males and females had been observed. The above mentioned different
results may due to the surveyed area size. In this chapter, sex ratios of all seals
from all four areas were discussed. However in this study, we compared the age
and sex composition between each areas of I-IV to obtain the more detailed knowledge on distribution of these species.
Harbour seal
As seen in Table 6, of all 390 harbour seals 122 seals were collected in area
I and 216, 39 and 13 were collected in areas II, III, IV respectively. Thesesample numbers generally reflect the abundance in these areas and indicate thatharbour seal were much abundant in area I and II. Concerning the age compositions, pups appeared to distribute in all areas, with age class ratio between
15.4%-30.8% of totals, and it was presumed that pups showed the scattered distribution in all areas, even in areas III and IV where this seal was less abundant.
Immature class (1-2 year class) seals appeared in high frequency in areas I andII with ratio 46. 7 % and 28. 7 % and low frequency in areas II and IV with ratio
both 7.7%. This may indicate that the immature seals concentrate to a certain
degree in areas I and II. The subadult and adult class seals seemed to appear
randomly, and old class appeared in areas I and II less frequently with ratio 4.1 %and 11.6%, and more frequently in areas III and IVwith ratio 41.0% and 53.8%.
It may be true that the distribution of pup widely spreads out and scatters.
As already mentioned pups stay on ice floes for a while even after weaning andhave ice dependent life (Naito and Nishiwaki 1972) and as a consequence, pup
occurrs in far south water along the coast or Japan (Naito 1976). These facts
may support above result. In this study some tendency of concentrations byimmature seals occurred in southern area and it was not widely known that immature harbour seal tend to concentrate. However the fact that concentrationof young harbour seals was annually observed near St. Mathew Island in the
Bering Sea (Tikhomirov and Kosygin 1966) may support the result obtained in thisstudy. Furthermore, the imformations from hunters that the immature harbour
seals are distributed in concentration should be taken into the considerations.
However, we are still short in data to obtain the clear result.
Concerning the sex ratio, there obtained the clear result in this study thatolder female seals appeared in high frequency in southern area of I and II andboth sex of immature seals appeared almost same frequency (Fig. 4 and Table
6). However in northern areas of III and IV, such phenomena were not observed
due to shortage of data.
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

~

"'

~~

<:.

<c ~~

"tc; ~......... ~;_,, ::0

Total

11-

7-10

~

12
(41.4)

17(58 .6)

29

0

1

3

58.6

17

1-2

27.6

8

100.0

0

3.5

10.3

% in
total
sample2
6(25 .0) (75 .0)
89(52.9) (47 .1)
2
l(33. 3) (67. 7)l0(100. 0)(0)0
0

- - - ·- ----- ----

0

Area I

Total
No.

3-6

~

100.0

No. of No. ofmales females
(%)
(%)

::0

:-'·

122

4.1

3.3

4

5

19.7

46.7

26.2

24

57

32

89
(41. 2)24(53.3)
28(45.2)
31(46.3)
4(23.5)
2(8.0)
127(58.8)

21(46. 7)34(54.8)
36(53. 7)
1::1(76.5)
23(92 .0)No. of No. ofmales females
(%)
(%)

216

25

17

67

62

45

Total
N
o.

Area II

100.0

11.6

7 .9

31.0

28. 7

20.8

% in
total
sample24(61. 5)

5(83.3)
3(100.0)3(37 .5)3(50.0)
10(62 .5)15(38.5)

1(16. 7)0(0)5(62.5)
3(50. 0)6(37. 5)

No. of No. ofmales females
(%)
(%)

39

16

6

8

3

6

Total
No.

Area III

100.0

41.0

15.4

20.5

7.7

15.4

% in
total
sample6(46. 2)

7(53.8)

2
2(50 .0) (50.0)01(0)(100.0)01(0)(100.0)
0
0
(0)
(0)
4
3
(57 .1) (42. 9)

No. of No. ofmales females
(%)
(%)

13

7

0

1

1

4

Total
N
o.

Area IV17(58.6)
99(49. 7)60(37 .5)16(45. 7)7(50.0)
199(45. 5)12(41.4)
100(50.3)
100(62. 5)19(54.3)
7(50.0)
238(54 . .5)

No. of No. ofmales females
(%)
(%)

90
(51.4)

100.0
437

14

35

3.2

8.0

36.6

45.5

199
160

4
(66. 7)26(54.2)
34(48. 5)15(50.0)
11(52 .4)85(48.6)

2(33. 3)22(45.8)
36(51.5)
15(50.0)
10(47 .6)

17.5

21

30

100.0

12.0

17.2

40.0

27.4

48
70

3.4

% in
total
sample---------- ---

6

Total
No.

Area IIINo. of No. ofmales females
(%)
(%)

6. 7

% in
total
sample-------~----

29

Total
No.

-~-------··--------------·

Area II

100.0

53.8

0

7.7

7.7

30.8

% in
total
sample44(:{6. 7)76
(63.3)

05(0)(100.0)26
28(48. l) (51.9)933(21.4) (78.6)
55(50.0) (50.0)
4
5(44.4) (55 .6)

No. of No, of
males females
(%)
(%)

120

9

10

42

54

5

Total
No.

100.0

7.5

8.3

35.0

45.0

4.2

% in
total
sample------- ----- ---------· · - - - - - -Area IVAGE CLASS COMPOSITION AND SEX RATIOS OF EACH AGE CLASS RIBBON SEAL IN EACH AREA

73
(59.8)TABLE 7.

49
(40.2)

16
16(50.0) (50.0)
2730(47.4) (52.6)
18
6(25.J) (75 .0)
40(0)(100.0)50(100.0)
(0)

% in
total
sample

Age
class

Total

11-

7-10

3-6

1-2

0

TJtal
o.

Area I

AGE CLASS COMPOSITION AND SEX RATIOS OF EACH AGE CLASS HARBOUR SEAL IN EACH AREA

Age No. of No. ofclass males females(%)(%)

TABLE 6.

z

z0z

0

~

zt:I

;i..

0

......
t-"'I

;i..

-....

HARBOUR SEAL AND RIBBON SEAL

age
group

Area ll

Area I
males

115

N.(49)

femalesN (73)

males

femalesN(89)

N(127)

11-

23

7-10

3-6

31

1-2

27

0

16

24

1CQ 80 60 40 20 0

20 40 60 80 100('/,)

Area lll

age
group

males

100 80 60 40 20 0 20 40 60

eo 100(%)

Area IVN(24)females

N(15)

males

femcilesN(G)N( 7 )

117-103-6

100 80 60 40 20 0

20 40 60 80 100(%)

100 80 60 40 20 0

20 40 60 80 100('!.)

Fig. 4. Sex ratios for age groups of harbour seal in each area, 0: pup class, 1-2:
immature class, 3-6: subadult class, 7-10: adult class and 11-: old classArea nArea I

age
group

males

N(12)

females

N(17)

mates

N(238)

femalesN(199)

117-10

3-6

I)()

1-2
0
100 80 60 40 20 0204060 eo 100

100 80 60 40 20 0 20 40 60 In 1<f.1,)

Area IllArea IV

('!.)

age
group

males

N(90)

N(8S)

females

11-

lO

7 -10

15

3-6

mates

N(44)

females

N(76)

36

-2

22

0
100 80 60 40 20 0

20 40 60 80 100('!.)

Fig. 5. Sex ratios for age groups of ribbon seal in each area, 0: pup class, 1-2:
immature class, 3-6: subadult class, 7-10: adult class and 11-: old class

Sci. Rep. Whales Res. Inst.,
No. 31, 1979

116

NAITO AND KONNO

According to Tikhomirov and Kosygin (1966), south ofNunivak Island in the
Bering Sea they found every spring aggregations of female which had recently
given birth to their pups. In our study, pups had been weaned almost 1.5-2month before and such female agregations were not observed. Although our result
may indicate that adult females keep some aggregation to a certain degree evenafter weaning, it may be more reasonable to understand that the female adultseals remain longer in ice floe region while adult male seals shift their habitate fromice region to the coastal region.

Ribbon seal
In the case of ribbon seal, the relative larger number of samples (761 seals)
were obtained. Out of 761 seals, 29 seals were collected from area I and 437,
175, 120 seals were from areas II, III, IV respectively. And in area II ribbon
seal was most abundant and least abundant in area I (Table 7). Concerning theage composition in each area, as seen in Table 7 appearance frequencies of pupclass and immature class were relatively high in area I compared with other threeareas. Especially pup class seals appeared high frequency in area I. On the contrary subadult class, adult class and old class seals appeared less frequently in arae
I and more frequently in other 3 areas, especially in area III.
From above, it was supposed that the southern area of area I was less abundant area and young seals were predominant like as harbour seal. The area II
was the most abundant area and immature and subadult seals were dominant.
The area III was relatively abundant and older seals were dominant. The area
IV was widely operated area (Fig. 1) and the seals was not abundant, while similar
age composition as in the area II of the most abundant area was observed.
At this stage of study, we could not obtain any reasonable explanation forabove phenomena.
Concerning the sex ratio, as seen in Fig. 5, ratios for all age classes in area
I were fluctuated very much due to the small sample size. From same reason,sex ratio for pup class also fluctuated among each area.
The male and female seals of immature, adult and old adult classes appearedin similar ratio among areas except area I showing the ratios of about 50% forboth sexes. However, only subadult seals showed the different ratio among areas
II, III and IV. In area II, sex ratio with 37.5% for females were observed, on
the other hand, in area IV females were dominant, and ratio of 78.6% for females
was observed. Being still uncertain, it was possibly considered that ribbon sealshows sexual segregation of some certain degree. It is necessary to obtain the datafrom much smaller areas in order to analyze the above problems. In this study,
the several hunting boats operated in wide spreaded areas. These hunting boats
made operations about twice a day, from 4 AM to noon and from 13 PM to 18 PM.
After operation, the seals were boarded on the mother boat where we collected
specimens. Therefore we could not make any more detailed analysis for ribbon
seal distribution.
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GROUP SIZEIn this study we also performed the observations on group size of both species.
The group size was estimated by the number of seals laying themselves on the sameice floe. The size of ice floes showed much variations. The largest size was about150-200 m in diameter.
Harbour seal
The group size of the two species was studied in order to know the social
structure of each species. The observations were performed from the top of themother boat bridge while the hunting boats were out from it. As shown in Table
8, 58.9% of the harbour seal groups was composed of a single seal, and 22.3%
was composed of two seals. The larger the group size, the frequency gradually
decreased so far, and the maximum group size was 8 individuals. The averagegroup was composed of 1.8 seals.
TABLE 8. GROUP SIZE AND FREQUENCY OF EACH GROUP SIZE OF HARBOUR SEAL
AND RIBBON SEAL. TOTAL NUMBER OF INDIVIDUALS OF EACH GROUP SIZEWAS ALSO SHOWN IN THIS TABLE. GROUP SIZE WAS ESTIMATED BY THE
NUMBER OF INDIVIDUALS THAT HAULED OUT ON THE SAME ICE FLOE
No. of individuals

No. of sighting
Group size---------·

Harbour seal

Ribbon seal

Harbour seal

Ribbon seal

(%)

(%)

(%)

(%)

2
3
4
5
6
7
8

66(58.9)25 (22. 3)
10 ( 8.9)
5 ( 4. 5)
2 ( 1.8)2 ( 1.8)
1 ( 0.9)l ( 0.9)90 (97 .8)
2 ( 2.2)
0
0
0
0
0
0

66 (32 .5)
50 (24 .6)30(14.8)
20 ( 9 .9)10 ( 4 .9)
12 ( 5 .9)
7 ( 3.4)
8 ( 3.9)90(95. 7)
4 ( 4. 3)
0
0
0
0
0
0

Total

112(100)

92 (100)

203 (100)

94 (100)

The group size observed in this study seemed to be rather small than what
it was under the natural condition, because we obtained the data from the area
where some hunting boats operated around and more or less the seals were disturbed. As the matter of fact, the largest group size was smaller than the groupsize of 12 young seals obtained in the same season and same area in 1971 (Naito
and Nishiwaki 1975). From above result, unfortunately, we could not analyze
the group size from the point of view of sex, age or areas in this study.
Ribbon seal
The ribbon seal showed the distinguishable aspect from the point of view ofgroup size. The group of this seal was composed of a single seal in 97.8% frequency, and only twice a couple of seals laying themselves on same ice floe wasobserved (Table 8). Even in these two cases, it was not supposed that they were
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NAITO AND KONNO

either associated each other or located close together, but observed laying themselves apart on the ice floe.
From these facts, we can conclude that ribbon seal does not form the groupbut keep a distance each other on ice floes in this season. Thikhomirov (1961)
also reported that ribbon seal appear solitary on ice floe.
DISCUSSION

It is generally said that ribbon seal and harbour seal occupy almost similar ice
region. However Burns (1970) reported that in the ice edge zone of the central
Bering Sea, ribbon seal had been most numerous along the inner (northern) edgeof the zone and gradually outcomed by harbour seal towards the southern edge.
In the same sense, we also attained the similar result. However, in our study theseason was different from the above suggestion as already mentioned high frequency of ribbon seal in northern area may be only due to the longer staying
on ice floes compared with harbour seal as suggested by Tikhomirov (1961). We
need the much more data for long period in order to obtain the reasonable conclusions.
Concerning the age and sex ratios, we obtained the result that female adultharbour seals occurred in high frequency and to a certain degree age and sexual
segregation were observed. On the other hand ribbon seals showed no age andsexual segregation as a whole. However, in some areas, sexual segregation wasobserved in subadult class. Burns (1978) described that he had seldom seen maleand female ribbon seal together. We also observed same as he described. Our
above result was based on the data from relatively larger area and the degrees ofsexual segregation seemed to be lower estimation than what it was, because wesometimes observed the adult males dominant zone and adult female dominantzone separately in smaller areas. This sexual segregation seems to occur even in
breeding season (Tikhomirov, 1961). In Nemuro Strait of eastern Hokkaido, sex
ratio offemale adult seal exceeded 70% (Itoh unpublished data) in breeding season.
Burns (1978) also reported that in breeding season both sex of ribbon seal seldom
appear together. The reason why these segregation may occur on ribbon seal isstill uncertain. The peculiar pelage pattern of ribbon seal may play some role
between both sex as some communication like as reported in harp seal (Phoca
groenlandicus) (Evans and Bastian 1969). However, we found that not only matured
ribbon seal, but also immatured seal appeared in solitary. Probably this seal isecologically adapted and specialized as pelagic seal. In this study we were quite
uncertain on this problems.
ACKNOWLEDGMENT

We are very much grateful to Mr Kazuyuki Shibuta, the president of Shibuta sealing
company who kindly permitted us to go on board for collecting samples. We werealso much indebted to Captain T. Ono and his crew who kindly helped us in
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119

the field work.
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EVANS, W. E. and J. BASTIAN, 1969. Marine mammal communication: Social and ecological factors.
pp. 425-476. Jn; H. T. Anderson (ed.) The biology ef marine mammals. Academic Press, New York.
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Sci. Rep. Whales Res. Inst.,
No. 31, 1979