distribution and abundance of spotted seals phoca largha and ribbon seals phoca fasciata in the...

INTRODUCTION

The distribution and abundance of wildlife forman important basis in their ecology; they are closelyrelated to factors such as population dynamics,movement, and habitat selection. Such informa-tion is also necessary for the conservation and man-agement of wildlife. These data should be obtainedperiodically and applied to management planningbefore the wildlife population is unexpectedlyinfluenced by human activities such as hunting,damage control kills, incidental takes/catches,habitat degradation, and epidemic diseases.

Many pinnipeds have been observed in winterand spring off the coast of Hokkaido, Japan, alongthe southern edge of the pack ice in the Sea ofOkhotsk. Spotted (Phoca largha) and ribbon seals (Phoca fasciata) are most common (Uno &Yamanaka 1988) and some incidental catches anddamage control kills occur (Deguchi 1999; Goto1999; Mizuno et al. 2001). The distribution andabundance of spotted and ribbon seals in the Seaof Okhotsk have been assessed by aerial censusseveral times previously (Fedoseev 1970, 1984;Lagerev 1988; Uno & Yamanaka 1988), but theseestimates have not included confidence intervals,and the censuses did not systematically cover theareas off Hokkaido. No follow-up investigationshave been conducted despite the importance ofperiodic surveys for mammals’ management andconservation planning.

Ecological Research (2002) 17, 79–96

Distribution and abundance of spotted seals Phoca larghaand ribbon seals Phoca fasciata in the southern

Sea of OkhotskAyako W. Mizuno,1* Akihiko Wada,2 Tsuyoshi Ishinazaka,1 Kaoru Hattori,1

Yukiko Watanabe3 and Noriyuki Ohtaishi1

1Laboratory of Wildlife Biology, Graduate School of Veterinary Medicine, Hokkaido University,N18-W9, Kita-ku, Sapporo, Hokkaido 060-0818, Japan 2Hokkaido Wakkanai Fisheries

Experiment Station, Suehiro 4-5-15, Wakkanai, Hokkaido 097-0001, Japan and 3Department ofVeterinary Anatomy, Obihiro University, Inada-cho, Obihiro, Hokkaido, 080-8555, Japan

The distribution and abundance of spotted seals (Phoca largha) and ribbon seals (Phoca fasciata) wereassessed in March and April, 2000, by aerial line-transect surveys along the southern edge of thepack ice off the coast of Hokkaido (southern Sea of Okhotsk), Japan. Five hundred and seventeenspotted seals and 107 ribbon seals were found on the total 2944 km survey line. Total abundancewas estimated to be 13 653 spotted (95% CI = 6167–30 252) and 2260 ribbon seals (95% CI =783–6607) in March, and 6545 spotted (95% CI = 3284–815 644) and 3134 ribbon seals (95% CI= 1247–17 802 512) in April. The pack ice area off Hokkaido had higher densities (0.54 seals km-2

and 0.58 seals km-2 in March and April, respectively) of spotted seals than those reported in easternSakhalin, whereas densities (0.09 seals km-2 in March and 0.28 seals km-2 in April) of ribbon sealswere lower than those in eastern Sakhalin. The large number of spotted seal pups suggests that thestudy area is an important breeding center. A greater number of female spotted seals with pupstended to be found in the center of larger and rougher floes than in other categories, and they weremore abundant in stable pack ice areas. Observations of ribbon seals were limited because the surveyperiod preceded the peak of pupping season. Ribbon seal surveys were also hampered by the inabil-ity to fly over the main breeding area between the Shiretoko Peninsula and Kunashiri Island.

Key words: aerial survey; distance; habitat selection; Hokkaido; line-transect method.

*Author to whom correspondence should beaddressed. Email: [email protected]

Received 25 October 2000.Accepted 20 September 2001.

Further studies are also needed to clarify whichhabitats are important for seal reproduction.Habitat characteristics described for these specieshave been fragmentary and inconsistent, such asocean depth and type of ice preferred for haul-outareas. In terms of ocean depth, some studies haveconcluded that spotted seals are abundant in oceandepths of 200 m or less, whereas ribbon seals preferdeeper water, near the continental shelf (Naito & Nishiwaki 1975; Braham et al. 1984; Uno &Yamanaka 1988). Fedoseev (1984), however, hasreported that both seals breed on the ice over‘greater’ depths. These descriptions seem to beinconsistent, which might have been caused by thelittle consideration given to the survey efforts usedand/or intraspecies variation for habitat preference.If we evaluate precisely the inter- and intraspeciesvariation for habitat preference, a better under-standing of the factors influencing reproductionmight be obtained.

80 A.W. Mizuno et al.

The present study aims to estimate the dis-tribution and abundance of spotted and ribbonseals, and to elucidate the habitat preference ofeach species, based on the data from aerial surveysoff the Okhotsk coast of Hokkaido.

MATERIALS AND METHODS

Study area

Aerial surveys were conducted over the waters offthe northeastern coast of Hokkaido, Japan; that is,in the southern Sea of Okhotsk and along the coastof Shiretoko Peninsula and Nemuro Peninsula(Fig. 1), and were limited to the area within the Japan Outer Air Defense Identification Zone(ADIZ). Because the study area was not expectedto be homogeneous, we divided the survey areainto two strata and one coastal strip based on

Fig. 1. Survey areas for spotted and ribbon seals in the southern Sea of Okhotsk, spring 2000. Parallel and dottedlines show transect lines and strata boundary at 144∞E, respectively. Line transects were selected based on a sys-tematic sampling scheme (Cochran 1977; Burnham et al., 1980).

ocean depths, seasonal movements of sea ice, andice concentrations (Marine Department JapanMeteorological Agency (JMA) 1988).

Two survey areas were bounded to the west byice-free waters, to the north by a line designated by the Japanese Self-Defense Forces, to the east byCape Shiretoko, and to the south by ice-free watersor the ice-packed coastline, depending on the iceconditions (Fig. 1). This area was then divided by the 144∞E longitudinal line; the western half(stratum A) was relatively shallow (<200 m depth),and the eastern half (stratum B) included a conti-nental shelf break with a steep slope down to adepth of 3000 m. The waters off the Hokkaido coastare usually covered completely by ice during thewinter (January–February) (JMA 1988). The icecover begins to melt and break apart at the begin-ning of March, forming areas of open water alongthe northern coast of Hokkaido (stratum A),although the ice cover remains along the coast ofstratum B until the beginning of April. Because thesea ice was stable, during the present study we wereable to thoroughly survey both strata in March, butin April the ice had broken up rapidly, open waterhad expanded southward along the coast, and theeastern half of stratum A was almost completelyopen water. Thus, we were unable to survey all areasof stratum A.

As a follow up to many reported sightings, aswell as fisheries control kills (Nevedomskaya et al.1998; Deguchi 1999), we also conducted aerialsurveys along the northeastern coastline betweenthe Shiretoko Peninsula and the Nemuro Peninsula (course C) (Fig. 1). However, because ofdiplomatic problems between Japan and Russia,we were able to survey only within approximately5 km offshore from the coastline. In March, courseC had fast ice with a very close concentration ormany small fishing boats in open waters along thecoasts. In April, no ice was seen along course C.We observed no seals in March and only threespotted seals swimming offshore in April. There-fore, the surveys along course C were not as sys-tematic as those of strata A and B, although Table1 includes the survey summary in this course.

Survey season and timing

Spotted and ribbon seals are counted most accu-rately during the pupping and breeding seasons,

whereas the aerial identification of pinnipeds inopen water is unreliable (Braham et al. 1984).Therefore, the survey was limited to sightings onthe sea ice in early spring. The first set of surveyswas conducted at the end of March 2000 and thesecond set at the beginning of April 2000.

In other species which use sea ice as breedingareas, seals generally prefer to haul-out duringclear and calm weather, and the maximum numberof hauling-out animals is normally reached byearly to mid-afternoon (Lunn et al. 1997). Feedingactivities of spotted and ribbon seals are highestfrom evening to morning in winter along theNemuro Strait (Deguchi 1999; Goto 1999). As aresult of this behavioral information and to ensuresafe flights, we conducted aerial surveys only in theearly to mid-afternoon on days (Table 1) withfavorable weather conditions (visibility >10 km,cloud ceiling >450 m, no precipitation, and <7 ms-1 lateral wind and <15 m s-1 headwind duringtake-off).

Arrangement of transect lines in strata A and B

The study was designed as a medium altitude, systematic, line-transect, aerial sample survey.Transects were flown along longitudinal lines tofacilitate aircraft navigation and to reduce the vari-ance between the number of sightings along atransect by moving parallel to the density gradi-ent of animals (Hammond 1986; Miyashita 1990).Spotted and ribbon seals use the southern edge ofthe seasonal pack ice (Burns 1970; Braham et al.1984), and seal density is expected to follow anorth–south gradient with the ice as it fills off-shore waters of Hokkaido.

Reliable population estimates require at leastfive survey lines (Burnham et al. 1980) and a sys-tematic sampling scheme (Cochran 1977). We sur-veyed a minimum of six transects in each stratum,except for stratum A in April, when the sea ice hadalready decayed (Table 1). Transects were spaced at intervals of 7.5∞ longitude, approximately10 km apart (Fig. 1). Strata were then subdividedinto blocks of three and two transects each instratum A (labeled d, e, and f) and statum B(labeled d and e), respectively (Fig. 1). Before eachsurvey, a transect label was chosen randomly foreach strata block. When fuel and flight budgets

Distribution and abundance of seals 81

allowed, in order to increase the accuracy of ourestimate of the effective perpendicular distance, wesurveyed more than one transect in a given block.While traveling to and from transect startingpoints, we also surveyed across transects (Lines xand y) (Fig. 2).


Data collection

We used a high-wing, 6-seat Cessna 206 with asingle engine, and flew at the lowest practicablespeed (90 knots) and altitude (target, 130 m;recorded mean, 129.3 m, SD ± 6.3 m) in order to

Table 1 A summary of aerial sighting surveys for seals in the southern Sea of Okhotsk, spring 2000

Spotted Ribbon UnknownTime seal seal seal

Date Line I.D. Start Finish Stratum L (km) m n m n m n

18 March d7 12 : 13 12 : 44 B 80.0 0 0 3 4 7 18e7* 11 : 22 12 : 06 B 106.3 1 5 19 22 8 10

20 March f4 13 : 49 14 : 38 A 147.4 11 27 (4) 1 1 3 3f3 12 : 35 13 : 32 A 141.7 5 6 1 1 1 1f2 11 : 41 12 : 19 A 108.1 1 1 4 5 5 6f1 11 : 07 11 : 28 A 35.6 0 0 0 0 1 1

22 March e5 11 : 48 12 : 34 A 125.3 22 40 (1) 0 0 1 1e4 12 : 55 13 : 51 A 152.7 13 18 (2) 1 1 2 3e7 10 : 04 10 : 44 B 98.7 1 2 0 0 4 5e6 10 : 59 11 : 39 B 112.8 3 5 0 0 2 2

23 March C* 10 : 01 12 : 13 C 347.1 0 0 0 0 0 028 March d11 10 : 18 10 : 54 B 99.7 24 30 (2) 6 6 8 8

d10 11 : 04 11 : 51 B 111.1 35 58 (11) 6 6 7 8d9 12 : 00 12 : 38 B 110.9 35 77 (20) 2 2 5 5d8 12 : 46 13 : 24 B 97.2 33 72 (17) 14 14 15 25 (5)x* 9 : 45 10 : 13 B 77.2 13 21 0 0 2 2

Pooled A 710.7 52 92 (7) 7 8 13 15B 893.8 145 270 (50) 50 54 58 83 (5)C 347.1 0 0 0 0 0 0

Total 1951.6 197 362 (57) 57 62 71 98 (5)

4 April d5 11 : 28 12 : 00 A 66.0 11 14 (2) 7 8 5 5d4 10 : 44 11 : 14 A 88.4 5 5 1 1 6 6e7 12 : 48 13 : 24 B 59.0 16 25 (6) 12 12 6 9 (2)e6 12 : 13 12 : 34 B 52.2 17 26 (6) 2 2 5 5

7 April d11 10 : 48 11 : 10 B 74.0 10 17 (4) 6 6 3 3d10 11 : 24 11 : 57 B 71.8 8 13 (3) 6 6 2 2d9 12 : 08 12 : 24 B 53.3 12 22 (6) 8 9 2 3 (1)d8 12 : 32 12 : 39 B 15.5 3 6 (2) 1 1 4 7 (2)e8* 12 : 49 13 : 11 B 37.5 13 24 (6) 0 0 7 8 (1)y* 10 : 00 10 : 36 B 78.8 0 0 0 0 0 0

9 April C* 10 : 16 12 : 50 C 395.4 3 3 0 0 0 0Pooled A 154.4 16 19 (2) 8 9 11 11

B 442.1 79 133 (33) 35 36 29 37 (6)C 395.4 3 3 0 0 0 0

Total 991.9 98 155 (35) 43 45 40 48 (6)

*Sightings on these lines were not used for estimating abundance.L, distance searched; m, number of groups detected; n, number of individuals detected, including pups. Values in parenthe-

ses are number of pups.

enhance the detection and identification of seals.In a preliminary survey done before the breedingseason, we found it difficult to identify speciesfrom higher altitudes. Although some spottedseals dived into the sea when the aircraftapproached at 130 m altitude, observers were ableto detect and count almost all disturbed sealsbefore they disappeared. Because this behavioralresponse was limited to spotted seals, it also helpedus identify species, as has been reported by Brahamet al. (1984).

Two observers, T. Ishinazaka and A. W. Mizuno,always sat on the left and right rear seats, respec-tively, and searched the area perpendicular to the transect lines without any optical tool aids. Straight-line search ranges were limited tobetween 65 m and 550 m because of impaired visibility just below the aircraft and the diffi-culty of observing seals beyond 550 m in the preliminary survey. Aircraft location was recordedautomatically every 30 s by an aircraft-mounted

global positioning system (GPS) unit. Oceandepth at each sighting point, and distance along the transect were obtained from the Geographic Information System of the HokkaidoInstitute of Environmental Sciences (Hokkaido,Japan).

The following environmental data along eachtransect were recorded at 5 min intervals: ice concentration (in tenths) (Erickson et al. 1993);proportion of differentially sized ice floes [large (L, >500 m), medium (M, 100–500 m), small (S,10–100 m), cake (C, <10 m), brash (Erickson et al.1993), and young (very thin)]; ice surface [smooth(s), rough (r), and hummocked (h) (Erickson et al.1993)]; altitude; ground speed; and weather conditions.

When seals were detected, we recorded thenumber, species, time of observation, location, alti-tude, ice conditions (concentration, size, surfacecharacteristics), and positions of seals on the floes(i.e. center/edge). We tried to record the original


Spotted seal Ribbon seal

Line xCourse C

March

Line xCourse C

March

Line yCourse C

April

Line yCourse C

April

Fig. 2. Track lines and sighting locations of spotted and ribbon seal groups in the southern Sea of Okhotsk, spring2000. (�), Seal groups without pups; (�), seal groups with pups.

location of seals when they moved toward thewater at the approach of the aircraft. Species wasidentified based on body shape and colorationaccording to Braham et al. (1984). We sometimesflew circularly above a seal group to ensure accu-rate species identification. When identification was uncertain, we recorded the individuals asunknown, and they were not included in the analy-sis. Incidental sightings of other marine mammalswere also recorded even if they were outside thetransect.

Calculation of perpendicular distance inaerial surveys of seals

For aerial surveys of seals, the strip-transectmethod is common (Braham et al. 1984; Kingsleyet al. 1985; Lunn et al. 1997). This is because sealsare often numerous and clumped, and there isinsufficient time to measure the perpendicular distance between the aircraft and each seal groupusing an inclinometer as in the line-transectmethod. However, the line-transect approach isrecommended by Burnham and Anderson (1984)for reasons of efficiency and validity, and it hasbeen used successfully for cetacean surveys(Yoshida et al. 1997, 1998; Dahlheim et al. 2000)and ship-based surveys of seals (Leopold et al.1997; Gelatt & Siniff 1999).

We developed a line-transect survey and calcu-lated the perpendicular distance from the transectline to observed seals following the methods ofYoshida et al. (1998). Using their approach, calcu-lations are made from the altitude of the aircraftand the declination angle to the sighting point (q). For consistent measures of q, observers main-tained a constant distance between their eyes and the windows of the aircraft by placing theirheads in a fixed stand. Their eyes were also fixedat the height of a mark on the windows; observersmoved only their eyes to search for seals. When aseal was detected, observers put a dot on a trans-parent sheet on the window; each dot was num-bered in order of sightings. After finishing asurvey, q was calculated for the horizontal and vertical distance, whereby the horizontal distancewas from the observer’s eyes to the fixed mark onthe window and the vertical distance was from the mark to each dot (illustrated in Yoshida et al. 1998; Fig. 2).


Data analysis for abundance estimate

We used only seals that we observed on sea icealong longitudinal transects in strata A and B asthe primary sightings, for abundance estimate, but excluded sightings on the duplicated lines(Table 1).

Following Burnham et al. (1980), abundance(N) was estimated as:

(1)

whereby m is the number of groups detected, s isthe mean group size, A is the size of the study area,m is the effective strip width, and L is the distanceof the transect line searched. Assuming no corre-lation among the three estimation components,and that all seals on the sea ice at 65 m from thetransect line were detected, by fitting these modelsto grouped sighting data the variance was esti-mated as follows:

(2)

We used the program distance 3.5 (Thomas et al. 1998) to estimate m. Twelve models were con-sidered: the uniform, hazard rate, half-normal, andnegative exponential curves either with cosine,polynomial, or hermite polynomial adjustments.To improve the precision of m, we pooled allprimary sighting data from both strata. Values ofm were determined among models based on the Akaike Information Criterion (AIC) and c2

goodness-of-fit test.The confidence interval (CI) of N was calculated

with the assumption of a log-normal distributionof N, because a small level of m skews the distri-bution of N positively (Buckland et al. 1993). A95% CI was obtained by following the calculationof Burnham et al. (1987):

(3)

where

(4)C tN

N= ( ) ◊ + ( )Ê

Ëˆ¯

È

ÎÍ

˘

˚˙exp . log

var ˆ

ˆ .d.f. e0 025 12

ˆ, ˆ ,

N

CN C◊È

ÎÍ˘˚

var ˆ ˆvar var

var.

ˆ

ˆ

N N

m

Lm

L

S

S( ) =

ÊË

ˆ¯

ÊË

ˆ¯

+( )

+

ÊË

ˆ¯

ÊË

ˆ¯

È

Î

ÍÍÍÍ

˘

˚

˙˙˙˙

2

2 2 2

1

1

m

m

ˆˆ

NmsA

L=

2m

The degrees of freedom for the t-distribution werecalculated following the formula of Yoshida(1994):

(5)

whereby R is the total number of transect lines,and k is the number of observed groups used toestimate m.

Data analysis for habitat preferences

Primary sighting data as well as the data collectedon lines x and y were used for the analysis ofhabitat preferences such as ocean depth and icecharacteristics. For evaluation of habitat prefer-ences, we classified all sightings into three cat-egories: (i) Pl-P (spotted seal groups with pup); (ii)Pl-A (spotted seal groups without pup); and (iii)Pf-A (ribbon seal groups without pup). Note thatribbon seal groups with pups were not detected.

The distribution of ocean depth at observationpoints (represented by the number of sightings)was compared between Pl-A and Pf-A for inter-species difference, and between Pl-P and Pl-A forintraspecies difference. In order to test the state-ment discussed earlier that spotted seals are abun-dant in shallower waters, and to confirm thatribbon seals prefer deeper waters, each group dis-tribution of depth was compared to those recordedevery 30 s on the survey line, taking samplingefforts into consideration.

Ice conditions (ice concentration, floe size, andice surface) at each haul-out site were also com-pared to those recorded every 5 min. For this analy-sis, we grouped ice concentration data into fourclasses: (i) 1/10th £ I < 4/10th; (ii) 4/10th £ II <7/10th; (iii) 7/10th £ III < 9/10th; and (iv) 9/10th£ IV £ 10/10th. Areas with ice concentrationsbelow 1/10th were regarded as open sea and wereexcluded from analysis. We recorded floe size every5 min and calculated cumulative proportions bysize class to compare relative availability and useby seal groups. We also excluded young ice floesfrom the analysis because we observed no seals

d.f.cv

cv cvcv

= ( ){ }ÊË

ˆ¯{ }

-+

ÊË

ˆ¯{ }

+( ){ }-

È

Î

ÍÍÍÍÍÍ

˘

˚

˙˙˙˙˙˙

intˆ

ˆ

N

m

LR k

s

m

4

4 4

4

1

1

1m

in such areas during transect flights. We analyzedthe haul-out ice features (ice surface, floe size, and haul-out position) between Pl-A and Pf-A,and between Pl-P and Pl-A for the inter- andintraspecies comparison, respectively.

RESULTS

Sightings

Surveys required 19.3 h in March and 10.1 h inApril to be completed (Table 1). We detected 295groups of spotted seals and 517 individuals,including 92 pups; 100 groups of ribbon seals and107 individuals; and 111 groups and 146 individ-ual unknown seals, including 11 pups, during allsurveys (Table 1; Fig. 2).

Table 2 shows the appearance (frequency per-centage) of newborn pups within the total numberof spotted seals observed. More pups were observedin stratum B groups than in stratum A groups, andtheir appearance increased daily, up to 25.6%.

Seal responses to the aircraft varied, includingno response or just looking up, twisting of theneck, moving a few meters on the ice, and divinginto the water. Most recognizable responses wereobserved in the vicinity of transects.

Abundance and density

Prior to estimating the effective strip width (m),we examined interobserver differences and groupsize effect on detectability using all seal sightingsfrom strata A and B (primary sightings and


Table 2 Appearance of pups (frequency in %) of thetotal of spotted seals

Date Stratum A Stratum B Course C All

18 March 0.0 0.020 March 11.8 11.822 March 5.2 0.0 4.623 March 0.0 0.028 March 19.4 19.44 April 10.5 23.5 20.07 April 25.6 25.69 April 0.0 0.0

March 7.6 18.5 0.0 15.7April 10.5 24.8 0.0 22.6

sightings in course X), including those ofunknown species. Differences in the number ofgroups detected by each observer were not signif-icant (paired t-test, t = – 1.469, d.f. = 23, n = 235and 268 for left and right, respectively, P > 0.05).The distribution of perpendicular distances toobserved seals was also not significant betweenobservers (Kolmogorov–Smirnov two-sample test,n = 234 and 268 for left and right, respectively, D= 0.095, P > 0.05). Based on these results wepooled the data from both observers (Table 3).

We examined the relationship between distanceand group size to test whether large groups aremore detectable than small groups and found thecorrelation not to be significant (r = 0.08, n = 500,P = 0.87). We concluded that detection probabil-ity remained unaffected by group size because thevariation in group size was usually small (x = 1.76,1.07, and 1.32; SD ± 0.947, ± 0.293, and ± 0.738;range = 1–9, 1–3, and 1–6 seals per group; n =292, 100 and 111 for spotted, ribbon andunknown seals, respectively).

We grouped the seals observed between 65 mand 550 m from the transect line into seven dis-tance categories at 70 m intervals for spotted (n =291), ribbon (n = 100), and unknown seals (n =108) in order to analyze the effects of perpen-dicular distances on the distribution of sightings(Fig. 3). There were no significant differencesbetween spotted and ribbon seals (Kolmogorov–Smirnov two-sample test, D = 0.137, P > 0.05),whereas unknown seals showed a significant dif-ference from both species (Kolmogorov–Smirnovtwo-sample test, D = 0.318 for spotted seal and D = 0.430 for ribbon seal, P < 0.01). The por-tion of unknown seals increased with perpen-dicular distance from 10.2% at 65–135 m up to83.3% at 415–485 m, and 100.0% at 485–550 m.

When we applied the spotted and ribbon sealdata to the distance program, a uniform modelwith cosine adjustments and a half-normal modelwithout adjustments were selected as the best-fitmodels for spotted and ribbon seals, respectively(AIC = 818.66 and 262.73; c2 = 1.59 and 0.51;d.f. = 4 and 3; P = 0.81 and 0.92, respectively)(Fig. 4). Values for m were estimated as 210 m and158 m (coefficient of variation, CV = 1.5% and7.8%, 95% CI = 204–216 and 136–185 m) forspotted and ribbon seals, respectively.


100

80

60

40

20

065–135 135–205 205–275 275–345 345–415 415–485 485–550

Perpendicular distance (m)

No.

gro

ups

Fig. 3. Distribution of perpendicular distances ofseals from transect lines. (�), Spotted seals; ( ), ribbonseals; (�), unknown seals.

1.0

0.8

0.6

0.4

0.2

0.065 135 205 275 345 415 485

100

80

60

40

20

0

Frequency

Det

ectio

n pr

obab

ility

(a)

1.0

0.8

0.6

0.4

0.2

0.065 135 205 275 345 415

50

40

30

20

10

0

Frequency

Det

ectio

n pr

obab

ility

(b)

Perpendicular distance (m)

Fig. 4. Perpendicular distances recorded and detec-tion functions estimated from the uniform model for (a)spotted seals and the half-normal model for (b) ribbonseals.


Tab

le3

Abu

ndan

ce (

N)

of s

pott

ed a

nd r

ibbo

n se

als

in t

he s

outh

ern

Sea

of O

khot

sk e

stim

ated

fro

m a

eria

l si

ghti

ng s

urve

ys

Mon

thSt

ratu

mA

rea

(km

2 )Tr

anse

ct (

km)

nm

CV

(m

L–1

) (%

)s

(n/m

)C

V (

s) (

%)

NC

V (

N)

(%)

95%

CI

D

Spot

ted

seal

Mar

chA

1422

071

0.7

9252

33.8

1.8

10.1

438

335

.318

94–1

014

10.

31B

1128

871

0.4

244

131

32.2

1.9

4.2

927

032

.542

73–2

011

10.

82

Tota

l25

509

1421

.033

118

313

653

24.8

6167

–30

252

0.54

Apr

ilA

482

715

4.4

1916

52.5

1.2

8.5

141

553

.22–

807

624

0.29

B6

440

325.

810

966

17.4

1.7

5.8

513

018

.432

82–8

020

0.80

Tota

l11

267

480.

212

882

654

518

.532

84–8

1564

40.

58

Rib

bon

seal

Mar

chA

1422

071

0.7

87

53.7

1.1

12.5

505

55.7

142–

1803

0.04

B11

288

710.

432

3143

.01.

08.

91

755

44.6

641–

4804

0.16

Tota

l25

509

1421

.040

382

260

36.8

783–

6607

0.09

Apr

ilA

482

715

4.4

98

90.4

1.1

11.1

888

91.5

0–17

798

446

0.18

B6

440

325.

836

3523

.21.

02.

82

246

24.6

124

1–40

660.

35

Tota

l11

267

480.

245

433

134

31.3

124

1–17

802

512

0.28

L,

dist

ance

sea

rche

d; n

and

m,

num

ber

of i

ndiv

idua

ls a

nd g

roup

s de

tect

ed,

resp

ecti

vely

; s,

mea

n gr

oup

size

; D

, de

nsit

y of

sea

ls (

indi

vidu

als

km–2

); C

V, c

oeffi

cien

t of

va

riat

ion.


tributions during both months were similar tothose of water depth (Fig. 5).

Seal distribution throughout various oceandepths was compared between Pl-A and Pf-A.There was a significant difference in March (Kolmogorov–Smirnov two-sample test, D =0.293, P < 0.01), but no significant difference inApril (Kolmogorov–Smirnov two-sample test, D =0.265, P > 0.05). We also compared the ratios ofseal distribution over the two categorized oceandepths (i.e. shallower or deeper than 300 m)between Pl-A and Pf-A, and found no differencesfor both months (c2 test, c2 = 1.73 for March, c2

= 2.18 for April; d.f. = 1, P > 0.05).Distribution of ocean depth was compared

between Pl-A and Pl-P. There were significant dif-ferences for both months (Kolmogorov–Smirnovtwo-sample test; D = 0.465, P < 0.01 for March;D = 0.295, P < 0.05 for April). Also, the ratios ofseal distribution throughout the two categorizedocean depths (shallower or deeper than 300 m)were significantly different between Pl-A and Pl-P for both months (c2 test, c2 = 11.55 forMarch and c2 = 7.40 for April, d.f. = 1, P < 0.01).The ratios of Pl-P sightings in deeper waters weregreater than those of Pl-A.

Ice preferences

Concentration

Seals of both species showed no significant prefer-ences for any particular category of ice coverage (c2

tests, c2 = 5.51, 6.05 and 1.97 for Pl-P, Pl-A andPf-A in March, and c2 = 3.18, 0.31 and 2.94 forPl-P, Pl-A and Pf-A in April, respectively; d.f. =3, P > 0.05) (Fig. 6), although at least 60.0% ofeach seal group was observed on the ice over 7/10thconcentrations (III and IV). Interspecies differencesbetween the groups without pups were not signif-icant for both months (c2 tests, c2 = 5.41 and 4.11for March and April, respectively; d.f. = 3, P >0.05). Intraspecies differences among spotted sealswas significant in March (c2 tests, c2 = 15.1 and3.01, d.f. = 3, P < 0.01 and P > 0.05 for Marchand April, respectively).

Surface

Although smooth ice surfaces were observed mostfrequently for both months, all seal groups showeda preference for rough ice during March (c2 test,

The abundance of seals in the southern Sea ofOkhotsk was estimated to be 13 653 spotted seals(95% CI = 6167–30 252) and 2260 ribbon seals (95% CI = 783–6607) in March 2000 and6545 spotted seals (95% CI = 3284–815 644) and 3134 ribbon seals (95% CI = 1241–17 802512) in April 2000 (Table 3). Density was esti-mated to be 0.54 for spotted seals and 0.09 forribbon seals per km2 in March 2000, and as 0.58for spotted seals and 0.28 for ribbon seals per km2

in April 2000 (Table 3). Both species occurred athigher densities in stratum B compared withstratum A.

Distribution and ocean depth

Within waters shallower than 300 m throughoutthe survey area, there were approximately one-third of sightings of all spotted seal groupswithout pups (Pl-A) during both months (n = 43and 28, or 30.3% and 44.4% for March and April,respectively), compared with only a few sightingsof spotted seal groups with pups (Pl-P) (n = 4 and6, or 7.3% and 17.1% for March and April, respec-tively) (Fig. 5). Most of the Pl-P sightingsoccurred in areas where ocean depth was between1000 m and 2000 m, especially in March (n = 47 and 20, or 85.5% and 57.1% for Marchand April, respectively) (Fig. 5). Twelve sightingsin March (21.1%) and 13 sightings in April(30.2%) of ribbon seal groups without pups (Pf-A) were observed within waters shallower than300 m (Fig. 5).

Taking survey efforts into consideration, therewere significant differences between the sightingsof all groups and ocean depth during March (Kolmogorov–Smirnov two-sample test, D =0.594, 0.261 and 0.407 for Pl-P, Pl-A and Pf-A,respectively; n = 55, 142 and 57; P < 0.05). Allgroups in March, especially Pl-P and Pf-A, werefound more frequently in deeper waters and lessfrequently in shallow waters (Fig. 5). Conversely,no significant differences were observed duringApril (Kolmogorov–Smirnov two-sample test, D =0.221, 0.124 and 0.195; n = 35, 63 and 43 for Pl-P, Pl-A and Pf-A, respectively; P > 0.05).Although there was no statistical significance, Pl-P and Pf-A showed a similar distributionduring April to that during March; that is, sealswere seen in shallow waters less frequently and indeeper waters more frequently (Fig. 5). Pl-A dis-

c2 = 31.03, 13.47 and 4.47, P < 0.01, P < 0.01and P < 0.05, for Pl-P, Pl-A and Pf-A, respec-tively; d.f. = 1). In April, no such preference wasevident (c2 test, c2 = 2.49, 0.88 and 0.004 for Pl-P, Pl-A and Pf-A, respectively; d.f. = 1, P > 0.05).Interspecies differences between groups withoutpups were not significant for both months (c2 test,c2 = 1.28 and 1.04 for March and April, respec-tively; d.f. = 1, P > 0.05). Intraspecies differencesamong spotted seals were significant for bothmonths (c2 test, c2 = 6.44 and 6.87, d.f. = 1, P <0.05 and P < 0.01 for March and April, respec-tively), and the Pl-P group used ice with a roughsurface more frequently than the Pl-A group (Fig. 6).

Floe size

Seals without pups (Pl-A and Pf-A) showed a pref-erence for small and medium ice floes in March (c2

test, c2 = 16.57 and 16.0 for Pl-A and Pf-A,respectively; d.f. = 3, P < 0.01), whereas the othergroups did not (c2 test, c2 = 3.02 for Pl-P inMarch, and 1.71, 0.70 and 0.25 for Pl-P, Pl-A andPf-A in April, respectively; d.f. = 3, P > 0.05)(Fig. 6). Interspecies differences between groupswithout pup were not significant for both months(c2 test, c2 = 1.33 and 2.56 for March and April,respectively; d.f. = 3, P > 0.05). For intraspeciescomparison among spotted seals, Pl-P used thelarger floes more frequently than Pl-A in March,although there was no significant difference in April (c2 test, c2 = 12.10 and 3.39; d.f. = 3, P < 0.01 and P > 0.05 for March and April,respectively).

Position and floe size

The ratios of seals’ positions (edge or center) werecompared between unobstructed cake ice (C) and


March March

30

20

10

0

Gro

ups

–3000 –2000 –1000 00

600

400

200 Env

ironm

ent

Depth (m)

30

20

10

0G

roup

s–3000 –2000 –1000 0

0

600

400

200 Env

ironm

ent

Depth (m)

March

30

20

10

0

Gro

ups

–3000 –2000 –1000 00

600

400

200 Env

ironm

ent

Depth (m)

Pf-APl-APl-P

April

15

5

0

10

Gro

ups

Gro

ups

200

100

0–3000 –2000 –1000 0

Depth (m)–3000 –2000 –1000 0

Depth (m)

Env

ironm

ent

200

100

0

Env

ironm

ent

15

April

10

5

0G

roup

s–3000 –2000 –1000 0

Depth (m)

200

100

0

Env

ironm

ent

15

April

10

5

0

Fig. 5. Distribution of ocean depths at observation points (number of sightings represented as bars) and (�) thoserecorded at regular 30 s intervals. Seal observations are divided into three categories: (i) Pl-P (spotted seal groupswith pups); (ii) Pl-A (spotted seal groups without pups); and (iii) Pf-A (ribbon seal groups without pups).

the larger floes (S, M, and L) for each seal group. ForPl-P, the seals’ positions were not different betweenfloe sizes for both months (Fisher’s exact probabil-ity test, Fisher’s P = 0.13 and 0.29 for March andApril, respectively; d.f. = 1, P > 0.05), and thisgroup tended to haul-out onto the floe edge or thecenter equally (Table 4). Seals’ positions for Pl-Aand Pf-A were also not different in March betweenfloe sizes (Fisher’s exact probability test, Fisher’s P= 0.54 and 0.09 for Pl-A and Pf-A, respectively; d.f.= 1, P > 0.05), and both groups were observed onthe floe edges of all sizes (Table 4). Seals’ positionsfor Pl-A and Pf-A were different in April whencomparing floe sizes (Fisher’s exact probability test,Fisher’s P = 0.02 and 0.04 for Pl-A and Pf-A,respectively; d.f. = 1, P < 0.05), and both groups


tended to be found either on the center of cake icesor on the edge of larger floes (Table 4).

Combining the data of all floe sizes, interspeciesdifferences between groups without pups were notsignificant for both months (c2 test, c2 = 1.01 and2.46 for March and April, respectively; d.f. = 1, P > 0.05). For intraspecies comparisons amongspotted seals, Pl-P groups hauled themselves outto the center of floes more frequently than Pl-A inMarch, whereas there was no significant differencein April (c2 test, c2 = 13.81 and 0.60; d.f. = 1, P< 0.01 and P > 0.05 for March and April, respec-tively) (Table 4). When the cake ice data wereexcluded and the positions on the larger floes (S,M, and L) were compared, interspecies differenceswere not significant for both months (c2 test, c2 =

Ice concentration Ice surface Floe size

March March March

100%

50%

0%

100%

50%

0%

100%

50%

0%

118 55 142 57IVIIIIII

Env. Pl-P Pl-A Pf-A

Group

Pl-P Pl-A Pf-A

Group

Env. Pl-P Pl-A Pf-A

Group

** ** * Env. Pl-P Pl-A Pf-A

Group

** *

107 55 138 57hrs

58 55 57140LMSC

April43 34 60 43

IVIIIIII

100%

50%

0%

Env. Pl-P Pl-A Pf-A

Group

April

100%

50%

0%

April

100%

50%

0%

Env. Pl-P Pl-A Pf-A

Group

Env.

40 33 58 43hrs

LMSC

43583423

Fig. 6. Comparison of the proportion of available ice types (Env.), recorded at regular 5 min intervals, and ice typesused by seal(s) for haul-out (Pl-P, spotted seal groups with pups; Pl-A, spotted seal groups without pups; and Pf-A,ribbon seal groups without pups). Ice concentration data is divided into four classes: (i) 1/10th £ I < 4/10th; (ii)4/10th £ II < 7/10th; (iii) 7/10th £ III < 9/10th; and (iv) 9/10th £ IV £ 10/10th. Areas with ice concentrationsbelow 1/10th were regarded as open sea and were excluded from analysis. Ice surfaces are classified as smooth (s),rough (r) and hummocked (h). Floe sizes are large (L, > 500 m), medium (M, 100–500 m), small (S, 10–100 m) andcake (C, < 10 m). *Distribution significantly different according to c2 test (P < 0.05). **Distribution significantlydifferent according to c2 test (P < 0.01).

0.14 and 2.17 for March and April, respectively;d.f. = 1, P > 0.05). Intraspecies differences wereonly significant in March (c2 test, c2 = 8.83 and0.77; d.f. = 1, P < 0.01 and P > 0.05 for Marchand April, respectively).

Incidental sightings of other marine mammals

During the flights, we observed six bearded seals(Erignathus barbatus) (including a pup with itsmother), three Steller sea lions (Eumetopias jubatus),one minke whale (Balaenoptera acutorostrata), andone Baird’s beaked whale (Berardius bairdii).

DISCUSSION

Possible biases affecting abundanceestimates

The population estimate in the present study maybe minimal for three reasons: (i) any individuals of

‘unknown species’ on the ice may have beenspotted or ribbon seals; (ii) individuals in the waterwere not recorded; and (iii) some seals showed anapparent avoidance to the aircraft’s approach.

Unknown seal groups comprised 21.6% of thetotal number of observations in strata A and B.Because we did not observe many other species,such as bearded or ringed seals, we assume thatmost of the unknown seals were either spotted orribbon seals. Consequently, our estimates may bemore than 20% lower than the real values.

During the present study, because of the diffi-culty of species identification, we excluded indi-viduals in the water and did not correct for thisbias. Diurnal activity in spotted and ribbon sealsmay differ, and a time-of-day correction factormight be applicable, as has been developed forAntarctic seals (Erickson et al. 1989). For example,Gelatt and Siniff (1999) have reported that theestimated density of 0.50 crabeater seals km-2

increased to 0.70 km-2 when they applied thetime-of-day correction factor.

Similarly, a correction factor for avoidancebehavior of seals would increase the accuracy of ourestimates. During aerial surveys, diving responsesare the most troublesome. Although we observedfleeing seals, some seals may have dived into thewater before they were sighted. In some cases,pilots detected seals from the front window of theaircraft that were not seen by the rear-seatobservers. Of course, some of the seals observedonly by the pilots may have been under the air-craft, meaning that they were not actually ‘missed’,but were outside the survey area.

The sensitivity of spotted seals to aircraft dis-turbance may depend on season and/or the sexualmaturity of individuals. This species was observedto be more sensitive in a preliminary survey done before the breeding season compared withduring the main surveys, and the responses werestronger for grouped, small-bodied immature seals than for solitary mature seals. Although for ribbon seals we were unable to discern a patternof response because of the limited group cat-egories observed, ribbon seals reacted little to thepresence of aircraft, as reported also by Braham et al. (1984). Consequently, our detection proba-bility at 65 m from the transect line should be <1 and vary according to species, gender, seasonand maturity.


Table 4 Haul-out location in the ice floe

Location in the Seal Floe ice floe

group size Center Edge Total

March Pl-P C 5 2S, M, L 19 28

Total 24 (44.4) 30 (55.6) 54

Pl-A C 5 21S, M, L 19 87

Total 24 (18.2) 108 (81.8) 132

Pf-A C 4 4S, M, L 10 39

Total 14 (24.6) 43 (75.4) 57

April Pl-P C 9 6S, M, L 8 10

Total 17 (51.5) 16 (48.5) 33

Pl-A C 12 6S, M, L 13 27

Total 25 (43.1) 33 (56.9) 58

Pf-A C 8 10S, M, L 4 21

Total 12 (27.9) 31 (72.1) 43

Values in parentheses are percentages.

Seal abundance in the southern Sea of Okhotsk

Simultaneous surveys of waters off eastern Sakhalinand Hokkaido are required to estimate the totalabundance and distribution of seals throughout thesouthern Sea of Okhotsk. However, recent esti-mates of local abundance near eastern Sakhalin arenot available, and there is only limited informa-tion from earlier studies. Fedoseev (1970) esti-mated that there were 12 000–13 000 spottedseals and 57 000–77 000 ribbon seals off easternSakhalin in spring, based on aerial surveys done in1968 and 1969. He estimated the total populationin the Sea of Okhotsk at 170 000 for spotted sealsand 130 000 for ribbon seals. Between 1976 and1979, after commercial harvesting was limited, hereported an increase in population to 200 000 forspotted seals and 345 000 for ribbon seals basedon aerial surveys, although the local abundancedata near eastern Sakhalin was not reported(Fedoseev 1984). He suggested that this largeincrease in the number of ribbon seals might bedue to their early age at the time of first repro-duction and a high annual production rate equalto 30% of the total population. In contrast, infor-mation about seal abundance in Hokkaido watersis not available. Therefore, we cannot compare sealabundance between Hokkaido and Sakhalin, nordiscuss recent seal population status in the south-ern Sea of Okhotsk; hence, future censuses inSakhalin, as well as in Hokkaido, are needed withthe comparable methodology.

Density during the ice season

Densities for both species were higher in stratumB than in A, and were similar to the highest den-sities reported by Uno and Yamanaka (1988), whofound 0.81 spotted seals km-2 and 0.25 ribbonseals km-2 on the Kitami Yamato Bank. Thesevalues are much higher than those reported for theBering Sea off Alaska (0.11 spotted seals km-2 and0.002 ribbon seals km-2; Braham et al. 1984).

The higher density in stratum B was expected,given two major conditions for parturition and thesurvival of weaned pups: (i) the stability of sea ice generated by the Shiretoko Peninsula, whichobstructs sea currents from the northwest to south-east (Aota & Kawamura 1979) and the seasonal


wind (Tabata et al. 1970); and (ii) the high primaryproductivity of the spring plankton bloom justafter the decay of sea ice (Nishihama et al. 1989).

Fedoseev (1970) recorded a lower density ofspotted seals off the eastern coast of Sakhalin(0.03–0.17 seals km-2) where only a few newbornpups were encountered (0.013 seals km-2 in Terpeniya Bay), but recorded a higher density in Mamiya Strait (1.06 seals km-2), suggesting animportant breeding concentration in the Sea ofJapan. Although his data cannot be compareddirectly with ours due to the differences inmethodology, much higher densities off Hokkaidothan off the eastern coast of Sakhalin suggest thatthe former area may be an important breeding areain the southern Sea of Okhotsk.

Conversely, the density of ribbon seals was loweroff Hokkaido than that off Sakhalin (0.32–1.0adult seals km-2; Fedoseev 1970). This finding isin accordance with the description by Fedoseev(1970) that density was greater in the northeast-ern region off Sakhalin than in the southeasternregion, which adjoins the present study area.

Parturition season

The parturition of spotted seals has been thoughtto occur from the middle to the end of March inthe southern Sea of Okhotsk, based on the growthof fetuses and newborn pups (Naito & Nishiwaki1972) and the appearance of fetuses obtained fromdamage control kills in 1997 and 1998 (Goto1999). The present study supports this statement.In stratum B, most parturition appeared to occuraround the 28th of March, which is when the icewas still stable but just before ice decay and thesubsequent plankton bloom (Nishihama et al.1989), which attracts prey species of seals. Thissurge in available food is likely to be important foradult seals, especially females, as suggested byLowry et al. (2000) in explaining the habitat preferences of spotted seals along the ice front of the Bering Sea.

Alternatively, data on the reproduction ofribbon seals are insufficient. Surveys of the presentstudy may have been conducted prior to the birthseason, which is considered to begin in early April(Tikhomirov 1968; Fedoseev 1970), when theOkhotsk coast of Hokkaido has almost no ice.Naito (1971) has reported previously that ribbon

seals breed in this region, mainly between theShiretoko Peninsula and Kunashiri Island, wherewe did not survey. The area surrounding the inter-national border between Russia and Japan shouldbe surveyed for a better understanding of ribbonseal reproduction.

Ocean depth preferences

During March, spotted seals without pups (Pl-A)were more abundant in shallow waters comparedwith ribbon seals without pups (Pf-A). This prob-ably reflects differences in food availability andpreferences by the two species; prey species forspotted seal are distributed in a wide area rangefrom the surface to the benthos in coastal waters(shallower area), whereas those for ribbon seals aredistributed near the continental shelf and near thesurface of offshore waters (deeper area) (Kato1977). The distribution pattern of ribbon sealgroups with pups, and the foraging activities ofboth species during the reproductive season wouldgive a possible explanation for habitat selection ofboth species.

Taking into account survey efforts, not onlywere ribbon seal sightings in March biased towarddeeper water rather than shallower water, but alsofor spotted seal sightings; whereas, in April, sealdistribution depended on survey efforts, andgreater efforts in shallow waters would make sealsightings more common there. Therefore, it is con-sidered that spotted seals do not necessarily prefera shallow water habitat. This might also apply toprevious reports of higher spotted seal occurrencesin waters 200 m deep or shallower (Naito &Nishiwaki 1975; Braham et al. 1984; Uno &Yamanaka 1988), because these studies did notconsider survey efforts. The conclusion thatspotted seals are more abundant in shallow watersmay be biased because of higher survey efforts insuch areas. For ribbon seals, we were able toconfirm previous statements made that ribbonseals haul-out on ice in deeper waters (Braham etal. 1984; Uno & Yamanaka 1988).

Spotted seals with pups (Pl-P) were more abun-dant in deeper waters than Pl-A for both months.Spotted seals may select deeper areas for reproduc-tion. However, ice is more stable in deeper waters,and ice stability may be a more important factorfor seal birthing (Burns 1970). Future study of

waters with different ice stability should clarifywhich factor is more important for spotted sealreproduction.

Ice preferences

Uno and Yamanaka (1988) have reported that thetype of ice chosen for haul-out sites did not differbetween spotted and ribbon seals during thebreeding season, although they did not separateseal groups according to those ‘with’ or ‘without’pups. In the present study, for those groupswithout pups, both seal species also showed asimilar preference for ice characteristics. However,ice preferences between the two groups of spottedseals were different in some features (i.e. icesurface, floe size, and position).

All seal groups, particularly Pl-P, showed a pref-erence for rough ice surfaces, which is reasonableconsidering that female seals with newborn pupswould need shelter from predators, such as eagles(Tikhomirov 1966; Riedman 1990), and/or strongwind and snow.

During the present study, aircraft disturbancesmight have made any seals without pups on theedge dive into the sea, or those seals on the centerof ‘cake ice’ to move to the edge. Conversely, sealswithout pups on the center of ‘larger’ floes (S, M,and L) were assumed not to have their positionsinfluenced because these floes were too large for theseals to move from the center to the edge. We con-sider that aircraft disturbances did not influencethe positions of the Pl-P group because of the poormobility of newborn pups.

Compared with Pl-P, the Pl-A group tended tohaul-out onto the edge of smaller floes duringMarch. If Pl-A that were positioned in the centerdid not move to the edges and the positions of Pl-P remained unaffected by aircraft disturbance,we could consider that there were intraspecies dif-ferences; that is, Pl-P was more abundant in thecenter of larger floes than Pl-A. A precise under-standing of habitat preferences needs more infor-mation about the effects of the reproductive stageof seals, aircraft disturbances, and sea ice quality.

CONCLUSIONS

The distribution and abundance of spotted sealsand ribbon seals were assessed during March and


April 2000, off the Okhotsk coast of Hokkaido.The large number of spotted seal pups observedsuggests that the study area may be an importantbreeding center. Compared with seal densityreports in eastern Sakhalin by Fedoseev (1970), thedensity of spotted seals in the present study areawas higher and that of ribbon seals was lower.

After considering the survey efforts, it wasregarded that spotted seals do not necessarilyprefer a shallow water habitat, and that ribbonseals prefer to haul-out on ice in deeper waters.Female spotted seals with pups tended to be found in the center of larger and rougher floes overdeeper waters compared with other categories ofseals.

For estimates and monitoring of the total sealabundance and distribution throughout the south-ern Sea of Okhotsk, simultaneous surveys of watersoff eastern Sakhalin and Hokkaido are required.When conducting ribbon seal studies, it is neces-sary to fly over the main breeding area just afterthe peak birthing season; that is, between theShiretoko Peninsula and Kunashiri Island (Naito1971) in the middle of April (Tikhomirov 1968;Fedoseev 1970). Studies on seals’ foraging activi-ties during the reproductive season would providesome explanation about the habitat selection ofboth species.

ACKNOWLEDGEMENTS

We are grateful to H. Yoshida, K. Frost, V.Burkanov, Y. Naito, and H. Uno for providingvaluable advice on survey design. We thank T.Isono for assistance in recording data in the field;Y. Yoshizawa, S. Ogawa, M. Takegahara, and H.Hirose of Hokkaido Aviation Co., for providingsafe and successful survey flights; S. Shimada andM. Kaneko for GIS analysis; H. Hirakawa for statistical advice; M. Suzuki, K. Mitsuta, T.Murakami, R. Tamura, the Shari City Governmentand the members of the Krill Seal Research Groupfor their survey assistance; D. Rugh and S.Kromann for collecting references; and J. Moll for assistance with English. The manuscript wasimproved by the constructive comments and sug-gestions from K. Frost and T. Saitoh. This researchwas partly supported by the Environment Agencyof Japan.


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distribution and abundance of spotted seals phoca largha and ribbon seals phoca fasciata in the...

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