haul-out activity of ringed seals (phoca hispida) determined from satellite telemetry

MARINE MAMMAL SCIENCE, 18( 1): 167-1 8 1 (January 2002) 0 2002 by the Society for Marine Mammalogy

HAUL-OUT ACTIVITY OF RINGED SEALS (PHOCA HISPIDA) DETERMINED FROM

SATELLITE TELEMETRY E. W. Bowl

Greenland Institute of Natural Resources, P. 0. Box 570, DK-3900 Nuuk, Greenland

J. TEILMANN National Environmental Research Institute,

P. 0. Box 358, DK-Roskilde, Denmark

F. RIGET National Environmental Research Institute,

P. 0. Box 358, DK-Roskilde, Denmark

ABSTRACT

The haul-out activity of 15 ringed seals (Phoca hispidu) equipped with satellite-linked radio transmitters was studied in N W Greenland (ca. 73”- 78”N). Between 19 June 1997 and 30 June 1999, telemetry data on haul- out activity were obtained by the “Land-Sea-Reporter’’ (LSR), “Time-at- Depth” (TAD), and “Timelines” (TIM) systems housed within the satellite transmitters. The haul-out activity (% of total time hauled out) reported by the TIM system, which is specifically designed for collecting haul-out data, was about 1.4 times higher than that inferred from the LSR, but only about 0.7 of that inferred from TAD data. The TIM were used to describe haul-out activity. A total of 1,011 d with TIM were obtained (64.5% of a total of 1,568 “seal-days” monitored) representing data from nearly an entire annual cycle. No differences were found in percentage of time hauled out per month among various age categories. At all seasons the haul-out time showed considerable individual variation. There were no trends in percentage of time hauled out per month during late summer, fall, and winter (August-Febm- ary). During the High Arctic winter darkness (November-January) the percentage of haul-out per month ranged between 3.9% in an adult (SD = 2.44, range: 1.1%-5.7%, n = 3 mo) and 15.7% in a subadult (SD = 1.95, range: 13.7%-17.6%, n = 3 mo). From late March there was a significant increase in haul-out time. Between 1 and 30 June, when aerial surveys of basking ringed seals usually are conducted, the haul-out time (% per day) increased from about 25% to about 57%. No tendencies in die1 haul-out activity were revealed.

Current address: ’% National Environmental Research Institute, Frederiksborgvej 399, P.O. Box 358, DK-Roskilde, Denmark; e-mail: [email protected].

167

168 MARINE MAMMAL SCIENCE, VOL. 18, NO. 1, 2002

Key words: ringed seal, Phoca hispida, N W Greenland, North Water Polynya, satellite telemetry, haul-out activity.

Ringed seals (Phoca hispida) are widely distributed in the ice-covered Arctic Ocean, the Baltic Sea, and in some inland lakes in Russia, Finland, and Canada (reviewed by Reeves 1998). Because of their abundance in arctic areas year- round, ringed seals are important to the subsistence economy of the Inuit (e.g,, Reeves et al. 1998, Teilmann and Kapel 1998, Belikov and Boltunov 1998).

Ringed seals occupy areas that have a stable, dense layer of sea ice for the majority of the year. Their preferred breeding habitat is the annual land-fast ice with good snow cover in fjords and bays of complex coast lines (e.g., McLaren 1958), but they are also widely distributed in offshore pack ice where some breeding occurs (e.g., Finley et al. 1983, Wiig et al. 1999). During winter the seals spend most of their time in the water or in subnivean lairs on the stable ice (e.g., Smith and Stirling 1975). After the breeding season (March- early May) they haul out to molt on the exposed surface of the ice between late March until ice breakup, which usually occurs in July (e.g., Vibe 1950, McLaren 1958, Smirh and Hammill 1981). Food consumption rates vary seasonally (e .g . , Ryg et al. 1990, Ryg and aritsland 1991). The general pattern is for seals to feed intensively from late summer to early spring. During the molt the seals feed less intensively and spend proportionally more time hauled out (e.g., Smith 1973, 1987; Finley 1979).

Various methods have been used to estimate population size of ringed seals, including ship-based surveys, land-based counts, visual and photographic aerial surveys, and extrapolation from densities of breeding lairs or breathing holes (reviewed by Reeves 1998). The most common method of estimating ringed seal densities has been visual aerial surveys flown between late May and mid-July, with most effort concentrated in June (e.g., McLaren 1966, Burns and Harbo 1972, Smith 1975, Stirling et al. 1977, Finley et al. 1983, Kingsley et al. 1985, Born et al. 1998).

Knowing what proportion of the population is hauled out on the ice during the surveys is crucial for extrapolation to total abundance. Information about ringed seal haul-out activity has been collected during ground-based visual observations (Smith 1975, Finley 1979, Smith and Hammill 1981, Hammill and Smith 1990) and VHF radio telemetry (Kelly and Quakenbush 1990, Lydersen 1991, Lydersen and Hammill 1993, Lydersen et al. 1993). The percentage of time hauled out was estimated from satellite-linked radio-tagged ringed seals in NW Greenland between early June and October (two seals; Heide-Jorgensen et al. 1992) and between August and December (four seals; Teilmann et al. 1999).

In this paper we present information about seasonal and die1 haul-out activity of 15 ringed seals equipped with satellite-linked radio transmitters in NW Greenland, 1997-1999. Our purposes are to (1) compare methods of recording data on haul-out and activity at the surface, and (2) investigate

BORN ETAL.: HAUL-OUT ACTIVITY 169

annual variation in haul-out activity, in particular during June when aerial surveys are usually conducted.

METHODS

Field Activities and Study AnimalJ

In June and August 1997 (n = 5) and July and August 1998 (n = 10) satellite-linked radio transmitters were glued to the back of 15 ringed seals (9 M, 6 F) that had been netted between 76’30” and 77”30’N in the Thule area (NW Greenland) (Table 1). The capture and handling methods were as described by Kapel et al. (1998) and Teilmann et al. (1999).

We hypothesized that the overall haul-out activity of subadult seals would differ from that of adult seals because the two groups occur in different hab- itats and behave differently during the mating season (e.g., McLaren 1958). Therefore, the study animals were categorized as either subadults or adults based on body length and mass. According to Kingsley (1998), mean body masses of ringed seals at age in August are: 2 yr of age = 31.5 kg, 3 yr = 38 kg, 4 yr = 44 kg, and 5 yr and older 2 50 kg. Ringed seals generally become sexually mature when 3-5 yr old (e.g., Smith 1973, Finley et al. 1983, Smith 1987, Kingsley and Byers 1998, Lydersen 1998). We considered females and males with a body mass of 33 kg or less at capture (late July and August), to be about 2 yr old and therefore subadults. Seals that were >47 kg were classified as adults. The reproductive status of three seals with a body mass of between 38 and 40 kg, and estimated to be slightly more than 3 yr old (June and August), could not be determined. Hence these seals were categorized as “pubescent” (Table 1).

Satellite Transmitters and Their Sampling Protocol

Three types of satellite transmitters with somewhat different sampling protocol were used (Table 1). The configuration of the transmitters was basically similar to that described in Teilmann et al. (1999). All had a transmission rate of 45 sec in water and 90 sec when the seal was hauled out. To extend battery life, all but four (3985, 11272, 11273, 11274) of the transmitters were programmed to suspend transmission after four hours of continuous haul- out (Table 1).

All transmitters collected three types of data that could be used to describe the haul-out (or at-surface) activity of an individual seal: “Land-Sea-Reporter” = LSR, “Timelines” = TIM, and “Time-at-Depth” = TAD (Wildlife Com- puters 1997). The LSR reported during each transmission whether the transmitter’s salt-water switch (SWS) was dry or wet. However, we programmed the transmitters so that a “dry reading” would be transmitted only after 10 consecutive dry recordings (i.e., 7.5 min). Hence, in a string of LSR “wet” and “dry” readings, the time elapsed between the first and the last “dry” reading was considered to represent the duration of haul-out (7.5 min was


TabCe 1. Basic data on 15 ringed seals equipped with satellite-liked radio transmitters and used in studies of haul-out activity in NW Greenland between 19 June 1997 and 30 June 1998 Percentages of time hauled out per month (in parentheses: total number of “seal-days” monitored per month) are shown. Haul-out time determined from “Timelines.”

Percentage of time hauled out (“seal-days” monitored

per month) determined

Fall

No. from “Timelines” Body days %

ID no. tagging Sex (kg) class” typeb tore& daysd Aug Sep Oct

1857-97 19 June 1997 m 38 P A 17 71 - - - 1856-97 21 June 1997 m 39 P A 31 74 - - - 1858-97 22 June 1997 f 83 A A 13 15 - - -

Date of mass Age PTT moni- TIM

- - 3984-97 27 June 1997 m 47 A A 8 25 - 11271-97 12 August 1997 m 96 A A 253 63 3.8 (16) 9.7 (27) 5.5 (21) 11273-98 30 July 1998 m 24 S B 85 85 21.6 (31) 5.0 (30) 13.9 (11) 4347-98 2 August 1998 m 33 S C 98 88 3.4 (28) 3.9 (30) 4.7 (28) 11272-98 2 August 1998 m 33 S B 70 97 5.8 (29) 2.0 (30) 8.9 (9) 1857-98 3 August 1998 f 27 S C 332 52 2.2 (20) 0.4 (29) 11.4 (30)

3985-98 10 August 1998 f 40 P B 95 100 4.2 (27) 8.6 (30) 1.6 (28)

3984-98 11 August 1998 m 26 S C 98 76 14.6 (19) 21.2 (30) 12.9 (25) 1858-98 11 August 1998 m 31 S C 320 42 9.7 (21) 5.0 (28) 8.1 (24) 1856-98 13 August 1998 f 28 S C 124 73 28.9 (22) 1.7 (8) 0.9 (29)

1859-98 5 August 1998 f 29 S C 12 67 23.4 (8) - -

11274-98 10 August 1998 f 22 S B 12 92 10.1 (11) - -

a Age class (see Material and methods): P = Pubescent; A = Adult; S = Subadult. PTT (Platform terminal transmitter; satellite-radio) type: A = SDR-T10, 0.4 W

output, transmission suspended after 4 h of haul-out; B = As “A” but transmission continuous; C = SSC3, 1 W out-put, transmission suspended after 4 h of haul-out.

Total number of days with contact. % of days with TIM (= Timelines) of total number of days with contact.

added to account for the lag time). This approach to determining haul-out activity could be applied only to the four seals with a continuous duty cycle. In TIM, information on haul-out activity during 24 h was collected in 20- min increments. If the SWS was dry for more than 50% of a 20-min interval, a “dry” reading was recorded. Otherwise, a “wet” reading was recorded. We programmed the transmitters so that TIM information for the previous 24 h was recorded and relayed to the satellites every 48th transmission. We excluded from the analyses of the TIM those individual haul-out periods that apparently either were initiated prior to the sampling day or were aborted at the end of the sampling period. TIM were recorded for all seals. However, only the data from the four seals with transmitters set for continuous duty cycles were used to compare haul-out activity as recorded by both TIM and LSR .

The 10 transmitters deployed in 1998 also collected information on haul- out, or at-surface, time via the Time-at-Depth (TAD) system. These trans-


Table I. Extended.

Percentage of time hauled out (“seal-days” monitored per month) determined from “Timelines”

Winter Spring Nov Dec Jan Feb Mar Apr May Jun JuI

- - - - 64.0 (11) 75.0 (1) - - -

- - - - - - - 56.1 (8) 36.5 (15) - - 62.0 (2) -

- 69.4 (2) - 4.8 (23) 1.1 (21) 5.7 (29) 5.0 (10) 17.3 (6) 5.6 (7) - - -

- - - - - - - - - - -

mitters were programmed so that the first bin of the TAD summarized the time when the SWS was dry (ie., first depth interval in TAD = 0). In this way, the cumulative time that the SWS had been out of the water was recorded in 6-h blocks per 24-h period, and then relayed to the satellites every 13th transmission.

To make information on haul-out activity comparable to that reported by Teilmann et al. (1999) the TIM, TAD, and LSR data were summarized in the following 6-h periods: 2100 through 0200,0300 through 0800,0900 through 1400, and 1500 through 2000 local time ( i e . , 2 h after GMT).

Statistical Analyses

Owing to the non-normal distribution of data, we used non-parametric statistics (Spearman rank correlation coefficient, Friedman two-way analysis of variance, Wilcoxon matched-pairs signed rank test, Kolmogorov-Smirnov two- sample test, and Mann-Whitney U test). For some of the analyses four periods were defined: fall (August-October), dark period (November-January), spring (February-April), and molt (May-July). Analyses were carried out using Stat- view for Windows 4.5 (1992-1997, Abacus Concepts), Systat 9.0 (1999, SPSS Inc.), and SAS (1990, SAS Inst. Inc.).


RESULTS

Study Area and the Weather Conditions

In Inglefield Bredning (approximately 77"25 'N, 67"-69"W) and Wolsten- holme Fjord (approximately 76"38'N, 68"-70"W), where the ringed seals were tagged and spent most of their time, fast ice usually forms during late October and breaks up from mid- to late July (Vibe 1950, Teilmann et al. 1999). Information from NOAA satellite images, ice charts, and local residents in- dicates that in 1997 breakup occurred in Inglefield Bredning on 28 July and the fast ice reformed around 10 November. In 1998 ice breakup occurred after the transmitters stopped transmitting. The 1998 tagging took place in late July-early August in Wolstenholme Fjord where the fast ice formed on 9 November. Breakup occurred in this area on 17 July 1999.

Overall, there were no marked differences in temperature and precipitation between the 1997/1998 and the 1998/1999 study periods. Air temperatures dropped gradually from August (% = ca. 4°C) and remained well below 0°C until late in spring. Between 19 June and 20 July 1997 (when five seals were monitored) the air temperature measured in Inglefield Bredning averaged 5.5"C (minimum/maximum: -4.7"/16.OoC). In Wolstenholme Fjord, where two seals were monitored in June 1999, the mean temperature during this month was 2.0"C (minimum/maximum: -2.0/10°C; data courtesy of the Asiaq and Thule Air Base meteorological office).

Movement of the Seals and Duration of the Periods with Data on Haul-out Activity

The four seals tagged in Inglefield Bredning in June 1997 stayed in this fast-ice covered fjord until their last transmission on 20 July 1997. A 96-kg male tagged in Inglefield Bredning in August 1997 moved south in November and spent the winter in an area with consolidated pack ice (ca. 73"30'N, 60"W) offshore of the Upernavik area ( N W Greenland) until transmissions stopped on 21 April 1998. The ten seals that were instrumented in late July and August 1998 in Wolstenholme Fjord remained in the North Water Po- lynya, in the northern Baffin Bay-Smith Sound area, until the last transmission was received on 30 June 1999. Hence, the majority of the haul-out activity reported in this study relates to seals that remained in, or relatively close (< lo0 km) to, the areas where they were tagged.

The transmitters provided information on haul-out activity during periods of variable length. Maximum longevity on a seal was 132 d (Table 1).

Haul-out Time Determined via Three Diffevent Systems

In the four transmitters (Table 1) that were programmed to transmit con- tinuously, we compared estimates of haul-out activity inferred from the three systems during the period 30 July-12 November 1998.

Haul-out activity from the LSR vs. TIM--If only days with information on


haul-out activity from both the LSR and the TIM (n = 64 during August- November) were included in the analyses, the daily haul-out time (h) recorded via the TIM and the LSR was positively correlated for all four seals (P 0.05, Spearman correlations). On average, the daily haul-out time estimated from the LSR was about 0.71 of that inferred from the TIM (SD = 0.534, range: 0.03-3.37, n = 64). This apparent discrepancy was not caused by a difference in the duration of individual haul-out bouts, as their frequency distribution obtained via the LSR and the TIM did not differ statistically (x2 = 1.129, P = 0.99, df = 2; Kolmogorov-Smirnov). Overall, the mean duration of haul- out bouts estimated from the LSR and the TIM was 3.1 h (SD = 3.1, range: 0.1-15.8 h; n = 213). During days when reports from both LSR and TIM were received, the number of haul-out periods in the LSR averaged only 0.87 of that in the TIM.

Haul-oat activity inferred from the TAD-For the four seals with continuous duty cycle the estimates of percentage of haul-out time per day obtained from the TAD was positively correlated with that obtained from the LSR ( P < 0.05, Spearman correlations). However, the daily haul-out time estimated from the TIM averaged only about 0.69 of that inferred from the TAD (SD = 0.362, range: 0.06-1.81, n = 77 d). From this relationship, we excluded a total of 118 days where there was no haul-out according to the TIM. However, on these days the percentage of time at the surface in the TAD averaged 10.4% (SD = 4.8, range: 4.6%-34.0%, n = 118). Hence, this percentage might represent an overall estimate of the proportion of time that the ringed seals spent at the surface of the sea during the fall.

Seasonal Haul-out Activity

The “timelines” (TIM) system is specifically designed for collecting haul- out data and, therefore, the description of ringed seal haul-out activity is based only on information from the TIM. Between 19 June 1997 and 30 June 1999, TIM were obtained for a total of 1,011 d from the 15 seals. Days with TIM represented between 15% and 100% of the time where individual seals were monitored (Table 1). Overall, TIM were received for 64.5% of all days where seals were monitored.

Information on individual haul-out activity was collected from nearly an entire annual cycle except for the period 21-31 July. The percentage of haul- out time per month (Table 1) did not differ significantly among adult, pubescent, and subadult seals (Friedman test statistic = 0.4, P = 0.82). At all seasons there was a pronounced difference between individuals in overall time hauled out per month (Table 1). For example, total individual haul-out times during August varied between 2.2% and 28.9%. Similarly, during the High Arctic winter darkness (November-January; the sun is over the horizon on 15 February) the percentage of haul-out per month ranged between 3.9% in an adult (SD = 2.44, range: 1.1%-5.796, n = 3 mo) and 15.7% in a subadult (SD = 1.95, range: 13.7%-17.6%, n = 3 mo; Table 1). In three seals that transmitted through the winter until April (11271-97) and June (1857-98,


1858-98), there was a tendency for haul-out time to increase after January (Table 1). However, these tendencies were not statistically significant (P > 0.05; Spearman correlations). Similarly, an apparent tendency of seal 1857-98 to spend a greater proportion of time hauled out during winter than 1858- 98 and 11271-97 (Table 1) was not statistically significant (P > 0.10, Wil- coxon signed rank).

In northwestern Greenland, ringed seals begin to haul out on the exposed surface of the ice in late March or April (Vibe 1950). From late March until the last transmission in April (11271-97) and June (1857-98, 1858-98), three seals increased the time hauled out (% of day). However, only in seal 1857- 98 was this tendency statistically significant (Y = 0.377, P < 0.01; Spearman correlation).

Relevant to surveys of basking ringed seals, haul-out time (% hauled out per day) during the late winter and spring is given in Table 2. Individual proportion of time hauled out per day also varied considerably, but during March-July there was a general increase in haul-out time.

Haul-out activity peaked in late June (Fig. 1). During the last week of June (24-30 June) there was information about the haul-out activity of one subadult and three adult seals (Fig. 1). During this period there was no difference among the seals in percentage of haul-out time per day (P > 0.05; Mann- Whitney). On average, the seals were hauled out for about 57% of the time (SD = 16.9, range: 33.3%-91.7% of the day, n = 22 d).

Mean duration of haul-out bouts did not differ among six individuals that transmitted in June-July ( P > 0.05; Kolmogorov-Smirnov). During these months the mean duration of a haul-out bout was 3.3 h (SD = 4.19, range: 0.33-28.1 h, n = 214).

Diel Haul-out Activity

The seals showed no tendencies in die1 haul-out activity during periods of continuous daylight (May-August) or day and night (March-April and Sep- tember-October) (Light x2 = 0.750, P = 0.87; df = 3. Darkness x2 = 2.700, P = 0.44; df = 3; Friedman). A tendency for the seals to haul out more between 2100 and 0800 local time during the dark season (November-Feb- ruary) was not statistically significant (x2 = 7.125, P = 0.07, df = 3; Fried- man).

DISCUSSION

Comparisons among the Three Different Ways of Recording Haul-out Time

Of the three methods used in this study to record haul-out activity, the TIM system is specifically devised to collect haul-out information, whereas the LSR and TAD systems can potentially be used for such a purpose. Overall, the haul-out times inferred from the LSR were about 70% of those estimated from the TIM, suggesting that the LSR-based haul-out percentages reported

Tabl

e 2.

D

aily

mea

n ha

ul-o

ut t

ime

(% p

er d

ay)

Mar

ch-J

uly

1998

and

199

9 by

sev

en r

inge

d se

als

equi

pped

with

sat

ellit

e-lin

ked

radi

o tra

nsm

itter

s in

NW

Gre

enla

nd.

Hau

l-ou

t tim

e de

term

ined

fro

m “

Tim

elin

es” (

see

Met

hods

).

ID

1127

1-97

” 18

58-9

7 18

5 6-9

7 18

57-9

7 39

84-9

7 19

57-9

8 18

58-9

8 A

ll se

als

Mon

th

Age

cla

ss

Adu

lt A

dult

Pube

scen

t Pu

besc

ent

Suba

dult

Suba

dult

Suba

dult

Mar

ch

Mea

n SD

R

ange

n

(dat

e)b

SD

Ran

ge

n (d

ate)

SD

Ran

ge

n (d

ate)

M

ean

SD

Ran

ge

n (d

ate)

SD

Ran

ge

n (d

ate)

Apr

il M

ean

May

M

ean

July

M

ean

June

17.1

21

.2

0.0-

54.2

6

(16-

21)

4.4

10.4

0.

0-27

.8

7 (1

-14)

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

55.9

20

.0

33.3

-88.

9 8

(23-

30)

36.5

16

.3

13.9

-68.

1 15

(2-

20)

-

63.8

16

.9

37.5

-91.

7 11

(20-

30)

-

-

10.1

7.

7 0.

0-20

.8

8 (1

-25)

16

.4

16.7

0.

0-52

.8

12 (4

-26)

18

.7

18.2

0.

0-5

0.0

17 (

1-31

) 29

.7

20.7

0.

0-7

0.4

19 (1

-29)

-

-

-

-

20.5

22

.8

0.0-

58.3

14

(13-

31)

14.3

21

.3

0.0-

54.2

7

(1-2

7)

20.2

19

.6

0.0-

56.6

13

(10-

31)

24.0

24

.1

0.0-

84.5

15

(1-2

1)

75.0

-

-

1 (2

)

16.8

19

.2

0.0-

58.3

28

(1-

31)

12.1

16

.7

0.0-

5 4.

2 26

(1-

27)

20.0

18

.5

0.0-

5 6.

6

40.7

26

.0

30 (

1-31

)

0.0-

91.7

57

(1-

30)

38.9

18

.5

13.9

-68.

1 16

(2-

20)

a Su

ffix

indi

cate

s ye

ar o

f ta

ggin

g.

n =

no.

of

days

with

“Ti

mel

ines

” (se

e: M

etho

ds);

(dat

e) =

dat

es b

etw

een

whi

ch t

he n d

ays

wer

e m

onito

red

in t

hat

parti

cula

r m

onth

.

176 MARINE MAMMAL SCIENCE, VOL. 18, N O . 1, 2002

U Y

1 40 a

Z 30

100

80 901

A ‘ A A

A A 0 A

0 AA A 0 0 AA A A

A

20:

10:

A

A

A 0 0

A A A A A

0 0 A o A A

A *

*O: rn 0

* ’ A * . *.A

0 .

A

A

Figure 1. Percentage of time hauled out per day by six ringed seals between 1 June (Julian day number 152) and 20 July (day 201) in 1997 (filled symbols) and 1999 (open symbols). Legend: Seal ID (suffix idicates year of tagging; for detailed information about these seals see Table 1). Haul-out time determined from “Timelines” (see Methods).

by Teilmann et al. (1999) were underestimated. A string of repeated “dry” readings in the LSR is a positive indication that the seal hauled out for a minimum of 7.5 min during the period in question. However, if, at the end of such a haul-out period, the seal, for example, lay on its back or otherwise changed position so that its transmitter lost contact with the satellites, or there was no satellite coverage, the string of “dry” readings would underestimate the actual duration. Furthermore, the discrepancy between estimates of total haul-out time in the LSR and the TIM was partly caused by the fact that fewer haul-out bouts could be extracted from the LSR than from the TIM. This difference between the two systems is probably due to the inade- quacy of satellite coverage, which means that some haul-out bouts are “missed” in the LSR.

Estimates of monthly haul-out time inferred from the TIM were about 70% of those estimated by the TAD. However, in both the LSR and the TIM, a seal had to be hauled out for a minimum period of time before a “haul-out” was registered. In contrast, the TAD accumulated all of the time when the SWS was dry, regardless of whether the seal was hauled out or was at the water surface. Therefore, the TAD will overestimate the haul-out time. Hence, our analyses indicate that the LSR tends to underestimate haul-out time whereas the TAD may overestimate it.

It can not be excluded that in the TIM, a single “dry” period (ie., a 20- min interval) surrounded by wet periods does not represent a real haul-out bout, but rather a period in which the SWS was out of the water for more than 10 min during the interval. For the seals with a continuous duty cycle,


such single 20-min periods represented an average of 20.9% (SD = 9.97, range: 12.1%-33.0%, n = 4 seals) of all haul-out periods during August- November, but only 3.1% (SD = 1.89, range: 1.0%-5.0%, n = 4 seals) of the time hauled out (9.2 h of 429.7 h hauled out). Hence, accepting these single 20 intervals as true haul-out bouts might overestimate individual haul- out times (in hours) by about 3%. Therefore, the potential error by including these short periods was neglected in the analyses of haul-out activity.

Overall, there was a fairly good coverage of seal activity as days with TIM represented 64.5% of all days monitored by the transmitters. The TIM represented the haul-outlin water activity of a seal during the 24 h prior to the day when the transmitter attempted to up-link this information to the satellites. Hence, the sampling as such was not affected by the behavior of the seal, whereas the behavior during transmission may have influenced the prob- ability of a TIM being up-linked to the satellites. Fewer TIM were received during November-April (Table 1) when ringed seals usually do not haul out on the exposed surface of the sea ice. For ringed seals in water, under the sea ice, or in a Iair, the chances of up-linking were generally low. Despite this, it seems fair to assume that the received data in a general fashion represented the haul-out activity of an animal.

Seasonal Haul-out Activity

In a study involving LSR data, haul-out time per month varied between 0.5% (November) and 3.9% (October) (Teilmann et al. 1999). Heide- Jorgensen et al. (1992) also used the activity of the SWS to determine haul- out activity. They found that a young ringed seal hauled out for 17% of the time in August, 1% of the time in September, and 4% of the time in October. Despite the different ways of monitoring haul-out activities in these and our study, they all revealed considerable individual variation in haul-our time both within the same month and seasonally,

We found that the ringed seals may haul out during all months of the year. This is in agreement with observations by Freuchen (1935) in Hudson Bay (Canada), but in contrast to his statement that in the Thule area ringed seals hauled out in October and November, but not in the darkest period of the year. However, in the Thule area it is not unusual to observe ringed seals on the ice even during the dark period.2 It is likely that seals often lay in their lairs under the snow on the ice when hauled out during the dark period. Activity data from satellite transmitters can be up-linked through a layer of snow as demonstrated in studies of denning polar bears (Messier et al. 1992).

In the present study it was assumed that a string of dry readings in the TIM and the LSR represented a period of time when the seals had been hauled out. However, we can not entirely preclude that in some cases the seals were lying motionless at the water surface. According to Vibe (1950) it is not unusual to meet ringed seals sleeping when floating at the surface of the water.

* Personal communication from Kurt Thomsen, Thule Air Base, 1998.

178 MARINE MAMMAL SCIENCE. VOL. 18. NO. 1. 2002

We found that the haul-out time increased during the spring. Relevant to most aerial surveys of ringed seals, our results indicate that the proportion of time on the ice doubled from the first to the last week of June (Fig. 1). This finding is in agreement with land-based visual observations (McLaren 1966, Smith 1973, Finley 1979).

Visual observations resulting in estimates of the fraction of the ringed seal population that is on the ice during the daily peak haul-out varied from about 50% in the latter half of May (Smith and Hammill 1981) and late June (Smith 1973) to about 70% in June and early July (Finley 1979). Using VHF telemetry Lydersen et al. (1993) estimated that two ringed seal pups hauled out for 10.4% and 63.7% of the time between mid-April and mid-May. Three ringed seal pups equipped with time-depth recorders and VHF radios spent an average of 49.7% of the time hauled out between 25 April and 15 May (Lydersen and Hammill 1993), whereas a lactating female hauled out for about 45% of the time (Lydersen 1991). Heide-JQrgensen et al. (1992) found that an adult ringed seal hauled out for an average of 53% of the time between 6 and 12 June and about 70% of the time between 13 and 20 June. A subadult ringed seal hauled out for about 60% of the time in June compared with about 15% in July (&id.). The studies involving telemetry have shown that the proportion of time that individual ringed seals haul out per day can vary substantially during all seasons. However, our study also indicated that the fraction of haul- out time increased significantly during May-June. Therefore, the timing of the surveys and the application of a period-specific correction factor are nec- essary.

We found an indication that during June subadults hauled out less than adults and pubescent seals (Table 1). However, the small sample size and the fact that the two age groups were studied in different years and places prevent us from pursuing the hypotheses of age-specific differences in haul-out patterns further.

Based on analyses of the LSR, Teilmann et al. (1999) reported that the mean duration of a haul-out period was 2.98 h (SD = 4.58, n = 54) during August- December. This is similar to the findings in the present study where the mean haul-out period was 3.12 h during the fall and 3.25 h during June-July. However, this latter mean duration of a haul-out period is considerably shorter than those reported in other studies of ringed seals during the spring. Smith (1973) observed that individual seals stayed on the ice for a minimum of 7.6 h per haul-out period during June. At Bathurst Island (ca. 76”N in Canada) the median of haul-out bouts between 9 June and 3 July was 5.1 h (Finley 1979). For ringed seal pups from late April to mid May, Lydersen and Ham- mill (1993) reported a mean haul-out period of 6.3 h, whereas an adult female seal hauled out for an average of only 0.7 h (range: 0.004-15.9 h) during May (Lydersen 1991). The differences between our observations and those of Smith (1973), Finley (1979), and Lydersen and Hammill (1993) are not readily ex- plained. If, for example, we assume that single 20-min “dry” periods in the TIM did not represent true haul-out sessions, but rather were “at water surface” periods, these intervals can been excluded. However, by excluding them


the mean of individual haul-out bouts increases only to 4.2 h, still somewhat shorter than those indicated by visual observations of haul-out activity. It might be that visual observations tend to overestimate the duration of individual haul-outs because the effort during visual observations concentrates on periods with favorable observation conditions-meaning periods of favorable weather when the seals stay up for a longer time.

Diel Haul-out Activity

Most visual studies of the die1 haul-out pattern in ringed seals have indicated that the haul-out activity peaks during the afternoon (e.g., Smith 1973, Finley 1979, Belikov and Boltunov 1998). However, in a study by Smith and Hammill (1980) continuous observations around mid-June revealed a relatively greater variation in the time of peak densities compared with earlier in the season, and maximum numbers were seen in the late hours of the night. Telemetered information on haul-out activity has shown that the individual variation is substantial. In Lydersen’s study (1991) of an adult seal, haul-out peaked at 1000-1 100 local time. In neither of the two seals studied by Heide- JGrgensen et al. (1992) did time of day significantly affect the amount of time hauled out. We found a tendency (although not statistically significant) for the seals to spend more time hauled out between 2100 and 0800 during the season of winter darkness. This is in accordance with the finding by Teilmann et al. (1999) that during the fall, ringed seals had a tendency to dive during the day and evening (until 2100) and to spend a relatively large amount of time at the surface between 2100 and 0900.

The present study showed considerable variation in individual haul-out activity at all seasons, and confirmed that the haul-out activity peaks in late June when the ringed seals spend about 57% of the time hauled-out. The study suggests that in the High Arctic a correction factor of 1.75 may be used to account for the proportion of seals that are not visible during aerial surveys conducted in late June.

ACKNOWLEDGMENTS

This study was a part of the International North Water Polynya Study (NOW), which is a part of the International Arctic Polynya Program (IAPP). The study was supported by the Danish Ministry of the Environment (DANCEA Programme) and the Greenland Institute of Natural Resources. We thank Kristian Eipe, Kurt Thomsen, Tornge Qaviaq, Sven-Erik Ascanius, and Kim Diget Christensen for valuable help during the field work. Also thanks to Randall R. Reeves (Okapi Wildlife Associates) for correcting our English. The constructive criticism offered by two reviewers and the editor is gratefully acknowledged.

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Received: 9 May 2000 Accepted: 9 April 2001

haul-out activity of ringed seals (phoca hispida) determined from satellite telemetry

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